Trends in Cancer, Vol. 2Issue 8p429–442, 2016

Adaptive Stress Responses During Tumor Metastasis and Dormancy

Daniela SenftZe'ev A. Ronai

To survive inhospitable environments, tumor cells are forced to remodel their signaling pathways by altering transcription, translation, and post-translational modifications. This adaptation is regulated in a spatial and temporal manner and gives rise to individual tumor cells with distinct gene expression and metabolic signatures. Such phenotypic heterogeneity is the result of tumor cell plasticity, which–together with the genetic background of the tumor–determines whether cells resist environmental stress, enter dormancy, or metastasize. This review summarizes our understanding of how tumor cells exploit the cellular stress response to balance proliferation, differentiation, and survival signals, as well as to remodel local and distant environments. We focus in particular on tumor metastasis, which is the greatest impediment to clinical management of cancers today.

Proc Natl Acad Sci U S A. 2016 Aug 9;113(32):8894-6. 

Monoubiquitination in proteasomal degradation.

Ronai ZA


Sci Signal. 2015 Dec 8;8(406):ra124. 

The transcription factor ATF2 promotes melanoma metastasis by suppressing protein fucosylation.

Lau E, Feng Y, Claps G, Fukuda MN, Perlina A, Donn D, Jilaveanu L, Kluger H, Freeze HH, Ronai ZA.

Melanoma is one of the most lethal skin cancers worldwide, primarily because of its propensity to metastasize. Thus, the elucidation of mechanisms that govern metastatic propensity is urgently needed. We found that protein kinase Cε (PKCε)-mediated activation of activating transcription factor 2 (ATF2) controls the migratory and invasive behaviors of melanoma cells. PKCε-dependent phosphorylation of ATF2 promoted its transcriptional repression of the gene encoding fucokinase (FUK), which mediates the fucose salvage pathway and thus global cellular protein fucosylation. In primary melanocytes and cell lines representing early-stage melanoma, the abundance of PKCε-phosphorylated ATF2 was low, thereby enabling the expression of FUK and cellular protein fucosylation, which promoted cellular adhesion and reduced motility. In contrast, increased expression of the gene encoding PKCε and abundance of phosphorylated, transcriptionally active ATF2 were observed in advanced-stage melanomas and correlated with decreased FUK expression, decreased cellular protein fucosylation, attenuated cell adhesion, and increased cell motility. Restoring fucosylation in mice either by dietary fucose supplementation or by genetic manipulation of murine Fuk expression attenuated primary melanoma growth, increased the number of intratumoral natural killer cells, and decreased distal metastasis in murine isograft models. Tumor microarray analysis of human melanoma specimens confirmed reduced fucosylation in metastatic tumors and a better prognosis for primary melanomas that had high abundance of fucosylation. Thus, inhibiting PKCε or ATF2 or increasing protein fucosylation in tumor cells may improve clinical outcome in melanoma patients.

PMID: 26645581

Cancer Research 2015 November 24: 

SBI-0640756 Attenuates the Growth of Clinically Unresponsive Melanomas by Disrupting the eIF4F Translation Initiation Complex.

Feng Y, Pinkerton AB, Hulea L, Zhang T, Davies MA, Grotegut S, Cheli Y, Yin H, Lau E, Kim H, De SK, Barile E, Pellecchia M, Bosenberg M, Li JL, James B, Hassig CA, Brown KM, Topisirovic I, Ronai ZA.

Disrupting the eukaryotic translation initiation factor 4F (eIF4F) complex offers an appealing strategy to potentiate the effectiveness of existing cancer therapies and to overcome resistance to drugs such as BRAF inhibitors (BRAFi). Here, we identified and characterized the small molecule SBI-0640756 (SBI-756), a first-in-class inhibitor that targets eIF4G1 and disrupts the eIF4F complex. SBI-756 impaired the eIF4F complex assembly independently of mTOR and attenuated growth of BRAF-resistant and BRAF-independent melanomas. SBI-756 also suppressed AKT and NF-κB signaling, but small-molecule derivatives were identified that only marginally affected these pathways while still inhibiting eIF4F complex formation and melanoma growth, illustrating the potential for further structural and functional manipulation of SBI-756 as a drug lead. In the gene expression signature patterns elicited by SBI-756, DNA damage, and cell-cycle regulatory factors were prominent, with mutations in melanoma cells affecting these pathways conferring drug resistance. SBI-756 inhibited the growth of NRAS, BRAF, and NF1-mutant melanomas in vitro and delayed the onset and reduced the incidence of Nras/Ink4a melanomas in vivo. Furthermore, combining SBI-756 and a BRAFi attenuated the formation of BRAFi-resistant human tumors. Taken together, our findings show how SBI-756 abrogates the growth of BRAF-independent and BRAFi-resistant melanomas, offering a preclinical rationale to evaluate its antitumor effects in other cancers.

PMID: 26603897

Pigment Cell Melanoma Res. 2015 Oct 30. doi: 10.1111/pcmr.12432. [Epub ahead of print] Review.

Immunogenic, cellular, and angiogenic drivers of tumor dormancy - a melanoma view.

Senft D, Ronai ZA.

In tumor cells the ability to maintain viability over long time periods without proliferation is referred to as a state of dormancy. Maintenance of dormancy is controlled by numerous cellular and environmental factors, from immune surveillance and tumor-stroma interaction to intracellular signaling. Interference of dormancy (to an "awaken" state) is associated with reduced response to therapy, resulting in relapse or in metastatic burst. Thus, maintaining a dormant state should prolong therapeutic responses and delay metastasis. Technical obstacles in studying tumor dormancy have limited our understanding of underlying mechanisms and hampered our ability to target dormant cells. In this review, we summarize progress in research in the field of immunogenic, angiogenic and cellular dormancy in diverse malignancies with particular attention to our current understanding of melanoma. 


Cell Reports. 2015 Jun; doi:10.1016/j.celrep.2015.04.049. 

Downregulation of the Ubiquitin Ligase RNF125 Underlies Resistance of Melanoma Cells to BRAF Inhibitors via JAK1 Deregulation

Kim H, Frederick DT, Levesque MP, Cooper ZA, Feng Y, Krepler C, Brill L Samuels Y, Hayward NK, Perlina A, Piris A, Zhang T, Halaban R, Herlyn MM, Brown KM, Wargo JA, Dummer R, Flaherty KT, Ronai ZA.

Despite the remarkable clinical response of melanoma harboring BRAF mutations to BRAF inhibitors (BRAFi), most tumors become resistant. Here, we identified the downregulation of the ubiquitin ligase RNF125 in BRAFi-resistant melanomas and demonstrated its role in intrinsic and adaptive resistance to BRAFi in cultures as well as its association with resistance in tumor specimens. Sox10/MITF expression correlated with and contributed to RNF125 transcription. Reduced RNF125 was associated with elevated expression of receptor tyrosine kinases (RTKs), including EGFR. Notably, RNF125 altered RTK expression through JAK1, which we identified as an RNF125 substrate. RNF125 bound to and ubiquitinated JAK1, prompting its degradation and suppressing RTK expression. Inhibition of JAK1 and EGFR signaling overcame BRAFi resistance in melanoma with reduced RNF125 expression, as shown in culture and in in vivo xenografts. Our findings suggest that combination therapies targeting both JAK1 and EGFR could be effective against BRAFi-resistant tumors with de novo low RNF125 expression.    

PMID:  Forthcoming

Cancer Cell 2015 Mar 9;27(3):354-69. 

Regulation of Glutamine Carrier Proteins by RNF5 Determines Breast Cancer Response to ER Stress-inducing Chemotherapies

Jeon YJ, Khelifa S, Ratnikov B, Scott DA, Feng Y, Parisi F, Ruller C, Lau E, Kim H, Brill LM, Jiang T, Rimm DL, Cardiff RD, Mills GB, Smith JW, Osterman AL, Kluger Y, Ronai ZA.

Many tumor cells are fueled by altered metabolism and increased glutamine (Gln) dependence. We identify regulation of the L-glutamine carrier proteins SLC1A5 and SLC38A2 (SLC1A5/38A2) by the ubiquitin ligase RNF5. Paclitaxel-induced ER stress to breast cancer (BCa) cells promotes RNF5 association, ubiquitination, and degradation of SLC1A5/38A2. This decreases Gln uptake, levels of TCA cycle components, mTOR signaling, and proliferation while increasing autophagy and cell death. Rnf5-deficient MMTV-PyMT mammary tumors were less differentiated and showed elevated SLC1A5 expression. Whereas RNF5 depletion in MDA-MB-231 cells promoted tumorigenesis and abolished paclitaxel responsiveness, SLC1A5/38A2 knockdown elicited opposing effects. Inverse RNF5(hi)/SLC1A5/38A2(lo) expression was associated with positive prognosis in BCa. Thus, RNF5 control of Gln uptake underlies BCa response to chemotherapies. 

PMID: 25759021

Oncogene 2015 Mar 2. doi: 10.1038. 

Transcriptional repression of IFNb1 by ATF2 confers melanoma resistance to therapy.

Lau E, Sedy J, Sander C, Shaw MA, Feng Y, Scortegagna M, Claps G, Robinson S, Cheng P, Srivas R, Soonthornvacharin S, Ideker T, Bosenberg M, Gonzalez R, Robinson W, Chanda SK, Ware C, Dummer R, Hoon D, Kirkwood JM, Ronai ZA.

The resistance of melanoma to current treatment modalities represents a major obstacle for durable therapeutic response, and thus the elucidation of mechanisms of resistance is urgently needed. The crucial functions of activating transcription factor-2 (ATF2) in the development and therapeutic resistance of melanoma have been previously reported, although the precise underlying mechanisms remain unclear. Here, we report a protein kinase C-ɛ (PKCɛ)- and ATF2-mediated mechanism that facilitates resistance by transcriptionally repressing the expression of interferon-β1 (IFNβ1) and downstream type-I IFN signaling that is otherwise induced upon exposure to chemotherapy. Treatment of early-stage melanomas expressing low levels of PKCɛ with chemotherapies relieves ATF2-mediated transcriptional repression of IFNβ1, resulting in impaired S-phase progression, a senescence-like phenotype and increased cell death. This response is lost in late-stage metastatic melanomas expressing high levels of PKCɛ. Notably, nuclear ATF2 and low expression of IFNβ1 in melanoma tumor samples correlates with poor patient responsiveness to biochemotherapy or neoadjuvant IFN-α2a. Conversely, cytosolic ATF2 and induction of IFNβ1 coincides with therapeutic responsiveness. Collectively, we identify an IFNβ1-dependent, cell-autonomous mechanism that contributes to the therapeutic resistance of melanoma via the PKCɛ-ATF2 regulatory axis.

PMID: 25728676

Cancer Research 2015 Apr 1;75(7):1399-412. 

PDK1 and SGK3 Contribute to the Growth of BRAF-Mutant Melanomas and Are Potential Therapeutic Targets.

Scortegagna M, Lau E, Zhang T, Feng Y, Sereduk C, Yin H, De SK, Meeth K, Platt JT, Langdon CG, Halaban R, Pellecchia M, Davies MA, Brown K, Stern DF, Bosenberg M, Ronai ZA.

Melanoma development involves members of the AGC kinase family, including AKT, PKC, and, most recently, PDK1, as elucidated recently in studies of Braf::Pten mutant melanomas. Here, we report that PDK1 contributes functionally to skin pigmentation and to the development of melanomas harboring a wild-type PTEN genotype, which occurs in about 70% of human melanomas. The PDK1 substrate SGK3 was determined to be an important mediator of PDK1 activities in melanoma cells. Genetic or pharmacologic inhibition of PDK1 and SGK3 attenuated melanoma growth by inducing G1 phase cell-cycle arrest. In a synthetic lethal screen, pan-PI3K inhibition synergized with PDK1 inhibition to suppress melanoma growth, suggesting that focused blockade of PDK1/PI3K signaling might offer a new therapeutic modality for wild-type PTEN tumors. We also noted that responsiveness to PDK1 inhibition associated with decreased expression of pigmentation genes and increased expression of cytokines and inflammatory genes, suggesting a method to stratify patients with melanoma for PDK1-based therapies. Overall, our work highlights the potential significance of PDK1 as a therapeutic target to improve melanoma treatment.

PMID: 25712345

Trends Biochem Sci. 2015 Mar;40(3):141-8. 

UPR, autophagy, and mitochondria crosstalk underlies the ER stress response.

Senft D, Ronai ZA.

Cellular stress, induced by external or internal cues, activates several well-orchestrated processes aimed at either restoring cellular homeostasis or committing to cell death. Those processes include the unfolded protein response (UPR), autophagy, hypoxia, and mitochondrial function, which are part of the global endoplasmic reticulum (ER) stress (ERS) response. When one of the ERS elements is impaired, as often occurs under pathological conditions, overall cellular homeostasis may be perturbed. Further, activation of the UPR could trigger changes in mitochondrial function or autophagy, which could modulate the UPR, exemplifying crosstalk processes. Among the numerous factors that control the magnitude or duration of these processes are ubiquitin ligases, which govern overall cellular stress outcomes. Here we summarize crosstalk among the fundamental processes governing ERS responses. 

PMID: 25656104

PLoS Genetics 2014 May 8. 10(5):e1004348. doi: 10.1371. 

Fine tuning of the UPR by the ubiquitin ligases Siah1/2.

Scortegagna M, Kim H, Li JL, Yao H, Brill LM, Han J, Lau E, Bowtell D, Haddad G, Kaufman RJ, Ronai ZA.

The endoplasmic reticulum (ER) responds to changes in intracellular homeostasis through activation of the unfolded protein response (UPR). Yet, it is not known how UPR-signaling coordinates adaptation versus cell death. Previous studies suggested that signaling through PERK/ATF4 is required for cell death. We show that high levels of ER stress (i.e., ischemia-like conditions) induce transcription of the ubiquitin ligases Siah1/2 through the UPR transducers PERK/ATF4 and IRE1/sXBP1. In turn, Siah1/2 attenuates proline hydroxylation of ATF4, resulting in its  stabilization, thereby augmenting ER stress output. Conversely, ATF4 activation is reduced upon Siah1/2 KD in cultured cells, which attenuates ER stress-induced cell death. Notably, Siah1a(+/-)::Siah2(-/-) mice subjected to neuronal ischemia exhibited smaller infarct volume and were protected from ischemia-induced death, compared with the wild type (WT) mice. In all, Siah1/2 constitutes an obligatory fine-tuning mechanism that predisposes cells to death under severe ER stress conditions.

PMID: 24809345

Oncogene. 2014 Aug 21;33(34):4330-9.

Genetic inactivation or pharmacological inhibition of Pdk1 delays development and inhibits metastasis of BrafV600E::Pten-/- melanoma.

Scortegagna M, Ruller C, Feng Y, Lazova R, Kluger H, Li JL, De SK, Rickert R, Pellecchia M, Bosenberg M, Ronai ZA.

Phosphoinositide-dependent kinase-1 (PDK1) is a serine/threonine protein kinase that phosphorylates members of the conserved AGC kinase superfamily, including AKT and protein kinase C (PKC), and is implicated in important cellular processes including survival, metabolism and tumorigenesis. In large cohorts of nevi and melanoma samples, PDK1 expression was significantly higher in primary melanoma, compared with nevi, and was further increased in metastatic melanoma. PDK1 expression suffices for its activity, owing to auto-activation, or elevated phosphorylation by phosphoinositide 3'-OH-kinase (PI3K). Selective inactivation of Pdk1 in the melanocytes of BrafV600E::Pten-/- or BrafV600E::Cdkn2a-/-::Pten-/- mice delayed the development of pigmented lesions and melanoma induced by systemic or local administration of 4-hydroxytamoxifen. Melanoma invasion and metastasis were significantly reduced or completely prevented by Pdk1 deletion. Administration of the PDK1 inhibitor GSK2334470 (PDKi) effectively delayed melanomagenesis and metastasis in BrafV600E::Pten-/- mice. Pdk1-/- melanomas exhibit a marked decrease in the activity of AKT, P70S6K and PKC. Notably, PDKi was as effective in inhibiting AGC kinases and colony forming efficiency of melanoma with Pten wild-type (WT) genotypes. Gene expression analyses identified Pdk1-dependent changes in FOXO3a-regulated genes, and inhibition of FOXO3a restored proliferation and colony formation of Pdk1-/- melanoma cells. Our studies provide direct genetic evidence for the importance of PDK1, in part through FOXO3a-dependent pathway, in melanoma development and progression

PMID: 24037523

Chem Biol. 2013 Aug 22;20(8):973-82. 

Structure-based design of covalent Siah inhibitors.

Stebbins JL, Santelli E, Feng Y, De SK, Purves A, Motamedchaboki K, Wu B, Ronai ZA, Liddington RC, Pellecchia M.

The E3 ubiquitin ligase Siah regulates key cellular events that are central to cancer development and progression. A promising route to Siah inhibition is disrupting its interactions with adaptor proteins. However, typical of protein-protein interactions, traditional unbiased approaches to ligand discovery did not produce viable hits against this target, despite considerable effort and a multitude of approaches. Ultimately, a rational structure-based design strategy was successful for the identification of Siah inhibitors in which peptide binding drives specific covalent bond formation with the target. X-ray crystallography, mass spectrometry, and functional data demonstrate that these peptide mimetics are efficient covalent inhibitors of Siah and antagonize Siah-dependent regulation of Erk and Hif signaling in the cell. The proposed strategy may result useful as a general approach to the design of peptide-based inhibitors of other protein-protein interactions. 

PMID: 23891150

Genes Cancer. 2013 Sep;4(9-10):369-77. 

RACK1 Function in Cell Motility and Protein Synthesis.

Gandin V, Senft D, Topisirovic I, Ronai ZA.

The receptor for activated C kinase 1 (RACK1) serves as an adaptor for a number of proteins along the MAPK, protein kinase C, and Src signaling pathways. The abundance and near ubiquitous expression of RACK1 reflect its role in coordinating signaling molecules for many critical biological processes, from mRNA translation to cell motility to cell survival and death. Complete deficiency of Rack1 is embryonic lethal, but the recent development of genetic Rack1 hypomorphic mice has highlighted the central role that RACK1 plays in cell movement and protein synthesis. This review focuses on the importance of RACK1 in these processes and places the recent work in the larger context of understanding RACK1 function. 

PMID: 24349634

Trends Biochem Sci. 2013 Jul 16. pii: S0968-0004(13)00100-X. doi: 10.1016/j.tibs.2013.06.008. [Epub ahead of print]

Emerging roles of E3 ubiquitin ligases in autophagy

Kuang EQi JRonai Z.

Autophagy is an evolutionarily conserved intracellular catabolic process that delivers cytoplasmic components to lysosomes for degradation and recycling. Although originally considered to be a non-selective pathway, it is now recognized that autophagy is involved in selective processes, including the turnover of organelles, removal of protein aggregates, and elimination of intracellular pathogens. This specificity implies that cargo recognition and processing by the autophagy machinery are tightly regulated processes. In support of this, various forms of post-translational modification have been implicated in the regulation of autophagy, one of which is the ubiquitin-proteasome system. Here we review current understanding of the role of ubiquitylation in the control of autophagy.

PMID: 23870665

Cell Biochem Biophys. 2013 May 23. [Epub ahead of print]

Regulators and effectors of Siah ubiquitin ligases

Qi JKim HScortegagna MRonai ZA.

The Siah ubiquitin ligases are members of the RING finger E3 ligases. The Siah E3s are conserved from fly to mammals. Primarily implicated in cellular stress responses, Siah ligases play a key role in hypoxia, through the regulation of HIF-1α transcription stability and activity. Concomitantly, physiological conditions associated with varying oxygen tension often highlight the importance of Siah, as seen in cancer and neurodegenerative disorders. Notably, recent studies also point to the role of these ligases in fundamental processes including DNA damage response, cellular organization and polarity. This review summarizes the current understanding of upstream regulators and downstream effectors of Siah.

PMID: 23700162

Oncogene. 2013 May 6. doi: 10.1038/onc.2013.149. [Epub ahead of print]

Siah2 regulates tight junction integrity and cell polarity through control of ASPP2 stability

Kim HClaps GMöller ABowtell DLu XRonai ZA.

Changes in cell adhesion and polarity are closely associated with epithelial cell transformation and metastatic capacity. The tumor suppressor protein ASPP (Apoptosis-Stimulating Proteins of p53) 2 has been implicated in control of cell adhesion and polarity through its effect on the PAR complex. Here we demonstrate that under hypoxic conditions, the ubiquitin ligase Siah (seven in absentia homolog)2 controls ASPP2 availability, with concomitant effect on epithelial cell polarity. LC-MS/MS analysis identified ASPP2 and ASPP1 as Siah2-interacting proteins. Biochemical analysis confirmed this interaction and mapped degron motifs within ASPP2, which are required for Siah2-mediated ubiquitination and proteasomal-dependent degradation. Inhibition of Siah2 expression increases ASPP2 levels and enhances ASPP2-dependent maintenance of tight junction (TJ) integrity, and polarized architecture in three dimensional (3D) organotypic culture. Conversely, increase of Siah2 expression under hypoxia decreases ASPP2 levels and the formation of apical polarity in 3D culture. In all, our studies demonstrate the role of Siah2 in regulation of TJ integrity and cell polarity under hypoxia, through its regulation of ASPP2 stability.

PMID: 23644657

Mol Cell Biol. 2013 Jul;33(13):2510-26. doi: 10.1128/MCB.01362-12. Epub 2013 Apr 22.

Degradation of newly synthesized polypeptides by ribosome-associated RACK1/c-Jun N-terminal kinase/eukaryotic elongation factor 1A2 complex.

Gandin VGutierrez GJBrill LMVarsano TFeng YAza-Blanc PAu QMcLaughlan SFerreira TAAlain TSonenberg NTopisirovic IRonai ZA.

Folding of newly synthesized polypeptides (NSPs) into functional proteins is a highly regulated process. Rigorous quality control ensures that NSPs attain their native fold during or shortly after completion of translation. Nonetheless, signaling pathways that govern the degradation of NSPs in mammals remain elusive. We demonstrate that the stress-induced c-Jun N-terminal kinase (JNK) is recruited to ribosomes by the receptor for activated protein C kinase 1 (RACK1). RACK1 is an integral component of the 40S ribosome and an adaptor for protein kinases. Ribosome-associated JNK phosphorylates the eukaryotic translation elongation factor 1A isoform 2 (eEF1A2) on serines 205 and 358 to promote degradation of NSPs by the proteasome. These findings establish a role for a RACK1/JNK/eEF1A2 complex in the quality control of NSPs in response to stress.

PMID: 23608534

Clin Cancer Res. 2013 May 15;19(10):2710-22. doi: 10.1158/1078-0432.CCR-12-2689. Epub 2013 Apr 15.

Inhibition of melanoma growth by small molecules that promote the mitochondrial localization of ATF2.

Varsano TLau EFeng YGarrido MMilan LHeynen-Genel SHassig CARonai ZA.

Effective therapy for malignant melanoma, the leading cause of death from skin cancer, remains an area of significant unmet need in oncology. The elevated expression of PKCε in advanced metastatic melanoma results in the increased phosphorylation of the transcription factor ATF2 on threonine 52, which causes its nuclear localization and confers its oncogenic activities. The nuclear-to-mitochondrial translocation of ATF2 following genotoxic stress promotes apoptosis, a function that is largely lost in melanoma cells, due to its confined nuclear localization. Therefore, promoting the nuclear export of ATF2, which sensitizes melanoma cells to apoptosis, represents a novel therapeutic modality. We conducted a pilot high-throughput screen of 3,800 compounds to identify small molecules that promote melanoma cell death by inducing the cytoplasmic localization of ATF2. The imaging-based ATF2 translocation assay was conducted using UACC903 melanoma cells that stably express doxycycline-inducible GFP-ATF2. We identified two compounds (SBI-0089410 and SBI-0087702) that promoted the cytoplasmic localization of ATF2, reduced cell viability, inhibited colony formation, cell motility, and anchorage-free growth, and increased mitochondrial membrane permeability. SBI-0089410 inhibited the 12-O-tetradecanoylphorbol-l3-acetate (TPA)-induced membrane translocation of protein kinase C (PKC) isoforms, whereas both compounds decreased ATF2 phosphorylation by PKCε and ATF2 transcriptional activity. Overexpression of either constitutively active PKCε or phosphomimic mutant ATF2(T52E) attenuated the cellular effects of the compounds. The imaging-based high-throughput screen provides a proof-of-concept for the identification of small molecules that block the oncogenic addiction to PKCε signaling by promoting ATF2 nuclear export, resulting in mitochondrial membrane leakage and melanoma cell death.

PMID: 23589174

Cancer Cell. 2013 Mar 18;23(3):332-46. doi: 10.1016/j.ccr.2013.02.016.

The E3 ubiquitin ligase Siah2 contributes to castration-resistant prostate cancer by regulation of androgen receptor transcriptional activity.

Qi JTripathi MMishra RSahgal NFazli LEttinger SPlaczek WJClaps GChung LWBowtell DGleave MBhowmick NRonai ZA.

Understanding the mechanism underlying the regulation of the androgen receptor (AR), a central player in the development of castration-resistant prostate cancer (CRPC), holds promise for overcoming the challenge of treating CRPC. We demonstrate that the ubiquitin ligase Siah2 targets a select pool of NCOR1-bound, transcriptionally-inactive AR for ubiquitin-dependent degradation, thereby promoting expression of select AR target genes implicated in lipid metabolism, cell motility, and proliferation. Siah2 is required for prostate cancer cell growth under androgen-deprivation conditions in vitro and in vivo, and Siah2 inhibition promotes prostate cancer regression upon castration. Notably, Siah2 expression is markedly increased in human CRPCs. Collectively, we find that selective regulation of AR transcriptional activity by the ubiquitin ligase Siah2 is important for CRPC development.

PMID: 23518348

PLoS Genet. 2012;8(10):e1003007. doi: 10.1371/journal.pgen.1003007. Epub 2012 Oct 18.

Regulation of ATG4B stability by RNF5 limits basal levels of autophagy and influences susceptibility to bacterial infection

Kuang E, Okumura CY, Sheffy-Levin S, Varsano T, Shu VC, Qi J, Niesman IR, Yang HJ, López-Otín C, Yang WY, Reed JC, Broday L, Nizet V, Ronai ZA.

Autophagy is the mechanism by which cytoplasmic components and organelles are degraded by the lysosomal machinery in response to diverse stimuli including nutrient deprivation, intracellular pathogens, and multiple forms of cellular stress. Here, we show that the membrane-associated E3 ligase RNF5 regulates basal levels of autophagy by controlling the stability of a select pool of the cysteine protease ATG4B. RNF5 controls the membranal fraction of ATG4B and limits LC3 (ATG8) processing, which is required for phagophore and autophagosome formation. The association of ATG4B with-and regulation of its ubiquitination and stability by-RNF5 is seen primarily under normal growth conditions. Processing of LC3 forms, appearance of LC3-positive puncta, and p62 expression are higher in RNF5(-/-) MEF. RNF5 mutant, which retains its E3 ligase activity but does not associate with ATG4B, no longer affects LC3 puncta. Further, increased puncta seen in RNF5(-/-) using WT but not LC3 mutant, which bypasses ATG4B processing, substantiates the role of RNF5 in early phases of LC3 processing and autophagy. Similarly, RNF-5 inactivation in Caenorhabditis elegans increases the level of LGG-1/LC3::GFP puncta. RNF5(-/-) mice are more resistant to group A Streptococcus infection, associated with increased autophagosomes and more efficient bacterial clearance by RNF5(-/-) macrophages. Collectively, the RNF5-mediated control of membranalATG4B reveals a novel layer in the regulation of LC3 processing and autophagy.

PMID: 23093945

Pigment Cell Melanoma Res. 2013 Jan;26(1):136-42. doi: 10.1111/pcmr.12033. Epub 2012 Nov 2.

Inhibition of melanoma develoopment in the Nras((Q61K))::Ink4a(-/-) mouse model by the small molecule BI-69A11

Feng Y, Lau E, Scortegagna M, Ruller C, De SK, Barile E, Krajewski S, Aza-Blanc P, Williams R, Pinkerton AB, Jackson M, Chin L, Pellecchia M, Bosenberg M, Ronai ZA.

To date, there are no effective therapies for tumors bearing NRAS mutations, which are present in 15-20% of human melanomas. Here we extend our earlier studies where we demonstrated that the small molecule BI-69A11 inhibits the growth of melanoma cell lines. Gene expression analysis revealed the induction of interferon- and cell death-related genes that were associated with responsiveness of melanoma cell lines to BI-69A11. Strikingly, the administration of BI-69A11 inhibited melanoma development in genetically modified mice bearing an inducible form of activated Nras and a deletion of the Ink4a gene (Nras((Q61K)) ::Ink4a(-/-) ). Biweekly administration of BI-69A11 starting at 10 weeks or as late as 24 weeks after the induction of mutant Nras expression inhibited melanoma development (100 and 36%, respectively). BI-69A11 treatment did not inhibit the development of histiocytic sarcomas, which constitute about 50% of the tumors in this model. BI-69A11-resistant Nras((Q61K)) ::Ink4a(-/-) tumors exhibited increased CD45 expression, reflective of immune cell infiltration and upregulation of gene networks associated with the cytoskeleton, DNA damage response, and small molecule transport. The ability to attenuate the development of NRAS mutant melanomas supports further development of BI-69A11 for clinical assessment.

PMID: 23035722

PLoS One. 2012;7(4):e35520. doi: 10.1371/journal.pone.0035520. Epub 2012 Apr 23.

The anaphase-promoting complex or cyclosome supports cell survival in response to endoplasmic reticulum stress 

Chen M, Gutierrez GJ, Ronai ZA.

The anaphase-promoting complex or cyclosome (APC/C) is a multi-subunit ubiquitin ligase that regulates exit from mitosis and G1 phase of the cell cycle. Although the regulation and function of APC/C(Cdh1) in the unperturbed cell cycle is well studied, little is known of its role in non-genotoxic stress responses. Here, we demonstrate the role of APC/C(Cdh1) (APC/C activated by Cdh1 protein) in cellular protection from endoplasmic reticulum (ER) stress. Activation of APC/C(Cdh1) under ER stress conditions is evidenced by Cdh1-dependent degradation of its substrates. Importantly, the activity of APC/C(Cdh1) maintains the ER stress checkpoint, as depletion of Cdh1 by RNAi impairs cell cycle arrest and accelerates cell death following ER stress. Our findings identify APC/C(Cdh1) as a regulator of cell cycle checkpoint and cell survival in response to proteotoxic insults.

PMID: 22539978

Cell. 2012 Feb 3;148(3):543-55. doi: 10.1016/j.cell.2012.01.016.

PKCℇ promotes oncogenic functions of ATF2 in the nucleus while blocking its apoptotic function at mitochondria 

Lau E, Kluger H, Varsano T, Lee K, Scheffler I, Rimm DL, Ideker T, Ronai ZA.

The transcription factor ATF2 elicits oncogenic activities in melanoma and tumor suppressor activities in nonmalignant skin cancer. Here, we identify that ATF2 tumor suppressor function is determined by its ability to localize at the mitochondria, where it alters membrane permeability following genotoxic stress. The ability of ATF2 to reach the mitochondria is determined by PKCε, which directs ATF2 nuclear localization. Genotoxic stress attenuates PKCε effect on ATF2; enables ATF2 nuclear export and localization at the mitochondria, where it perturbs the HK1-VDAC1 complex; increases mitochondrial permeability; and promotes apoptosis. Significantly, high levels of PKCε, as seen in melanoma cells, block ATF2 nuclear export and function at the mitochondria, thereby attenuating apoptosis following exposure to genotoxic stress. In melanoma tumor samples, high PKCε levels associate with poor prognosis. Overall, our findings provide the framework for understanding how subcellular localization enables ATF2 oncogenic or tumor suppressor functions.

PMID: 22304920

F1000 » Evaluated Articles » ranked - 8


Mol Cell. 2011 Nov 18;44(4):532-44. doi: 10.1016/j.molcel.2011.08.045.

Fine-tuning of Drp1/Fis1 availability by AKAP121/Siah2 regulates mitochondrial adaptation to hypoxia 

Kim H, Scimia MC, Wilkinson D, Trelles RD, Wood MR, Bowtell D, Dillin A, Mercola M, Ronai ZA.

Defining the mechanisms underlying the control of mitochondrial fusion and fission is critical to understanding cellular adaptation to diverse physiological conditions. Here we demonstrate that hypoxia induces fission of mitochondrial membranes, dependent on availability of the mitochondrial scaffolding protein AKAP121. AKAP121 controls mitochondria dynamics through PKA-dependent inhibitory phosphorylation of Drp1 and PKA-independent inhibition of Drp1-Fis1 interaction. Reduced availability of AKAP121 by the ubiquitin ligase Siah2 relieves Drp1 inhibition by PKA and increases its interaction with Fis1, resulting in mitochondrial fission. High AKAP121 levels, seen in cells lacking Siah2, attenuate fission and reduce apoptosis of cardiomyocytes under simulated ischemia. Infarct size and degree of cell death were reduced in Siah2(-/-) mice subjected to myocardial infarction. Inhibition of Siah2 or Drp1 in hatching C. elegans reduces their life span. Through modulating Fis1/Drp1 complex availability, our studies identify Siah2 as a key regulator of hypoxia-induced mitochondrial fission and its physiological significance in ischemic injury and nematode life span.

PMID: 22099302

Cancer Cell. 2010 Jul 13;18(1):23-38.

Siah2-dependent concerted activity of HIF and FoxA2 regulates formation of neuroendocrine phenotype and neuroendocrine prostate tumors.

Qi JNakayama KCardiff RDBorowsky ADKaul KWilliams RKrajewski SMercola DCarpenter PMBowtell DRonai ZA.

Comment in:

Neuroendocrine (NE) phenotype, seen in >30% of prostate adenocarcinomas (PCa), and NE prostate tumors are implicated in aggressive prostate cancer. Formation of NE prostate tumors in the TRAMP mouse model was suppressed in mice lacking the ubiquitin ligase Siah2, which regulates HIF-1alpha availability. Cooperation between HIF-1alpha and FoxA2, a transcription factor expressed in NE tissue, promotes recruitment of p300 to transactivate select HIF-regulated genes, Hes6, Sox9, and Jmjd1a. These HIF-regulated genes are highly expressed in metastatic PCa and required for hypoxia-mediated NE phenotype, metastasis in PCa, and the formation of NE tumors. Tissue-specific expression of FoxA2 combined with Siah2-dependent HIF-1alpha availability enables a transcriptional program required for NE prostate tumor development and NE phenotype in PCa.

PMID: 20609350

Nat Cell Biol. 2010 Jul;12(7):686-95. Epub 2010 Jun 27.

Interplay between Cdh1 and JNK activity during the cell cycle

Gutierrez GJTsuji TChen MJiang WRonai ZA.

The ubiquitin ligase APC/C(Cdh1) coordinates degradation of key cell cycle regulators. We report here that a nuclear-localized portion of the stress-activated kinase JNK is degraded by the APC/C(Cdh1) during exit from mitosis and the G1 phase of the cell cycle. Expression of a non-degradable JNK induces prometaphase-like arrest and aberrant mitotic spindle dynamics. Moreover, JNK phosphorylates Cdh1 directly, during G2 and early mitosis, changing its subcellular localization and attenuating its ability to activate the APC/C during G2/M. This regulatory mechanism between JNK and Cdh1 reveals an important function for JNK during the cell cycle.

PMID: 20581839

Genes Cancer. 2010 Apr 1;1(4):316-330.

Radiation Sensitivity and Tumor Susceptibility in ATM Phospho-Mutant ATF2 Mice.

Li SEzhevsky SDewing ACato MHScortegagna MBhoumik ABreitwieser WBraddock DEroshkin AQi JChen MKim JYJones SJones NRickert RRonai ZA.

The transcription factor ATF2 was previously shown to be an ATM substrate. Upon phosphorylation by ATM, ATF2 exhibits a transcription-independent function in the DNA damage response through localization to DNA repair foci and control of cell cycle arrest. To assess the physiological significance of this phosphorylation, we generated ATF2 mutant mice in which the ATM phosphoacceptor sites (S472/S480) were mutated (ATF2(KI)). ATF2(KI) mice are more sensitive to ionizing radiation (IR) than wild-type (ATF2 (WT)) mice: following IR, ATF2(KI) mice exhibited higher levels of apoptosis in the intestinal crypt cells and impaired hepatic steatosis. Molecular analysis identified impaired activation of the cell cycle regulatory protein p21(Cip/Waf1) in cells and tissues of IR-treated ATF2(KI) mice, which was p53 independent. Analysis of tumor development in p53(KO) crossed with ATF2(KI) mice indicated a marked decrease in amount of time required for tumor development. Further, when subjected to two-stage skin carcinogenesis process, ATF2(KI) mice developed skin tumors faster and with higher incidence, which also progressed to the more malignant carcinomas, compared with the control mice. Using 3 mouse models, we establish the importance of ATF2 phosphorylation by ATM in the acute cellular response to DNA damage and maintenance of genomic stability.

Nat Rev Cancer. 2010 Jan;10(1):65-76.

Emerging roles of ATF2 and the dynamic AP1 network in cancer.

Lopez-Bergami PLau ERonai Z.

Cooperation among transcription factors is central for their ability to execute specific transcriptional programmes. The AP1 complex exemplifies a network of transcription factors that function in unison under normal circumstances and during the course of tumour development and progression. This Perspective summarizes our current understanding of the changes in members of the AP1 complex and the role of ATF2 as part of this complex in tumorigenesis.

PMID: 20029425


J Biol Chem. 2009 Nov 12. [Epub ahead of print]

C-JUN regulates PDK1 transcription: implication for AKT and PKC activities and melanoma tumorigenesis.

Lopez-Bergami PKim HDewing AGoydos JAaronson SRonai ZA.

Mutations in N-Ras and B-Raf, which commonly occur in melanomas, result in constitutive activation of the Mitogen-Activated Protein Kinase (MAPK)/ Extracellular signal-Regulated protein Kinase (ERK) signaling. Active ERK increases expression and activity of the c-Jun transcription factor, linking ERK and Jun N-terminal Kinase (JNK) cascades. Here, we show that c-Jun regulates transcription of Phosphoinositide-Dependent Kinase 1 (PDK1) with a concomitant impact on Akt and Protein Kinase C (PKC) activity and related substrates. Inhibition of c-Jun reduces PDK1 expression and attenuates Akt and PKC activity, which can be restored by exogenous PDK1. c-Jun regulation of PDK1 in melanoma contributes to growth rate and the ability to form tumors in mice. Correspondingly, increased levels of c-Jun in melanoma cell lines coincide with upregulation of PDK1 and phosphorylation of PKC and Akt. The identification of c-Jun as a transcriptional regulator of PDK1 expression highlights key mechanism underlying c-Jun oncogenic activity, and provides new insight into the nature of upregulated Akt and PKC in melanoma.

PMID: 19910471


Proc Natl Acad Sci U S A. 2009 Aug 4;106(31):12676-81. Epub 2009 Jul 27.

Control of p53 multimerization by Ubc13 is JNK-regulated.

Topisirovic IGutierrez GJChen MAppella EBorden KLRonai ZA.

The p53 tumor suppressor protein is a key regulator of cellular proliferation and survival whose function is tightly regulated at the levels of transcription and protein stability. Here, we unveil the fine control of p53 on translationally active polysomes. We have previously reported that Ubc13, an E2 ubiquitin-conjugating enzyme, directly regulates p53 localization and transcriptional activity. We now demonstrate that the association of p53 and Ubc13 on polysomes requires ongoing translation and results in p53 ubiquitination that interferes with its tetramerization. JNK phosphorylation of p53 at Threonine 81 occurring on polysomes is required for the dissociation of Ubc13 from p53, leading to p53 multimerization and transcriptional activation. Inhibition of JNK activity or expression of a nonphosphorylatable mutant of p53 maintains an Ubc13-p53 complex that inhibits p53 multimerization. Our findings reveal a layer in the regulation of p53 multimerization that requires the concerted action of JNK and Ubc13 on polysome-bound p53.

PMID: 19651615 


Proc Natl Acad Sci U S A. 2008 Oct 28;105(43):16713-8. Epub 2008 Oct 22.

The ubiquitin ligase Siah2 regulates tumorigenesis and metastasis by HIF-dependent and -independent pathways.

Qi JNakayama KGaitonde SGoydos JSKrajewski SEroshkin ABar-Sagi DBowtell DRonai Z.

The ubiquitin ligase Siah2 has been shown to regulate prolyl hydroxylase 3 (PHD3) stability with concomitant effect on HIF-1alpha availability. Because HIF-1alpha is implicated in tumorigenesis and metastasis, we used SW1 mouse melanoma cells, which develop primary tumors with a propensity to metastasize, in a syngeneic mouse model to assess a possible role for Siah2 in these processes. Inhibiting Siah2 activity by expressing a peptide designed to outcompete association of Siah2-interacting proteins reduced metastasis through HIF-1alpha without affecting tumorigenesis. Conversely, inhibiting Siah2 activity by means of a dominant-negative Siah2 RING mutant primarily reduced tumorigenesis through the action of Sprouty 2, a negative regulator of Ras signaling. Consistent with our findings, reduced expression of PHD3 and Sprouty2 was observed in more advanced stages of melanoma tumors. Using complementary approaches, our data establish the role of Siah2 in tumorigenesis and metastasis by HIF-dependent and -independent mechanisms.

PMID: 18946040 


Mol Biol Cell. 2008 Nov;19(11):5019-28. Epub 2008 Sep 10.

JAMP optimizes ERAD to protect cells from unfolded proteins.

Tcherpakov MBroday LDelaunay AKadoya TKhurana AErdjument-Bromage HTempst PQiu XBDeMartino GNRonai Z.

Clearance of misfolded proteins from the ER is central for maintenance of cellular homeostasis. This process requires coordinated recognition, ER-cytosol translocation, and finally ubiquitination-dependent proteasomal degradation. Here, we identify an ER resident seven-transmembrane protein (JAMP) that links ER chaperones, channel proteins, ubiquitin ligases, and 26S proteasome subunits, thereby optimizing degradation of misfolded proteins. Elevated JAMP expression promotes localization of proteasomes at the ER, with a concomitant effect on degradation of specific ER-resident misfolded proteins, whereas inhibiting JAMP promotes the opposite response. Correspondingly, a jamp-1 deleted Caenorhabditis elegans strain exhibits hypersensitivity to ER stress and increased UPR. Using biochemical and genetic approaches, we identify JAMP as important component for coordinated clearance of misfolded proteins from the ER.

PMID: 18784250


PLoS ONE. 2008 Feb 13;3(2):e1609.

The ER-Bound RING Finger Protein 5 (RNF5/RMA1) Causes Degenerative Myopathy in Transgenic Mice and Is Deregulated in Inclusion Body Myositis.

Delaunay ABromberg KDHayashi YMirabella MBurch DKirkwood BSerra CMalicdan MCMizisin APMorosetti RBroccolini AGuo LTJones SNLira SAPuri PLShelton GDRonai Z.

Growing evidence supports the importance of ubiquitin ligases in the pathogenesis of muscular disorders, although underlying mechanisms remain largely elusive. Here we show that the expression of RNF5 (aka RMA1), an ER-anchored RING finger E3 ligase implicated in muscle organization and in recognition and processing of malfolded proteins, is elevated and mislocalized to cytoplasmic aggregates in biopsies from patients suffering from sporadic-Inclusion Body Myositis (sIBM). Consistent with these findings, an animal model for hereditary IBM (hIBM), but not their control littermates, revealed deregulated expression of RNF5. Further studies for the role of RNF5 in the pathogenesis of s-IBM and more generally in muscle physiology were performed using RNF5 transgenic and KO animals. Transgenic mice carrying inducible expression of RNF5, under control of beta-actin or muscle specific promoter, exhibit an early onset of muscle wasting, muscle degeneration and extensive fiber regeneration. Prolonged expression of RNF5 in the muscle also results in the formation of fibers containing congophilic material, blue-rimmed vacuoles and inclusion bodies. These phenotypes were associated with altered expression and activity of ER chaperones, characteristic of myodegenerative diseases such as s-IBM. Conversely, muscle regeneration and induction of ER stress markers were delayed in RNF5 KO mice subjected to cardiotoxin treatment. While supporting a role for RNF5 Tg mice as model for s-IBM, our study also establishes the importance of RNF5 in muscle physiology and its deregulation in ER stress associated muscular disorders.

PMID: 18270596 


Proc Natl Acad Sci U S A. 2008 Feb 5;105(5):1674-9.  

Suppressor role of activating transcription factor 2 (ATF2) in skin cancer.

Bhoumik AFichtman BDerossi CBreitwieser WKluger HMDavis SSubtil AMeltzer PKrajewski SJones NRonai Z.

Activating transcription factor 2 (ATF2) regulates transcription in response to stress and growth factor stimuli. Here, we use a mouse model in which ATF2 was selectively deleted in keratinocytes. Crossing the conditionally expressed ATF2 mutant with K14-Cre mice (K14.ATF2(f/f)) resulted in selective expression of mutant ATF2 within the basal layer of the epidermis. When subjected to a two-stage skin carcinogenesis protocol [7,12-dimethylbenz[a]anthracene/phorbol 12-tetradecanoate 13-acetate (DMBA/TPA)], K14.ATF2(f/f) mice showed significant increases in both the incidence and prevalence of papilloma development compared with the WT ATF2 mice. Consistent with these findings, keratinocytes of K14.ATF2(f/f) mice exhibit greater anchorage-independent growth compared with ATF2 WT keratinocytes. Papillomas of K14.ATF2(f/f) mice exhibit reduced expression of presenilin1, which is associated with enhanced beta-catenin and cyclin D1, and reduced Notch1 expression. Significantly, a reduction of nuclear ATF2 and increased beta-catenin expression were seen in samples of squamous and basal cell carcinoma, as opposed to normal skin. Our data reveal that loss of ATF2 transcriptional activity serves to promote skin tumor formation, thereby indicating a suppressor activity of ATF2 in skin tumor formation.


Cancer Res. 2007 Sep 1;67(17):8172-9.

Increased expression of the E3 ubiquitin ligase RNF5 is associated with decreased survival in breast cancer.

Bromberg KDKluger HMDelaunay AAbbas SDiVito KAKrajewski SRonai Z.

Signal Transduction Program, The Burnham Institute for Medical Research, La Jolla, CA 92037, USA.

The selective ubiquitination of proteins by ubiquitin E3 ligases plays an important regulatory role in control of cell differentiation, growth, and transformation and their dysregulation is often associated with pathologic outcomes, including tumorigenesis. RNF5 is an E3 ubiquitin ligase that has been implicated in motility and endoplasmic reticulum stress response. Here, we show that RNF5 expression is up-regulated in breast cancer tumors and related cell lines. Elevated expression of RNF5 was seen in breast cancer cell lines that became more sensitive to cytochalasin D- and paclitaxel-induced apoptosis following its knockdown with specific short interfering RNA. Inhibition of RNF5 expression markedly decreased cell proliferation and caused a reorganization of the actin cytoskeleton in response to stress in MCF-7 but not in p53 mutant breast cancer cells, suggesting a p53-dependent function. Significantly, high levels of RNF5 were associated with decreased survival in human breast cancer specimens. Similarly, RNF5 levels were higher in metastatic melanoma specimens and in melanoma, leukemia, ovarian, and renal tumor-derived cell lines, suggesting that increased RNF5 expression may be a common event during tumor progression. These results indicate that RNF5 is a novel regulator of breast cancer progression through its effect on actin cytoskeletal alterations, which also affect sensitivity of breast cancer cells to cytoskeletal targeting antineoplastic agents.

PMID: 17804730 

Cancer Cell  2007 May 11(5):447-60

Rewired ERK-JNK signaling pathways in melanoma.

Lopez-Bergami P, Huang C, Goydos JS, Yip D, Bar-Eli M, Herlyn M, Smalley KS, Mahale A, Eroshkin A, Aaronson S, Ronai Z
 Constitutive activation of MEK-ERK signaling is often found in melanomas. Here, we identify a mechanism that links ERK with JNK signaling in human melanoma. Constitutively active ERK increases c-Jun transcription and stability, which are mediated by CREB and GSK3, respectively. Subsequently, c-Jun increases transcription of target genes, including RACK1, an adaptor protein that enables PKC to phosphorylate and enhance JNK activity, enforcing a feed-forward mechanism of the JNK-Jun pathway. Activated c-Jun is also responsible for elevated cyclin D1 expression, which is frequently overexpressed in human melanoma. Our data reveal that, in human melanoma, the rewired ERK signaling pathway upregulates JNK and activates the c-Jun oncogene and its downstream targets, including RACK1 and cyclin D1.

PMID: 17482134

 Mol Cell. 2005 Aug 19;19(4):578-9.

RACK1 mediates activation of JNK by protein kinase C

Lopez-Bergami PHabelhah HBhoumik AZhang WWang LHRonai Z.

Activation of the Jun-N-terminal kinase (JNK) signaling cascade by phorbol esters (TPA) or protein kinase C (PKC) is well documented, although the underlying mechanism is not known. Here, we demonstrate that the receptor for activated C kinase 1 (RACK1) serves as an adaptor for PKC-mediated JNK activation. Phosphorylation of JNK by PKC occurs on Ser129 and requires the presence of RACK1. Ser129 phosphorylation augments JNK phosphorylation by MKK4 and/or MKK7 and is required for JNK activation by TPA, TNFalpha, UV irradiation, and PKC, but not by anisomycin or MEKK1. Inhibition of RACK1 expression by siRNA attenuates JNK activation, sensitizes melanoma cells to UV-induced apoptosis, and reduces their tumorigenicity in nude mice. In finding the role of RACK1 in activation of JNK by PKC, our study also highlights the nature of crosstalk between these two signal-transduction pathways.

PMID: 16061178 

Mol Cell. 2005 May 27;18(5):577-87.

ATM-dependent phosphorylation of ATF2 is required for the DNA damage response.

Bhoumik ATakahashi SBreitweiser WShiloh YJones NRonai Z.

Activating transcription factor 2 (ATF2) is regulated by JNK/p38 in response to stress. Here, we demonstrate that the protein kinase ATM phosphorylates ATF2 on serines 490 and 498 following ionizing radiation (IR). Phosphoantibodies to ATF2(490/8) reveal dose- and time-dependent phosphorylation of ATF2 by ATM that results in its rapid colocalization with gamma-H2AX and MRN components into IR-induced foci (IRIF). Inhibition of ATF2 expression decreased recruitment of Mre11 to IRIF, abrogated S phase checkpoint, reduced activation of ATM, Chk1, and Chk2, and impaired radioresistance. ATF2 requires neither JNK/p38 nor its DNA binding domain for recruitment to IRIF and the S phase checkpoint. Our findings identify a role for ATF2 in the DNA damage response that is uncoupled from its transcriptional activity.

PMID: 15916964 

Sci STKE. 2005 Apr 26;2005(281):re5. 

Ubiquitin chains in the ladder of MAPK signaling.

Laine ARonai Z.

With a better understanding of the cellular stress response, it has become evident that catalytic modules consisting of kinases that mediate the activation of downstream effector components are subject to multiple layers of regulation. Such regulatory mechanisms are not limited to those involving scaffold proteins or protein phosphatases, and they appear to include a growing number of modifications by ubiquitin and ubiquitin-like proteins. The role of ubiquitin in the regulation of mitogen-activated protein kinase (MAPK) emerges as a paradigm for understanding the role of ubiquitination in regulating other signal transduction pathways. Ubiquitination influences signal diversification and limits the duration of the signal through its role in the assembly of protein kinase complexes, subcellular localization, and the actual degradation of the kinase or its substrate. This review summarizes our current understanding of the roles of ubiquitin in regulating MAPK signaling.

PMID: 15855411

Cancer Res. 2004 Nov 15;64(22):8222-30.

Inhibition of melanoma growth and metastasis by ATF2-derived peptides.

Bhoumik AGangi LRonai Z.

The resistance of melanoma to apoptosis, as well as its growth and metastasis capabilities, can be overcome by expression of a peptide derived from amino acid (aa) 51 to 100 of ATF2. Here we show that expression of ATF2((51-100)) in human melanoma cells reduced their growth in nude mice, which was additionally inhibited upon treatment with protein kinase inhibitors UCN-01 or SB203580. Injection of a fusion protein consisting of HIV-TAT and aa 51 to 100 of ATF2 into SW1 melanomas efficiently inhibits their growth and their metastasis up to complete regression. Additionally, expression of a 10aa peptide that corresponds to aa 51 to 60 of ATF2 sensitizes melanoma cells to spontaneous apoptosis, which coincides with activation of caspase 9 and poly(ADP-ribose) polymerase cleavage, and inhibit their growth in vivo. The 10aa peptide increases the association of c-Jun NH(2)-terminal kinase with c-Jun but not with ATF2, resulting in concomitant increase in TRE-mediated transcription. Our study points to mechanisms underlying the activities of the ATF2 peptide while highlighting its possible use in drug design.

PMID: 15548688 

Genes Dev. 2004 Oct 1;18(19):2380-91.

The small ubiquitin-like modifier (SUMO) is required for gonadal and uterine-vulval morphogenesis in Caenorhabditis elegans.

Broday LKolotuev IDidier CBhoumik AGupta BPSternberg PWPodbilewicz BRonai Z.

The small ubiquitin-like modifier (SUMO) modification alters the subcellular distribution and function of its substrates. Here we show the major role of SUMO during the development of the Caenorhabditis elegans reproductive system. smo-1 deletion mutants develop into sterile adults with abnormal somatic gonad, germ line, and vulva. SMO-1::GFP reporter is highly expressed in the somatic reproductive system. smo-1 animals lack a vulval-uterine connection as a result of impaired ventral uterine pi-cell differentiation and anchor cell fusion. Mutations in the LIN-11 LIM domain transcription factor lead to a uterine phenotype that resembles the smo-1 phenotype. LIN-11 is sumoylated, and its sumoylation is required for its activity during uterine morphogenesis. Expression of a SUMO-modified LIN-11 in the smo-1 background partially rescued pi-cell differentiation and retained LIN-11 in nuclear bodies. Thus, our results identify the reproductive system as the major SUMO target during postembryonic development and highlight LIN-11 as a physiological substrate whose sumoylation is associated with the formation of a functional vulval-uterine connection. Copyright 2004 Cold Spring Harbor Laboratory Press

PMID: 15466489

Cell. 2004 Jun 25;117(7):941-52.

Comment in:

·         Cell. 2004 Jun 25;117(7):851-3.

Siah2 regulates stability of prolyl-hydroxylases, controls HIF1alpha abundance, and modulates physiological responses to hypoxia.

Nakayama KFrew IJHagensen MSkals MHabelhah HBhoumik AKadoya TErdjument-Bromage HTempst PFrappell PBBowtell DDRonai Z.

Hypoxia-inducible factor-1alpha (HIF1alpha) is a central regulator of the cellular response to hypoxia. Prolyl-hydroxylation of HIF1alpha by PHD enzymes is prerequisite for HIF1alpha degradation. Here, we demonstrate that the abundance of PHD1 and PHD3 are regulated via their targeting for proteasome-dependent degradation by the E3 ubiquitin ligases Siah1a/2, under hypoxia conditions. Siah2 null fibroblasts exhibit prolonged PHD3 half-life, resulting in lower levels of HIF1alpha expression during hypoxia. Significantly, hypoxia-induced HIF1alpha expression was completely inhibited in Siah1a/2 null cells, yet could be rescued upon inhibition of PHD3 by RNAi. Siah2 targeting of PHD3 for degradation increases upon exposure to even mild hypoxic conditions, which coincides with increased Siah2 transcription. Siah2 null mice subjected to hypoxia displayed an impaired hyperpneic respiratory response and reduced levels of hemoglobin. Thus, the control of PHD1/3 by Siah1a/2 constitutes another level of complexity in the regulation of HIF1alpha during hypoxia.

PMID: 15210114 

Proc Natl Acad Sci U S A. 2004 Mar 23;101(12):4222-7. Epub 2004 Mar 9.

Transcriptional switch by activating transcription factor 2-derived peptide sensitizes melanoma cells to apoptosis and inhibits their tumorigenicity.

Bhoumik AJones NRonai Z.

The notorious resistance of melanoma cells to drug treatment can be overcome by expression of a 50-aa peptide derived from activating transcription factor 2 (ATF2(50-100)). Here we demonstrate that ATF2(50-100) induced apoptosis by sequestering ATF2 to the cytoplasm, thereby inhibiting its transcriptional activities. Furthermore, ATF2(50-100) binds to c-Jun N-terminal kinase (JNK) and increases its activity. Mutation within ATF2(50-100) that impairs association with JNK and the inhibition of JNK or c-Jun expression by RNA interference (RNAi) reduces the degree of ATF2(50-100)-induced apoptosis. In contrast, TAM67, a dominant negative of the Jun family of transcription factors, or JunD RNAi attenuates sensitization of melanoma cells expressing ATF2(50-100) to apoptosis after treatment with anisomycin, which is used as a model drug. Mutations within the JNK binding region of ATF2(50-100) or expression of TAM67 or JunD RNAi attenuates inhibition of melanoma's tumorigenicity by ATF2(50-100). We conclude that inhibition of ATF2 in concert with increased JNK/Jun and JunD activities is central for the sensitization of melanoma cells to apoptosis and inhibition of their tumorigenicity.

PMID: 15010535

EMBO J. 2004 Jan 28;23(2):322-32. Epub 2004 Jan 8.

Ubiquitination and translocation of TRAF2 is required for activation of JNK but not of p38 or NF-kappaB.

Habelhah HTakahashi SCho SGKadoya TWatanabe TRonai Z.

TRAF2 is a RING finger protein that regulates the cellular response to stress and cytokines by controlling JNK, p38 and NF-kappaB signaling cascades. Here, we demonstrate that TRAF2 ubiquitination is required for TNFalpha-induced activation of JNK but not of p38 or NF-kappaB. Intact RING and zinc finger domains are required for TNFalpha-induced TRAF2 ubiquitination, which is also dependent on Ubc13. TRAF2 ubiquitination coincides with its translocation to the insoluble cellular fraction, resulting in selective activation of JNK. Inhibition of Ubc13 expression by RNAi resulted in inhibition of TNFalpha-induced TRAF2 translocation and impaired activation of JNK but not of IKK or p38. TRAF2 aggregates in the cytoplasm, as seen in Hodgkin-Reed-Sternberg lymphoma cells, resulting in constitutive NF-kappaB activity but failure to activate JNK. These findings demonstrate that the TRAF2 RING is required for Ubc13-dependent ubiquitination, resulting in translocation of TRAF2 to an insoluble fraction and activation of JNK, but not of p38 or NF-kappaB. Altogether, our findings highlight a novel mechanism of TRAF2-dependent activation of diverse signaling cascades that is impaired in Hodgkin-Reed-Sternberg cells.

PMID: 14713952 

© Ronai Lab 2017