These authors contributed equally to this work.Keywords: cancer, genotoxic stress, miRNA, p53, post-translational modifications Abbreviations: miRNA, microRNA; ROS, reactive oxygen species; UTR, untranslated region p53, the revered savior of genomic integrity, receives signals from diverse stress sensors and strategizes to maintain cellular homeostasis. However, the predominance of p53 overshadows the fact that this herculean task is no one-man show; rather, there is a huge army of regulators that reign over p53 at various levels to avoid an unnecessary surge in its levels and sculpt it dynamically to favor one cellular outcome over another. This governance starts right at the time of p53 translation, which is gated by proteins that bind to p53 mRNA and keep a stringent check on p53 protein levels. The same effect is also achieved by ubiquitylases and deubiquitylases that fine-tune p53 turnover and miRNAs that modulate p53 levels, adding precision to this entire scheme. In addition, extensive covalent modifications and differential protein interactions allow p53 to trigger a tailor-made response for a given circumstance. To magnify the marvel, these various tiers of regulation operate simultaneously and in various combinations. In this review, we have tried to provide a glimpse into this bewildering labyrinth. We believe that further studies will result in a better understanding of p53 regulation and that new insights will help unravel many aspects of cancer biology.Regulation of any cellular pathway is essential to coordinate the heterogeneity and complexity of functions in multicellular organisms. Among the tumor suppressors, decoding the bewildering number of pathways that p53 is involved in has long been the holy grail of scientists. p53 is a master regulator that integrates signals from diverse nodes and thus it is of no surprise that it is the most commonly mutated gene in a huge array of cancers with varied origins. p53 has many weapons at its disposal to combat stress including cell cycle arrest, senescence, apoptosis, autophagy, and metabolic reprogramming. Paradoxically, some of the outcomes of p53 activation are disparate and contradictory, such as cell cycle arrest, which is prosurvival, versus apoptosis and senescence, which are directed toward eliminating irreversibly damaged cells. This indicates that p53 needs to be educated to sense the extent and type of damage and make an appropriate choice of the kind of response it is going to elicit. Extensive research on the regulation of p53 under diverse kinds of stresses including genotoxic stress, starvation, hypoxia, and oncogene activation clearly indicate that p53 protein is regulated at diverse levels, including synthesis, degradation, covalent modifications, subcellular localization, and differential interaction with other proteins. Moreover, all possible permutations and combinations of these are employed to modulate p53 specificity, tissue heterogeneity, and diversity of function. In light of this, we restrict this review to exclusively dis...

UBE3A is an E3 ubiquitin ligase well known for its role in the proteasomal degradation of p53 in human papillomavirus (HPV)-associated cancers. Here we report that UBE3A ubiquitylates and triggers degradation of the tumor-suppressive sirtuin SIRT6 in hepatocellular carcinoma. UBE3A ubiquitylated the highly conserved Lys160 residue on SIRT6. FOXO1-mediated transcriptional repression of was sufficient to stabilize SIRT6 and to epigenetically repress, a key mediator of UBE3A oncogenic function. Thus, UBE3A-mediated SIRT6 degradation promoted the proliferative capacity, migration potential, and invasiveness of cells. In mouse models of hepatocellular carcinoma, SIRT6 downregulation and consequent induction of ANXA2 were critical for UBE3A-mediated tumorigenesis. Furthermore, in clinical specimens, increased UBE3A levels correlated with reduced SIRT6 levels and elevated ANXA2 levels in increasing tumor grades. Overall, our findings show how the tumor suppressor SIRT6 is regulated in hepatocellular carcinoma and establish the mechanism underlying UBE3A-mediated tumorigenesis in this disease. These findings provide mechanistic insights into regulation of the tumor suppressive sirtuin SIRT6 and its implications for the development of hepatocellular carcinoma. .

Background: UBE3A is the founding member of the HECT family of E3 ubiquitin ligases, implicated in cervical, prostate and breast cancer. A recent study has reported that UBE3A promotes proliferation and invasiveness of cancer cells by inducing Annexin A2 (ANXA2) levels. We have delineated a cascade in which UBE3A brings about the proteasomal degradation of SIRT6. Furthermore, we have also established that SIRT6 transcriptionally regulates ANXA2 expression. Here, we have confirmed that UBE3A-mediated SIRT6 downregulation and consequent derepression of ANXA2 promotes tumor metastasis. Methods: HepG2, a Hepatocellular carcinoma cell line, was used to study the effects of the UBE3A-SIRT6-AXA2 axis. HepG2 UBE3A knockdown, SIRT6 knockdown, UBE3A/SIRT6 double knockdown and SIRT6/ANXA2 double knockdown cells were used to examine in-vitro cell proliferation, invasion, plasmin generation and MMP activation. These cell lines were also used for obtaining subcutaneous and orthotopic xenografts in immunodeficient mice and multiple parameters were analyzed to determine tumor growth and metastases. Results: We observed that upon UBE3A abrogation, cells exhibited reduced tumor-forming potential and resulted in smaller tumors with low metastasis. On the other hand, increased tumorigenesis, and much larger tumors with increased metastasis were observed upon SIRT6 knockdown, which was significantly reversed upon simultaneous knockdown of UBE3A. Furthermore, smaller tumors were observed in case of SIRT6/ANXA2 double knockdown cells as compared to UBE3A/SIRT6 double knockdown cells. These results suggest that UBE3A-mediated ANXA2 upregulation, via SIRT6 degradation, is critical for its oncogenic functions Conclusion: UBE3A mediated SIRT6 degradation leads to ANXA2 upregulation and hence enhances tumorigenesis. It would be beneficial to identify drugs targeting UBE3A as a therapeutic approach in hepatocellular carcinoma. Alternatively, mechanisms which inhibit ANXA2 activity must be deciphered to identify druggable targets. Citation Format: Saishruti Kohli, Sanjeev Das. UBE3A regulates SIRT6 levels to promote tumorigenesis in an ANXA2 dependent manner [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 97.