Abstract 3734: Preclinical evaluation of Rad6 inhibition to overcome platinum resistance in ovarian cancer
Authors: Spencer Sebastian M. , Somasagara Ranganatha R. , Tripathi Kaushlendra , Clark David W. , Kothayer Hend , Westwell Andrew D. , Rocconi Rodney P. , Palle Komaraiah 
Published: July 2016

Abstract
Ovarian cancer (OC) is the most lethal gynecological cancer in women in the United States. Advances in surgery and chemotherapy have not significantly changed the overall survival rate of OC for the last few decades, which highlights the need for new therapeutic strategies. Platinum drug resistance and refractory disease pose major challenges in treating this disease and are major factors contributing to the poor survival rate of OC patients. Although most patients initially respond to platinum based chemotherapy, about 80% of cases present with recurrent disease, develop platinum resistance, and die with the advanced disease. Considering the heterogeneity, small fractions of the cells could be inherently resistant to chemotherapy and/or dormant and exhibit stem-like cell properties, contributing to the resistant phenotype and disease recurrence. Although the Cancer stem cell (CSC) theory of therapeutic resistance proposes that the proportion of CSCs correlate to enhanced chemoresistance and early disease recurrence, the specific molecular mechanisms that regulate tumor cell behavior (stemness) and integrate signaling networks with aberrant oncogenic signaling in OC cells are not known. Our analysis of clinical samples revealed upregulation of Rad6, an E2 ubiquitin conjugating enzyme, in more than 80% of ovarian tumors compared to normal ovarian tissues. Upregulation of Rad6 also correlated well with tumor progression. Further analysis of molecular pathways in OC cells revealed a strong correlation between Rad6 upregulation and increased β-catenin and hedgehog signaling, stem cell like characteristics and platinum resistance. Downregulation of Rad6 using siRNAs or inhibition of its catalytic activity by a small molecule inhibitor, attenuated carboplatin induced monoubiquitination of its target proteins such as histone 2B, PCNA and proteins of the Fanconi anemia pathway thereby sensitizing OC cells to carboplatin. Interestingly, inhibition of Rad6 alone in OC cells induced replication stress and reduced cell survival and proliferation by arresting cells in the G2/M phase. Moreover, inhibition of Rad6 in various OC cell lines reduced expression of β-catenin, Gli1 and several OC stem cell markers. Moreover, Rad6 plays an important role in the activation of the trans-lesion synthesis (TLS) pathway by monoubiquitinating PCNA and in the activation of the Fanconi Anemia (FA) DNA repair pathway. These are critical mechanisms for cells to repair DNA crosslinks induced by platinum drugs. Together with these observations, our data suggest that inhibition of Rad6 could be a viable therapeutic target for overcoming platinum resistance and disease recurrence in ovarian cancer.
Citation Format: Sebastian M. Spencer, Ranganatha R. Somasagara, Kaushlendra Tripathi, David W. Clark, Hend Kothayer, Andrew D. Westwell, Rodney P. Rocconi, Komaraiah Palle. Preclinical evaluation of Rad6 inhibition to overcome platinum resistance in ovarian cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3734.