Abstract 1669: DIRAS family members inhibit cell growth and proliferation while inducing autophagy in human ovarian cancer cells.

Abstract Among the three members of the DIRAS family, DIRAS3 (also known as AplasiaRasHomolog Member I; ARHI) has been best characterized. ARHI is a maternally expressed imprinted tumor suppressor gene that encodes a 26 kDa GTPase with 60% homology to Ras and Rap. ARHI is downregulated in many tumor types including ovarian cancer where its re-expression at physiologic levels inhibits cancer cell growth, reduces motility, induces autophagy and promotes tumor dormancy. ARHI induces autophagy by multiple mechanism including:1) blocking PI3K-Akt signaling and inhibiting mTOR activity, 2) displacing Bcl-2 from Beclin and participating directly in the autophagy initiation complex; and 3) inducing expression of Atg4 and participating directly in the formation of autophagic vesicles, co-localizing with MAP-LC3-II in the vesicle membrane. In human cells, knockdown experiments have demonstrated that ARHI is essential for the induction of autophagy. During evolution, murine cells have lost the ARHI (DIRAS3) gene, yet can still undergo autophagy. Mice and humans express two closely related Ras-related GTPases,DIRAS1 and DIRAS2. These 22 kDa GTPases, have 30-40% overall homology with H-Ras, and 50-60% homology with ARHI (DIRAS3) with the only major difference being the truncation of the N-terminal extension. In this study, we show that transfection of DIRAS1, 2 or 3 with a selectable marker inhibits clonogenic growth of SKOv3 human ovarian cancer cells. Whereas DIRAS3 produced complete inhibition, DIRAS 1 produced 50% inhibition and D IRAS2 35% inhibition of clonogenic growth. One possible mechanism for the growth inhibition observed could be attributed to the induction of autophagy which has been shown by the formation of LC3 punctae in SKOV3 cells and the increased conversion of LC3-I to LC3-II by western blot analysis when DIRAS1 and DIRAS2 were overexpressed by transfection. These findings suggest that these DIRAS family homologs may have similar functions in limiting human tumor progression and in inducing autophagy. Future studies will examine whether DIRAS1 and DIRAS2 serve as surrogates forARHI (DIRAS3) in murine cells. Citation Format: Margie N. Sutton, Zhen Lu, Robert C. Bast. DIRAS family members inhibit cell growth and proliferation while inducing autophagy in human ovarian cancer cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1669. doi:10.1158/1538-7445.AM2013-1669