Ovarian cancer is the most lethal of all gynecological malignancies, due in part to the diagnosis at an advanced stage caused by the lack of specific signs and symptoms and the absence of reliable tests for screening and early detection. Furthermore, survival is reduced by recurrence of chemo-resistant tumors. The insulin-like growth factor 1 (IGF1)/IGF1 receptor (IGFR) signaling system exerts a broad anti-apoptotic function and plays a crucial role in human cancer progression, including ovarian cancers. The small GTP-binding protein RAC1 regulates a diverse array of cellular events, including the control of cell growth, cytoskeleton reorganization for motility, and the activation of protein kinases. Although RAC1 is expressed in most tissues, it can be elevated in tumors at various stages of neoplastic progression. This study was conducted to explore the role of RAC1 in IGF1-stimulated ovarian cancer cell growth. While IGFR was expressed in normal human ovarian surface epithelial (HOSE) cells and all OSE cancer cell lines examined, the IGF1R was over expressed in IGROV1 ovarian cancer cells. Exposure of IGROV1 cells to IGF1 rapidly (<0.5 min) induced IGFR phosphorylation which triggered the phosphorylation of AKT (S473) (≥0.5 min), ERK1[[Unsupported Character - &#8260;]] 2 (>1.0 min) and p70S6K (T389) (≥1.0 min); indicating that IGF1 actives PI3K and MAPK signaling pathways that are known to be important in cell growth. We observed a maximal effect of IGF1 on IGROV1 cell growth at 50 ng/ml (2.2 fold increase, p<0.001) after 24 h. In contrast, exposure of IGROV1 cells to the PI3K inhibitor LY294002 or the MEK1/2 inhibitor UO126 decreased cell viability. Pull-down assays revealed that IGF1 stimulated GTP loading of RAC1 in less than 1.0 min with maximal increases observed after 5 min (2.6 fold, p<0.05). Treatment with the PI3K inhibitor LY294002 significantly reduced RAC1 activation in response to IGF1. Pre-treatment of IGROV1 cells with the RAC1 inhibitor (NSC23766) for 1 h inhibited IGF1-induced activation of RAC1; and decreased cell survival in a concentration-dependent manner (5- 50 µM NSC23766, p<0.05). The reduction in viability after 48 hr was accompanied by an increase in apoptotic cells, cleaved PARP and elevated cytosolic cytochrome C. In contrast, treatment of HOSE cells with NSC23766 did not alter cell viability. Of interest, following 6 h treatment of IGROV1 cells with 20-50 µM NSC23766 we observed increases in the phosphorylation of Akt (S473), GSK3β (S9), and p70S6K (T389), suggesting the activation of PI3K/AKT pathway was not sufficient to rescue cells from death. Furthermore, treatment with NSC23766 (50 µM) and cisplatin (20 µM) reduced cell viability to an extent greater than achieved by either agent alone (p<0.05). Thus, disruption of RAC1 activation reduces IGF1-stimulated cell growth, induces apoptosis, and enhances cell death by anti-cancer agents in ovarian cancer cells.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3915. doi:1538-7445.AM2012-3915