Abstract 1247: ARHI downregulates PI3K and Ras-MAP signaling and induces autophagy by promoting integrin and EGFR membrane protein degradation

Abstract

Abstract Autophagy selectively degrades cellular proteins, protein aggregates and organelles, providing energy in a nutrient-deprived environment. We have reported that autophagy can be induced by re-expression of ARHI (DIRAS3), a maternally imprinted Ras-related tumor suppressor gene that is down regulated in more than 60% of ovarian cancers. Whereas prolonged expression of ARHI in cultured cells induces autophagic cell death, ARHI expression in xenografts sustains dormant cancer cell survival for weeks. When ARHI levels were reduced in dormant cells, xenografts grew rapidly. Inhibition of ARHI-induced autophagy with chloroquine dramatically reduces regrowth of xenografts, suggesting that autophagy contributes to the survival of dormant cells. Re-expression of ARHI inhibits both PI3K and Ras-MAPK signaling. Growth factors (VEGF, IL-8, IGF) and ECM components (fibronectin) found in the cancer microenvironment can rescue ovarian cancer cells from ARHI-induced autophagic cell death in culture, associated with partial reversal of ARHI-mediated inhibition of PI3K signaling. Consequently, we have hypothesized that autophagy together with growth factors and ECM in the microenvironment are all required to maintain dormancy, where autophagy provides energy for xenografts and survival factors prevent autophagic death by partially reversing inhibition of PI3K-AKT signaling. The mechanism by which ARHI regulates PI3K and Ras-MAPK signaling has not been fully elucidated. Here, we show that re-expression of ARHI in SKOv3 ovarian cancer cells inhibits expression of integrin β1 and EGFR. ARHI expression enhanced EGF-mediated receptor endocytosis and colocalization of EGFR with Rab11 and Rab7, markers for multivesicular endosomes and lysosomes, respectively. Our results suggest that ARHI enhances membrane receptor turnover, in part, by facilitating the internalization and association of membrane receptors with vesicular trafficking proteins targeted for degradation in the lysosome. ARHI-mediated downregulation of membrane receptors reduces both PI3K-AKT and Ras-MAPK signaling, ultimately contributing to the inhibition of tumor growth. 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 1247. doi:1538-7445.AM2012-1247