Abstract A6: Targeting fatty acid synthase abrogates PI3K signaling in ovarian cancer by dephosphorylation and ubiquitin-mediated proteasomal degradation of downstream effectors

Abstract

Abstract Ovarian carcinoma is fourth leading cause of cancer death in women and accounts for highest mortality of all gynecological malignancies. The phosphatidylinositol-3 kinase (PI3K) cascade controls proliferation, differentiation, tumorigenesis, angiogenesis and apoptosis. Many ovarian carcinomas harbor aberrations within the PI3K pathway. Amplification of PI3K is observed in ∼40% of ovarian carcinomas and cell lines. The PI3K downstream target AKT phosphorylates mTOR, which is hyperactivated in many cancers. mTOR activates S6 via p70S6K, which is frequently activated in ovarian cancer. S6 protein being a component of the 40S ribosomal subunit is involved in translation control. mTOR also phosphorylates eukaryotic translation initiation factor 4E (eIF4E) inhibitor binding protein 1 (4EBP1). Phosphorylated 4EBP1 dissociates from eIF4E and activates cap-dependent mRNA translation. In addition, many ovarian carcinomas harbor aberrations of the ErbB receptors ErbB1 (EGFR; 55%) or ErbB2 (HER2/neu; 35%), respectively. Importantly, PI3K signaling plays crucial roles in transmitting ErbB-derived signals and stimulating cancer growth. Irrespectively, clinical studies yet reveal that monotherapies with ErbB1 or ErbB2 inhibitors or antibodies are largely inefficient in ovarian carcinomas. Therefore, additional molecular targeting strategies are urgently needed. Fatty acid synthase (FASN) being overexpressed in ∼80% of ovarian carcinomas is a marker for poor prognosis. It supports formation of lipid rafts in the plasma membranes, which accommodate transmembrane growth factor receptors incl. ErbB proteins. Thereby, FASN facilitates signal generation at the cell membranes. Most importantly, inhibition of FASN delays disease progression of ovarian carcinoma xenografts. Recently, we reported that the FASN inhibitor C75 downregulates ErbB1 and ErbB2 in ovarian cancer and sensitizes the cells against ErbB targeting drugs (Grunt et al., BBRC, 385, 454). We now demonstrate that C75 abrogates A2780 ovarian cancer cell growth. This correlates with silencing of PI3K downstream signaling as evidenced by reduced phosphorylation of AKT, mTOR, p70S6K and 4EBP1 in Western blot analyses, which is caused by both specific protein dephosphorylation/deactivation and by ubiquitin-mediated proteasomal degradation of these PI3K effector proteins. In contrast, specific phosphorylation/activation of the mitogen-activated protein kinase ERK1/2 is increased, although ERK1/2 steady-state levels are concurrently decreased by C75. In comparison, the PI3K inhibitor LY294002 blocks phosphorylation while concurrently upregulating steady-state levels of AKT, mTOR, p70S6K, and 4EBP1, and it activates ERK1/2. This suggests i) that PI3K/AKT normally cross-inhibits ERK1/2, which can be abrogated by silencing of PI3K/AKT, and ii) that PI3K, but not ERK1/2, signaling is crucial for growth arrest of ovarian cancer cells. Notably, our data demonstrate for the first time that C75-mediated silencing of PI3K signaling is caused by reduced phosphorylation and diminished protein stability due to increased ubiquitination and proteasomal degradation. Thus, C75 provides additive anticancer action, when compared to the PI3K inhibitor LY294002, which directly targets PI3K and downstream signaling, but does not stimulate effector protein degradation. In summary, FASN represents a promising anticancer drug target, which should be further developed for clinical use in ovarian carcinoma. Supp. ‘Med.-Wiss. Fonds Bürgerm. Wien’ Citation Information: Clin Cancer Res 2010;16(7 Suppl):A6