Blockade of Fatty Acid Synthase Induces Ubiquitination and Degradation of Phosphoinositide-3-Kinase Signaling Proteins in Ovarian Cancer

Abstract

Aberrations within the phosphoinositide-3-kinase (PI3K) pathway occur in greater than 45% of ovarian carcinomas. The PI3K cascade transmits signals from ErbB receptors downstream to S6 and 4EBP1, which are involved in protein biosynthesis. Many ovarian carcinomas reveal hyperactivation of ErbB1 (epidermal growth factor receptor) or ErbB2 (HER2/neu). Unfortunately, the benefit of anti-ErbB drugs is yet rather limited in ovarian carcinomas. Thus, novel targeting strategies are needed for ovarian carcinomas. The lipogenic enzyme fatty acid synthase (FASN) is overexpressed in approximately 80% of ovarian carcinomas. It stimulates cell growth and signifies poor prognosis. FASN inhibition impedes (ErbB) membrane receptor signaling and sensitizes cells against anti-ErbB drugs. Here, we show that the FASN inhibitor C75 and FASN-targeting siRNAs abrogate growth, induce apoptosis, and downregulate phosphorylation/expression of the PI3K effectors AKT, mTOR, p70S6K, S6, and 4EBP1. In contrast, FASN inhibition impairs expression but only weakly affects phosphorylation of ERK1/2 mitogen-activated protein kinases in ovarian carcinoma cells. Cycloheximide-mediated blockade of protein translation reveals that C75-or FASN siRNA-induced shutdown of FASN accelerates decomposition of signaling proteins. This effect is caused by C75-or FASN siRNA-dependent stimulation of ubiquitination followed by lysosomal-autophagosomal proteolysis. In contrast, PI3K inhibitor LY294002 blocks phosphorylation but does not reduce expression/stability of PI3K effectors. Forced expression of hyperactive (HA) AKT1, unlike HA-MEK1, impairs the growth-inhibitory action of C75. We provide first evidence that the anticancer action of FASN inhibitors is at least partially mediated by drug-dependent proteolysis of PI3K effectors. FASN is a promising cancer target, whose inhibition not only abrogates lipogenesis, which is indispensable for cancer growth, but also downregulates oncogenic PI3K signaling.

Blocking fatty acid synthase inhibits tumor progression of human osteosarcoma by regulating the human epidermal growth factor receptor 2/phosphoinositide 3-kinase/protein kinase B signaling pathway in xenograft models

“…A study by Tomek et al (34) indicated that the inhibition of FASN induces ubiquitination and degradation of PI3K signaling proteins in ovarian cancer. A previous study demonstrated that small interfering RNA silencing of FASN decreased the PI3K/Akt/NF-κB signaling pathway in vitro (32); however, the mechanism (s) responsible for the inhibition of Akt activity due to the downregula tion of FASN have not been investigated.…”

Mechanisms and Targets Involved in Dissemination of Ovarian Cancer

“…Fatty acid synthase (FASN) promotes proliferation and EMT via transcriptional regulation of E-cadherin and Ncadherin and its overexpression correlates with dissemination and peritoneal metastasis of ovarian cancer (136). In this context, it was shown that blockage of FASN induces ubiquitination and degradation of PI3K signaling proteins in ovarian cancer (137). The involvement of FASN in the generation of phospholipids involved in the formation of membrane microdomains that accommodate receptor tyrosine kinases, including HER1 and HER2, has also been demonstrated (138).…”

Guidelines for the use and interpretation of assays for monitoring autophagy

“…For example, blocking proteasome function may activate autophagy. [476][477][478][479] Thus, when using inhibitors it is critical to know whether the inhibitors being used alter autophagy in the particular cell type and context being examined. In addition, because 3-MA could have some autophagy-independent effects in particular settings it is advisable to verify that the 3-MA-sensitive degradation is also sensitive to general lysosomal inhibitors (such as NH 4 Cl or leupeptin).…”