Fatty acid synthase inhibition induces apoptosis independent of lipid metabolism

Abstract

Fatty acid synthase (FAS) is the sole eukaryotic enzyme responsible for condensing acetyl-CoA with malonyl-CoA to form palmitate, the precursor of long chain fatty acids. FAS expression is upregulated in a broad range of cancers and its inhibition has been shown to induce tumor cell cycle arrest and cell death. The decrease in palmitate and the accumulation of malonyl-CoA accompanying FAS inhibition have both been hypothesized as mediators of these antitumorigenic effects. In the present study, inhibition of FAS activity by FAS siRNA and Orlistat led to apoptosis in various cultured tumor cell lines as shown by activation of caspase 8 and subsequent DNA fragmentation. Co-transfection with caspase 8 siRNA, but not caspase 9 siRNA, rescued tumor cells from undergoing FAS induced cell death. Interestingly, transfection with either acetyl-CoA carboxylase (ACACA) siRNA to block malonyl-CoA production or ATP citrate lyase (ACLY) siRNA to block acetyl-CoA production failed to activate caspase 8 and did not lead to apoptosis. Knockdown of ACACA led to an inhibition in palmitate similar to that observed with FAS siRNA indicating that suppression of fatty acid production on its own is not sufficient to promote cell death. Co-transfection of ACACA or ACLY siRNA with FAS siRNA to block accumulation of acetyl-CoA or malonyl-CoA failed to prevent cells from undergoing FAS induced apoptosis. Collectively, these data show that FAS inhibition induces apoptosis through caspase 8. Suppression of palmitate does not account for the ability of FAS inhibition to induce tumor cell death. Research was funded by DoD Grant DAMD17-02-0693 and CBCRP Grants CA6982713, CA69306 and 8FB-0107.