Fatty acid synthase (FAS) has been found to be overexpressed in a wide range of epithelial
tumors, including
breast cancer. Pharmacologic inhibitors of FAS cause apoptosis of
breast cancer cells and result in decreased
tumor size in vivo. However, how the inhibition of FAS induces apoptosis in
tumor cells remains largely unknown. To understand the apoptotic pathway resulting from direct inhibition of FAS, we treated
breast tumor cells with or without FAS
small interfering RNA (
siRNA) followed by a microarray analysis. Our results indicated that the proapoptotic genes BNIP3,
tumor necrosis factor-related apoptosis-inducing
ligand (TRAIL), and
death-associated protein kinase 2 (
DAPK2) were significantly up-regulated on direct inhibition of the FAS gene. We also found that the knockdown of FAS expression significantly increased
ceramide level in the
tumor cells, and this increase was abrogated by
acetyl-CoA carboxylase inhibitor. In addition,
carnitine palmitoyltransferase-1 (CPT-1) inhibitor up-regulated the
ceramide and BNIP3 levels in these cells, whereas treatment of
tumor cells with FAS
siRNA in the presence of a
ceramide synthase inhibitor abrogated the up-regulation of BNIP3 and inhibited apoptosis. Furthermore, we found that treatment of cells with BNIP3
siRNA significantly counteracted the effect of FAS
siRNA-mediated apoptosis. Consistent with these results, a significant inverse correlation was observed in the expression of FAS and BNIP3 in clinical samples of human
breast cancer. Collectively, our results indicate that inhibition of FAS in
breast cancer cells causes accumulation of
malonyl-CoA, which leads to inhibition of CPT-1 and up-regulation of
ceramide and induction of the proapoptotic genes BNIP3, TRAIL, and
DAPK2, resulting in apoptosis.