This article is available online at http://www.jlr.org in normal nonadipose tissues/cells. Cancer cells, however, synthesize more than 90% of their triacylglycerol de novo, requiring increased FASN expression ( 1 ). The important role of FASN in cancer cell proliferation/survival and its potential oncogenic function have been demonstrated in several previous studies ( 2-8 ).Overexpression of FASN has been associated with poor prognosis and higher risk of recurrence of human cancers of the breast ( 9-12 ) and prostate ( 13 ), as well as many other types of cancers ( 1 ), suggesting that increased FASN expression may also contribute to disease progression and treatment failure. In particular, FASN overexpression has been found in a drug-selected breast cancer cell line, and this elevation contributes to cellular resistance to Adriamycin (doxorubicin) and mitoxantrone in breast ( 14 ) and to gemcitabine in pancreatic cancer cells ( 15 ). However, the mechanism by which FASN contributes to drug resistance is currently unknown.In this study, we investigated the molecular mechanism of FASN-mediated drug resistance using FASN overexpression stable clones of MCF7 cells and found that FASN overexpression causes resistance to multiple anticancer drugs as well as to ␥ -irradiation via inhibiting treatmentinduced ceramide production, caspase 8 activation, and apoptosis. Further studies showed that FASN overexpression suppresses tumor necrosis factor (TNF)-␣ production, nuclear factor (NF)-B activation, and drug-induced activation of neutral sphingomyelinase (nSMase). Hence, FASN overexpression may cause drug resistance by inhibiting TNF-␣ expression, which in turn suppresses activation of NF-B and nSMase and, thus, reduced ceramide production, caspase 8 activation, and apoptosis.Abstract Fatty acid synthase (FASN) is a key enzyme in the synthesis of palmitate, the precursor of major nutritional, energetic, and signaling lipids. FASN expression is upregulated in many human cancers and appears to be important for cancer cell survival. Overexpression of FASN has also been found to associate with poor prognosis and higher risk of recurrence of human cancers. Indeed, elevated FASN expression has been shown to contribute to drug resistance. However, the mechanism of FASN-mediated drug resistance is currently unknown. In this study, we show that FASN overexpression causes resistance to multiple anticancer drugs via inhibiting drug-induced ceramide production, caspase 8 activation, and apoptosis. We also show that FASN overexpression suppresses tumor necrosis factor-␣ production and nuclear factor-B activation as well as drug-induced activation of neutral sphingomyelinase. Thus, TNF-␣ may play an important role in mediating FASN function in drug resistance. Mammalian FASN is a homo-dimer of a multifunctional protein of ف 270 kDa containing seven catalytic domains:  -ketoacyl synthase, malonyl/acetyltransferase, dehydrogenase, enoyl reductase,  -ketoacyl reductase, acyl carrier protein, and thioesterase . Under normal physiologi...