Clinical observations in patients with
peroxisomal disorders and studies employing corresponding mouse models have shown that supraphysiological concentrations of dietary branched chain
fatty acids (BCFAs) are associated with a high level of toxicity, which is poorly understood at present. Here we show that phytanic and
pristanic acid, two BCFAs that are metabolized in peroxisomes, promote apoptosis in cultured vascular smooth muscle cells of human, rat, and porcine origin. Under the conditions used, the apoptosis-promoting effect of BCFAs was neither shared by saturated or unsaturated straight chain
fatty acids nor by artificial
peroxisome proliferators, which, like phytanic and
pristanic acid, have been shown to activate the
peroxisome proliferator-activated receptor alpha (
PPARalpha). We could demonstrate, however, that BCFA induced
tumor necrosis factor alpha (
TNFalpha) activation and secretion, which is an obligatory step required for induction of apoptosis by BCFAs. Furthermore, incubation of VSMCs with BCFA increased
inducible nitric-oxide synthase (iNOS)
mRNA and
protein concentrations markedly within 2 h of treatment. Correspondingly, apoptosis was significantly reduced when the cells were co-treated with the competitive NOS inhibitors monomethyl-
L-arginine monoacetate and
aminoguanidine. Moreover, co-incubation with TGFbeta1, previously shown to destabilize iNOS
mRNA, also abolished apoptosis. These results establish a new signaling cascade in which natural BCFA induced NO-dependent apoptosis, which is apparently triggered by autocrine secretion of
TNFalpha in cultured VSMCs.