FATTY-ACID CYCLING IN HUMAN HEPATOMA-CELLS AND THE EFFECTS OF TROGLITAZONE

Fatty acid cycling by chain shortening/elongation in the peroxisomes i s an important source of fatty acids for membrane lipid synthesis. Its role in the homeostasis of nonessential fatty acids is poorly underst ood. We report here a study on the cycling of saturated fatty acids an d the effects of troglitazone in HepG2 cells in culture using [U-C-13] stearate or [U-C-13]oleate and mass isotopomer analysis. HepG2 cells w ere grown in the presence of 0.7 mmol/liter [U-C-13]stearate or [U-C-1 3]oleate, and in the presence and absence of 50 mu M troglitazone for 72 h. Fatty acids extracted from cell pellets after saponification wer e analyzed by gas chromatography/mass spectrometry. Peroxisomal beta-o xidation of uniformly C-13-labeled stearate (C18:0) and oleate (C18:1) resulted in chain shortening and produced uniformly labeled palmitate (C16:0) and palmitoleate (C16:1). In untreated cells, 16% of C16:0 wa s derived from C18:0 and 26% of C16:1 fi om C18:1 by chain shortening. Such contributions were significantly increased by troglitazone to 23 .6 and 36.6%, respectively (p < 0.001). Desaturation of stearate contr ibuted 67% of the oleate, while reduction of oleate contributed little to stearate (2%). The desaturation of C18:0 to C18:I was not affected by troglitazone. Our results demonstrated a high degree of recycling of C18:0 and C18:1 to C16:0 and C16:1 through chain shortening and des aturation. Chain shortening was accompanied by chain elongation in the synthesis of other long chain fatty acids. Troglitazone specifically increased recycling by peroxisomal beta-oxidation of C18 to C16 fatty acids, and the interconversion of long chain fatty acids was associate d with reduced de novo lipogenesis.