The turnover of cytoplasmic triacylglycerols in human fibroblasts involves two separate acyl chain length-dependent degradation pathways.

Cultured fibroblasts from patients affected with the genetic metabolic disorder named neutral lipid storage disease (NLSD) exhibit a dramatic accumulation of cytoplasmic triacylglycerols (Radom, J., Salvayre, R., Nègre, A., Maret, A., and Douste-Blazy, L. (1987) Eur. J. Biochem. 164, 703-708). We compared here the metabolism of radiolabeled short-, medium- and long-chain fatty acids in these cells. Short/medium-chain fatty acids (C4-C10) were incorporated into polar lipids (60-80%) and triacylglycerols (20-40%) at a lower rate (5-10 times lower) than long-chain fatty acids. Pulse-chase experiments allowed to evaluate the degradation rate of cytoplasmic triacylglycerols in normal and NLSD fibroblasts and to discriminate between two catabolic pathways of cytoplasmic triacylglycerols. Short/medium-chain (C4-C10) triacylglycerols were degraded at a normal rate in NLSD fibroblasts, whereas long-chain (C12 and longer) triacylglycerols remained undegraded. These data are confirmed by mass analysis. The use of diethylparanitrophenyl phosphate (E600) and parachloromercuribenzoate (PCMB) inhibitors allows to discriminate between the two triacylglycerol degradation pathways. E600 inhibited selectively the in situ degradation of short/medium-chain triacylglycerols without inhibition of the degradation of long-chain triacylglycerols, whereas PCMB inhibited selectively the in situ hydrolysis of long-chain triacylglycerols without affecting the degradation of long-chain triacylglycerols. This was correlated with the in vitro properties of cellular triacylglycerol-hydrolyzing enzymes characterized by their substrate specificity and their susceptibility to inhibitors; the neutral lipase specific to long-chain triacylglycerols is inhibited by PCMB, but not by E600, in contrast to short/medium-chain lipase, which is inhibited by E600 but not by PCMB. The data of in vitro and in situ experiments suggest the existence in fibroblasts of two separate acyl chain length-dependent pathways involved in the degradation of cytoplasmic triacylglycerols, one mediated by a neutral long-chain lipase and another one mediated by a short/medium-chain lipase.

Oxidizability and subsequent cytotoxicity of chylomicrons to monocytic U937 and endothelial cells are dependent on dietary fatty acid composition.

Oxidized chylomicrons may be a metabolic factor involved in the injury of the arterial wall and may constitute a potential link between postprandial lipemia and atherogenesis. It was of interest to study the influence of dietary fatty acid composition on the oxidizability and subsequent cytotoxicity of chylomicrons on cultured cells. Human chylomicrons were obtained from healthy volunteers 3 h after ingestion of a triglyceride-rich meal containing mainly either polyunsaturated fatty acids (soya oil) or monounsaturated fatty acids (olive oil) or saturated fatty acids (partly hydrogenated palm oil). Polyunsaturated fatty acid (PUFA)-rich chylomicrons exhibited a high oxidizability, whereas chylomicrons enriched with monounsaturated or saturated fatty acids were relatively resistant to oxidation. The cytotoxicity of various types of chylomicrons submitted to oxidation has been tested comparatively on cultured human monocytic U937 cells and endothelial cells. Chylomicrons enriched with saturated and monounsaturated fatty acids were not or only slightly cytotoxic to cultured cells, whereas PUFA-rich chylomicrons (highly susceptible to oxidation) were highly cytotoxic. The influence of cholesterol on the oxidizability and subsequent cytotoxicity of PUFA-rich chylomicrons has been investigated by using comparatively a soya diet supplemented or not with cholesterol. PUFA-rich cholesterol-rich chylomicrons were slightly more oxidizable and more cytotoxic than PUFA-rich (cholesterol-poor) chylomicrons, thus suggesting that the cytotoxicity of PUFA-rich chylomicrons may be due to oxidation derivatives of PUFA (for the major part) and to oxysterols (for a minor part). Furthermore, the cytotoxic effects of oxidized PUFA-rich chylomicrons and of mildly oxidized LDL were in similar range (even higher for PUFA-rich chylomicrons when expressed per lipoprotein particle), thus suggesting that oxidized PUFA-rich chylomicrons may play a nonnegligible role in cytotoxic events occurring during atherogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)