Metabolism of neutral lipids in cultured fibroblasts from multisystemic (or type 3) lipid storage myopathy.

The lipid metabolism in cultured fibroblasts from multisystemic (type 3) lipid storage myopathy (MLSM) and controls has been studied through pulse-chase experiments using radiolabelled oleic acid and acetate precursors. The uptake of radiolabelled oleic acid by MLSM fibroblasts was slightly higher than in controls but did not seem to be the primary defect of the multisystemic lipid storage myopathy. The uptake of radiolabelled acetate was quite similar in MLSM and in control cells. During short-time pulse periods, using either radiolabelled oleic acid or acetate as precursors, we observed no significant difference in lipid composition between MLSM and controls. In contrast, pulse experiments using radiolabelled oleic acid as precursor showed a major accumulation of radiolabelled triacylglycerols in MLSM (around 1000% of controls); no significant increase of other neutral or polar lipids was noticed. A similar triacylglycerol storage was observed by using radiolabelled acetate as precursor, but in this case the difference between MLSM and controls was more pronounced; we also observed in MLSM cells a higher amount of polar lipids which can be due to an increased rate of fatty acid biosynthesis (from radiolabelled acetate). Chase experiments, after pulse by low concentration of exogenous radiolabelled oleic acid or acetate, showed similar features: the biosynthesized triacylglycerols were not at all degraded in MLSM, but on the contrary increased, probably by accumulation of radiolabelled triacylglycerols newly synthesized from radiolabelled fatty acids liberated during the phospholipid turnover. Similarly, the triacylglycerol storage induced by high doses of fatty acids was not degraded in MLSM cells, in contrast to control cells. This suggested that the triacylglycerols synthesized in the presence of low and high levels of fatty acids were accumulated in only one subcellular cytoplasmic compartment without relationship with the lysosomal compartment, since these cells were not deficient in acid lysosomal lipase. The more probable hypothesis is a deficiency of the cytoplasmic catabolism of triacylglycerols in MLSM cells.