Lipoprotein metabolism in a coculture system with eel skeletal muscle cells and hepatocytes

Abstract

: Eel muscle contains lipid as much as 15–20% of wet weight of muscle. However, it is uncertain whether lipid is synthesized in muscle or transported from another tissue such as liver. We prepared isolated muscle cells to investigate the synthetic ability of lipid by muscle cell itself and studied lipoprotein metabolism in coculture with eel hepatocytes. Isolated eel muscle cells were prepared by collagenase digestion of skeletal muscle and their length and width were approximately 2.2 mm and 5–20 µm, respectively. The length corresponded to that of myotome between myocommata. Cultured muscle cells synthesized lipid and protein from 14C-acetate, but the ability of lipid synthesis by muscle cells was below 1/100 that of eel hepatocytes. In the absence of lipoprotein lipase (LPL), the highest component among 14C-lipid changed from 14C-phospholipid in muscle cells cultured alone to 14C-triacylglycerol (14C-TG) in cocultured muscle cells. These results suggest that cocultured muscle cells incorporate the very low-density lipoprotein (VLDL)-like lipoprotein secreted by cocultured hepatocytes, of which the main lipid is 14C-TG. In the presence of LPL in the coculture system, 14C-lipid in muscle cells increased 1.6-fold during 48 h incubation compared to that without LPL. 14C-Free fatty acids to total 14C-lipid in muscle cells increased from 1.1 to 32.6% by addition of 6 U/mL of LPL. Furthermore, the secreted VLDL-like lipoprotein was decreased and a gel filtration column separated formed high-density lipoprotein (HDL)-like lipoprotein. These results suggest that 14C-free fatty acids produced by LPL digestion of 14C-VLDL-like lipoprotein were transported to muscle cells. We concluded that there are two metabolisms of the secreted lipoprotein dependent or independent on LPL.