Biogenesis of plasma lipoproteins in rat hepatoma McA-RH7777: Importance of diffusion-mediated events during cell growth

Summary Cultured McA-RH7777 rat hepatoma cells actively synthesize and secrete plasma lipoproteins. However, synthesis of [¹⁴C]triglyceride declines monotonically throughout the early growth period and remains low in postconfluent cultures; and net secretion of [¹⁴C]triglyceride is 10-fold more efficient in logarithmically growing cultures than in postconfluent cultures. Secretion of apolipoproteins associated with very low density and low density lipoproteins is selectively reduced in postconfluent cultures. The temporal reductions in [¹⁴C]triglyceride production are related more strongly to increasing cell concentration (cells/cm³ medium) than to increasing cell density (cells/cm² growth surface). We have allowed cells to grow either retained within small circular corrals or unrestricted in culture dishes. When seeded at equal density (10⁴ cells/cm²) but at one-fifth the cell concentration, corralled cells synthesize twice as much [¹⁴C]triglyceride per cell after 2 and 4 d, and are 10 times as efficient in [¹⁴C]triglyceride secretion by 6 d of growth, as noncorralled cells. When seeded at equal cell concentration (10⁵ cells/dish) but at 5 times the cell density, corralled cells are only 20% less efficient at [¹⁴C]triglyceride synthesis and secretion than noncorralled cells. Conditioned medium depresses synthesis and secretion efficiency of [¹⁴C]triglyceride. Orotic acid exposure also inhibits synthesis of [¹⁴C]triglyceride and secretion of certain [³⁵S]apolipoproteins in early cultures, but it has no significant effect on late cultures. We conclude that diffusion-mediated events are important regulators of triglyceride and apolipoprotein production in growing rat hepatoma cells, but that events associated with formation of cell-to-cell contacts play a minor role in regulation of plasma lipoprotein biogenesis. This research was supported by grants to R. H. from the American Heart Association (85-805), from NHLBI (15062, Specialized Center of Research in Atherosclerosis), and from the Louis Block Fund. S. T. was supported as an American Heart Association Medical Student Research Fellow. S. T. and H. S. are recipients of predoctoral training grants from the National Institutes of Health, Bethesda, MD (HL 723711, HD 07009). This work has been presented elsewhere in preliminary form (14, 48, 49).