Increased synthesis and accumulation of phospholipids during differentiation of 3T3-L1 cells into adipocytes

Conversion of 3T3-L1 preadipocytes to fully developed adipocytes in culture under the influence of dexamethasone, 1-methyl-3-isobutyl xanthine, and insulin offers a unique system to investigate differentiation-related changes in lipid metabolism. Depending on the type of isotopic precursors ([3H2]O, 32Pi, and [1-14C]acetate) used, a 10-170-fold increase in the rate of incorporation into lipid was observed in 3T3-L1 adipocytes which contained 5-10-fold higher amounts of cellular protein/culture than preadipocytes. In preadipocytes and adipocytes, the major lipids synthesized were phospholipids, triglycerides, and fatty acids. The rate of 32Pi incorporation into total lipids was 21-fold higher in adipocytes than that in preadipocytes, and 90% of the total radioactivity in both preadipocytes and adipocytes was contributed by phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). The content of phospholipids was 7-8-fold higher/culture in adipocytes than that in preadipocytes, and 65 and 80% of the total phospholipids of preadipocytes and adipocytes, respectively, were composed of PC and PE. Based on DNA measurements, there was a 3-fold increase in phospholipids and an approximately 3-fold increase in protein/cell. During 3T3-L1 adipose conversion, a detectable increase in cellular protein, triglycerides, and phospholipids was observed on day 3 after induction and, thereafter, these constituents increased continuously up to day 11. Among the different phospholipids of both preadipocytes and differentiating 3T3-L1 cells, PC, PE, and PI exhibited significant changes in their rate of 32Pi incorporation during adipogenesis. While there was a continuous increase in the incorporation of 32Pi into PI, there was preferential incorporation of 32Pi into PC and PE, the significance of which is not clear. The increase in phospholipids and protein content during adipogenesis suggests that phospholipids are required for membrane biosynthesis.