LIPOPROTEIN-LIPASE DEGRADATION BY ADIPOCYTES - RECEPTOR-ASSOCIATED PROTEIN (RAP)-SENSITIVE AND PROTEOGLYCAN-MEDIATED PATHWAYS

Lipoprotein lipase (LPL), the major enzyme responsible for tile hydrol ysis of triglycerides, is primarily synthesized by adipocytes and myoc ytes. In addition to synthesis, degradation of cell surface-associated LPL is thought to be important in regulating production of the enzyme . We studied LPL metabolism in the LPL synthesizing adipocyte cell lin e BFG-1 beta and assessed the contributions of cell surface heparan su lfate proteoglycans (HSPG), low density lipoprotein receptor related p rotein (LRP), and glycosylphosphatidylinositol (GPI)-linked proteins t o LPL uptake and degradation by these cells. Adipocytes degraded 10-12 % of total cell surface I-125-labeled LPL in 2 h and 23-28% in 4 h. In 1 h, 30-54% of the degradation was inhibited by the 39 kDa receptor a ssociated protein (RAP), an inhibitor of ligand and binding to LRP. At 4 h, only 19-23% of the LPL degradation was RAP inhibitable. This sug gested that two pathways with different kinetics were important for LP L degradation. Heparinase/heparitinase treatment of cells showed that most LPL degradation required the presence of HSPG. Treatment with pho sphatidylinositol-specific phospholipase C (PIPLC) inhibited I-125-lab eled LPL degradation by 13%. However, neither RAP nor PIPLC treatment of adipocytes significantly increased the amount of endogenously produ ced LPL activity in the media. To determine whether direct uptake of L PL bound to HSPG could account for the non-RAP sensitive LPL uptake an d degradation, proteoglycan metabolism was assessed by labeling cells with (SO4)-S-35. Of the total pericellular proteoglycans, 14% were PIP LC releasable; surprisingly, 30% were dissociated from the cells with hepar-in. The amount of labeled pericellular proteoglycans decreased 2 6% in 2 h and 50% in 8 h, rapid enough to account for at least half of the degradation of cell surface LPL. We conclude that adipocytes degr ade a fraction of the cell surface LPL, and that this process is media ted by both proteoglycans and RAP-sensitive receptors.