ENDOTHELIAL-CELL HEPARANASE MODULATION OF LIPOPROTEIN-LIPASE ACTIVITY- EVIDENCE THAT HEPARAN-SULFATE OLIGOSACCHARIDE IS AN EXTRACELLULAR CHAPERONE

A unique feature of lipoprotein lipase (LpL), the ratelimiting enzyme in the hydrolysis of circulating triglycerides, is its movement from i ts cell of synthesis, adipocyte or myocyte, to its site of action, the luminal endothelial surface. This involves processes that allow LpL t o be released from the adipocyte cell surface and transferred against the flow of interstitial fluid to the luminal surface of endothelial c ells, LpL, an unstable enzyme, must retain its activity during this pr ocess, Whether a chaperone-like molecule is involved in LpL stabilizat ion and transport is unclear. In the present study, we tested the hypo thesis that endothelial cells secrete factors that release LpL and pro mote its transfer to the luminal endothelial surface. Incubation of ad ipocytes with endothelial cell conditioned medium (ECCM) led to releas e of about a-fold more LpL activity than control medium. Medium from e ndothelial cells exposed to lysophosphatidylcholine (lyso-ECCM), a pro duct of LpL lipolysis of lipoproteins, released ap proximately 3-fold more LpL than ECCM. Concomitant with the release of LpL, adipocyte cel l surface heparan sulfate (HS) proteoglycans were degraded suggesting that lyso ECCM contained a heparanase-like activity. More heparanase w as found in media from the basolateral than the apical side of lysolec ithin-stimulated polarized endothelial cells. In coculture experiments , lipolysis and lysolecithin stimulation of endothelial cells increase d LpL release from adipocytes. LpL released by lyso-ECCM remained stab le and did mot lose enzymatic activity at 37 degrees C for 1 h. LPL ac tivity was also stabilized, by heparanase-digested fragments of HS (HS oligosaccharide) and by purified LPL binding decasaccharide. Moreover , LpL HS oligosaccharide complexes crossed endothelial cell monolayers and bound to the apical side of the cells. Thus, an endothelial hepar anase may play a critical role in releasing subendothelial HS bound pr oteins, and specific HS oligosaccharides produced by this enzyme may s erve as extracellular chaperones.