Lipoprotein lipase release from BFC-1 beta adipocytes. Effects of triglyceride-rich lipoproteins and lipolysis products.

A Sasaki,I.J Goldberg

doi:10.1016/s0021-9258(18)42165-5

Abstract

Lipoprotein lipase (LPL), synthesized by adipocytes and myocytes, must be transported to the luminal endothelial cell surface where it then interacts with circulating lipoproteins. The first step in this extracellular LPL transport pathway is LPL release from the surface of LPL-synthesizing cells. Because hydrolysis of triglyceride (TG)-rich lipoproteins releases LPL from the apical surface of endothelial cells, we hypothesized that the same substances dissociate LPL from adipocytes. 125I-LPL was bound to the surface of brown adipocytes (BFC-1 beta). LPL binding to the adipocyte surface was greater than to endothelial cell surfaces. Using low concentrations of heparin, more LPL was released from endothelial cells than BFC-1 beta, suggesting that the affinity of LPL binding to the adipocytes was greater than LPL affinity for endothelial cells. Greater than 3-fold more LPL was released from the cell surface when very low density lipoproteins (VLDL) were added to culture medium containing 3% bovine serum albumin. LPL remaining on the cell surface decreased with VLDL addition. Endogenously produced LPL activity was also released from the cells by VLDL. Low and high density lipoproteins did not release 125I-LPL or LPL activity from the adipocytes. To assess whether lipolysis was necessary for LPL release, BFC-1 beta were incubated with TG-rich lipoproteins from a patient with apoCII deficiency. The apoCII-deficient lipoproteins did not release LPL unless an exogenous source of apoCII was added. Apolipoproteins E and Cs and high molar ratios of oleic acid:bovine serum albumin did not release surface-associated LPL. Lysolecithin (25 and 100 microM), but not lecithin, monoglycerides, or diglycerides, released adipocyte surface LPL. Because lysolecithin also released LPL during a 4 degrees C incubation, cellular metabolic functions are not required for LPL dissociation from the cells. Lysolecithin also inhibited LPL binding to endothelial cells; however, this effect was abrogated by addition of bovine serum albumin. We hypothesize that lipolysis products from TG-rich lipoproteins release adipocyte surface LPL, which can then be transported to the luminal endothelial cell surface.

Highlights

From the Department of Medicine and Specialized Center of Research in Arteriosclerosis, Columbia University College of Physicians and Surgeons
Lipoprotein lipase (LPL), synthesized by adipocytes Lipoprotein lipase (LPL)’ is the major enzyme responsible and myocytes, musbt e transported to the luminal endo- for hydrolysis of circulating triglyceride (TG)
Because hydrolysis of LPL must leave the adipocyte, travel to the abluminal triglyceride (TG)-rich lipoproteins releases LPL from the apical surface of endothelial cells, we hypothesized that the same substances dissociate LPL from adipocytes. ‘2sI-LPL was bound to the surface of brown adipocytes (BFC-18)

Summary

MATERIALS ANDMETHODS

Brown Fat Cell Cultures-Brown fat cells (BFC-1P) were a kind gift from Dr C. The reaction mixture (1mg of purified LPL, 20 mM glucose, 1 mCiofNa'"1 (Amersbam Corp.), 7.5 mg/ml of lactoperoxidase, and 1mg/ml glucoseoxidase (Sigma))was incubated in 50 mM Tris-HC1buffer, pH 7.4, for 7 min on ice. the mixture was applied to a column containing 3 mlof heparin-Sepharose gel (Pharmacia LKB Biotechnology Inc.). Media containing the unbound Iz5I-LPLwere removed and the cells were washed three times with cold DMEM-3%BSA. Heparinreleasable LPL associated with the cell surface was assessed after incubating the cells in 1ml of DMEM-3%BSA, 100 units/ml heparin (Elkins-Sinn,Inc., Cherry Hill, NJ) for 10 min a t 37 "C. "'I-LPL released into themedium was determined, and LPLremaining on the cell surface was assessed after washing the cells with DMEM-3%BSA and releasing "?-LPL with heparin-containing medium. Endogenous or exogenous LPL activity was assessed in the medium or after LPL release from the cell surface. The description of this patienthas been published [27]

RESULTS

Lipoprotein Lipase Release "

DISCUSSION