Lipoprotein lipase (LPL) is responsible for hydrolysis of plasma triglycerides making their component fatty acids available for metabolism or synthesis into lipid. It plays a major role in the synthesis of milk fat. Mammary LPL is synthesized and secreted by adipocytes in the interstitial space of the mammary gland and secreted LPL is dispersed to be bound by receptors in the capillary wall, the basement membrane and the basal surface of the mammary alveolar cell. We will characterize the LPL binding elements present on the surfaces of mammary alveolar cells and in the mammary extracellular matrix and determine their role in LPL internalization. We will test the hypothesis that the plasma concentration of free fatty acids is an important factor in the activity of mammary LPL. We will characterize the biochemical nature and the mode of action of an inhibitor of adipocyte LPL secretion found in conditioned medium from mammary epithelial cells. Studies from other laboratories suggest that LPL has a second mode of action, binding lipoproteins to both cell surface and interstitial heparan sulfate proteogIycans. This observation suggests the exciting possibility that the high concentrations of LPL observed immunocytochemically in the lactating mammary gland may play a role in the transfer of sterol rich VLDL and LDL into the mammary alveolar cell providing insight into the previously uncharacterized passage of cholesterol, Vitamin E, beta-carotene and other lipid-soluble molecules into milk. We will use our model mammary culture systems to evaluate this hypothesis. This laboratory is the only one addressing the regulation of LPL in the mammary gland. An understanding of the regulation of mammary LPL should give us insight into the regulation of the lipid content of both human and cow's milk. If the inhibitor of adipocyte LPL that we have identified in our culture system is active in vivo it may have utility in the reduction of obesity. Should LPL be involved in the transfer of sterols into the lactating mammary cell, the possibility of regulating the cholesterol content of bovine milk can be entertained. Finally, insight into the interactions of LPL with mammary extracellular matrix molecules may increase our understanding of the general nature and function of this important compartment of all secretory organs through which chemical agents, be they hormones, drugs or toxins, must pass as they travel from the blood stream to the secretory epithelium.
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