The enzyme hepatic lipase (HL) is central to cholesterol homeostasis. HL participates in the regulation of plasma cholesterol levels, a major risk factor for atherosclerosis. It also mediates steroid hormone production, by controlling access to exogenous cholesterol in steroid-producing tissues. In turn, increased production of the adrenal steroid cortisol (corticosterone in mice) as occurs in stress, depression and certain tumors of adrenal and pituitary origin, modulates the development of atherosclerosis: increased cortisol levels are linked to an increased risk for atherosclerosis. However, HL's precise role in atherosclerosis is unknown. HL exerts its role via at least two functions. Its catalytic function processes lipoproteins for both receptor-mediated endocytosis and selective cholesterol uptake. Its bridging function facilitates interactions between lipoproteins, receptors, and the plasma membrane, thereby modulating lipoprotein cholesterol flux. This proposal seeks to establish the respective contributions of the catalytic and bridging functions to the cellular uptake of lipoprotein cholesterol and atherosclerosis by generating transgenic mice that express wildtype HL and functional mutants of HL. These mice will be bred with mice deficient in endogenous HL(hl-/-) and with h1-/- mice that are also deficient in the low density lipoprotein (LDL) receptor to generate Ldlr-/-hl-/- mice. (The Ldlr-/- mice serves as a model for diet-induced atherosclerosis). Thus, we will determine the effect of the expression of wildtype and mutant HLs on plasma lipid and lipoprotein concentrations, composition and size, and lipoprotein turnover. The mechanisms of lipoprotein cholesterol uptake will be examined in primary hepatocyte cultures from the livers of these mice by determining the effects of wildtype and mutant HL on cellular binding and uptake of labeled lipoproteins in the presence of specific inhibitors of cellular receptors. The effect of expression of wildtype and mutant HL on atherosclerosis development will be established in high-fat diet fed wildtype and mutant HL expressing mice on the Ldlr-/-hl-/-background. In addition this proposal seeks to establish the role of HL in regulating the adrenal steroidogenic response to stress and to identify the contribution of the catalytic function of HL to the adrenal steroidogenic response to stress. This will be accomplished by determining plasma corticosterone response, adrenal cholesterol content and adrenal expression of receptors and enzymes involved in cholesterol metabolism, in response to chronic pharmacologic stimulation of corticosterone production in wildtype- and mutant HL-expressing mice and compared to nontransgenic mice.
