Plasma lipoproteins are a major source of cholesterol used in ovarian steroidogenesis. But, the mechanisms responsible for the uptake and metabolism of cholesterol-carried by high density lipoproteins (HDL) remain unclear. Circumstantial evidence implies that hepatic lipase (HL), an enzyme produced in liver and accumulated by binding sites on endothelial cells (ECs) of ovarian blood vessels (BVs), may have an important role in HDL metabolism. HL, by perturbing the HDL phospholipid:cholesterol ratio, may facilitate delivery of sterol to cells, including those of the ovary. No data exist substantiating this role of HL in the ovary. It is striking, however, that HL is preferentially concentrated in BVs of corpora lutea, a main steroidogenic compartment, and sparse in those of the stroma. this implies that the corpus luteum, perhaps luteal cells (LCs), influences the appearance of HL binding sites in BVs. Moreover, gonadotropins (GN) may regulate the association of HL with specific subsets of ovarian BVs, perhaps through their action on LCs. These data are the first to suggest that interaction between luteal cells and ECs alters the differentiated phenotype of ECs, by causing expression of HL binding sites and thus the ability to accumulate HL. This, in turn, may impact on the steroidogenic capacity of the ovary. Of equal importance, our pilot data are the first to imply that the interaction between GCs and ECs represents a two-way communication, with ECs promoting development of steroidogenic organelles in GCs. Thus, the objective of this proposal is to clarify the role of HL in ovarian steroidogenesis and the nature of the two-way interaction between ovarian parenchymal and endothelial cells. Specifically, we will utilize in vivo and in vitro approaches, including co-cultures and conditioned medium, and a mix of biochemical and structural methods (microscopy, morphometry, immunocytochemistry, etc.) to assess the ability of (a) HL-treated HDL to support steroidogenesis in situ perfused ovaries and primary cultures of GCs, (b) GCs + hormones/second message systems to promote expression of HL binding sites on ECs and/or cause changes in EC structure, and (c) ECs to enhance GC steroidogenic machinery and steroid secretion. We will determine if the interaction depends on direct physical contact of the cells or on diffusible factors produced by the cells. Data emerging from these studies will provide the first insights into the two- way communication between luteal cells and endothelial cells and how this interaction may impact on steroidogenic function of the ovary.
