REGULATORY ROLES OF HIGH-DENSITY AND LOW-DENSITY LIPOPROTEINS IN CELLULAR PROLIFERATION AND SECRETION OF PROGESTERONE AND INSULIN-LIKE-GROWTH-FACTOR-I BY ENRICHED CULTURES OF BOVINE SMALL AND LARGE LUTEAL CELLS

We tested the hypotheses that bovine high-density (HDL) and low-densit y (LDL) Lipoproteins differentially influence cellular proliferation a nd progesterone and IGF-I production by bovine small and large luteal cells. Unit gravity sedimentation was used to produce enriched culture s of small (> 95% pure) and large (75 to 90% pure) luteal cells from c orpora lutea (CL) on d 4 and 10 of the estrous cycle. Addition of LDL, HDL, or both resulted in the maintenance of higher (P < .05) numbers of 3 beta-hydroxysteroid dehydrogenase (HSD)-positive small and large cells in culture and produced a marked proliferation of 3 beta-HSD-neg ative small luteal cells compared to control medium. Low-density lipop rotein and HDL each stimulated greater (P < .01) progesterone secretio n in enriched large cell cultures on both days of the cycle, and by sm all luteal cells on d 10, compared to the control. Together, LDL and H DL maximized this response. Lipoproteins markedly stimulated (P < .01) the secretion of IGF-I by bovine large luteal cells, and secretion wa s greater (P < .05) by cells from d 10 CL compared to d 4 CL. Results suggest that the actions of Lipoproteins in. bovine luteal cells exten d beyond their widely recognized roles in steroidogenesis and include remarkable effects on cellular proliferation and IGF-I secretion. Type of lipoprotein (LDL vs HDL) did not have differential effects on any of the variables measured.