Plasma Lipoprotein Regulation of Progesterone Biosynthesis by Human Corpus Luteum Tissue in Organ Culture

The role of plasma lipoproteins in supplying cholesterol for progesterone biosynthesis by human corpus luteum tissue in culture was investigated. Progesterone secretion by tissue fragments maintained in organ culture reached a maximum rate by the third day and subsequently declined. Maximal secretion of progesterone was dependent on the presence of both low density lipoprotein (LDL) and hCG in the culture medium, whereas high density lipoprotein (HDL) was ineffective in supporting progesterone biosynthesis. Human corpus luteum tissue degraded [^l25I]iodo-LDL by a mechanism which was saturable, and degradation of [¹²⁵I]iodo-LDL was stimulated by hCG. Although 3-hydroxy-3-methylglutaryl coenzyme A reductase activity was present in microsomes prepared from fresh human corpus luteum tissue, the activity of this enzyme in microsomes prepared from tissue maintained in culture for 3 days was virtually undetectable. Fresh human corpus luteum tissue contained 3 times more unesterified cholesterol than esterified cholesterol. It is concluded that LDL, but not HDL, is the major source of cholesterol used by the human corpus luteum for progesterone biosynthesis.