Molecular control of luteal secretion of progesterone.

Cholesterol provided by low- or high-density lipoprotein is the precursor for biosynthesis of progesterone. Once inside the cell, cholesterol can be used for steroidogenesis or esterified with long-chain fatty acids and stored as cholesterol esters in lipid droplets. When it is needed for steroidogenesis, free cholesterol is transported to the mitochondrion via a mechanism that involves cytoskeletal elements and sterol carrier proteins. Cytochrome P450 cholesterol side chain cleavage enzyme complex converts the cholesterol to pregnenolone, which is then converted to progesterone by 3beta-hydroxysteroid dehydrogenase/delta5,delta4 isomerase in the smooth endoplasmic reticulum. Transport of cholesterol from the cytoplasm to the inner mitochondrial membrane is both the rate-limiting step in progesterone biosynthesis and the step most acutely influenced by second messengers. Steroidogenic acute regulatory protein (StAR) and peripheral-type benzodiazepine receptors (PBR) are involved in this transport. StAR may bind cholesterol in the cytosol and transport it to the mitochondrial membrane where PBR is involved in transport from the outer to the inner mitochondrial membrane. Phosphorylation of StAR by protein kinase A (PKA) stimulates cholesterol transport, whereas phosphorylation by PKC may inhibit this process. Endozepine, the natural ligand for PBR, also appears to be involved in regulation of the rate of cholesterol transport to the inner mitochondrial membrane and to play a role in the stimulatory effects of PKA on steroidogenesis. Increased concentrations of endozepine were detected in large luteal cells, and may explain the increased progesterone secretion from this type of cell. Fluorescence energy transfer procedures indicate that StAR associates with PBR in mitochondrial membranes. A model is presented for the proposed interactions of StAR, PBR and endozepine in the transport of cholesterol from the outer to the inner mitochondrial membrane.