Steroidogenesis in adult Leydig cells is controlled by the pituitary gonadotropic hormones. Of the two gonadotropins, luteinizing hormone (LH) is the main stimulus for androgen biosynthesis. LH signal transduction is initiated on binding of the hormone to specific receptors on the Leydig cell surface. The receptor for LH (LH-R) belongs to a superfamily of G protein-coupled receptor (GPCR) (78).
Fig. 5. Acute regulation of Leydig cell steroidogenesis by luteinizing hormone (LH). The complex of LH and LHRs triggers adenylate cyclase-mediated increases cytoplasmic cyclic adenosine monophosphate (cAMP) levels. Activation of protein kinase A and cholesterol esterase results in cholesterol mobilization from extracellular lipoprotein (low-density lipoprotein) and intercellular sources (including de novo synthesis) in the smooth endoplasmic reticulum (SER) and released from lipid droplet stores. Cholesterol moves across the outer to the inner membrane of the mitochondrion via shuttle proteins steroidogenic acute response (StAR) and peripheral benzodiazepine receptor (PBR). Cholesterol side-chain cleavage enzyme catalyzes the initial step in steroid biosynthesis. The subsequent steps are catalyzed by P45017a, 3PHSD, and 17PHSD in the SER. The ultimate product, testosterone, moves out of the Leydig cell down a concentration gradient and is carried to androgen-responsive tissues by androgen-binding protein (rodents) and sex hormone-binding globulin (SHBG) (humans). The response to LH also involves an increased cytoplasmic calcium, synthesis of arachidomic acid, and efflux of chloride ions. The mechanism of Ca2+ action involves a calcium/calmodulin (Ca/CaM) protein kinase. Activation of phospholipase A2(PLA2) produces arachidonic acid (AA), which has a stimulatory effect on cholesterol mobilization to mitochondria. (Courtesy of P.N. Schlegel and M.P. Hardy (2002)  with the publisher's permission.)
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