Rapid mechanisms of glucocorticoid signaling in the Leydig cell

Abstract

Stress-mediated elevations in circulating glucocorticoid levels lead to corresponding rapid declines in testosterone production by Leydig cells in the testis. In previous studies we have established that glucocorticoids act on Leydig cells directly, through the classic glucocorticoid receptor (GR), and that access to the GR is controlled prior to the GR by a metabolizing pathway mediated by the type 1 isoform of 11beta-hydroxysteroid dehydrogenase (11betaHSD1). This enzyme is bidirectional (with both oxidase and reductase activities) and in the rat testis is exclusively localized in Leydig cells where it is abundantly expressed and may catalyze the oxidative inactivation of glucocorticoids. The predominant reductase direction of 11betaHSD1 activity in liver cells is determined by an enzyme, hexose-6-phosphate dehydrogenase (H6PDH), on the luminal side of the smooth endoplasmic reticulum (SER). Generation of the pyridine nucleotide cofactor NADPH by H6PDH stimulates the reductase direction of 11betaHSD1 resulting in increased levels of active glucocorticoids in liver cells. Unlike liver cells, steroidogenic enzymes including 17beta-hydroxysteroid dehydrogenase 3 (17betaHSD3) forms the coupling with 11betaHSD1. Thus the physiological concentrations of androstenedione serve as a substrate for 17betaHSD3 utilizing NADPH to generate NADP+, which drives 11betaHSD1 in Leydig cells primarily as an oxidase; thus eliminating the adverse effects of glucocorticoids on testosterone production. At the same time 11betaHSD1 generates NADPH which promotes testosterone biosynthesis by stimulating 17betaHSD3 in a cooperative cycle. This enzymatic coupling constitutes a rapid mechanism for modulating glucocorticoid control of testosterone biosynthesis. Under stress conditions, glucocorticoids also have rapid actions to suppress cAMP formation thus to lower testosterone production.

Figure 1

Rapid inhibition of testosterone production in rat Leydig cells with corticosterone- BSA conjugate. When 0.5 × 106 Leydig cells cultured with control (n = 6) or different concentrations from 0.1 nM to 100 nM (n = 3, respectively) for 30 minutes. Media were collected for testosterone measurement. *designates the significant difference compared to control at P<0.05.

Figure 2

Models for different couplings between 11βHSD1 and other enzymes in liver and Leydig cells. In liver cells (Panel A), in SER luminal side, H6PDH forms coupling with 11βHSD1: G6P is exported into SER luminal side by G6P exporting protein which can be inhibited by the drug S3483. G6P and NADP+ are used by H6PDH to generate NADPH which renders 11βHSD1 primary as a reductase to activate 11DHC to CORT. In Leydig cells (Panel B), both 11βHSD1 and 17βHSD3 are located in SER membrane and form the coupling. The androgen intermediate androstenedione (DIONE) and NADPH are used by 17βHSD3 to generate NADP+ which renders 11βHSD1 to function primarily as an oxidase to inactivate CORT to 11DHC.