Dexamethasone treatment of stallions impairs testosterone biosynthesis at a late step: Glutathione S-transferase A3 isomerization of Δ5-androstenedione

Abstract

Dexamethasone, a synthetic glucocorticoid, is used as an anti-inflammatory agent in horses. Similar to endogenous stress hormones, dexamethasone lowers serum testosterone within 4 h with levels recovering as early as 24 h post-injection. Weeks later, numbers of motile sperm with normal morphology decrease greatly. To explore the mechanism by which dexamethasone down-regulates testosterone biosynthesis, four adult Quarter Horse stallions were injected i.v. with 0.1 mg/kg dexamethasone while four others received saline. Within 10 h of dexamethasone injection, serum testosterone and cortisol were decreased by 60% and 87%, respectively. Twelve h after injection, stallions were castrated and testes recovered. Testis RNA samples were analyzed on Codelink microarrays carrying 55,000 probes for human mRNAs, as described in Davidson et al. (Cancer Res 64: 6797-6804, 2004). A novel gene, glutathione S-transferase A3 (GSTA3), was discovered to be down-regulated by dexamethasone. GSTA3 is a member of a closely related family of enzymes. While GSTA1, GSTA2 and GSTA4 genes arewidely expressed across tissues, GSTA3 gene expression is restricted to steroidogenic tissues in species using the D5-pathway of steroidogenesis, which include livestock species and humans but not rodents. Quantitative PCR, using primers that distinguish GSTA3mRNA from the other GSTAmRNAs, detected a 49% decrease in GSTA3 mRNA concentrations in testes of dexamethasone-treated stallions. The same primers were