Endocrine Abstracts

Metabolism of glucocorticoids to dihydro and tetrahydro derivatives via hepatic 5alpha- and 5beta-reductases has been assumed to be a pathway of irreversible inactivation of glucocorticoids. However, 5alpha-reduced metabolites of other steroids (testosterone, aldosterone) have significant affinity for the parent hormone receptor. We have investigated whether 5alpha-reduced metabolites of corticosterone (B) are GR agonists.
In hepatocytes from male lean Zucker rats (n=6) competition for binding of ³H₄dexamethasone was determined. B and 5alpha-tetrahydrocorticosterone (5alphaTHB) had equal affinity for GR (Kd,nM: B, 158 plus/minus 43 vs 5alphaTHB, 258 plus/minus 78, mean/SEM), which was greater than 5alphadihydrocorticosterone (397 plus/minus 100nM, p<0.01) and 5beta-reduced metabolites (5betaDHB, 7798 plus/minus 1901, p<0.001; 5betaTHB, 3590 plus/minus 802, p<0.001). Activation of GR was assessed with hGR transiently transfected into HeLa cells with a luciferase reporter. By comparison with B (1micromolar; 37.5 plus/minus 6.9 fold induction), 5alphaTHB was active (1micromolar; 28 plus/minus 4.8 fold induction, n=6) and additive. 5beta-Reduced metabolites did not activate GR. In addition, in H4IIE cells, 5alphaTHB induced tyrosine aminotransferase (TAT) mRNA expression (Northern blot; 0.62 plus/minus 0.03 vs 0.2 plus/minus 0.09 (vehicle) arbitrary units corrected for U1 (AU), p=0.04) albeit to a lesser extent than B (1.35 plus/minus 0.3 AU, p=0.04). To test relevance in pathology, we examined 5alpha-reductase1 in obese versus lean Zucker rats. 5alpha-Reductase1 mRNA and protein were increased (Northern blot, 0.58 plus/minus 0.12 vs 0.24 plus/minus 0.03 AU, p<0.01; Western blot, 0.70 plus/minus 0.05 vs 0.40 plus/minus 0.07 relative optical density, p<0.005).
We conclude that 5a-reduced glucocorticoids bind GR with significant affinity and are endogenous agonists at this receptor. Transcription of glucocorticoid regulated genes will thus be influenced by intracellular levels of corticosterone and its 5alpha-reduced metabolites. Increased levels of these metabolites in obesity may amplify glucocorticoid action in the liver independently of circulating concentrations of corticosterone.