Endocrine Abstracts

Glucocorticoid excess results in central obesity and insulin resistance/diabetes mellitus. At a pre-receptor level, 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) modulates glucocorticoid levels and has been implicated in the pathogenesis of the metabolic syndrome. 11beta-HSD1 is a bi-directional NADP(H) dependant enzyme, but keto-reductase activity predominates in-vivo. Recent studies indicate that the enzyme H6PDH ensures reductive metabolism (cortisone to cortisol in man, 11-dehydrocorticosterone (A) to corticosterone (B) in rodents) by providing NADPH to 11beta-HSD1 in the endoplasmic reticulum. To investigate this hypothesis further, mice were produced with a targeted inactivation of the H6PDH gene.

Homozygous knockout (KO) mice were shown to be null for the H6PDH locus. Mice are viable, have a normal gestation and litter size and show a Mendelian distribution of genotypes from heterozygote crosses. H6PDH enzyme activity was assessed in liver microsomes from WT and KO animals (n=4). As expected, KO microsomes were devoid of activity whilst WT microsomes were able to generate 5.5nm NADPH (plus/minus0.41) /min/mg protein. Microsomal 11B-HSD1 reductase activity (conversion of A to B) was 3.66 nmol B (plus/minus0.34) /mg protein/h in WT and 0.16 nmol B (plus/minus0.003) /mg protein/h in KO mice (p<0.0001). Conversion of B to A (dehydrogenase activity) was 0.23 nmol B (plus/minus0.006) /mg protein/h in WT to 1.66 nmol B (plus/minus0.11) /mg protein/h in KO mice (p<0.001). 24 hour urine collections were analyzed for A and B metabolites. WT mice exclusively excreted steroids with an 11beta-hydroxyl group; the excretion of 11-carbonyl metabolites was <2% in WT versus 62% in KO mice. The ratio of THA/THB (a marker of 11beta-HSD1 activity) was 0.01 in WT and 0.75 in KO mice.

These data demonstrate that H6PDH confers 11-keto-reductase activity upon 11beta-HSD1 in the generation of active glucocorticoid in liver, a finding that has implications for the future therapy of patients with the metabolic syndrome.