Endocrine Abstracts

Bile acids (BA) are potent steroid hormones that mediate a variety of metabolic effects. They play a pivotal role in cholesterol catabolism, intestine nutrient absorption, and regulate lipid, glucose and energy metabolism. 5-Beta-Reductase (AKR1D1) is a key enzyme in the BA synthesis pathway, required for cholesterol metabolism into bile acids. We generated a novel global AKR1D1 knockout (KO) mouse which, as expected, has decreased total serum and hepatic BAs and altered BA composition. Here we demonstrate that AKR1D1 deletion leads to increased insulin sensitivity in mature male mice. Male WT and AKR1D1 KO mice were maintained on a control diet with metabolic assessments undertaken at 30-weeks of age. AKR1D1 KO mice presented with enhanced insulin sensitivity as measured by intraperitoneal insulin tolerance test compared to WT littermates (WT 793; KO 647 mMol/min). At a molecular level, qPCR analysis of skeletal muscle insulin signalling pathway genes showed increased mRNA expression of insulin receptor substrate 1 (IRS-1) in AKR1D1 KO mice compared to WT (WT 0.46±0.03; KO 0.71±0.07 relative expression n=12 P<0.05) but no changes in phosphoinositide 3-kinases (PI3Ks), protein kinase B (total-AKT) or insulin receptor subunit β (INSRβ). Despite the unchanged mRNA levels, western blotting revealed increased protein levels of skeletal muscle INSRβ (WT 1.82±0.13 n=9; KO 1.24±0.21 protein/total protein n=9 P<0.05), total-AKT (WT 1.83±0.21 n=9; KO 0.9864±0.2504 protein/total protein n=9 P<0.05) and mTOR (WT 1.86±0.05 n=3; KO 1.13±0.11 protein/total protein n=3 P<0.05). Muscle glycogen levels were also elevated suggesting increased anabolic metabolism in the KO animals. QPCR analysis of the hepatic IRS-1, PI3Ks, total-AKT and INRβ showed no differences in mRNA expression levels. Western blotting detected elevated hepatic total-AKT protein levels (WT 1.05±0.09 n=9; KO 0.60±0.07 protein/total protein n=9 P<0.05) although INRβ, mTOR, and glycogen levels were unchanged. Despite the increased insulin sensitivity, there was no change in intraperitoneal glucose tolerance. However, in the fed state, circulating insulin levels were significantly reduced in AKR1D1 KO mice (WT 1.31±0.22 n=9; KO 0.57±0.01 ng/ml n=6, P<0.05) with normal blood glucose levels (WT 14.36±0.7 n=14; KO 14.21±0.52 mMol/l n=12 P<0.05). Taken together, our results suggest that AKR1D1 plays an important role in glucose homeostasis by mediating insulin sensitivity in liver and skeletal muscle. Further studies are required to define the underpinning mechanisms.