Endocrine Abstracts

In adipose tissue, glucocorticoids (GC) are known to regulate lipogenesis and lipolysis. Hexose-6-phosphate dehydrogenase (H6PDH), a protein located in the endoplasmic reticulum (ER) provides co-factor for the enzyme 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1), thus regulating the set point of its activity and allowing for tissue specific activation of GCs. The aim of this study was to examine the effect of lack of H6PDH on adipose tissue biology using the H6PDH null (H6PDH/KO) mouse model. Real-time PCR analysis confirmed similar mRNA levels of 11β-HSD1 and glucocorticoid receptor (GRα) in wild type (WT) and H6PDH/KO mice in liver and adipose tissue. H6PDH mRNA was absent in liver and adipose tissues of H6PDH/KO mice. Despite similar amounts of 11β-HSD1 protein expressed in gonadal fat (GF), the enzyme directionality in GF explants of H6PDH/KO mice was changed from oxo-reductase (corticosterone generation) in WT (219±55 pmol/g wet tissue/h) to dehydrogenase (corticosterone inactivation) (300±29 pmol/g wet tissue/h, mean±S.D., P<0.001, n=4). A similar switch was also observed in stromal–vascular cultures from GF of H6PDH/KO mice. In vivo lipogenesis rates were significantly lower in H6PDH/KO mice (8.7±3.3 vs 12.5±3.9 μmol fatty acids/g tissue/2 h, P<0.05, n=5). This may be explained by lower mRNA expression levels of the key lipogenic enzyme acetyl CoA carboxylase (AU: 0.040±0.017 vs 0.008±0.003 mean±S.D., P<0.01). Overnight fasting revealed impaired rates of lipolysis in the H6PDH/KO as determined by no change in serum free fatty acids concentrations together with significantly lower mRNA expression of hormone sensitive lipase (AU: 0.48±0.08 vs 1.55±0.22, mean±S.D., P<0.001, n=4). Fat mobilisation in WT mice was shown as the reduction of GF mass (1.02±0.51 vs 0.44±0.34 g, mean±S.E.M., P<0.05, n=6) and adipocyte size. Equally, fasted H6PDH/KO had no change in GF mass and had larger average adipocyte size compared to WT (85.1±2.7 vs 73.1±3.0 μm, mean±S.E.M., P<0.01, n=6) despite similar adipocyte size in the fed state. In conclusion, in the absence of H6PDH the set point of 11β-HSD1 enzyme activity is ‘switched’ from oxo-reductase to dehydrogenase activity in adipose tissue – as a consequence this leads to an impairment of fat storage and mobilisation.