Modelling altered Glucocorticoid Sensitivity: From HPA axis to metabolic abnormalities in mice and humans

摘要

The primary determinants of tissue glucocorticoid action are glucocorticoid receptorud(GR) density and intracellular levels of ligand, the latter determined both by activityudof the hypothalamic-pituitary-adrenal (HPA) axis and cellular activity of 11beta-udhydroxysteroid dehydrogenase (11beta-HSD) enzymes that interconvert active 11-udhydroxy (corticosterone, cortisol) and inactive 11-keto (11-dehydrocorticosterone,udcortisone) glucocorticoids. Here, the contribution of GR density and ligand levels inuddetermining body composition and metabolic phenotype have been investigated inudmice and in humans.udGenetic evidence in humans implicates variations in the GR gene in the regulation ofudthe HPA axis as well as the control of body fat distribution, metabolic parametersudand blood pressure. Although GR deficient mouse models have been previouslyudgenerated (with homozygous nulls dying at birth), the effects of altered GR densityudupon fat distribution and blood pressure have not been described. This studyudaddresses the relationship between GR density and metabolic parameters, includingudbody fat distribution, insulin resistance and hypertension. A novel line of miceudharbouring a null mutation in the GR gene (GR+/-) was generated from an ES cell lineudin which a beta-galactosidase-neomycin phosphotransferase (beta geo) reporter cassetteudwas fused with GR. The resulting fusion protein lacks part of the DNA bindinguddomain and the entire ligand binding domain and is transcriptionally inactive. Inudaddition, the beta-galactosidase enzyme activity “reports” activity of the GR geneudpromoter. GR-/- mice are present in a normal Mendelian ratio before birth.udIntriguingly, 1 (of 36/146 expected if null allele not lethal) survived to adulthoodudsuggesting this might be a hypomorphic rather than a null allele. Heterozygousud15ud(GR+/-) mice showed 40-45% reductions in GR mRNA levels in the hippocampus,udparaventricular nucleus of the hypothalamus, pituitary gland and adipose tissue, 30%udin liver, 56% in muscle and 67% in adrenals. X-gal staining of GR+/- brain sectionsudshowed that GR-beta gal is present throughout, mirroring GR mRNA expression. AdultudGR+/- mice had larger adrenals, higher evening plasma corticosterone levels andudgreater corticosterone responses following 10 minute restraint suggesting audhyperactive HPA axis. Compared to GR+/+ littermates, GR+/- mice had similar bodyudweight gain on normal chow or high fat diet, with unaltered fat depot (inguinal,udepididymal, mesenteric) weights and similar glucose and insulin tolerance. However,udGR+/ - mice had higher (10%) systolic blood pressure, associated with activation ofudthe renin-angiotensin system. Thus GR haploinsufficiency in mice causes increasedudblood pressure and accords with data associating GR polymorphisms withudhypertension in humans.udThe role of altered GC sensitivity was also investigated in a mouse model of HPAudaxis hypoactivity pro-opiomelanocortin null (POMC) mice. POMC-null mice areudobese due to central melanocortin deficiency. In contrast to most rodent models ofudobesity, POMC-null mice are also glucocorticoid deficient due to ACTH deficiency.udPrevious data have shown that glucocorticoid replacement in POMC-null miceudexaggerated hyperphagia, obesity and insulin resistance and caused hypertension.udHere, the contribution of peripheral glucocorticoid sensitivity was investigated.udPOMC-null mice have increased liver and retroperitoneal fat GR mRNA levels but,udspecifically in adipose tissue, decreased levels of mRNA encoding 11beta-HSD1, audreductase which regenerates active glucocorticoids, thus amplifying their action.