|dc.description.abstract||The primary determinants of tissue glucocorticoid action are glucocorticoid receptor
(GR) density and intracellular levels of ligand, the latter determined both by activity
of the hypothalamic-pituitary-adrenal (HPA) axis and cellular activity of 11beta-
hydroxysteroid dehydrogenase (11beta-HSD) enzymes that interconvert active 11-
hydroxy (corticosterone, cortisol) and inactive 11-keto (11-dehydrocorticosterone,
cortisone) glucocorticoids. Here, the contribution of GR density and ligand levels in
determining body composition and metabolic phenotype have been investigated in
mice and in humans.
Genetic evidence in humans implicates variations in the GR gene in the regulation of
the HPA axis as well as the control of body fat distribution, metabolic parameters
and blood pressure. Although GR deficient mouse models have been previously
generated (with homozygous nulls dying at birth), the effects of altered GR density
upon fat distribution and blood pressure have not been described. This study
addresses the relationship between GR density and metabolic parameters, including
body fat distribution, insulin resistance and hypertension. A novel line of mice
harbouring a null mutation in the GR gene (GR+/-) was generated from an ES cell line
in which a beta-galactosidase-neomycin phosphotransferase (beta geo) reporter cassette
was fused with GR. The resulting fusion protein lacks part of the DNA binding
domain and the entire ligand binding domain and is transcriptionally inactive. In
addition, the beta-galactosidase enzyme activity “reports” activity of the GR gene
promoter. GR-/- mice are present in a normal Mendelian ratio before birth.
Intriguingly, 1 (of 36/146 expected if null allele not lethal) survived to adulthood
suggesting this might be a hypomorphic rather than a null allele. Heterozygous
(GR+/-) mice showed 40-45% reductions in GR mRNA levels in the hippocampus,
paraventricular nucleus of the hypothalamus, pituitary gland and adipose tissue, 30%
in liver, 56% in muscle and 67% in adrenals. X-gal staining of GR+/- brain sections
showed that GR-beta gal is present throughout, mirroring GR mRNA expression. Adult
GR+/- mice had larger adrenals, higher evening plasma corticosterone levels and
greater corticosterone responses following 10 minute restraint suggesting a
hyperactive HPA axis. Compared to GR+/+ littermates, GR+/- mice had similar body
weight gain on normal chow or high fat diet, with unaltered fat depot (inguinal,
epididymal, mesenteric) weights and similar glucose and insulin tolerance. However,
GR+/ - mice had higher (10%) systolic blood pressure, associated with activation of
the renin-angiotensin system. Thus GR haploinsufficiency in mice causes increased
blood pressure and accords with data associating GR polymorphisms with
hypertension in humans.
The role of altered GC sensitivity was also investigated in a mouse model of HPA
axis hypoactivity pro-opiomelanocortin null (POMC) mice. POMC-null mice are
obese due to central melanocortin deficiency. In contrast to most rodent models of
obesity, POMC-null mice are also glucocorticoid deficient due to ACTH deficiency.
Previous data have shown that glucocorticoid replacement in POMC-null mice
exaggerated hyperphagia, obesity and insulin resistance and caused hypertension.
Here, the contribution of peripheral glucocorticoid sensitivity was investigated.
POMC-null mice have increased liver and retroperitoneal fat GR mRNA levels but,
specifically in adipose tissue, decreased levels of mRNA encoding 11beta-HSD1, a
reductase which regenerates active glucocorticoids, thus amplifying their action.||en