The effects of insulin on hepatic glucocorticoid receptor content in the diabetic rat

Hepatocyte nuclear factor-1alpha regulates glucocorticoid receptor expression to control postnatal body growth

Hepatocyte nuclear factor 1alpha (HNF-1alpha) is a homeodomain-containing transcription factor and is important in postnatal growth and development in mice. In the HNF-1alpha-deficient liver, the expressions of a large set of growth hormone (GH)-responsive genes were significantly downregulated. By analyzing various HNF-1alpha mutant mice, we disclosed a mechanism by which hepatic HNF-1alpha regulates the expression of GH-responsive genes that are crucial for growth and development. We found that HNF-1alpha is required for the normal expression of glucocorticoid receptor (GR) specifically in livers. In the liver, GR, together with STAT5, is known to mediate the GH action by transactivating the GH-responsive genes that function in body growth and development. We further demonstrated that HNF-1alpha modulated GR gene expression by directly transactivating the GR gene promoter via a cryptic regulatory element located 3 bp upstream of the translation start site in exon 2 of the GR gene locus.

Hepatic gene expression profiling of streptozotocin-induced diabetes

Diabetes induces biochemical, morphological, and functional alterations in the liver. The liver is a major target of insulin action, and plays a critical role in maintaining blood glucose homeostasis. We investigated the effects of streptozotocin-induced diabetes (SID) on global hepatic gene expression in mice. We induced SID in mice by intraperitoneal injection of streptozotocin. Affymetrix (Santa Clara, CA) microarrays containing probe sets for approximately 11,000 murine genes and expressed sequence tags were used to assess the effects of SID on hepatic gene expression in mice. SID significantly altered the expression of 87 known genes in the liver. SID increased the expression of genes associated with cytoprotective stress responses, oxidative and reductive xenobiotic metabolism, cell cycle inhibition, growth arrest, apoptosis induction, and protein degradation. SID decreased the expression of genes associated with cell proliferation, growth factor signaling, protein synthesis, and xenobiotic metabolism. The novel results reported here should open new areas of investigation in diabetes research and facilitate the development of novel strategies for gene therapy and drug discovery.

Streptozotocin-induced diabetes and glucocorticoid receptor regulation: tissue- and age-specific variation

Streptozotocin (STZ) -induced diabetic effects were analyzed for glucocorticoid receptor (GR) level and for in vitro activation of GR by specific binding analysis, using [3H]dexamethasone, a synthetic glucocorticoid, and by DNA cellulose and nuclear binding assay, in the liver and kidney of 15- (immature) and 120-day-old (mature) male mice. Comparison of GR level (fmol/mg protein) among the control mice reveals decreased (22-33%) specific binding in the liver and kidney of mature mice compared with immature ones. Scatchard analyses, however, reveal no change in the affinity (K(d)) of receptor at these two ages of mice. STZ-induced diabetes did not alter the level of GR in either of the tissues at both the ages studied. The GR from both the tissues underwent thermal activation, albeit the extent of activation was more pronounced in mature liver compared to immature, with no such difference of activation in the kidney. In diabetic mice, the activation of hepatic GR exhibits reduced DNA cellulose ( approximately 20-23%) and nuclear (24-30%) binding compared to control mice. In contrast, thermal activation of kidney GR does not show marked differences in diabetic mice at either of the ages studied. Cross-mixing experiments (i.e. binding of activated GR from diabetic mice to nuclei of control and vice-versa) performed on the mature liver, indicate receptor specificity. These findings reveal tissue- and age- specific variations in the level of GR that is not influenced under diabetic conditions. However, the activation of hepatic GR is reduced during STZ-induced diabetes that might play a role in controlling glucose homeostasis in diabetic animals.

Regulation of glucocorticoid receptor by the tyrosine kinase inhibitor herbimycin A in the cytosolic fraction of primary cultured rat hepatocytes

The participation of tyrosine kinase in the regulation of the glucocorticoid receptor (GR) was studied in primary cultured rat hepatocytes using the tyrosine kinase inhibitor herbimycin A. Herbimycin A decreased the number of high-affinity binding sites of glucocorticoids in the cytosolic fraction and increased the equilibrium dissociation constant (Kd). Western blot analysis revealed that it also decreased the amount of GR protein. On the other hand, cycloheximide did not affect the GR protein level. Although herbimycin A slightly increased the amount of GR protein in the nuclear fraction, the increase was much lower than that of its decrease in the cytosolic fraction. Therefore, the decrease of GR protein in the cytosolic fraction was not caused by the inhibition of GR protein synthesis nor the translocation of GR from cytosol to nuclei. As herbimycin A also suppressed the dexamethasone (Dex)-dependent induction of tyrosine aminotransferase (TAT) activity, the decrease of GR protein was followed by the suppression of the GR-mediated biological response. These findings indicate that tyrosine kinase is necessary for the maintenance of the level of GR protein and its affinity of binding sites in the cytosolic fraction. Our results also suggested that the increase of GR protein stability is the most probable explanation for the maintenance of its level.

Optimal ligand binding by the recombinant human glucocorticoid receptor and assembly of the receptor complex with heat shock protein 90 correlate with high intracellular ATP levels in Spodoptera frugiperda cells

The full-length human glucocorticoid receptor (hGR), overexpressed in Spodoptera frugiperda (Sf9) cells, associates with heat shock protein 90 (hsp90) and hsp70 and binds dexamethasone with high affinity. Baculovirus infection of Sf9 cells grown in TNM-FH medium results in the rapid depletion of glucose from the medium within 24 h. Noting a discrepancy between hGR protein levels and ligand binding capacity in such cultures, we hypothesized that the depletion of glucose from the medium could result in intracellular ATP depletion and consequently affect the ligand binding capacity of the recombinant hGR. Supplementation of the Sf9 culture medium with additional glucose resulted in a three-fold increase in intracellular ATP levels, and a three-fold increase in 3H-dexamethasone binding capacity, without altering the protein levels of hGR, hsp90 or hsp70. However, more hsp90 co-immunoprecipitated with hGR from cells grown in glucose supplemented medium. Our data support the hypothesis that high-affinity ligand binding by hGR requires the ATP-dependent formation of the hGR:hsp90 heterocomplex. Besides having practical consequences for the production of recombinant GR and other related proteins, our findings could ultimately have relevance in diseases such as diabetes mellitus.

Glucocorticoid binding during the differentiation of 3T3-F442A fibroblasts into adipocytes. A possible regulatory effect of insulin

Until recently, few studies had been carried out on receptors for glucocorticoids in adipocytes, although the role of these steroids is considerable. In the present studies, we chose the pre-adipocyte line 3T3-F442A, which constitutes an excellent model for investigating the differentiation and function of adipocytes. Using a whole cell assay system, we showed the existence of a homogenous class of sites with the characteristics of glucocorticoid receptors, that is, high-affinity binding which is reversible, specific and saturable. Whatever the state of cellular differentiation, the affinity of the receptor for dexamethasone did not vary, although we observed an increase in the number of sites during differentiation. When cells were differentiated in the presence of insulin, there was a further increase in the binding capacity; moreover, insulin deprivation of such adipocytes caused a decrease in the number of sites. Our results therefore suggest that factors other than the glucocorticoids themselves influence dexamethasone binding. It is suggested that insulin plays a role in the regulation of the number of glucocorticoid receptors.