Steroid Receptor Coactivator-1 Expression Research Articles

Thyroid hormone exerts site-specific effects on SRC-1 and NCoR expression selectively in the neonatal rat brain

Thyroid hormone receptors (TRs) are ligand-gated transcription factors. Recently, many coregulator proteins have been identified that interact with steroid/TRs and are required for the activation or repression of hormone sensitive genes. We tested whether steroid receptor coactivator-1 (SRC-1) and nuclear corepressor (N-CoR) expression is altered by hypothyroidism in rat brains on gestational day 16 and postnatal day 15. We found that both SRC-1 and N-CoR mRNA levels were decreased in the cortex and dentate gyrus of 6- n-propyl-2 thiouracil treated rats only on P15, while mRNA levels for both genes were increased in the same CA3 region of the brains. These findings do not support the idea that cofactors are involved in the compensatory mechanisms for conserving TH action, but they do suggest that hypothyroidism affects the responsiveness of tissues to steroid hormones by altering the expression of necessary cofactors.

Effects of gender and stress on the regulation of steroid receptor coactivator-1 expression in the rat brain and pituitary

Steroid/thyroid actions in the brain are exerted through their receptors which belong to the nuclear receptor superfamily. Transcriptional transactivation mediated by these receptors depends on recruited co-activators, among which steroid receptor co-activators (SRCs) seem to be restricted to the nuclear receptor family. By using Northern and Western blot analysis we have estimated the mRNA and protein levels, respectively, of SRC-1 in the brain and pituitary of male and female rats, under physiological conditions and following restraint stress. Under basal conditions, SRC-1 is expressed at higher levels in the hippocampus and the pituitary of male, compared to female rats. Acute stress results in decreased, compared to the control, SRC-1 levels in the hypothalamus of both sexes, in the pituitary and frontal cortex of male rats, and in increased SRC-1 levels in the hippocampus of female rats. The observed changes at the mRNA level are supported by analogous changes at the protein level. The apparent regulation of SRC-1 gene expression in the nervous system by the endocrine status of the animal, adds another level of complexity in the mechanism controlling steroid hormone actions. Furthermore, the variability in SRC-1 expression within the brain provides a means to explain the cell-specificity of steroid hormone actions in this tissue.

Gene amplification and expression of the steroid receptor coactivator SRC3 (AIB1) in sporadic breast and endometrial carcinomas.

Steroid receptor coactivator 3 (SRC3) functions as a coactivator for nuclear receptor mediated transcriptional activation. It binds to nuclear receptors in a ligand-dependent fashion and recruits other factors such as CBP and p300 to the transactivation complex. Due to its function as activator of nuclear receptors, overexpression of SRC3 might enhance their effects. Gene amplification is a common mechanism that causes overexpression, already described for oncogenes like c-erbB2, c-myc and int2. In this study, SRC3 gene amplification and expression levels were analyzed in 127 sporadic breast carcinomas, 30 endometrial carcinomas and different cell lines (MCF7, HeLa, Ishikawa, T47D, BT-20, SK-BR-3, HEC-1a, RL 95-2, OVCAR3 and A-431). To determine gene amplification and mRNA expression levels, quantitative differential PCR and RT-PCR were performed in combination with fluorescent DNA technology. Gene amplification was not found in any of the breast and endometrial carcinomas, but was found in the carcinoma cell lines MCF7 (10-fold) and HeLa (3-fold). SRC3 overexpression was detected in 13% (3/23) of breast carcinomas and 17% (5/30) of endometrial carcinomas, as well as in MCF7 and HeLa cells. Thus, SRC3 overexpression found in breast and endometrial tumors is not caused by SRC3 gene amplification. A carcinogenic potential provided by SRC3 overexpression has to be elucidated in further studies.

Steroid receptor co-activator-1 mediates 1,25-dihydroxyvitamin D(3)-stimulated alkaline phosphatase in human osteosarcoma cells.

For steroid hormone function to occur, nuclear receptors interact with a series of coactivators including steroid receptor coactivator-1 (SRC-1). The SRC-1 binds the vitamin D receptor (VDR) in the presence of ligand in an activation function 2 (AF-2)-dependent manner. In order to understand the role of this interaction in 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]-mediated gene expression, the level of SRC-1 expression was altered in MG-63 cells. Previous studies had demonstrated that MG-63 cells express the VDR and that 1,25(OH)(2)D(3) regulates expression of alkaline phosphatase (ALP). Analysis of MG-63 cells demonstrated that SRC-1 is expressed. A full-length cDNA coding for SRC-1 was inserted in antisense orientation into an expression vector (anti-SRC-1). The MG-63 cells were transfected with anti-SRC-1 or mock vector and stable transformants were selected. Western blot analysis showed a 95% reduction in SRC-1 protein as compared with mock cells. We determined the effect of normal and reduced SRC-1 expression in MG-63 cells on 1,25(OH)(2)D(3)-mediated stimulation of ALP. Whereas 10(-8) M 1,25(OH)(2)D(3) produced a 3.6-fold stimulation in ALP in mock cells expressing normal levels of SRC-1, it did not alter ALP in cells expressing reduced levels of SRC-1. Thus, SRC-1 is required for 1,25(OH)(2)D(3)-mediated gene expression of ALP by human MG-63 cells.

Segregation of steroid receptor coactivator-1 from steroid receptors in mammary epithelium.

Steroid receptor coactivator-1 (SRC-1) family members interact with steroid receptors, including estrogen receptor alpha (ERalpha) and progesterone receptor (PR), to enhance ligand-dependent transcription. However, the expression of ERalpha and SRC-1 was found to be segregated in distinct subsets of cells within the epithelium of the estrogen-responsive rat mammary gland. This finding was in contrast to the finding for the stroma, where significant numbers of cells coexpressed ERalpha and SRC-1. Treatment of animals with estrogen induced PR expression in the ERalpha-expressing mammary epithelial cells in the absence of detectable SRC-1 and did not affect the segregated pattern of SRC-1 and ERalpha expression. PR was neither expressed nor induced by estrogen treatment in stroma, despite the coexpression of ERalpha and SRC-1. These results suggest that SRC-1 is not necessary for ERalpha-mediated induction of PR in mammary epithelial cells and is also not sufficient for PR induction in stromal cells expressing both ERalpha and SRC-1. Furthermore, the expression of SRC-1 in a subpopulation of mammary epithelial cells distinct from those expressing ERalpha or PR raises the possibility that SRC-1 has cell type-specific functions other than simply to act as coactivator for ERalpha or PR in the mammary epithelium.

COUP-TFI expression in human adrenocortical adenomas: possible role in steroidogenesis.

Chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) is an orphan nuclear receptor essential for neurogenesis, organogenesis, and cell fate determination. CYP17 gene transcription has recently been shown to be activated by SF-1 (steroidogenic factor-1) binding to a cyclic AMP-responsive sequence within the promoter region of the gene, and inhibited by COUP-TF binding to the sequence. Thus, COUP-TF and SF-1 act as a transcriptional repressor and activator, respectively, of CYP17 gene expression. Transcriptional repression by COUP-TFI is mediated by corepressors, N-CoR (nuclear receptor-corepressor) and SMRT (silencing mediator for retinoid and thyroid hormone-receptor), whereas transcriptional activation by SF-1 is mediated by coactivator SRC-1 (steroid receptor coactivator-1). We therefore examined the expression of COUP-TFI, SF-1, SRC-1, N-CoR, and SMRT in a variety of adrenocortical adenomas and compared the results with CYP17 mRNA levels. We found significantly high COUP-TFI mRNA expression in nonfunctional adenomas (n=8: 220+/-16%; normal 96+/-4%), a deoxycorticosterone-producing adenoma (n=1: 200%), and a pre-clinical Cushing's adenoma (n=1: 280%), intermediate COUP-TFI expression in cortisol-producing adenomas (n=8: 63+/-5%), and low COUP-TFI expression in aldosterone-producing adenomas (n=8: 49+/-4%). In contrast to COUP-TFI, SF-1 mRNA expression did not vary significantly among adrenals. We did not detect the expected negative correlation between COUP-TFI and CYP17 mRNA levels in adrenocortical adenomas. High COUP-TFI expression was associatedwith a nonfunctioning phenotype. Interestingly, the pattern of COUP-TFI expression was similar to the profile of N-CoR expression, but not of SMRT expression. These results indicate that COUP-TFI and N-CoR may play a role in steroidogenesis by human adrenocortical adenomas.

Differential Expression of the Peroxisome Proliferator-Activated Receptor γ (PPARγ) and Its Coactivators Steroid Receptor Coactivator-1 and PPAR-Binding Protein PBP in the Brown Fat, Urinary Bladder, Colon, and Breast of the Mouse

Peroxisome proliferator-activated receptors (PPARs) regulate genes involved in lipid metabolism and adipocyte differentiation. Steroid receptor coactivator-1 (SRC-1) and PPAR-binding protein (PBP) interact with PPARgamma and act as coactivators to enhance ligand-dependent transcription. We report here that PPARgamma, SRC-1, and PBP are differentially expressed in the brown fat, transitional epithelium of the urinary bladder, colonic mucosa, and mammary epithelium of the adult mouse. PPARgamma and PBP are expressed in the transitional epithelium of urinary bladder and in brown adipose tissue, but not SRC-1. In the colonic mucosa, PPARgamma expression occurs throughout the villi, whereas the expression of both SRC-1 and PBP is confined mostly to the crypts. The expression of both SRC-1 and PBP is prominent in the breast epithelium of nonpregnant, pregnant, and lactating mice, whereas PPARgamma expression appeared prominent during lactation. During early embryonic development, PPARgamma, SRC-1, and PBP are differentially expressed, with only limited cell types displaying overlapping expression. PPARgamma and PBP expression overlapped in the brown fat and urogenital sinus at stage E15.5 of embryogenesis, whereas SRC-1 expression occurred mostly in neuroepithelium and cartilage between stages E9.5 and E13.5 of embryogenesis.