Repression of the Human Sex Hormone-binding Globulin Gene in Sertoli Cells by Upstream Stimulatory Transcription Factors

Abstract

Expression of the sex hormone-binding globulin gene (SHBG) in the liver produces SHBG, which transports sex steroids in the blood. In rodents, the SHBG gene is also expressed in Sertoli cells giving rise to the testicular androgen-binding protein, which is secreted into the seminiferous tubule where it presumably controls testosterone action. Evidence that the SHBG gene functions in this way in the human testis is lacking, and mice containing a human SHBG transgene (shbg4) under the control of its own promoter sequence are characterized by SHBG gene expression in the liver but not in the testis. A potential cis-element, defined as footprint 4 (FP4) within the human SHBG promoter, is absent in SHBG promoters of mammals that produce the testicular androgen-binding protein, and we have produced mice harboring a shbg4 transgene in which FP4 was deleted to evaluate its functional significance. Remarkably, these mice express the modified human SHBG transgene in the testis as well as the liver. Human SHBG transcripts were found within their Sertoli cells, primary cultures of which secrete human SHBG, and this was increased by treatment with follicle-stimulating hormone, retinoic acid, and estradiol but not testosterone. We have also found that the upstream stimulatory factors (USF-1 and USF-2) bind FP4 in vitro by electromobility shift assay of Sertoli cell nuclear extracts and in vivo by chromatin immunoprecipitation assay and conclude that USF transcription factors repress human SHBG transcription in Sertoli cells through an interaction with FP4 within its proximal promoter.

Intensive insulin therapy increases sex hormone-binding globulin in newly diagnosed type 2 diabetic patients

“…Some in vitro and in vivo studies have indicated that metabolic disorders reduce production of SHBG via downregulating the expression of HNF4a in HepG2 cells, and exogenous insulin stimulates the production of SHBG via repressing chicken ovalbumin upstream promotertranscription factor II (COUP-TFII) expression in the liver (16,30,31). Thus, we speculate that a collective effect of IT on systemic decrease in glucose and lipid levels, in addition to the improved liver function, may be responsible for the drastic and rapid (in 2 weeks) restoration of serum SHBG.…”

Sex hormone-binding globulin gene expression in the liver: Drugs and the metabolic syndrome

“…An Upstream Stimulatory Factor (USF-1/2) binding site present in the human SHBG promoter is lacking in the SHBG promoters of subprimate species and has recently been shown to account for phylogenetic differences in testicular SHBG expression between rodents and humans (Selva et al, 2005b). This site has little effect on hepatic SHBG expression, as transgenic mice expressing a human SHBG transgene lacking this USF binding site, maintain a high level of SHBG expression in the liver (Selva et al, 2005b).…”

“…This site has little effect on hepatic SHBG expression, as transgenic mice expressing a human SHBG transgene lacking this USF binding site, maintain a high level of SHBG expression in the liver (Selva et al, 2005b). Interestingly, whereas the wild type 4.3 kb SHBG transgene is not expressed in the testis, the deletion of this USF1/2 binding site permitted the expression of SHBG in the testes of transgenic mice harboring the modified SHBG transgene (Selva et al, 2005b). Furthermore, Sertoli cell expression of the modified human SHBG transgene is responsive to FSH and retinoic acid which stimulate its expression.…”

“…In this testis enriched alternatively spliced transcripts comprise an alternative exon 1 sequence that replaces the exon 1 sequence found in liver and kidney SHBG mRNA, and in humans this alternative transcript is expressed specifically in the germ cells of the testis (Selva et al, 2005a, Selva et al, 2005b. The alternative exon 1 sequence contains no apparent initiating methionine residue, and is generally believed to be non-coding.…”

Thyroid hormones act indirectly to increase sex hormone-binding globulin production by liver via hepatocyte nuclear factor-4α

“…The 299 bp human SHBG promoter sequence also contains a binding site for USF transcription factors ( Fig. 2A) that play a key role in controlling the expression of the SHBG gene in testicular cell types, but has no obvious role in hepatocytes (Selva et al 2005).…”