Single nucleotide polymorphisms in the human corticosteroid-binding globulin promoter alter transcriptional activity

Abstract

Corticosteroid-binding globulin (CBG) is a high-affinity plasma protein that transports glucocorticoids and progesterone. Others and we have reported non-synonymous single nucleotide polymorphisms (SNPs) that influence CBG production or steroid-binding activity. However, no promoter polymorphisms affecting the transcription of human CBG gene (Cbg) have been reported. In the present study we investigated function implications of six promoter SNPs, including 26 C/G, 54 C/T, 144 G/C, 161 A/G, 205 C/A, and 443/444 AG/, five of which are located within the first 205 base pairs of 5′-flanking region and close to the highly conserved footprinted elements, TATA-box, or CCAAT-box. Luciferase reporter assays demonstrated that basal activity of the promoter carrying 54 T or 161 G was significantly enhanced. The first three polymorphisms, 26 C/G, 54 C/T, and 144 G/C located close to the putative hepatic nuclear factor (HNF) 1 binding elements, altered the transactivation effect of HNF1. We also found a negative promoter response to dexamethasone-activated glucocorticoid receptor (GR) , although none of the SNPs affected its transrepression function. Our results suggest that human Cbg 26 C/G, 54 C/T, 144 G/C, and 161 A/G promoter polymorphisms alter transcriptional activity, and further studies are awaited to explore their association with physiological and pathological conditions. , both of which are well-known hormones for pregnancy maintenance or labor onset. While CBG mainly binds >80% of cortisol in human peripheral blood [3], at the maternal-fetal interface CBG will be occupied by the very high concentrations of progesterone, which is several fold higher than those of cortisol [6]. In the circulation of women during mid-late pregnancy, approximately 10% of CBG steroid-binding sites are occupied by progesterone, versus <1% in non-pregnant women [5]. Plasma CBG levels rise progressively through gestation until term pregnancy [7], and so do blood cortisol and progesterone levels. Maternal plasma CBG, total and free cortisol concentrations are reduced in pre-eclampsia and gestational hypertension patients, and this may be a consequence of decreased Cbg synthesis driven by cytokines or increased degradation driven by inflammation [7]. SPECIAL TOPIC

Inhibition of Corticosteroid-Binding Globulin Gene Expression by Glucocorticoids Involves C/EBPβ

“…However, DNase I foot-printing of the rat Cbg proximal promoter, has identified five protein-binding sites (P1–P5) within −236 bp from the transcription start site in rat liver nuclear extract [25] . These five protein-binding sites are also highly conserved in the human and mouse Cbg gene ( Figure 1 ) [28] . They resemble recognition sequences for hepatocyte nuclear factor-1 beta (HNF1β; P1), CCAAT-binding protein-2 (CP-2; P2), D-site-binding protein (DBP; P3), hepatocyte nuclear factor-3 alpha (HNF3α; P4) and CAAT/enhancer binding protein beta (C/EBPβ or NF/IL6; P5), respectively ( Figure 1 ) [25] .…”

“…Although the molecular mechanism by which GCs influence CBG levels is unclear, the Cbg promoter has been shown to be transcriptionally regulated via the GR [30] – [32] . However, while the Cbg promoter is modulated by GCs [30] , [32] , [33] , no glucocorticoid response elements (GREs) seem to be present in the mouse, rat or human CBG gene proximal promoters [25] , [28] , suggesting tethering of the GR to other transcription factors rather than binding directly to DNA. Footprints P3–P5 in the Cbg promoter resemble recognition sequences for DBP, HNF3α and C/EBPβ or NF-IL6 [25] , and, although binding of these transcription factors has not been confirmed, it is interesting to note that HNF3α and C/EBPβ have been reported to be important in GR-mediated signaling [34] , [35] .…”