1,25-Dihydroxyvitamin D3-Independent Stimulatory Effect of Estrogen on the Expression of ECaC1 in the Kidney

Abstract

ABSTRACT. Estrogen deficiency results in a negative Ca2+balance and bone loss in postmenopausal women. In addition to bone, the intestine and kidney are potential sites for estrogen action and are involved in Ca2+handling and regulation. The epithelial Ca2+channel ECaC1 (or TRPV5) is the entry channel involved in active Ca2+transport. Ca2+entry is followed by cytosolic diffusion, facilitated by calbindin-D28Kand/or calbindin-D9k, and active extrusion across the basolateral membrane by the Na+/Ca2+-exchanger (NCX1) and plasma membrane Ca2+-ATPase (PMCA1b). In this transcellular Ca2+transport, ECaC1 probably represents the final regulatory target for hormonal control. The aim of this study was to determine whether 17β-estradiol (17β-E2) is involved in Ca2+reabsorption via regulation of the expression of ECaC1. The ovariectomized rat model was used to investigate the regulation of ECaC1, at the mRNA and protein levels, by 17β-E2replacement therapy. Using real-time quantitative PCR and immunohistochemical analyses, this study demonstrated that 17β-E2treatment at pharmacologic doses increased renal mRNA levels of ECaC1, calbindin-D28K, NCX1, and PMCA1b and increased the protein abundance of ECaC1. Furthermore, the involvement of 1,25-dihydroxyvitamin D3in the effects of 17β-E2was examined in 25-hydroxyvitamin D3-1α-hydroxylase-knockout mice. Renal mRNA expression of calbindin-D9K, calbindin-D28K, NCX1, and PMCA1b was not significantly altered after 17β-E2treatment. In contrast, ECaC1 mRNA and protein levels were both significantly upregulated. Moreover, 17β-E2treatment partially restored serum Ca2+levels, from 1.63 ± 0.06 to 2.03 ± 0.12 mM. In conclusion, this study suggests that 17β-E2is positively involved in renal Ca2+reabsorption via the upregulation of ECaC1, an effect independent of 1,25-dihydroxyvitamin D3.