1,25-Dihydroxyvitamin D₃-independent stimulatory effect of estrogen on the expression of ECaCl in the kidney

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 ECaCl (or TRPV5) is the entry channel involved in active Ca²⁺ transport. Ca²⁺ entry is followed by cytosolic diffusion, facilitated by calbindin-D_28K and/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, ECaCl 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 ECaCl. The ovariectomized rat model was used to investigate the regulation of ECaCl, at the mRNA and protein levels, by 17β-E₂ replacement therapy. Using real-time quantitative PCR and immunohistochemical analyses, this study demonstrated that 17β-E₂ treatment at pharmacologic doses increased renal mRNA levels of ECaCl, calbindin-D_28K, NCX1, and PMCA1b and increased the protein abundance of ECaCl. Furthermore, the involvement of 1,25-dihydroxyvitamin D₃ in the effects of 17β-E₂ was examined in 25-hydroxyvitamin D₃-1α-hydroxylase-knockout mice. Renal mRNA expression of calbindin-D_9K, calbindin-D_28K, NCX1, and PMCA1b was not significantly altered after 17β-E₂ treatment. In contrast, ECaCl mRNA and protein levels were both significantly upregulated. Moreover, 17β-E₂ treatment partially restored serum Ca²⁺ levels, from 1.63 ± 0.06 to 2.03 ± 0.12 mM. In conclusion, this study suggests that 17β-E₂ is positively involved in renal Ca²⁺ reabsorption via the upregulation of ECaCl, an effect independent of 1,25-dihydroxyvitamin D₃.