1,25-dihydroxyvitamin D3-independent stimulatory effect of estrogen on the expression of ECaC1 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-D-9k, 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 17beta-estradiol (17beta-E-2) is involved in Ca2+ reabsorption via regulation of the expression of ECaCl. The ovariectornized rat model was used to investigate the regulation of ECaCl, at the mRNA and protein levels, by 17beta-E-2 replacement therapy. Using real-time quantitative PCR and immunohistochemical analyses, this study demonstrated that 17beta-E-2 treatment at pharmacologic doses increased renal mRNA levels of ECaC1, calbindin-D-28K, NCX1, and PMCA1b and increased the protein abundance of ECaC I. Furthermore, the involvement of 1,25-dihydroxyvitamin D-3 in the effects of 17beta-E-2 was examined in 25-hydroxyvitamin D-3-1alpha-hydroxylase-knockout mice. Renal mRNA expression of calbindin-D-9K, calbindin-D-9K, NCX1, and PMCA1b was not significantly altered after 17beta-E-2 treatment. In contrast, ECaC1 mRNA and protein levels were both significantly upregulated. Moreover, 17beta-E-2 treatment partially restored serum Ca2+ levels, from 1.63 +/- 0.06 to 2.03 +/- 0.12 mM. In conclusion, this study suggests that 17beta-E-2 is positively involved in renal Ca2+ reabsorption via the upregulation of ECaCl, an effect independent of 1,25-dihydroxyvitamin D-3.