Vitamin D₃ upregulates plasma membrane Ca²⁺-ATPase expression and potentiates apico-basal Ca²⁺ flux in MDCK cells

Plasma membrane Ca²⁺-ATPases (PMCAs) are a ubiquitous system for the expulsion of Ca²⁺ from eukaryotic cells. In tight monolayers of polarized Madin-Darby canine kidney (MDCK) cells representing a distal kidney tubule model, PMCAs are responsible for about one-third of the vectorial Ca²⁺ transport under resting conditions, with the remainder being provided by the Na⁺/ Ca²⁺ exchanger. Vitamin D ₃ (VitD) is known to increase PMCA expression and activity in Ca ²⁺-transporting tissues such as the intestine, as well as in osteoblasts and Madin-Darby bovine kidney epithelial cells. We found that VitD upregulated the expression of the PMCAs (mainly PMCA4b) in MDCK cell lysates at the RNA and protein level in a time- and dose-dependent manner. Interestingly, VitD caused a decrease of the PMCAs in the apical plasma membrane fraction and a concomitant increase of the pumps in the basolateral membrane. Functional studies demonstrated that transcellular ⁴⁵Ca²⁺ flux from the apical-to-basolateral compartment was significantly enhanced by VitD. These findings demonstrate that VitD is a positive regulator of the PMCAs in MDCK epithelial cells. The correlation of decreased apical/increased basolateral expression of the PMCAs with an increase in transcellular Ca²⁺ flux from the apical (urine) toward the basolateral (blood) compartment indicates the physiological relevance of VitD function in kidney tubular Ca²⁺ reabsorption.