1,25-Dihydroxyvitamin D3 induces the synthesis of vitamin D-dependent calcium-binding protein in cultured chick kidney cells.

A 28,000 mol wt vitamin D-dependent calcium-binding protein (CaBP), first isolated from avian intestine, has recently been shown to exist in kidney and other tissues. To study the mechanism regulating the production of renal CaBP, we used primary cultures of chick kidney cells as an in vitro model. Renal cortical tubules isolated from vitamin D-deficient chicks were grown in serum-free, hormone-supplemented medium. Confluent cells were epithelioid and expressed CaBP, as demonstrated by immunocytochemistry and a specific RIA. 1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] increased cellular CaBP content in a dose-dependent manner (10(-10)-10(-7) M) from basal concentrations of 50-240 ng/mg protein to maximal concentrations of 600-1200 ng/mg protein 48 h after dosing. Cycloheximide (2 microM) inhibited 1,25-(OH)2D3 induction of CaBP, indicating that the mechanism requires new protein synthesis. Western blotting of cell extracts confirmed the identity of the inducible protein as the 28,000 mol wt CaBP. 25-Hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 were effective but somewhat less potent inducers of CaBP, whereas vitamin D3 was without significant effect. The activities of 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 are attributed to their rapid conversion by kidney cells to 1 alpha-hydroxylated metabolites. The dose-response profiles for two additional bioresponses in these cells, namely induction of 24-hydroxylase and inhibition of 1 alpha-hydroxylase activity, were comparable to those for CaBP induction. To our knowledge, this is the first description of a cell culture system that exhibits 1,25-(OH)2D3-inducible CaBP in vitro. This model should permit the study of certain aspects of the physiology of 1,25-(OH)2D3 and CaBP in kidney that are not possible in vivo.