Tissue-specific regulation of avian vitamin D-dependent calcium-binding protein 28-kDa mRNA by 1,25-dihydroxyvitamin D3.

We have studied the regulation, by 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), of vitamin D-dependent calcium-binding protein (28-kDa CaBP) mRNA in chick tissues in vivo. Northern analysis of poly(A)+ RNA was carried out using, as hybridization probes, synthetic oligonucleotides complementary to chick 28-kDa CaBP mRNA. In vitamin D-deficient chicks, 28-kDa CaBP mRNA was virtually undetectable in intestine, was clearly detectable in kidney, and present at the highest levels in cerebellum. After a single intravenous dose of 500 ng of 1,25-(OH)2D3, intestinal 28-kDa CaBP mRNA levels were increased 50-fold, kidney levels were increased 4-fold, and cerebellum levels were unchanged. Increased levels of 28-kDa CaBP mRNA were appreciated 2 h after induction and were maximal at 12 h. Pretreatment of vitamin D-deficient chicks with actinomycin D had little effect on the acute phase of the 1,25-(OH)2D3 induction of 28-kDa CaBP mRNA in intestine but blunted the induction in kidney. Pretreatment with cycloheximide caused a delayed response to 1,25-(OH)2D3 in the intestine, although control (noninhibition) levels of 28-kDa CaBP mRNA were present 12 h after hormone administration. By contrast, in the kidney, cycloheximide pretreatment resulted in an increased steady-state (vitamin D-deficient) level of 28-kDa CaBP mRNA, but completely abolished the induction of 1,25-(OH)2D3. Our studies indicate that, whereas 1,25-(OH)2D3 does not regulate 28-kDa CaBP mRNA levels in the brain, the hormone modulates 28-kDa CaBP gene expression in intestine and kidney in a tissue-specific manner, by acting through both transcriptional and post-transcriptional mechanisms.