Regulation of calbindin-D28K gene expression by 1,25-dihydroxyvitamin D3 is correlated to receptor occupancy.

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) induces the synthesis of calcium binding protein (calbindin-D28K) and calbindin-D28K-mRNA in the chick intestine. We have examined the relationship between nuclear uptake of 1,25-(OH)2D3, 1,25-(OH)2D3 receptor occupancy levels, and transcription of the calbindin-D28K gene and found all three parameters to be highly correlated. All three events occur rapidly (within 15-30 min) following a single dose of 6.5 nmol of 1,25-(OH)2D3 to vitamin D-deficient chicks, reaching peak values by 1-2 h; by 4 h, values of all three parameters start to decline. Calbindin-D28K-mRNA begins to accumulate by 3-5 h but does not peak until 12 h following hormone administration. The levels of calbindin-D28K start to increase by 5-8 h and do not peak until 48 h after the 1,25-(OH)2D3 dose. These observations suggest that post-transcriptional regulatory mechanisms may be involved. Measurements of basal levels of calbindin-D28K gene transcription show that there is virtually no transcription in the vitamin D-deficient chick intestine, and a 12-fold induction in the intestine of vitamin D-replete chicks. This basal level of transcription in vitamin D-replete chick intestine is not inhibited by cycloheximide pretreatment. These results confirm the thesis that a major component of the mechanism of action of 1,25-(OH)2D3 is functioning as a steroid hormone, effected through the direct action of the seco-steroid-receptor complex on the initiation of transcription of specific genes.