Regulation of calbindin D9k mRNA levels and promoter activity by differentiation. A: regulation of calbindin D9k promoter activity during TC7 cell differentiation using a full-length calbindin D9k promoter-reporter gene construct (CaBP4600). TC7 cells were transfected with a CaBP4600 at 1 day in culture, and reporter activity was examined at 2, 5, 9, and 13 days in culture by measuring luciferase activity. B: vitamin D (VD) regulated promoter activity in 15-day cultures of TC7 cells. Cells were transfected with either CaBP4600 or a 24-hydroxylase promoter construct (CYP24), treated for 24 h with 100 nM 1,25(OH)2 vitamin D, and promoter activity was assessed by measuring luciferase activity. Bars represent the means ± SE. Bars with different letters (in A) are significantly different (P < 0.05). In B, the bar with an asterisk (*) indicates significant difference from vehicle treatment (P < 0.05).