VARIABLE IN-VIVO REGULATION OF RAT VITAMIN-D-DEPENDENT GENES (OSTEOPONTIN, CA,MG-ADENOSINE TRIPHOSPHATASE, AND 25-HYDROXYVITAMIN-D-3 24-HYDROXYLASE) - IMPLICATIONS FOR DIFFERING MECHANISMS OF REGULATION AND INVOLVEMENT OF MULTIPLE FACTORS

In these studies, the regulation of renal osteopontin (OPN), intestinal calcium pump, and renal and intestinal 25-hydroxyvitamin D-3 24-hydroxylase (24OHase) was compared to the regulation of calbindin by Northern blot analysis. The time course of induction of the calbindins and calcium pump messenger RNAs (mRNAs) in vitamin D-deficient rats in response to 1,25-hydroxyvitamin D-3 [1,25-(OH)(2)D-3] was similar [first induction 3 h after 1,25-(OH)(2)D-3 (200 ng/100 g BW), reaching a maximum at 12-24 h]. However, the maximal induction of calbindin mRNA was greater (8.2 +/- 1.2-fold) than that of intestinal calcium pump mRNA (2.3 +/- 0.5-fold). 24-Hydroxylase mRNA showed a different time course of induction by 1,25-(OH)(2)D-3. 24-Hydroxylase mRNA was undetectable in vitamin D-deficient rat intestine and kidney. In intestine, 24OHase mRNA was maximally induced early [3-6 h after 1,25-(OH)(2)D-3] and did not accumulate long after 1,25-(OH)(2)D-3 administration. Unlike the other vitamin D-regulated genes, OPN was uninduced by 1,25-(OH)(2)D-3 in the kidney of D-deficient rats. However, in vitamin D-replete rats given 1,25-(OH)(2)D-3 for 5 days, a significant induction in renal OPN was observed (6.0-fold; P < 0.01). Developmental studies indicated that renal 24OHase and intestinal calcium pump mRNAs, similar to calbindin-D-9k mRNA, were induced at 3 weeks of age, the time of weaning when active intestinal calcium absorption is induced. A comparison of the effect of calcium status showed that low dietary calcium resulted in an induction of intestinal calcium pump as well as calbindin-D-9k mRNA, consistent with the requirement for an increase in intestinal calcium absorption during dietary calcium restriction. We next examined gender- and age-related changes in the expression of these genes. Our results indicate that renal OPN mRNA increases markedly in males with aging (10-fold between 3-20 months; P < 0.01), whereas OPN mRNA levels are unchanged in females. In both males and females, significant increases (2.7- to 4.5-fold) in renal 24OHase mRNA were observed at 20 months compared to 3 months of age. In summary, 1) the most pronounced effect of 1,25-(OH)(2)D-3 in vitamin D-deficient rats was increased synthesis of calbindin and 24OHase; 2) the lack of an effect of 1,25-(OH)(2)D-3 on renal OPN mRNA in vitamin D-deficient rats and the induction in vitamin D-replete rats suggest that multiple factors may be involved in vitamin D-mediated OPN regulation; and 3) the increase in 24OHase mRNA with age suggests that increased renal metabolism of 1,25-(OH)(2)D-3 may contribute to the decrease in serum 1,25-(OH)(2)D-3 levels observed in both aging and postmenopausal osteoporosis.