Studies on the mode of action of vitamin D XXXVII 1 alpha-hydroxyvitamin D3: a long-acting 1,25-dihydroxyvitamin D3 analog

1alpha-hydroxyvitamin D2 is less toxic but not bone selective relative to 1alpha-hydroxyvitamin D3 in ovariectomized rats

Identification of bone selective vitamin D analogues would provide an interesting substance class for the treatment of osteoporosis. The synthetic prodrug 1alpha-hydroxyvitamin D2 [1alpha(OH)D2] has been shown to combine equal bone-preserving activity with distinctly reduced calcemic effects relative to 1alpha-hydroxyvitamin D3 [1alpha(OH)D3] in 3-month-old ovariectomized (OVX) rats. Therefore, 1alpha(OH)D2 may be a bone-selective compound. The aim of this study was to compare the bone protective and the calcemic activities of chronically administered 1alpha(OH)D2 and 1alpha(OH)D3 in 6-month-old OVX rats over a broad dose range from ineffective to toxic doses. Ninety-six female 6-month-old Fischer-344 rats were used for this experiment. Eighty rats were bilaterally OVX, 8 rats were sham-operated (SHAM), and 8 rats were killed at the time of surgery as a baseline control. Groups of OVX rats received vehicle alone (n = 16) or daily doses in the diet of 0.025, 0.05, 0.1, and 0.2 microg of 1alpha(OH)D2 or 1alpha(OH)D3 per kg body weight (BW) per day (n = 8 each). After calcein double-labeling, all animals were killed 3 months post-OVX. Orally administered 1alpha(OH)D2 was significantly less toxic compared with 1alpha(OH)D3 in terms of BW gain and kidney calcium content. The effects of 1alpha(OH)D2 and 1alpha(OH)D3 on serum calcium and urinary calcium excretion were generally similar at all doses in this study. Both 1alpha(OH)D2 and 1alpha(OH)D3 prevented the estrogen deficiency-induced bone loss in OVX rats, and induced profound bone anabolic effects at high dosages. 1alpha(OH)D3 and 1alpha(OH)D2 also dose-dependently increased total bone mineral density (BMD), cortical area, and cortical thickness in the tibial diaphysis of OVX rats. Bone resorption as assessed by osteoclast numbers (Oc.Ns) in vertebral cancellous bone and urinary excretion of deoxypyridinoline (DPD) was dose-dependently suppressed by 1alpha(OH)D2 and 1alpha(OH)D3. These data show that although 1alpha(OH)D2 was slightly but significantly less toxic compared with 1alpha(OH)D3, it did not have increased skeletal effects at any dose. Taken together, our findings argue against selective metabolic activation of 1alpha(OH)D2 in bone.

Regulation of metallothionein gene expression by 1 alpha,25-dihydroxyvitamin D3 in cultured cells and in mice

1 alpha,25-Dihydroxyvitamin D3 [1 alpha,25(OH)2D3], a hormonally active form of vitamin D3, has been shown to modulate cell differentiation and tumor promotion. This report demonstrates that mRNA of the metallothionein (MT) gene was induced by 1 alpha,25(OH)2D3 in cultured epidermal keratinocytes and also in liver, kidney, and skin tissues when 1 alpha-hydroxyvitamin D3, a synthetic precursor of 1 alpha,25(OH)2D3, was applied in vivo. Exposure of FRSK cells, a cell line derived from fetal rat skin keratinocytes, to 1 alpha,25(OH)2D3 at 5 ng/ml (12 nM) increased MT mRNA to almost the same extent as that induced by 10 microM dexamethasone or 1 microM CdCl2. This increase in the level of MT mRNA was evident within 2 hr and was maximal 12-24 hr after the addition of 1 alpha,25(OH)2D3. The induction was dose-dependent with concentrations of 1 alpha,25(OH)2D3 from 0.05 to 5.0 ng/ml. Amounts of MT increased with the increase of MT mRNA induced by 1 alpha,25(OH)2D3. Of the derivatives of vitamin D3 tested, only 1 alpha,25(OH)2D3 caused marked induction. Treatment with cycloheximide did not inhibit MT mRNA induction by 1 alpha,25(OH)2D3. 1 alpha,25(OH)2D3 induced MT mRNA in primary cultures of mouse epidermal keratinocytes but not in IAR-20, a liver cell line. 1 alpha,25(OH)2D3 had a similar effect in vivo: oral administration of 1 alpha-hydroxyvitamin D3 to mice resulted in increased levels of MT mRNA in the liver, kidney, and skin 24 hr later. Increase in the level of MT mRNA may be relevant to some biological actions of 1 alpha,25(OH)2D3.

The effects of betamethasone on plasma and intestinal calcium binding protein and on 25-hydroxyvitamin D3 metabolism in the pig

Betamethasone (50 micrograms/kg body weight/day) given to young pigs reduced calcium absorption, growth and plasma vitamin D dependent calcium binding protein (CaBP) concentration. No changes occurred in plasma 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and intestinal CaBP concentrations. 1,25(OH)2D3 (0.1 microgram/kg body weight/day) given with betamethasone increased calcium absorption although growth and plasma CaBP concentrations remained low. Intestinal CaBP levels remained unchanged. Plasma CaBP concentrations were not consistently related to intestinal CaBP or calcium absorption in the presence of betamethasone. We conclude that betamethasone-induced depression of calcium absorption was not mediated by alterations in intestinal CaBP, but the mechanism remains obscure.