COMPARATIVE EFFECTS OF 1,25-DIHYDROXYVITAMIN D-3 AND EB-1089 ON MOUSERENAL AND INTESTINAL 25-HYDROXYVITAMIN D-3-24-HYDROXYLASE

EB 1089 is a vitamin D analog that is less potent than 1,25-dihydroxyv itamin D-3 (1,25(OH)(2)D-3) in its calcemic action but more potent in its antiproliferative action. We characterized the interaction of 1,25 (OH)(2)D-3 and EB 1089 with renal 25-hydroxyvitamin D-3-24-hydroxylase (24-hydroxylase), the first enzyme in the C-24 oxidation pathway, and compared the effects of 1,25(OH)(2)D-3 and EB 1089 on induction of 24 -hydroxylase mRNA in mouse kidney and intestine. 1,25(OH)(2)D-3 and EB 1089 were competitive inhibitors of 24-hydroxylase activity. However, the K-i for 1,25(OH)(2)D-3 (5.2 +/- 2.5 nM) was significantly lower t han that for EB 1089 (286 +/- 59 nM). In the kidney, the time course a nd extent of 24-hydroxylase mRNA induction, relative to 18S rRNA, was similar for 1,25(OH)(2)D-3 and EB 1089 with a peak response at approxi mate to 6 h that was sustained for at least 16 h. In the intestine, ho wever, induction of 24-hydroxylase mRNA, relative to 18S rRNA, was sim ilar to 50% lower for EB 1089 than for 1,25(OH)(2)D-3 at 3 h (p < 0.05 ) and 6 h (p < 0.05) while at 16 h 24-hydroxylase mRNA was no longer d etectable. Moreover, while both 1,25(OH)(2)D-3 and EB 1089 elicited a similar dose-dependent induction of 24-hydroxylase mRNA in the kidney (EC(50) = 0.4 +/- 0.13 and 0.3 +/- 0.08 ng/g for EB 1089 and 1,25(OH)( 2)D-3, respectively), the EC(50) for EB 1089 (6.6 +/- 1.7 ng/g) was si gnificantly higher than that for 1,25(OH)(2)D-3 (0.9 +/- 0.32 ng/g) in the intestine (p < 0.01). EB 1089 was also less effective than 1,25(O H)(2)D-3 in the induction of intestinal but not renal calbindin-D-9k m RNA. To determine the mechanism for tissue-specific differences in pot ency, we determined the binding affinity of 1,25(OH)(2)D-3 and EB 1089 for the vitamin D receptor. In the kidney, K-d values for 1,25(OH)(2) D-3 (0.40 +/- 0.05 nM) and EB 1089 (0.48 +/- 0.04 nM) were not differe nt. However, in the intestine, the K-d for EB 1089 (1.43 +/- 0.19 nM) was significantly higher than that for 1,25(OH)(2)D-3 (0.85 +/- 0.06 n M; p < 0.05). Our results demonstrate that: (i) EB 1089 has a 50-fold lower affinity than 1,25(OH)(2)D-3 for renal 24-hydroxylase, suggestin g that it is more resistant to catabolism by the C-24 oxidation pathwa y; and (ii) EB 1089 and 1,25(OH)(2)D-3 exhibit tissue-specific differe nces in vitamin D receptor-mediated responses in vivo that may be ascr ibed, at least in part, to differences in binding affinities for the v itamin D receptor.