Comparative effects of 1,25‐dihydroxyvitamin D 3 and EB 1089 on mouse renal and intestinal 25‐hydroxyvitamin D 3 ‐24‐hydroxylase

EB 1089 is a vitamin D analog that is less potent than 1,25‐dihydroxyvitamin 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 1 for 1,25(OH) 2 D 3 (5.2 ± 2.5 nM) was significantly lower than that for EB 1089 (286 ± 59 nM). In the kidney, the time course and 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 ≈ 6 h that was sustained for at least 16 h. In the intestine, however, induction of 24‐hydroxylase mRNA, relative to 18S rRNA, was ∼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 detectable. 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 significantly 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(OH) 2 D 3 in the induction of intestinal but not renal calbindin‐D 9k mRNA. To determine the mechanism for tissue‐specific differences in potency, 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 different. 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 nM; 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, suggesting that it is more resistant to catabolism by the C‐24 oxidation pathway; and (ii) EB 1089 and 1,25(OH) 2 D 3 exhibit tissue‐specific differences in vitamin D receptor‐mediated responses in vivo that may be ascribed, at least in part, to differences in binding affinities for the vitamin D receptor.