The vitamin D prodrugs 1alpha(OH)D2, 1alpha(OH)D3 and BCI-210 suppress PTH secretion by bovine parathyroid cells.
BACKGROUND
Active vitamin D compounds are widely used in the treatment of secondary hyperparathyroidism associated with renal failure. These compounds reduce PTH secretion through vitamin D receptor (VDR)-dependent repression of PTH gene transcription. In previous studies, 1alpha(OH)D3, a vitamin D prodrug, inhibited PTH secretion in cultured bovine parathyroid cells, but it was unclear whether 1alpha(OH)D3 itself or an active metabolite produced this inhibition.


METHODS
We determined the effectiveness of the vitamin D prodrugs 1alpha(OH)D3, 1alpha(OH)D2 and 1alpha(OH)-24(R)-methyl-25-ene-D2 (BCI-210) at inhibiting PTH secretion in bovine parathyroid cell cultures, and examined the metabolism of [3H]1alpha(OH)D2 in these cells.


RESULTS
All three prodrugs suppressed PTH secretion with approximately 10% of the activity of 1,25(OH)2D3; much higher activity than expected based on the VDR affinities of these prodrugs (0.25% of 1,25(OH)2D3). Parathyroid cells activated [3H]1alpha(OH)D2 to both 1,25(OH)2D2 and 1,24(OH)2D2. 1,24(OH)2D2 was detectable at 4 h, increased to a maximum at 8 h, and then decreased. In contrast, 1,25(OH)2D2 levels increased linearly with time, suggesting the presence of constitutively active vitamin D-25-hydroxylase not previously reported in parathyroid cells. The cytochrome P-450 inhibitor ketoconazole (50 microM) reduced 1alpha(OH)D2 metabolism to below detectable levels, but did not significantly affect suppression of PTH by 1alpha(OH)D2.


CONCLUSIONS
The vitamin D prodrugs 1alpha(OH)D3, 1alpha(OH)D2 and BCI-210 suppressed PTH production by cultured parathyroid cells. The ability of 1alpha(OH)D2 to reduce PTH despite inhibition of its metabolism suggests a direct action of this 'prodrug' on the parathyroid gland, but the mechanism underlying this activity is not yet known.
Brown
Alex J
AJ
Renal Division, Washington University School of Medicine, St. Louis, MO 63110, USA. abrown@imgate.wustl.edu
Ritter
Cynthia S
CS
Knutson
Joyce C
JC
Strugnell
Stephen A
SA
D000818:Animals; D050071:Bone Density Conservation Agents / Q000494:pharmacology; D002417:Cattle; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004797:Enzyme-Linked Immunosorbent Assay; D004872:Ergocalciferols / Q000494:pharmacology*; D015870:Gene Expression / Q000235:genetics; D006887:Hydroxycholecalciferols / Q000494:pharmacology*; D066298:In Vitro Techniques; D007654:Ketoconazole / Q000494:pharmacology; D010280:Parathyroid Glands / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000494:pharmacology
C506649:BCI-210; D050071:Bone Density Conservation Agents; D065607:Cytochrome P-450 Enzyme Inhibitors; D004872:Ergocalciferols; D006887:Hydroxycholecalciferols; D010281:Parathyroid Hormone; D012333:RNA, Messenger; D014807:Vitamin D; C042533:1 alpha-hydroxyergocalciferol; D003577:Cytochrome P-450 Enzyme System; D007654:Ketoconazole; C008088:alfacalcidol