1α,25‐Dihydroxy‐3‐Epi‐vitamin D3, a natural metabolite of 1α,25‐dihydroxyvitamin D3, is a potent suppressor of parathyroid hormone secretion

Abstract

1α,25(OH)2D3 is an important negative regulator of parathyroid hormone (PTH) gene transcription. In parathyroid cells, as in other target tissues, 1α,25(OH)2D3 is degraded by side chain oxidation by the inducible 24-hydroxylase. We have previously shown that one metabolite of this pathway, 1α,23(S),25-(OH)3-24-oxo-D3, potently suppresses PTH synthesis and secretion in cultured bovine parathyroid cells (bPTC). Further examination of the metabolites of 1α,25(OH)2D3 in bPTC has revealed another compound that is less polar than 1α,25(OH)2D3. By HPLC analysis and mass spectrometry, this metabolite was identified as 1α,25(OH)2-3-epi-D3. The activity of this metabolite on PTH gene transcription was assessed by the steady-state PTH secretion by bPTC after 72-h treatment with concentrations from 10−11 M to 10−7 M. 1α,25(OH)2-3-epi-D3 was found to be only slightly, but not significantly, less active than the native 1α,25(OH)2D3 in suppressing PTH secretion despite having 30 times lower affinity for the bPTC VDR. Both 1α,25(OH)2D3 and 1α,25(OH)2-3-epi-D3 maximally suppressed PTH secretion by 50%. Along with 1α,25(OH)2-3-epi-D3, the activities of the other two A-ring diastereomers were assessed. 1β,25(OH)2D3 suppressed PTH only at 10−7 M with a decrease of only 30%, in good agreement with its low VDR affinity. Surprisingly, 1β,25(OH)2-3-epi-D3 stimulated PTH secretion by 30–50% at concentrations from 10−11 M to 10− 8M and fell to control (untreated) rates at 10−7M. The mechanism for this increase in PTH secretion is under investigation. Metabolism studies performed in bPTC cells using high concentrations of 1α,25(OH)2D3 substrate showed that in some incubations, the concentration of 1α,25(OH)2-3-epi-D3 was even higher than that of the parent substrate 1α,25(OH)2D3. This finding indicates a slower rate of metabolism for this diastereomer. Thus, production and accumulation of 1α,25(OH)2-3-epi-D3, as a major stable metabolite of 1α,25(OH)2D3 in parathyroid glands, may contribute to the prolonged suppressive effect of 1α,25(OH)2D3 on PTH gene transcription. J. Cell. Biochem. 73:106–113, 1999. © 1999 Wiley-Liss, Inc.