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

Abstract

1Alpha,25(OH)2D3 is an important negative regulator of parathyroid hormone (PTH) gene transcription. In parathyroid cells, as in other target tissues, 1alpha,25(OH)2D3 is degraded by side chain oxidation by the inducible 24-hydroxylase. We have previously shown that one metabolite of this pathway, 1alpha,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 1alpha,25(OH)2D3 in bPTC has revealed another compound that is less polar than 1alpha,25(OH)2D3. By HPLC analysis and mass spectrometry, this metabolite was identified as 1alpha,25(OH)2-3-epi-D3. The activity of this metabol ite 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. 1Alpha,25(OH)2-3-epi-D3 was found to be only slightly, but not significantly, less active than the native 1alpha,25(OH)2D3 in suppressing PTH secretion despite having 30 times lower affinity for the bPTC VDR. Both 1alpha,25(OH)2D3 and 1alpha,25(OH)2-3-epi-D3 maximally suppressed PTH secretion by 50%. Along with 1alpha,25(OH)2-3-epi-D3, the activities of the other two A-ring diastereomers were assessed. 1beta,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, 1beta,25(OH)2-3-epi-D3 stimulated PTH secretion by 30-50% at concentrations from 10(-11) M to 10(-8)M and fell to control (untreated) rates at 10(-7) M. The mechanism for this increase in PTH secretion is under investigation. Metabolism studies performed in bPTC cells using high concentrations of 1alpha,25(OH)2D3 substrate showed that in some incubations, the concentration of 1alpha,25(OH)2-3-epi-D3 was even higher than that of the parent substrate 1alpha,25(OH)2D3. This finding indicates a slower rate of metabolism for this diastereomer. Thus, production and accumulation of 1alpha,25(OH)2-3-epi-D3, as a major stable metabolite of 1alpha,25(OH)2D3 in parathyroid glands, may contribute to the prolonged suppressive effect of 1alpha,25(OH)2D3 on PTH gene transcription.

Two Novel Metabolic Pathways of 22-Oxacalcitriol (OCT)

“…These findings imply that C-3 epimerization pathway is cell-selective and contributes to the metabolism of vitamin D in concert with the C-23/C-24 hydroxylation pathways. It is interesting to note that 3-epi-1␣,25(OH) 2 D 3 was almost equipotent to 1␣,25(OH) 2 D 3 in suppressing parathyroid hormone secretion in bovine parathyroid cells (36) and in inhibiting keratinocyte proliferation (24,37), and more potent than 1␣,25(OH) 2 D 3 in inducing HL-60 cell apoptosis (38). In addition, a high metabolic stability of 3-epi-1␣,25(OH) 2 D 3 in target cells has been proposed by Reddy et al (30).…”

DISPOSITION AND METABOLISM OF F6-1α,25(OH)2 VITAMIN D3 AND 1α,25(OH)2 VITAMIN D3 IN THE PARATHYROID GLANDS OF RATS DOSED WITH TRITIUM-LABELED COMPOUNDS

“…Parathyroid glands are reported to contain VDR (Naveh-Many et al, 1990;Brown et al, 1992bBrown et al, , 1995Demay et al, 1992;Hellman et al, 1999), and therefore the observed distribution of radioactivity may reflect binding of [1␤-3 H]F 6 -1␣,25(OH) 2 VD 3 and [1␤-3 H]1␣,25(OH) 2 VD 3 . Brown et al (1992aBrown et al ( , 1999 earlier reported that in parathyroid cells, as in other target tissues, 1␣,25(OH) 2 VD 3 was degraded by side chain oxidation, suggesting that the inducible 24-hydroxylase is responsible. Recently, reverse-transcription polymerase chain reaction and immunohistochemical analyses showed expression of 1␣-hydroxylase in both normal and pathological parathyroid tissue, and the findings imply that in addition to feedback control by circulating 1␣,25(OH) 2 VD 3 levels, parathyroid cells may also be influenced by local 1␣-hydroxylase activity with possible growthregulatory effects (Segersten et al, 2002).…”

C-3 Epimerization of Vitamin D3 Metabolites and Further Metabolism of C-3 Epimers

“…The metabolism of C-3 epimers has not been fully understood. Reddy et al (32) reported that 3-epi-1␣,25(OH) 2 D 3 was metabolized to three polar metabolites, 3-epi-1␣,24,25 (OH) It was reported that 3-epi-1␣,25(OH) 2 D 3 suppresses parathyroid hormone secretion in cultured bovine parathyroid cells (12) and induces surfactant phospholipid synthesis in pulmonary alveolar type II cells (34) irrespective of its low binding affinity to VDR. High metabolic stability of 3-epi-1␣,25(OH) 2 D 3 has been proposed as a possible explanation for the parathyroid hormone-suppressing and surfactant phospholipid-inducing effects of 3-epi-1␣,25(OH) 2 D 3 in vitro.…”

“…1␣,25(OH) 2 D 3 has potent anti-proliferative and cell differentiation-inducing activities in addition to its role in calcium homeostasis. After the expression of various biological activities, 1␣,25(OH) 2 D 3 is further metabolized through the C-24 (3-6)/C-23 (7-10) oxidation pathways and the C-3 epimerization pathway (11)(12)(13)(14). The C-24 oxidation pathway, initiated by C-24 hydroxylation, leads to the conversion of 1␣,25(OH) 2 D 3 into a side chain cleavage product, calcitroic acid (4,5).…”

“…The C-3 epimerization of 1␣,25(OH) 2 D 3 was observed in human colon carcinoma-derived Caco-2 cells (11), bovine parathyroid cells (12), rat osteoblastic UMR 106 and Ros17/2.8 cells (13), and various cultured cell lines (14). From these studies, the C-3 epimerization pathway is assumed to be cell-selective.…”