1α,25-Dihydroxy-3-epi-vitamin D 3, a natural metabolite of 1α,25-dihydroxy vitamin D 3: production and biological activity studies in pulmonary alveolar type II cells

Abstract

Pulmonary alveolar type II cells have been shown to be a possible target for the secosteroid hormone, 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2D 3], during perinatal transition. At present, there is great interest to isolate and identify the metabolites of 1α,25(OH) 2D 3 produced in its target tissues and to determine the contribution of each individual metabolite of 1α,25(OH) 2D 3 to the final expression of the pleiotropic actions attributed to 1α,25(OH) 2D 3. Of all the known metabolites of 1α,25(OH) 2D 3, 1α,25(OH) 2-3-epi-D 3 has gained most attention as it is produced only in specific tissues and possesses significant activity in tissues in which it is produced. Furthermore, in vivo studies indicate that this metabolite when compared to 1α,25(OH) 2D 3 is less calcemic. Therefore, we performed the present study to identify production of 1α,25(OH) 2-3-epi-D 3 in alveolar type II cells, and to evaluate its effect on surfactant synthesis. We incubated NCI-H441 cells, an alveolar type II cell line, with 1α,25(OH) 2D 3 and demonstrated that these cells metabolize 1α,25(OH) 2D 3 to various previously well-characterized polar metabolites, and to a less polar metabolite which was unequivocally identified as 1α,25(OH) 2-3-epi-D 3 by GC/MS and HPLC analysis. Further, biological activity studies in H441 cells indicated that 1α,25(OH) 2-3-epi-D3 possesses significant activity in terms of its ability: (i) to increase surfactant phospholipid synthesis, (ii) to induce surfactant SP-B mRNA gene expression, and (iii) to increase surfactant SP-B protein synthesis. However, the activity of 1α,25(OH) 2-3-epi-D 3 when compared to 1α,25(OH) 2D 3 in generating VDR-mediated transcriptional activity in ROS 17/2.8 cells transfected with human osteocalcin VDRE/growth hormone gene construct, was significantly reduced. The high metabolic stability of 1α,25(OH) 2-3-epi-D 3, as previously proposed by us, may be a possible explanation for the high in vitro activity in spite of the reduced VDR-mediated transcriptional activity. In summary, we report for the first time the pathways of 1α,25(OH) 2D 3 metabolism in pulmonary alveolar type II cells and indicate that 1α,25(OH) 2-3-epi-D 3, a natural intermediary metabolite of 1α,25(OH) 2D 3 possesses significant activity in stimulating surfactant synthesis in alveolar type II cells.