Extrahepatic 25-Hydroxylation of Vitamin D(3) in an Engineered Osteoblast Precursor Cell Line Exploring the Influence on Cellular Proliferation and Matrix Maturation during Bone Development

Osteoblastic precursors experience distinct stages during differentiation and bone development, which include proliferation, extracellular matrix (ECM) maturation, and ECM mineralization. It is well known that vitamin D plays a large role in the regulation of bone mineralization and homeostasis via the endocrine system. The activation of vitamin D requires two sequential hydroxylation steps, first in the kidney and then in the liver, in order to carry out its role in calcium homeostasis. Recent research has demonstrated that human-derived mesenchymal stem cells (MSCs) and osteoblasts can metabolize the immediate vitamin D precursor 25-dihydroxyvitamin D(3) (25OHD(3)) to the active steroid lα,25-dihydroxyvitamin D(3) (1,25OH(2)D(3)) and elicit an osteogenic response. However, reports of extrahepatic metabolism of vitamin D(3), the parental vitamin D precursor, have been limited. In this study, we investigated whether osteoblast precursors have the capacity to convert vitamin D(3) to 1,25OH(2)D(3) and examined the potential of vitamin D(3) to induce 1,25OH(2)D(3) associated biological activities in osteoblast precursors. It was demonstrated that the engineered osteoblast precursor derived from human marrow (OPC1) is capable of metabolizing vitamin D(3) to 1,25OH(2)D(3) in a dose-dependent manner. It was also demonstrated that administration of vitamin D(3) leads to the increase in alkaline phosphatase (ALP) activity associated with osteoblast ECM maturation and calcium deposits and a decrease in cellular proliferation in both osteoblast precursor cell lines 0PC1 andOMC3T3-E1. These findings provide a two-dimensional culture foundation for future three-dimensional engineered tissue studies using the OPC1 cell line.