24R,25-Dihydroxyvitamin D3 [24R,25(OH)2D3] controls growth plate development by inhibiting apoptosis in the reserve zone and stimulating response to 1α,25(OH)2D3 in hypertrophic cells

Previously we showed that costochondral growth plate resting zone (RC) chondrocytes response primarily to 24R,25(OH)(2)D-3 whereas prehypertrophic and hypertrophic (GC) cells respond to 1 alpha,25(OH)(2)D-3. 24R,25(OH)(2)D-3 increases RC cell proliferation and inhibits activity of matrix processing enzymes, suggesting it stabilizes cells in the reserve zone, possibly by inhibiting the matrix degradation characteristic of apoptotic hypertrophic GC cells. To test this, apoptosis was induced in rat RC cells by treatment with exogenous inorganic phosphate (Pi). 24R,25(OH)(2)D-3 blocked apoptotic effects in a dose-dependent manner. Similarly, apoptosis was induced in ATDC5 cell cultures and 24R,25(OH)(2)D-3 blocked this effect. Further studies indicated that 24R,25(OH)(2)D-3 acts via at least two independent pathways. 24R,25(OH)(2)D-3 increases LPA receptor-1 (LPA R1) expression and production of lysophosphatidic acid (LPA), and subsequent LPA R1/3-dependent signaling, thereby decreasing p53 abundance. LPA also increases the Bcl-2/Bax ratio. In addition, 24R,25(OH)(2)D-3 acts by increasing PKC activity. 24R,25(OH)(2)D-3 stimulates 1-hydroxylase activity, resulting in increased levels of 1,25(OH)(2)D-3, and it increases levels of phospholipase A2 activating protein, which is required for rapid 1 alpha,25(OH)(2)D-3-dependent activation of PKC in GC cells. These results suggest that 24R,25(OH)(2)D-3 modulates growth plate development by controlling the rate and extent of RC chondrocyte transition to a GC chondrocyte phenotype. (C) 2010 Elsevier Ltd. All rights reserved.