Treatment of resting zone chondrocytes with bone morphogenetic protein-2 induces maturation into a phenotype characteristic of growth zone chondrocytes by downregulating responsiveness to 24,25(OH)2D3 and upregulating responsiveness to 1,25-(OH)2D3

To determine if bone morphogenetic protein-2 (BMP-2) can induce the endochondral maturation of resting zone (RC) chondrocytes, confluent fourth-passage cultures of these cells were pretreated for 24, 36, 48, 72, or 120 h with recombinant human BMP-2. At the end of pretreatment, the media were replaced with new media containing 10⁻¹⁰–10⁻⁸ M 1,25-(OH)₂D₃ or 10⁻⁹–10⁻⁷ M 24,25-(OH₂)D₃, and the cells incubated for an additional 24 h. This second treatment was chosen, because prior studies had shown that the more mature growth zone (GC) chondrocytes and RC cells respond to 1,25-(OH)₂D₃ and 24,25-(OH)₂D₃ in distinctly different ways with respect to the parameters examined. The effect of BMP-2 pretreatment on cell maturation was assessed by measuring alkaline phosphatase specific activity (ALPase). In addition, changes in matrix protein production were assessed by measuring collagen synthesis, as well as [³⁵S]-sulfate incorporation into proteoglycans. When RC cells were pretreated for 72 or 120 h with BMP-2, treatment with 1,25-(OH)₂D₃ caused a dose-dependent increase in ALPase specific activity and collagen synthesis, with no effect on proteoglycan sulfation. RC cells pretreated with 1,25-(OH)₂D₃ responded like RC cells that had not received any pretreatment. RC cells normally respond to 24,25-(OH)₂D₃; however, RC cultures pretreated for 72 or 120 h with BMP-2 lost their responsiveness to 24,25-(OH)₂D₃. These results indicate that BMP-2 directly regulates the differentiation and maturation of RC chondrocytes into GC chondrocytes. These observations support the hypothesis that BMP-2 plays a significant role in regulating chondrocyte maturation during endochondral ossification.