Bone morphogenetic protein‐7 in growth‐plate chondrocytes: Regulation by retinoic acid is dependent on the stage of chondrocyte maturation

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Abstract
Although the bone morphogenetic proteins stimulate chondrogenesis, little is known regarding their expression and regulation in growth-plate chondrocytes. The expression of bone morphogenetic protein-7 was examined in chick growth-plate chondrocyte cultures. Low basal levels of bone morphogenetic protein-7 mRNA and protein expression were stimulated by increasing doses of all-trans retinoic acid, a metabolite of vitamin A. The addition of 10 μM retinoic acid resulted in approximately a 6-fold increase in bone morphogenetic protein-7 mRNA levels. In contrast, other growth regulators, including basic fibroblast growth factor, transforming growth factor-β. vitamin D, bone morphogenetic protein-6, bone morphogenetic protein-7, and parathyroid hormone-related peptide. did not alter bone morphogenetic protein-7 transcript levels. The increase in bone morphogenetic protein-7 transcripts, although present at 6 hours, was maximal following a 12-hour exposure to retinoic acid. Retinoic acid induction of Done morphogenetic protein-7 transcript levels was dependent on protein synthesis because the induction could be blocked by cyclohexamide. In maturation-ally distinct subpopulations of chondrocytes separated by countercurrent centrifugal elutriation, retinoic acid markedly induced bone morphogenetic protein-7 mRNA levels in the least differentiated chondrocytes but had no effect in the most terminally differentiated hypertrophic chondrocytes. Immunohistochemical localization of bone morphogenetic protein-7 demonstrates its expression throughout the developing and adolescent growth plate consistent with the constitutive pattern of expression seen in isolated chondrocytes. The addition of exogenous bone morphogenetic protein-7 to chondrocyte cultures stimulated maturation in undif-ferentiated chondrocyte populations. The data support a role for bone morphogenetic protein-7 as an autocrine regulator of chondrocyte maturation in the growth plate. Regulation of bone morphogenetic protein-7 by retinoic acid may be important in normal growth and development as well as in pathologic conditions of an excess or deficiency of vitamin A.

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