GROWTH-HORMONE BINDS TO A SINGLE HIGH-AFFINITY RECEPTOR-SITE ON MOUSEOSTEOBLASTS - MODULATION BY RETINOIC ACID AND CELL-DIFFERENTIATION

Growth hormone (GH) exerts direct differentiative and proliferative ef fects on osteoblasts. We studied I-125-labeled human (h) GH binding to primary mouse osteoblasts derived from collagenase-treated 18-day fet al mouse calvaria. Scatchard analysis of the data revealed a single cl ass of high affinity GH receptors (apparent K-a = 5.74 x 10(9) M(-1)) with 2200 sires per cell. Affinity crosslinking and SDS-PAGE electroph oresis showed two bands with apparent molecular masses of 120 and 70 k Da. Mouse osteoblasts express GH receptor mRNA with gene transcripts o f 4.2 and 1.2 kb, at levels which reach approximately 1/6 of those in mouse liver and 1/3 of those in mouse muscle. Two populations of undif ferentiated and diffentiated osteoblasts, obtained by sequential colla genase digestion of mouse calvaria, were used to study the relationshi p between osteoblastic phenotype and GH receptor expression. Although tile affinity of the receptors in undifferentiated and differentiated cells was the same, the capacity was significantly higher (1.45 +/- 1. 0% vs 2.39 +/- 0.9%, P = 0.03) in differentiated cells. This stresses the specific importance of the osteoblast as a target cell for GH. The differentiating potential oi the vitamin A derivative retinoic acid w as subsequently used experimentally to induce differentiation in the c ells. Retinoic acid increased I-125-hGH binding to preosteoblasts (153 %, P = 0.02). Together, these data demonstrate the presence of a high affinity GH receptor in mouse osteoblasts which is related to differen tiation.