REGULATION OF PARATHYROID-HORMONE PARATHYROID HORMONE-RELATED PROTEIN-RECEPTOR EXPRESSION BY OSTEOBLAST-DEPOSITED EXTRACELLULAR-MATRIX IN AHUMAN OSTEOBLAST-LIKE CELL-LINE

Parathyroid hormone (PTH) receptors and the biological response to PTH in osteoblasts have been shown to be influenced by glucocorticoids, g rowth factors, cytokines or PTH itself. Furthermore, components of ext racellular matrix (ECM) appear to regulate the response to PTH as well . We investigated the effects of osteoblast-deposited ECM on PTH-relat ed protein (PTHrP)-stimulated cAMP production, PTHrP binding and PTH/P THrP receptor mRNA in the human osteoblast-like Cell line SaOS-2. ECM was laid down by the human osteoblastic cell line MG-63. At confluence , maximal cAMP stimulation induced by 100 nmol/l PTHrP (1-34) was decr eased in SaOS-2 cells grown on ECM as compared with cultures on plasti c dishes, without any change in PTHrP concentration producing half-max imal stimulation. In contrast, cAMP production stimulated by PGE2 was increased in cells on ECM. Saturable I-125-PTHrP binding (as evaluated by Scatchard plot analysis) was markedly diminished in cells grown on ECM (5,600 +/- 2,010 vs. 20,700 +/- 1,710 binding sites/cell, x +/- S .E.M., P < 0.01, n = 4 experiments), without any significant change in affinity (1.3 +/- 0.4 vs. 2.5 +/- 0.5 nmol/I (NS), in cells on ECM an d plastic, respectively). This apparent decrease in membrane receptor density was associated with markedly lower steady state PTH/PTHrP rece ptor mRNA levels as assessed by Northern blot analysis (ECM/control: 0 .4 +/- 0.1). A difference in PTH/PTHrP receptor mRNA levels between ce lls on ECM or on plastic dishes was detectable by 8 hours, but not by 4 hours, after seeding the cells at high density. By 24 hours after pl ating, PTH/PTHrP receptor mRNA levels were maximally decreased in cell s on ECM. These results in the human osteoblast-like cell line SaOS-2 indicate that PTH/PTHrP receptors are down-regulated by growth on ECM. Thus, attachment of bone cells to bone surface could influence differ entiation and function of osteoblasts. (C) 1995 Wiley-Liss, Inc.