Age-dependent expression of osteoblastic phenotypic markers in normal human osteoblasts cultured long-term in the presence of dexamethasone.

Abstract

We have previously shown that osteoblasts derived from trabecular bone explants and cultured long term in 10 nM dexamethasone ((HOB + DEX) cells) exhibited properties consistent with a more differentiated phenotype compared with those grown in the absence of dexamethasone ((HOB-DEX) cells). To characterize these two cell models further, we measured the steady-state mRNA levels of the phenotypic markers alkaline phosphatase (ALP), collagen type I (COLL) and osteocalcin (OC), OC production, and the activities of ALP and parathyroid hormone (PTH)-stimulated adenylate cyclase. These findings were then correlated with the age and sex of the bone donors. Long-term culture in dexamethasone significantly increased ALP and OC mRNA levels and the activities of ALP and PTH-stimulated adenylate cyclase but not OC production, in (HOB + DEX) compared with (HOB-DEX) cells (p < 0.05). When the data were examined with respect to the age of the bone donor, age-dependent differences in the expression and responses to dexamethasone were apparent. ALP and PTH-stimulated adenylate cyclase activities decreased with increasing age of the bone donor in (HOB-DEX) and (HOB + DEX) cells (p < 0.05). There were no significant correlations between phenotypic marker mRNA levels and bone donor age in (HOB-DEX) and ((HOB + DEX) cells. All age-dependent decreases in ALP and PTH-stimulated cyclase activities were enhanced in the (HOB + DEX) cells. However, when the data were examined according to the sex of the bone donor, there were no differences in mRNA levels, OC production, or ALP and cyclase activities between cells from male and female donors. These results indicate an age dependence in the expression of osteoblastic markers in human bone cells at different stages of differentiation: thus osteoblastic cultures derived from older donors are likely to contain fewer osteoprogenitor cells, lower levels of glucocorticoid receptors or represent more differentiated osteoblasts compared with those derived from younger donors.