Glucocorticoids Regulate the Expression of the Human Osteoblastic Endothelin A Receptor Gene

The endothelial cell–derived peptide endothelin 1 (ET1) stimulates cell proliferation and differentiated functions of human osteoblastic cells (HOC), and HOC constitutively express the endothelin A receptor (ETR(A)). Therefore, ET1 may play an important role in the regulation of bone cell metabolism. As glucocorticoids (GC) exert a profound influence on bone metabolism and increase the effects of ET1 on bone cell metabolism in vitro, the effects of GC on ETR(A) expression in HOC were investigated. Dexamethasone (DEX) increased ETR(A) mRNA levels in a dose- and time-dependent fashion. The effects of dexamethasone, prednisolone, and deflazacort on the increase of ETR(A) mRNA levels correlate positively with their binding affinity to the GC receptor. Scatchard analysis of ET1 binding data to HOC revealed that DEX increased the binding capacity for ET1 from 25,300 to 62,800 binding sites per osteoblastic cell, leading to an enhanced mitogenic effect of ET1 on HOC after preincubation with DEX. Transiently transfected primary HOC with a reporter gene construct, containing the 5′-flanking region of the ETR(A) gene fused to luciferase gene, showed a promoter-dependent expression of the reporter gene and the induction of reporter gene expression by DEX treatment. Total RNA extracts of femoral head biopsies with osteonecrotic lesions from GC-treated patients showed threefold higher ETR(A) mRNA levels compared with extracts of bone biopsies from patients with traumatically induced osteonecrosis and coxarthrosis. Furthermore, GC treatment increased plasma ET1 levels by 50% compared with pretreatment values. These findings suggest that GC induced upregulation of ETR(A), and ET1 plasma levels enhance ET1's anabolic action on bone cell metabolism. Increased ET1 concentrations may also impair bone perfusion by vasoconstriction in a metabolically activated skeletal region.