Am. Kitten et al., OSTEOGENIC PROTEIN-1 DOWN-REGULATES ENDOTHELIN-A RECEPTORS IN PRIMARYRAT OSTEOBLASTS, American journal of physiology: endocrinology and metabolism, 35(6), 1997, pp. 967-975
Osteogenesis is a complex process whereby growth factors and mediators
from both local and systemic sources modulate the bone-forming activi
ties of osteoblasts. In the present study we utilized primary cultures
of fetal rat calvarial cells to characterize osteoblast responsivenes
s to the vascular mediator endothelin-l (ET-1) and to investigate whet
her ET-I responses are regulated by osteogenic protein-1 (OP-1). We fo
und that a 1- to 2-day exposure to OP-1 diminished ET-1 receptor ligan
d binding and signal transduction by downregulating ET-1 receptor mRNA
expression. ET-l-mediated calcium signaling and ligand binding were c
ompletely abolished by the ETA receptor antagonist BQ-123, suggesting
that ET-1 effects are mediated by this receptor. Northern analysis of
total RNA revealed that ETA mRNA expression was inhibited similar to 5
0% by OP-1 treatment, whereas ETB receptor mRNA was not detected by th
is method of analysis. In OP-1-treated cultures, the magnitude and dur
ation of ET-1 calcium signals varied among individual cells. This find
ing may be related to a heterogeneous OP-1 response, indicated by alka
line phosphatase induction in only a subpopulation of cells. These res
ults suggest that modulation of osteoblast function by ET-1 occurs dur
ing distinct periods of phenotypic development and imply that downregu
lation of ET-1 responsiveness may be necessary for optimal bone format
ion in vivo.