AJP - Endocrinology and Metabolism, Vol 272, Issue 6 E967-E975, Copyright © 1997 by American Physiological Society
Osteogenic protein-1 downregulates endothelin A receptors in primary rat osteoblasts
A. M. Kitten, S. A. Harvey, N. Criscimagna, M. Asher, J. C. Lee and M. S. Olson
Department of Biochemistry, University of Texas Health Science Center, San Antonio 78284-7760, USA.
Osteogenesis is a complex process whereby growth factors and mediators from
both local and systemic sources modulate the bone-forming activities of
osteoblasts. In the present study we utilized primary cultures of fetal rat
calvarial cells to characterize osteoblast responsiveness to the vascular
mediator endothelin-1 (ET-1) and to investigate whether ET-1 responses are
regulated by osteogenic protein-1 (OP-1). We found that a 1- to 2-day
exposure to OP-1 diminished ET-1 receptor ligand binding and signal
transduction by downregulating ET-1 receptor mRNA expression. ET-1-mediated
calcium signaling and ligand binding were completely 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 approximately 50% by OP-1 treatment, whereas ETB
receptor mRNA was not detected by this method of analysis. In OP-1-treated
cultures, the magnitude and duration of ET-1 calcium signals varied among
individual cells. This finding may be related to a heterogeneous OP-1
response, indicated by alkaline phosphatase induction in only a
subpopulation of cells. These results suggest that modulation of osteoblast
function by ET-1 occurs during distinct periods of phenotypic development
and imply that downregulation of ET-1 responsiveness may be necessary for
optimal bone formation in vivo.
