Department of Research , St. Francis Hospital and Medical Center , Hartford , Connecticut 06105 .
Endocrinology 134 : 2541-6 ( 1994)
Abstract
Platelet-derived growth factor ( PDGF ) , an agent with important mitogenic effects for bone cells , exists in three isoforms , PDGF-AA , -BB , and -AB .
PDGF-AB and -BB are the prevalent circulating isoforms , whereas normal unstimulated cells of the osteoblast lineage synthesize primarily PDGF-AA .
We examined the effects of PDGF-BB on PDGF-A mRNA expression and PDGF-AA polypeptide concentrations in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae ( Ob cells ) .
In a selected number of experiments we compared the effects of PDGF-BB with those of PDGF-AA on PDGF-A mRNA levels .
Steady state PDGF-A mRNA levels were determined by Northern blot analysis , and PDGF-AA concentrations were determined in acidified and fractionated culture medium by a specific RIA for PDGF-A chains .
Treatment of Ob cells with PDGF-AA or -BB at 0.3-3.3 nM caused a dose-dependent increase in steady state PDGF-A mRNA , an effect that was initially observed after 2 h . Treatment with PDGF-BB at 1-3.3 nM for 24 h increased PDGF-AA polypeptide concentrations by 2- to 5-fold .
The effects of PDGF on PDGF-A mRNA and polypeptide levels were prevented by the protein synthesis inhibitor cycloheximide at 3.6 microM .
Phorbol 12-myristate -acetate at 1 microM increased PDGF-A mRNA after 2-6 h and PDGF-AA polypeptide levels after 24 h by 2-fold .
However , the protein kinase-C inhibitor staurosporine at 50 nM did not modify basal PDGF-A mRNA levels and did not prevent the stimulatory effect of PDGF-AA or -BB on PDGF-A mRNA or PDGF-AA polypeptide levels .
In conclusion , PDGF-BB and -AA increase skeletal PDGF-A synthesis , an effect that reveals autoinduction of PDGF in bone cells .