Platelet-derived growth factor-AA and -BB (PDGF-AA and -BB) enhance the synthesis of PDGF-AA in bone cell cultures

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 13-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.