PLATELET-DERIVED GROWTH FACTOR-AA AND FACTOR-BB (PDGF-AA AND PDGF-BB)ENHANCE THE SYNTHESIS OF PDGF-AA IN BONE CELL-CULTURES

Platelet-derived growth factor (PDGF), an agent with important mitogen ic 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 primari ly PDGF-AA. We examined the effects of PDGF BB on PDGF-A mRNA expressi on and PDGF-AA polypeptide concentrations in cultures of osteoblast-en riched cells from 22-day-old fetal rat calvariae (Ob cells). In a sele cted number of experiments we compared the effects of PDGF-BB with tho se of PDGF-AA on PDGF-A mRNA levels. Steady state PDGF-A mRNA levels w ere determined by Northern blot analysis, and PDGF-AA concentrations w ere determined in acidified and fractionated culture medium by a speci fic RIA for PDGF-A chains. Treatment of Ob cells with PDGF-AA or -BB a t 0.3-3.3 nm caused a dose-dependent increase in steady state PDGF-A m RNA, an effect that was initially observed after 2 h. Treatment with P DGF-BB at 1-3.3 nM for 24 h increased PDGF-AA polypeptide concentratio ns by 2- to 5-fold. The effects of PDGF on PDGF-A mRNA and polypeptide levels were prevented by the protein synthesis inhibitor cycloheximid e at 3.6 mu M. Phorbol 12-myristate 13-acetate at 1 mu M increased PDG F-A mRNA after 2-6 h and PDGF-AA polypeptide levels after 24 h by 2-fo ld. However, the protein kinase-C inhibitor staurosporine at 50 nM did not modify basal PDGF-A mRNA levels and did not prevent the stimulato ry effect of PDGF-AA or -BB on PDGF-A mRNA or PDGF-AA polypeptide leve ls. In conclusion, PDGF-BB and -AA increase skeletal PDGF-A syn thesis , an effect that reveals autoinduction of PDGF in bone cells.