Migration of medial smooth muscle cells ( SMCs ) and their proliferation in the intima contribute to thickening of injured and atherosclerotic vessels . These events have been proposed to be regulated in part by platelet-derived growth factor ( PDGF ) . Two separate PDGF receptors have been identified , PDGF-R alpha and PDGF-R beta . To study the functions of PDGF-R alpha and PDGF-R beta in vascular SMCs , neutralizing monoclonal antibodies ( mAbs ) specific for each of the two receptors were used . These antibodies allowed us to evaluate the role of each receptor for PDGF-induced proliferation and migration of cultured baboon SMCs . Both PDGF-AA and PDGF-BB stimulated SMC growth , with PDGF-BB being more potent than PDGF-AA . Studies with anti-PDGF-R alpha and anti-PDGF-R beta mAbs revealed that both PDGF receptors promoted the stimulatory signals for proliferation . In contrast , PDGF-BB stimulated SMC migration , whereas PDGF-AA had no stimulatory activity on its own . Additionally , PDGF-AA was able to suppress migration induced by PDGF-BB or fibronectin in modified Boyden 's chamber assay . When PDGF-BB-induced migration was separated into chemotactic and chemokinetic activities , only the chemotactic component was inhibited by PDGF-AA . The suppression of SMC migration by PDGF-AA was eliminated by anti-PDGF-R alpha mAb . In addition , PDGF-BB , in the presence of anti-PDGF-R beta , bound only to PDGF-R alpha and caused suppression of SMC migration induced by fibronectin . These results suggest that when activated by ligand binding , both PDGF-R alpha and PDGF-R beta stimulate proliferation . In contrast , only activation of PDGF-R beta stimulates migration , whereas ligand binding to PDGF-R alpha leads to inhibition of cell migration . These observations provide support for the conclusion that PDGF-R alpha and PDGF-R beta may play different roles in SMC function and may be involved in different regulatory mechanisms during vascular remodeling . 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 . In cultured rat vascular smooth muscle cells , platelet-derived growth factor ( PDGF ) beta receptor was expressed at a high level , whereas PDGF-alpha receptor was not detected . PDGF-BB showed a high binding activity at 4 degrees C in the cells and was not displaced by PDGF-AA or -AB . This result indicates that PDGF-AB as well as PDGF-AA does not bind to the cells lacking PDGF-alpha receptor at 4 degrees C. However , at 37 degrees C , PDGF-AB bound to the cells and induced the internalization of PDGF-beta receptor . DNA synthesis was also stimulated potentially by PDGF-AB as well as PDGF-BB in the cells , although PDGF-AA was completely inactive . At 37 degrees C , PDGF-AB caused tyrosine phosphorylation of a group of proteins including PDGF-beta receptor and phospholipase C-gamma 1 , but at a slower rate than PDGF-BB . At 4 degrees C , PDGF-AB did not stimulate protein tyrosine phosphorylation , whereas PDGF-BB did . A chemical cross-linking experiment showed that PDGF-beta receptor was dimerized by PDGF-AB as well as PDGF-BB . These results indicate that PDGF-beta receptor binds PDGF-AB without participation of PDGF-alpha receptor at 37 degrees C ( but not at 4 degrees C ) , and PDGF-AB as well as PDGF-BB acts as a potent mitogen in the vascular smooth muscle cells . Platelet-derived growth factor ( PDGF ) , a potent mitogen for mesenchymal cells , consists of PDGF-1 and PDGF-2 polypeptide chains which are linked by disulfide bonds . Sequence analysis has revealed that : ( a ) the PDGF-2 chain is encoded by the c-sis protooncogene , the cellular counterpart of the simian sarcoma viral oncogene ; ( b ) the PDGF-1 and PDGF-2 chains are related ; and ( c ) the PDGF-1 gene has no known viral homologue . We have previously shown that the PDGF-2 gene is expressed during 12-O-tetradecanoylphorbol-13-acetate ( TPA ) induced monocytic differentiation of human HL-60 leukemia cells . In the present study , PDGF-1 and PDGF-2 gene expression was compared in HL-60 cells , human THP-1 monocytic leukemia cells , and human monocytes . Uninduced HL-60 cells , uninduced THP-1 cells , and resting monocytes had no detectable PDGF-1 or PDGF-2 mRNA . In contrast , both PDGF-1 and PDGF-2 transcripts were detected in HL-60 cells and monocytes induced with TPA , while only PDGF-1 mRNA was found in TPA-treated THP-1 cells . Moreover , neither of these transcripts were found during drug induced granulocytic differentiation of HL-60 cells . Cycloheximide , an inhibitor of protein synthesis : ( a ) failed to increase PDGF-1 and PDGF-2 mRNA levels in uninduced HL-60 cells ; ( b ) increased PDGF-2 , but not PDGF-1 , mRNA in resting monocytes ; and ( c ) increased levels of PDGF-1 and PDGF-2 mRNA in HL-60 cells and monocytes treated with TPA . This effect of cycloheximide was related in part to stabilization of both transcripts . Thus , PDGF-1 and PDGF-2 genes are regulated in myeloid cells , although they share common control mechanisms at the post-transcriptional level . Differential regulation of PDGF gene expression would result in altered chain composition of the PDGF protein and possibly changes in biological activity . Platelet-derived growth factor ( PDGF ) exists as a homodimer or a heterodimer comprising either PDGF-A or PDGF-B subunits , and each isoform occurs in various tissues , including bone . Although the stimulatory effects of PDGF-BB have been studied in cultures of bone cells and intact bone fragments , the influence of other isoforms that may arise locally or systematically in vivo , has not been reported . Therefore recombinant human PDGF-BB , PDGF-AB , and PDGF-AA were evaluated in osteoblast-enriched cultures from fetal rat bone . Within 24 hours these factors produced a graded response in bone cell DNA and protein synthesis , with half-maximal effects at approximately 0.6 , 2.1 , and 4.8 nM PDGF-BB , PDGF-AB , and PDGF-AA , respectively . Increases in collagen and noncollagen protein synthesis were abrogated when DNA synthesis was blocked with hydroxyurea . Furthermore , each factor reduced alkaline phosphatase activity , PDGF-BB being the most inhibitory . Binding studies with 125I-PDGF-BB or 125I-PDGF-AA and each unlabeled PDGF isoform produced discrete ligand binding and displacement patterns : 125I-PDGF-BB binding was preferentially displaced by PDGF-BB ( Ki approximately 0.7 nM ) , by PDGF-AB ( Ki approximately 2.3 nM ) and poorly by PDGF-AA . In contrast , 125I-PDGF-AA binding was measurably reduced by PDGF-AA ( Ki approximately 4.0 nM ) , but was more effectively displaced by PDGF-BB or PDGF-AB ( each with Ki approximately 0.7 nM ) . These studies indicate that each PDGF isoform produces biochemical effects proportional to binding site occupancy and suggest that receptors that favor PDGF-B subunit binding preferentially mediate these results in osteoblast-enriched bone cell cultures .