PDGF-INDUCED EGR-1 EXPRESSION IN RAT MESANGIAL CELLS IS MEDIATED THROUGH UPSTREAM SERUM RESPONSE ELEMENTS

Platelet-derived growth factor (PDGF) has been implicated in the proce ss of mesangial cell (MC) proliferation in vitro and in vivo. To inves tigate early changes in gene expression that couple biochemical events with changes in phenotype in PDGF-stimulated cultured MC, we studied expression of the early growth response gene 1 (Egr-1), a member of th e family of immediate early genes. Our findings show that protein tyro sine phosphorylation is required for induction of Egr-1 mRNA and proli feration by PDGF in MC. Nuclear run-off assays show that Egr-1 inducti on occurs at the transcriptional level. An 11.3-fold increase in Egr-1 transcription rate was observed as early as 5 min after PDGF stimulat ion of MC. Promoter deletion analysis revealed that the region critica l for Egr-1 inducibility by PDGF contains serum response element (SRE) consensus sequences. Sequential deletion of the Egr-1 SREs led to a s tepwise drop in promoter activity, suggesting that PDGF induces Egr-1 transcription through SREs in the Egr-1 promoter region. Interestingly , electrophoretic mobility shift assays, with an Egr-I SRE as probe, d emonstrate that protein-SRE complexes of differing size undergo modest quantitative changes following PDGF stimulation. These data in MC sug gest that the upstream SREs mediate the transcriptional induction of E gr-1 by PDGF.