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 process of mesangial cell (MC) proliferation in vitro and in vivo. To investigate 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 the family of immediate early genes. Our findings show that protein tyrosine phosphorylation is required for induction of Egr-1 mRNA and proliferation by PDGF in MC. Nuclear run-off assays show that Egr-1 induction occurs at the transcriptional level. An 11.3-fold increase in Egr-1 transcription rate was observed as early as 5 min after PDGF stimulation of MC. Promoter deletion analysis revealed that the region critical for Egr-1 inducibility by PDGF contains serum response element (SRE) consensus sequences. Sequential deletion of the Egr-1 SREs led to a stepwise 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-1 SRE as probe, demonstrate that protein-SRE complexes of differing size undergo modest quantitative changes following PDGF stimulation. These data in MC suggest that the upstream SREs mediate the transcriptional induction of Egr-1 by PDGF.