V-sis induces Egr-1 expression by a pathway mediated by c-Ha-Ras

The early growth response gene, Egr-1, is up-regulated transiently by mitogens and many other stimuli in all cells tested. Using NIH3T3 cells conditionally expressing v-sis from a metallothionein promoter, we show that the addition of Zn2+ stimulates the production of PDGF-B (v-sis) and elicits the expression of Egr-1 in a dose-dependent and time-regulated manner. The signal is likely independent of protein kinase C, but depends on tyrosine kinase and other kinase activities and is mediated by c-Ha-Ras since the presence of dominant-negative mutants of Ras and Raf abrogates the induction of Egr-1 expression by Zn2+. Transiently activated Ras expression in NIH3T3 cells also stimulates the transient expression of Egr-1, but cells that constitutively express Ras do not have elevated levels of Egr-1. Transient assays also demonstrated that Zn2+ or activated Ras expression stimulate the activity of a 950 bp Egr-1 promoter-reporter gene construct and this is abrogated in the presence of mutant Ras and Raf. The accumulated data show that Egr-1 gene expression is regulated by multiple mechanisms, as would be needed for putative roles in cell proliferation, in suppression of transformation and in differentiation.