Insulin-regulated expression of Egr-1 and Krox20: dependence on ERK1/2 and interaction with p38 and PI3-kinase pathways.

In addition to its ability to rapidly alter metabolism, insulin is also able to regulate the expression of numerous genes via activation of the PI3-kinase (PI3-K), MAPK kinase (MEK)-ERK, or p38 pathways. Using differential screening of H4IIE cells, we have identified two members of the Egr zinc-finger transcription factor family of early response genes, Egr-1 and Krox20, whose transcription is induced by insulin treatment. Egr-1 may be involved in insulin's regulation of hepatic gene expression. Krox20 regulation and expression have been primarily studied in neural cells and tissues, but little has been previously reported on the presence of Krox20 in cells of hepatic origin or its regulation by insulin. In the present studies, insulin treatment rapidly increased transcription of both Egr-1 and Krox20. In cells pretreated with a PI3-K inhibitor, there was no reduction in the effect of insulin on Egr-1 and Krox20, but an increase in Egr-1 transcription. The rapid induction of ERK1/2 phosphorylation was completely blocked by pretreatment with a MEK1 inhibitor and was associated with a nearly complete inhibition of insulin-stimulated induction of both Egr-1and Krox20, indicating this pathway is necessary for insulin's effect on these genes. Finally, inhibition of the p38 pathway, followed by insulin addition, caused an additive induction of both Egr-1and Krox20. In conclusion, these genes are induced by insulin via coordinated regulation of the MEK-ERK and p38 pathways and, in the case of Egr-1, the PI3-K pathway.