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Nuclear signaling by endothelin-1. A Ras pathway for activation of the c-fos serum response element

Endothelin-1 (ET-1) regulates gene expression and growth of vascular cells by triggering signals that link its cognate, G protein-coupled receptor in the plasma membrane to transcriptional activation of immediate early genes in the nucleus. To define the nature of these signals, we asked whether Ras proteins contribute to activation of the c-fos serum response element (SRE) by ET-1 in mesangial cells, a microvascular cell from the renal glomerulus. ET-1 stimulated Ras by increasing Ras GTP loading. Addition of ET-1 or transfection with a plasmid expressing v-Ha-Ras stimulated SRE-dependent transcription. Activation of the c-fos SRE by ET-1 was blocked by a dominant negative Asn-17 c-Ha-Ras mutant. Expression of v-Ha-Ras reversed inhibition of ET-1-stimulated SRE transcriptional activity by Asn-17 c-Ha-Ras. ET-1 also stimulated kinase activity of c-Raf-1, a downstream effector in Ras signaling cascades. Activation of the c-fos SRE by transfection with a plasmid expressing constitutively activated delta Raf-1 was consistent with a role for Ras-Raf-1 in ET-1 signaling. Interestingly, Ras-dependent SRE activation in cells treated with ET-1 was blocked by point mutations in the SRE CArG DNA sequence, which binds the serum response factor, but not by mutations that inhibit binding of ternary complex factors (p62TCF) to the Ets DNA sequence of the SRE. Thus, Ras contributes to a nuclear signaling cascade linking ET-1 receptors to transcriptional activation through the CArG cis-element of the c-fos SRE.