We have previously shown that the myocardium is a target tissue for estrogen. Here, we have identified rapid non-nuclear estrogen effects on the expression of the early growth response gene-1 (Egr-1) in cardiomyocytes. Egr-1 mRNA and protein were rapidly and strongly induced by estrogen in an estrogen receptordependent manner via the extracellular signal-regulated kinase, ERK1/2. A promoter analysis study of a 1.2-kilobase Egr-1 promoter fragment revealed that the serum response elements (SREs) but not the estrogen response elements or AP-1 sites are responsible for Egr-1 induction by estrogen, identifying a novel mechanism of estrogen receptor-dependent gene activation in the myocardium. Both estrogen receptor-␣ and -␤ induced the Egr-1 promoter via the SREs as well as an artificial promoter consisting of only five SREs in cardiomyocytes. Electrophoretic mobility shift assays showed that a protein complex containing serum response factor or an antigenically related protein was recruited to the SREs by estrogen treatment of primary cardiomyocytes. The recruitment of the protein complex was inhibited by the specific estrogen receptor antagonist ICI 182,780 as well as the MEK inhibitor PD 98059. Taken together, these results identify SREs as important promoter control elements for an estrogen receptordependent mechanism of gene activation in the myocardium.Gender-specific differences in the occurrence of cardiac disease and the protective role of estrogens in the heart have been established in numerous clinical studies (1, 2). Hormone replacement therapy may prevent the primary development of coronary artery disease in postmenopausal women (1) but have not been effective in secondary prevention trials (3). These findings call for a better understanding of the role and function of estrogen and estrogen-like substances in the heart. The vast majority of studies have concentrated on vascular hormone effects (4, 5). However, cardioprotection by estrogen is not necessarily restricted to the vasculature. We have previously shown that functional estrogen receptors in the myocardium regulate the expression of relevant target genes such as connexin 43 (6) and ␣-myosin heavy chain, one of the major contractile proteins in the heart (7).In addition to the classical genomic regulation of target gene expression, a new role for estrogens as mediators of rapid non-genomic effects has recently been identified. These rapid effects include the stimulation of nitric oxide release in vascular cells (8, 9) and the activation of Ras and Raf-1 kinase (10), resulting in the activation of ERK1/2 1 and the phosphorylation of transcription factors in both non-myocytes (11-15) and cardiomyocytes (16). In the present work we have for the first time analyzed in detail one of the mechanisms of rapid gene induction by estrogen in the myocardium by performing a detailed promoter analysis of a bona fide estrogen target gene identified by us, the early growth response gene 1 (Egr-1).Surprisingly, a cluster of serum response elements (SREs) turned o...