RAF-1 KINASE-ACTIVITY IS NECESSARY AND SUFFICIENT FOR GENE-EXPRESSIONCHANGES BUT NOT SUFFICIENT FOR CELLULAR MORPHOLOGY CHANGES ASSOCIATEDWITH CARDIAC MYOCYTE HYPERTROPHY

Around the time of birth, cardiac muscle cells lose the capacity to di vide and, from this time on, growth of the heart occurs by hypertrophy where each cell gets bigger. The hypertrophic response is characteriz ed by changes in gene expression including expression of the atrial na triuretic factor (ANF) and myosin light chain-2 (MLC-2) genes. In cult ured neonatal ventricular myocytes, hypertrophy also involves reorgani zation of contractile proteins into sarcomeric units. We have investig ated the role of the Raf-1 kinase in this response. Activation of an e stradiol-regulated Raf-1 protein kinase led to activation of mitogen-a ctivated protein (MAP) kinase and activated expression from the ANF an d MLC-2 promoters. Raf-1-induced activation of these genes was inhibit ed by a kinase deficient mutant of the 44-kDa MAP kinase, Erk1 indicat ing a requirement for MAP kinases in the Raf-1-induced response. Howev er, activation of Raf-1 was not sufficient to induce the organization of actin into sarcomeric units. Transfection of dominant negative Raf- 1 inhibited phenylephrine-induced activation of the ANF and MLC-2 prom oters. Transactivation was rescued by the introduction of increased am ounts of c-Raf suggesting a role for Raf-1 in the response to alpha-ad renergic agonists. These results suggest that activation of Raf-1 kina se is a critical component of the signal transduction pathway leading to changes in gene expression associated with hypertrophy but that Raf -1 is not sufficient for the regulation of actin organization during t he hypertrophic response.