ONCOGENIC SRC, RAF, AND RAS STIMULATE A HYPERTROPHIC PATTERN OF GENE-EXPRESSION AND INCREASE CELL-SIZE IN NEONATAL RAT VENTRICULAR MYOCYTES

In response to hormones and growth factors, cultured neonatal ventricu lar myocytes increase in profile, exhibit myofibrillogenesis, and re-e xpress genes whose expression is normally restricted to the fetal stag e of ventricular development. These include atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), and skeletal muscle (SkM)-a lpha-actin. By using luciferase reporter plasmids, we examined whether oncogenes that activate the extracellular signal-regulated kinase cas cade (src(F527), Ha-ras(V12), and v-raf) increased expression of ''fet al'' genes. Transfection of myocytes with src(F527) stimulated express ion of ANF, SkM-alpha-actin, and beta-MHC by 62-, 6.7-, and 50-fold, r espectively, but did not induce DNA synthesis. Stimulation of ANF expr ession by src(F527) was greater than by Ha-ras(V12), which in turn was greater than by v-raf. General gene expression was also increased but to a lesser extent. The response to src(F527) was inhibited by domina nt-negative Ha-ras(N17). Myocyte area was increased by src(F527), Ha-r as(V12), and v-raf, and although it altered myocyte morphology by caus ing a pseudopodial appearance, src(F527) did not detectably increase m yofibrillogenesis either alone or in combination with Ha-ras(V12). A k inase-dead src mutant increased myocyte size to a much lesser extent t han src(F527), and also did not inhibit ANF-luciferase expression in r esponse to phenylephrine. We conclude that members of the Src family o f tyrosine kinases may be important in mediating the transcriptional c hanges occurring during cardiac myocyte hypertrophy and that Ras and R af may be downstream effectors.