Cardiac-specific gene expression is intricately regulated in response to developmental, hormonal, and hemodynamic stimuli. To test whether cardiac muscle might be a target for regulation by peptide growth factors, the effect of three growth factors on the actin and myosin gene families was investigated by Northern blot analysis in cultured neonatal rat cardiac myocytes.Transforming growth factor-ftI (TGFfl1, 1 ng/ml) and basic fibroblast growth factor (FGF, 25 ng/ml) elicited changes corresponding to those induced by hemodynamic load. The "fetal" ,B-myosin heavy chain (MHC) was up-regulated about fourfold, whereas the "adult" aMHC was inhibited > 50-60%; expression of a-skeletal actin increased approximately twofold, with little or no change in a-cardiac actin. Thus, peptide growth factors alter the program of differentiated gene expression in cardiac myocytes, and are sufficient to provoke fetal contractile protein gene expression, characteristic of pressureoverload hypertrophy. Acidic FGF (25 ng/ml) produced sevento eightfold reciprocal changes in MHC expression but, unlike either TGF-fil or basic FGF, inhibited both striated a-actin genes by 70-90%. Expression of vascular smooth muscle aactin, the earliest a-actin induced during cardiac myogenesis, was increased by all three growth factors. Thus, three a-actin genes demonstrate distinct responses to acidic vs. basic FGF. (J. Clin. Invest. 1990. 85:507-514.) actin -myosin * fibroblast growth factor * transforming growth factor-#1l -cardiac hypertrophy

Cardiac hypertrophy triggered by mechanical load possesses features in common with growth factor signal transduction. A hemodynamic load provokes rapid expression of the growth factor-inducible nuclear oncogene, c-fos, and certain peptide growth factors specifically stimulate the "fetal" cardiac genes associated with hypertrophy, even in the absence of load. These include the gene encoding vascular smooth muscle a-actin, the earliest a-actin expressed during cardiac myogenesis; however, it is not known whether reactivation of the smooth muscle a-actin gene occurs in ventricular hypertrophy. We therefore investigated myocardial expression of the smooth muscle a-actin gene after hemodynamic overload. Smooth muscle a-actin mRNA was discernible 24 h after coarctation and was persistently expressed for up to 30 d. In hypertrophied hearts, the prevalence of smooth muscle a-actin gene induction was 0.909, versus 0.545 for skeletal muscle a-actin (P < 0.05). Ventricular mass after 2 d or more of aortic constriction was more highly correlated with smooth muscle a-actin gene activation (r = 0.852; P = 0.0001) than with skeletal muscle a-actin (r = 0.532; P = 0.009); P < 0.0005 for the difference in the correlation coefficients. Thus, smooth muscle a-actin is a molecular marker of the presence and extent of pressure-overload hypertrophy, whose correlation with cardiac growth at least equals that of skeletal a-actin. Induction of smooth muscle a-actin was delayed and sustained after aortic constriction, whereas the nuclear oncogenes c-jun and junB were expressed rapidly and transiently, providing potential dimerization partners for transcriptional control by c-fos. (J. Clin. Invest. 1991. 88:1581-1588

Cardiac hypertrophy, in response to mechanical load or growth factors, characteristically entails the induction of a socalled fetal program of cardiac gene expression, superimposed on a generalized increase in cellular RNA and protein content. Signaling pathways leading to the transcription of fetal genes have been extensively studied (19,26,32,35,45,47,48,50,(56)(57)(58)(59), but information is still lacking for the underlying molecular mechanisms that augment total protein content. Despite evidence from gene transfer in vitro and in vivo implicating the proto-oncoprotein Ras in cardiac hypertrophy (1,24,56,57), there is only meager information on the exact mechanism(s) by which this GTP-binding molecule might augment cardiac growth.Our previous finding that Ras can enhance expression of a generalized set of promoters, including constitutive ones, led us to speculate that Ras may be a candidate molecule that regulates global gene expression during cardiac hypertrophy (1). In support of this inference, a transgenic mouse expressing activated Ras in the heart manifested cardiac hypertrophy (23,24), although the exact mechanism for Ras-dependent growth was not established, and an indirect effect, inherent with a chronic model, cannot be excluded. Through mutational analysis of the effector domain of Ras, we have shown that a GTPase-activating protein (GAP) binding site is necessary for Ras-dependent gene induction in the ventricular myocytes, suggesting that GAP predominantly exercises an effector role in the cardiac cells (2). This conclusion was corroborated by the fact that full-length GAP and the N-terminal region of GAP (nGAP) both mimicked the global effect of Ras on cardiac gene expression.While GAP may thus mediate the generalized effects of Ras on gene expression, one Ras effector protein, Raf, has been implicated more specifically in the regulation of fetal genes that are reexpressed during ventricular hypertrophy, such as ANF and MLC-2 (56). A possible dissociation between the signaling pathways that lead to an increase in total cellular protein and the fetal phenotype was recently suggested in connection with angiotensin II (AII) stimulation (49): rapamycin blocked the increase in ribosomal p70 kinase (S6K) activity, and consequently the increase in total cell protein, but did not impair the reactivation of fetal genes (skeletal ␣-actin gene and ANF) or the increase in mitogenactivated protein kinase activity.An increase in total protein per cell (the sine qua non of hypertrophy) is itself a complex process that involves regulation of multiple cellular functions. Cardiac hypertrophy is accompanied by enhanced activity of RNA polymerase I (pol I) (38, 39), pol II, and pol III (10), which regulate synthesis of rRNA, mRNA, and tRNA, respectively, as well as by enhanced p70 S6K (49) and eukaryotic translation initiation factor 4E (eIF-4E) (61) phosphorylation and activities, which each contribute to the regulation of overall protein synthesis. However, the precise signaling pathways involved in mediating t...