A Ras-Dependent Pathway Regulates RNA Polymerase II Phosphorylation in Cardiac Myocytes: Implications for Cardiac Hypertrophy

Abstract

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...

An Alliance between Ras GTPase-activating Protein, Filamin C, and RasGTPase-activating Protein SH3 Domain-binding Protein Regulates MyocyteGrowth

“…We also found that this effect was associated with an increase in Cdk7 and hypo-and hyperphosphorylated RNA polymerase II (3). In this study we attempt to identify partners of RasGAP that participate in this pathway.…”

“…Our previous data implicate RasGAP in a pathway that appears to regulate both mRNA and protein synthesis. Overexpression of RasGAP 1 or a mutant lacking the GTPase activating domain results in enhanced global mRNA and protein synthesis that is accompanied by up-regulation of Cdk7 and hypo-and hyperphosphorylated RNA polymerase II (3). The present study attempts to identify other members of this pathway.…”

A transcriptional autoregulatory loop for KIN28–CCL1 and SRB10–SRB11, each encoding RNA polymerase II CTD kinase–cyclin pair, stimulates the meiotic development of S. cerevisiae

“…It is likely that the transcriptional regulation of the general transcription factor affected expressions of many genes, although we examined for only a small number of genes in this study. Global regulation of transcription by modulating CTD kinases has been observed in transcriptionally silent germ cells of invertebrates, such as the nematode Caenorhabditis elegans, and the fruit¯y, Drosophila melanogaster (Seydoux and Dunn, 1997), in negative regulation of meiotic maturation in the frog Xenopus laevis (Shuttleworth et al, 1990), and also in mitotic repression of transcription (Akoulitchev and Reinberg, 1998;Long et al, 1998), and mitogen- (Dubois et al, 1994) and oncogene-stimulated (Abdellatif et al, 1998;Baskaran et al, 1996) transcriptional upregulation in mammalian cells. Therefore, the global control of transcription by regulating CTD kinases is generally conserved from yeast to mammals.…”

Hypertrophy of the Heart

“…84 Likewise, expression of this Ras mutant in neonatal rat cardiomyocytes results in hypertrophic gene expression, 85 whereas dominant-negative Ras mutants blunt phenylephrine-mediated increases in cell size and protein synthesis. 86,87 The Rho family of small G proteins, consisting of RhoA, Rac, and Cdc42 subfamilies, regulates cytoskeletal organization in cardiomyocytes. 88 RhoA activates several protein kinases, specifically Rho-associated kinase (ROCK), and potentiates GATA4 transcriptional activity to induce a hypertrophic phenotype in neonatal rat cardiomyocytes.…”