Mammalian target of rapamycin (mTOR) is a key regulator of cell growth acting via two independent targets, ribosomal protein S6 kinase 1 (S6K1) and 4EBP1. While each is known to regulate translational efficiency, the mechanism by which they control cell growth remains unclear. In addition to increased initiation of translation, the accelerated synthesis and accumulation of ribosomes are fundamental for efficient cell growth and proliferation. Using the mTOR inhibitor rapamycin, we show that mTOR is required for the rapid and sustained serum-induced activation of 45S ribosomal gene transcription (rDNA transcription), a major rate-limiting step in ribosome biogenesis and cellular growth. Expression of a constitutively active, rapamycininsensitive mutant of S6K1 stimulated rDNA transcription in the absence of serum and rescued rapamycin repression of rDNA transcription. Moreover, overexpression of a dominant-negative S6K1 mutant repressed transcription in exponentially growing NIH 3T3 cells. Rapamycin treatment led to a rapid dephosphorylation of the carboxy-terminal activation domain of the rDNA transcription factor, UBF, which significantly reduced its ability to associate with the basal rDNA transcription factor SL-1. Rapamycin-mediated repression of rDNA transcription was rescued by purified recombinant phosphorylated UBF and endogenous UBF from exponentially growing NIH 3T3 cells but not by hypophosphorylated UBF from cells treated with rapamycin or dephosphorylated recombinant UBF. Thus, mTOR plays a critical role in the regulation of ribosome biogenesis via a mechanism that requires S6K1 activation and phosphorylation of UBF.Cell growth (increased cell mass and size) is a prerequisite for proliferation (increased cell number), since a cell will divide only after it has reached a critical mass (38,49,55,58). Thus, factors that govern cell cycle progression must also regulate growth in an interrelated fashion. Cell growth is not however, unconditionally dependent on cell cycle progression, as mutations in the budding yeast Saccharomyces cerevisiae and the fruit fly Drosophila melanogaster that block or disrupt cell division do not necessarily arrest cell growth (34,44). Recent studies have demonstrated that cell growth and cell cycle progression in proliferating mammalian cells, like lower organisms, are also separable processes (8, 50, 63). Thus, detailed knowledge of the biochemical and molecular mechanisms governing cell size will be essential to understanding how the cell division cycle is coupled to growth and how this process is uncoupled during differentiation or is perturbed during diseases associated with deregulated growth. Our knowledge of cell cycle regulatory mechanisms has advanced considerably over the past decade. In contrast, information on the mechanisms of regulating cell growth in mammalian cells is limited.Increased protein synthesis is one of the major anabolic events required for the growth response (28). Recent studies suggest that one of the key nodal points upon which signaling pathwa...

Abstract-Angiotensin II (Ang II) may cause cardiac hypertrophy via type 1 Ang II receptors (AT 1 ) on cardiomyocytes and through growth factors released from cardiac fibroblasts. Whereas cardiomyocyte-specific AT 1 receptor expression produces cardiac hypertrophy and remodeling in vivo, delineation of the signals that mediate growth to Ang II is challenging because the prevailing in vitro model (cultured neonatal cardiomyocytes) expresses low levels of AT 1 receptor and responds inconsistently to Ang II. In this study, when AT 1A receptors were expressed using adenovirus in cultured neonatal cardiomyocytes, Ang II stimulated a robust hypertrophy that was not secondary to the release of cardiac fibroblast-derived factors, specifically endothelin-1. Hypertrophy was accompanied by the induction of the immediate-early response genes, c-fos and c-jun, and reexpression of atrial natriuretic peptide (ANP). Ang II-induced activation of an ANP promoter-reporter was inhibited by the dominant/negative mutants, G␣qI and N17Ras, indicating that hypertrophic signaling by the AT 1A receptor is via heterotrimeric G protein coupling and downstream Ras pathways. AT 1A -mediated cardiomyocyte hypertrophy and mitogen-activated protein kinase (MAPK) activation were inhibited by the MAPK kinase inhibitor, PD98059, and the epidermal growth factor (EGF) receptor kinase antagonist, AG1478, but not by PKC inhibitor, bisindolylmaleimide-1. Moreover, Ang II-induced MAPK activation was prevented by treatment with a matrix metalloproteinase inhibitor, consistent with the tyrosine phosphorylation of the EGF receptor in response to AT 1A receptor activation. These data unequivocally demonstrate that Ang II can directly promote cardiac myocyte growth via AT 1A receptors expressed on these cells and reveal for the first time the important contribution of EGF receptor-transactivated MAPK signaling to this process. Key Words: AT 1A receptor Ⅲ angiotensin Ⅲ cardiomyocyte hypertrophy Ⅲ adenovirus Ⅲ EGF receptor transactivation A ngiotensin II (Ang II) regulates blood pressure, cardiovascular homeostasis, and cellular growth via type 1 (AT 1 ) angiotensin receptors, which activate G␣ q/11 -phospholipase C-␤1 to generate inositol trisphosphate and diacylglycerol, and thereby increase intracellular calcium and activate protein kinase C (PKC). 1 Activation of PKC and the calcium-dependent phosphatase, calcineurin, has been linked to Ang II-mediated growth. 2,3 Activation of AT 1 receptors also promotes tyrosine phosphorylation, stimulation of mitogen-activated protein kinases, and growth, particularly in vascular and cardiac cells. 1 How AT 1 receptors, which lack intrinsic tyrosine kinase activity, induce tyrosine phosphorylation, MAP kinase, and growth factor signaling pathways is not clear, but recent evidence from vascular smooth muscle cells, 4 liver epithelial cells, 5 and cardiac fibroblasts 6 suggest that a major mechanism is the "transactivation" 7 of the EGF receptor (EGFR).Cardiac hypertrophy involves cardiomyocyte growth as well as extracell...

The regulation of cell mass (cell growth) is often tightly coupled to the cell division cycle (cell proliferation). Ribosome biogenesis and the control of rDNA transcription through RNA polymerase I are known to be critical determinants of cell growth. Here we show that granulocytic cells deficient in the c‐MYC antagonist MAD1 display increased cell volume, rDNA transcription and protein synthesis. MAD1 repressed and c‐MYC activated rDNA transcription in nuclear run‐on assays. Repression of rDNA transcription by MAD1 was associated with its ability to interact directly with the promoter of upstream binding factor (UBF), an rDNA regulatory factor. Conversely, c‐MYC activated transcription from the UBF promoter. Using siRNA, UBF was shown to be required for c‐MYC‐induced rDNA transcription. These data demonstrate that MAD1 and c‐MYC reciprocally regulate rDNA transcription, providing a mechanism for coordination of ribosome biogenesis and cell growth under conditions of sustained growth inhibition such as granulocyte differentiation.

2. Regardless of the initiating cause, the actual increase in chamber enlargement is, in each case, due to an increase in size of a pre-existing cardiomyocyte population, with little or no change in their number; a process referred to as cellular hypertrophy.3. An accelerated rate of global protein synthesis is the primary mechanism by which protein accumulation increases during cardiomyocyte hypertrophy. In turn, increased rates of synthesis are a result of increased translational rates of existing ribosomes (translational efficiency) and/or synthesis and recruitment of additional ribosomes (translational capacity).4. The present review examines the relative importance of translational capacity and translational efficiency in the response of myocytes to acute and chronic demands for increased protein synthesis and the role of these mechanisms in the development of LVH.

Recent evidence suggests that increased translational efficiency of existing ribosomes alone is insufficient to account for the hypertrophic growth of cardiomyocytes and that synthesis of new functional ribosomes must occur. The rate-limiting step in ribosome accumulation is the transcription of the ribosomal 45S genes (rDNA) by RNA polymerase I. Our previous studies have demonstrated that increases in the expression of the rDNA transcription factor UBF correlated with hypertrophy of neonatal cardiomyocytes. These studies expand this observation to examine directly the hypothesis that increased UBF levels are an essential requirement for the initiation of cardiac hypertrophy. We demonstrate that the introduction of UBF antisense RNA into myocytes, using adenovirus approaches, efficiently inhibits UBF accumulation during induction of cardiomyocyte hypertrophy. Moreover, this approach results in a significant reduction in rDNA transcription, rRNA levels, and protein accumulation, which are all the hallmarks of cardiac growth. Furthermore, UBF antisense RNA expression did not alter re-expression of the fetal gene program, which confirmed that the effect was specific for transcription by RNA polymerase I. These findings demonstrate that an increase in rRNA synthesis is required for hypertrophy of cardiomyocytes and also implicate UBF as a major regulatory factor in this process. Approaches that target UBF activity may be of therapeutic use in the regression of pathophysiological cardiac hypertrophy.