School of Kinesiology, The University of Illinois at Chicago, Chicago, Illinois
Submitted 7 April 2005 ; accepted in final form 27 July 2005
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
muscle growth; ribosome biogenesis
Although it is clear that mTOR is important for the regulation of skeletal muscle hypertrophy, the mechanisms by which this regulation occurs are not fully understood. Several lines of evidence have demonstrated that mTOR exerts its role in protein synthesis at the initiation step by phosphorylating and inhibiting the eukaryotic initiation factor 4 (EIF4)-binding protein 1 (4E-BP1) and thereby relieving repression of EIF4E and cap-dependent translation (10, 35, 37). Another mTOR-regulated mechanism important for protein synthesis is the regulation of ribosome biogenesis (12, 26). mTOR controls this process by at least two different mechanisms: 1) modulation of the activation of the 70-kDa ribosomal protein S6 kinase (S6K-1/p70S6K), which in turn plays a role in the translation of ribosomal proteins and other 5'-TOP mRNA (5, 21, 22); and 2) modulation of the synthesis of ribosomal RNA (rRNA) (12, 26).
Ribosome accumulation during cellular hypertrophy has been studied well in several systems and is mainly regulated by de novo ribosome synthesis (27, 28). Transcription of the 45S ribosomal DNA (rDNA) genes is thought to be the rate-limiting step of ribosome synthesis (30, 32). Efficient transcription of rDNA genes is supported by the RNA polymerase I (RNA Pol I) holoenzyme complex, and a series of accessory proteins such the trans-activating factors selectivity factor 1 (SL-1) and upstream binding factor (UBF) (23, 32). The best characterized of these factors is UBF, and its availability for transcriptional activity at the 45S rDNA promoter is regulated in part through sequestration by the tumor suppressor protein retinoblastoma (Rb) (13). Rb is in turn modulated by the concerted action of cyclin-dependent kinases (CDK); their catalytic partners, cyclins (e.g., cyclin D); and cyclin-dependent kinase inhibitors (CKI; e.g., p21) (29, 46).
Given the importance of mTOR in skeletal muscle hypertrophy, the goal of this project was to identify downstream molecular steps by which mTOR may regulate components of protein synthesis and subsequent muscle cell growth. The general hypothesis was that signaling through mTOR modulates increases in rRNA associated with myotube growth. Experiments were performed to determine whether mTOR signaling is necessary for the accumulation of rRNA during growth and whether steps such as availability of UBF, inhibition of Rb, and activation of cyclin D1-dependent CDK-4 activity are mTOR sensitive. The results presented herein demonstrate that mTOR-dependent growth of myotubes is associated with the accumulation of rRNA and an increase in cyclin D1-dependent CDK-4 activity, Rb phosphorylation, and UBF availability. On the basis of these findings, we suggest that the mechanisms modulating mTOR-dependent hypertrophy of differentiated myotubes resemble those necessary for growth during the G1 phase of the cell cycle.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Cell culture. Myoblasts were grown to confluence in growth medium (GM; DMEM supplemented with 10% FBS) and were induced to fuse into myotubes by being incubated in differentiation medium (DM; DMEM supplemented with 2% HS) for 4 days. At day 4 postdifferentiation, myotubes were either maintained in DM (control group) or stimulated with GM high-serum medium (20% FBS) for 3, 6, 12, 24, and 48 h. All experiments were performed in a humidified environment at 37°C in a 5% CO2-95% O2 atmosphere. For RNA, DNA, and protein analysis, cells were cultured in six-well plastic plates, of which three wells were used for protein or RNA and the other three were used for DNA quantitation. Protein and RNA experiments were performed in triplicate (n = 3/plate). For coimmunoprecipitation and kinase assays, cells were grown in 10-mm plates.
RNA, DNA, and protein quantitation.
Cells were washed with cold PBS and homogenized in TRIzol reagent by being passed several times through a 21-gauge needle. RNA concentration was determined spectrophotometrically at 260 nm. The integrity of the RNA was assessed visually using agarose gel fractionation of the 28S and 18S rRNA subunits. DNA was isolated using the DNA isolation Wizard kit (Promega) according to the manufacturer's guidelines and quantified spectrophotometrically at 260 nm. Total protein was isolated after the myotubes were washed as described above, collected in lysis buffer [50 mM HEPES-NaOH, pH 7.5, 1% Nonidet P-40 (NP-40), 150 mM NaCl2, 1 mM EDTA, 2.5 mM EGTA, 50 mM -glycerophosphate, 200 mM PMSF, 10 µg/ml aprotinin, 10 µg/ml leupeptin, 10 nM okadaic acid, 100 µM Na3VO4, and 1 µM microcystin-LR] and homogenized by being passed several times through a 27-gauge needle. Total protein content was determined using the Bio-Rad DC protein assay.
Western blot analysis. Protein homogenates were mixed 1:1 with 2x Laemmli sample buffer for SDS-PAGE and Western blot analysis. Primary antibody dilutions were for Rb (1:500 dilution), UBF (1:5,000 dilution), cyclin D1 (1:2,000 dilution), p21 (1:1,000 dilution), and CDK-4 (1:500 dilution), and rpS6, PO4-rpS6, ERK, and PO4-ERK (1:1,000 dilution). After being incubated with primary antibodies, membranes were washed three times with Tris-buffered saline with Tween 20 and incubated with the corresponding anti-rabbit or anti-mouse secondary antibodies conjugated to horseradish peroxidase (1:10,000 dilution). Protein immunoblots were visualized using ECL, and bands were quantified by performing scanning densitometry.
Analysis of Rb-UBF interaction.
Myotubes were lysed in 500 µl of immunoprecipitation buffer (50 mM HEPES-NaOH, pH 7.5, 0.1% NP-40, 150 mM NaCl2, 1 mM EDTA, 2.5 mM EGTA, 10% glycerol, 50 mM -glycerophosphate, 200 mM PMSF, 10 µg/ml aprotinin, 10 µg/ml leupeptin, 10 nM okadaic acid, and 100 µM sodium orthovanadate) and homogenized by being passed through a 21-gauge needle. Homogenates were rotated for 30 min at 4°C and spun at 10,000 g for 10 min, and protein content was determined. After centrifugation, 500 µg of protein were precleared for 1 h with 50 µl of 50% protein Sepharose A slurry. After preclearing, 5 µg of anti-Rb antibody were added and incubated overnight with gentle rotation at 4°C. The antigen-antibody complex was precipitated by adding 50 µl of protein A beads with rotation for 30 min at 4°C and spun at 12,000 g for 20 s. Pellets were washed three times with immunoprecipitation buffer, once with 50 mM Tris-Cl2, pH 8.0, boiled in 2x Laemmli sample buffer, and resolved using SDS-PAGE as described above. A negative control with a nonspecific antibody was prepared in each individual experiment.
Analysis of CDK-4 activity. CDK-4 activity was determined as described by Pestov et al. (33). Myotubes were lysed, and 500 µg of lysate were precleared for 1 h with 50 µl of 50% protein Sepharose G slurry. After preclearing, 2 µg of anti-CDK-4 antibody were added and incubated for 2.5 h with gentle rotation at 4°C. The antigen-antibody complex was precipitated by adding 50 µl of protein G beads, rotated for 30 min at 4°C, and spun at 12,000 g for 20 s, and then the pellets were washed three times with 1 ml of kinase buffer (Cell Signaling Technology). The kinase reaction was performed by incubating the immune complex in 30 µl of kinase buffer with the addition of 1 mM ATP and 2 µg of maltose-binding protein Rb fusion protein (amino acids 701928) as a substrate. After being incubated for 10 min at 30°C, the reaction was stopped by boiling the sample in 30 µl of 2x Laemmli sample buffer (25). Reaction products were resolved by performing SDS-PAGE (12%), and CDK-4 activity was determined using Western blot analysis with a phosphospecific anti-Rb antibody (1:2,000 dilution) that detects Rb phosphorylation only at Ser780 and does not cross react with any other phosphorylation site in Rb. A negative control with a nonspecific antibody was prepared in each individual experiment.
Rapamycin treatment. Rapamycin was resuspended in absolute ethanol and used at a final concentration of 5 ng/ml. Before being incubated with rapamycin, myotubes were washed twice with PBS and incubated with serum-free media, rapamycin or medium, and vehicle for 30 min. Myotubes were then treated for 24 h under one of the following four conditions: DM plus vehicle, DM plus rapamycin, GM plus vehicle, or GM plus rapamycin.
BrdU labeling and confocal microscopy.
For BrdU visualization, both myoblasts and myotubes were grown on collagen-coated glass plates. BrdU labeling (10 µM) of myotubes was performed for 1 h after 24 h of serum stimulation. As a positive control, asynchronously proliferating myoblasts at 5060% confluence were incubated for 1 h with BrdU after 24 h of serum stimulation. After being incubated with BrdU, cells were washed with ice-cold PBS followed by fixation in 15 mM glycine-methanol solution for 20 min at 20°C. Fixed cells were incubated with working solution for 30 min at 37°C, incubated with an anti-BrdU mouse antibody for 30 min at 37°C, allowed to dry at room temperature, and then covered with VectaShield medium containing 1.5 µg/ml 4',6'-diamidino-2-phenylindole HCl (DAPI). Visualization was performed using a multichannel Zeiss LSM-510 confocal microscope.
Immunohistochemical localization of cyclin D1. Localization of cyclin D1 expression was performed as previously described (43). Briefly, myotubes were grown as described above and coimmunostained for cyclin D1, MHC, and nuclei (DAPI staining). Cultured cells were fixed in 4% paraformaldehyde, permeabilized with 0.5% Triton X-100, blocked, and incubated overnight at 4°C with antibodies against cyclin D1 (1:50 dilution) and MHC (1:50 dilution). Cells were then washed and incubated with mouse anti-rabbit rhodamine-conjugated (tetramethylrhodamine isothiocyanate; 1:200 dilution) and donkey anti-goat FITC (1:200 dilution) secondary antibodies for cyclin D1 and MHC, respectively. Nuclei were stained using DAPI, and immunoreactivity was visualized using a Nikon LabShot-2 fluorescence microscope with Spot RT imaging software (Diagnostic Imaging, Sterling Heights, MI).
Statistical analysis. All data are expressed as means ± SE. Statistical differences between groups were determined using two-way ANOVA, and significant differences between groups identified using Tukey's honestly significant difference post hoc test. Statistical significance was established at P < 0.05. All data were analyzed using SigmaStat software (version 2.03; SPSS, Chicago, IL).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Serum stimulation of terminally differentiated L6 myotubes resulted in myotube hypertrophy as evidenced by an increase in protein mass without detectable changes in DNA content or BrdU uptake. This finding is consistent with previous reports in which stimulation of myotubes with IGF-1 induced an increase in myotube diameter without an increase in DNA synthesis as measured by [3H]thymidine uptake (36, 38), and it indicates that hypertrophy of differentiated myotubes does not require an increase in DNA accumulation.
As demonstrated during in vivo skeletal muscle hypertrophy (1), myotube hypertrophy was associated with a significant increase total RNA levels. Because >85% of total cellular RNA is rRNA, the increase in RNA levels represents an increase in ribosomal RNA. This finding can be interpreted as an increase in ribosome content, because previous studies have shown that ribosome biogenesis is regulated in part by an increase in rRNA synthesis (6, 8, 11). The relative contribution of rRNA synthesis and degradation to rRNA accumulation has been studied during cardiomyocyte hypertrophy (27, 28). The results from these studies showed that rRNA accumulation resulted from accelerated rates of rRNA synthesis because assessment of 45S rDNA transcription using nuclear run-on analysis revealed that the increase in rRNA could be accounted for by increased transcriptional activity (27, 28). Therefore, assuming similarities between models of striated muscle cell growth, the increase in total RNA observed during serum-induced myotube hypertrophy is likely a consequence of increased rates of rDNA transcription.
Consistent with increased rDNA transcription, the results from this study show an increased availability of the rDNA transcription factor UBF at times associated with increased total RNA levels. This finding is consistent with other findings reported for cardiomyocyte hypertrophy in which an increase in rDNA transcription by RNA Pol I was modulated by an increase in UBF availability (1416) or by changes in UBF phosphorylation (44, 45). Increased amounts of free UBF can be achieved by increasing UBF protein content or by increasing its availability via hyperphosphorylation of Rb. In the present study, UBF protein content did not increase in parallel with the increase in rRNA. However, increases in Rb phosphorylation were detected and were associated with an increase in UBF availability. This suggests that in myotubes, the increase in RNA is in part a result of an increase in rDNA transcription via UBF availability.
In the absence of cellular proliferation, serum-stimulated myotube hypertrophy was associated with a significant increase in cyclin D1 protein expression and an increase in cyclin D1-dependent CDK-4 activity. In agreement with previous studies, the increase in cyclin D1 protein expression was detected in the nuclei of myotubes, indicating that reexpression of this cell cycle gene can occur in the multinucleated myotube and is not limited to unfused myoblasts (20). Coimmunoprecipitation analysis of CDK-4 demonstrated that both cyclin D1 and p21 were functionally associated with CDK-4. Even though p21 has been viewed as a CDK inhibitor (39, 40), the identification of p21 in the CDK-4 complex was not surprising, because recent evidence has suggested that p21 is required for cyclin D1/CDK-4 activity (7). Fibroblasts lacking p21 are unable to form active cyclin D1/CDK-4 complexes, and p21 has been found to remain bound to active cyclin/CDK complexes in proliferating cells (47), indicating that the functional interaction between p21 and cyclin D1/CDK-4 complexes is required for their kinase activity.
An association between cell cycle genes and cellular hypertrophy has previously been demonstrated in cardiomyocytes (31, 42). For example, stimulation of cardiomyocytes with serum or ANG II resulted in cellular hypertrophy, upregulation of various G1 phase cyclin/CDK complexes, and increased kinase activity toward Rb even in the absence of DNA synthesis (31, 42). In animal and human models of skeletal muscle hypertrophy, increases in cyclin D mRNA and other cell cycle regulators have been shown (1, 2, 18, 19). These reports, together with the data in the present study, provide strong evidence for a role of cell cycle genes in the growth process of terminally differentiated myotubes/muscle fibers. Although in vitro the cell cycle genes likely function in the modulation of protein synthetic regulation, the in vivo expression of cell cycle regulators likely contribute to both hypertrophy and proliferation on the basis of localization studies (18, 19).
Inhibition of serum-stimulated increases in RNA by rapamycin suggests that ribosomal biogenesis is one critical function regulated by mTOR in skeletal muscle. This conclusion in myotubes is consistent with the findings reported in other studies because rapamycin has been shown to 1) prevent the increase in muscle mass in response to muscle overload in both skeletal and cardiac muscles (3, 17, 41), 2) inhibit ribosome biogenesis in yeast (34), and 3) modulate RNA levels during cardiomyocyte hypertrophy (4).
On the basis of the results of the present study, a model is proposed for the increased protein synthetic changes that occur during skeletal muscle hypertrophy (Fig. 6). In this model, activation of mTOR leads to an increase in the translational efficiency of cyclin D1 mRNA, resulting in increased cyclin D1 content and an increase in cyclin D1-dependent CDK-4 activity. These changes lead to the phosphorylation of Rb with a concomitant increase in UBF availability, resulting in enhanced rDNA transcription and the accumulation of rRNA. The activity of mTOR also results in increased S6K-1 activity that in turn modulates an increased translation of ribosomal protein mRNA and mRNA associated with the regulation of translation, such as elongation factor-1 (21, 22, 24, 35). Consequently, the increase rRNA and ribosomal protein content results in increased ribosomal biogenesis, which supports the protein synthetic demands of the growing cell.
|
![]() |
GRANTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
ACKNOWLEDGMENTS |
---|
Present address of T. J. McLoughlin: Department of Kinesiology, MS 201, The University of Toledo, 2801 W. Bancroft St., Toledo, OH 43606 (e-mail: tmcloug@Utnet.utoledo.edu).
![]() |
FOOTNOTES |
---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
2. Bamman MM, Ragan RC, Kim J, Cross JM, Hill VJ, Tuggle SC, and Allman RM. Myogenic protein expression before and after resistance loading in 26- and 64-yr-old men and women. J Appl Physiol 97: 13291337, 2004.
3. Bodine SC, Stitt TN, Gonzalez M, Kline WO, Stover GL, Bauerlein R, Zlotchenko E, Scrimgeour A, Lawrence JC, Glass DJ, and Yancopoulos GD. Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo. Nat Cell Biol 3: 10141019, 2001.[CrossRef][ISI][Medline]
4. Boluyt MO, Zheng JS, Younes A, Long X, O'Neill L, Silverman H, Lakatta EG, and Crow MT. Rapamycin inhibits 1-adrenergic receptor-stimulated cardiac myocyte hypertrophy but not activation of hypertrophy-associated genes: evidence for involvement of p70 S6 kinase. Circ Res 81: 176186, 1997.
5. Brown EJ, Beal PA, Keith CT, Chen J, Shin TB, and Schreiber SL. Control of p70 s6 kinase by kinase activity of FRAP in vivo. Nature 377: 441446, 1995.[CrossRef][ISI][Medline]
6. Camacho JA, Peterson CJ, White GJ, and Morgan HE. Accelerated ribosome formation and growth in neonatal pig hearts. Am J Physiol Cell Physiol 258: C86C91, 1990.
7. Cheng M, Olivier P, Diehl JA, Fero M, Roussel MF, Roberts JM, and Sherr CJ. The p21Cip1 and p27Kip1 CDK "inhibitors" are essential activators of cyclin D-dependent kinases in murine fibroblasts. EMBO J 18: 15711583, 1999.
8. Chua BH, Russo LA, Gordon EE, Kleinhans BJ, and Morgan HE. Faster ribosome synthesis induced by elevated aortic pressure in rat heart. Am J Physiol Cell Physiol 252: C323C327, 1987.[Abstract]
9. Dennis PB, Fumagalli S, and Thomas G. Target of rapamycin (TOR): balancing the opposing forces of protein synthesis and degradation. Curr Opin Genet Dev 9: 4954, 1999.[CrossRef][ISI][Medline]
10. Fingar DC and Blenis J. Target of rapamycin (TOR): an integrator of nutrient and growth factor signals and coordinator of cell growth and cell cycle progression. Oncogene 23: 31513171, 2004.[CrossRef][ISI][Medline]
11. Goldspink DF, Cox VM, Smith SK, Eaves LA, Osbaldeston NJ, Lee DM, and Mantle D. Muscle growth in response to mechanical stimuli. Am J Physiol Endocrinol Metab 268: E288E297, 1995.
12. Hannan KM, Brandenburger Y, Jenkins A, Sharkey K, Cavanaugh A, Rothblum L, Moss T, Poortinga G, McArthur GA, Pearson RB, and Hannan RD. mTOR-dependent regulation of ribosomal gene transcription requires S6K1 and is mediated by phosphorylation of the carboxy-terminal activation domain of the nucleolar transcription factor UBF. Mol Cell Biol 23: 88628877, 2003.
13. Hannan RD, Taylor L, Cavanaugh A, Hannan K, and Rothblum LI. UBF and the regulation of ribosomal DNA transcription. In: Transcription of Ribosomal RNA Genes by Eukaryotic RNA Polymerase I, edited by Paule MR. Berlin, Germany: Springer-Verlag, 1998, p. 221232.
14. Hannan RD, Luyken J, and Rothblum LI. Regulation of rDNA transcription factors during cardiomyocyte hypertrophy induced by adrenergic agents. J Biol Chem 270: 82908297, 1995.
15. Hannan RD, Luyken J, and Rothblum LI. Regulation of ribosomal DNA transcription during contraction-induced hypertrophy of neonatal cardiomyocytes. J Biol Chem 271: 32133220, 1996.
16. Hannan RD and Rothblum LI. Regulation of ribosomal DNA transcription during neonatal cardiomyocyte hypertrophy. Cardiovasc Res 30: 501510, 1995.[CrossRef][ISI][Medline]
17. Hornberger TA, McLoughlin TJ, Leszczynski JK, Armstrong DD, Jameson RR, Bowen PE, Hwang ES, Hou H, Moustafa ME, Carlson BA, Hatfield DL, Diamond AM, and Esser KA. Selenoprotein-deficient transgenic mice exhibit enhanced exercise-induced muscle growth. J Nutr 133: 30913097, 2003.
18. Ishido M, Kami K, and Masuhara M. In vivo expression patterns of MyoD, p21, and Rb proteins in myonuclei and satellite cells of denervated rat skeletal muscle. Am J Physiol Cell Physiol 287: C484C493, 2004.
19. Ishido M, Kami K, and Masuhara M. Localization of MyoD, myogenin and cell cycle regulatory factors in hypertrophying rat skeletal muscles. Acta Physiol Scand 180: 281289, 2004.[CrossRef][ISI][Medline]
20. Jahn L, Sadoshima J, and Izumo S. Cyclins and cyclin-dependent kinases are differentially regulated during terminal differentiation of C2C12 muscle cells. Exp Cell Res 212: 297307, 1994.[CrossRef][ISI][Medline]
21. Jefferies HB, Fumagalli S, Dennis PB, Reinhard C, Pearson RB, and Thomas G. Rapamycin suppresses 5' TOP mRNA translation through inhibition of p70s6k. EMBO J 16: 36933704, 1997.
22. Jefferies HB, Reinhard C, Kozma SC, and Thomas G. Rapamycin selectively represses translation of the "polypyrimidine tract" mRNA family. Proc Natl Acad Sci USA 91: 44414445, 1994.
23. Kihm AJ, Hershey JC, Haystead TA, Madsen CS, and Owens GK. Phosphorylation of the rRNA transcription factor upstream binding factor promotes its association with TATA binding protein. Proc Natl Acad Sci USA 95: 1481614820, 1998.
24. Kubica N, Bolster DR, Farrell PA, Kimball SR, and Jefferson LS. Resistance exercise increases muscle protein synthesis and translation of eukaryotic initiation factor 2B mRNA in a mammalian target of rapamycin-dependent manner. J Biol Chem 280: 75707580, 2005.
25. Laemmli UK. Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature 227: 680685, 1970.[ISI][Medline]
26. Mahajan PB. Modulation of transcription of rRNA genes by rapamycin. Int J Immunopharmacol 16: 711721, 1994.[CrossRef][ISI][Medline]
27. McDermott PJ, Carl LL, Conner KJ, and Allo SN. Transcriptional regulation of ribosomal RNA synthesis during growth of cardiac myocytes in culture. J Biol Chem 266: 44094416, 1991.
28. McDermott PJ, Rothblum LI, Smith SD, and Morgan HE. Accelerated rates of ribosomal RNA synthesis during growth of contracting heart cells in culture. J Biol Chem 264: 1822018227, 1989.
29. Mita MM, Mita A, and Rowinsky EK. The molecular target of rapamycin (mTOR) as a therapeutic target against cancer. Cancer Biol Ther 2, Suppl 1: S169S177, 2003.[ISI][Medline]
30. Moss T and Stefanovsky VY. Promotion and regulation of ribosomal transcription in eukaryotes by RNA polymerase I. Prog Nucleic Acid Res Mol Biol 50: 2566, 1995.[ISI][Medline]
31. Nozato T, Ito H, Tamamori M, Adachi S, Abe S, Marumo F, and Hiroe M. G1 cyclins are involved in the mechanism of cardiac myocyte hypertrophy induced by angiotensin II. Jpn Circ J 64: 595601, 2000.[CrossRef][ISI][Medline]
32. Paule MR. Transcription of Ribosomal RNA Genes by Eukaryotic RNA polymerase I. Berlin, Germany: Springer-Verlag, 1998.
33. Pestov DG, Strezoska Z, and Lau LF. Evidence of p53-dependent cross-talk between ribosome biogenesis and the cell cycle: effects of nucleolar protein Bop1 on G1/S transition. Mol Cell Biol 21: 42464255, 2001.
34. Powers T and Walter P. Regulation of ribosome biogenesis by the rapamycin-sensitive TOR-signaling pathway in Saccharomyces cerevisiae. Mol Biol Cell 10: 9871000, 1999.
35. Reiter AK, Anthony TG, Anthony JC, Jefferson LS, and Kimball SR. The mTOR signaling pathway mediates control of ribosomal protein mRNA translation in rat liver. Int J Biochem Cell Biol 36: 21692179, 2004.[CrossRef][ISI][Medline]
36. Rommel C, Bodine SC, Clarke BA, Rossman R, Nunez L, Stitt TN, Yancopoulos GD, and Glass DJ. Mediation of IGF-1-induced skeletal myotube hypertrophy by PI(3)K/Akt/mTOR and PI(3)K/Akt/GSK3 pathways. Nat Cell Biol 3: 10091013, 2001.[CrossRef][ISI][Medline]
37. Schmelzle T and Hall MN. TOR, a central controller of cell growth. Cell 103: 253262, 2000.[CrossRef][ISI][Medline]
38. Semsarian C, Sutrave P, Richmond DR, and Graham RM. Insulin-like growth factor (IGF-I) induces myotube hypertrophy associated with an increase in anaerobic glycolysis in a clonal skeletal-muscle cell model. Biochem J 339: 443451, 1999.[CrossRef][ISI][Medline]
39. Sherr CJ, Kato J, Quelle DE, Matsuoka M, and Roussel MF. D-type cyclins and their cyclin-dependent kinases: G1 phase integrators of the mitogenic response. Cold Spring Harb Symp Quant Biol 59: 1119, 1994.[ISI][Medline]
40. Sherr CJ and Roberts JM. Inhibitors of mammalian G1 cyclin-dependent kinases. Genes Dev 9: 11491163, 1995.[CrossRef][ISI][Medline]
41. Shioi T, McMullen JR, Tarnavski O, Converso K, Sherwood MC, Manning WJ, and Izumo S. Rapamycin attenuates load-induced cardiac hypertrophy in mice. Circulation 107: 16641670, 2003.
42. Tamamori M, Ito H, Hiroe M, Terada Y, Marumo F, and Ikeda MA. Essential roles for G1 cyclin-dependent kinase activity in development of cardiomyocyte hypertrophy. Am J Physiol Heart Circ Physiol 275: H2036H2040, 1998.
43. Torgan CE and Daniels MP. Regulation of myosin heavy chain expression during rat skeletal muscle development in vitro. Mol Biol Cell 12: 14991508, 2001.
44. Voit R and Grummt I. Phosphorylation of UBF at serine 388 is required for interaction with RNA polymerase I and activation of rDNA transcription. Proc Natl Acad Sci USA 98: 1363113636, 2001.
45. Voit R, Schäfer K, and Grummt I. Mechanism of repression of RNA polymerase I transcription by the retinoblastoma protein. Mol Cell Biol 17: 42304237, 1997.[Abstract]
46. Yamasaki L. Role of the RB tumor suppressor in cancer. Cancer Treat Res 115: 209239, 2003.[Medline]
47. Zhang H, Hannon GJ, and Beach D. p21-containing cyclin kinases exist in both active and inactive states. Genes Dev 8: 17501758, 1994.[Abstract]
|
HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
Visit Other APS Journals Online |