This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Sugden, P. H. Search for Related Content PubMed PubMed Citation Articles by Sugden, P. H.

(Circulation. 2001;103:1375.)
© 2001 American Heart Association, Inc.

Editorials

Mechanotransduction in Cardiomyocyte Hypertrophy

Peter H. Sugden, DPhil

From the National Heart and Lung Institute Division, Imperial College School of Medicine, London, UK.

Correspondence to Peter H. Sugden, DPhil, NHLI Division (Cardiac Medicine), Imperial College School of Medicine, Dovehouse St, London SW3 6LY, UK. E-mail p.sugden{at}ic.ac.uk

Key Words: Editorials • hypertrophy • myocytes • signal transduction

The study of ventricular hypertrophy has recently been a fruitful research area. From studies principally in neonatal rat ventricular myocytes in primary culture but now increasingly in transgenic mice, the molecular mechanisms involved in this important pathophysiological adaptation are becoming clearer.¹ In culture, cardiomyocyte hypertrophy is characterized by numerous transcriptional and morphological changes, including increased expression of atrial and B-type natriuretic peptides (ANP and BNP, respectively), increased ß-myosin heavy chain (ß-MHC) expression, and increased sarcomere deposition.¹ Similar changes are evident in hypertrophied hearts in vivo, where increased sarcomere deposition in particular allows cardiomyocytes to accommodate demands for elevated contractile power.

Wall Strain and Left Ventricular Hypertrophy

In vivo, pathological or experimental left ventricular hemodynamic overload induces patterns of hypertrophy that can be broadly divided into concentric or eccentric. In concentric (pressure-overload) hypertrophy, peak-systolic and end-diastolic pressures are increased, but meridional ventricular wall stress is normal throughout the cardiac cycle, the additional wall stress being counterbalanced by wall thickening in the absence of chamber enlargement. In eccentric (volume-overload) hypertrophy, chamber volume is enlarged, and wall thickness is increased approximately in proportion to the increase in chamber diameter. End-diastolic pressure and wall stress are increased, but peak systolic pressure and wall stress are normal.

The orientation of cardiomyocytes in heart is primarily circumferential. Thus, cardiomyocytes from pressure- or volume-overloaded hearts differ in their relative dimensions. Cross-sectional area is proportionally expanded over length in pressure overload, whereas length is proportionally increased in volume overload. Sarcomere deposition parallel to the long axes of cells is greater in pressure overload than in volume overload, in which deposition of sarcomeres in series predominates. Changes in gene and protein expression in vivo may not be identical in the 2 paradigms. One study showed that although increases in prepro-ANP mRNA are comparable, increases in ß-MHC mRNA occur only in pressure overload² (although others, using protein expression, have detected increases in ß-MHC in both situations³ ).

Cardiomyocyte culture models have provided little information about mechanisms underlying development of these distinct cellular morphologies or the signaling pathways responsible for the differences. The -adrenergic agonist phenylephrine, which acts through G protein–coupled receptors (GPCRs), induces hypertrophy reminiscent of a pressure-overloaded cardiomyocyte, whereas the cytokine-related agonist cardiotrophin-1 produces a volume-overloaded morphology.⁴ To gain information about the differing responses to pressure overload and volume overload, Yamamoto et al,⁵ in this issue of Circulation, use cultured cardiomyocytes to focus on differences in intracellular signaling and gene expression induced by strain imposed specifically at the systolic and diastolic phases of the contractile cycle.

Signal Transduction in Cardiac Hypertrophy

Reversible protein phosphorylation and/or dephosphorylation play a central role in signal transduction in cardiomyocyte hypertrophy. Multiple signaling pathways probably mediate hypertrophy (particularly in vivo), with 2 important groups of regulatory proteins having been identified: the mitogen-activated protein kinases (MAPKs)¹ and Ca²⁺/calmodulin-activated protein kinases/phosphatases.^{1 6 7} The latter include Ca²⁺/calmodulin-dependent protein kinases (CaM kinases) and calcineurin, but Yamamoto et al⁵ focused only on MAPK signaling. MAPKs are final members of 3-component protein kinase cascades in which a MAPK kinase kinase (MKKK) phosphorylates and activates a MAPK kinase (MKK), which finally phosphorylates and activates a MAPK.⁸ After their activation, MAPKs phosphorylate other signaling proteins (notably transcription factors and other protein kinases) to change their biological activities. Through these phosphorylations, MAPKs regulate cell growth and survival and other biological processes.

The simplest subclassification of MAPKs into 3 subfamilies [extracellular-signal–regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38-MAPKs] is based on the primary sequence of their phosphorylation motifs. p42/p44-MAPKs (ERK2 and ERK1, respectively) were the first to be identified and remain the best characterized, being strongly stimulated by growth-promoting stimuli in all cells. In cardiomyocytes, they are powerfully activated by hypertrophic agonists that couple to GPCRs of the G_q-coupled subclass.¹ These include phenylephrine, endothelin-1 (ET-1), and angiotensin II (Ang II). The linkage between G_qPCR activation and stimulation of the p42/p44-MAPK cascade is unclear. It probably involves the phospholipase Cß/protein kinase C pathway and activation of the 21-kDa GTPase Ras.¹ This last step leads to activation of the Raf MKKKs, thence to MEK1/2 (the MKKKs and MKKs of the p42/p44-MAPK cascade) and p42/p44-MAPK.^{1 8} Three other ERK subfamily members have been identified, the poorly characterized ERK3, ERK5 [or "big" (100-kDa) MAPK, BMK1], and the recently identified ERK7. PD98059 and U0126 inhibit activation of p42/p44-MAPK (and BMK1) by inhibiting at the MKKK/MKK levels and have proved useful in identifying ERK-dependent responses.

JNKs and p38-MAPKs are more strongly activated by cytotoxic cellular stresses (reactive oxygen species, hyperosmotic stress, anisomycin, ischemia/reperfusion) than by ET-1 or phenylephrine in cardiomyocytes or isolated hearts.⁹ In humans, mRNAs transcribed from 3 JNK genes are alternatively spliced, producing 10 JNK isoforms with molecular masses of either 46 or 54 kDa. p38-MAPKs (, ß, , and isoforms) are encoded by 4 genes, with the - and ß-isoforms each being alternatively spliced. SB203580 (<1 µmol/mL) directly and reversibly inhibits activated p38-MAPK/ß2 (but does not inhibit p38-MAPK/) and is useful in implicating p38-MAPK/ß2 in biological processes. Although this is still controversial, all 3 MAPK subfamilies have been associated with hypertrophic signaling,¹ although a view exists that JNKs and p38-MAPK may be detrimental to cardiomyocyte survival.¹ I have emphasized the primacy of p42/p44-MAPK activation in compensated hypertrophy, with JNKs and p38-MAPKs possibly more involved in development/maintenance of decompensated states. Recent work in transgenic mice supports this.¹⁰

Mechanical Strain and Signal Transduction

Although the terms "stress" and "strain" are used interchangeably, they have different physical meanings. Stress is a force that induces strain, which is the change in shape or the deformation produced. Static or intermittent stress/strain of cultured cardiomyocytes on deformable Silastic membranes induces many of the transcriptional and signaling responses of hypertrophy (including activation of MAPKs).^{1 11} Such findings may be relevant to mechanotransduction (the coupling of mechanical forces on cells to biological responses) in hemodynamic overload in vivo. Yamamoto et al⁵ refined the mechanical deformational model to examine differences in intracellular signaling and gene expression induced by spatially isotropic (ie, uniform) biaxial strain imposed at the systolic (see pressure overload in vivo) or diastolic (see volume overload) phase of the contractile cycle in electrically paced cardiomyocytes. Control experiments involved strain/no pacing or pacing/no strain. (Note that these strained/unpaced cardiomyocytes still contract spontaneously but at a lower frequency than paced cells. Contraction is therefore out of phase with strain.)

Using antibodies specific for phosphorylated (activated) MAPKs, Yamamoto et al⁵ showed that systolic strain increased p42/p44-MAPK phosphorylation by up to 10-fold. This increase was significantly greater than those elicited by strain/no pacing or pacing/no strain (2- to 3-fold). The increase in p42/p44-MAPK phosphorylation with diastolic strain was intermediate, being at most 6-fold. Likewise, differential phosphorylation of MEK1/2 was demonstrable. PD98059 completely inhibited systolic strain–induced phosphorylation of p42/p44-MAPK. Although p38-MAPKs and JNKs were also phosphorylated (maximally 3- and 6-fold, respectively) with systolic or diastolic strain, there were no detectable differences in the extent of phosphorylation between the 2 conditions. p38-MAPKs (but not JNKs) were also phosphorylated in strained/unpaced or paced/unstrained cardiomyocytes. These results imply that the p42/p44-MAPK cascade (but not the JNK or p38-MAPK cascade) responds differentially to cardiac cycle phase–specific strains.

BNP mRNA was rapidly induced by systolic strain (6-fold) and diastolic strain (3-fold), although only the former was significantly greater than in strained/unpaced or paced/unstrained cardiomyocytes.⁵ Systolic strain was more effective in inducing mRNA encoding the extracellular matrix protein tenascin C. Although this was not quantified, the authors suggest that BNP mRNA induction by systolic strain was essentially completely inhibited by PD98059. In contrast, SB203580 appeared to be less effective, although the absence of accurate quantification of blots to take into account the 18S rRNA loading control makes this difficult to assess unambiguously. Finally, systolic strain for 1 hour was more effective in increasing leucine incorporation into protein over the subsequent 24 hours than other treatments, although these also significantly stimulated leucine incorporation. Because leucine incorporation was measured over a prolonged period, such incorporation will reflect both the total protein pool (equilibrium labeling) and the initial rate of protein synthesis. Its physical significance (total protein versus initial rate) is therefore difficult to assess. PD98059 (present throughout the incubation) inhibited leucine incorporation by 60%, suggesting that activation of p42/p44-MAPKs may be responsible. Inhibition (30%) by SB203580 was not statistically significant. Because of the possible role of tonic p42/p44-MAPK activity in cell survival, prolonged exposure of cells to PD98059 may reduce viability. Indeed, PD98059 alone increases TUNEL-positivity and DNA laddering in cardiomyocytes.¹² Experiments involving prolonged use of PD98059 should thus be interpreted cautiously.

The overall conclusions of Yamamoto et al⁵ are that systolic strain is more effective than diastolic strain in activating the p42/p44-MAPK cascade and in stimulating expression of BNP and protein synthesis. This may be due to the sensitivity of cardiomyocytes to strain energy. Cell stiffness is greater during systole than diastole, and hence, systolic strain may deliver more strain energy. Such considerations, however, still do not identify the molecular events responsible for the differential responses.

Mechanotransduction-Sensing Mechanisms

Although the sensor mechanisms for strain are unclear, they must be differentially affected by cardiac cycle phase–specific strain to account for the findings of Yamamoto et al.⁵ Information here is sparse. In studies of static strain, use of receptor antagonists suggests the autocrine/paracrine involvement of Ang II or ET-1 and MAPKs.^{11 13} Whenever strain is mentioned, the nebulous entity of the mechanosensitive ion channel is invoked.¹⁴ These are cation-selective channels (but are not particularly selective for individual biological cations), which are often sensitive to inhibition by Gd³⁺ or streptomycin. Hypertrophic changes in statically strained cardiomyocytes, however, are not sensitive to Gd³⁺.¹¹ Strain stimulates release of the prostanoid precursor arachidonate,¹¹ possibly through MAPK-mediated activation of phospholipase A₂. Prostaglandin F₂ is reportedly hypertrophic in cardiomyocytes,¹ but signal transduction here may involve JNKs rather than p42/p44-MAPKs.¹⁵ Extracellular matrix– (integrin-) based signaling could be involved,¹¹ and signaling through Ca²⁺ (which could promote hypertrophy through CaM kinases or calcineurin^{6 7} ) cannot be excluded.

Reactive oxygen species are increasingly recognized to be signaling molecules in their own right (especially at low concentrations), and they certainly activate MAPKs in cardiomyocytes.¹⁶ They inhibit protein synthesis, however, and are demonstrably apoptotic at higher concentrations.^{17 18} The earlier experiments of Yamamoto and colleagues¹⁹ suggested that biaxial strain produces reactive oxygen species, which are responsible for induction of tenascin C mRNA through the transcription factor NF-B. In contrast, induction of BNP mRNA is sensitive to Ang II AT₁ receptor antagonists.¹⁹ Conversely, others have shown that BNP expression in strained cardiomyocytes is regulated by p38-MAPKs and NF-B.²⁰ Whether any of the changes induced by phasic strain are inhibited by antioxidants, calmodulin-directed inhibitors, or receptor and integrin antagonists should be examined. Whether systolic and diastolic strains produce morphologically distinct responses in cardiomyocytes needs to be established before data are extrapolated too widely. If prolonged strain is necessary for such changes to be apparent, this experiment may not be trivial, because cardiomyocytes are unlikely to remain attached to the substrate.

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

1. Sugden PH, Clerk A. Cellular mechanisms of cardiac hypertrophy. J Mol Med. 1998;76:725–746.[Medline] [Order article via Infotrieve]
   
2. Calderone A, Takahashi N, Izzo NJ Jr, et al. Pressure- and volume-induced left ventricular hypertrophies are associated with distinct myocyte phenotypes and differential induction of peptide growth factor mRNAs. Circulation. 1995;92:2385–2390.[Abstract/Free Full Text]
   
3. Dool JS, Mak AS, Friberg P, et al. Regional myosin heavy chain expression in volume and pressure overload induced hypertrophy. Acta Physiol Scand. 1995;155:396–404.[Medline] [Order article via Infotrieve]
   
4. Wollert KC, Taga T, Saito M, et al. Cardiotrophin-1 activates a distinct form of cardiac hypertrophy: assembly of sarcomeric units in series via gp130/leukemia inhibitory factor receptor-dependent pathways. J Biol Chem. 1996;271:9535–9545.[Abstract/Free Full Text]
   
5. Yamamoto K, Dang QN, Maeda Y, et al. Regulation of cardiomyocyte mechanotransduction by the cardiac cycle. Circulation. 2001;103:1459–1464.[Abstract/Free Full Text]
   
6. Molkentin JD, Lu J-R, Antos C, et al. A calcineurin-dependent transcriptional pathway for cardiac hypertrophy. Cell. 1998;93:215–228.[Medline] [Order article via Infotrieve]
   
7. Passier R, Zeng H, Frey N, et al. CaM kinase signaling induces cardiac hypertrophy and activates the MEF2 transcription factor in vivo. J Clin Invest. 2000;105:1395–1406.[Medline] [Order article via Infotrieve]
   
8. Cohen P. The search for physiological substrates of MAP and SAP kinases in mammalian cells. Trends Cell Biol. 1997;7:353–361.
   
9. Sugden PH, Clerk A. "Stress-responsive" mitogen-activated protein kinases in the myocardium. Circ Res. 1998;83:345–352.[Free Full Text]
   
10. Bueno OF, de Windt LJ, Tymitz KM, et al. The MEK1-ERK1/2 signaling pathway promotes compensated hypertrophy in transgenic mice. EMBO J. 2000;19:6341–6350.[Medline] [Order article via Infotrieve]
    
11. Sadoshima J, Izumo S. The cellular and molecular response of cardiac myocytes to mechanical stress. Annu Rev Physiol. 1997;59:551–571.[Medline] [Order article via Infotrieve]
    
12. Aikawa R, Komuro I, Yamazaki T, et al. Oxidative stress activates extracellular signal-regulated kinases through Src and Ras in cultured cardiac myocytes of neonatal rats. J Clin Invest. 1997;100:1813–1821.[Medline] [Order article via Infotrieve]
    
13. Yamazaki T, Komuro I, Kudoh S, et al. Endothelin-1 is involved in mechanical stress-induced cardiomyocyte hypertrophy. J Biol Chem. 1996;271:3221–3228.[Abstract/Free Full Text]
    
14. Hu H, Sachs F. Stretch-activated ion channels in the heart. J Mol Cell Cardiol. 1997;29:1511–1523.[Medline] [Order article via Infotrieve]
    
15. Adams JW, Sah VP, Henderson SA, et al. Tyrosine kinase and c-Jun NH₂-terminal kinase mediate hypertrophic responses to prostaglandin F₂ in cultured neonatal rat ventricular myocytes. Circ Res. 1998;83:167–178.[Abstract/Free Full Text]
    
16. Clerk A, Michael A, Sugden PH. Stimulation of multiple mitogen-activated protein kinase sub-families by oxidative stress and phosphorylation of the small heat shock protein, HSP25/27, in neonatal ventricular myocytes. Biochem J. 1998;333:581–589.
    
17. Cook SA, Sugden PH, Clerk A. Regulation of Bcl-2 family proteins during development and in response to oxidative stress in cardiac myocytes: association with changes in mitochondrial membrane potential. Circ Res. 1999;85:940–949.[Abstract/Free Full Text]
    
18. Pham FH, Sugden PH, Clerk A. Regulation of protein kinase B and 4E-BP1 by oxidative stress in cardiac myocytes. Circ Res. 2000;86:1252–1258.[Abstract/Free Full Text]
    
19. Yamamoto K, Dang QN, Kennedy SP, et al. Induction of tenascin-C in cardiac myocytes by mechanical deformation: role of reactive oxygen species. J Biol Chem. 1999;274:21840–21846.[Abstract/Free Full Text]
    
20. Liang F, Gardner DG. Mechanical strain activates BNP gene transcription through a p38/NF-B-dependent mechanism. J Clin Invest. 1999;104:1603–1612.[Medline] [Order article via Infotrieve]
    

This article has been cited by other articles:

				M. A Frias, S. Somers, C. Gerber-Wicht, L. H Opie, S. Lecour, and U. Lang The PGE2-Stat3 interaction in doxorubicin-induced myocardial apoptosis Cardiovasc Res, July 9, 2008; (2008) cvn171v2. [Abstract] [Full Text] [PDF]

				A. M. D. Gonzalez, J. C. Osorio, C. Manlhiot, D. Gruber, S. Homma, and S. Mital Hypertrophy signaling during peripartum cardiac remodeling Am J Physiol Heart Circ Physiol, November 1, 2007; 293(5): H3008 - H3013. [Abstract] [Full Text] [PDF]

				K. Sekiguchi, X. Li, M. Coker, M. Flesch, P. M Barger, N. Sivasubramanian, and D. L Mann Cross-regulation between the renin-angiotensin system and inflammatory mediators in cardiac hypertrophy and failure Cardiovasc Res, August 15, 2004; 63(3): 433 - 442. [Abstract] [Full Text] [PDF]

				P. A. Modesti, S. Vanni, I. Bertolozzi, I. Cecioni, C. Lumachi, A. M. Perna, M. Boddi, and G. F. Gensini Different Growth Factor Activation in the Right and Left Ventricles in Experimental Volume Overload Hypertension, January 1, 2004; 43(1): 101 - 108. [Abstract] [Full Text] [PDF]

				H. A. Baba, J. Stypmann, F. Grabellus, P. Kirchhof, A. Sokoll, M. Schafers, A. Takeda, M. J. Wilhelm, H. H. Scheld, N. Takeda, et al. Dynamic regulation of MEK/Erks and Akt/GSK-3{beta} in human end-stage heart failure after left ventricular mechanical support: myocardial mechanotransduction-sensitivity as a possible molecular mechanism Cardiovasc Res, August 1, 2003; 59(2): 390 - 399. [Abstract] [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Sugden, P. H. Search for Related Content PubMed PubMed Citation Articles by Sugden, P. H.