Our results suggest a model whereby ERK1/2 signaling is necessary to prevent eccentric cardiomyocyte growth. Indeed, Erk1/2 double null ventricular myocytes lengthen and become slightly thinner with aging and pathologic stimulation to cause whole organ eccentric growth. Antithetically, activation of ERK1/2 with aMek1 preferentially programmed concentric cardiac growth while at the same time partially inhibiting eccentric growth. These results suggest a refined hypothesis whereby ERK1/2 are not technically necessary for whole organ hypertrophy, as measured by heart weight normalized to body weight, but are necessary for promoting a properly coordinated growth response that also allows myocyte thickening and addition of sarcomeres at the periphery (). Thus, ERK1/2 signaling is necessary for facilitating a select type of cardiac growth in vivo, concentric hypertrophy, while at the same time preventing eccentric growth and addition of sarcomeres in series ().
Model of eccentric versus concentric myocyte growth due to MEK1-ERK1/2 signaling
The model of growth proposed above is also consistent with previous work in which we inhibited the ERK1/2 pathway in vivo
using a transgene to overexpress Dusp6 in the heart.12
Dusp6 transgenic mice showed complete loss of ERK1/2 phosphorylation and activity, although the cardiac hypertrophic response was not reduced following TAC or agonist infusion.12
In fact, Dusp6 transgenic mice subjected to long-term TAC showed even larger increases in heart weights compared with control mice, an increase that was associated with chamber dilation suggesting greater eccentric growth. Thus, while we had not previously measured cellular lengths and widths in Dusp6 transgenic hearts, their increased propensity towards dilation with pathologic stimulation suggests a similar mechanism in play, whereby loss of ERK1/2 signaling promotes cardiomyocyte lengthening.
To our knowledge the MEK1-ERK1/2 signaling pathway appears to be unique in its ability to regulate both the length and width decision of a myocyte with aging or stress stimulation. Previously, activation of the MEK5-ERK5 signaling pathway was reported to produce eccentric cardiac hypertrophy by lengthening myocytes, although the reciprocal relationship of increased width was not examined in Mek5 or Erk5 gene-deleted mice.23
Moreover, many transgenic mouse models of cardiomyopathy and heart failure lead to spontaneous dilation with myocyte lengthening, such as with myocyte enhancer factor-2 (MEF2) overexpression.24–26
The data we present with the MEK1-ERK1/2 pathway are unique because activation of this pathway leads to myocyte thickening, while loss leads to myocyte lengthening without cellular dysfunction, suggesting it is a primary effect. Moreover, growth of adult myocytes in culture over 3 days showed the exact same relationship, and even myocytes from beating and cyclically stretched EHTs showed the same effect. Since nearly all other transgenic models of heart failure appear to cause secondary dilation, we believe that the MEK1 effect in driving concentric hypertrophy is highly unique and suggestive a true biologic signaling pathway whose role is to program myocyte thickening.
Ventricular dilation can result from either myocardial cell elongation through an addition of contractile-protein units in series, or it can also result from myocyte slipage and changes in wall architecture.27
Either event, or a mixture therein, changes the geometry of the ventricle by increasing the internal radius and thinning of the free wall, resulting in greater wall tension and secondary neuroendocrine stress signaling.28
In fact, according to Laplace’s law, our echocardiographic measurements in hearts from Erk1−/−
mice suggest an almost doubled load at the end of diastole as compared with control mice. This significantly increased load could explain the reduced FS observed in hearts from Erk1−/−
mice, leading to secondary induction of hypertrophic marker gene expression, fibrosis, and mild desensitization to β-adrenergic receptor agonists. Indeed, volume overload induced eccentric hypertrophy due to mitral regurgitation, can also lead to mechanical dysfunction.29
However, this reduction in whole organ function and secondary disease manifestations are unlikely the result of a primary defect in myocyte contractility in the absence of ERK1/2, as adult myocytes from these hearts showed even better functional performance in isolation. Such results from Erk1/2 deleted adult myocytes are in stark contrast to the observed slowing of contraction in unloaded myocytes from other models of heart failure.30
Such a dramatic increase in series sarcomeres without matching concentric growth of sarcomeres on the periphery of the fibers likely promotes whole organ dysfunction through dilation and dramatically increased wall tension with less myocyte-myocyte connectivity.
Concentric hypertrophy usually results from pressure overload, while eccentric hypertrophy usually results from situations of volume overload or forms of exercise. However, almost all hypertrophic stimuli appear to activate and induce phosphorylation of ERK1/2.7
Together these observations may suggest two divergent points of view. The more simplistic view suggests that the ERK pathway is activated in all types of hypertrophy to program concentric growth of myocytes. Therefore, in pressure overload the MEK1-ERK1/2 pathway would work with other signaling pathways to add sarcomeres in parallel and increase the width of the cells, while in volume overload situations the ERK pathway would work against the machinery that drives the cell towards elongation. Alternatively, it is possible that volume overload does not lead to continual recruitment and activation of ERK1/2 in the myocyte, or more provocatively, could even lead to mild inhibition of ERK1/2 to permit preferential addition of sarcomeres in series. Unfortunately, the relationship between types of cardiac growth (concentric vs eccentric) and the associated profile of ERK1/2 activation has not been carefully mapped in vivo at multiple time points, although Toischer et al recently reported ERK1/2 activation at 24 hrs of pressure overload hypertrophy (with concentric myocyte growth) but not with an eccentric hypertrophy promoting shunting procedure.31
The precise intracellular targets that are phosphorylated by ERK1/2 to mediate concentric growth and suppress eccentric growth have not been identified. In theory, ERK1/2 might phosphorylate one or more regulatory proteins that control sarcomeric assembly, such that ERK1/2 activation inhibits the construction of series units while assembly of sarcomeres in parallel is enhanced. In addition to affecting assembly of sarcomeres, ERK1/2 might regulate the site of new protein synthesis within the sarcomeres, at either the periphery of a fiber area or within the middle of a fiber to produce lengthening (). In addition to acute regulation of sarcomerogenesis, ERK1/2 activation might affect the type of hypertrophy through a transcriptional mechanism that secondarily affects how myocytes grow. ERK1/2 might also impact other signaling pathways that control other aspects of cardiomyocyte growth, although analysis AKT, GSK3β, JAK/STAT and calcineurin did not reveal differences between control, Mek1 transgenic or Erk1−/−
mice (Online Figure VI
Interestingly, autophosphorylation of ERK1/2 on Thr188, which directs ERK1/2 to phosphorylate nuclear targets, did not affect the growth response32
suggesting that MEK1-ERK1/2 signaling within the cytoplasm may be more germane in controlling the type of growth that occurs. Indeed, ERK1/2 are known to directly phosphorylate the cytoplasmic ribosomoal proteins p90 ribosomal S6 kinase (p90RSK) and p70S6K that could control protein synthesis.33
Clearly, additional studies are needed to identify the ERK1/2 effectors in hypertrophy and study them through gain- and loss- of function approaches in vivo
to assess their roles in concentric and eccentric hypertrophic growth.
Novelty and significance
What is known?
- An increase in afterload or preload induces cardiac concentric and eccentric hypertrophy, respectively
- The extracellular signal-regulated kinases (ERK) pathway is activated by stimuli inducing concentric hypertrophy
- Constitutive activation of the ERK pathway in the heart results in concentric hypertrophy
What new information does this article contribute?
- Increases in MEK1-ERK1/2 signaling directly programs myocyte thickening in vitro and in vivo.
- Decreased or absent ERK1/2 signaling induces myocyte lengthening and eccentric hypertrophic growth.
- Mice lacking ERK1/2 from the heart show eccentric cardiac growth at baseline and with stimulation