A novel inhibitory role for CREG-mediated signalling in cardiac hypertrophy?

Mechanotransduction in cardiac tissue is a process linking changes of wall stress to adaptive tissue responses, including cellular growth and extracellular matrix accumulation. These mechanisms may serve to regulate growth of the heart during development and maintain cardiac function at an appropriate level during changing physiological demands or, when more extensive in nature, may disrupt function and contribute to pathology in the heart. In hypertension, the increased wall thickness in response to increased pressure is sufficient to reduce wall stress and resolve the stimulus, but the adaptive response can lead to a chronically hypertrophied heart and an associated increased risk of sudden death or progression to heart failure [1,2]. In heart failure, the activation of cell death pathways is an additionally imposed factor that can contribute to chronic dysfunction [3]. The increasing incidence of hypertension and heart failure has generated a strong interest in these mechanisms of hypertrophy and cardiac dysfunction. The addition of biomechanical and/or neurohumoral load to cardiac myocytes can produce a reactive cellular hypertrophy through several alternate signalling pathways [4,5]. These pathways and their complex interactions are not fully understood but are thought to include stretch response elements that directly activate transcription in the nucleus, as well as neurohumoral mechanisms that activate intracellular signalling cascades. The latter include an important group that signal through secondary release of autocrine and paracrine factors (e.g. angiotensin II, endothelin-1, vascular endothelial growth factor, transforming growth factor β), which may amplify the initial growth stimulus. These appear to be acting, at least in part, through the mitogen-activated protein kinase family (MAPK), of which extracellular-signal regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs) and p38 MAP kinases influence cardiac hypertrophy. MAPKs phosphorylate multiple proteins in the nucleus to produce changes in transcriptional programming associated with hypertrophy [4,5]. There are likely different roles for the major MAPK signalling pathways [6,7]. All three MAPK subgroups appear to be responsive to anabolic agents and G protein-coupled receptors, while p38 and JNK are also activated by ischaemia and cytotoxic agents and may play a different role in hypertrophy [4,5,7]. ERKs can be activated by raf via ras, phospholipase C and protein kinase C, and protein kinase A and G protein-coupled receptors stimulated by hormones or growth factors [8]. Some controversy exists in that an ERK-independent hypertrophy pathway has been identified in a Gq overexpression model [9] whereas overexpression of MEK1, downstream from Gq, can produce hypertrophy through ERK activation without activating JNK or p38 MAPKs [10]. The understanding gained from this complexity is that differential activation of MAPK hypertrophy signalling pathways, and non-MAPK pathways, likely generates specific hypertrophy signals for physiological, adaptive and pathological hypertrophy responses. Recent gains in our knowledge of this area include a partial understanding of transcriptional effects of these signalling pathways mediating hypertrophy. The transcriptional response in the heart is considered to be a potentially important target for intervention against hypertrophy because it is thought to arise from regulation of a relatively small group of mediators, perhaps allowing for effective development of agents that can inhibit the hypertrophic mechanisms arising from multiple signalling pathways [4]. The transcription factors MEF2, NFAT and GATA4 represent major components, and their regulation by phosphorylation [11] and acetylation [12,13] provide further attractive targets for an inhibitory approach. In this issue of the journal, Xu et al. [14] define a novel role for cellular repressor of E1A-stimulated genes (CREG) as a potential inhibitory regulator of cardiac hypertrophy, signalling through inhibition of ERK. CREG is a widely expressed glycoprotein in adult tissues [15,16] and is thought to be important in maintaining differentiation and reducing proliferation by antagonizing cellular transformation by E1A and ras [17,18]. The oncogene E1A is of significant interest in cardiac growth because it can reinitiate cell cycle entry and promote apoptosis in cardiac tissue [19,20]. However, little is known of the role of CREG in the heart, either in normal cardiac function or in cardiac hypertrophy/failure. The study by Xu et al. [14] addresses this issue, and shows that CREG mRNA expression is specifically reduced in the left ventricle during pressure overload in the rat. Although it was not determined whether reduced expression of CREG was due directly to pressure elevation, the change was evident early on, before significant pressure elevation and cardiac myocyte hypertrophy could be measured, suggesting some degree of independence from pressure. The time course of CREG expression may also suggest a potential for contribution to the development of hypertrophy observed later in this model. Interestingly, the reduction of mRNA levels was only 50% at maximum, suggesting that either partial loss of CREG is sufficient to influence this modest hypertrophy response, or that some proportion of cardiac cells continue to produce CREG at significant levels. The relative contributions of myocytes and non-myocytes to cardiac CREG mRNA levels are not known; however, when addressed in vitro, myocyte and fibroblast populations both responded to CREG inhibition with hypertrophy and proliferation, respectively. Some insight into the signalling mechanism by which CREG influenced hypertrophy was found when anti-sense inhibition or overexpression of CREG in vitro influenced levels of ERK1/2 and increased or decreased the myocyte hypertrophy responses, respectively. The absence of effects of manipulating CREG on protein expression in other signalling pathways, including protein kinase C, other MAPKs (p38 and JNK), and apoptotic signalling pathways, in either cardiac myocytes or fibroblasts, is suggestive of an independent and specific ERK1/2-mediated pathway that enables increased transcriptional regulation of hypertrophy upon loss of the inhibitory effects of CREG. Further testing of this hypothesis will require confirmation using methods that involve a more detailed analysis of activity of those alternate pathways, and further determination of signalling elements in the novel CREG/MAPK pathway. If the signalling mechanism is confirmed, future work that identifies the means of regulating these endogenous inhibitors of hypertrophic signalling may provide useful therapeutic approaches. Aside from its role in hypertrophy, CREG may be a gene associated with maintaining differentiated function in the heart and other tissues [18]. The concept that cells re-express a fetal gene program during hypertrophy has been widely accepted. The study by Xu et al. [14] has described a novel mechanism whereby expression of genes normally associated with an adult expression pattern may be downregulated during the process of hypertrophy. Maintaining a higher level of CREG in the presence of hypertrophic stimuli may maintain a resistance to ERK signalling and cellular hypertrophy. Together with other recent publications, these data suggest a role for E1A-associated transcriptional regulation in hypertrophic mechanisms [12,13, 19,20], potentially through the acetylation/deacetylation of transcription factors critical to hypertrophy and differentiation [12,21]. Collectively, these data show that CREG likely has a significant role in regulating cellular functions in the heart, but precisely how that relates to the transcriptional programming of cardiac hypertrophy is not yet clear. The study by Xu et al. [14] may provide a useful starting point for understanding the novel mechanism regulating or promoting differentiated cardiac function and maintaining ventricular performance.