GATA4, a new regulator of cardiac fibroblasts, is sensitive to natriuretic peptides

Cardiac fibroblasts are the most prevalent cell type in the heart. These cells exert a critical role in regulating normal myocardial function and in adverse myocardial remodelling that occur with hypertension, myocardial infarction, and heart failure.1 In normal cardiac tissue, fibroblasts are found throughout the cardiac tissue in parallel to the prevailing direction of muscle fibres. In essence, every cardiac myocyte is therefore in direct contact with fibroblasts. Adverse myocardial remodelling includes enlargement of myocytes, proliferation of fibroblasts, and increased procollagen synthesis. Natriuretic peptides, including atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), released from myocytes have been shown to act as autocine/paracrine factors that reduce myocyte hypertrophy and exert antifibrotic and antiproliferative effects on cardiac fibroblasts.2,3 Antifibrotic effects are attributed to C-type natriuretic peptide (CNP) released from endothelial cells and cardiac fibroblasts.4 Natriuretic peptides initiate their signalling via guanylyl cyclase receptors (NPR-A, NPR-B) and C-type receptor (NPR-C) all expressed in cardiac fibroblasts.3,4 These effects may be of particular importance during pathological conditions, when the fibrosis and increased deposition of collagen results in increased myocardial stiffness and diastolic dysfunction. Myocardial hypertrophy is also characterized by activation of transcription factors. A number of stimuli that induce myocyte hypertrophy are known to enhance GATA4 transcriptional activity through phosphorylation at serine 105 due to the direct action of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogen-activated protein kinase (MAPK).5 GATA4 is essential for several developmental processes and a number of adaptive responses in the heart, including myocyte survival and cardiac hypertrophy. As a survival factor, GATA4 contributes to cytoprotection in myocytes induced to hypertrophy by endothelin-1 (ET-1).6 GATA4 is recognized as a key transcriptional regulator of numerous peptides synthesized in cardiac myocytes, including ANP, BNP, α- and β-myosin heavy chain, and ET-1. In clinical and experimental models of heart failure, myocardial levels of ET-1 are elevated as a result increased ET-1 synthesis by myocytes, vascular endothelial cells, and cardiac fibroblasts.7 Because ET-1 regulates cardiac myocyte function and its expression in rat cardiac fibroblasts can be induced by ET-1 itself,8 ET-1 may contribute to the cross-talk between cardiac fibroblasts and myocytes. Glenn et al.'s9 intriguing findings suggest that GATA4 is produced in cardiac fibroblasts. This study is a logical continuation of their previous work, demonstrating that natriuretic peptides inhibit DNA synthesis in cardiac fibroblasts.4 In their current paper, the authors demonstrate that ANP inhibits ET-1 gene expression and ET-1 promoter activity in cultured cardiac fibroblasts. Using elegant molecular tools they reveal that the site of inhibition is localized to a specific GATA4-binding site on the ET-1 gene promoter. They also demonstrate in cardiac fibroblasts that ANP suppresses ET-induced phosphorylation of ERK2 and GATA4. Experiments showing that inhibition of endogenous GATA4 by specific siRNA reduced ET-1 expression, to a level similar to that seen in the presence of ANP, strengthen the evidence that ANP suppresses GATA4-dependent transcriptional activity in cardiac fibroblasts. The observation by Glenn et al.,9 which nicely demonstrates the importance of the signalling from cardiac myocytes to fibroblasts, is consistent with the hypothesis that in addition to an intracellular function, GATA4 is also implicated in intercellular cross-talk. In this context, the GATA4 produced by myocytes stimulates angiogenic factor VEGF-A, targeting the endothelial cells.10 Based on the study of Gleen et al., it is possible that GATA4 orchestrates signaling pathways from cardiac fibroblasts to myocytes. However, because of the developmental stage of the cells investigated by Glenn et al., this can potentially change the interpretation of the results. Recently, Ieda et al.11 reported that in a co-culture system, embryonic cardiac fibroblasts induced proliferation of myocytes, in contrast to adult cardiac fibroblasts that promoted myocyte hypertrophy. Consequently, the experimental in vitro culture setting may render the findings of Glenn et al. not applicable to the adult heart in vivo because of the neonatal age of the cells studied. This problem is resolved, in part, by a very recent report showing that fibroblasts in the adult rat heart also express the transcription factor GATA4.12 GATA4 expression in the fibroblasts appears to be a particular property of the heart, because in contrast to cardiac fibroblasts, the fibroblasts that are also derived from the splanchnopleuric mesoderm, like those of the gut, or fibroblasts of different embryological origin, such as those of skin and skeletal muscle, lack GATA4.12 Therefore, it is likely that GATA4 acts as an organ-specific marker that controls cardiac and non-cardiac cell fate in hypertrophy. Consistent with this line of thinking, the modest reduction of GATA4 levels in mice within a range relevant for human heart failure stimulated mechanisms involving myocyte cell death and increased fibrosis.13 The signalling pathways responsible for natriuretic peptide-mediated inhibition of hypertrophy are not entirely established. The results of Glenn et al.9 are consistent with observations that ANP inhibits p38 MAPK and ERK activity in smooth muscle cells in response to angiotensin II and that NPR-A-deficient mice have increased basal phosphorylation of p38, ERK, and Akt in the heart.14 All three natriuretic peptides induce cGMP production, which appears to be the major mediator of hypertrophy inhibition. However, a specific role for NPR-C in this signalling cascade has been proposed. Specifically, BNP has been shown to exert an antiproliferative action on human fibroblasts based on a BrdU cell proliferation assay.15 This effect was not affected by the application of HS-142-1 (which blocks the NPR-A and NPR-B receptors), but was antagonized by the application of ANP clearance receptor-binding peptide, suggesting a distinct role for NPR-C in the regulation of fibroblast proliferation. In summary, Glenn et al.9 have defined the potential role for GATA4 as a new regulator of cardiac fibroblast function. Their findings compel us to re-evaluate the function of natriuretic peptides as co-factors regulating gene transcription in these cells. Although the current study focused on myocyte communication with fibroblasts in heart pathology, it is still unknown whether GATA4 plays a role in developmental regulation in cardiac fibroblasts and their effects on the proliferation of cardiac myocytes or whether it has a function in the maintenance of cardiac-resident progenitors. This work was supported by the Canadian Institute for Health Research grant (CIHR, MOP-53217, NET Program) and the Canadian Heart and Stroke Foundation (CHSF, NET Program). Special thanks to Jolanta Gutkowska and Tom Broderick for their helpful comments. Conflict of interest: none declared.