Role of scleraxis in mechanical stretch-mediated regulation of cardiac myofibroblast phenotype

Abstract

-The phenotype conversion of fibroblasts to myofibroblasts plays a key role in the pathogenesis of cardiac fibrosis. Numerous triggers of this conversion process have been identified, including plating of cells on solid substrates, cytokines such as transforming growth factor-␤, and mechanical stretch; however, the underlying mechanisms remain incompletely defined. Recent studies from our laboratory revealed that the transcription factor scleraxis is a key regulator of cardiac fibroblast phenotype and extracellular matrix expression. Here we report that mechanical stretch induces type I collagen expression and morphological changes indicative of cardiac myofibroblast conversion, as well as scleraxis expression via activation of the scleraxis promoter. Scleraxis causes phenotypic changes similar to stretch, and the effect of stretch is attenuated in scleraxis null cells. Scleraxis was also sufficient to upregulate expression of vinculin and F-actin, to induce stress fiber and focal adhesion formation, and to attenuate both cell migration and proliferation, further evidence of scleraxis-mediated regulation of fibroblast to myofibroblast conversion. Together, these data confirm that scleraxis is sufficient to promote the myofibroblast phenotype and is a required effector of stretch-mediated conversion. Scleraxis may thus represent a potential target for the development of novel antifibrotic therapies aimed at inhibiting myofibroblast formation. transcription factor; cardiac fibroblast; stretch; migration; proliferation MYOFIBROBLASTS, AS THE ACTIVATED form of fibroblasts, are major mediators of tissue fibrosis in the heart, lungs, dermis, kidneys, and gastrointestinal tract (17,26,29,36,42). Excess deposition of fibrillar collagens in the extracellular matrix (ECM) of these tissues imparts increased stiffness and reduced organ function. Cardiac fibrosis entails a poor prognosis, negatively impacting both systolic and diastolic function and eventually leading to heart failure and death (43). There currently exists no treatment for the arrest or reversal of cardiac fibrosis (38). However, alteration of the myofibroblast phenotype may provide a novel means for the treatment and even reversal of fibrotic lesions in various tissue types (20,21,23,26,34,37,49).In response to myocardial injury, the release of damage factors [such as profibrotic transforming growth factor-␤ (TGF-␤)] and mechanical strain resulting from the loss of ECM integrity induce activation of cardiac fibroblasts and subsequent conversion to the myofibroblast phenotype (11,14,22,25,32). Cardiac myofibroblasts are characterized by hypersynthesis of fibrillar collagens type I and III, increased expression of ␣-smooth muscle actin (␣-SMA), increased adhesions and cell size, reduced proliferation and migration, and the formation of stress fibers (12,18,39,44,46). The morphological and functional changes that cardiac fibroblasts undergo during their conversion to myofibroblasts are critical to the wound healing process following myocardial injury. The loca...

Cardiac Fibroblast Activation Post-Myocardial Infarction: Current Knowledge Gaps

“…Of note, the transcription factor scleraxis induces a myofibroblast phenotype that mimics the effects of stretch [59]. Moreover, the effect of stretch is attenuated in scleraxis null fibroblasts, indicating that scleraxis mediates stretch-induced fibroblast phenotypic conversion to myofibroblasts.…”

Defining the Cardiac Fibroblast

“…Two of these proteins are scleraxis 46,87,88 , which is downstream of TGFβ signaling and involved in ECM synthesis, and myocardin-related transcription factors (MRTFs) 89 , which are involved in cytoskeletal changes and upregulation of αSMA expression during fibroblast activation. This information suggests that rather than a differentiation process, the changes in gene expression of fibroblasts after cardiac injury is more likely to be a response to changes in growth factor signaling and an increase in tissue stiffness (reviewed by van Putten 90 ).…”

The Transcription Factor SCX is a Potential Serum Biomarker of Fibrotic Diseases

“…For example, during mitral heart valve prolapse, high SCX levels promote the abnormal expression of proteoglycans and other ECM components [38]. Similarly, in cardiac fibrosis, the SCX dysregulation promotes exacerbated ECM synthesis, the formation of focal adhesions and tension fibers, and diminished migration and proliferation of cardiac cells [19,21,31,39]. In diabetic nephropathies, SCX also activates the expression of α-SMA/ACTA2 and bone morphogenetic protein 4, to promote differentiation of mesangial cells into activated myofibroblasts [40].…”