Department of Pathology and Laboratory Medicine, The James Hogg iCAPTURE Centre for Cardiovascular and Pulmonary Research, St Paul's Hospital-University of British Columbia, 1081 Burrard Street, Vancouver, British Columbia, Canada V6Z IY6
* Corresponding author. Tel: +604 682 2344 ext. 62872; fax: +604 806 9274. E-mail address: dyang{at}mrl.ubc.ca
This editorial refers to Elevated expression of PDGF-C in coxsackievirus B3-induced chronic myocarditis
by K. Grün et al., on page 728
Coxsackievirus B3 (CVB3) is the most common causative agent of myocarditis.1 Acute myocarditis caused by CVB3 infection either recovers completely without any functional and morphological defects or progresses to chronic myocarditis, which is characterized by chronic inflammation and interstitial hyperplasia that may be related to progressive intrinsic dysfunction, degeneration, and loss of cardiomyocyte viability.2 In later stages of this disease, CVB3 persists in the myocardium and results in chronic activation of fibroblasts and progressive fibrosis of the myocardium, reflected by the accumulation of connective tissue and extracellular matrix, which is characteristic of dilated cardiomyopathy (DCM).3,4 However, the molecular mechanisms by which acute myocarditis progresses to chronic myocarditis and DCM still remain unclear.
Fibrotic diseases usually occur in a variety of organ systems including lung, liver, kidney, intestine, heart, skin, and bone marrow.5 Several polypeptide mediators are central to the fibrotic process, including interleukin-1 (IL-1), tumor necrosis factor- (TNF-
), platelet-derived growth factor (PDGF), and transforming growth factor-ß1. They primarily function in fibrogenesis to transform interstitial fibroblasts into myofibroblasts and stimulate collagen deposition by newly replicated myofibroblasts. PDGF was discovered as a component of whole blood serum and initially purified from human platelets. It is well documented that PDGF, which acts primarily on connective tissue and is involved in cellular proliferation and tissue repair, is a potent mitogenic polypeptide and chemoattractant that functions as an important mediator in the pathogenesis of cardiovascular diseases.6
Grün et al.7 report a link between increased expression of PDGF to the development of myocardial fibrosis. The authors utilized a mouse model with immunodeficient major histocompatibititing complex (MHC) class II knockout and chronic myocarditis caused by CVB3 infection. Their data indicated that immunodeficient mice developed a higher titre of CVB3, prominent inflammatory cell infiltration, and moderately elevated expression of the inflammatory cytokines IL-1, IL-1ß, and TNF-
in mouse hearts, and showed severe cardiac fibrosis.
The PDGF family consists of four isoforms (A, B, C, and D). These PDGF isoforms have a common structure with the typical growth factor domain involved in the dimerization of the two subunits and in receptor binding and activation. They exist as four disulfide-linked homodimeric proteins, PDGF-AA, PDGF-BB, PDGF-CC, and PDGF-DD, and one heterodimeric protein, PDGF-AB, and differentially bind homo- and heterodimer combinations of two receptor tyrosine kinases, PDGF-R and PDGF-Rß. Both PDGF-A and PDGF-B chain dimeric isoforms (PDGF-AA, PDGF-AB, and PDGF-BB) have important roles in the pathogenesis of fibrosis. The roles of PDGF-C and PDGF-D in the cardiovascular system are just at the beginning of being explored; much less is known regarding the significance of the more recently discovered PDGF-C and PDGF-D isoforms to fibrogenesis.
To explore which factor may play a major role in CVB3-induced myocarditis and fibrosis, the study of Grün et al.7 demonstrated, using immunohistochemistry, PCR, and in situ hybridization, that all analysed isoforms of PDGF-A, PDGF-B, and PDGF-C were upregulated in close relation with the inflammatory process. High levels of the growth factors persisted only in CVB3-infected MHC class II knockout mice when compared with the wild-type C57/BL/6 control. The location of an elevated PDGF expression localized to areas of inflammatory cell infiltration, adjacent to fibrotic areas, indicate that infiltrating immune cells are likely the source of PDGF production.7 In this context, the elevation of PDGF-C expression is of particular interest because most of the previous studies focused on other PDGF isoforms in other organs such as liver and kidney. The findings of this study are corroborated by a recent report which described PDGF-C-mediated cardiac fibrogenesis in PDGF-C transgenic mice.8 In the latter study, transgenic mice were generated using the -myosin heavy chain promoter to drive PDGF-C overexpression in the heart. PDGF-C expression was dramatically upregulated and associated with a three-fold increase in PDGF receptor mRNA levels, its enhanced phosphorylation, and an increase in fibroblast proliferation. As a result, the transgenic overexpression of PDGF-C in the mouse heart induced progressive cardiac fibrosis, hypertrophy, and DCM by 3 months of age. Taken together, these two studies provided new insights into the role of increased expression of PDGF-C in inducing cardiac fibrosis.79
The PDGFs are potent chemotactic and mitogenic stimuli for fibroblasts, monocytes, and endothelial cells, produce their cellular effects by binding to structurally similar receptors (PDGF-R and PDGF-Rß) with different affinities. They also support the growth of connective tissue and the production of extracellular matrix.10,11 Thus, PDGF receptor activation in the heart is another important indicator to support the link between the elevated expression of PDGF-C and cardiac fibrosis. The investigators of this study have provided solid evidence to demonstrate the activation of PDGF-R
in the region of inflammation and fibrosis, which parallels the expression of PDGF-C. However, the receptor expression is prominent not in the cardiomyocytes, but in small cells within and also outside the inflammatory lesions, which likely represent infiltrating immune cells and also resident fibroblasts. These data were further established by determining the activation of a downstream signalling mediator of PDGF receptors, protein kinase B (Akt/PKB).7 They found that antibodies recognizing only the activated form of Akt/PKB revealed signals in the heart of infected MHC class II knockout animals with a similar distribution as in the case of PDGF-R
staining.7
The unique aspect of this study is the involvement of CVB3 infection. To further confirm the connection between the PDGF-C-induced fibrosis and the CVB3 infection of the heart, the current study also determined the viral titre in infected tissue and correlated it to the data of immunostaining. The authors found that tissues containing enhanced PDGF levels had highest virus load. This data was further verified by in situ hybridization, demonstrating that the inflamed areas showing PDGF-C RNA signals are surrounded by RNA of the CVB3 capsid protein VP1 gene.7 This distribution pattern strongly suggests that CVB3 infection is the cause of the PDGF-C-mediated cardiac fibrosis.
As mentioned earlier, PDGF and its receptor system play a crucial role in cardiac fibrogenesis, which provides new insights into the strategies to control PDGF-dependent pathophysiologic processes. In recent years, new therapeutic strategies for the treatment of fibrotic heart disease are aimed at the inhibition of PDGF receptor activity. Perhaps, the most promising drugs for blocking fibrosis development are the PDGF receptor tyrosine kinase inhibitors. These drugs can inhibit tyrosine phosphorylation within the intracellular domain of PDGF receptors and thus can be used to reduce fibrogenic responses by limiting myofibroblast replication, and thereby reduce the major cell type involved in scar formation in a variety of fibrotic diseases.5 Although several such inhibitors have been reported, such as AG1296 and STI571, the current mouse model may be useful to evaluate more new inhibitors for the therapeutic intervention of chronic viral myocarditis.
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References
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