A gain-of-function mutation in the PDGFR-beta alters the kinetics of injury response in liver and skin

Quantitative proteomics reveals the protective effects of Yinchenzhufu decoction against cholestatic liver fibrosis in mice by inhibiting the PDGFRβ/PI3K/AKT pathway

Introduction: Yinchenzhufu decoction (YCZFD) is a traditional Chinese medicine formula with hepatoprotective effects. In this study, the protective effects of YCZFD against cholestatic liver fibrosis (CLF) and its underlying mechanisms were evaluated. Methods: A 3, 5-diethoxycarbonyl-1, 4-dihydro-collidine (DDC)-induced cholestatic mouse model was used to investigate the amelioration of YCZFD on CLF. Data-independent acquisition-based mass spectrometry was performed to investigate proteomic changes in the livers of mice in three groups: control, model, and model treated with high-dose YCZFD. The effects of YCZFD on the expression of key proteins were confirmed in mice and cell models. Results: YCZFD significantly decreased the levels of serum biochemical, liver injury, and fibrosis indicators of cholestatic mice. The proteomics indicated that 460 differentially expressed proteins (DEPs) were identified among control, model, and model treated with high-dose YCZFD groups. Enrichment analyses of these DEPs revealed that YCZFD influenced multiple pathways, including PI3K-Akt, focal adhesion, ECM-receptor interaction, glutathione metabolism, and steroid biosynthesis pathways. The expression of platelet derived growth factor receptor beta (PDGFRβ), a receptor associated with the PI3K/AKT and focal adhesion pathways, was upregulated in the livers of cholestatic mice but downregulated by YCZFD. The effects of YCZFD on the expression of key proteins in the PDGFRβ/PI3K/AKT pathway were further confirmed in mice and transforming growth factor-β-induced hepatic stellate cells. We uncovered seven plant metabolites (chlorogenic acid, scoparone, isoliquiritigenin, glycyrrhetinic acid, formononetin, atractylenolide I, and benzoylaconitine) of YCZFD that may regulate PDGFRβ expression. Conclusion: YCZFD substantially protects against DDC-induced CLF mainly through regulating the PDGFRβ/PI3K/AKT signaling pathway.

Cleavage of proteoglycans, plasma proteins and the platelet-derived growth factor receptor in the hemorrhagic process induced by snake venom metalloproteinases

Envenoming by viperid snakes results in a complex pattern of tissue damage, including hemorrhage, which in severe cases may lead to permanent sequelae. Snake venom metalloproteinases (SVMPs) are main players in this pathogenesis, acting synergistically upon different mammalian proteomes. Hemorrhagic Factor 3 (HF3), a P-III class SVMP from Bothrops jararaca, induces severe local hemorrhage at pmol doses in a murine model. Our hypothesis is that in a complex scenario of tissue damage, HF3 triggers proteolytic cascades by acting on a partially known substrate repertoire. Here, we focused on the hypothesis that different proteoglycans, plasma proteins, and the platelet derived growth factor receptor (PDGFR) could be involved in the HF3-induced hemorrhagic process. In surface plasmon resonance assays, various proteoglycans were demonstrated to interact with HF3, and their incubation with HF3 showed degradation or limited proteolysis. Likewise, Western blot analysis showed in vivo degradation of biglycan, decorin, glypican, lumican and syndecan in the HF3-induced hemorrhagic process. Moreover, antithrombin III, complement components C3 and C4, factor II and plasminogen were cleaved in vitro by HF3. Notably, HF3 cleaved PDGFR (alpha and beta) and PDGF in vitro, while both receptor forms were detected as cleaved in vivo in the hemorrhagic process induced by HF3. These findings outline the multifactorial character of SVMP-induced tissue damage, including the transient activation of tissue proteinases, and underscore for the first time that endothelial glycocalyx proteoglycans and PDGFR are targets of SVMPs in the disruption of microvasculature integrity and generation of hemorrhage.

PDGF in organ fibrosis

Fibrosis is part of a tissue repair response to injury, defined as increased deposition of extracellular matrix. In some instances, fibrosis is beneficial; however, in the majority of diseases fibrosis is detrimental. Virtually all chronic progressive diseases are associated with fibrosis, representing a huge number of patients worldwide. Fibrosis occurs in all organs and tissues, becomes irreversible with time and further drives loss of tissue function. Various cells types initiate and perpetuate pathological fibrosis by paracrine activation of the principal cellular executors of fibrosis, i.e. stromal mesenchymal cells like fibroblasts, pericytes and myofibroblasts. Multiple pathways are involved in fibrosis, platelet-derived growth factor (PDGF)-signaling being one of the central mediators. Stromal mesenchymal cells express both PDGF receptors (PDGFR) α and β, activation of which drives proliferation, migration and production of extracellular matrix, i.e. the principal processes of fibrosis. Here, we review the role of PDGF signaling in organ fibrosis, with particular focus on the more recently described ligands PDGF-C and -D. We discuss the potential challenges, opportunities and open questions in using PDGF as a potential target for anti-fibrotic therapies.

Genetic lineage tracing defines myofibroblast origin and function in the injured heart

Cardiac fibroblasts convert to myofibroblasts with injury to mediate healing after acute myocardial infarction (MI) and to mediate long-standing fibrosis with chronic disease. Myofibroblasts remain a poorly defined cell type in terms of their origins and functional effects in vivo. Here we generate Postn (periostin) gene-targeted mice containing a tamoxifen-inducible Cre for cellular lineage-tracing analysis. This Postn allele identifies essentially all myofibroblasts within the heart and multiple other tissues. Lineage tracing with four additional Cre-expressing mouse lines shows that periostin-expressing myofibroblasts in the heart derive from tissue-resident fibroblasts of the Tcf21 lineage, but not endothelial, immune/myeloid or smooth muscle cells. Deletion of periostin⁺ myofibroblasts reduces collagen production and scar formation after MI. Periostin-traced myofibroblasts also revert back to a less-activated state upon injury resolution. Our results define the myofibroblast as a periostin-expressing cell type necessary for adaptive healing and fibrosis in the heart, which arises from Tcf21⁺ tissue-resident fibroblasts.

The origin and fate of myofibroblasts, the cells responsible for cardiac remodelling and fibrosis, is controversial. Here the authors show that cardiac myofibroblasts express periostin, derive exclusively from tissue-resident fibroblasts, are necessary for scar formation after injury, and can revert back to a less-activated state upon injury resolution.

Activation of platelet-derived growth factor receptor alpha contributes to liver fibrosis

Chronic liver injury leads to fibrosis, cirrhosis, and loss of liver function. Liver cirrhosis is the 12th leading cause of death in the United States, and it is the primary risk factor for developing liver cancer. Fibrosis and cirrhosis result from activation of hepatic stellate cells (HSCs), which are the primary collagen producing cell type in the liver. Here, we show that platelet-derived growth factor receptor α (PDGFRα) is expressed by human HSCs, and PDGFRα expression is elevated in human liver disease. Using a green fluorescent protein (GFP) reporter mouse strain, we evaluated the role of PDGFRα in liver disease in mice and found that mouse HSCs express PDGFRα and expression is upregulated during carbon tetrachloride (CCl₄) induced liver injury and fibrosis injection. This fibrotic response is reduced in Pdgfrα heterozygous mice, consistent with the hypothesis that liver fibrosis requires upregulation and activation of PDGFRα. These results indicate that Pdgfrα expression is important in the fibrotic response to liver injury in humans and mice, and suggest that blocking PDGFRα–specific signaling pathways in HSCs may provide therapeutic benefit for patients with chronic liver disease.

Structural and functional properties of platelet-derived growth factor and stem cell factor receptors

The receptors for platelet-derived growth factor (PDGF) and stem cell factor (SCF) are members of the type III class of PTK receptors, which are characterized by five Ig-like domains extracellularly and a split kinase domain intracellularly. The receptors are activated by ligand-induced dimerization, leading to autophosphorylation on specific tyrosine residues. Thereby the kinase activities of the receptors are activated and docking sites for downstream SH2 domain signal transduction molecules are created; activation of these pathways promotes cell growth, survival, and migration. These receptors mediate important signals during the embryonal development, and control tissue homeostasis in the adult. Their overactivity is seen in malignancies and other diseases involving excessive cell proliferation, such as atherosclerosis and fibrotic diseases. In cancer, mutations of PDGF and SCF receptors-including gene fusions, point mutations, and amplifications-drive subpopulations of certain malignancies, such as gastrointestinal stromal tumors, chronic myelomonocytic leukemia, hypereosinophilic syndrome, glioblastoma, acute myeloid leukemia, mastocytosis, and melanoma.

A new hypothesis for the cancer mechanism

Several observations have led us to a new hypothesis for cancer mechanism. First, that cancer appears only on those multicellular organisms with complicated wound-healing capacities. Second, that wounds considered as risk factors can be identified in all cancers in clinics. And finally, that oncogene activation appears not only in cancer, but also in normal physiology and noncancer pathology processes. Our proposed hypothesis is that cancer is a natural wound healing-related process, which includes oncogene activations, cytokine secretions, stem cell recruitment differentiation, and tissue remodeling. Wounds activate oncogenes of some cells and the latter secrete cytokines to recruit stem cells to heal the wounds. However, if the cause of the wound or if the wound persists, such as under the persistent UV and carcinogen exposures, the continuous wound healing process will lead to a clinical cancer mass. There is no system in nature to stop or reverse the wound healing process in the middle stage when the wound exists. The outcome of the cancer mechanism is either healing the wound or exhausting the whole system (death). The logic of this cancer mechanism is consistent with the rationales of the other physiological metabolisms in the body-for survival. This hypothesis helps to understand many cancer mysteries derived from the mutation theory, such as why cancer only exists in a small proportion of multicellular organisms, although they are all under potential mutation risks during DNA replications. The hypothesis can be used to interpret and guide cancer prevention, recurrence, metastasis, in vitro and in vivo studies, and personalized treatments.

Hypoxic pulmonary hypertension in mice with constitutively active platelet-derived growth factor receptor-β

Platelet-derived growth factor (PDGF) has been implicated in the pathobiology of vascular remodeling. The multikinase inhibitor imatinib that targets PDGF receptor (PDGFR), c-kit and Abl kinases, shows therapeutic efficacy against experimental pulmonary hypertension (PH); however, the role of PDGFR-b in experimental PH has not been examined by genetic approach. We investigated the chronic hypoxia-induced PH in mice carrying an activating point mutation of PDGFR-β (D849N) and evaluated the therapeutic efficacy of imatinib. In addition, we studied pulmonary global gene expression and confirmed the expression of identified genes by immunohistochemistry. Chronically hypoxic D849N mice developed PH and strong pulmonary vascular remodeling that was improved by imatinib (100 mg/kg/day) as evident from the significantly reduced right ventricular systolic pressure, right ventricular hypertrophy and muscularization of peripheral pulmonary arteries. Global gene expression analysis revealed that stromal cell derived factor SDF)-1α was significantly upregulated, which was confirmed by immunohistochemistry. Moreover, an enhanced immunoreactivity for SDF-1α, PDGFR-β and CXCR4, the receptor for SDF-1α was localized to the α-smooth muscle cell (SMC) actin positive pulmonary vascular cells in hypoxic mice and patients with idiopathic pulmonary arterial hypertension (IPAH). In conclusion, our findings substantiate the major role of PDGFR activation in pulmonary vascular remodeling by a genetic approach. Immunohistochemistry findings suggest a role for SDF-1α/CXCR4 axis in pulmonary vascular remodeling and point to a potential interaction between the chemokine SDF-1 and the growth factor PDGF signaling. Future studies designed to elucidate an interaction between the chemokine SDF-1 and the PDGF system may uncover novel therapeutic targets.