Background Urokinase-type plasminogen activator receptor (uPAR) is a key component of the fibrinolytic system which plays an important role in the degradation of the extracellular matrix (ECM). uPAR concentrates the serine protease activity of its ligand, uPA, at the cell-matrix interface and promotes ECM and vascular remodelling. Moreover, uPAR not only functions as a uPA receptor, but also plays a role in cell differentiation, adhesion and migration through intracellular signalling. The cleavage/inactivation of uPAR was shown to be necessary for the transdifferentiation of fibroblasts into myofibroblasts, which are most responsible for the excessive ECM production and deposition in fibrotic disorders. In addition, previous studies have implicated uPAR cleavage/inactivation in systemic sclerosis (SSc) microvasculopathy and impaired angiogenesis.
Objectives The aim of the present study was to investigate whether inactivation of uPAR gene in mice could result in tissue fibrosis and peripheral microvasculopathy resembling human SSc.
Methods The expression of the native full-length form of uPAR in skin biopsies from SSc patients (n=15) and healthy controls (n=10) was determined by immunohistochemistry. Skin and lung sections from uPAR-deficient mice (n=14) and wild-type littermates (n=13) at 12 and 24 weeks of age were processed for haematoxylin-eosin, Masson’s trichrome and Picrosirius red stainings. Dermal thickness and hydroxyproline content were quantified. The number of myofibroblasts and microvessels were determined in skin sections immunostained for alpha-smooth muscle actin and the pan-endothelial cell marker CD31, respectively.
Results Expression of full-length uPAR was significantly decreased in SSc skin compared with controls, especially in fibroblasts and endothelial cells. uPAR-deficient mice developed dermal fibrosis that was paralleled by a severe loss of dermal microvessels. Dermal thickness, Picrosirius red-positive area, hydroxyproline content and the number of dermal myofibroblasts were significantly greater in uPAR-deficient mice than in wild-type littermates. Moreover, in uPAR-deficient mice the subcutaneous adipose tissue was partly replaced by connective tissue with severe perivascular fibrosis, presence of inflammatory cell infiltrates and mast cell degranulation. A significant decrease in dermal capillary density could be observed in uPAR-deficient mice. No significant differences in skin histopathology were observed between uPAR-deficient mice at 12 and 24 weeks of age. Lung specimens from uPAR-deficient mice displayed the typical features of non-specific interstitial pneumonia with uniform interstitial involvement characterised by both diffuse cellular inflammation and collagen deposition. In uPAR-deficient mice pulmonary pathology worsened significantly from 12 to 24 weeks of age, as shown by a significant increase in alveolar septal width and hydroxyproline content.
Conclusions The absence of uPAR induces dermal and pulmonary fibrosis and peripheral microvasculopathy in mice closely resembling human SSc. uPAR-deficient mice are a promising animal model to study the mechanisms and therapeutic approaches of fibrosis and peripheral microvasculopathy in SSc.
Disclosure of Interest None Declared
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