Vasculopathy, including altered vasoreactivity and abnormal large vessel biomechanics, is a hallmark of systemic sclerosis (SSc). However, the pathogenic link with other aspects of the disease is less clear. To assess the potential role of transforming growth factor beta (TGF-beta) overactivity in driving these cardiovascular abnormalities, we studied a novel transgenic mouse model characterized by ligand-dependent activation of TGF-beta signaling in fibroblasts.
The transgenic mouse strain Tbeta RIIDeltak-fib is characterized by balanced ligand-dependent upregulation of TGF-beta signaling. Aortic and cardiac tissues were examined with histologic, biochemical, and isolated organ bath studies. Vascular and perivascular architecture was examined by hematoxylin and eosin (H&E) and special stains including immunostaining for TGF-beta1 and phospho-Smad2/3 (pSmad2/3). Confirmatory aortic smooth muscle cell proliferation, phenotype, and functional assays, including signaling responses to exogenous TGF-beta and endothelin-1, were performed. Aortic ring contractile responses to direct and receptor-mediated stimulation were assessed.
Aortic ring contractility and relaxation were diminished compared with wild-type controls, and this was associated with aortic adventitial fibrosis confirmed histologically and with Sircol assay. TGF-beta1 and pSmad 2/3 expression was increased in the adventitia and smooth muscle layer of the aorta. Aortic smooth muscle cells from transgenic animals showed significant upregulation of TGF-beta- responsive genes important for cytoskeletal function, such as transgelin and smoothelin, which were then resistant to further stimulation with exogenous TGF-beta1. These cells promoted significantly more contraction of free floating type I collagen lattices when compared with the wild-type, but were again resistant to exogenous TGF-beta1 stimulation. Aortic ring responses to receptor-mediated contraction were reduced in the transgenic animals. Specifically, bosentan reduced endothelin-mediated contraction in wild-type animals, but had no effect in transgenic animals, and endothelin axis gene expression was altered in transgenic animals. Transgenic mice developed cardiac fibrosis.
The histologic, biochemical, and functional phenotype of this transgenic mouse model of scleroderma offers insight into the altered biomechanical properties previously reported for large elastic arteries in human SSc and suggests a role for perturbed TGF-beta and endothelin activity in this process.