Vascular alterations upon activation of TGFβ signaling in fibroblasts - implications for systemic sclerosis

Tissue fibrosis and vascular disease are hallmarks of systemic sclerosis (SSc). Transforming growth factor β (TGFβ) is a key-player in fibroblast activation and tissue fibrosis in SSc. In contrast to fibrosis, evidence for a role of TGFβ in vascular disease of SSc is scarce. Using a transgenic mouse model with fibroblast-specific expression of a kinase-deficient TGFβ receptor type II, Derrett-Smith and colleagues demonstrate that aberrant TGFβ signaling in fibroblasts might result in activation of vascular smooth muscle cells and architectural changes of the vessel wall of the aorta.

Using the TβRIIΔk-fi b transgenic mouse model, Derrett-Smith and colleagues [1] analyzed a potential role of transforming growth factor β (TGFβ) signaling in the vascular pathogenesis of systemic sclerosis (SSc).
SSc is a chronic autoimmune disease that aff ects the skin and various internal organs. Th e most obvious histopathological alteration of SSc is an extensive accumulation of extracellular matrix [2]. Th e resulting fi brosis disrupts the physiological tissue structure and frequently leads to dysfunction of the aff ected organs. Th e accumulation of extracellular matrix in SSc patients is caused by activated fi broblasts [3]. In addition to fi brosis, vascular changes are a major hallmark of SSc. Th ese may be classifi ed into a destructive-and a proliferative vasculopathy. Th e destructive vasculopathy aff ects small vessels and manifests early in the course of SSc as progressive loss of capillaries and insuffi cient angiogenesis. Th e clinical correlates of the destructive vasculopathy are Raynaud's phenomenon and fi ngertip ulcers. In contrast, the proliferative vasculopathy is characterized by prolifera tion of vascular cells with obstruction of the lumen, aff ects larger vessels like the pulmonary arteries and often manifests later in the course of the disease as pulmonary arterial hypertension [2].
Th e key-role of TGFβ in fi brosis is well established as TGFβ signaling is activated in SSc. Activated TGFβ signaling stimulates the release of collagen in cultured fi broblasts and overexpression of a constitutively active TGFβ receptor type I in fi broblasts results in progressive fi brosis [3]. Moreover, inhibition of TGFβ signaling exerted potent anti-fi brotic eff ects in diff erent preclinical models of SSc [4].
In contrast to fi brosis, only few data suggest a role of TGFβ in the vascular pathogenesis of SSc. First data from mouse models suggest that aberrant TGFβ signaling might not result in only fi brosis, but also in vascular alterations. Vascular changes have been described in several models with activated TGFβ signaling, such as caveolin-1 knockout mice and fos-related antigen (Fra-2) transgenic mice [5][6][7][8]. However, apart from Fra-2 transgenic mice, the type of vessels involved and the histological changes diff er from those observed in human SSc.
Derrett-Smith and colleagues describe macrovascular changes in the thoracic aorta with altered gene expression in vascular smooth-muscle cells (vSMCs) in TβRIIΔk-fi b mice [1]. TβRIIΔk-fi b mice selectively express a kinasedefi cient TGFβ receptor type II (TβRIIΔk) in fi broblasts under a fi broblast-specifi c pro-α2(I) collagen promoter [9]. Although overexpression of the kinase-defi cient TβRIIΔk construct interferes with TGFβ signaling in cultured fi broblasts in vitro, TβRIIΔk transgenic mice are characterized by activated TGFβ signaling and develop dermal and pulmonary fi brosis. Th e molecular mechanism underlying this paradoxical activation of TGFβ signaling in TβRIIΔk transgenic mice is incompletely characterized. Potential explanations include upregulation of wild-type TβRII and TGFβ1 [9]. Th e authors observed signs of activated TGFβ signaling in the aortas

Abstract
Tissue fi brosis and vascular disease are hallmarks of systemic sclerosis (SSc). Transforming growth factor β (TGFβ) is a key-player in fi broblast activation and tissue fi brosis in SSc. In contrast to fi brosis, evidence for a role of TGFβ in vascular disease of SSc is scarce. Using a transgenic mouse model with fi broblast-specifi c expression of a kinase-defi cient TGFβ receptor type II, Derrett-Smith and colleagues demonstrate that aberrant TGFβ signaling in fi broblasts might result in activation of vascular smooth muscle cells and architectural changes of the vessel wall of the aorta.
of TβRIIΔk-fi b mice with increased expression of latency-associated peptide-TGFβ1 (LAP-TGFβ1) and TGFβ1 in the adventitia and accumulation of phosphorylated Smad 2/3. Of note, TGFβ signaling was not restricted to fi broblasts, but was also observed in other cell types, such as smooth muscle cells. Consistent with activated TGFβ signaling, the collagen content of the thoracic aorta was increased and the adventitial and the smooth muscle cell layers were thickened. Th ese changes were functionally relevant and resulted in increased vascular stiff ness. Th e contractility of isolated aortic rings upon incubation with KCl, α-adrenoreceptor agonists or thromboxane analogues was reduced in TβRIIΔk-fi b mice. Surprisingly, a partial TGFβ gene signature and increased contractility was also observed in vitro in early passage cultured aortic vSMCs, even though the TβRIIΔk transgene was not detectable in vSMCs [1].
Although the authors elegantly demonstrate vascular alterations in TβRIIΔk-fi b mice, additional studies are needed to establish increased TGFβ signaling in fi broblasts as a molecular mediator of the vascular disease in SSc. Th e molecular mechanisms by which the expression of the kinase-defi cient TβRIIΔk construct in fi broblasts activates TGFβ signaling in other cell types such as vSMCs are poorly understood. Th us, confi rmation of the altered phenotype of vSMCs in other models with fi broblast-specifi c activation of TGFβ signaling such as TβRI CA Cre-ER mice would be important and might provide further mechanistic insights [10]. Furthermore, localization and the kinds of vascular changes in TβRIIΔk-fi b mice and also in most other animal models diff er from those in SSc patients. Derrett-Smith and coauthors describe vascular changes in the aorta of TβRIIΔk-fi b mice. However, the clinically relevant vascular manifestations in SSc aff ect the pulmonary arteries and the smaller vessels. Moreover, the histological changes described in TβRIIΔk-fi b mice do not resemble the features of the destructive or proliferative vasculopathy in SSc. Does altered TGFβ signaling in fi broblasts also result in alterations of the pulmonary arteries, the small arteries and the capillaries and do the histological changes in these vessels resemble those observed in human SSc more closely? Th e demonstration of typical SSc-like changes in these vessels would further strengthen the importance of TGFβ signaling in the vascular pathology of SSc.