Whereas RA is characteristically a disease of increased angiogenesis, there is evidence for an insufficient angiogenic response in SSc. The dysregulation of angiogenesis leads to failure to replace damaged vessels, resulting in a reduction in capillary density in patients with SSc [36
]. Clinically, these changes in the capillary network are evident on nail-fold capillaroscopy and may manifest as ischaemic fingertip ulcers.
Studies in peripheral blood mononuclear cells support the concept of decreased and inefficient angiogenesis in SSc. Isolated peripheral blood mononuclear cells induce less angiogenesis than do peripheral blood mononuclear cells from healthy control individuals [37
], whereas supernatants from SSc peripheral blood mononuclear cells decreased endothelial chemotaxis [38
], and serum samples from patients with SSc failed to enhance normal mononuclear angiogenic capability [39
]. Thus, in contrast to the marked angiogenic response seen in peripheral blood monocytes from normal individuals, those from SSc patients are effectively inert, leading to a defective or dysregulated angiogenic response. Comparisons of skin biopsies from SSc patients with normal skin samples have shown the former to have fewer blood vessels, providing further evidence for decreased angiogenesis in SSc [40
Although an overall decrease in the angiogenic response is well documented, the mechanisms that lead to the disturbed angiogenesis are much less clear. Several antiangiogenic or angiostatic mediators have been reported to be upregulated in SSc, including endostatin, platelet factor 4, thrombospondin, and IL-4 [41
] (Table ). However, results on angiostatic factors in SSc are conflicting [42
] or flawed by low patient numbers or poor clinical characterization, and heterogeneity of the study population. In addition, the functional contribution of angiostatic factors to the disturbed angiogenesis in SSc has not been addressed.
Some antiangiogenic and proangiogenic mediators in systemic sclerosis
Evidence for a mechanism of dysregulated angiogenesis in SSc has emerged from two recent experimental studies. The first of these showed decreased urokinase plasminogen activator dependent invasion, proliferation, and capillary morphogenesis, and increased MMP-12 in SSc endothelial cells [46
]. In this study the authors were able to restore the defect in the endothelium with an anti-MMP-12 antibody. They also discovered that urokinase plasminogen activator undergoes truncation between domains 1 and 2, thus impairing its function. The second experimental study showed evidence for decreased expression of tissue kallikreins 9, 11 and 12 in endothelial cells isolated from patients with SSc in comparison with normal endothelial cells [47
]. There is also evidence that circulating endothelial cells are increased in SSc as a marker of endothelial cell activation in comparison with normal controls, and that this increase correlates with disease activity scores [48
]. Recent studies indicated that, in addition to angiogenesis, the formation of new vessels from precursor cells (vasculogenesis) is impaired in SSc [49
]. However, findings regarding the role of vasculogenesis in SSc are inconsistent [50
], and this must be addressed by further studies.
Paradoxically, there is upregulation or increased expression of a large number of proangiogenic mediators despite the defective angiogenesis in SSc. In addition, the bushy and tortuous capillaries seen on nail-fold capillaroscopy could be interpreted as futile attempts to build new vessels after stimulation with angiogenic factors. Examples of upregulated proangiogenic factors in SSc include soluble VCAM-1 in peripheral blood monocytes, intercellular adhesion molecule-1 in peripheral blood monocytes and skin fibroblasts, E-selectin and P-selectin in skin, ET-1 in monocytes and skin fibroblasts, and monocyte chemoattractant molecule-1/CC chemokine ligand 2 in different cell types in the skin [51
] (Table ). Some adhesion molecules such as VCAM-1 and P-selectin, as well as cytokines such as TNF-α and IL-8/CXCL8, have more commonly been found in skin biopsies taken from patients with SSc of duration 1 year or less, whereas other molecules, for example IL-6, are upregulated in late stage disease [54
]. Taken together, these findings suggest that certain cellular adhesion molecules and cytokines may play differential roles in the early, inflammatory and later fibrotic stages of SSc.
As an additional example, the potent angiogenic factor VEGF is markedly over-expressed in various cell types in the skin of SSc patients [55
]. In parallel, VEGF receptors are upregulated on endothelial cells in SSc. In agreement with this finding, a number of studies confirmed that serum levels of VEGF are significantly increased in SSc patients throughout different disease stages [42
]. The serum levels of VEGF correlate significantly with the development of fingertip ulcers. SSc patients with fingertip ulcers have increased serum levels of VEGF as compared with healthy individuals, but they are lower than in SSc patients without fingertip ulcers [42
]. These data indicate that there might be a functional deficit of VEGF in SSc patients, which can be overcome if the levels of VEGF exceed an individual threshold.
The mechanisms that lead to increased expression of VEGF in SSc are unclear. In contrast to the situation in RA, hypoxia induced expression of hypoxia inducible factor-1α does not appear to play a major role in the induction of VEGF in SSc [55
], whereas induction by cytokines such as platelet-derived growth factor and TGF-β appear to be more important.
A crucial point in the interpretation of the upregulation of angiogenic factors in this disease is that angiogenic factors must be expressed in a strictly controlled temporal and spatial distribution if they are to lead to the development of functionally adequate new vessels. For instance, although the upregulation of VEGF and other angiogenic factors might be a compensatory mechanism for the initial effect of unidentified angiostatic factors, the temporal kinetics of its expression appear to be critical in overcoming the inhibitory effects of angiostatic factors. In this regard, it has been shown that a brief upregulation of VEGF results in instability of newly formed vessels [60
]. On the other hand, prolonged over-expression of VEGF, as is seen in SSc patients throughout various disease stages, also has deleterious effects because the vessels fuse in an uncontrolled manner and form a chaotic vessel network that is strikingly similar to the disturbed capillary network observed in SSc [60
]. In addition, isolated microvascular endothelial cells from patients exhibit an impaired response to VEGF in the Matrigel capillary morphogenesis assay [46
], indicating that VEGF receptor signalling might be impaired in endothelial cells of SSc patients. In contrast, functionally important gene polymorphisms that lead to an impairment in biological properties of VEGF do not exist in SSc patients [61
The individual functional consequences of the upregulated angiogenic factors for defective angiogenesis in SSc has not yet been addressed in detail. In this regard, it is important to highlight the results of a randomized placebo controlled trial conducted to evaluate the effects of the dual endothelin receptor antagonist bosentan on wound healing of fingertip ulcers in patients with SSc (RAPIDS-2 [RAndomized, double-blind, Placebo-controlled, multi-center study to assess the effect of bosentan on healing and prevention of Ischemic Digital ulcers in patients with systemic Sclerosis-2]) [62
]. Wound healing requires angiogenesis, and ET-1 is among the group of angiogenic factors that are upregulated in SSc (Table ). Interestingly, inhibition of ET-1 did not affect wound healing in this clinical trial, indicating that ET-1 is of little functional importance for the angiogenic responses in patients with SSc.