Increased circulating concentrations of the counteradhesive proteins SPARC and thrombospondin-1 in systemic sclerosis (scleroderma). Relationship to platelet and endothelial cell activation.

OBJECTIVE: To determine whether circulating concentrations of the counteradhesive proteins SPARC (secreted protein acidic and rich in cysteine) and thrombospondin-1 (TSP-1) are elevated in scleroderma (systemic sclerosis, SSc). The relationship of these counteradhesive proteins to measures of platelet and endothelial cell activation was examined.
METHODS: Plasma from 45 patients with SSc (26 limited form, 19 diffuse) and 22 age and sex matched controls was assayed for SPARC, TSP-1, beta-thromboglobulin (betaTG), and platelet factor 4 (PF4), 2 distinct platelet a-granule products, and soluble E-selectin, a marker of endothelial cell activation.
RESULTS: The mean (+/- SE) SPARC concentration was greater in patients with limited SSc (124.0 +/- 9.6 ng/ml) compared to controls (66.8 +/- 8.0 ng/ml) (p = 0.0005), whereas in patients with diffuse SSc (74.1 +/- 7.9 ng/ml) it was not. Elevated SPARC concentrations in the limited SSc group could not be ascribed to either platelet or endothelial cell activation. TSP-1 concentrations were also increased in SSc patients (n = 29) compared to controls (n = 11) (2.98 +/- 0.12 vs 2.4 +/- 0.21 log transformed ng/ml; p < 0.02). Unlike SPARC, TSP-1 concentrations correlated with both betaTG (r = 0.57, p = 0.0014) and PF4 (r = 0.41, p = 0.026) levels, indicating that increased TSP-1 could, in part, be explained through elevated platelet a-granule release in SSc patients. Plasma levels of betaTG, PF4, and E-selectin were each similarly elevated (p < 0.003) in patients with both limited and diffuse SSc compared to controls.
CONCLUSION: That circulating SPARC and TSP-1 are elevated in patients with SSc raises the possibility that counteradhesive proteins, which regulate vascular organization and remodeling, might contribute to the pathogenesis of SSc vasculopathy.