Constitutive Thrombospondin-1 Overexpression Contributes to Autocrine Transforming Growth Factor-β Signaling in Cultured Scleroderma Fibroblasts

Abstract

The extracellular matrix (ECM) glycoprotein thrombospondin-1 (TSP-1) has been reported to activate the latent complex of transforming growth factor-beta (TGF-beta), the major effects of which in mesenchymal cells is stimulation of the synthesis of ECM. Previous reports suggested the involvement of an autocrine TGF-beta loop in the pathogenesis of scleroderma. In this study, we examined whether TSP-1 plays a role in maintaining the autocrine TGF-beta loop in scleroderma. TSP-1 expression was increased in scleroderma patients compared with in healthy controls in vivo and in vitro. TGF-beta blocking antibody or TGF-beta1 antisense oligonucleotide markedly reduced the up-regulated TSP-1 expression in scleroderma fibroblasts but had little effect on normal fibroblasts. The expression of TSP-1 is up-regulated in scleroderma fibroblasts, possibly at the post-transcriptional level just like in normal fibroblasts stimulated with exogenous TGF-beta1. TSP-1 blocking peptide or antisense oligonucleotide had an inhibitory effect on the up-regulated alpha2I collagen and phosopho-Smad3 levels in scleroderma fibroblasts but had little effects on normal fibroblasts. The transient overexpression of TSP-1 up-regulated alpha2I collagen and phospho-Smad3 levels in normal fibroblasts but had no major effect on scleroderma fibroblasts. Furthermore, these effects of transiently overexpressed TSP-1, which possibly occurred via the activation of latent TGF-beta1, were abolished by the TGF-beta1 antisense oligonucleotide. These results indicate that the constitutive overexpression of TSP-1 may play an important role in autocrine TGF-beta signaling and accumulation of ECM in scleroderma fibroblasts.

Altered dynamics of transforming growth factor (TGF- ) receptors in scleroderma fibroblasts

“…Collectively, these findings suggest that cell surface molecules recruiting endogenous TGF-β to its receptors may be a promising candidate involved in the establishment of autocrine TGF-β signalling in SSc fibroblasts. Our previous reports indicated that latent TGF-β receptors, such as integrin αvβ517 and thrombospondin-118 were upregulated, and such abnormalities contribute to the establishment of autocrine TGF-β signalling in SSc fibroblasts 12 13 18. These cell surface molecules and impaired Smad7–Smurfs-mediated negative regulation of TGF-β signalling may co-ordinately contribute to the establishment of autocrine TGF-β signalling in SSc fibroblasts.…”

Systemic sclerosis: a prototypic multisystem fibrotic disorder

“…The levels of receptors for TGF-β are elevated on SSc fibroblasts, permitting them to mount a robust response to endogenously produced TGF-β or to subthreshold levels of exogenous TGF-β in their environment (112,113). Furthermore, thrombospondin and α v β 3 integrins, both of which can mediate latent TGF-β activation, are elevated on SSc fibroblasts (114,115). Consistent with the autocrine TGF-β hypothesis, SSc fibroblasts show evidence of ligand-independent intracellular TGF-β signaling, with elevated expression and nuclear accumulation of activated SMAD3 (99,116) and its constitutive interaction with the p300/CBP coactivator (117,118).…”

The cytokines of pulmonary fibrosis: Much learned, much more to learn

“…To become functionally active, TGF-b needs to be activated from its latent state into a biologically active cytokine; such activation may occur through a spectrum of molecular processes, among which activation by aV-containing integrins plays the most important role, making them an ideal target for innovative antifibrotic therapies [108,109]. Thrombospondin is also an important activator of latent TGF-b [110,111], but is not required in all cases of tissue fibrosis, including in lung [112,113].…”