Recent advances in fibroblast signaling and biology in scleroderma

Systemic sclerosis is a complex disease manifesting itself by fibrosis of skin and other internal organs. Fibroblasts isolated from scleroderma lesions and cultured in vitro are characterized by increased synthesis of collagen and other extracellular matrix proteins, consistent with the disease phenotype. Cultured systemic sclerosis fibroblasts therefore serve as a principal experimental model for studying the molecular and cellular mechanisms involved in collagen overproduction in this disease. This review will discuss recent findings related to intracellular signal transduction pathways implicated in deregulated extracellular matrix deposition by systemic sclerosis fibroblasts.

Recent findings suggest that constitutively elevated synthesis of extracellular matrix by cultured systemic sclerosis fibroblasts is, at least in part, due to the aberrant activation of the autocrine transforming growth factor-beta signaling. Enhanced constitutive transforming growth factor-beta signaling may result from the elevated levels of transforming growth factor-beta receptor type I and/or inappropriate activation of Smad3. These alterations of the transforming growth factor-beta signaling in systemic sclerosis fibroblasts may facilitate increased collagen production in vivo even under conditions of low ligand availability. However, there exist many inconsistencies among published reports regarding the detailed mechanisms of this pathway in systemic sclerosis fibroblasts, and additional studies in this area are needed. Other signaling molecules implicated in fibrotic phenotype include several members of the protein kinase C family, mammalian target of rapamycin, mitogen-activated protein kinase, necdin, reactive oxygen species, and sphingolipids. These signaling pathways may work in conjunction with transforming growth factor-beta signaling to regulate the behavior of systemic sclerosis fibroblasts.

Alterations in multiple signaling pathways contribute to elevated extracellular matrix synthesis by systemic sclerosis fibroblasts. Improved understanding of the key signaling molecules may provide a novel avenue for therapeutic interventions.