MEDIATORS OF FIBROSIS IN VITRO AND IN AN IN VIVO RAT MODEL OF REPETITIVE OVERUSE INJURY

Abstract

Key clinical features of Work-Related Musculoskeletal Disorders (WMSDs) include pain, tendinosis and tissue fibrosis, although the etiology is still under investigation. Here, we characterized expressions of key mediators of fibrosis in an in vivo rat model of repetitive overuse injury and the effects of these mediators on fibroblast proliferation and collagen deposition in vitro. We first evaluated levels of anti-fibrogenic and fibrogenic proteins in young adult female rats performing high repetitive high force (HRHF) reaching and grasping task for 18 weeks. The anti-fibrogenic protein IFNγ increased at week 18 in muscle. Transforming growth factor beta-1 (TGFß-1) and CCN2 (formerly known as connective tissue growth factor, or CTGF) also increased in and around flexor digitorum muscles and extracellular matrix in the mid-forearm at 18 weeks, suggesting that TGFß-1 may serve as a bio-target to inhibit fibrosis development occurring as a consequence of WMSDs. TGFß-1 and CCN2 are important mediators of tissue fibrosis by their stimulatory effect on extracellular matrix deposition, with CCN2 functions as a downstream mediator of TGFß-1. Substance P (SubP), a neuropeptide, has also been linked to tissue fibrosis, although little work has been done to understand whether SubP directly causes fibrotic responses in tenocytes. In the in vitro study, we used primary tenocytes (obtained from flexor digitorum tendons of normal young adult female rats) and a cell line of primary rat dermal fibroblasts (RDF cells). We hypothesized that SubP either acts directly or indirectly through CCN2 or acts independently to increase fibroblast proliferation and extracellular matrix production in vitro. We demonstrated that SubP and TGFß-1 can induce CCN2 production in both rat tenocytes and RDF cells. Our data indicated that SubP-induced CCN2 production may be TGFß-1 independent since no TGFß-1 expression was detected after SubP treatment. We further demonstrated that SubP and TGFß-1 did not act in an additive or synergistic fashion to promote fibroblast proliferation and collagen production. Our findings suggest that both SubP and TGFß-1 have distinct fibrotic actions on tenocytes and both may be involved in tendinosis observed in animal models and patients.