Regulation of matrix turnover: fibroblasts, forces, factors and fibrosis.

Fibroblasts are multifunctional cells that are responsible for matrix homoeostasis, continuously synthesizing and degrading a diverse group of extracellular molecules and their receptors. Rates of turnover of matrix molecules and the proteases that degrade them are normally under the control of diverse chemical and mechanical cues, with cytokines, growth factors, proteases, lipid mediators and mechanical forces playing roles. The maintenance of this homoeostasis is vital to the preservation of normal tissue function and is clearly lost in chronic diseases of the joints, skin and internal organs where destruction and excessive deposition is seen. Current research is focusing on defining the key pathways of activation either in resident fibroblasts, matrix-producing cells derived from circulating fibrocytes, or from transdifferentiation of resident cells. The common downstream signalling pathways are also being defined, as well as the gene interactions leading to altered cell phenotype. The present article reviews these findings and our current concepts of the key molecular events leading to tissue damage and excessive matrix deposition in tissue fibrosis. These studies are leading to an appreciation of the complexity of events with multiple pathways involved, but, as the facts emerge, we are finding promising new ways to treat fibrosis and halt the inexorable progression that is a feature of so many fibrotic and remodelling disorders.