The TAK1/ROS pathway: a mediator of adhesive signaling?

点击翻译按钮获取中文摘要

Tissue architecture is maintained by the underlying connective tissue. In skin, the structure and organization of connective tissue provides support for surface epithelia and generates the skin’s elasticity and barrier functions. Connective tissue is comprised largely of extracellular matrix (ECM, which includes collagen). Within connective tissue, fibroblasts, which are responsible for producing and remodeling the ECM and hence for connective tissue function. In response to injury, a form of differentiated fibroblast, the myofibroblast (so called as it expresses the highly contractile protein alpha-smooth muscle actin), is responsible for producing and remodeling new ECM; myofibroblasts disappear during normal repair but, in scarred (fibrotic) tissue abnormally. Understanding how fibroblasts contribute to dermal homeostasis and repair is essential not only for understanding fibroblast function and skin homeostasis but also how to properly engineer replacement skin tissue which should integrate within the normal tissue without contractions (i.e., without scarring). The cytokine transforming growth factor (TGF) beta potently induces fibroblasts to produce and remodel ECM via both canonical pathway and non-canonical pathways, the latter of which involves adhesive signaling operating through integrin beta1 and focal adhesion kinase (FAK). Loss of integrin beta1 by fibroblasts results in impaired myofibroblast formation and delayed tissue repair kinetics. Moreover, expression of the phosphatase PTEN (Phosphatase and tensin homolog), which inactivates adhesive signaling by dephosphorylating FAK, is decreased in activated dermal fibroblasts; loss of PTEN expression by dermal fibroblasts significantly increased dermal thickness in vivo and elevated ECM production and contractile responses by fibroblasts both in vivo and in vitro. TGFbeta activated kinase (TAK)1 pathway operates downstream of FAK to promote ECM remodeling in response to TGFbeta. TAK1 expression by fibroblasts is required for maintaining normal skin thickness and for the induction of myofibroblasts in response to tissue injury. This non-canonical TGF beta pathway appears to specifically control contractile responses to TGF beta. However, the fundamental signaling mechanisms through which adhesion/TAK1 mediates the ability of fibroblasts to produce and remodel ECM are unclear. We also showed that fibroblasts actively undergoing ECM production/remodeling have integrin beta 1-mediated ROS generation was required for the expression of genes involved with ECM production and remodeling. Collectively, these data suggest the intriguing idea that the ability of fibroblasts to actively engage in ECM remodeling in response to factors such as TGF beta occurs via an adhesive signaling/TAK/ROS-dependent mechanism. However, this hypothesis has not been evaluated, and is the subject of this proposal.