Integrins and the activation of latent transforming growth factor beta1 - an intimate relationship.

Integrins are crucial for the ability of cells to sense mechanical perturbations and to transmit intracellular stress to their environment. We here review the more recently discovered role of integrins in activating the pleiotrophic cytokine transforming growth factor beta 1 (TGF-beta1). TGF-beta1 controls tissue homeostasis in embryonic and normal adult tissues and contributes to the development of fibrosis, cancer, autoimmune and vascular diseases when being mis-regulated. In most of these conditions, active TGF-beta1 is generated by dissociation from a large latent protein complex that sequesters latent TGF-beta1 in the extracellular matrix (ECM). Two main models are proposed how integrins contribute to latent TGF-beta1 activation: (1) In a protease-dependent mechanism, integrins alphavbeta8 and alphavbeta3 are suggested to simultaneously bind the latent TGF-beta1 complex and proteinases. This close vicinity at the cell surface improves enzymatic cleavage of the latent complex to release active TGF-beta1. (2) Integrins alphavbeta3, alphavbeta5, alphavbeta6, and alphavbeta8 appear to change the conformation of the latent TGF-beta1 complex by transmitting cell traction forces. This action requires association of the latent complex with a mechanically resistant ECM and is independent from proteolysis. Understanding that different integrins use different mechanisms to activate latent TGF-beta1 opens new possibilities to develop cell-specific therapeutic strategies for TGF-beta1-induced pathologies.