Tissue inhibitors of metalloproteinases in cell signaling: metalloproteinase-independent biological activities

Abstract

Over the past 20 years, the tissue inhibitors of metalloproteinases (TIMPs) have been implicated in direct regulation of cell growth and apoptosis. However, the mechanisms of these effects have been controversial. Recent work by several laboratories has identified specific signaling pathways and cell surface binding partners for members of the TIMP family. TIMP-2 binding to the integrin alpha(3)beta(1) is the first description of a cell surface receptor for a TIMP family member. TIMP-2 has been shown to induce gene expression, to promote G(1) cell cycle arrest, and to inhibit cell migration. TIMP-1 binding to CD63 inhibits cell growth and apoptosis. These new findings suggest that TIMPs are multifunctional and can act either directly through cell surface receptors or indirectly through modulation of protease activity to direct cell fate. The emerging concept is that TIMPs function in a contextual fashion so that the mechanism of action depends on the tissue microenvironment.

Fig. 1

Multiple Pathways of TIMP-2-α3β1 signaling. TIMP-2 binding to α3β1 integrin initiates receptor tyrosine kinase inactivation through the action of the protein tyrosine phosphatase, Shp-1, which binds to SH2 sites created by phosphorylation of specific tyrosine residues. Cell cycle arrest is mediated by the de novo synthesis and nuclear translocation (gray circle) of p27^Kip1, which down-regulates activity of the cyclin-dependent kinases 4 and 2 (Cdk4/2). This results in hypophosphorylation of pRb and cell cycle arrest in G₁. Paxillin (PAX) is a scaffolding protein associated with integrin receptors that is phosphorylated downstream of activated Src. TIMP-2 binding to α3β1 activates c-terminal Src kinase (Csk) which is responsible for inactivation of Src kinase activity. TIMP-2 also mediates activation of the small GTPase Rap1 through a mechanism involving altered association of guanidine exchange factors (C3G and Crk) with PAX), ultimately resulting in increased abundance of RECK, a cell surface associate metalloproteinase inhibitor. Enhanced RECK expression at both the mRNA and protein level is associated with inhibition of cell migration. This suggests that, in addition to inhibiting cellular proliferation, TIMP-2 also seems to promote expression of cellular differentiation markers, i.e. RECK.

Fig. 2

Multiple Pathways of TIMP-1-CD63 signaling. TIMP-1 and TIMP-4 interact with the tetraspanin CD63, which associates with the β1 subunit of integrin receptors. Downstream of CD63, TIMP-1 inhibits both the intrinsic and extrinsic cell death pathways through activation of the FAK-PI3K pathway. These effects are mediated by activation of the extracellular regulated kinase (ERK). In addition, TIMP-1 inhibits cell growth through a mechanism similar to that of TIMP-2, involving suppression of cyclin D₁ and up regulation of p27^Kip1. The net effect is hypophosphorylation of pRB and cell cycle arrest in the G₁ phase of the cell cycle.