6533b861fe1ef96bd12c5a9e

RESEARCH PRODUCT

Integrin alpha 2 beta 1 promotes activation of protein phosphatase 2A and dephosphorylation of Akt and glycogen synthase kinase 3 beta.

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The integrins are a large family of heterodimeric transmembrane receptors composed of α and β subunits (22). In addition to mediating cell-matrix interactions, integrins have been shown to activate intracellular signaling pathways which, in collaboration with growth factor-induced signals, regulate cellular functions (46). Some integrin signaling cascades are activated via the β subunit cytoplasmic domain, and they are therefore triggered by several integrin heterodimers. These signals include the activation of protein tyrosine kinases of the Src and focal adhesion kinase (FAK) families (9, 47). More-recent studies have revealed signaling events that are activated specifically by an α subunit (19). Integrins may associate with other membrane proteins, such as caveolin-1, and a subset of integrins can activate extracellular signal-related kinase, one of the mitogen-activated protein kinases, via Fyn and Shc (53, 54). Some integrins interact with other membrane proteins to regulate distinct signaling cascades. For example laminin receptor α3β1 associates with tetraspanin proteins and activates phosphatidylinositol-3-kinase (PI-3K) and PI-4K (4). We have shown that α2β1 integrin specifically activates the p38 pathway via a mechanism involving the α2 cytoplasmic tail and Cdc42 (25). The p38 signaling pathway seems to regulate the expression of type I collagen and collagenase-3 (25, 42), and it is required for cell migration on collagen (29). The PI-3K/Akt pathway is activated by a wide range of extracellular stimuli, including the integrins (12), and it has been linked to cell survival (13). Recently it was demonstrated that the different variants of the cytoplasmic domain in the β1 subunit can equally activate Akt (14, 16) and that the binding of α5β1 to fibronectin activates Akt, unlike the binding of α2β1 to monomeric collagen (15). Thus the activation of Akt may be dependent on the integrin α subunit. Reversible phosphorylation of proteins is a major mechanism for the control of cellular signaling pathways and maintenance of homeostasis (21). Although numerous kinases have been implicated in integrin signaling, the function and possible regulation of the corresponding phosphatases are largely unknown. Adhesion of cultured fibroblasts to extracellular matrix proteins has been shown to induce recruitment and activation of SHP-2, a nontransmembrane protein tyrosine phosphatase (39, 51). SHP-2 seems to play an active role in integrin-mediated signaling events, such as cell adhesion and migration (36, 62). Very little is known about the role of protein serine/threonine phosphatases in integrin signaling. Recent data have indicated a positive role for protein serine/threonine phosphatase 2A (PP2A) in integrin inside-out signaling. Inhibition of PP2A activity induces a selective loss of β1 integrins from focal adhesion sites (38) and inhibits cell adhesion (11); in addition PP2A has been shown to colocalize with β1 integrin at adhesion sites (38). However, the role of serine/threonine phosphatases in modulating integrin outside-in signals remains to be studied. Many studies have demonstrated the importance of PP2A in regulating a variety of cellular functions (52). Therefore it is likely that PP2A activity is tightly controlled in vivo. Cell adhesion to three-dimensional (3D) fibrillar collagen, unlike adhesion to monomeric two-dimensional collagen, inhibits cell proliferation in different cell types (15, 20, 30) and induces specific integrin-mediated signals, which regulate gene expression (25, 42, 44). Here, a novel α2β1-mediated signaling mechanism is introduced. Using human primary fibroblasts and human osteosarcoma (Saos-2) cell clones expressing either the wild-type α2 subunit or a signaling-deficient α2/α1 chimera, we have analyzed the ability of α2β1 integrin to regulate signals that have been linked with cell proliferation and survival. We and others have shown that α2β1 integrin is not involved in the regulation of the extracellular signal-related kinase mitogen-activated protein kinase pathway in response to collagen (25, 42, 53). However, here we show that cell adhesion to 3D collagen attenuates Akt and glycogen synthase kinase 3β (GSK3β) phosphorylation by a mechanism involving α2β1-induced activation of PP2A.