Integrin α1β1 Promotes Caveolin-1 Dephosphorylation by Activating T Cell Protein-tyrosine Phosphatase

Integrin alpha 1 beta 1 is a collagen receptor that down-regulates collagen and reactive oxygen species (ROS) production, and mice lacking this receptor show increased ROS levels and exacerbated glomerular sclerosis following injury. Caveolin-1 (Cav-1) is a multifunctional protein that is tyrosine-phosphorylated in response to injury and has been implicated in ROS-mediated injury. Cav-1 interacts with integrins, and integrin alpha 1 beta 1 binds/activates T cell protein-tyrosine phosphatase (TCPTP), which is homologous to the tyrosine phosphatase PTP1B known to dephosphorylate Cav-1. In this study, we analyzed whether phosphorylated Cav-1 (pCav-1) is a substrate of TCPTP and if integrin alpha 1 beta 1 is essential for promoting TCPTP-mediated Cav-1 dephosphorylation. We found that Cav-1 phosphorylation is significantly higher in cells lacking integrin alpha 1 beta 1 at base line and following oxidative stress. Overexpression of TCPTP leads to reduced pCav-1 levels only in cells expressing integrin alpha 1 beta 1. Using solid phase binding assays, we demonstrated that 1) purified Cav-1 directly interacts with TCPTP and the integrin alpha 1 subunit, 2) pCav-1 is a substrate of TCPTP, and 3) TCPTP-mediated Cav-1 dephosphorylation is highly increased by the addition of purified integrin alpha 1 beta 1 or an integrin alpha 1 cytoplasmic peptide to which TCPTP has been shown to bind. Thus, our results demonstrate that pCav-1 is a new substrate of TCPTP and that integrin alpha 1 beta 1 acts as a negative regulator of Cav-1 phosphorylation by activating TCPTP. This could explain the protective function of integrin alpha 1 beta 1 in oxidative stress-mediated damage and why integrin alpha 1-null mice are more susceptible to fibrosis following injury.