| Thrombospondin-1 (TSP1) is a matricellular protein that is up-regulated in areas of active tissue remodeling. Amino acids 17-35 of TSP1 bind to cell surface calreticulin complexed with LRP1, induce focal adhesion disassembly and promote cell motility. This transition to an intermediate adhesive phenotype is proposed to be a dynamic state in cells which allows adaptation to changing environments and evasion of apoptotic stimuli. Here we show that TSP1 or a peptide of comprised of amino acids 17-35 (hep I) activates Akt, a pro-survival kinase and inhibits apoptotic stimuli induced by oxygen deprivation and loss of matrix contact. TSP1/hep I dose dependently activate Akt in a PI3K-dependent manner in endothelial cells and fibroblasts. TSP1/hep I binding to calreticulin is required for Akt activation, since calreticulin null fibroblasts or fibroblasts expressing calreticulin lacking the TSP1 binding site are unable to activate Akt by TSP1. A peptide that inhibits binding of TSP1 to calreticulin also inhibited Akt activation. Hep I, but not a control peptide, modified-hep I, inhibits apoptotic signaling in response to oxygen deprivation. Furthermore, TSP1/hep I inhibits apoptotic signaling in response to loss of matrix contact, a specialized form of apoptosis called anoikis. Suspended cells treated with TSP1 or hep I have increased viability, decreased caspase-3 activity and reduced levels of cleavage PARP1. Anoikis resistance induced by TSP1/hep I requires calreticulin and is PI3K-dependent. Together these data show that TSP1 binding to calreticulin/LRP1 through its hep I sequence activates anti-apoptotic signaling. These data suggest that TSP1 plays an important role in cellular response to injury and tissue remodeling by inhibiting apoptotic signaling and promoting anchorage-independent survival. |
