Evidence is provided showing that a) elevated cell surface expression levels of 1 integrins inevitably require elevated amounts of ligands to act in an antiapoptotic manner, and, b) a complex formation of 1 1 integrin with Akt prevents procaspase-8-mediated apoptosis PI3K-dependently. cleavage. Conclusions/Significance The presented data suggest that the ligand status of 1 1 integrins is critical for their antiapoptotic effect in leukemia cells treated with Ara-C, FasL or ionizing radiation. The antiapoptotic actions involve formation of a 1 integrin/Akt complex, which signals to prevent procaspase-8-mediated induction of apoptosis in a PI3K-dependent manner. Antagonizing agents targeting 1 integrin and PI3K/Akt signaling in conjunction with conventional therapies might effectively reduce radiation- and drug-resistant tumor populations and treatment failure in hematological malignancies. Introduction Integrin-mediated interactions of cells with extracellular matrix (ECM) are well known to confer resistance to clinically administered chemotherapeutic drugs or ionizing radiation C. These interactions mediate a substantial survival advantage particularly in isolated tumor cell niches. These residual tumor cell islands are likely to represent the starting base for the propagation of highly chemo and radiation resistant clonal cells in hematological neoplasias as well as solid tumors . Twenty-four different / heterodimeric transmembrane TBK1/IKKε-IN-5 integrin receptors are formed by 18 and 8 integrin subunits, which control survival, apoptosis, proliferation and differentiation among other functions in cooperation with receptor-mediated signaling from soluble growth factors or cytokines . As integrins lack intrinsic kinase activity, different cytoplasmic protein kinases recruited to cytoplasmic integrin domains such as integrin-linked kinase (ILK), focal adhesion kinase (FAK) and phosphatidylinositol-3 kinase (PI3K)/Akt have been reported to transmit signals in normal epithelial cells directly via the PI3K/Akt cascade to prevent anoikis (apoptosis upon detachment from ECM) C. FAK- and NFB-dependently, integrin-mediated adhesion regulates the expression of several members of the antiapoptotic Bcl-2 protein family C. By downregulating Bim and Bax and upregulating Bcl-2-like proteins, integrin-mediated cell adhesion confers resistance in leukemia cells to genotoxic brokers such as Ara-C, bleomycin, fludarabine or ionizing radiation , C. The first cue that procaspase-8 might play a critical role in integrin-mediated survival came from studies focusing on integrin-mediated death, which is usually induced by unligated integrins . It was reported that procaspase-8 binds to the cytoplasmic tail of integrins. Inhibition of procaspase-8 cleavage via enhanced binding of c-Fas-associated death domain-like interleukin-1-converting enzyme-like inhibitory protein-long (c-FLIPL) to Fas-associated death domain protein (FADD) also essentially contributes to adhesion-mediated survival in endothelial cells  or mediates drug resistance in myeloma cell lines . Concerning the role of 1 1 integrins in adherent growing tumor and normal cells, we uncovered a signaling pathway different from the apoptosis cascades. A PI3K-dependent signaling cascade from 1 integrin to the p130Cas/Paxillin/c-Jun N2-terminal kinase complex has demonstrated to confer an advantage of clonogenic cell survival in genotoxically stressed normal fibroblasts and cells from solid tumors , . With regard to drug- or radiation-induced apoptosis in leukemia cells TBK1/IKKε-IN-5 such as HL60, ILK promotes apoptosis upon irradiation via caspase-8 or -9 in an adhesion-dependent manner . In HL60 cells, Kasahara et al.  have found that FAK, as another critical mediator of integrin signals, functions in a prosurvival manner upon exposure to X-rays. Despite of this interesting discrepancy between ILK and FAK for cell survival after genotoxic stress, we strongly focused on 1 integrin and the intrinsic and extrinsic apoptotic pathways in Rabbit Polyclonal to PTGIS this study. TBK1/IKKε-IN-5 In addition to anoikis, there is a large number of different apoptosis-inducing stimuli such as ionizing radiation or cytotoxic drugs. Radiation-induced genotoxic injury mainly triggers the mitochondrial cascade involving release of cytochrome c, dATP, Apaf-1 and procaspase-9 upon Bax translocation to the mitochondrial membrane that, subsequently, results in breakdown of the mitochondrial transmembrane potential (m) and autoproteolytic cleavage of caspases C. The extrinsic.