BackgroundChemo- and radiotherapeutic responses of leukemia cells are modified by integrin-mediated adhesion to extracellular matrix. To further characterize the molecular mechanisms by which β1 integrins confer radiation and chemoresistance, HL60 human acute promyelocytic leukemia cells stably transfected with β1 integrin and A3 Jurkat T-lymphoma cells deficient for Fas-associated death domain protein or procaspase-8 were examined.Methodology/Principal FindingsUpon exposure to X-rays, Ara-C or FasL, suspension and adhesion (fibronectin (FN), laminin, collagen-1; 5–100 µg/cm2 coating concentration) cultures were processed for measurement of apoptosis, mitochondrial transmembrane potential (MTP), caspase activation, and protein analysis. Overexpression of β1 integrins enhanced the cellular sensitivity to X-rays and Ara-C, which was counteracted by increasing concentrations of matrix proteins in association with reduced caspase-3 and -8 activation and MTP breakdown. Usage of stimulatory or inhibitory anti β1 integrin antibodies, pharmacological caspase or phosphatidylinositol-3 kinase (PI3K) inhibitors, coprecipitation experiments and siRNA-mediated β1 integrin silencing provided further data showing an interaction between FN-ligated β1 integrin and PI3K/Akt for inhibiting procaspase-8 cleavage.Conclusions/SignificanceThe presented data suggest that the ligand status of β1 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 β1 integrin/Akt complex, which signals to prevent procaspase-8-mediated induction of apoptosis in a PI3K-dependent manner. Antagonizing agents targeting β1 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.

Integrin-mediated adhesion of leukemia cells to extracellular matrix proteins reduces apoptosis following radiation-induced genotoxic injury. To evaluate the role of integrin-linked kinase (ILK) in this process, HL60 human acute promyelocytic leukemia cells were stably transfected with ILK wild-type or kinase-hyperactive overexpression vectors. Suspension or fibronectin (FN) adhesion cultures were irradiated with X-rays and processed for measurement of apoptosis, mitochondrial transmembrane potential and caspase activation. Adhesion to FN pronouncedly reduced radiation-induced apoptosis of HL60 cells and vector controls. Intriguingly, overexpressed ILK enhanced apoptosis after irradiation by combined activation of caspase-3 through caspase-8 and -9 in irradiated FN cultures. Irradiation of ILK suspension cultures lacked caspase-8 activation, but showed serial cleavage of caspase-9, -3 and poly (ADPribose) polymerase. These findings further characterize the cell death-promoting function of ILK in DNAdamaged cells. Moreover, ILK might represent a potential therapeutic target for innovative chemo-and radiooncological approaches in hematological malignancies.

In a previous publication, we were able to show that irradiation of KupVer cells, the liver resident macrophages, leads to an increased TNF-concentration in the culture medium. The pathomechanisms underlying this phenomenon, however, remained to be elucidated. Here, we show that following irradiation of KupVer cells, the apoptosis rate increased drastically within 48 h. At the same time, the total TNF-concentration in cell lysates of KupVer cells attached to the culture plate decreased. However, normalization of the TNF-concentration with respect to cell number revealed that TNF-concentration per attached cell remained constant during the observation period. Western blot analysis showed that heat shock protein 27 (Hsp27) is strongly downregulated and bax is upregulated in irradiated KupVer cells as compared to sham-irradiated cells. Overexpression of Hsp27 in KupVer cells was shown to prevent the eVect of irradiation on bax expression, apoptosis and, at the same time, on increase of TNF-concentration in the KupVer cell medium. We conclude that irradiation of KupVer cells leads to apoptosis because of downregulation of Hsp27 and consecutive upregulation of bax expression. Furthermore, we suggest that apoptosis of KupVer cells leads to an increase of TNFconcentration in the culture medium which may be due to cell death rather than active release or synthesis.

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