This study was carried out to dissect the mechanism by which beta1 integrins promote resistance to radiation. For this purpose, we conditionally ablated beta1 integrins in the prostatic epithelium of transgenic adenocarcinoma of mouse prostate (TRAMP) mice. The ability of beta1 to promote resistance to radiation was also analyzed by using an inhibitory antibody to beta1, AIIB2, in a xenograft model. The role of beta1 integrins and of a beta1 downstream target, c-Jun amino-terminal kinase 1 (JNK1), in regulating radiation-induced apoptosis in vivo and in vitro was studied. We show that beta1 integrins promote prostate cancer (PrCa) progression and resistance to radiation in vivo. Mechanistically, beta1 integrins are shown here to suppress activation of JNK1 and, consequently apoptosis, in response to irradiation. Downregulation of JNK1 is necessary to preserve the effect of beta1 on resistance to radiation in vitro and in vivo. Finally, given the established cross-talk between beta1 integrins and type1 insulin-like growth factor receptor (IGF-IR), we analyzed the ability of IGF-IR to modulate beta1 integrin levels. We report that IGF-IR regulates the expression of beta1 integrins, which in turn confer resistance to radiation in PrCa cells. In conclusion, this study demonstrates that beta1 integrins mediate resistance to ionizing radiation through inhibition of JNK1 activation.

The cells' ability to proliferate in response to growth factor stimulation is significantly altered during cancer progression. To investigate the mechanisms underlying these alterations in prostate cancer, the role and expression of beta1A integrin and type 1 insulin-like growth factor receptor (IGF-IR), known to contribute to cell proliferation and transformation, were analyzed. Using small interfering RNA oligonucleotides to down-regulate beta1A, we show that beta1A expression is required for IGF-IR-mediated prostate cancer cell proliferation and anchorage-independent growth. In vivo, using age-matched transgenic adenocarcinoma of mouse prostate (TRAMP) mice at different stages of prostate cancer [prostatic intraepithelial neoplasia, PIN; well-differentiated adenocarcinoma, WD; and poorly differentiated adenocarcinoma, PD], the expression of beta1A and of IGF-IR was studied. beta1A and IGF-IR expression levels were concurrently up-regulated in high PIN and WD, whereas their expression did not correlate in late-stage PD. In contrast to the up-regulated expression of beta1A, the levels of beta1C, a beta1 cytoplasmic variant that inhibits cell proliferation, were down-regulated in all stages of prostate cancer. A similar expression pattern was observed for a beta1C downstream effector, Grb2-associated binder-1 (Gab1) which is known to inhibit IGF-IR phosphorylation. To analyze in vitro the mechanistic implications of beta1A, beta1C, and Gab1 deregulation in prostate cancer, we investigated whether expression of either beta1 variant in beta1-null cells affected IGF-IR localization. We found that IGF-IR and beta1A were colocalized in highly specialized integrin signaling compartments, designated focal contacts. However, in the presence of beta1C, IGF-IR remained diffuse on the cell surface and did not localize to focal contacts. The findings that beta1 integrins and IGF-IR are concurrently deregulated and that expression of beta1 integrins is necessary to achieve appropriate IGF-IR intracellular distribution point to the important role that the cross-talk between these receptors may have during prostate cancer progression and will be helpful in formulating new therapeutic strategies.