Loss of PTEN-assisted G2/M checkpoint impedes homologous recombination repair and enhances radio-curability and PARP inhibitor treatment response in prostate cancer.

Here we report that PTEN contributes to DNA double-strand break (DSB) repair via homologous recombination (HR), as evidenced by (i) inhibition of HR in a reporter plasmid assay, (ii) enhanced sensitivity to mitomycin-C or olaparib and (iii) reduced RAD51 loading at IR-induced DSBs upon PTEN knockdown. No association was observed between PTEN-status and RAD51 expression either in-vitro or in-vivo in a tissue microarray of 1500 PTEN-deficient prostate cancer (PC) samples. PTEN depletion and sustained activation of AKT sequestered CHK1 in the cytoplasm, thus impairing the G2/M-checkpoint after irradiation. Consistently, AKT inhibition recovered the G2/M-checkpoint and restored HR efficiency in PTEN-depleted cells. We show that, although PTEN loss correlates with a worse prognosis, it may predict for improved response of PC patients to radiotherapy. Further, we provide evidence for the use of PTEN as a biomarker for predicting the response to PARP inhibitors as radiosensitizing agents in prostate cancer. Collectively, these data implicate PTEN in maintaining genomic stability by delaying G2/M-phase progression of damaged cells, thus allowing time for DSB repair by HR. Furthermore, we identify PTEN-status in PC as a putative predictor of (i) radiotherapy response and (ii) response to treatment with PARP inhibitor alone or combined with radiotherapy.

Targeting of ß1 integrins impairs DNA repair for radiosensitization of head and neck cancer cells.

ß1 Integrin-mediated cell-extracellular matrix interactions allow cancer cell survival and confer therapy resistance. It was shown that inhibition of ß1 integrins sensitizes cells to radiotherapy. Here, we examined the impact of ß1 integrin targeting on the repair of radiation-induced DNA double-strand breaks (DSBs). ß1 Integrin inhibition was accomplished using the monoclonal antibody AIIB2 and experiments were performed in three-dimensional cell cultures and tumor xenografts of human head and neck squamous cell carcinoma (HNSCC) cell lines. AIIB2, X-ray irradiation, small interfering RNA-mediated knockdown and Olaparib treatment were performed and residual DSB number, protein and gene expression, non-homologous end joining (NHEJ) activity as well as clonogenic survival were determined. ß1 Integrin targeting impaired repair of radiogenic DSB (γH2AX/53BP1, pDNA-PKcs T2609 foci) in vitro and in vivo and reduced the protein expression of Ku70, Rad50 and Nbs1. Further, we identified Ku70, Ku80 and DNA-PKcs but not poly(ADP-ribose) polymerase (PARP)-1 to reside in the ß1 integrin pathway. Intriguingly, combined inhibition of ß1 integrin and PARP using Olaparib was significantly more effective than either treatment alone in non-irradiated and irradiated HNSCC cells. Here, we support ß1 integrins as potential cancer targets and highlight a regulatory role for ß1 integrins in the repair of radiogenic DNA damage via classical NHEJ. Further, the data suggest combined targeting of ß1 integrin and PARP as promising approach for radiosensitization of HNSCC.

The absence of Ku but not defects in classical non-homologous end-joining is required to trigger PARP1-dependent end-joining.

Classical-non-homologous end-joining (C-NHEJ) is considered the main pathway for repairing DNA double strand breaks (DSB) in mammalian cells. When C-NHEJ is defective, cells may switch DSB repair to an alternative-end-joining, which depends on PARP1 and is more erroneous. This PARP1-EJ is suggested to be active especially in tumor cells contributing to their genomic instability. Here, we define conditions under which cells would switch the repair to PARP1-EJ. Using the end jining repair substrate pEJ, we revealed that PARP1-EJ is solely used when Ku is deficient but not when either DNA-PKcs or Xrcc4 is lacking. In the latter case, DSB repair, however, could be shuttled to PARP1-EJ after additional Ku80 down-regulation, which partly rescued the DSB repair in these mutants. We demonstrate here that PARP-EJ may work on DSB ends at high fidelity manner, as evident from the unchanged efficiency upon blocking end resection by either roscovitin or mirin. Furthermore, we demonstrate for that PARP-EJ is likewise involved in the repair of multiple DSBs (I-PpoI- and IR-induced). Importantly, we identified a chromatin signature associated with the switch to PARP1-EJ which is characterized by a strong enrichment of both PARP1 and LigIII at damaged chromatin. Together, these data indicate that Ku is the main regulator for the hierarchal organization between C-NHEJ and PARP1-EJ.

Expression of insulin-like growth factor II mRNA-binding protein 3 in squamous cell carcinomas of the head and neck.

BACKGROUND: Insulin-like growth factor II mRNA-binding protein 3 (IMP3) was found overexpressed in various cancer types suggesting its possible role in carcinogenesis. Analysis of IMP3 expression in head and neck squamous cell carcinomas (HNSCC) is rare so that we evaluated it using tissue microarray method. METHOD: Immunohistochemical analysis of IMP3 was performed on samples from over 400 patients. The expression was measured semiquantitative, subsequently divided into four categories (negative, weak, medium, or strong) and correlated with several available clinicopathologic parameters. RESULTS: For HNSCC, positive IMP3 expression was observed in patients with all tumor stages (pT1-4) and nodal stages (pN0-3), showing also significant statistical correlation (P=0.023 and P=0.0013, respectively). No further correlations were found. Separate analysis according to tumor localization (oral cavity, oropharyngeal, and laryngeal) showed a significant correlation of positive IMP3 expression and overall survival (P=0.038) only in patients with tumors of the oral cavity. Multivariate analysis showed IMP3 as an independent predictive marker for oral squamous cell carcinomas (OSCC). CONCLUSION: Insulin-like growth factor II mRNA-binding protein 3 (IMP3) expression might be used as an independent prognostic factor in the subgroup of OSCC.

Role of activated astrocytes in neuronal damage: potential links to HIV-1-associated dementia.

HIV-1-associated dementia (HAD) is an important complication of HIV-1 infection. Reactive astrogliosis is a key pathological feature in HAD brains and in other central nervous system (CNS) diseases. Activated astroglia may play a critical role in CNS inflammatory diseases such as HAD. In order to test the hypothesis that activated astrocytes cause neuronal injury, we stimulated primary human fetal astrocytes with HAD-relevant pro-inflammatory cytokine IL-1beta. IL-1beta-activated astrocytes induced apoptosis and significant changes in metabolic activity in primary human neurons. An FITC-conjugated pan-caspase inhibitor peptide FITC-VAD-FMK was used for confirming caspase activation in neurons. IL-1beta activation enhanced the expression of death protein FasL in astrocytes, suggesting that FasL is one of the potential factors responsible for neurotoxicity observed in HAD and other CNS diseases involving glial inflammation. Our data presented here add to the developing picture of role of activated glia in HAD pathogenesis.

HIV-1 and IL-1 beta regulate Fas ligand expression in human astrocytes through the NF-kappa B pathway.

Reactive astrogliosis is a prominent pathological feature of HIV-1-associated dementia (HAD). We hypothesized that in HAD, astrocytes activated with proinflammatory stimuli such as IL-1beta express Fas ligand (FasL), a death protein. IL-1beta and HIV-1-activated astrocytes expressed FasL mRNA and protein. Luciferase reporter constructs showed that IL-1beta and HIV-1 upregulated FasL promoter activity (p<0.001). The NF-kappaB pathway was involved as shown by inhibition with SN50 and dominant negative IkappaBalpha mutants. Brain extracts from HAD patients had significantly elevated FasL levels compared to HIV-seropositive (p<0.001) and seronegative individuals (p<0.01). We propose that astrocyte expression of FasL may participate in neuronal injury in HAD.