Keyword:
Insulin-like growth factor I Receptor (IGF-IR) signaling system, Human polyomavirus JC, Homologous recombination directed DNA repair (HRR), Non-homologous end joining (NHEJ), Medulloblastoma and Glioblastoma.
Details:
Molecular strategies aiming against Insulin-like Growth Factor I Receptor (IGF-IR) in Brain Tumors. Work from other laboratories and our recent findings strongly indicate that the signal from activated IGF-IR supports growth and survival of cancer cells, and the functional IGF-IR is required for supporting malignant transformation mediated by different cellular and viral oncogenic proteins, including Polyomavirus large T-antigens. In this respect, we have shown that medulloblastomas, which are aggressive cerebellar tumors of the childhood, are particularly sensitive to the inhibition of the IGF-IR. Therefore, we are developing and testing different strategies aiming at the IGF-IR in vitro and in experimental animals. These include: use of dominant negative mutants, antisense strategies, neutralizing antibodies, small inhibitory RNAs (siRNAs), and recently in collaboration with Novartis, the use of small molecular weight inhibitors of the IGF-IR tyrosine kinase activity. In addition, we are testing, different ways of drug delivery, into the tumors including: subcutaneous, intraperitoneal, and intracranial injections using different animal models of the tumor growth. Among delivery systems, we are testing retroviral and lentiviral expression vectors; lipid and nanoparticle bound oligodnucleotides; producer cells engineered to express and release viral vectors, dominant negative mutants, and synthetic peptides equipped with stearic acid residue, or Tat –nuclear localization sequence, to facilitate cellular uptake of the drug/s.
The contribution of IGF-IR to JC virus T-antigen –mediated cellular transformation. Polyomaviruses are highly suspected to participate in the development of cancer. In fact, substantial body of evidence points to the role of human polyomavirus JC in brain tumors. The viral oncoprotein, JCV T-antigen, has the ability to transform cells in culture, is tumorigenic in experimental animals, and has been found in a significant number of brain tumor clinical samples including medulloblastomas and glioblastomas. In addition, epidemiologic studies show that up to 70% of the human population is seropositive for JC virus, raising a possibility that this virus may be a common factor in tumor formation worldwide. By investigating functional association between JCV T-antigen and the IGF-IR system, we have found that: (i) cells with targeted disruption of the IGF-IR gene are resistant to JCV T-antigen –induced transformation; (ii) 1.5 x104 IGF-IR molecules per cell fully supports JCV T-antigen –mediated anchorage-independent growth in vitro; (iii) the major IGF-IR signaling molecule, IRS-1, directly interacts with JCV T-antigen, and translocates IRS-1 to the nucleus. In addition, we have determined specific regions within IRS-1 and JCV T-antigen molecules responsible for this interaction; and have established that the interaction with IRS-1 plays a critical role in JCV T-antigen -mediated transformation. In this project, we have discovered a new mechanism by which the signal from the IGF-IR affects the process of homologous recombination directed repair of DNA breaks (HRR). Our results indicate that HRR, which is a fundamental process responsible for DNA repair fidelity, is differentially affected by the IGF-IR and JCV T-antigen. We have shown that IGF-I stimulates and T-antigen inhibits fidelity of DNA repair. Importantly, IRS-1 binds directly to the major HRR protein, Rad51. The interaction has been localized within perinuclear region of the cell. In the presence of IGF-I, IRS-1 becomes tyrosine phosphorylated and loses the affinity to Rad51. Once released from the IRS-1 complex, Rad51 can translocation into the proximity of damaged DNA, triggering HRR. Importantly, JCV T-antigen translocates IRS-1 to the nucleus. In this “wrong” subcellular compartment IRS-1 is still capable of interacting with Rad51, but in the nucleus IRS-1 can interfere with Rad51 function during DNA repair. This leads to the inhibition of HRR, and may contribute to the increased involvement of unfaithful DNA repair mechanisms. In consequence, accumulation of spontaneous mutations, selection of new cellular adaptation and tumor progression and/or recurrences may develop.
Targeting PPARa and tumor chemoprevention: Recent studies suggest a potential role of lipid lowering drugs, fibrates and statins, in anticancer treatment. One candidate for tumor chemoprevention is fenofibrate, which is a potent agonist of peroxisome proliferator activated receptor alpha (PPARa). Our results demonstrate elevated expression of PPARa in the nuclei of neoplatic cells in 12 out of 13 cases of medulloblastoma, and of PPAR? in 6 out of 13 cases. Further analysis demonstrated that aggressive medulloblastoma and glioblastoma cell lines express PPARa. Both cell types responded to fenofibrate treatment by a significant increase of PPAR–mediated transcriptional activity, and by a gradual accumulation of cells in G1 and G2/M phase of the cell cycle, leading to the inhibition of cell proliferation and elevated apoptosis. In addition, pre-incubation with fenofibrate attenuated IGF-I–induced IRS-1, Akt, ERKs and GSK3ß phosphorylation, and inhibited their clonogenic growth. Importantly, simultaneous delivery of fenofibrate with low dose of the IGF-IR inhibitor, NVP-AEW541, completely abolished clonogenic growth and survival of medulloblastoma and partially inhibited glioblastoma cell motility lines in vitro and strongly attenuated subcutaneous growth of theses malignant cells in nude mice. These results indicate a strong supportive role of fenofibrate in chemoprevention against IGF-I-induced growth responses in medulloblastoma and in glioblastoma.
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