Inhibition of human pancreatic cancer growth by the adenovirus-mediated introduction of a novel growth suppressing gene, tob, in vitro

Transducer of ERBB2.1 (TOB1) as a Tumor Suppressor: A Mechanistic Perspective

Transducer of ERBB2.1 (TOB1) is a tumor-suppressor protein, which functions as a negative regulator of the receptor tyrosine-kinase ERBB2. As most of the other tumor suppressor proteins, TOB1 is inactivated in many human cancers. Homozygous deletion of TOB1 in mice is reported to be responsible for cancer development in the lung, liver, and lymph node, whereas the ectopic overexpression of TOB1 shows anti-proliferation, and a decrease in the migration and invasion abilities on cancer cells. Biochemical studies revealed that the anti-proliferative activity of TOB1 involves mRNA deadenylation and is associated with the reduction of both cyclin D1 and cyclin-dependent kinase (CDK) expressions and the induction of CDK inhibitors. Moreover, TOB1 interacts with an oncogenic signaling mediator, β-catenin, and inhibits β-catenin-regulated gene transcription. TOB1 antagonizes the v-akt murine thymoma viral oncogene (AKT) signaling and induces cancer cell apoptosis by activating BCL2-associated X (BAX) protein and inhibiting the BCL-2 and BCL-XL expressions. The tumor-specific overexpression of TOB1 results in the activation of other tumor suppressor proteins, such as mothers against decapentaplegic homolog 4 (SMAD4) and phosphatase and tensin homolog-10 (PTEN), and blocks tumor progression. TOB1-overexpressing cancer cells have limited potential of growing as xenograft tumors in nude mice upon subcutaneous implantation. This review addresses the molecular basis of TOB1 tumor suppressor function with special emphasis on its regulation of intracellular signaling pathways.

Adenovirus-mediated expression of Tob1 sensitizes breast cancer cells to ionizing radiation

AIM

To investigate the effect of the Tob1 gene, a member of the Transducing Molecule of ErbB2/B-cell Translocation Ggene (TOB/BTG) family, by using the adenovirus-mediated expression of Tob1 on radiosensitivity in a human breast cancer cell line MDA-MB-231.

METHODS

Cell survival was determined by clonogenic assay. Apoptosis was evaluated by DNA fragmentation gel electrophoresis and terminal deoxynucleotidyl transferase-mediated nick end labeling assay. Protein expression was analyzed by Western blot assay and DNA repair was measured by a host cell reactivation assay.

RESULTS

We demonstrated that pre-irradiation treatment with Ad5-Tob1 significantly increased radiosensitivity, accompanying the increased induction of apoptosis and the repression of DNA damage repair. Furthermore, Ad5-Tob1-mediated radiosensitivity correlates with the upregulation of the pro-apoptotic protein Bax and the downregulation of several DNA double strand break repair proteins, including DNA-dependent protein kinases, Ku70 and Ku80, and X-ray-sensitive complementation group 4.

CONCLUSION

Tob1, as a new radiosensitizer, is a new target in the radiotherapy of breast cancer via increasing apoptosis and suppressing DNA repair.

Identification of a potential role for POU2AF1 and BTG4 in the deletion of 11q23 in chronic lymphocytic leukemia

Deletions of 11q in chronic lymphocytic leukemia (CLL) are usually associated with progressive disease and poor prognosis. A novel translocation within the previously identified 11q minimal region has been defined in a patient with CLL. The breakpoint is between genes POU2AF1 and BTG4. POU2AF1 is a B-cell-specific transcriptional coactivator, and BTG4 is a member of the BTG family of negative regulators of the cell cycle, making both of them good candidate genes for the pathogenesis of 11q- CLL. POU2AF1 was observed to be differentially expressed in the cells of patients with CLL compared to its expression in normal B cells in the absence of mutations. This may reflect ongoing stimulation and active accessory signaling in CLL cells. BTG4 could contribute to CLL pathogenesis following inactivation by haploinsufficiency.

Mice lacking a transcriptional corepressor Tob are predisposed to cancer

tob is a member of antiproliferative family genes. Mice lacking tob are prone to spontaneous formation of tumors. The occurrence rate of diethylnitrosamine-induced liver tumors is higher in tob(-/-) mice than in wild-type mice. tob(-/-)p53(-/-) mice show accelerated tumor formation in comparison with single null mice. Expression of cyclin D1 mRNA is increased in the absence of Tob and is reduced by Tob. Tob acts as a transcriptional corepressor and suppresses the cyclin D1 promoter activity through an interaction with histone deacetylase. Levels of tob mRNA are often decreased in human cancers, implicating tob in cancer development.

The gene PC3(TIS21/BTG2), prototype member of the PC3/BTG/TOB family: regulator in control of cell growth, differentiation, and DNA repair?

PC3(TIS21/BTG2) is the founding member of a family of genes endowed with antiproliferative properties, namely BTG1, ANA/BTG3, PC3B, TOB, and TOB2. PC3 was originally isolated as a gene induced by nerve growth factor during neuronal differentiation of rat PC12 cells, or by TPA in NIH3T3 cells (named TIS21), and is a marker for neuronal birth in vivo. This and other findings suggested its implication in the process of neurogenesis as mediator of the growth arrest before differentiation. Remarkably, its human homolog, named BTG2, was shown to be p53-inducible, in conditions of genotoxic damage. PC3(TIS21/BTG2) impairs G(1)-S progression, either by a Rb-dependent pathway through inhibition of cyclin D1 transcription, or in a Rb-independent fashion by cyclin E downregulation. PC3(TIS21/BTG2) might also control the G(2) checkpoint. Furthermore, PC3(TIS21/BTG2) interacts with carbon catabolite repressor protein-associated factor 1 (CAF-1), a molecule that associates to the yeast transcriptional complex CCR4 and might influence cell cycle, with the transcription factor Hoxb9, and with the protein-arginine methyltransferase 1, that might control transcription through histone methylation. Current evidence suggests a physiological role of PC3(TIS21/BTG2) in the control of cell cycle arrest following DNA damage and other types of cellular stress, or before differentiation of the neuron and other cell types. The molecular function of PC3(TIS21/BTG2) is still unknown, but its ability to modulate cyclin D1 transcription, or to synergize with the transcription factor Hoxb9, suggests that it behaves as a transcriptional co-regulator.