Targeting the oncogenic protein beta-catenin to enhance chemotherapy outcome against solid human cancers.

BACKGROUND: Beta-catenin is a multifunctional oncogenic protein that contributes fundamentally to cell development and biology. Elevation in expression and activity of ß-catenin has been implicated in many cancers and associated with poor prognosis. Beta-catenin is degraded in the cytoplasm by glycogen synthase kinase 3 beta (GSK-3ß) through phosphorylation. Cell growth and proliferation is associated with ß-catenin translocation from the cytoplasm into the nucleus. This laboratory was the first to demonstrate that selenium-containing compounds can enhance the efficacy and cytotoxicity of anticancer drugs in several preclinical xenograft models. These data provided the basis to identify mechanism of selenium action focusing on ß-catenin as a target. This study was designed to: (1) determine whether pharmacological doses of methylseleninic acid (MSeA) have inhibitory effects on the level and the oncogenic activity of ß-catenin, (2) investigate the kinetics and the mechanism of ß-catenin inhibition, and (3) confirm that inhibition of ß-catenin would lead to enhanced cytotoxicity of standard chemotherapeutic drugs. RESULTS: In six human cancer cell lines, the inhibition of total and nuclear expression of ß-catenin by MSeA was dose and time dependent. The involvement of GSK-3ß in the degradation of ß-catenin was cell type dependent (GSK-3ß-dependent in HT-29, whereas GSK-3ß-independent in HCT-8). However, the pronounced inhibition of ß-catenin by MSeA was independent of various drug treatments and was not reversed after combination therapy.Knockout of ß-catenin by ShRNA and its inhibition by MSeA yielded similar enhancement of cytotoxicity of anticancer drugs.Collectively, the generated data demonstrate that ß-catenin is a target of MSeA and its inhibition resulted in enhanced cytotoxicity of chemotherapeutic drugs. CONCLUSIONS: This study demonstrates that ß-catenin, a molecule associated with drug resistance, is a target of selenium and its inhibition is associated with increased multiple drugs cytotoxicity in various human cancers. Further, degradation of ß-catenin by GSK-3ß is not a general mechanism but is cell type dependent.

Expression of RASSF1A, an epigenetically silenced tumor suppressor, overcomes resistance to apoptosis induction by interferons.

Resistance of human renal cell carcinoma (RCC) and melanoma to the apoptosis-inducing effects of IFNs was postulated to result from epigenetic silencing of genes by DNA methylation, a common feature of human cancers. To reverse silencing, 5-AZA-deoxycytidine (5-AZA-dC) or selective depletion of DNA methyltransferase 1 (DNMT1) by phosphorothioate oligonucleotide antisense (DNMT1 AS) were employed in cells resistant (<5% terminal deoxynucleotidyl transferase-mediated nick-end labeling positive) to apoptosis induction by IFN-alpha2 and IFN-beta (ACHN, SK-RC-45, and A375). 5-AZA-dC and DNMT1 AS similarly depleted available DNMT1 protein and, at doses that did not cause apoptosis alone, resulted in apoptotic response to IFNs. The proapoptotic tumor suppressor RASSF1A was reactivated by DNMT1 inhibitors in all three cell lines. This was associated with demethylation of its promoter region. IFNs augmented RASSF1A protein expression after reactivation by DNMT1 inhibition. In IFN-sensitive WM9 melanoma cells, expression of RASSF1A was constitutive but also augmented by IFNs. RASSF1A small interfering RNA reduced IFN-induced apoptosis in WM9 cells and in DNMT1-depleted ACHN cells. Conversely, lentiviral expression of RASSF1A but not transduction with empty virus enabled IFN-induced apoptosis. IFN induced tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and TRAIL-neutralizing antibody inhibited apoptotic response to IFN in RASSF1A-expressing ACHN cells. Accordingly, RASSF1A markedly sensitized to recombinant TRAIL. Normal kidney epithelial cells, although expressing RASSF1A, did not undergo apoptosis in response to IFN or TRAIL but had >400-fold higher TRAIL decoy receptor 1 expression than transduced ACHN cells (real-time reverse transcription-PCR). Results identified RASSF1A as regulated by IFNs and participating in IFN-induced apoptosis at least in part by sensitization to TRAIL.