Induction of duplication reversion in human fibroblasts, by wild-type and mutated SV40 T antigen, covaries with the ability to induce host DNA synthesis

Targeting PI3K and RAD51 in Barrett's adenocarcinoma: impact on DNA damage checkpoints, expression profile and tumor growth

Phosphatidylinositol 3-kinase (PI3K)/v-akt murine thymoma viral oncogene homolog 1 (AKT) signaling in cancer is implicated in various survival pathways including regulation of recombinase (RAD51). In this study, we evaluated PI3K and RAD51 as targets in Barrett's adenocarcinoma (BAC) cells both in vitro and in vivo. BAC cell lines (OE19, OE33, and FLO-1) were cultured in the presence of PI3K inhibitor (wortmannin) and the impact on growth and expression of AKT, phosphorylated-AKT (P-AKT), and RAD51 was determined. Wortmannin induced growth arrest and apoptosis in two BAC cell lines (OE33 and OE19), which had relatively higher expression of AKT. FLO-1 cells, with lower AKT expression, were less sensitive to treatment and investigated further. In FLO-1 cells, wortmannin suppressed ataxia telangiectasia and Rad3-related protein (ATR)-checkpoint kinase 1 (CHK1)-mediated checkpoint and multiple DNA repair genes, whereas RAD51 and CHK2 were not affected. Western blotting confirmed that RAD51 was suppressed by wortmannin in OE33 and OE19 cells, but not in FLO-1 cells. Suppression of RAD51 in FLO-1 cells down-regulated the expression of CHK2 and CHK1, and reduced the proliferative potential. Finally, the suppression of RAD51 in FLO-1 cells, significantly increased the anticancer activity of wortmannin in these cells, both in vitro and in vivo. We show that PI3K signaling and hsRAD51, through distinct roles in DNA damage response and repair pathways, provide survival advantage to BAC cells. In cells with inherent low expression of AKT, RAD51 is unaffected by PI3K suppression and provides an additional survival pathway. Simultaneous suppression of PI3K and RAD51, especially in cells with lower AKT expression, can significantly reduce their proliferative potential.

Repetitive sequences, genomic instability and Barrett's esophageal adenocarcinoma

Barrett's esophageal adenocarcinoma (BAC) is a cancer associated with heartburn. If gastroesophageal reflux is not treated, the exposure to acid over the years, leads to a premalignant condition known as Barrett's esophagus (BE) which then progresses through low grade and high grade dysplasias to Barrett's adenocarcinoma. Genomic instability, which seems to arise early at BE stage, leads to accrual of mutational changes which underlie the the succession of histological and physiological changes associated with this disease. Genomic instability is therefore an important target for prevention and treatment of cancer and it is important to elucidate the mechanisms associated with this problem. We have shown that elevated/deregulated homologous recombination mediates genomic instability in cancer. Recently we also demonstrated that the mutational rates of individual chromosomes in BAC cells correlate with their ALU frequency. The aims of this article are to briefly discuss different types of repetitive sequences and highlight their importance in physiology of normal and cancer cells, especially BAC.

Genomic evolution in Barrett's adenocarcinoma cells: critical roles of elevated hsRAD51, homologous recombination and Alu sequences in the genome

A prominent feature of most cancers including Barrett's adenocarcinoma (BAC) is genetic instability, which is associated with development and progression of disease. In this study, we investigated the role of recombinase (hsRAD51), a key component of homologous recombination (HR)/repair, in evolving genomic changes and growth of BAC cells. We show that the expression of RAD51 is elevated in BAC cell lines and tissue specimens, relative to normal cells. HR activity is also elevated and significantly correlates with RAD51 expression in BAC cells. The suppression of RAD51 expression, by short hairpin RNA (shRNA) specifically targeting this gene, significantly prevented BAC cells from acquiring genomic changes to either copy number or heterozygosity (P<0.02) in several independent experiments employing single-nucleotide polymorphism arrays. The reduction in copy-number changes, following shRNA treatment, was confirmed by Comparative Genome Hybridization analyses of the same DNA samples. Moreover, the chromosomal distributions of mutations correlated strongly with frequencies and locations of Alu interspersed repetitive elements on individual chromosomes. We conclude that the hsRAD51 protein level is systematically elevated in BAC, contributes significantly to genomic evolution during serial propagation of these cells and correlates with disease progression. Alu sequences may serve as substrates for elevated HR during cell proliferation in vitro, as they have been reported to do during the evolution of species, and thus may provide additional targets for prevention or treatment of this disease.

Dysfunctional homologous recombination mediates genomic instability and progression in myeloma

A prominent feature of most if not all cancers is a striking genetic instability, leading to ongoing accrual of mutational changes, some of which underlie tumor progression, including acquisition of invasiveness, drug resistance, and metastasis. Thus, the molecular basis for the generation of this genetic diversity in cancer cells has important implications in understanding cancer progression. Here we report that homologous recombination (HR) activity is elevated in multiple myeloma (MM) cells and leads to an increased rate of mutation and progressive accumulation of genetic variation over time. We demonstrate that the inhibition of HR activity in MM cells by small inhibitory RNA (siRNAs) targeting recombinase leads to significant reduction in the acquisition of new genetic changes in the genome and, conversely, the induction of HR activity leads to significant elevation in the number of new mutations over time and development of drug resistance in MM cells. These data identify dysregulated HR activity as a key mediator of DNA instability and progression of MM, with potential as a therapeutic target.

Growth arrest, apoptosis, and telomere shortening of Barrett's-associated adenocarcinoma cells by a telomerase inhibitor

BACKGROUND & AIMS

Barrett's esophageal adenocarcinoma (BEAC) is a complication of gastroesophageal reflux disease, with no effective chemotherapy and poor prognosis. BEAC cells, like many other types of cancers, may reactivate telomerase to achieve unlimited proliferative potential, making telomerase a unique therapeutic target. The purpose of this study was to evaluate effects of telomerase inhibition on BEAC.

METHODS

We examined the effect of a selective G-quadruplex intercalating telomerase inhibitor, 2,6-bis[3-(N-Piperidino)propionamido]anthracene-9,10-dione (PPA), on telomerase activity, telomere length, colony size distribution, and proliferative potential in 2 BEAC cell lines, BIC-1 and SEG-1.

RESULTS

Telomerase activity was >10-fold and >600-fold elevated in the adenocarcinoma cells as compared with normal gastric/intestinal cells and normal diploid fibroblasts, respectively. Telomeres were short, being less than 4 kilobase pair in both tumor cell lines. Exposure to PPA effectively inhibited telomerase activity and shortened telomeres. PPA also arrested cell proliferation and reduced colony number and size after a lag period of about 10 cell generations, consistent with the attrition of telomeres. The growth arrest was not due to senescence but was due to apoptosis. Expression analysis of the cells following PPA treatment did not show significant change in the expression of genes involved in cell-cycle proliferation and apoptosis. Exposure to PPA had no effect on proliferative potential of normal intestinal cells.

CONCLUSIONS

We conclude that telomerase inhibition by PPA induces cell growth arrest in BEAC cells and demonstrate the potential of telomerase inhibitors in chemoprevention and treatment of Barrett's-associated esophageal adenocarcinoma.

Telomerase inhibition by peptide nucleic acids reverses 'immortality' of transformed human cells

Telomerase activity, the ability to add telomeric repeats to the ends of chromosomes, has been detected in most immortal cell lines including tumor cells, but is low or absent in most diploid, mortal cells such as those of somatic tissues. Peptide nucleic acids (PNAs), analogs of DNA or RNA which bind to complementary nucleic acids with very high affinity, were co-electroporated into immortal human cells along with a selectable plasmid. Introduction of PNAs inverse-complementary to telomerase RNA effectively inhibited telomerase activity in intact cells, shortened telomeres, reduced colony size, and arrested cell proliferation after a lag period of 5-30 cell generations, consistent with suppression of their 'immortality'. Electroporation of selection plasmid alone had no effect, while PNAs of altered sequence were markedly less effective in each assay. This constitutes the first demonstration of cell growth arrest through telomerase inhibition, upon treatment of intact cells with an exogenous compound which can be efficiently delivered in vivo. The phenotype of telomerase-inhibited transformed cells differs from senescence of normal diploid fibroblasts, but rather resembles the crisis state of incompletely transformed cells.

Carcinogens stimulate intrachromosomal homologous recombination at an endogenous locus in human diploid fibroblasts

Mitotic recombination is believed to play an important role in the development of many cancers. An improved system has been developed to detect reversion of an intragenic DNA duplication, as a model for intrachromosomal homologous recombination. The 'LNtd' strain of human fibroblasts, derived from a Lesch-Nyhan donor, produces no detectable hypoxanthine phosphoribosyltransferase (HPRT) activity due to a 13.7-kilobase-pair DNA insertion duplicating exons 2 and 3 of the HPRT locus. These cells are therefore sensitive to selection in HAT medium, against cells lacking functional HPRT enzyme. Clonal reversion to HAT resistance occurs spontaneously at 1-3 x 10(-5)/cell/generation, and can be induced by brief exposure to a variety of carcinogenic agents. Six known carcinogens, including two (diethylstilbestrol and nickel chloride) which were non-mutagenic in Salmonella by Ames HIS-reversion tests, showed dose-dependent induction of LNtd reversion by a maximum of 2.4- to > 11-fold over controls (each p < 0.01). In contrast, 5 non-carcinogenic agents, including two 'Ames-positive' chemicals, sodium azide and 8-hydroxyquinoline, evoked no more than a 1.7-fold increase in reversion (not significant). The molecular events associated with reversion to HAT-resistance were characterized, relative to the parental strain, in HATR clones derived from either untreated or carcinogen-treated cells. Both the intron-3:intron-1 junction situated between the duplicated HPRT segments in LNtd cells (amplified by polymerase chain reaction), and a restriction fragment corresponding to the duplicated HPRT DNA (assessed by Southern-blot hybridization), were lost from the majority of HATR revertant clones, whether they arose spontaneously or following exposure to Cr(VI) or ultraviolet light. These results imply that HATR reversion is induced in LNtd cells by carcinogenic treatments, through a mechanism consistent with homologous recombination, and is highly concordant with induction of in vivo carcinogenesis by the same agents.