Absence of stimulation of poly(ADP-ribose) polymerase activity in patients predisposed to colon cancer

DNA damage response pathways and cell cycle checkpoints in colorectal cancer: current concepts and future perspectives for targeted treatment

Although several drugs have been designed in the last few years to target specific key pathways and functions in colorectal cancer (CRC), the backbone of CRC treatment is still made up of compounds which rely on DNA damage to accomplish their role. DNA damage response (DDR) and checkpoint pathways are intertwined signaling networks that arrest cell cycle, recognize and repair genetic mistakes which arise during DNA replication and transcription, as well as through the exposure to chemical and physical agents that interact with nucleic acids. The good but highly variable activity of DNA damaging agents in the treatment of CRC suggests that intrinsic alterations in DDR pathways and cell cycle checkpoints may contribute differentially to the way cancer cells react to DNA damage. In the present review, our aim is to depict the recent advances in understanding the molecular basis of the activity of DNA damaging agents used for the treatment of CRC. We focus on the known and potential drug targets that are part of these complex and intertwined pathways. We describe the potential role of the checkpoints in CRC, and how their pharmacological manipulation could lead to chemopotentiation or synergism with currently used drugs. Novel therapeutic agents playing a role in DDR and checkpoint inhibition are assessed. We discuss the possible rationale for combining PARP inhibition with DNA damaging agents, and we address the link between DDR and EGFR pathways in CRC.

AMP-activated protein kinase is a positive regulator of poly(ADP-ribose) polymerase

AMPK acts as a cellular fuel gauge and responds to decreased cellular energy status by inhibiting ATP-consuming pathways and increasing ATP-synthesis. The aim of this study was to examine the role of AMPK in modulating poly(ADP-ribose) polymerase (PARP), a nuclear enzyme involved in maintaining chromatin structure and DNA repair. HT-29 cells infected with constitutively active AMPK demonstrated increased PARP automodification and an increase in bioNAD incorporation. AMPK and PARP co-immunoprecipitated under basal conditions and in response to H(2)O(2), suggesting a physical interaction under both resting and stress-induced conditions. Incubation of PARP with purified AMPK resulted in the phosphorylation of PARP; and the inclusion of AMP as an AMPK activator potentiated PARP phosphorylation. Using immobilized PARP, the incorporation of bioNAD by PARP was dramatically increased following the addition of AMPK. These data suggest a novel role for AMPK in regulating PARP activity through a direct interaction involving phosphorylation.

X-ray induced L02 cells damage rescued by new anti-oxidant NADH

AIM: To explore molecular mechanism of nicotinamide adenine dinucleotide (NADH) antagonization against X-ray induced L02 cells damage.

METHODS: L02 liver cells were cultured in RPMI 1640, exposed to X-ray irradiation and continued to culture in the presence or absence of NADH. Cellular viability was analyzed by routine MTT methods. The percent age of apoptotic cells and positive expressions of p53, bax and bcl-2, fas, fasL proteins were determined by FCM. Level of intracellular ROS was determined by confocal microscope scanning. Morphological change was detected by scanning electron micrograph.

RESULTS: The viability of L02 cells was decreased with increasing dose of X-ray irradiation. NADH could not only eliminate the apoptosis induced by X-ray irradiation, but also up-regulate expression of bcl-2 protein and down-regulate expression of p53, bax, fas and fasL proteins (P < 0.05). At the same time, NADH could reduce level of intracellular ROS in radiated L02 cells.

CONCLUSION: NADH has marked anti-radiation effect, its mechanism may be associated with up-regulation of bcl-2 expression and down-regulation of p53, bax fas and fasL expression, as well as decline of intracellular ROS. However, further investigation of its mechanism is worthwhile.

Patchy field defects of apoptosis resistance and dedifferentiation in flat mucosa of colon resections from colon cancer patients

BACKGROUND

Abnormal areas in normal-appearing flat colonic mucosa (field defects) may predispose individuals to colon cancer. Markers of field defects would indicate cancer risk.

METHODS

We evaluated apoptosis capability, dedifferentiation, frequency of simple aberrant crypts, aberrant crypt foci, microadenomas, and total nicotinamide adenine dinucleotide levels at locations within normal-appearing flat mucosa obtained from colon resections.

RESULTS

Among goblet cells from colonic mucosa samples of individuals without colonic neoplasia, there was a high mean deoxycholate-induced apoptotic index (AI) of 59.1% and high Dolichos biflorus agglutinin (DBA) lectin reactivity (differentiation) in 85.0% of samples. In contrast, flat mucosa samples from colon cancer patients had a significantly (P <.01) lower average AI of 37.4%, and a significantly (P =.03) lower percentage (40.5%) had high DBA reactivity. For colon cancer patients, AI and DBA reactivity values were patchy within a resection. Nicotinamide adenine dinucleotide levels were highly variable among individuals without neoplasia, and aberrant crypt foci and microadenomas were rare.

CONCLUSIONS

AI and aberrant DBA reactivity are promising indicators of colon cancer risk. Our results attest to the importance of obtaining multiple samples to assess colon cancer risk because of the patchy nature of field defects.

Reduced poly(ADP-ribosyl)ation in lymphocytes of laryngeal cancer patients: results of a case-control study

Poly(ADP-ribose) polymerase (PARP), a nuclear enzyme that is catalytically activated by DNA strand breaks, plays a complex role in DNA repair. Using NAD(+) as a precursor, it catalyzes the formation of ADP-ribose polymers, which are attached to various proteins. Defects in DNA repair pathways have been associated with increased risks for cancer in humans. We investigated whether differences in the activity of PARP are associated with the risk for laryngeal cancer. In a case-control study on genetic, lifestyle and occupational risk factors for laryngeal cancer, PARP activity was assessed as DNA damage-induced poly(ADP-ribose) formation in human peripheral blood lymphocytes by quantitative immunofluorescence analysis. Polymer formation was determined as the cellular response to bleomycin, a well-known inducer of DNA strand breaks, in lymphocytes from 69 laryngeal cancer patients and 125 healthy controls. The frequency of bleomycin-induced polymer formation, measured as mean pixel intensity, was significantly lower in cases (74.6, SE = 3.7) than in controls (94.5, SE = 3.5) and not influenced by smoking, age or sex. There was no significant difference between cases (59.1, SE = 5.2) and controls (50.5, SE = 3.7) in basal polymer formation (in cells not treated with bleomycin). When the highest tertile of polymer formation was used as the reference, the odds ratio for the lowest tertile of bleomycin-induced polymer formation was 3.79 (95% confidence interval 1.37-10.47, p = 0.01). Peripheral blood lymphocytes from laryngeal cancer patients thus showed significantly less bleomycin-induced poly(ADP-ribose) formation. Our results suggest that a reduced capacity of somatic cells to synthesize poly(ADP-ribose) might be associated with an increased risk for laryngeal cancer. The underlying mechanism remains to be investigated.

DNA repair pathways and cancer prevention

This article describes the five main classes of DNA repair processes that occur in humans with respect to their mechanism of action, major substrates, and role in protection against endogenous and environmental DNA damaging agents. The importance of all of these processes in protection from the initiation of neoplastic growth has been established either in studies of inheritable diseases affecting DNA repair or experiments with transgenic animals or both. The capacity of DNA repair pathways to deal with DNA damage is therefore a critical factor in the cellular response to environmental, and dietary carcinogens. DNA repair activity and factors affecting this activity either directly or indirectly must be taken into account in risk assessment.

Functions of poly(ADP-ribose) polymerase in controlling telomere length and chromosomal stability

In most eukaryotes, poly(ADP-ribose) polymerase (PARP) recognizes DNA strand interruptions generated in vivo. DNA binding by PARP triggers primarily its own modification by the sequential addition of ADP-ribose units to form polymers; this modification, in turn, causes the release of PARP from DNA ends. Studies on the effects of the disruption of the gene encoding PARP (Adprt1, formerly Adprp) in mice have demonstrated roles for PARP in recovery from DNA damage and in suppressing recombination processes involving DNA ends. Telomeres are the natural termini of chromosomes and are, therefore, potential targets of PARP. Here, by the use of two different techniques, we show that mice lacking PARP display telomere shortening compared with wild-type mice. Telomere shortening is seen in different genetic backgrounds and in different tissues, both from embryos and adult mice. In vitro telomerase activity, however, is not altered in Adprt1-/- mouse fibroblasts. Furthermore, cytogenetic analysis of mouse embryonic fibroblasts reveals that lack of PARP is associated with severe chromosomal instability, characterized by increased frequencies of chromosome fusions and aneuploidy. The absence of PARP does not affect the presence of single-strand overhangs, naturally present at the ends of telomeres. This study therefore reveals an unanticipated role for PARP in telomere length regulation and provides insights into its functions in maintaining genomic integrity.