Poly (ADP-ribose) polymerase 3 (PARP3), a potential repressor of telomerase activity.

BACKGROUND: Considering previous result in Non-Small Cell Lung Cancer (NSCLC), we investigated in human cancer cells the role of PARP3 in the regulation of telomerase activity. METHODS: We selected A549 (lung adenocarcinoma cell line) and Saos-2 (osteosarcoma cell line), with high and low telomerase activity levels, respectively. The first one was transfected using a plasmid construction containing a PARP3 sequence, whereas the Saos-2 cells were submitted to shRNA transfection to get PARP3 depletion. PARP3 expression on both cell systems was evaluated by real-time quantitative PCR and PARP3 protein levels, by Western-blot. Telomerase activity was determined by TRAP assay. RESULTS: In A549 cells, after PARP3 transient transfection, data obtained indicated that twenty-four hours after transfection, up to 100-fold increased gene expression levels were found in the transfected cells with pcDNA/GW-53/PARP3 in comparison to transfected cells with the empty vector. Moreover, 48 hours post-transfection, telomerase activity decreased around 33%, and around 27%, 96 hours post-transfection. Telomerase activity average ratio was 0.67 ± 0.05, and 0.73 ± 0.06, respectively, with significant differences. In Saos-2 cells, after shRNA-mediated PARP3 silencing, a 2.3-fold increase in telomerase activity was detected in relation to the control. CONCLUSION: Our data indicated that, at least in some cancer cells, repression of PARP3 could be responsible for an increased telomerase activity, this fact contributing to telomere maintenance and, therefore, avoiding genome instability.

Telomere dysfunction and genome instability.

The nucleoprotein complexes that cap the very ends of the eukaryotic chromosomes, named telomeres, are indispensable for cell viability. Telomeric DNA shortens in each cell division until it cannot exert end-protective functions in human somatic cells. Additionally, several proteins have been described to play a key role in telomere homeostasis preventing chromosome extremities to be recognized as double-stranded breaks (DSBs). When telomeres become dysfunctional, either through excessive shortening or due to defects in the proteins that form its structure, they trigger p53/pRb pathways what limits proliferative lifespan. Impairment of telomere function together with a compromised senescence/apoptosis response leads to chromosome instability. Fusions between dysfunctional telomeres or even between dysfunctional telomeres and DSBs can initiate breakage-fusion-bridge (BFB) cycles. Initially, telomere fusions were proposed to cause only structural abnormalities. Nevertheless, changes in chromosome number have also emerged as a possible consequence of alterations in end capping. Here we review the main aspects of telomeres and telomere-based chromosome instability, highlighting why they have been proposed as a driving force for tumourigenesis.

Progressive telomere dysfunction causes cytokinesis failure and leads to the accumulation of polyploid cells.

Most cancer cells accumulate genomic abnormalities at a remarkably rapid rate, as they are unable to maintain their chromosome structure and number. Excessively short telomeres, a known source of chromosome instability, are observed in early human-cancer lesions. Besides telomere dysfunction, it has been suggested that a transient phase of polyploidization, in most cases tetraploidization, has a causative role in cancer. Proliferation of tetraploids can gradually generate subtetraploid lineages of unstable cells that might fire the carcinogenic process by promoting further aneuploidy and genomic instability. Given the significance of telomere dysfunction and tetraploidy in the early stages of carcinogenesis, we investigated whether there is a connection between these two important promoters of chromosomal instability. We report that human mammary epithelial cells exhibiting progressive telomere dysfunction, in a pRb deficient and wild-type p53 background, fail to complete the cytoplasmatic cell division due to the persistence of chromatin bridges in the midzone. Flow cytometry together with fluorescence in situ hybridization demonstrated an accumulation of binucleated polyploid cells upon serial passaging cells. Restoration of telomere function through hTERT transduction, which lessens the formation of anaphase bridges by recapping the chromosome ends, rescued the polyploid phenotype. Live-cell imaging revealed that these polyploid cells emerged after abortive cytokinesis due to the persistence of anaphase bridges with large intervening chromatin in the cleavage plane. In agreement with a primary role of anaphase bridge intermediates in the polyploidization process, treatment of HMEC-hTERT cells with bleomycin, which produces chromatin bridges through illegimitate repair, resulted in tetraploid binucleated cells. Taken together, we demonstrate that human epithelial cells exhibiting physiological telomere dysfunction engender tetraploid cells through interference of anaphase bridges with the completion of cytokinesis. These observations shed light on the mechanisms operating during the initial stages of human carcinogenesis, as they provide a link between progressive telomere dysfunction and tetraploidy.

Role of telomere dysfunction in genetic intratumor diversity.

Most solid tumors are unable to maintain the stability of their genomes at the chromosome level. Indeed, cancer cells display highly rearranged karyotypes containing translocations, amplifications, deletions, and gains and losses of whole chromosomes, which reshuffle steadily. This chromosomal instability most likely occurs early in the development of cancer, and may represent an important step in promoting the multiple genetic changes required for the initiation and/or progression of the disease. Different mechanisms may underlie chromosome instability in cancer cells, but a prominent role for telomeres, the tip of linear chromosomes, has been determined. Telomeres are ribonucleoprotein structures that prevent natural chromosome ends being recognized as DNA double-strand breaks, by adopting a loop structure. Loss of telomere function appears from either alteration on telomere-binding proteins or from the progressive telomere shortening that normally occurs under physiological conditions in the majority of cells in tissues. Importantly, unmasked telomeres may either trigger the senescent phenotype that has been linked to the aging process or may initiate the chromosome instability needed for cancer development, depending on the integrity of the DNA damage checkpoint responses. Telomere dysfunction contributes to chromosome instability through end-to-end chromosome fusions entering breakage-fusion-bridge (BFB) cycles. Resolution of chromatin bridge intermediates is likely to contribute greatly to the generation of segmental chromosome amplification events, unbalanced chromosome rearrangements, and whole chromosome aneuploidy. Noteworthy is the fact that telomere length heterogeneity among individuals may directly influence the scrambling of the genome at tumor initiation. However, reiterated BFB cycles would randomly reorganize the cell karyotype, thus increasing the genetic diversity that characterizes tumor cells. Even though a direct link is still lacking, multiple evidence lead one to believe that telomere dysfunction directly contributes to cancer development in humans. The expansion of highly unstable cells due to telomere dysfunction enhances the genetic diversity needed to fuel specific mutations that may promote cell immortalization and the acquisition of a tumor phenotype.

Different outcomes of telomere-dependent anaphase bridges.

Chromosomal instability occurs early in the development of cancer and may represent an important step in promoting the multiple genetic changes required for the initiation and/or progression of the disease. Telomere erosion is one of the factors that contribute to chromosome instability through end-to-end chromosome fusions entering BFB (breakage-fusion-bridge) cycles. Uncapped chromosomes with short dysfunctional telomeres represent an initiating substrate for both pre- and post-replicative joining, which leads to unstable chromosome rearrangements prone to bridge at mitotic anaphase. Resolution of chromatin bridge intermediates is likely to contribute greatly to the generation of segmental chromosome amplification events, unbalanced chromosome rearrangements and whole chromosome aneuploidy. Accordingly, telomere-driven instability generates highly unstable genomes that could promote cell immortalization and the acquisition of a tumour phenotype.

Differential colorectal carcinogenesis: Molecular basis and clinical relevance.

Colorectal cancer (CCR) is one of the most frequent cancers in developed countries. It poses a major public health problem and there is renewed interest in understanding the basic principles of the molecular biology of colorectal cancer. It has been established that sporadic CCRs can arise from at least two different carcinogenic pathways. The traditional pathway, also called the suppressor or chromosomal instability pathway, follows the Fearon and Vogelstein model and shows mutation in classical oncogenes and tumour suppressor genes, such as K-ras, adenomatous polyposis coli, deleted in colorectal cancer, or p53. Alterations in the Wnt pathway are also very common in this type of tumour. The second main colorectal carcinogenesis pathway is the mutator pathway. This pathway is present in nearly 15% of all cases of sporadic colorectal cancer. It is characterized by the presence of mutations in the microsatellite sequences caused by a defect in the DNA mismatch repair genes, mostly in hMLH1 or hMSH2. These two pathways have clear molecular differences, which will be reviewed in this article, but they also present distinct histopathological features. More strikingly, their clinical behaviours are completely different, having the "mutator" tumours a better outcome than the "suppressor" tumours.

MMP-7 and SGCE as distinctive molecular factors in sporadic colorectal cancers from the mutator phenotype pathway.

Colorectal cancers (CRCs) from the suppressor and the mutator carcinogenic pathways display distinctive pathological and clinical features that remain not completely understood. In this context, the aim of this work was to study the differential expression of metalloproteinases and adhesion molecules related to cancer invasiveness in both groups of tumours. We analyzed 84 tissue specimens, 42 primary sporadic CRCs obtained from patients who underwent radical surgery, and its corresponding control tissues. According to microsatellite instability, 31 cancers showed low or null microsatellite instability (MSI-L/MSS) and 11 tumours displayed high microsatellite instability (MSI-H). Expression assays were established using the Oligo GEArray(R) human extracellular matrix and adhesion molecules microarray containing 114 genes. Real-time quantitative PCR (RT-qPCR) confirmed expression data from arrays, using TaqMan probes. Results from oligoarray expression analyses indicated that ITGA3, ITGA9, ITGB4, ITGB7 and MMP15 had significantly higher expression levels in MSI-H tumours versus MSS/MSI-L cancers, whereas COL12A1, CSPG2, FN1, MMP-7 and SGCE were down-regulated in tumours with high microsatellite instability when compared to the stable group. After RT-qPCR validation, two of these genes, MMP-7 and SGCE, were confirmed to have statistical differences between the two groups of tumours studied. In both cases, MSI-H tumours displayed significant lower expression levels than MSI-L/MSS tumours. In conclusion, these two distinctive molecular markers could be related to a diminished invasion in colorectal tumours from the mutator pathway, this may contribute to the understanding of the better patient prognosis conferred by this type of tumours.

Telomere function in colorectal cancer.

Colorectal cancer is the third most common form of cancer and the second leading cause of cancer-related death in the western world. Tumour cells acquire the hallmarks of cancer during the carcinogenic selection process. Cell immortality is one of the principal features acquired during this process which involves the stabilization of telomere length. It is achieved mainly, by telomerase activation. Thus, the discovery of telomeres and telomerase allowed an understanding of the mechanisms by which cells can become immortalized. Different studies have shown that tumour cells have shorter telomeres than nontumour cells and have detected telomerase activity in the majority of tumours. Survival studies have determined that telomere maintenance and telomerase activity are associated with poor prognosis. Taking into account all the results achieved by different groups, quantification and evaluation of telomerase activity and measurement of telomere length may be useful methods for additional biologic and prognostic staging of colorectal carcinoma.

Differential Wnt pathway gene expression and E-cadherin truncation in sporadic colorectal cancers with and without microsatellite instability.

PURPOSE: Alterations in the Wnt pathway play a major role in colorectal cancer with high (MSI-H) or low microsatellite instability (MSS/MSI-L). However, the differential impact of the Wnt pathway components on these tumors is poorly understood. MMP-3 (stromelysin-1) promoter is a target of the mutator phenotype in sporadic colorectal cancer. Among MMP-3 targets, we investigated E-cadherin integrity status in both groups of tumors. Because beta-catenin is the main effector of the Wnt pathway, we have also investigated the differential cellular status of beta-catenin. EXPERIMENTAL DESIGN: Expression profiles of 114 genes related to the Wnt pathway were analyzed by oligo microarrays in 48 tumors classified by their MSI status. In addition, we analyzed 48 sporadic colorectal cancers for E-cadherin integrity status. We performed investigation of beta-catenin and cyclin D1 by immunohistochemistry using tissue arrays containing 96 tumors. RESULTS: Our data show that a group of genes that negatively regulate Wnt signaling are downregulated in MSS/MSI-L as compared with MSI-H colorectal tumors. E-cadherin truncation was significantly higher in MSS/MSI-L as compared with MSI-H tumors. Moreover, MSI-H tumors showed low or null beta-catenin nuclear presence, whereas the group of tumors classified as MSS or MSI-L displayed a high content of the nuclear beta-catenin location. CONCLUSIONS: Our results suggest that the differential expression of genes that negatively regulate the Wnt pathway, as well as the status of E-cadherin and beta-catenin in MSI-H or MSS/MSI-L colorectal tumors, shed some light on the different clinical behavior showed by the two groups.

Telomere shortening is associated with poor prognosis and telomerase activity correlates with DNA repair impairment in non-small cell lung cancer.

BACKGROUND AND PURPOSE: Telomere function and DNA damage response pathways are frequently inactivated in cancer. Moreover, some telomere-binding proteins have been implicated in DNA repair. The purpose of this work consists of evaluating the prognostic impact of telomere dysfunction and its relationship with DNA repair systems in non-small cell lung cancer (NSCLC). PATIENTS AND METHODS: We analysed 83 NSCLCs and their corresponding control samples obtained from patients submitted to surgery. Telomere function was evaluated by determining telomerase activity and telomere length. DNA repair expression assays were established by using cDNA arrays containing 96 DNA-repair genes and by Real Time Quantitative PCR. RESULTS: Our data indicated that telomere attrition was significantly associated with poor clinical outcome of patients (P=0.02), being this parameter a significant prognostic factor independent of tumour stage (P=0.012; relative risk=1.887; 95% CI: 1.147-3.102). DNA-repair gene expression studies showed down regulation of DCLRE1C and GTF2H1 and a clear FLJ10858 up regulation in tumour tissues, as compared to controls. In addition, a number of genes related to DNA-repair were significantly down regulated in tumours that reactivated telomerase (DCLRE1C, GTF2H1, PARP-3, MLH1, and TRF2). CONCLUSIONS: Telomere shortening emerged as a poor clinical evolution parameter in NSCLC. Moreover, results from this work suggest a relationship between the loss of several DNA repair genes and telomerase activity, which may be of relevance in the pathogenesis of non-small lung cancer.

Biological and clinical significance of MMP-2, MMP-9, TIMP-1 and TIMP-2 in non-small cell lung cancer.

Our main aim consists of investigating the clinical usefulness of gelatinases and their tissue inhibitors in non-small cell lung cancer (NSCLC). Thus, we have analysed in 111 NSCLCs, levels and activity of MMP-2, MMP-9, TIMP-1 and TIMP-2, by Enzymoimmunoassay and Gelatine zymography, respectively. Our data revealed higher MMP-2 net activity in the NSCLC population analyzed in this study, this parameter showing a significant association with the TNM stage of tumours (P=0.002). Moreover, MMP-9 levels were significantly associated with poor clinical evolution of patients (P=0.02). Also, disease-free survival time was higher for patients whose tumours showed TIMP-1 increased levels (P=0.04). Of interest, Cox multivariate analysis revealed that TIMP-1 levels can be considered as an independent prognostic factor in NSCLC. Relative Risk (RR) to tumour relapse was more than two times lower for patients showing high TIMP-1 levels (RR=0.420, P=0.041). Therefore, according to our results, we conclude that MMP-9 and TIMP-1 levels of synthesis could be useful for the selection of patients with potentially unfavourable clinical evolution in order to establish adjuvant therapy protocols. Among these parameters, TIMP-1 level evaluation emerges as the main factor to predict the clinical outcome of patients.