Effect of transfection of human poly(ADP-ribose)polymerase in Chinese hamster cells on mutagen resistance

Programming of Cell Resistance to Genotoxic and Oxidative Stress

Different organisms, cell types, and even similar cell lines can dramatically differ in resistance to genotoxic stress. This testifies to the wide opportunities for genetic and epigenetic regulation of stress resistance. These opportunities could be used to increase the effectiveness of cancer therapy, develop new varieties of plants and animals, and search for new pharmacological targets to enhance human radioresistance, which can be used for manned deep space expeditions. Based on the comparison of transcriptomic studies in cancer cells, in this review, we propose that there is a high diversity of genetic mechanisms of development of genotoxic stress resistance. This review focused on possibilities and limitations of the regulation of the resistance of normal cells and whole organisms to genotoxic and oxidative stress by the overexpressing of stress-response genes. Moreover, the existing experimental data on the effect of such overexpression on the resistance of cells and organisms to various genotoxic agents has been analyzed and systematized. We suggest that the recent advances in the development of multiplex and highly customizable gene overexpression technology that utilizes the mutant Cas9 protein and the abundance of available data on gene functions and their signal networks open new opportunities for research in this field.

Poly(ADP-Ribose) Polymerase 1: Cellular Pluripotency, Reprogramming, and Tumorogenesis

Poly(ADP-ribos)ylation (PARylation) is the catalytic function of the Poly(ADP-ribose) polymerases (Parps) family for post-translational modification in cellular process. Being a major member of Parps, Parp1 is a crucial nuclear factor with biological significance in modulating DNA repair, DNA replication, transcription, DNA methylation and chromatin remodeling through PARylation of downstream proteins. In addition, high expression level and activity of Parp1 are correlated with pluripotent status, reprogramming, and cancer. Furthermore, epigenetic modulation of Parp1 is explored for regulating wide variety of gene expression. Genetic and pharmaceutical disruption of Parp1 further confirmed the importance of Parp1 in cell growth, DNA repair, and reprogramming efficiency. Taken together, the proximity toward the understanding of the modulation of Parp1 including interaction and modification in different fields will provide new insight for future studies. In this review, the biological significance of Parp1 in transcription and the epigenetic modulation of Parp1 in pluripotent status, reprogramming process and cancer will be summarized.

Overexpression of enzymes that repair endogenous damage to DNA

A significant contribution to human mutagenesis and carcinogenesis may come from DNA damage of endogenous, rather than exogenous, origin. Efficient repair mechanisms have evolved to cope with this. The main repair pathway involved in repair of endogenous damage is DNA base excision repair. In addition, an important contribution is given by O6-alkylguanine DNA alkyltranferase, that repairs specifically the miscoding base O6-alkylguanine. In recent years, several attempts have been carried out to enhance the efficiency of repair of endogenous damage by overexpressing in mammalian cells single enzymatic activities. In some cases (e.g. O6-alkylguanine DNA alkyltransferase or yeast AP endonuclease) this approach has been successful in improving cellular protection from endogenous and exogenous mutagens, while overexpression of other enzymatic activities (e.g. alkyl N-purine glycosylase or DNA polymerase beta) were detrimental and even produced a genome instability phenotype. The reasons for these different outcomes are analyzed and alternative enzymatic activities whose overexpression may improve the efficiency of repair of endogenous damage in human cells are proposed.

Transgenic systems in studies on genotoxicity of alkylating agents: critical lesions, thresholds and defense mechanisms

Transgenic systems, both cell lines and mice with gain or loss of function, are being used in order to modulate the expression of DNA repair proteins, thus allowing to assess their contribution to the defense against genotoxic mutagens and carcinogens. In this review, questions have been addressed concerning the use of transgenic systems in elucidating critical primary DNA lesions, their conversion into genotoxic endpoints, low-dose effects, and the relative contribution of individual cellular functions in defense. It has been shown that the repair protein alkyltransferase (MGMT) is decisive for protection against methylating and chloroethylating compounds. Protection pertains also to tumor formation, as revealed by the response of MGMT transgenic and knockout mice. Overexpression of genes involved in base excision repair (N-methylpurine-DNA glycosylase, apurinic endonuclease, DNA polymerase beta) is in most cases not beneficial in increasing the protection level, whereas their down-modulation or inactivation increases cellular sensitivity. This indicates that non-repaired base N-alkylation lesions and/or repair intermediates possess genotoxic potential. Modulation of mismatch repair and poly(ADP)ribosyl transferase has also been shown to affect the cellular response to alkylating agents. Furthermore, the role of Fos, Jun and p53 in cellular defense against alkylating mutagens is discussed.

Overexpression of human poly(ADP-ribose) polymerase in transfected hamster cells leads to increased poly(ADP-ribosyl)ation and cellular sensitization to gamma irradiation

Poly(ADP-ribosyl)ation is a posttranslational modification of nuclear proteins catalyzed by poly(ADP-ribose) polymerase (PARP), an enzyme which uses NAD+ as substrate. Binding of PARP to DNA single-strand or double-strand breaks leads to enzyme activation. Inhibition of poly(ADP-ribose) formation impairs the cellular recovery from DNA damage. Here we describe stable transfectants of the Chinese hamster cell line CO60 that constitutively overexpress human PARP (COCF clones). Immunofluorescence analysis of gamma-irradiation-stimulated poly(ADP-ribose) synthesis revealed consistently larger fractions of cells positive for this polymer in the COCF clones than in control clones, which failed to express human PARP. HPLC-based quantitative determination of in vivo levels of poly(ADP-ribose) confirmed this result and revealed that the basal polymer levels of undamaged cells were significantly higher in the COCF clones. The COCF clones were sensitized to the cytotoxic effects of gamma irradiation compared with control transfectants and parental cells. This effect could not be explained by depletion of cellular NAD+ or ATP pools. Together with the well-known cellular sensitization by inhibition of poly(ADP-ribosyl)ation, our data lead us to hypothesize that an optimal level of cellular poly(ADP-ribose) accumulation exists for the cellular recovery from DNA damage.

Biochemical changes associated with the adaptive response of human keratinocytes to N-methyl-N'-nitro-N-nitrosoguanidine

Exposure of cells to low doses of radiation or chemicals renders them more resistant to higher doses of these agents. This phenomenon, termed adaptive response, was studied in quiescent human keratinocytes exposed to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The cells were adapted with 2.5 nM MNNG for 60 min and challenged immediately thereafter with 2.5 microM MNNG for 30, 45 or 60 min. Clonogenic survival studies revealed that adapted cells were more resistant to the subsequent challenge treatment (up to 30% higher survival) than unadapted cells. In addition, formation of DNA strand breaks was lower in adapted cells. We monitored poly-ADP-ribosylation activity during expression of the adaptive response both at the substrate as well as the product level. NAD+ utilization in adapted and non-adapted cells exposed to the high dose of MNNG was similar, but recovery from NAD+ depletion was faster in low-dose pretreated cells. Induction of poly(ADP-ribose) formation was more than 2 times higher in low-dose adapted cells and this was associated with the formation of a distinct class of ADP-ribose polymers, i.e., branched polymers. These polymers exhibit a very high binding affinity for histones and can displace them from DNA. Elevated levels of poly(ADP-ribose) and, particularly, synthesis of branched polymers may play a critical role in low-dose adaptation.

Identification of a 116 kDa protein able to bind 1,3-bis(2-chloroethyl)-1-nitrosourea-damaged DNA as poly(ADP-ribose) polymerase

SKI-1 is a 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-resistant glioma cell line and SK-MG-1 is a BCNU-sensitive glioma cell line. Both cell lines do not express O6-methylguanine-DNA methyl transferase (MGMT) and exhibit comparable levels of 3-methyladenine DNA glycosylase. In order to detect DNA binding proteins involved in alternative DNA repair mechanisms of BCNU damage, we performed Southwestern analysis using a DNA probe damaged with BCNU and nuclear protein extracts from SKI-1 and SK-MG-1 cell lines. Both cell lines express a protein of M(r) 116,000 that is able to bind to BCNU-damaged DNA with higher specificity than to undamaged DNA. This protein was identified as poly(ADP-ribose) polymerase (PARP). Using glioma extracts depleted of PARP or using antibody to block the DNA binding domain of PARP no other protein binding to BCNU-treated probe was observed. Addition of methoxyamine, an inhibitor of DNA strand breaks, led to a significant reduction of PARP binding to BCNU-treated DNA. BCNU treatment of both glioma cell lines led to reduced nicotinamide adenine dinucleotide levels, indicating activation of PARP. Thus, the recognition and binding of PARP to BCNU-induced DNA nicks with concomitant PARP activation may be important processes that are involved in the initial stage of DNA repair of BCNU lesions in glial cells.