Loss of intragenomic DNA repair heterogeneity with cellular differentiation

The Emerging Role of GLP-1 Receptors in DNA Repair: Implications in Neurological Disorders

Glucagon-like peptide-1 (GLP-1) is originally found as a metabolic hormone (incretin) that is able to regulate blood-glucose levels via promoting synthesis and secretion of insulin. GLP-1 and many analogues are approved for treatment of type II diabetes. Accumulating results imply that GLP-1 performs multiple functions in various tissues and organs beyond regulation of blood-glucose. The neuroprotective function of GLP-1 has been extensively explored during the past two decades. Three of our previous studies have shown that apurinic/apyrimidinic endonuclease 1 (APE1) is the only protein of the base excision repair (BER) pathway able to be regulated by oxidative stress or exogenous stimulations in rat primary cortical neurons. In this article, we review the role of APE1 in neurodegenerative diseases and its relationship to neuroprotective mechanisms of the activated GLP-1 receptor (GLP-1R) in neurodegenerative disorders. The purpose of this article is to provide new insight, from the aspect of DNA damage and repair, for studying potential treatments in neurodegenerative diseases.

Modulation of DNA base excision repair during neuronal differentiation

Neurons are terminally differentiated cells with a high rate of metabolism and multiple biological properties distinct from their undifferentiated precursors. Previous studies showed that nucleotide excision DNA repair is downregulated in postmitotic muscle cells and neurons. Here, we characterize DNA damage susceptibility and base excision DNA repair (BER) capacity in undifferentiated and differentiated human neural cells. The results show that undifferentiated human SH-SY5Y neuroblastoma cells are less sensitive to oxidative damage than their differentiated counterparts, in part because they have robust BER capacity, which is heavily attenuated in postmitotic neurons. The reduction in BER activity in differentiated cells correlates with diminished protein levels of key long patch BER components, flap endonuclease-1, proliferating cell nuclear antigen, and ligase I. Thus, because of their higher BER capacity, proliferative neural progenitor cells are more efficient at repairing DNA damage compared with their neuronally differentiated progeny.

DNA repair in differentiated cells: some new answers to old questions

Terminally differentiated cells need never replicate their genomes and may therefore dispense with the daunting task of maintaining several repair systems to constantly scan their entire complement of DNA. Obviously, transcribed genes need to be repaired, so that cells can carry out their specialized functions, but dedicated mechanisms such as transcription-coupled repair and differentiation-associated repair can ensure the maintenance of those transcriptionally active domains. Many groups have studied DNA repair in differentiated cells, often with divergent results, possibly because there are distinct classes of differentiated cells, with unique properties. Thus neurons ought to last for a lifetime, whereas myocytes are backed by precursor cells, while white blood cells like macrophages are constantly being replaced. More importantly, different DNA repair systems can vary in their response to cellular differentiation, possibly depending on whether they can be coupled to transcription. Nucleotide excision repair (NER) is probably the most versatile DNA repair system and is coupled to transcription. NER was shown to be attenuated by differentiation in several cell types, including neurons. The attenuation occurs only at the global genome level, with transcribed genes still being efficiently repaired. We have determined that this attenuation results from the lack of ubiquitination of a NER factor, most likely owing to differences in phosphorylation of the ubiquitin-activating enzyme E1. Because there is only one E1 in human cells, it is likely that other metabolic pathways are similarly affected, depending on whether they rely on an E2 enzyme which is sensitive to the state of E1 phosphorylation.

Nucleotide excision repair in differentiated cells

Nucleotide excision repair (NER) is the principal pathway for the removal of a wide range of DNA helix-distorting lesions and operates via two NER subpathways, i.e. global genome repair (GGR) and transcription-coupled repair (TCR). Although detailed information is available on expression and efficiency of NER in established mammalian cell lines, little is known about the expression of NER pathways in (terminally) differentiated cells. The majority of studies in differentiated cells have focused on repair of UV-induced cyclobutane pyrimidine dimers (CPD) and 6-4-photoproducts (6-4PP) because of the high frequency of photolesions at low level of toxicity and availability of sensitive technologies to determine photolesions in defined regions of the genome. The picture that emerges from these studies is blurred and rather complex. Fibroblasts and terminally differentiated myocytes of the rat heart display equally efficient GGR of 6-4PP but poor repair of CPD due to the absence of p48 expression. This repair phenotype is clearly different from human terminal differentiated neurons. Furthermore, both cell types were found to carry out TCR of CPD, thus mimicking the repair phenotype of established rodent cell lines. In contrast, in intact rat spermatogenic cells repair was very inefficient at the genome overall level and in transcriptionally active genes indicating that GGR and TCR are non-functional. Also, non-differentiated mouse embryonic stem (ES) cells exhibit low levels of NER after UV irradiation. However, the mechanisms that lead to low NER activity are clearly different: in differentiated spermatogenic cells differences in chromatin compaction and sequestering of NER proteins may underlie the lack of NER activity in pre-meiotic cells, whereas in non-differentiated ES cells NER is impaired by a strong apoptotic response.

Deficient global genome repair of UV-induced cyclobutane pyrimidine dimers in terminally differentiated myocytes and proliferating fibroblasts from the rat heart

Nucleotide excision repair (NER) is the principal pathway for the removal of a wide range of DNA helix-distorting lesions. Two NER subpathways have been identified, i.e. global genome repair (GGR) and transcription-coupled repair (TCR). Little is known about the expression of NER pathways in differentiated cells. We assessed the repair of UV-induced cyclobutane pyrimidine dimers (CPD) and 6-4-photoproducts (6-4 PP) in terminally differentiated myocytes and proliferating fibroblasts isolated from the hearts of neonatal rats. Myocytes and fibroblasts were found to carry out efficient removal of 6-4 PP but display poor repair of CPD by GGR. Furthermore, both cell types were found to carry out TCR of CPD, thus mimicking the repair phenotype of established rodent cell lines. The inefficient repair of CPD at the genome overall level occurs in the absence of massive apoptosis, but goes along with an undetectable level of transcription of the p48 gene, known to be mutated in xeroderma pigmentosum group E (XP-E) patients and recently proposed to be essential for repair of CPD in nonexpressed DNA. Taken together, the results suggest that primary non-dividing cardiac myocytes and proliferating fibroblasts from rat heart selectively remove CPD from the transcribed strand of transcriptionally active genes. GGR of CPD is poor due to the absence of p48 expression.

Formation and repair of DNA double-strand breaks in gamma-irradiated K562 cells undergoing erythroid differentiation

Cellular differentiation is accompanied by gross changes in nuclear organization, metabolic pathways and gene expression characteristics. To investigate, whether the response to radiation damage is altered during cellular differentiation, we studied the formation and repair of DNA double-strand breaks in gamma-irradiated K562 erythroleukemia cells induced to differentiate by exposure to butyric acid. We applied an assay based on pulsed-field gel electrophoresis and Southern hybridization to measure break induction in several genomic restriction fragments. Pulsed-field gel electrophoresis of (14)C-labelled unrestricted DNA was used to study the rejoining of gamma-radiation-induced breaks in the whole genome. Total rejoining and joining of correct break ends in specific genomic regions was monitored by hybridization analysis of blots of unrestricted and restriction digested DNA with single-copy probes. The yields of gamma-ray-induced DNA double-strand breaks were found to decrease with differentiation by about 20%. Correct rejoining of radiation-induced breaks, as measured by the reconstitution of broken restriction fragments, was unaltered in differentiating cells compared to actively proliferating precursor cells. Total rejoining, however, appeared to be retarded in differentiating cells. The results suggest that in spite of the fundamental changes accompanying differentiation, the cellular damage response pathways are not essentially affected throughout erythroid differentiation.

Terminally differentiated human neurons repair transcribed genes but display attenuated global DNA repair and modulation of repair gene expression

Repair of UV-induced DNA lesions in terminally differentiated human hNT neurons was compared to that in their repair-proficient precursor NT2 cells. Global genome repair of (6-4)pyrimidine-pyrimidone photoproducts was significantly slower in hNT neurons than in the precursor cells, and repair of cyclobutane pyrimidine dimers (CPDs) was not detected in the hNT neurons. This deficiency in global genome repair did not appear to be due to denser chromatin structure in hNT neurons. By contrast, CPDs were removed efficiently from both strands of transcribed genes in hNT neurons, with the nontranscribed strand being repaired unexpectedly well. Correlated with these changes in repair during neuronal differentiation were modifications in the expression of several repair genes, in particular an up-regulation of the two structure-specific nucleases XPG and XPF/ERCC1. These results have implications for neuronal dysfunction and aging.

Characterization of gene-specific DNA repair by primary cultures of rat hepatocytes

At present, almost all the information on gene-specific DNA repair in mammals comes from studies with transformed cell lines and proliferating primary cells obtained from rodents and humans. In the present study, we measured the repair of specific DNA regions in primary cultures of nondividing rat hepatocytes (parenchymal cells). DNA damage was induced by irradiating the primary cultures of hepatocytes with ultraviolet (UV) light, and the presence of cyclobutane pyrimidine dimers (CPDs) was measured by using T4 endonuclease V in the following: a 21-kb BamHI fragment containing the albumin gene, a 14-kb BamHI fragment containing the H-ras gene, and the genome overall. The frequency of CPDs in the two BamHI fragments and the genome overall were similar and ranged from 0.5 to 1.3 CPDs per 10 kb for UV doses of 5-30 J/m2. However, the removal of CPDs from the DNA fragment containing the albumin gene was significantly higher than from that of the genome overall and the DNA fragment containing the H-ras gene. Within 24 hr, approximately 67% of the CPDs was removed from the DNA fragment containing the albumin gene versus less than 40% for the genome overall and the DNA fragment containing the H-ras gene. The lower repair observed for the 14-kb fragment containing the H-ras gene is probably indicative of repair of the nontranscribed region of this fragment because the H-ras gene makes up only 2.4 kb of the 14-kb fragment. Primary cultures of hepatocytes removed CPDs from the transcribed strand of albumin fragment more efficiently than from the nontranscribed strand; however, no differences were observed in the repair of the two strands of the fragment containing the H-ras gene. These results demonstrate that primary cultures of nondividing rat hepatocytes show differential repair of UV-induced DNA damage that is comparable to what has been reported for transformed, proliferating mammalian cell lines.

Gene-specific repair in human CD4+ lymphocytes reflects transcription and proliferation

We have measured the gene-specific repair of ultraviolet irradiation (UV)-induced cyclobutane pyrimidine dimers (CPD) in freshly isolated human peripheral blood CD4+ T-lymphocytes. Two populations of CD4+ lymphocytes were assayed: resting and proliferating cells. DNA repair was assessed in the essential gene dihydrofolate reductase (DHFR) as well as in each of its strands, in the proliferation inducible c-myc gene and in the inactive delta-globin gene. Transcription rates in these genes were determined by nuclear run-on assay in the two cell populations. The rate of DHFR transcription increased 10-fold from resting to proliferating lymphocytes. Transcripts from c-myc were present only in proliferating cells, and we detected no delta-globin transcripts in either cell population. During the 24-h period after UV irradiation, there was little or no repair in any of the genes in the resting cells; there was some repair in the transcribed strand of the DHFR gene, but no repair in its nontranscribed strand. In the proliferating cells where the transcription of DHFR was much increased, the repair was efficient. The delta-globin gene was not expressed in either cell population, but it was more efficiently repaired in the proliferating than in the resting cells. We suggest that the gene-specific repair activity in CD4+ lymphocytes can reflect the proliferative state of the cells as well as the transcriptional state of the gene.

Effects of UVC irradiation on cultured mouse embryonic limb bud cells

Effects of UVC irradiation (UVC) at a dose range from 0 to 30 J/m(2) were investigated on the cultured embryonic limb bud cells (LBC), isolated from fore- and hindlimbs of day 11 mouse embryos. Although dose-dependent inhibition was found for both cellular proliferation and chondrogenesis, the chondrogenic proteoglycan (PG) synthesis was more sensitive to UVC than cellular proliferation when compared at ID(50), the inhibitory dose that reduced assessment value by 50% of the control. No significant difference in induction and repair kinetics of UV-induced cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts ((6-4)PPs) were found between LBC and NIH3T3 mouse cell line. The fluorescent light (FL) treatment of LBC pre or post UVC irradiation did not affect repair kinetics of CPDs and (6-4)PPs, cellular proliferation, formation of PG-producing nodule and the PG synthesis.

DNA damage, mutation and fine structure DNA repair in aging

The primary focus of this review is on correlations found between DNA damage, repair, and aging. New techniques for the measurement of DNA damage and repair at the level of individual genes, in individual DNA strands and in individual nucleotides will allow us to gain information regarding the nature of these correlations. Fine structure studies of DNA damage and repair in specific regions, including active genes, telomeres, and mitochondria have begun. Considerable intragenomic DNA repair heterogeneity has been found, and there have been indications of relationships between aging and repair in specific regions. More studies are necessary, however, particularly studies of the repair of endogenous damage. It is emphasized that the information obtained must be viewed from a perspective that takes into account the total responses of the cell to damaging events and the inter-relationships that exist between DNA repair and transcription.

Lower mutation frequencies are induced by ENU in undifferentiated embryonic cells than in differentiated cells of the mouse in vitro

The pluripotent embryonic carcinoma cells of line P19 established from undifferentiated cells of the early mouse embryo and their differentiated progeny, the epithelioid ectoderm-like EPI-7 cells, were investigated for the induction of mutations at the HPRT locus by the alkylating agent N-ethyl-N-nitrosourea (ENU). We showed that the cytotoxic effects of ENU after a 5-h treatment were lower in undifferentiated P19 cells than in differentiated EPI-7 cells. The IC50 values of ENU in the two cell lines amounted to 0.6 mg/ml and 0.09 mg/ml for P19 and EPI-7 cells, respectively. The induction of 6-thioguanine-resistant mutants by ENU (1.0 mg/ml) determined after an expression time of 8 days for both cell lines resulted in similar mutation frequencies. Using expression times of 8 days for P19 and 11.75 days for EPI-7 cells, taking into account the longer generation time of differentiated EPI-7 cells (13.7 +/- 3.6 h) in comparison to undifferentiated P19 cells (9.3 +/- 0.9 h), ENU induced significantly higher mutant frequencies in EPI-7 cells (4865 mutants/10(6) cells) than in P19 cells (282 mutants/10(6) cells). Our results and data from the literature on UV irradiation-induced repair support the idea that the induction of lower mutation frequencies in embryonic cells may correlate with different proliferation capacities, cell cycle parameters and/or different mechanisms of DNA repair in embryonic stem cells and differentiated cells, respectively.

Influence of DNA repair capacity and cell differentiation on UV-induced gene expression

Terminally differentiated (TD) 3T3-T cells have a reduced capacity to repair ultraviolet (UV)-induced DNA damage, as compared with the repair capabilities of growth-arrested and cycling (stem) 3T3-T cells. In this study UV-induced expression of the immediate early response genes c-fos and c-jun and the secondary response gene type IV collagenase in TD, growth-arrested and stem 3T3-T cells was investigated by northern blot analysis. At each UV dose (0-10 J/m2) there was an increase in c-fos and, to a lesser extent, c-jun expression 0.5 h after irradiation in each 3T3-T phenotype as compared with unirradiated controls. Maximum induction of c-fos was reached at 0.5-1 J/m2 in TD and growth-arrested cells, which was 10-fold greater than in stem cells. The induction of c-jun in TD and growth-arrested cells reached maximums at 0.5 J/m2; at this dose each was greater than in stem cells. The increases in c-fos and c-jun expression were transient in each phenotype reaching maximums from 0.5 to 1 h after irradiation. The expression of type IV collagenase was increased in the non-cycling phenotypes at 2-8 h after irradiation. However, collagenase expression was not detected in unirradiated or irradiated stem cells. These results suggest that growth arrest, not differentiation or DNA repair capacity, is the primary influence on gene induction after UV irradiation.

Fine tuning of DNA repair in transcribed genes: mechanisms, prevalence and consequences

Cells fine-tune their DNA repair, selecting some regions of the genome in preference to others. In the paradigm case, excision of UV-induced pyrimidine dimers in mammalian cells, repair is concentrated in transcribed genes, especially in the transcribed strand. This is due both to chromatin structure being looser in transcribing domains, allowing more rapid repair, and to repair enzymes being coupled to RNA polymerases stalled at damage sites; possibly other factors are also involved. Some repair-defective diseases may involve repair-transcription coupling: three candidate genes have been suggested. However, preferential excision of pyrimidine dimers is not uniformly linked to transcription. In mammals it varies with species, and with cell differentiation. In Drosophila embryo cells it is absent, and in yeast, the determining factor is nucleosome stability rather than transcription. Repair of other damage departs further from the paradigm, even in some UV-mimetic lesions. No selectivity is known for repair of the very frequent minor forms of base damage. And the most interesting consequence of selective repair, selective mutagenesis, normally occurs for UV-induced, but not for spontaneous mutations. The temptation to extrapolate from mammalian UV repair should be resisted.