Influence of medium-chain fatty acids and short-chain organic acids on jejunal morphology and intra-epithelial immune cells in weaned piglets.

Medium-chain fatty acids (MCFA) and short-chain organic acids (SOA) are often used as feed additives in piglet diets. There are limited studies in pigs describing the impact of MCFA or SOA on gut morphology and the local immune system. The aim of this study was to investigate whether the supplementation of SOA (0.41% fumaric acid and 0.32% lactic acid), or the combination of SOA with MCFA (0.15% caprylic and capric acid) would have effects on gut morphology and intestinal immune cells in weaned piglets. A total number of 72 weaned piglets were randomly allocated into three experimental groups. Tissue samples of six animals per group were used to investigate the potential impact of the feed additives on villus length and crypt depth of the jejunum and to quantify intra-epithelial lymphocytes (IEL). CD3-positive IEL were determined via immunohistochemistry (IHC) and flow cytometry (FC), whereas CD2-, CD5-, CD8ß-, CD16- and γδ TCR-positive IEL were only analysed by FC. The supplementation of MCFA and SOA did not significantly affect morphometric data. The FC data indicated that SOA significantly increased the quantity of CD2- CD8- γδ T cells in the jejunum epithelium. Both IHC and FC analyses of pig jejunum confirmed that the majority of IEL expressed the surface marker CD3 and could be classified as cytotoxic T lymphocytes. In conclusion, the data indicated that SOA increased the proportion of CD2- CD8- γδ T cells in the jejunal epithelium. Thus, SOA might enable a beneficial effect on the local immunity by increasing the constitutive number of potential effector cells to defeat infectious diseases.

Effects of age and zinc supplementation on transport properties in the jejunum of piglets.

Zinc is effective in the prevention and treatment of post-weaning diarrhoea and in promoting piglet growth. Its effects on the absorption of nutrients and the secretory capacity of the intestinal epithelium are controversial. We investigated the effects of age, dietary pharmacological zinc supplementation and acute zinc exposure in vitro on small-intestinal transport properties of weaned piglets. We further examined whether the effect of zinc on secretory responses depended on the pathway by which chloride secretion is activated. A total of 96 piglets were weaned at 26 days of age and allocated to diets containing three different levels of zinc oxide (50, 150 and 2500 ppm). At the age of 32, 39, 46 and 53 days, piglets were killed, and isolated epithelia from the mid-jejunum were used for intestinal transport studies in conventional Ussing chambers, with 23 µm ZnSO4 being added to the serosal side for testing acute effects. Absorptive transport was stimulated by mucosal addition of d-glucose or l-glutamine. Secretion was activated by serosal addition of prostaglandin E2 , carbachol or by mucosal application of Escherichia coli heat-stable enterotoxin (Stp ). Jejunal transport properties showed significant age-dependent alterations (p < 0.03). Both absorptive and secretory responses were highest in the youngest piglets (32 d). The dietary zinc supplementation had no significant influence on jejunal absorptive and secretory responses. However, the pre-treatment of epithelia with ZnSO4 in vitro led to a small but significant decrease in both absorptive and secretory capacities (p < 0.05), with an exception for carbachol (p = 0.07). The results showed that, in piglets, chronic supplementation with zinc did not sustainably influence the jejunal transport properties in the post-weaning phase. Because transport properties are influenced by the addition of zinc in vitro, we suggest that possible epithelial effects of zinc depend on the acute presence of this ion.

Dose-dependent effects of dietary zinc oxide on bacterial communities and metabolic profiles in the ileum of weaned pigs.

Pharmacological levels of zinc oxide (ZnO) can improve the health of weaning piglets and influence the intestinal microbiota. This experiment aimed at studying the dose-response effect of five dietary concentrations of ZnO on small intestinal bacteria and metabolite profiles. Fifteen piglets, weaned at 25 ± 1 days of age, were allocated into five groups according to body weight and litter. Diets were formulated to contain 50 (basal diet), 150, 250, 1000 and 2500 mg zinc/kg by adding analytical-grade (>98% purity) ZnO to the basal diet and fed ad libitum for 14 days after a 7-day adaptation period on the basal diet. Ileal bacterial community profiles were analysed by denaturing gradient gel electrophoresis and selected bacterial groups quantified by real-time PCR. Concentrations of ileal volatile fatty acids (VFA), D- and L-lactate and ammonia were determined. Species richness, Shannon diversity and evenness were significantly higher at high ZnO levels. Quantitative PCR revealed lowest total bacterial counts in the 50 mg/kg group. Increasing ZnO levels led to an increase (p = 0.017) in enterobacteria from log 4.0 cfu/g digesta (50 mg/kg) to log 6.7 cfu/g digesta (2500 mg/kg). Lactic acid bacteria were not influenced (p = 0.687) and clostridial cluster XIVa declined (p = 0.035) at highest ZnO level. Concentration of total, D- and L-lactate and propionate was not affected (p = 0.736, p = 0.290 and p = 0.630), but concentrations of ileal total VFA, acetate and butyrate increased markedly from 50 to 150 mg/kg and decreased with further increasing zinc levels and reached low levels again at 2500 mg/kg (p = 0.048, p = 0.048 and p = 0.097). Ammonia decreased (p < 0.006) with increasing dietary ZnO level. In conclusion, increasing levels of dietary ZnO had strong and dose-dependent effects on ileal bacterial community composition and activity, suggesting taxonomic variation in metabolic response to ZnO.

Review: Bioavailability of trace elements in farm animals: definition and practical considerations for improved assessment of efficacy and safety.

Currently, the authorisation procedure of trace elements as feed additives in the European Union according to Regulation (EC) No. 1831/2003 does not consider the bioavailability of trace element sources. This manuscript provides framework conditions for in vivo experiments that aim to estimate differences in the relative bioavailability between supplements of essential trace elements. Framework conditions encompass necessary technical information on the test substance, the experimental design and diet composition as well as the suitability of status parameters that allow for relative comparisons of regression variables. This manuscript evolves recommendations for researchers to conduct solid and reliable experiments on the matter as well as decision makers to interpret the value of studies submitted with authorisation applications regarding a certain trace element supplement.

Transfer of Non-Dioxin-Like Polychlorinated Biphenyls (ndl-PCBs) from Feed and Soil into Hen Eggs.

Understanding the transfer of non-dioxin-like polychlorinated biphenyls (ndl-PCBs) into foods of animal origin is crucial for human health risk assessment. In two experiments, we investigated the transfer of ndl-PCBs from contaminated feed and soil into eggs and meat of laying hens. The transfer from the feed was investigated with 30 laying hens. The treated hens were divided into two groups fed a contaminated diet (12.8 µg/kg sum of indicator ndl-PCBs; 88% dry matter (DM)) for 28 and 63 days, respectively, and then experienced a depuration period of 100 days with control feed. The transfer from soil was investigated with 72 laying hens kept in three separate outdoor pens (with three levels of ndl-PCB soil contamination) for 168 days. In both experiments, eggs were collected and analyzed for ndl-PCBs. In the second experiment, animals (n = 3 at the beginning, n = 6 per group after 42, 84, and 168 days) were slaughtered to determine ndl-PCBs in meat (breast muscle tissue) fat. The transfer of ndl-PCB from both feed and soil was clearly measurable and concentrations in eggs quickly exceeded maximum levels. Clear differences between individual congeners were observed. In particular, the low-chlorinated ndl-PCBs 52 and 101 are hardly found in eggs, despite their relatively high concentration in feed and soil. PCBs 138, 153, and 180, on the other hand, were found in large proportions in eggs and meat.

Chemical analysis of materials used in pig housing with respect to the safety of products of animal origin.

Bedding, environmental enrichment materials and disinfectant powders in pig farming are meant to ensure a hygienic bedding environment or allow pigs to perform explorative behaviour. To our knowledge, no legal regulation exists, that established maximum contents for undesirable substances, such as toxic metals, dioxins or trace elements in these materials, although oral ingestion could be expected. In the present study, a total of 74 materials (disinfectant powders [n = 51], earth/peat [n = 12], biochar [n = 8], recycled manure solids [n = 3]) were analysed for their content of various toxic metals, trace elements, dioxins and polychlorinated biphenyls. The data suggest that, in some samples, trace elements like iron, copper and zinc might have been added intentionally in order to induce physiological effects (iron supply to piglets, copper and zinc as growth promoter in pigs). Moreover, some materials contained high levels of lead, cadmium or arsenic. Consequently, if farm animals repeatedly consume environmental enrichment and bedding materials or disinfectant powders in considerable amounts and these quantities are added to the daily ration, the amount of ingested undesirable substances and trace elements might exceed the maximum levels set for complete feedstuffs, and an elevated transfer into food of animal origin might occur. Future studies are required to address the possible quantitative contribution in the light of feed and food safety. Finally, the excretion of undesirable substances with manure needs to be considered due to their possible accumulation in soils.

Biochemical identification of a mutated human melanoma antigen recognized by CD4(+) T cells.

CD4(+) T cells play a critical role in generating and maintaining immune responses against pathogens and alloantigens, and evidence suggests an important role for them in antitumor immunity as well. Although major histocompatibility complex class II-restricted human CD4(+) T cells with specific antitumor reactivities have been described, no standard method exists for cloning the recognized tumor-associated antigen (Ag). In this study, biochemical protein purification methods were used in conjunction with novel mass spectrometry sequencing techniques and molecular cloning to isolate a unique melanoma Ag recognized by a CD4(+) tumor-infiltrating lymphocyte (TIL) line. The HLA-DRbeta1*0101-restricted Ag was determined to be a mutated glycolytic enzyme, triosephosphate isomerase (TPI). A C to T mutation identified by cDNA sequencing caused a Thr to Ile conversion in TPI, which could be detected in a tryptic digest of tumor-derived TPI by mass spectrometry. The Thr to Ile conversion created a neoepitope whose T cell stimulatory activity was enhanced at least 5 logs compared with the wild-type peptide. Analysis of T cell recognition of serially truncated peptides suggested that the mutated amino acid residue was a T cell receptor contact. Defining human tumor Ag recognized by T helper cells may provide important clues to designing more effective immunotherapies for cancer.

Bar-coded pyrosequencing of 16S rRNA gene amplicons reveals changes in ileal porcine bacterial communities due to high dietary zinc intake.

Feeding high levels of zinc oxide to piglets significantly increased the relative abundance of ileal Weissella spp., Leuconostoc spp., and Streptococcus spp., reduced the occurrence of Sarcina spp. and Neisseria spp., and led to numerical increases of all Gram-negative facultative anaerobic genera. High dietary zinc oxide intake has a major impact on the porcine ileal bacterial composition.

Effects of ensiling cereal grains (barley, wheat, triticale and rye) on total and pre-caecal digestibility of proximate nutrients and amino acids in pigs.

Inclemency of weather frequently causes critical water contents in cereal grains above 15%. Ensiling in pre-mature condition may be an alternative to other techniques of preservation. Aim of this study was to compare apparent total tract digestibility (D(t) ; barley, wheat, triticale, rye) of proximate nutrients and pre-caecal digestibility (D(pc); barley, wheat) of amino acids (AA), respectively, from cereal grains in ensiled and almost dry condition. Moistly harvested cereal grains (67-73% dry matter) were milled through a 4-mm sieve and ensiled with lactic acid bacteria (LAB, 3 × 10(5) colony forming units/g Lactobacillus plantarum DSMZ 8862 and 8866). To investigate D(t), two trials were conducted with six Mini-Lewe pigs and four German Landrace pigs, respectively. D(pc) of AA was determined using four German Landrace pigs with ileo-rectal anastomosis. D(t) of proximate nutrients did not differ between cereal grains and their silages, except for ether extract, which was more digestible in ensiled than dry wheat, triticale and rye (p < 0.05). Lysine content was lower in ensiled than dry barley and wheat. In barley, ensiling was accompanied by reduced D(pc) of lysine and histidine (p < 0.05). In wheat, ensiling increased D(pc) of lysine, methionine, threonine and leucin (p < 0.05). Ensiling of pre-mature cereal grains with LAB can serve as a reasonable storage alternative. However, as limited data are yet available, further research is required to understand completely the impact of ensiling on nutritional value as indicated, for example, by the lysine content and the D(pc) of certain AA.

Digital impressions--easier than ever.

The technical features of the new Cerec Bluecam greatly simplify the optical impression-taking process and allow the dentist to concentrate on the patient and on the clinical aspects of the procedure. The level of precision attained with the Cerec Bluecam compares very well with the results achieved with laboratory scanners. With Cerec Bluecam, digital impressions become a clinical reality even for laboratory-made indirect restorations.

DNA damaging agent-induced autophagy produces a cytoprotective adenosine triphosphate surge in malignant glioma cells.

Although autophagy enhances cell survival in nutrient-deprived cells by increasing adenosine triphosphate (ATP) production, it remains unclear if autophagy functions similarly in cells treated with cytotoxic chemotherapy agents. To address this issue, we measured both the ability of DNA damaging agents (Temozolomide, and Etoposide) to induce an autophagy-dependent production of ATP, and the effects of modulation of autophagy on drug-induced cell death. Both drugs induced an autophagy-associated increase in ATP production in multiple glioma cell lines. The drug-induced ATP surge could not be blocked by glucose starvation, but could be blocked by preincubation with the autophagy inhibitor 3-methyladenine (3-MA), an siRNA targeting beclin 1, or the mitochondrial inhibitor oligomycin. Inhibition of autophagy-induced ATP production increased non-apoptotic cell death associated with micronucleation, while restoration of the 3-MA-inhibited ATP surge by addition of pyruvate suppressed cell death. These results show that DNA damaging agents induce an autophagy-associated ATP surge that protects cells and may contribute to drug resistance.

Influence of differently processed yeast (Kluyveromyces fragilis) on feed intake and gut physiology in weaned pigs.

Two feeding trials were conducted to investigate the effects of hydrolyzed (HY) or non-hydrolyzed (NHY) yeast (Kluyveromyces fragilis) in isoenergetic and isonitrogeneous diets in the postweaning period. In experiment 1, a total of 550 unsexed pigs (6.5 ± 0.5 kg BW), weaned at 24 ± 2 d of age, were allocated to five treatment groups, receiving either a control diet (CON) or diets with 1%, 3%, and 5% HY (groups HY1, HY3, and HY5, respectively), or a diet with 3% NHY (group NHY3). In experiment 2, a total of 48 male and female pigs (6.2 ± 0.3 kg BW, weaned at d 25) were allocated to three dietary groups (n = 8 replicates with two pigs) receiving a control diet (CON) or diets with 1% NHY or 1% HY. Eight animals were sacrificed 2 wk after weaning for histological investigations in the jejunum and colon, determination of apparent ileal digestibility (AID) of CP and ether extract (EE), and electrophysiological measurements in the jejunal tissue after addition of carbachol or l-glutamine using Ussing chambers. In experiment 1, different treatments had no significant effect on pig performance, but diet HY1 tended to increase ADG and G:F in wk 2 after weaning (P < 0.1). In experiment 2, diet HY1 increased feed intake in wk 2 (P < 0.05), whereas NHY yeast had no effect on feed intake. Villus height, villus/crypt ratio in jejunum (P < 0.05), and crypt depth in colon (P < 0.01) were increased in group HY1. Crypt depth in jejunum and small intestinal length were not affected by different treatments. The AID of CP and EE tended to increase in group HY1 (P < 0.1) compared with groups CON and NHY. In the Ussing chamber experiments, no changes in basal electrophysiological parameters were observed, and the reactions of the treatment groups to carbachol and l-glutamine were comparable. ADFI was positively correlated with different parameters of intestinal morphology (villus height, villus/crypt ratio, crypt depth in colon, length of small intestine), AID of CP, EE, and performance. The results suggest that a supplementation of 1% HY based on K. fragilis to pig diets may positively influence ADFI and intestinal morphology in pig in the early postweaning period (d 1 to 14).

Aberrant silencing of the CpG island-containing human O6-methylguanine DNA methyltransferase gene is associated with the loss of nucleosome-like positioning.

Tumor-associated aberrant silencing of CpG island-containing genes has been correlated with increased cytosine methylation, a "closed" chromatin structure, and exclusion of transcription factor binding in the CpG island/promoter regions of affected genes. Given the lack of understanding of what constitutes a closed chromatin structure in CpG islands, however, it has been difficult to assess the relationship among cytosine methylation, chromatin structure, and inappropriate gene silencing. In this study, nuclease accessibility analysis was used to more clearly define the chromatin structure in the CpG island of the human O6-methylguanine DNA methyltransferase (MGMT) gene. Chromatin structure was then related to in vivo DNA-protein interactions and cytosine methylation status of the MGMT CpG island in human glioma cells varying in MGMT expression. The results of these studies indicated that the "open" chromatin structure associated with the MGMT CpG island in MGMT+ cells consisted of an approximately 250-bp transcription factor-binding, nuclease-accessible, nucleosome-free region of DNA, whose formation was associated with at least four flanking, precisely positioned nucleosome-like structures. In MGMT- cells, this precise nucleosomal array was lost and was replaced by randomly positioned nucleosomes (i.e., the closed chromatin structure), regardless of whether methylation of the CpG island was spread over the entire island or limited to regions outside the transcription factor binding region. These results suggest that CpG islands facilitate the expression of housekeeping genes by facilitating nucleosomal positioning and that the conditions that alter the formation of this array (such as perhaps methylation) may indirectly affect CpG island-containing gene expression.

Methylation-related chromatin structure is associated with exclusion of transcription factors from and suppressed expression of the O-6-methylguanine DNA methyltransferase gene in human glioma cell lines.

There is considerable interest in identifying factors responsible for expression of the O-6-methylguanine DNA methyltransferase (MGMT) gene, as MGMT is a major determinant in the response of glioma cells to the chemotherapeutic agent 1,3 bis(2-chloroethyl)-1-nitrosourea. Recently we have shown that MGMT expression is correlated in a direct, graded fashion with methylation in the body of the MGMT gene and in an inverse, graded fashion with promoter methylation in human glioma cell lines. To determine if promoter methylation is an important component of MGMT expression, this study addressed the complex interactions between methylation, chromatin structure, and in vivo transcription factor occupancy in the MGMT promoter of glioma cell lines with different levels of MGMT expression. Our results show that the basal promoter in MGMT-expressing glioma cell lines, which is 100% unmethylated, was very accessible to restriction enzymes at all sites tested, suggesting that this region may be nucleosome free. The basal promoter in glioma cells with minimal MGMT expression, however, which is 75% unmethylated, was much less accessible, and the basal promoter in nonexpressing cells, which is 50% unmethylated, was entirely inaccessible to restriction enzymes. Despite the presence of the relevant transcription factors in all cell lines examined, in vivo footprinting showed DNA-protein interactions at six Sp1 binding sites and one novel binding site in MGMT-expressing cell lines but no such interactions in nonexpressors. We conclude that in contrast to findings of previous in vitro studies, Sp1 is an important component of MGMT transcription. These correlations also strongly suggest that methylation and chromatin structure, by determining whether Sp1 and other transcription factors can access the MGMT promoter, set the transcriptional state of the MGMT gene.

Methylation of discrete regions of the O6-methylguanine DNA methyltransferase (MGMT) CpG island is associated with heterochromatinization of the MGMT transcription start site and silencing of the gene.

O6-Methylguanine DNA methyltransferase (MGMT) repairs the mutagenic and cytotoxic O6-alkylguanine lesions produced by environmental carcinogens and the chemotherapeutic nitrosoureas. As such, MGMT-mediated repair of O6-alkylguanine lesions constitutes a major form of resistance to nitrosourea chemotherapy and makes control of MGMT expression of clinical interest. The variability of expression in cell lines and tissues, along with the ease with which the MGMT phenotype reverts under various conditions, suggests that MGMT is under epigenetic control. One such epigenetic mechanism, 5-methylation of cytosines, has been linked to MGMT expression. We have used an isogenic human multiple myeloma tumor cell line model composed of an MGMT-positive parent cell line, RPMI 8226/S, and its MGMT-negative variant, termed 8226/V, to study the control of MGMT expression. The loss of MGMT activity in 8226/V was found to be due to the loss of detectable MGMT gene expression. Bisulfite sequencing of the MGMT CpG island promoter revealed large increases in the levels of CpG methylation within discrete regions of the 8226/V MGMT CpG island compared to those in 8226/S. These changes in CpG methylation are associated with local heterochromatinization of the 8226/V MGMT transcription start site and provide a likely mechanism for the loss of MGMT transcription in 8226/V.

Mycobacterium tuberculosis in central Ethiopia: drug sensitivity patterns and association with genotype.

Drug resistance tuberculosis (TB) and the emergence of multidrug resistant (MDR) isolates are significant concerns regarding TB control programs in several countries. This study was undertaken to evaluate the drug sensitivity of Mycobacterium tuberculosis and to assess its association with strains and lineages of M. tuberculosis. A total of 279 M. tuberculosis strains isolated from Central Ethiopia were tested for their drug sensitivity patterns to first line TB drugs using the conventional proportion method on Löwenstein Jensen media. The association between drug sensitivity and strain type was assessed on 263 isolates of the 279 isolates. Of the 268 M. tuberculosis isolates obtained from new cases, 209 (78%) were susceptible to first line TB drugs, and 59 (22.2%) bacterial isolates were resistant to at least one of the first line drugs. The highest mono-resistance (7.5%) pertained to streptomycin (STM). Remarkably, seven of eleven isolates (63.6%) previous treatment for TB were resistant to at least one of the first line drugs. The prevalence of MDR-TB was 1.5% (4/268) for newly identified TB cases, all of which were members of the Euro-American Lineage. There was no statistically significant association (P > 0.05) between drug sensitivity, and either strains, sub-lineages or main lineages of M. tuberculosis. A significant proportion of M. tuberculosis was resistant to at least one first line anti-TB drug. Moreover, the frequencies of resistance to either isoniazid or rifampicin were high compared to data that were previously reported in some part of the country.

UBE2S, a novel substrate of Akt1, associates with Ku70 and regulates DNA repair and glioblastoma multiforme resistance to chemotherapy.

Glioblastoma multiforme (GBM) is the most common primary malignant brain cancer in adults. However, the molecular events underlying carcinogenesis and their interplay remain elusive. Here, we report that the stability of Ubiquitin-conjugating enzyme E2S (UBE2S) is regulated by the PTEN/Akt pathway and that its degradation depends on the ubiquitin-proteasome system. Mechanistically, Akt1 physically interacted with and phosphorylated UBE2S at Thr 152, enhancing its stability by inhibiting proteasomal degradation. Additionally, accumulated UBE2S was found to be associated with the components of the non-homologous end-joining (NHEJ) complex and participated in the NHEJ-mediated DNA repair process. The association of Ku70 with UBE2S was enhanced, and the complex was recruited to double-stranded break (DSB) sites in response to etoposide treatment. Furthermore, knockdown of UBE2S expression inhibited NHEJ-mediated DSB repair and rendered glioblastoma cells more sensitive to chemotherapy. Overall, our findings provide a novel drug target that may serve as the rationale for the development of a new therapeutic approach.

Presetting of chromatin structure and transcription factor binding poise the human GADD45 gene for rapid transcriptional up-regulation.

GADD45 has been suggested to coordinate cell cycle regulation with the repair of DNA damage following ionizing radiation (IR). Although the GADD45 gene is transcriptionally up-regulated in response to IR, alterations in in vivo transcription factor (TF) binding or chromatin structure associated with up-regulation have not been defined. To understand how chromatin structure might influence TF binding and GADD45 up-regulation, key regulatory regions of the gene were identified by in vivo DNase I hypersensitivity (HS) analysis. Chromatin structure and in vivo TF binding in these regions were subsequently monitored in both non-irradiated and irradiated human ML-1 cells. In non-irradiated cells expressing basal levels of GADD45, the gene exhibited a highly organized chromatin structure with distinctly positioned nucleosomes. Also identified in non-irradiated cells were DNA-protein interactions at octamer binding motifs and a CCAAT box in the promoter and at consensus binding sites for AP-1 and p53 within intron 3. Upon irradiation and a subsequent 15-fold increase in GADD45 mRNA levels, neither the chromatin structure nor the pattern of TF binding in key regulatory regions was altered. These results suggest that the GADD45 gene is poised for up-regulation and can be rapidly induced independent of gross changes in chromatin structure or TF binding.

Confluence-induced alterations in CpG island methylation in cultured normal human fibroblasts.

Growth constraint of bacterial and human cells has been shown to trigger genetic mutation. We questioned whether growth constraint might also trigger epigenetic mutation in the form of CpG island methylation. Logarithmically growing normal human fibro-blasts (NHF) displayed little (0-15%) CpG methylation in select regions of three CpG islands [estrogen receptor (ER), E-cadherin (ECAD) and O (6)-methylguanine-DNA methyltransferase (MGMT)] examined. NHF grown to and left at confluence for 2-21 days showed little (<10%) CpG methylation in the ER and ECAD CpG islands. These confluent, growth-arrested cells, however, displayed extensive ( approximately 50%) methylation of the MGMT CpG island. CpG methylation in the MGMT CpG island was not associated with cellular senescence. The methylation was, however, heritable, but not permanent, as the level of CpG methylation in the MGMT CpG island of cells 4 population doublings following replating after confluence were no different from those in confluent cultures, but returned to levels noted in logarithmically growing cells by 10 population doublings following replating. These results suggest that growth constraint can trigger transient epigenetic change even in normal non-senescent human cells.

DNA-directed actions of 3-deazaguanine: effects on DNA integrity and DNA elongation/ligation.

The cytotoxic action of the guanine analogue, 3-deazaguanine, was shown previously to be closely associated with deazaguanine-induced inhibition of DNA synthesis and incorporation of deazaguanine into DNA. The DNA-directed effects of the compound have been further investigated by studying the effect of deazaguanine on DNA integrity, and on the ability of pulse-labeled L1210 cells to synthesize full length DNA. Deazaguanine caused DNA single strand breaks in newly synthesized DNA but not in preformed DNA. The amount of DNA single strand breaks correlated with both deazaguanine exposure and with the amount of deazaguanine incorporated into the DNA. When cells were allowed to recover in drug-free medium for 12 or 24 h after drug exposure little effect on either the amount of DNA single strand breaks or cell viability relative to controls was observed. Deazaguanine also inhibited the ability of L1210 cells to synthesize full length DNA after pulse labeling of DNA. This effect was temporally related to the inhibition by deazaguanine of total DNA synthesis.