Implication of protein kinase C in the regulation of DNA mismatch repair protein expression and function

Nrf2 overexpression increases risk of high tumor mutation burden in acute myeloid leukemia by inhibiting MSH2

Nuclear factor erythroid 2-related factor 2 (Nrf2, also called NFE2L2) plays an important role in cancer chemoresistance. However, little is known about the role of Nrf2 in tumor mutation burden and the effect of Nrf2 in modulating DNA mismatch repair (MMR) gene in acute myeloid leukemia (AML). Here we show that Nrf2 expression is associated with tumor mutation burden in AML. Patients with Nrf2 overexpression had a higher frequency of gene mutation and drug resistance. Nrf2 overexpression protected the AML cells from apoptosis induced by cytarabine in vitro and increased the risk of drug resistance associated with a gene mutation in vivo. Furthermore, Nrf2 overexpression inhibited MutS Homolog 2 (MSH2) protein expression, which caused DNA MMR deficiency. Mechanistically, the inhibition of MSH2 by Nrf2 was in a ROS-independent manner. Further studies showed that an increased activation of JNK/c-Jun signaling in Nrf2 overexpression cells inhibited the expression of the MSH2 protein. Our findings provide evidence that high Nrf2 expression can induce gene instability-dependent drug resistance in AML. This study demonstrates the reason why the high Nrf2 expression leads to the increase of gene mutation frequency in AML, and provides a new strategy for clinical practice.

Genomic Instability Promoted by Overexpression of Mismatch Repair Factors in Yeast: A Model for Understanding Cancer Progression

Mismatch repair (MMR) proteins act in spellchecker roles to excise misincorporation errors that occur during DNA replication. Curiously, large-scale analyses of a variety of cancers showed that increased expression of MMR proteins often correlated with tumor aggressiveness, metastasis, and early recurrence. To better understand these observations, we used The Cancer Genome Atlas and Gene Expression across Normal and Tumor tissue databases to analyze MMR protein expression in cancers. We found that the MMR genes MSH2 and MSH6 are overexpressed more frequently than MSH3, and that MSH2 and MSH6 are often cooverexpressed as a result of copy number amplifications of these genes. These observations encouraged us to test the effects of upregulating MMR protein levels in baker's yeast, where we can sensitively monitor genome instability phenotypes associated with cancer initiation and progression. Msh6 overexpression (two- to fourfold) almost completely disrupted mechanisms that prevent recombination between divergent DNA sequences by interacting with the DNA polymerase processivity clamp PCNA and by sequestering the Sgs1 helicase. Importantly, cooverexpression of Msh2 and Msh6 (∼eightfold) conferred, in a PCNA interaction-dependent manner, several genome instability phenotypes including increased mutation rate, increased sensitivity to the DNA replication inhibitor HU and the DNA-damaging agents MMS and 4-nitroquinoline N-oxide, and elevated loss-of-heterozygosity. Msh2 and Msh6 cooverexpression also altered the cell cycle distribution of exponentially growing cells, resulting in an increased fraction of unbudded cells, consistent with a larger percentage of cells in G1. These novel observations suggested that overexpression of MSH factors affected the integrity of the DNA replication fork, causing genome instability phenotypes that could be important for promoting cancer progression.

Understanding how mismatch repair proteins participate in the repair/anti-recombination decision

Mismatch repair (MMR) systems correct DNA mismatches that result from DNA polymerase misincorporation errors. Mismatches also appear in heteroduplex DNA intermediates formed during recombination between nearly identical sequences, and can be corrected by MMR or removed through an unwinding mechanism, known as anti-recombination or heteroduplex rejection. We review studies, primarily in baker's yeast, which support how specific factors can regulate the MMR/anti-recombination decision. Based on recent advances, we present models for how DNA structure, relative amounts of key repair proteins, the timely localization of repair proteins to DNA substrates and epigenetic marks can modulate this critical decision.

Ubiquitin-specific Peptidase 10 (USP10) Deubiquitinates and Stabilizes MutS Homolog 2 (MSH2) to Regulate Cellular Sensitivity to DNA Damage

MSH2 is a key DNA mismatch repair protein, which plays an important role in genomic stability. In addition to its DNA repair function, MSH2 serves as a sensor for DNA base analogs-provoked DNA replication errors and binds to various DNA damage-induced adducts to trigger cell cycle arrest or apoptosis. Loss or depletion of MSH2 from cells renders resistance to certain DNA-damaging agents. Therefore, the level of MSH2 determines DNA damage response. Previous studies showed that the level of MSH2 protein is modulated by the ubiquitin-proteasome pathway, and histone deacetylase 6 (HDAC6) serves as an ubiquitin E3 ligase. However, the deubiquitinating enzymes, which regulate MSH2 remain unknown. Here we report that ubiquitin-specific peptidase 10 (USP10) interacts with and stabilizes MSH2. USP10 deubiquitinates MSH2 in vitro and in vivo Moreover, the protein level of MSH2 is positively correlated with the USP10 protein level in a panel of lung cancer cell lines. Knockdown of USP10 in lung cancer cells exhibits increased cell survival and decreased apoptosis upon the treatment of DNA-methylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and antimetabolite 6-thioguanine (6-TG). The above phenotypes can be rescued by ectopic expression of MSH2. In addition, knockdown of MSH2 decreases the cellular mismatch repair activity. Overall, our results suggest a novel USP10-MSH2 pathway regulating DNA damage response and DNA mismatch repair.

Triapine-mediated ABCB1 induction via PKC induces widespread therapy unresponsiveness but is not underlying acquired triapine resistance

Although triapine is promising for treatment of advanced leukemia, it failed against solid tumors due to widely unknown reasons. To address this issue, a new triapine-resistant cell line (SW480/tria) was generated by drug selection and investigated in this study. Notably, SW480/tria cells displayed broad cross-resistance against several known ABCB1 substrates due to high ABCB1 levels (induced by promoter hypomethylation). However, ABCB1 inhibition did not re-sensitize SW480/tria cells to triapine and subsequent analysis revealed that triapine is only a weak ABCB1 substrate without significant interaction with the ABCB1 transport function. Interestingly, in chemo-naive, parental SW480 cells short-time (24 h) treatment with triapine stimulated ABCB1 expression. These effects were based on activation of protein kinase C (PKC), a known response to cellular stress. In accordance, SW480/tria cells were characterized by elevated levels of PKC. Together, this led to the conclusion that increased ABCB1 expression is not the major mechanism of triapine resistance in SW480/tria cells. In contrast, increased ABCB1 expression was found to be a consequence of triapine stress-induced PKC activation. These data are especially of importance when considering the choice of chemotherapeutics for combination with triapine.

Transcriptional regulation of human DNA repair genes following genotoxic stress: trigger mechanisms, inducible responses and genotoxic adaptation

DNA repair is the first barrier in the defense against genotoxic stress. In recent years, mechanisms that recognize DNA damage and activate DNA repair functions through transcriptional upregulation and post-translational modification were the focus of intensive research. Most DNA repair pathways are complex, involving many proteins working in discrete consecutive steps. Therefore, their balanced expression is important for avoiding erroneous repair that might result from excessive base removal and DNA cleavage. Amelioration of DNA repair requires both a fine-tuned system of lesion recognition and transcription factors that regulate repair genes in a balanced way. Transcriptional upregulation of DNA repair genes by genotoxic stress is counteracted by DNA damage that blocks transcription. Therefore, induction of DNA repair resulting in an adaptive response is only visible through a narrow window of dose. Here, we review transcriptional regulation of DNA repair genes in normal and cancer cells and describe mechanisms of promoter activation following genotoxic exposures through environmental carcinogens and anticancer drugs. The data available to date indicate that 25 DNA repair genes are subject to regulation following genotoxic stress in rodent and human cells, but for only a few of them, the data are solid as to the mechanism, homeostatic regulation and involvement in an adaptive response to genotoxic stress.

Structural, molecular and cellular functions of MSH2 and MSH6 during DNA mismatch repair, damage signaling and other noncanonical activities

The field of DNA mismatch repair (MMR) has rapidly expanded after the discovery of the MutHLS repair system in bacteria. By the mid 1990s yeast and human homologues to bacterial MutL and MutS had been identified and their contribution to hereditary non-polyposis colorectal cancer (HNPCC; Lynch syndrome) was under intense investigation. The human MutS homologue 6 protein (hMSH6), was first reported in 1995 as a G:T binding partner (GTBP) of hMSH2, forming the hMutSα mismatch-binding complex. Signal transduction from each DNA-bound hMutSα complex is accomplished by the hMutLα heterodimer (hMLH1 and hPMS2). Molecular mechanisms and cellular regulation of individual MMR proteins are now areas of intensive research. This review will focus on molecular mechanisms associated with mismatch binding, as well as emerging evidence that MutSα, and in particular, MSH6, is a key protein in MMR-dependent DNA damage response and communication with other DNA repair pathways within the cell. MSH6 is unstable in the absence of MSH2, however it is the DNA lesion-binding partner of this heterodimer. MSH6, but not MSH2, has a conserved Phe-X-Glu motif that recognizes and binds several different DNA structural distortions, initiating different cellular responses. hMSH6 also contains the nuclear localization sequences required to shuttle hMutSα into the nucleus. For example, upon binding to O(6)meG:T, MSH6 triggers a DNA damage response that involves altered phosphorylation within the N-terminal disordered domain of this unique protein. While many investigations have focused on MMR as a post-replication DNA repair mechanism, MMR proteins are expressed and active in all phases of the cell cycle. There is much more to be discovered about regulatory cellular roles that require the presence of MutSα and, in particular, MSH6.

Characterization of the antiproliferative activity of Xylopia aethiopica

BACKGROUND

Xylopia aethiopica, a plant found throughout West Africa, has both nutritional and medicinal uses. The present study aims to characterize the effects of extracts of this plant on cancer cells.

RESULTS

We report that X. aethiopica extract prepared with 70% ethanol has antiproliferative activity against a panel of cancer cell lines. The IC50 was estimated at 12 μg/ml against HCT116 colon cancer cells, 7.5 μg/ml and > 25 μg/ml against U937 and KG1a leukemia cells, respectively. Upon fractionation of the extract by HPLC, the active fraction induced DNA damage, cell cycle arrest in G1 phase and apoptotic cell death. By using NMR and mass spectrometry, we determined the structure of the active natural product in the HPLC fraction as ent-15-oxokaur-16-en-19-oic acid.

CONCLUSION

The main cytotoxic and DNA-damaging compound in ethanolic extracts of Xylopia aethiopica is ent-15-oxokaur-16-en-19-oic acid.

Phosphorylated hMSH6: DNA mismatch versus DNA damage recognition

DNA mismatch repair (MMR) maintains genomic integrity by correction of mispaired bases and insertion-deletion loops. The MMR pathway can also trigger a DNA damage response upon binding of MutSα to specific DNA lesions such as O(6)methylguanine (O(6)meG). Limited information is available regarding cellular regulation of these two different pathways. Within this report, we demonstrate that phosphorylated hMSH6 increases in concentration in the presence of a G:T mismatch, as compared to an O(6)meG:T lesion. TPA, a kinase activator, enhances the phosphorylation of hMSH6 and binding of hMutSα to a G:T mismatch, though not to O(6)meG:T. UCN-01, a kinase inhibitor, decreases both phosphorylation of hMSH6 and binding of hMutSα to G:T and O(6)meG:T. HeLa MR cells, pretreated with UCN-01 and exposed to MNNG, undergo activation of Cdk1 and mitosis despite phosphorylation of Chk1 and inactivating phosphorylation of Cdc25c. These results indicate that UCN-01 may inhibit an alternative cell cycle arrest pathway associated with the MMR pathway that does not involve Cdc25c. In addition, recombinant hMutSα containing hMSH6 mutated at an N-terminal cluster of four phosphoserines exhibits decreased phosphorylation and decreased binding of hMutSα to G:T and O(6)meG:T. Taken together, these results suggest a model in which the amount of phosphorylated hMSH6 bound to DNA is dependent on the presence of either a DNA mismatch or DNA alkylation damage. We hypothesize that both phosphorylation of hMSH6 and total concentration of bound hMutSα are involved in cellular signaling of either DNA mismatch repair or MMR-dependent damage recognition activities.

G2/M checkpoint stringency is a key parameter in the sensitivity of AML cells to genotoxic stress

Acute myeloid leukemia (AML) cells exposed to genotoxic agents arrest their cell cycle at the G2/M checkpoint and are inherently chemoresistant. To understand the mechanism of this chemoresistance, we compared the ability of immature CD34+ versus mature CD34- AML cell lines (KG1a and U937, respectively) to recover from a DNA damage-induced cell cycle checkpoint in G2. Here, we report that KG1a cells have a more stringent G2/M checkpoint response than U937 cells. We show that in both cell types, the CDC25B phosphatase participates in the G2/M checkpoint recovery and that its expression is upregulated. Furthermore, we show that CHK1 inhibition by UCN-01 in immature KG1a cells allows checkpoint exit and induces sensitivity to genotoxic agents. Similarly, UCN-01 treatment potentializes genotoxic-induced inhibition of colony formation efficiency of primary leukemic cells from AML patients. Altogether, our results demonstrate that checkpoint stringency varies during the maturation process and indicate that targeting checkpoint mechanisms might represent an attractive therapeutic opportunity for chemoresistant immature AML cells.

Structure and function of the components of the human DNA mismatch repair system

DNA mismatch repair (MMR) is one of the several enzyme systems involved in DNA homeostasis. DNA MMR is involved in the repair of specific types of errors that occur during new DNA synthesis; loss of this system leads to an accelerated accumulation of potential mutations, and predisposes to certain types of cancers. Germline mutations in some of the DNA MMR genes cause the hereditary cancer predisposition, Lynch syndrome. This review addresses advances in the biochemistry of DNA MMR and its relationship to carcinogenesis.

Flow cytometric detection of tandem repeat mutations induced by various chemical classes

To facilitate detection of genotoxicity from environmental mutagen exposure, we generated an in vitro enhanced green fluorescence protein (EGFP) reactivation assay that quickly and effectively detects frameshift mutations in tandem repeat sequences (TRS). Two murine cell lines, C3H10T1/2 and mismatch repair deficient MC2a, were stably transfected with EGFP reporter plasmids in which the EGFP constructs contain TRS that put the EGFP sequence out of frame. These included several 2, 3, 4, 5 and 6 bp repeat sequences, a control non-repetitive sequence and a human gene sequence containing a 4 bp repeat motif. Transfected cultures were exposed to five model mutagens and carcinogens: hydrogen peroxide (H(2)O(2)), 12-O-tetradecanoyl-phorbol-13-acetate (TPA), benzo-a-pyrene-diol-epoxide (BPDE), ethyl nitrosourea (ENU), 9-aminoacridine (9AA) and two controls: acetone and ethanol. Frameshift mutations resulted in green fluorescent revertants, as determined by flow cytometry, and were confirmed, for 9AA treatments, by sequencing. All five treatments with model agents induced statistically significant sequence- and exposure-dependent responses in MC2a cells and a negative response with the two negative control treatments, acetone and ethanol. Similar responses were seen in a smaller panel of treatments and plasmids in C3H10T1/2 cells. The mutation frequencies were higher in cells transfected with the plasmids containing TRS than those harbouring the control construct lacking repeats. The highest mutation frequencies were observed with H(2)O(2) and 9AA treatments, yielding up to a 50-fold difference between vehicle and highest concentration treatment. ENU, BPDE, and to a lesser extent TPA treatments, also showed a statistically significant exposure response. Results from these experiments reveal that the assay responds robustly to various classes of mutagenic substances, as well as to rodent carcinogens that are inactive in conventional mutation assays, and that responses are not linked to cytotoxicity. This assay is a promising approach for detecting chemically induced frameshifts within certain DNA sequences of interest, but further characterization and validation are required prior to general use in genotoxicity screening.

hMutSα is Protected from Ubiquitin-proteasome-dependent Degradation by Atypical Protein Kinase Cζ Phosphorylation

The hMutS alpha (hMSH2-hMSH6) protein heterodimer plays a critical role in the detection of DNA mispairs in the mismatch repair (MMR) process. We recently reported that hMutS alpha proteins were degraded by the ubiquitin-proteasome pathway in a cell-type-dependent manner, indicating that one or several regulator(s) may interfere with hMutS alpha protein ubiquitination and degradation. On the other hand, we and others have shown that protein kinase C (PKC) is involved as a positive regulator of MMR activity. Here, we provide evidence that the atypical PKC zeta regulates ubiquitination, degradation, and levels of hMutS alpha proteins. Using both PKC zeta-transfected U937 and PKC zeta siRNA-transfected MRC-5 cell lines, we found that PKC zeta protein expression was correlated with that of hMutS alpha as well as with MMR activity, but was inversely correlated with hMutS alpha protein ubiquitination and degradation. Interestingly, PKC zeta interacts with hMSH2 and hMSH6 proteins and phosphorylates both. Moreover, in an in vitro assay PKCzeta mediates phosphorylation events decreasing hMutS alpha protein degradation via the ubiquitin-proteasome pathway. Altogether, our results indicate that PKC zeta modulates hMutS alpha stability and protein levels, and suggest a role for PKC zeta in genome stability by regulating MMR activity.

Atypical protein kinase C stimulates nucleotide excision repair activity

Nucleotide excision repair (NER) deals with bulky DNA damages. However, the regulation of this process is still unclear. Here, we show that both cell resistance to genotoxic agents that generate DNA lesions corrected by NER and in vitro NER activity are correlated with atypical protein kinase C (PKC) zeta expression levels. Moreover, repair intermediates are produced and eliminated more rapidly in UV-irradiated PKCzeta-overexpressing cells. The expression levels of XPC and hHR23B, two NER proteins, are correlated with PKCzeta expression. Altogether, these results strongly suggest that PKCzeta could act as a modulator of NER activity by regulating the expression of XPC/hHR23B heterodimer.

Degadration of mismatch repair hMutSalpha heterodimer by the ubiquitin-proteasome pathway

Mismatch repair plays a critical role in genome stability. This process requires several proteins including hMSH2/hMSH6 (hMutSalpha) heterodimer involved in the first stage of the process, the mispair recognition. We previously reported that in U937 and HL-60 cell lines, hMSH2 and hMSH6 protein expression was much lower than that in HeLa and KG1a cells. Here, we showed that the decreased expression of hMutSalpha results from differences in the degradation rate of both proteins by the ubiquitin-proteasome pathway. Our data suggest that in human cell lines, ubiquitin-proteasome could play an important role in the regulation of hMutSalpha protein expression, thereby regulating mismatch repair activity.

Distinct mechanisms lead to HPRT gene mutations in leukemic cells

Leukemias are considered malignant clonal disorders arising from the accumulation of mutations in hematopoietic cells; the majority of these mutations are thought to be acquired somatically. Measurement of mutation frequency (Mf) at the hypoxanthine phosphoribosyltransferase (HPRT) locus has been developed as a method for estimating genomic instability. We investigated the Mf in 16 leukemic cell lines to determine whether these cell lines showed evidence of genomic instability. Although some leukemic cell lines had markedly elevated Mfs, the Mfs at the HPRT locus in leukemic cell lines were not always higher than those of B-lymphoblastoid cell lines and T lymphocytes from normal individuals. We were able to identify the HPRT mutation for 159 of 160 individual HPRT mutants. The HPRT mutations were characterized at a molecular level and classified as either gross chromosomal rearrangements (GCRs) or point mutations, such as single-nucleotide substitutions, insertions, or deletions. With rare exceptions, individual leukemic cell lines showed either point mutations or GCR, but not both. Of note, all the cell lines that primarily showed point mutations are known to be defective in mismatch repair machinery.

BAT-25 and BAT-26, two mononucleotide microsatellites, are not sensitive markers of microsatellite instability in acute myeloid leukaemia

BAT-25 and BAT-26 are mononucleotide microsatellites with quasi-monomorphic allele length distribution in healthy controls but unstable, shortened alleles in solid organ tumours with a mutator phenotype (RER+). Both markers are highly sensitive and specific for RER+ colorectal cancer. This study evaluated three mononucleotide microsatellites, BAT-25, BAT-26 and BAT-40 (a polymorphic mononucleotide microsatellite) in RER+ acute myeloid leukaemia (AML). Sixteen [six therapy-related AML (t-AML), 10 de novo AML] known RER+ AML, 22 healthy controls and five known RER- AML samples were analysed. In healthy controls and RER- AML, BAT-25 and BAT-26 were quasi-monomorphic and BAT-40 was polymorphic. Of the RER+ AML samples, 0 of 16 had microsatellite instability (MSI) at BAT-25, 0 of 16 had MSI at BAT-26 and three of 16 had MSI at BAT-40. We conclude that BAT-25, BAT-26 and BAT-40 microsatellites are insensitive to RER+ AML. Further studies are required to determine a consensus panel of sensitive microsatellites for use in AML.

hMSH2 expression is driven by AP1-dependent regulation through phorbol-ester exposure

Mammalian mismatch repair (MMR) plays a prominent role in genomic stability and toxicity induced by some DNA damaging agents. Advance in the appreciation of regulation mechanisms of the key MMR protein hMSH2 would certainly lead to valuable information on its role and to a better understanding of MMR system dysfunctions with respect to their consequences in cells. We have previously reported that, in myeloid leukemic U937 cell line, the expression of hMSH2 MMR protein is regulated by protein kinase C (PKC) activity. Here we show that the increase of protein level following PKC activation by phorbol ester (TPA) treatment parallels that of hMSH2 mRNA. Our results support the view that the hMSH2 gene is prone to transcriptional regulation upon TPA induction, and that AP-1 is a factor implicated in the transactivation. When losing the AP-1-dependent hMSH2 promoter activity, either by mutating the AP-1 binding sites of the hMSH2 promoter or by using a dominant negative c-Jun factor, the hMSH2 overexpression induced by TPA is abolished both in vitro and in vivo. Thus the control of hMSH2 expression by PKC appears to be dependent, at least partially, on an up-regulation mediated by AP-1 transactivation.

Chelerythrin activates caspase-8, downregulates FLIP long and short, and overcomes resistance to tumour necrosis factor-related apoptosis-inducing ligand in KG1a cells

KG1a cells (CD34+/38-) express FAS and TRAIL (tumour-necrosis-factor-related apoptosis-inducing ligand) receptors but are resistant to FAS-ligand and TRAIL/APO2-L (apoptosis antigen-2 ligand)-induced apoptosis. KG1a cells are sensitized to FAS-induced apoptosis by chelerythrin, an inhibitor of protein kinase C (PKC). As cytoplasmatic adaptor molecules of FAS, e.g. FLIP [Fas-associated death domain protein (FADD)-like interleukin 1 beta-converting enzyme [FLICE (caspase-8)-inhibitory protein]], also modulate TRAIL signals, we determined whether chelerythrin affected TRAIL-mediated apoptosis. Chelerythrin by itself induced apoptosis in KG1a cells, and apoptosis was associated with activation of caspase-8. While TRAIL alone failed to activate caspase-8 or induce apoptosis, the addition of TRAIL to chelerythrin-treated cells significantly enhanced cleavage of caspase-8 and apoptosis. Chelerythrin-pretreated KG1a cells showed decreased phosphorylation of protein kinase C (PKC)-zeta and downregulation of both FLIP long and FLIP short proteins. Downregulation of FLIP and induction of apoptosis were partially abrogated by pretreatment with the specific caspase-8 inhibitor, Z-IETD-FMK. The decrease in FLIP protein expression induced by chelerythrin was accompanied by a progressive increase in mRNA levels of both FLIP long and FLIP short. CD34+ precursors from normal human marrow were also sensitive to chelerythrin but, in contrast to KG1a cells, were not sensitized to TRAIL-mediated apoptosis. Thus, resistance to TRAIL-induced apoptosis in leukaemic KG1a cells but not in normal CD34+ precursors was overcome in the presence of chelerythrin. The mechanism appeared to involve inhibition of PKC. Central targets were FLIP long and FLIP short, and their interactions with caspase-8. Whether such a pathway can be exploited to selectively target leukaemic progenitor cells remains to be determined.