Implication of Ref-1 in the repression of renin gene transcription by intracellular calcium

Additional functions of selected proteins involved in DNA repair

Protein moonlighting is a phenomenon in which a single polypeptide chain can perform a number of different unrelated functions. Here we present our analysis of moonlighting in the case of selected DNA repair proteins which include G:T mismatch-specific thymine DNA glycosylase (TDG), methyl-CpG-binding domain protein 4 (MBD4), apurinic/apyrimidinic endonuclease 1 (APE1), AlkB homologs, poly (ADP-ribose) polymerase 1 (PARP-1) and single-strand selective monofunctional uracil DNA glycosylase 1 (SMUG1). Most of their additional functions are not accidental and clear patterns are emerging. Participation in RNA metabolism is not surprising as bases occurring in RNA are the same or very similar to those in DNA. Other common additional function involves regulation of transcription. This is not unexpected as these proteins bind to specific DNA regions for DNA repair, hence they can also be recruited to regulate transcription. Participation in demethylation and replication of DNA appears logical as well. Some of the multifunctional DNA repair proteins play major roles in many diseases, including cancer. However, their moonlighting might prove a major difficulty in the development of new therapies because it will not be trivial to target a single protein function without affecting its other functions that are not related to the disease.

The long noncoding RNA ROCKI regulates inflammatory gene expression

Long noncoding RNAs (lncRNAs) can regulate target gene expression by acting in cis (locally) or in trans (non-locally). Here, we performed genome-wide expression analysis of Toll-like receptor (TLR)-stimulated human macrophages to identify pairs of cis-acting lncRNAs and protein-coding genes involved in innate immunity. A total of 229 gene pairs were identified, many of which were commonly regulated by signaling through multiple TLRs and were involved in the cytokine responses to infection by group B Streptococcus We focused on elucidating the function of one lncRNA, named lnc-MARCKS or ROCKI (Regulator of Cytokines and Inflammation), which was induced by multiple TLR stimuli and acted as a master regulator of inflammatory responses. ROCKI interacted with APEX1 (apurinic/apyrimidinic endodeoxyribonuclease 1) to form a ribonucleoprotein complex at the MARCKS promoter. In turn, ROCKI-APEX1 recruited the histone deacetylase HDAC1, which removed the H3K27ac modification from the promoter, thus reducing MARCKS transcription and subsequent Ca2+ signaling and inflammatory gene expression. Finally, genetic variants affecting ROCKI expression were linked to a reduced risk of certain inflammatory and infectious disease in humans, including inflammatory bowel disease and tuberculosis. Collectively, these data highlight the importance of cis-acting lncRNAs in TLR signaling, innate immunity, and pathophysiological inflammation.

On the top of ARB N/L type Ca channel blocker leads to less elevation of aldosterone

The activation of the renin–angiotensin system (RAS) is one of the unfavourable characteristics of calcium channel blocker (CCB). N type calcium channel is thought to be involved in renin gene transcription and adrenal aldosterone release. Accordingly, N/L type CCB has a possibility of less elevation of plasma aldosterone concentrations (PAC) among CCBs. In a monotherapy study, we had already demonstrated that N/L type CCB leads to less activation of the RAS compared with L type CCB. The objective of this study is to substantiate the hypothesis that at the condition of additive administration on the top of an angiotensin receptor blocker (ARB), still N/L type CCB leads to less elevation of PAC compared with L type one. Subjects were 60 hypertensives administered with valsartan. As an open label study, amlodipine (L type) or cilnidipine (N/L type) were administered on the top of valsartan (ARB) in a cross-over manner. Results were as follows (valsartan+amlodipine compared with valsartan+cilnidipine): systolic blood pressure (SBP)/diastolic blood pressure (DBP) (mmHg): 132±10/76±10 compared with 131±10/77±9, P=0.95/0.48, plasma renin activity (PRA) (ng/ml·h): 2.41±2.67 compared with 2.00±1.50 P=0.20, PAC (pg/ml): 77.3±31.0 compared with 67.4±24.8, P<0.05, urinary albumin excretion (UAE) (mg/gCr): 105.9±216.1 compared with 73.9±122.2, P<0.05. Thus, PAC at cilnidipine was significantly lower than those at amlodipine in spite of the comparable BP reductions. Besides, UAE was significantly lower at cilnidipine. In conclusion, on the top of the ARB, it is suggested that cilnidipine administration might lead to less elevation of PAC and reduction in UAE compared with amlodipine.

Dynamic Regulation of APE1/Ref-1 as a Therapeutic Target Protein

Apurinic/apyrimidinic endonuclease 1/redox factor-1 (APE1/Ref-1) is a multifunctional protein that plays a central role in the cellular response to DNA damage and redox regulation against oxidative stress. APE1/Ref-1 functions in the DNA base excision repair pathway, the redox regulation of several transcription factors, and the control of intracellular redox status through the inhibition of reactive oxygen species (ROS) production. APE1/Ref-1 is predominantly localized in the nucleus; however, its subcellular localization is dynamically regulated and it may be found in the mitochondria or elsewhere in the cytoplasm. Studies have identified a nuclear localization signal and a mitochondrial target sequence in APE1/Ref-1, as well as the involvement of the nuclear export system, as determinants of APE1/Ref-1 subcellular distribution. Recently, it was shown that APE1/Ref-1 is secreted in response to hyperacetylation at specific lysine residues. Additionally, post-translational modifications such as phosphorylation, S-nitrosation, and ubiquitination appear to play a role in fine-tuning the activities and subcellular localization of APE1/Ref-1. In this review, we will introduce the multifunctional role of APE1/Ref-1 and its potential usefulness as a therapeutic target in cancer and cardiovascular disease.

Determinants of plasma renin activity: role of a human renin gene variant as a genetic factor

The plasma renin activity (PRA) is affected by a number of environmental factors. However, significant heritability has been shown for the activity. A hypothesis that a candidate regulatory single-nucleotide polymorphism, C-5312T, of human renin gene should have a significant effect on PRA was elucidated and updating of independent determinants of PRA was attempted. Cross sectional study. Outpatient study. We enrolled consecutive 810 subjects who had consulted our hospitals for lifestyle-related diseases. Genotypes were assayed with genomic DNA for C-5312T. Among the genetic variants, the difference of PRA was evaluated. Monovariate linear regression analysis was performed to test the correlation between PRA and clinical variables. Finally, stepwise multiple regression analysis was performed to evaluate the independent determinants. On comparing 2 genotype groups, CC/CT and T allele homozygote, the geometric means of PRA were 0.778 and 0.941 ng/ml/h, respectively (F = 5.992, P = 0.015). Monovariate linear regression analysis revealed that a number of variables have a significant correlation with the activity, including urinary salt excretion. A stepwise multivariate regression analysis revealed that renin C-5312T variant (TT) is one of the independent determinants of PRA. Thus, for the first time, a human renin gene variant was associated with a significant increase in PRA as a genetic factor and the independent determinants for the activity were updated including genetic factor.

Functional variants of human APE1 rescue the DNA repair defects of the yeast AP endonuclease/3'-diesterase-deficient strain

Human APE1 is an essential enzyme performing functions in DNA repair and transcription. It possesses four distinct repair activities acting on a variety of base and sugar derived DNA lesions. APE1 has seven cysteine residues and Cys65, and to a lesser extent Cys93 and Cys99, is uniquely involved in maintaining a subset of transcription factors in the reduced and active state. Four of the cysteines Cys93, 99, 208 and 310 of APE1 are located proximal to its active site residues Glu96, Asp210 and His309 involved in processing damaged DNA, raising the possibility that missense mutation of these cysteines could alter the enzyme DNA repair functions. An earlier report documented that serine substitution of the individual cysteine residues did not affect APE1 ability to cleave an abasic site oligonucleotide substrate in vitro, except for Cys99Ser, although any consequences of these variants in the repair of in vivo DNA lesions were not tested. Herein, we mutated all seven cysteines of APE1, either singly or in combination, to alanine and show that none of the resulting variants interfered with the enzyme DNA repair functions. Cross-specie complementation analysis reveals that these APE1 cysteine variants fully rescued the yeast DNA repair deficient strain YW778, lacking AP endonucleases and 3'-diesterases, from toxicities caused by DNA damaging agents. Moreover, the elevated spontaneous mutations arising in strain YW778 from the lack of the DNA repair activities were completely suppressed by the APE1 cysteine variants. These findings suggest that the cysteine residues of APE1 are unlikely to play a role in the DNA repair functions of the enzyme in vivo. We also examine other APE1 missense mutations and provide the first evidence that the variant Asp308Ala with normal AP endonuclease, but devoid of 3'→5' exonuclease, displays hypersensitivity to the anticancer drug bleomycin, and not to other agents, suggesting that it has a defect in processing unique DNA lesions. Molecular modeling reveals that Asp308Ala cannot make proper contact with Mg(2+) and may alter the enzyme ability to cleave or disassociate from specific DNA lesions.

Human apurinic/apyrimidinic endonuclease 1

SIGNIFICANCE

Human apurinic/apyrimidinic endonuclease 1 (APE1, also known as REF-1) was isolated based on its ability to cleave at AP sites in DNA or activate the DNA binding activity of certain transcription factors. We review herein topics related to this multi-functional DNA repair and stress-response protein.

RECENT ADVANCES

APE1 displays homology to Escherichia coli exonuclease III and is a member of the divalent metal-dependent α/β fold-containing phosphoesterase superfamily of enzymes. APE1 has acquired distinct active site and loop elements that dictate substrate selectivity, and a unique N-terminus which at minimum imparts nuclear targeting and interaction specificity. Additional activities ascribed to APE1 include 3'-5' exonuclease, 3'-repair diesterase, nucleotide incision repair, damaged or site-specific RNA cleavage, and multiple transcription regulatory roles.

CRITICAL ISSUES

APE1 is essential for mouse embryogenesis and contributes to cell viability in a genetic background-dependent manner. Haploinsufficient APE1(+/-) mice exhibit reduced survival, increased cancer formation, and cellular/tissue hyper-sensitivity to oxidative stress, supporting the notion that impaired APE1 function associates with disease susceptibility. Although abnormal APE1 expression/localization has been seen in cancer and neuropathologies, and impaired-function variants have been described, a causal link between an APE1 defect and human disease remains elusive.

FUTURE DIRECTIONS

Ongoing efforts aim at delineating the biological role(s) of the different APE1 activities, as well as the regulatory mechanisms for its intra-cellular distribution and participation in diverse molecular pathways. The determination of whether APE1 defects contribute to human disease, particularly pathologies that involve oxidative stress, and whether APE1 small-molecule regulators have clinical utility, is central to future investigations.

P2Y2 receptor activation inhibits the expression of the sodium-chloride cotransporter NCC in distal convoluted tubule cells

Luminal nucleotide stimulation is known to reduce Na(+) transport in the distal nephron. Previous studies suggest that this mechanism may involve the thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC), which plays an essential role in NaCl reabsorption in the cells lining the distal convoluted tubule (DCT). Here we show that stimulation of mouse DCT (mDCT) cells with ATP or UTP promoted Ca(2+) transients and decreased the expression of NCC at both mRNA and protein levels. Specific siRNA-mediated silencing of P2Y2 receptors almost completely abolished ATP/UTP-induced Ca(2+) transients and significantly reduced ATP/UTP-induced decrease of NCC expression. To test whether local variations in the intracellular Ca(2+) concentration ([Ca(2+)]i) may control NCC transcription, we overexpressed the Ca(2+)-binding protein parvalbumin selectively in the cytosol or in the nucleus of mDCT cells. The decrease in NCC mRNA upon nucleotide stimulation was abolished in cells overexpressing cytosolic PV but not in cells overexpressing either a nuclear-targeted PV or a mutated PV unable to bind Ca(2+). Using a firefly luciferase reporter gene strategy, we observed that the activity of NCC promoter region from -1 to -2,200 bp was not regulated by changes in [Ca(2+)]i. In contrast, high cytosolic calcium level induced instability of NCC mRNA. We conclude that in mDCT cells: (1) P2Y2 receptor is essential for the intracellular Ca(2+) signaling induced by ATP/UTP stimulation; (2) P2Y2-mediated increase of cytoplasmic Ca(2+) concentration down-regulates the expression of NCC; (3) the decrease of NCC expression occurs, at least in part, via destabilization of its mRNA.

SIRT1 gene expression upon genotoxic damage is regulated by APE1 through nCaRE-promoter elements

APE1 is recruited to the transcription initiation site of the SIRT1 promoter during early cell response to oxidative stress. This reveals the importance of BER enzyme involvement in controlling specific gene expression at the transcriptional level.

Apurinic/apyrimidinic endonuclease 1 (APE1) is a multifunctional protein contributing to genome stability via repair of DNA lesions via the base excision repair pathway. It also plays a role in gene expression regulation and RNA metabolism. Another, poorly characterized function is its ability to bind to negative calcium responsive elements (nCaRE) of some gene promoters. The presence of many functional nCaRE sequences regulating gene transcription can be envisioned, given their conservation within ALU repeats. To look for functional nCaRE sequences within the human genome, we performed bioinformatic analyses and identified 57 genes potentially regulated by APE1. We focused on sirtuin-1 (SIRT1) deacetylase due to its involvement in cell stress, including senescence, apoptosis, and tumorigenesis, and its role in the deacetylation of APE1 after genotoxic stress. The human SIRT1 promoter presents two nCaRE elements stably bound by APE1 through its N-terminus. We demonstrate that APE1 is part of a multiprotein complex including hOGG1, Ku70, and RNA Pol II, which is recruited on SIRT1 promoter to regulate SIRT1 gene functions during early response to oxidative stress. These findings provide new insights into the role of nCaRE sequences in the transcriptional regulation of mammalian genes.

Dual regulatory roles of human AP-endonuclease (APE1/Ref-1) in CDKN1A/p21 expression

The human AP-endonuclease (APE1/Ref-1), an essential multifunctional protein involved in repair of oxidative DNA damage as well as in transcriptional regulation, is often overexpressed in tumor cells. APE1 was earlier shown to stimulate p53's DNA binding and its transactivation function in the expression of cyclin-dependent kinase inhibitor p21 (CDKN1A) gene. Here, we show APE1's stable binding to p53 cis elements which are required for p53-mediated activation of p21 in p53-expressing wild type HCT116 cells. However, surprisingly, we observed APE1-dependent repression of p21 in isogenic p53-null HCT116 cells. Ectopic expression of p53 in the p53-null cells abrogated this repression suggesting that APE1's negative regulatory role in p21 expression is dependent on the p53 status. We then identified APE1's another binding site in p21's proximal promoter region containing cis elements for AP4, a repressor of p21. Interestingly, APE1 and AP4 showed mutual dependence for p21 repression. Moreover, ectopic p53 in p53-null cells inhibited AP4's association with APE1, their binding to the promoter and p21 repression. These results together establish APE1's role as a co-activator or co-repressor of p21 gene, dependent on p53 status. It is thus likely that APE1 overexpression and inactivation of p53, often observed in tumor cells, promote tumor cell proliferation by constitutively downregulating p21 expression.

The changes of calretinin immunoreactivity in paraquat-induced nephrotoxic rats

Calcium-binding proteins are present in the kidneys: calbindin D-28k in the distal tubules and calretinin in the proximal tubules. Since paraquat causes degeneration in the brush border-bearing proximal tubule cells in rat kidneys, we investigated the changes of calretinin immunoreactivity in the proximal tubule cells of paraquat-induced nephrotoxicity in experimental male Sprague-Dawley rats following chitosan oligosaccharide pretreatment to investigate its protective properties. Paraquat (60 mg/kg) was administered intraperitoneally with or without chitosan oligosaccharide (500 mg/kg, p.o.) pretreatment. The changes on calretinin were compared with those of calbindin D-28k by immunohistochemistry and Western Blot analysis. Calretinin was immunolocalized on the apical surface of proximal tubule cells in the deeper cortex of normal kidney, and disappeared after paraquat administration with minor changes of calbindin D-28k immunoreactivity in the distal tubules and collecting ducts. Chitosan oligosaccharide pretreatment caused increased expression of calretinin and calbindin D-28k before paraquat injection and helped preserve proximal tubules after paraquat treatment. However, Western blot analysis on calretinin and calbindin D-28k could not explain the degeneration of the proximal tubule cells in paraquat-induced nephrotoxicity. These findings suggested that calretinin is a possible and more useful histopathological marker for proximal tubule cells in paraquat-induced nephrotoxic rats.

The role of apurinic/apyrimidinic endonuclease on the progression of streptozotocin-induced diabetic nephropathy in rats

Apurinic/apyrimidinic endonuclease (APE) acts as a regulator of p53 or vice versa in the cellular response to oxidative stress. Since oxidative stress-induced apoptosis is suggested in the pathophysiology of diabetic nephropathy, we proposed that APE may have a feasible role in the progression of diabetic complications. We investigated the interrelationship between APE and p53 in streptozotocin-induced diabetic rat kidneys. Variable parameters on kidneys were checked 12 weeks after streptozotocin administration with or without chitosan oligosaccharide (COS) treatment. Streptozotocin administration caused changes as seen in early diabetic nephropathy with increased kidney size, increased p53, decreased APE, and increased cleaved caspase-3. COS was not suspected as being detrimental to renal measurements, and caused the augmentation of APE after streptozotocin administration. The augmented APE, in association with increased p53, suppressed cleaved caspase-3. 8-OHdG was mainly immunolocalized in the distal tubules, but also in the proximal tubules after streptozotocin administration without COS treatment, while APE was observed in proximal tubules in all groups. These results suggested that p53-dependent apoptosis resulting in suppressed APE might be an underlying mechanism of streptozotocin-induced nephropathy.

Regulation of mouse-renin gene by apurinic/apyrimidinic-endonuclease 1 (APE1/Ref-1) via recruitment of histone deacetylase 1 corepressor complex

OBJECTIVES

Apurinic/apyrimidinic-endonuclease 1 (APE1) heterozygous mice have chronically elevated blood pressure. Renin of the renin-angiotensin (ANG) system for blood pressure maintenance regulates production of ANG II, a vasoactive hormone. Renin expression and secretion from kidney juxtaglomerular cells are regulated by intracellular calcium. Our objective in this study is to investigate APE1's regulatory role in renin expression.

METHODS

Effect of APE1 on calcium-mediated modulation of renin expression was examined by real-time reverse transcriptase-PCR, Western analysis and renin promoter-dependent luciferase activity in APE1-knockdown, APE1-overexpressing or control mouse kidney As4.1 cells. Furthermore, coimmunoprecipitation and chromatin immunoprecipitation assays were utilized to examine the association of APE1 with histone deacetylase (HDAC)1 corepressor complex and their recruitment to renin enhancer. Finally, kidney renin mRNA level and plasma-renin activity were measured in wild-type and APE1-heterozygous mice.

RESULTS

Here we show that APE1 is involved in calcium-mediated repression of renin gene. Our results further indicate that APE1 is a component of HDAC1 corepressor complex bound to renin-enhancer region. Increase in intracellular calcium ion concentration enhances the association of APE1 with HDAC1 corepressor complex and their recruitment to the enhancer region. Furthermore, APE1's N-terminal region is critical for formation and recruitment of the enhancer-bound corepressor complex. Increased renin expression in kidneys and higher plasma-renin activity in APE1 heterozygous mice further supports APE1's corepressor role in vivo.

CONCLUSION

This study uncovers APE1's function as a novel negative regulator of renin expression, and thereby in blood pressure maintenance.

A crossover comparison of urinary albumin excretion as a new surrogate marker for cardiovascular disease among 4 types of calcium channel blockers

BACKGROUND

At the intervention for cardiovascular disease (CVD), albuminuria is a new pivotal target. Calcium channel blocker (CCB) is one of the most expected agents. Currently CCBs have been classified by delivery system, half-life and channel types. We tested anti-albuminuric effect among 4 types of CCBs.

METHODS

Subjects were 50 hypertensives (SBP/DBP 164.7±17.1/92.3±12.2mmHg, s-Cr 0.81±0.37mg/dl, urinary albumin excretion (UAE) 69.4 (33.5-142.6) mg/gCr). Four CCBs were administered in a crossover setting: nifedipine CR, a long biological half-life L type by controlled release; cilnidipine, an N/L type; efonidipine, a T/L type; and amlodipine, a long biological half-life L type.

RESULTS

Comparable BP reductions were obtained. UAE at endpoints ware as follows (mg/gCr, *P<0.01): nifedipine CR 30.8 (17.3-81.1),* cilnidipine 33.9 (18.0-67.7),* efonidipine 51.0 (21.2-129.8), amlodipine 40.6 (18.7-94.7). By all agents, significant augmentations were observed in PRA, angiotensin I and angiotensin II (AngII). AngII at cilnidipine was significantly lower than that at amlodipine. PAC at cilnidipine and efonidipine was significantly lower than that at amlodipine. Nifedipine CR significantly reduced ANP concentration.

CONCLUSIONS

It is revealed that only nifedipine CR and cilnidipine could reduce albuminuria statistically. Thus, it is suggested that the 2 CCBs might be favorable for organ protection in hypertensives.

Ubiquitination of human AP-endonuclease 1 (APE1) enhanced by T233E substitution and by CDK5

Apurinic/apyrimidinic endonuclease-1 (APE1) is a multifunctional DNA repair/gene regulatory protein in mammalian cells, and was recently reported to be phosphorylated at Thr233 by CDK5. We here report that ubiquitination of T233E APE1, a mimicry of phospho-T233 APE1, was markedly increased in multiple cell lines. Expression of CDK5 enhanced monoubiquitination of endogenous APE1. Polyubiquitinated APE1 was decreased when K48R ubiquitin was expressed, suggesting that polyubiquitination was mediated mainly through Lys48 of ubiquitin. The ubiquitination activity of MDM2, consistent in its role for APE1 ubiquitination, was increased for T233E APE1 compared to the wild-type APE1. In mouse embryonic fibroblasts lacking the MDM2 gene, ubiquitination of T233E APE1 was still observed probably because of the decreased degradation activity for monoubiquitinated APE1 and because of backup E3 ligases in the cells. Monoubiquitinated APE1 was present in the nucleus, and analyzing global gene expression profiles with or without induction of a ubiquitin-APE1 fusion gene suggested that monoubiquitination enhanced the gene suppression activity of APE1. These data reveal a delicate balance of ubiquitination and phosphorylation activities that alter the gene regulatory function of APE1.

A genetic variation in APE1 is associated with gastric cancer survival in a Chinese population

Altered DNA repair can be associated with aggressive tumor biology and impact on survival of cancer patients. We investigated whether genetic variation of human apurinic/apyrimidinic (AP) endonuclease, a key multifunctional gene involved in the base excision repair pathway, would play a role in gastric cancer survival outcomes. We genotyped APE1 rs1760944 by the TaqMan method in 925 gastric cancer patients. Analyses of association between the polymorphism and survival outcomes were carried out using the Kaplan-Meier method, Cox proportional hazards models, and the log-rank test. Survival analyses for all patients showed that the differences in median survival time between gastric cancer carriers with APE1 rs1760944 TT (55 months) and those with GT/GG (78 months), were statistically significant (P = 0.025, log-rank test). Kaplan-Meier survival estimates revealed that gastric cancer patients carrying the GT/GG genotypes had a higher survival than TT, and this protective effect was also more pronounced among subgroups with tumor size >5 cm (hazard ratio = 0.66, 95% confidence interval = 0.49-0.88), diffuse-type gastric cancer (0.76, 0.60-0.97), T3 depth of invasion (0.73, 0.57-0.93), lymph node metastasis (0.73, 0.58-0.92), no distant metastasis (0.81, 0.66-0.99), and TNM stage III and IV (0.75, 0.58-0.99 for stage III; 0.50, 0.29-0.88 for stage IV). Our results showed that the genetic variant rs1760944 in APE1 was associated with gastric cancer survival in a Chinese population. Larger studies are needed to verify our findings in different populations.

Human AP endonuclease (APE1/Ref-1) and its acetylation regulate YB-1-p300 recruitment and RNA polymerase II loading in the drug-induced activation of multidrug resistance gene MDR1

The overexpression of human apurinic/apyrimidinic (AP) endonuclease 1 (APE1/Ref-1), a key enzyme in the DNA base excision repair (BER) pathway, is often associated with tumor cell resistance to various anticancer drugs. In this study, we examined the molecular basis of transcriptional regulatory (nonrepair) function of APE1 in promoting resistance to certain types of drugs. We have recently shown that APE1 stably interacts with Y-box-binding protein 1 (YB-1), and acts as its coactivator for the expression of multidrug resistance gene MDR1, thereby causing drug resistance. In this study, we show, to the best of our knowledge, for the first time that APE1 is stably associated with the basic transcription factor RNA polymerase II (RNA pol II) and the coactivator p300 on the endogenous MDR1 promoter. The depletion of APE1 significantly reduces YB-1-p300 recruitment to the promoter, resulting in reduced RNA pol II loading. Drug-induced APE1 acetylation, which is mediated by p300, enhances formation of acetylated APE1 (AcAPE1)-YB-1-p300 complex on the MDR1 promoter. Enhanced recruitment of this complex increases MDR1 promoter-dependent luciferase activity and its endogenous expression. Using APE1-downregulated cells and cells overexpressing wild-type APE1 or its nonacetylable mutant, we have demonstrated that the loss of APE1's acetylation impaired MDR1 activation and sensitizes the cells to cisplatin or etoposide. We have thus established the basis for APE1's acetylation-dependent regulatory function in inducing MDR1-mediated drug resistance.

A novel regulatory circuit in base excision repair involving AP endonuclease 1, Creb1 and DNA polymerase beta

DNA repair is required to maintain genome stability in stem cells and early embryos. At critical junctures, oxidative damage to DNA requires the base excision repair (BER) pathway. Since early zebrafish embryos lack the major polymerase in BER, DNA polymerase ß, repair proceeds via replicative polymerases, even though there is ample polb mRNA. Here, we report that Polb protein fails to appear at the appropriate time in development when AP endonuclease 1 (Apex), the upstream protein in BER, is knocked down. Because polb contains a Creb1 binding site, we examined whether knockdown of Apex affects creb1. Apex knockdown results in loss of Creb1 and Creb complex members but not Creb1 phosphorylation. This effect is independent of p53. Although both apex and creb1 mRNA rescue Creb1 and Polb after Apex knockdown, Apex is not a co-activator of creb1 transcription. This observation has broad significance, as similar results occur when Apex is inhibited in B cells from apex^+/− mice. These results describe a novel regulatory circuit involving Apex, Creb1 and Polb and provide a mechanism for lethality of Apex loss in higher eukaryotes.

Understanding different functions of mammalian AP endonuclease (APE1) as a promising tool for cancer treatment

The apurinic endonuclease 1/redox factor-1 (APE1) has a crucial function in DNA repair and in redox signaling in mammals, and recent studies identify it as an excellent target for sensitizing tumor cells to chemotherapy. APE1 is an essential enzyme in the base excision repair pathway of DNA lesions caused by oxidation and alkylation. As importantly, APE1 also functions as a redox agent maintaining transcription factors involved in cancer promotion and progression in an active reduced state. Very recently, a new unsuspected function of APE1 in RNA metabolism was discovered, opening new perspectives for this multifunctional protein. These observations underline the necessity to understand the molecular mechanisms responsible for fine-tuning its different biological functions. This survey intends to give an overview of the multifunctional roles of APE1 and their regulation in the context of considering this protein a promising tool for anticancer therapy.

Posttranslational modification of mammalian AP endonuclease (APE1)

A key issue in studying mammalian DNA base excision repair is how its component proteins respond to a plethora of cell-signaling mediators invoked by DNA damage and stress-inducing agents such as reactive oxygen species, and how the actions of individual BER proteins are attributed to cell survival or apoptotic/necrotic death. This article reviews the past and recent progress on posttranslational modification (PTM) of mammalian apurinic/apyrimidinic (AP) endonuclease 1 (APE1).

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Human apurinic/apyrimidinic endonuclease (APE1) is a prognostic factor in ovarian, gastro-oesophageal and pancreatico-biliary cancers

BACKGROUND

Altered DNA repair may be associated with aggressive tumour biology and impact upon response to chemotherapy and radiotherapy. We investigated whether expression of human AP endonuclease (APE1), a key multifunctional protein involved in DNA BER, would impact on clinicopathological outcomes in ovarian, gastro-oesophageal, and pancreatico-biliary cancer.

METHODS

Formalin-fixed human ovarian, gastro-oesophageal, and pancreatico-biliary cancers were constructed into TMAs. Expression of APE1 was analysed by IHC and correlated to clinicopathological variables.

RESULTS

In ovarian cancer, nuclear APE1 expression was seen in 71.9% (97 out of 135) of tumours and correlated with tumour type (P=0.006), optimal debulking (P=0.009), and overall survival (P=0.05). In gastro-oesophageal cancers previously exposed to neoadjuvant chemotherapy, 34.8% (16 out of 46) of tumours were positive in the nucleus and this correlated with shorter overall survival (P=0.005), whereas cytoplasmic localisation correlated with tumour dedifferentiation (P=0.034). In pancreatico-biliary cancer, nuclear staining was seen in 44% (32 out of 72) of tumours. Absence of cytoplasmic staining was associated with perineural invasion (P=0.007), vascular invasion (P=0.05), and poorly differentiated tumours (P=0.068). A trend was noticed with advanced stage (P=0.077).

CONCLUSIONS

Positive clinicopathological correlations of APE1 expression suggest that APE1 is a potential drug target in ovarian, gastro-oesophageal, and pancreatico-biliary cancers.

Novel mechanisms for the control of renin synthesis and release

Renin is the key regulated step in the enzymatic cascade that leads to angiotensin generation and the control of blood pressure and fluid/electrolyte homeostasis. In the adult unstressed animal, renin is synthesized and released by renal juxtaglomerular cells. However, when homeostasis is threatened, the number of cells that express and release renin increases and extends beyond the juxtaglomerular area; the result is an increase in circulating renin and the reestablishment of homeostasis. The increase in the number of renin cells, a process termed recruitment, is achieved by dedifferentiation and re-expression of renin in cells derived from the renin lineage. The mechanisms that regulate the related processes of reacquisition of the renin phenotype, renin synthesis, and renin release are beginning to be understood. Numerous studies point to cAMP as a central common factor for the regulation of renin phenotype. In addition, we are seeing the emergence of gap junctions and microRNAs as new and promising avenues for a more complete understanding of the complex regulation of the renin cell.

Human AP endonuclease 1 (APE1): from mechanistic insights to druggable target in cancer

DNA base excision repair (BER) is critically involved in the processing of DNA base damage induced by alkylating agents. Pharmacological inhibition of BER (using PARP inhibitors), either alone or in combination with chemotherapy has recently shown promise in clinical trials. Human apurinic/apyrimidinic endonuclease 1(APE1) is an essential BER protein that is involved in the processing of potentially cytotoxic abasic sites that are obligatory intermediates in BER. Here we provide a summary of the basic mechanistic role of APE1 in DNA repair and redox regulation and highlight preclinical and clinical data that confirm APE1 as a valid anticancer drug target. Development of small molecule inhibitors of APE1 is an area of intense research and current evidence using APE1 inhibitors has demonstrated potentiation of cytotoxicity of alkylating agents in preclinical models implying translational applications in cancer patients.

The role of calcium in the regulation of renin secretion

Renin is the enzyme which is the rate-limiting step in the formation of the hormone angiotensin II. Therefore, the regulation of renin secretion is critical in understanding the control of the renin-angiotensin-aldosterone system and its many biological and pathological actions. Renin is synthesized, stored in, and released from the juxtaglomerular (JG) cells of the kidney. While renin secretion is positively regulated by the "second messenger" cAMP, unlike most secretory cells, renin secretion from the JG cell is inversely related to the extracellular and intracellular calcium concentrations. This novel relationship is referred to as the "calcium paradox." This review will address observations made over the past 30 years regarding calcium and the regulation of renin secretion, and focus on recent observations which address this scientific conundrum. These include 1) receptor-mediated pathways for changing intracellular calcium; 2) the discovery of a calcium-inhibitable isoform of adenylyl cyclase associated with renin in the JG cells; 3) calcium-sensing receptors in the JG cells; 4) calcium-calmodulin-mediated signals; 5) the role of phosphodiesterases; and 6) connexins, gap junctions, calcium waves, and the cortical extracellular calcium environment. While cAMP is the dominant second messenger for renin secretion, calcium appears to modulate the integrated activities of the enzymes, which balance cAMP synthesis and degradation. Thus this review concludes that calcium modifies the amplitude of cAMP-mediated renin-signaling pathways. While calcium does not directly control renin secretion, increased calcium inhibits and decreased calcium amplifies cAMP-stimulated renin secretion.

SIRT1 deacetylates APE1 and regulates cellular base excision repair

Apurinic/apyrimidinic endonuclease-1 (APE1) is an essential enzyme in the base excision repair (BER) pathway. Here, we show that APE1 is a target of the SIRTUIN1 (SIRT1) protein deacetylase. SIRT1 associates with APE1, and this association is increased with genotoxic stress. SIRT1 deacetylates APE1 in vitro and in vivo targeting lysines 6 and 7. Genotoxic insults stimulate lysine acetylation of APE1 which is antagonized by transcriptional upregulation of SIRT1. Knockdown of SIRT1 increases cellular abasic DNA content, sensitizing cells to death induced by genotoxic stress, and this vulnerability is rescued by overexpression of APE1. Activation of SIRT1 with resveratrol promotes binding of APE1 to the BER protein X-ray cross-complementing-1 (XRCC1), while inhibition of SIRT1 with nicotinamide (NAM) decreases this interaction. Genotoxic insult also increases binding of APE1 to XRCC1, and this increase is suppressed by NAM or knockdown of SIRT1. Finally, resveratrol increases APE activity in XRCC1-associated protein complexes, while NAM or knockdown of SIRT1 suppresses this DNA repair activity. These findings identify APE1 as a novel protein target of SIRT1, and suggest that SIRT1 plays a vital role in maintaining genomic integrity through regulation of the BER pathway.

Acetylation of apurinic/apyrimidinic endonuclease-1 regulates Helicobacter pylori-mediated gastric epithelial cell apoptosis

BACKGROUND & AIMS

Helicobacter pylori-induced gastric epithelial cell (GEC) apoptosis is a complex process that includes activation of the tumor suppressor p53. p53-mediated apoptosis involves p53 activation, bax transcription, and cytochrome c release from mitochondria. Apurinic/apyrimidinic endonuclease-1 (APE-1) regulates transcriptional activity of p53, and H pylori induce APE-1 expression in human GECs. H pylori infection increases intracellular calcium ion concentration [Ca2+]i of GECs, which induces APE-1 acetylation. We investigated the effects of H pylori infection and APE-1 acetylation on GEC apoptosis.

METHODS

AGS cells (wild-type or with suppressed APE-1), KATO III cells, and cells isolated from gastric biopsy specimens were infected with H pylori. Effects were examined by immunoblotting, real-time reverse-transcription polymerase chain reaction, immunoprecipitation, immunofluorescence microscopy, chromatin immunoprecipitation, mobility shift, DNA binding, and luciferase assays.

RESULTS

H pylori infection increased [Ca2+]i and acetylation of APE-1 in GECs, but the acetylation status of APE-1 did not affect the transcriptional activity of p53. In GECs, expression of a form of APE-1 that could not be acetylated increased total and mitochondrial levels of Bax and induced release of cytochrome c and fragmentation of DNA; expression of wild-type APE-1 reduced these apoptotic events. We identified a negative calcium response element in the human bax promoter and found that poly (adenosine diphosphate-ribose) polymerase 1 recruited the acetylated APE-1/histone deacetylase-1 repressor complex to bax nCaRE.

CONCLUSIONS

H pylori-mediated acetylation of APE-1 suppresses Bax expression; this prevents p53-mediated apoptosis when H pylori infect GECs.

Ubiquitination of mammalian AP endonuclease (APE1) regulated by the p53-MDM2 signaling pathway

APE1/Ref-1 is an essential DNA repair/gene regulatory protein in mammals of which intracellular level significantly affects cellular sensitivity to genotoxicants. The apurinic/apyrimidinic endonuclease 1 (APE1) functions are altered by phosphorylation and acetylation. We here report that APE1 is also modified by ubiquitination. APE1 ubiquitination occurred specifically at Lys residues near the N-terminus, and was markedly enhanced by mouse double minute 2 (MDM2), the major intracellular p53 inhibitor. Moreover, DNA-damaging reagents and nutlin-3, an inhibitor of MDM2-p53 interaction, increased APE1 ubiquitination in the presence of p53. Downmodulation of MDM2 increased APE1 level, suggesting that MDM2-mediated ubiquitination can be a signal for APE1 degradation. In addition, unlike the wild-type APE1, ubiquitin-APE1 fusion proteins were predominantly present in the cytoplasm. Therefore, monoubiquitination not only is a prerequisite for degradation, but may also alter the APE1 activities in cells. These results reveal a novel regulation of APE1 through ubiquitination.

Transcriptional regulatory functions of mammalian AP-endonuclease (APE1/Ref-1), an essential multifunctional protein

The mammalian AP-endonuclease (APE1/Ref-1) plays a central role in the repair of oxidized and alkylated bases in mammalian genomes via the base excision repair (BER) pathway. However, APE1, unlike its E. coli prototype Xth, has two unique and apparently distinct transcriptional regulatory activities. APE1 functions as a redox effector factor (Ref-1) for several transcription factors including AP-1, HIF1-alpha, and p53. APE1 was also identified as a direct trans-acting factor for repressing human parathyroid hormone (PTH) and renin genes by binding to the negative calcium-response element (nCaRE) in their promoters. We have characterized APE1's post-translational modification, namely, acetylation which modulates its transcriptional regulatory function. Furthermore, stable interaction of APE1 with several other trans-acting factors including HIF-1alpha, STAT3, YB-1, HDAC1, and CBP/p300 and formation of distinct trans-acting complexes support APE1's direct regulatory function for diverse genes. Multiple functions of mammalian APE1, both in DNA repair and gene regulation, warrant extensive analysis of its own regulation and dissection of the mechanisms. In this review, we have discussed APE1's own regulation and its role as a transcriptional coactivator or corepressor by both redox-dependent and redox-independent (acetylation-mediated) mechanisms, and explore the potential utility of targeting these functions for enhancing drug sensitivity of cancer cells.

Ape1/Ref-1 induces glial cell-derived neurotropic factor (GDNF) responsiveness by upregulating GDNF receptor alpha1 expression

Apurinic/apyrimidinic endonuclease 1 (Ape1/Ref-1) dysregulation has been identified in several human tumors and in patients with a variety of neurodegenerative diseases. However, the function of Ape1/Ref-1 is unclear. We show here that Ape1/Ref-1 increases the expression of glial cell-derived neurotropic factor (GDNF) receptor alpha1 (GFRalpha1), a key receptor for GDNF. Expression of Ape1/Ref-1 led to an increase in the GDNF responsiveness in human fibroblast. Ape1/Ref-1 induced GFRalpha1 transcription through enhanced binding of NF-kappaB complexes to the GFRalpha1 promoter. GFRalpha1 levels correlate proportionally with Ape1/Ref-1 in cancer cells. The knockdown of endogenous Ape1/Ref-1 in pancreatic cancer cells markedly suppressed GFRalpha1 expression and invasion in response to GNDF, while overexpression of GFRalpha1 restored invasion. In neuronal cells, the Ape1/Ref-1-mediated increase in GDNF responsiveness not only stimulated neurite outgrowth but also protected the cells from beta-amyloid peptide and oxidative stress. Our results show that Ape1/Ref-1 is a novel physiological regulator of GDNF responsiveness, and they also suggest that Ape1/Ref-1-induced GFRalpha1 expression may play important roles in pancreatic cancer progression and neuronal cell survival.

Effect of calcium ionophore A23187 on prostaglandin synthase type 2 and 15-hydroxy-prostaglandin dehydrogenase expression in human chorion trophoblast cells

OBJECTIVE

Prostaglandins induce parturition in humans. Prostaglandin output is regulated by the synthetic and metabolic enzymes, prostaglandin synthase type 2 (PTGS2) and 15-hydroxyprostaglandin dehydrogenase (PGDH). The role of calcium in regulating PTGS2 and PGDH expression was investigated in chorion trophoblasts.

STUDY DESIGN

Cells were treated with calcium ionophore A23187 in the presence or absence of calcium chelators; changes in messenger ribonucleic acid expression were measured with real-time polymerase chain reaction and analyzed with analysis of variance. Protein expression was evaluated with Western blot and dual immunofluorescence.

RESULTS

A23187 stimulated PTGS2 and suppressed PGDH expression. Effects of A23187 were reversed by calcium chelators. PTGS2 had perinuclear and cytosolic distribution, whereas PGDH was cytosolic. Some cells expressed both enzymes, some neither enzyme, and some either PTGS2 or PGDH.

CONCLUSION

Chorion cells showed heterogeneity in the expression of PTGS2 and PGDH. Calcium influx regulates PTGS2 and PGDH expression, thereby promoting coordinated increased prostaglandin output in circumstances such as term and preterm labor.

Regulatory role of human AP-endonuclease (APE1/Ref-1) in YB-1-mediated activation of the multidrug resistance gene MDR1

Human AP-endonuclease (APE1/Ref-1), a central enzyme involved in the repair of oxidative base damage and DNA strand breaks, has a second activity as a transcriptional regulator that binds to several trans-acting factors. APE1 overexpression is often observed in tumor cells and confers resistance to various anticancer drugs; its downregulation sensitizes tumor cells to such agents. Because the involvement of APE1 in repairing the DNA damage induced by many of these drugs is unlikely, drug resistance may be linked to APE1's transcriptional regulatory function. Here, we show that APE1, preferably in the acetylated form, stably interacts with Y-box-binding protein 1 (YB-1) and enhances its binding to the Y-box element, leading to the activation of the multidrug resistance gene MDR1. The enhanced MDR1 level due to the ectopic expression of wild-type APE1 but not of its nonacetylable mutant underscores the importance of APE1's acetylation in its coactivator function. APE1 downregulation sensitizes MDR1-overexpressing tumor cells to cisplatin or doxorubicin, showing APE1's critical role in YB-1-mediated gene expression and, thus, drug resistance in tumor cells. A systematic increase in both APE1 and MDR1 expression was observed in non-small-cell lung cancer tissue samples. Thus, our study has established the novel role of the acetylation-mediated transcriptional regulatory function of APE1, making it a potential target for the drug sensitization of tumor cells.

Unusual role of a cysteine residue in substrate binding and activity of human AP-endonuclease 1

The mammalian AP-endonuclease (APE1) repairs apurinic/apyrimidinic (AP) sites and strand breaks with 3' blocks in the genome that are formed both endogenously and as intermediates during base excision repair. APE1 has an unrelated activity as a redox activator (and named Ref-1) for several trans-acting factors. In order to identify whether any of the seven cysteine residues in human APE1 affects its enzymatic function, we substituted these singly or multiply with serine. The repair activity is not affected in any of the mutants except those with C99S mutation. The Ser99-containing mutant lost affinity for DNA and its activity was inhibited by 10 mM Mg(2+). However, the Ser99 mutant has normal activity in 2 mM Mg(2+). Using crystallographic data and molecular dynamics simulation, we have provided a mechanistic basis for the altered properties of the C99S mutant. We earlier predicted that Mg(2+), with potential binding sites A and B, binds at the B site of wild-type APE1-substrate complex and moves to the A site after cleavage occurs, as observed in the crystal structure. The APE1-substrate complex is stabilized by a H bond between His309 and the AP site. We now show that this bond is broken to destabilize the complex in the absence of the Mg(2+). This effect due to the mutation of Cys99, approximately 16 A from the active site, on the DNA binding and activity is surprising. Mg(2+) at the B site promotes stabilization of the C99S mutant complex. At higher Mg(2+) concentration the A site is also filled, causing the B-site Mg(2+) to shift together with the AP site. At the same time, the H bond between His309 and the AP site shifts toward the 5' site of DNA. These shifts could explain the lower activity of the C99S mutant at higher [Mg(2+)]. The unexpected involvement of Cys99 in APE1's substrate binding and catalysis provides an example of involvement of a residue far from the active site.

A proximal direct repeat motif characterized as a negative regulatory element in the human renin gene

The regulation of renin gene expression is thought to be fundamental to regulation of the total renin-angiotensin system. The human renin gene contains a direct repeat (DR) motif AGGGGTCAC-AGGGCCA in the proximal region (-259/-245 bp), which contains similar sequence for nuclear receptor superfamily binding core motif, AGGTCA, and is the most similar to COUP-TFII consensus. The DR motif was evaluated as a functional cis-element with renal cortex and chorio-decidual cells by footprint assay, electromobility shift assay (EMSA) and reporter assay. The DR motif site was protected by footprint analysis with a clear hypersensitive and a minor hypersensitive region in good accordance with the DR of the consensus. One of the binding proteins was strongly suspected to be COUP-TFII-consensus-specific by EMSA. The DNA/protein complexes obtained with nuclear extract of renin producing cells could be completely blocked by homologous competitor and strongly blocked by the second-half mutant oligonucleotide of the DR motif but not by the first-half mutant oligonucleotide. Finally, the transcriptional activity of second-half mutant construct is slightly elevated and that first-half mutant construct is significantly stronger by twofold compared with wild type construct in reporter assay. These findings suggest that the DR motif site of the human renin gene functions as a negative regulatory element involved in a twofold repression of transcription and that member(s) of nucleic receptor superfamily bind the site and play important roles in the human renin gene expression with a possibility that one of the binding protein is COUP-TFII.

Regulation of the human AP-endonuclease (APE1/Ref-1) expression by the tumor suppressor p53 in response to DNA damage

The human AP-endonuclease (APE1/Ref-1), an essential multifunctional protein, plays a central role in the repair of oxidative base damage via the DNA base excision repair (BER) pathway. The mammalian AP-endonuclease (APE1) overexpression is often observed in tumor cells, and confers resistance to various anticancer drugs; its downregulation sensitizes tumor cells to those agents via induction of apoptosis. Here we show that wild type (WT) but not mutant p53 negatively regulates APE1 expression. Time-dependent decrease was observed in APE1 mRNA and protein levels in the human colorectal cancer line HCT116 p53^(+/+), but not in the isogenic p53 null mutant after treatment with camptothecin, a DNA topoisomerase I inhibitor. Furthermore, ectopic expression of WTp53 in the p53 null cells significantly reduced both endogenous APE1 and APE1 promoter-dependent luciferase expression in a dose-dependent fashion. Chromatin immunoprecipitation assays revealed that endogenous p53 is bound to the APE1 promoter region that includes a Sp1 site. We show here that WTp53 interferes with Sp1 binding to the APE1 promoter, which provides a mechanism for the downregulation of APE1. Taken together, our results demonstrate that WTp53 is a negative regulator of APE1 expression, so that repression of APE1 by p53 could provide an additional pathway for p53-dependent induction of apoptosis in response to DNA damage.

Control of core 2 beta1,6 N-acetylglucosaminyltransferase-I transcription by Sp1 in lymphocytes and epithelial cells

Core 2 beta1,6 N-acetylglucosaminyltransferase-I (C2GnT-I) catalyzes the synthesis of one of the major core structures in GalNAc alpha-Ser/Thr O-linked oligosaccharides, the core 2 branch. The production of the core 2 branch is required for the synthesis of glycoforms that are important for the cellular functions of lymphocytes, mucin-producing epithelial cells and other cell types. Therefore, proper molecular control of C2GnT-I expression is very important for different types of cells. C2GnT-I is transcribed from 4 promoters, with promoter 2 being the major promoter. C2GnT-I promoter 2 lacks a TATA box and is very GC rich. In this study, the analysis of this promoter finds that the transcription factor Sp1 is essential for transcription of C2GnT-I in both mesodermally derived T-cells (Jurkat) and in endodermal mucin producing epithelial cells (NCI H292). In Jurkat cells, all nine of the Sp1 binding sites within the minimal promoter region contribute to transcription, and there is a linear relationship between the number of Sp1 sites and the transcriptional activity of the promoter. In NCI H292 cells, only three of these Sp1 binding sites are required for transcription from promoter 2. Chromatin immunoprecipitation confirms that Sp1 binds to promoter 2 in NCI H292 cells in vivo.

Identification and characterization of mitochondrial abasic (AP)-endonuclease in mammalian cells

Abasic (AP)-endonuclease (APE) is responsible for repair of AP sites, and single-strand DNA breaks with 3′ blocking groups that are generated either spontaneously or during repair of damaged or abnormal bases via the DNA base excision repair (BER) pathway in both nucleus and mitochondria. Mammalian cells express only one nuclear APE, 36 kDa APE1, which is essential for survival. Mammalian mitochondrial (mt) BER enzymes other than mtAPE have been characterized. In order to identify and characterize mtAPE, we purified the APE activity from beef liver mitochondria to near homogeneity, and showed that the mtAPE which has 3-fold higher specific activity relative to APE1 is derived from the latter with deletion of 33 N-terminal residues which contain the nuclear localization signal. The mtAPE-sized product could be generated by incubating ³⁵S-labeled APE1 with crude mitochondrial extract, but not with cytosolic or nuclear extract, suggesting that cleavage of APE1 by a specific mitochondria-associated N-terminal peptidase is a prerequisite for mitochondrial import. The low abundance of mtAPE, particularly in cultured cells might be the reason for its earlier lack of detection by western analysis.

Effect of aging on intracellular distribution of abasic (AP) endonuclease 1 in the mouse liver

The abasic (AP) endonuclease (APE1) plays a central role in the base excision repair (BER) pathway for repairing oxidatively damaged bases and abasic sites in mammalian genomes. We have investigated age-dependent changes in APE activity, contributed primarily by APE1, in total extracts as well as in nuclear, mitochondrial, and cytoplasmic compartments of mouse hepatocytes. The APE1 protein and mRNA levels did not differ significantly between the livers of 4-mo (young), 10-mo (middle-aged), and 20-mo (old) mice, and corresponds with similar APE activity. However, we observed a 2-fold increase in specific activity of APE1 in the nucleus, a 2-fold decrease in the cytoplasm, and a 6-fold increase in the mitochondrial matrix of hepatocytes of the old relative to the young animals. Surprisingly, in the middle-age animals we observed 30% increase in APE activity in the nucleus but 6-fold in the mitochondrial matrix. These results indicate age-dependent accumulation of APE1 in the nucleus and mitochondria. Such redistribution occurred early in the mitochondria during the aging process and preferential accumulation of APE in the nucleus was more gradual which may reflect distinct levels of oxidative stress in these organelles.

Redox-dependent transcriptional regulation

Reactive oxygen species contribute to the pathogenesis of a number of disparate disorders including tissue inflammation, heart failure, hypertension, and atherosclerosis. In response to oxidative stress, cells activate expression of a number of genes, including those required for the detoxification of reactive molecules as well as for the repair and maintenance of cellular homeostasis. In many cases, these induced genes are regulated by transcription factors whose structure, subcellular localization, or affinity for DNA is directly or indirectly regulated by the level of oxidative stress. This review summarizes the recent progress on how cellular redox status can regulate transcription-factor activity and the implications of this regulation for cardiovascular disease.

Calcium inhibits renin gene expression by transcriptional and posttranscriptional mechanisms

The aim of this study was to investigate the role of cytosolic calcium for renin gene expression in juxtaglomerular cells. For this purpose, we used the immortalized juxtaglomerular mouse cell line As4.1. To increase cytosolic calcium concentration, we treated the cells with thapsigargin and cyclopiazonic acid, inhibitors of the endoplasmatic reticulum Ca- ATPase. Thapsigargin and cyclopiazonic acid inhibited renin gene expression in a characteristic time and concentration-dependent manner. This effect was concentration-dependently blocked by BAPTA-AM, an intracellular Ca2+ chelator. Pharmacological blocking of protein kinase C activity by calphostin, Gö6976, and Gö6983 did not change the effect of thapsigargin on renin gene expression. Experiments with renin1C-promoter-reporter constructs revealed that thapsigargin inhibited renin gene transcription. Analysis of deletion constructs of the renin1C promoter indicated that regulatory elements involved in the calcium-mediated inhibition of renin gene transcription are located in the enhancer region of the renin gene and that > or =3 transcription factor-binding sites are involved in this process. In addition, thapsigargin reduced the renin mRNA half-life from 10 hours (control conditions) to 4 hours. Knockdown studies with small interfering RNA directed to dynamin-1 mRNA revealed that dynamin-1 is likely to be involved in the calcium-mediated destabilization of renin mRNA. These data suggest that calcium inhibits renin gene expression in juxtaglomerular cells via a concerted action of inhibition of renin gene transcription and destabilization of renin mRNA.

Human AP endonuclease suppresses DNA mismatch repair activity leading to microsatellite instability

The multifunctional mammalian apurinic/apyrimidinic (AP) endonuclease (APE) participates in the repair of AP sites in the cellular DNA as well as participating in the redox regulation of the transcription factor function. The function of APE is considered as the rate-limiting step in DNA base excision repair. Paradoxically, an unbalanced increase in APE protein leads to genetic instability. Therefore, we investigated the mechanisms of genetic instability that are induced by APE. Here, we report that the overexpression of APE protein disrupts the repair of DNA mismatches, which results in microsatellite instability (MSI). We found that expression of APE protein led to the suppression of the repair of DNA mismatches in the normal human fibroblast cells. Western blot analysis revealed that hMSH6 protein was markedly reduced in the APE-expressing cells. Moreover, the addition of purified Mutalpha (MSH2 and MSH6 complex) to the extracts from the APE-expressing cells led to the restoration of mismatch repair (MMR) activity. By performing MMR activity assay and MSI analysis, we found that the co-expression of hMSH6 and APE exhibited the microsatellite stability, whereas the expression of APE alone generated the MSI-high phenotype. The APE-mediated decrease in MMR activity described here demonstrates the presence of a new and highly effective APE-mediated mechanism for MSI.

Role of CREB1 and NFκB-p65 in the Down-regulation of Renin Gene Expression by Tumor Necrosis Factor α

Tumor necrosis factor-alpha (TNFalpha) is a potent inhibitor of renin gene expression in renal juxtaglomerular cells. We have found that TNFalpha suppresses renin transcription via transcription factor NFkappaB, which targets a cAMP responsive element (CRE) in the renin promoter. Here we aimed to further clarify the role of NFkappaB and the canonical CRE-binding proteins of the CRE-binding protein/activating transcription factor (CREB/ATF) family in the inhibition of renin gene expression by TNFalpha in the juxtaglomerular cell line As4.1. TNFalpha caused a moderate decrease in the binding of CREB1 to its cognate CRE DNA binding site. On the other hand, NFkappaB-p65 transcriptional activity was substantially reduced by TNFalpha, which targeted a trans-activation domain at the very C terminus of the p65 molecule. Our results suggest that TNFalpha inhibits renin gene expression by decreasing the transactivating capacity of NFkappaB-p65 and partially by attenuating CREB1 binding to CRE.

Age-dependent modulation of DNA repair enzymes by covalent modification and subcellular distribution

Chronic oxidative stress is generally believed to be a major etiologic factor in the aging process. In addition to modulation of signaling processes and oxidation of cellular proteins and lipids, reactive oxygen species (ROS) induce multiple damages in both nuclear and mitochondrial genomes, most of which are repaired via the DNA base excision repair pathway. 8-Oxoguanine (8-oxoG), a major ROS product in the genome, is excised by 8-oxoG-DNA glycosylase (OGG1) and the resulting abasic (AP) site is cleaved by AP-endonuclease (APE1) in the initial steps of repair. Here, we provide data showing that differences between young and aged cells' efficiency in import of OGG1 and APE1 may be responsible for age-associated increase in DNA damage in both nuclear and mitochondrial compartments. It is also evident that age-dependent changes in covalent modifications of APE1 by acetylation regulate its action as a transcriptional repressor of many Ca(2+)-responsive genes by binding to nCaRE, in addition to its endonuclease activity. Thus, ROS-induced altered signaling is responsible for age-dependent changes in post-translational modifications and import of DNA repair enzymes into nuclei and mitochondria (mt), which in turn affect repair of their genomes.

Two essential but distinct functions of the mammalian abasic endonuclease

The mammalian abasic endonuclease, APE1, has two distinct roles in the repair of oxidative DNA damage and in gene regulation. Here we show that both functions are essential for cell survival. Deletion of the APE1 gene causes embryonic lethality in mice, and no nullizygous embryo fibroblasts have been isolated. We have now established nullizygous embryo fibroblast lines from APE1(-/-) mouse embryos that are transgenic with the "floxed" human APE1 (hAPE1) gene. Removal of hAPE1 by Cre expression through nuclear microinjection elicited apoptosis in these cells within 24 h, which was blocked by coinjection of the wild-type hAPE1 gene. In contrast, mutant hAPE1 alleles, lacking either the DNA repair or acetylation-mediated gene regulatory function, could not prevent apoptosis, although the combination of these two mutants complemented APE deficiency induced by Cre. These results indicate that distinct and separable functions of APE1 are both essential for mammalian cells even in vitro and provide the evidence that mammalian cells, unlike yeast or Escherichia coli, absolutely require APE for survival, presumably to protect against spontaneous oxidative DNA damage.

The intracellular localization of APE1/Ref-1: more than a passive phenomenon?

Human apurinic/apyrimidinic endonuclease 1/redox effector factor-1 (APE1/Ref-1) is a perfect paradigm of the functional complexity of a biological macromolecule. First, it plays a crucial role, by both redox-dependent and -independent mechanisms, as a transcriptional coactivator for different transcription factors, either ubiquitous (i.e., AP-1, Egr-1, NF-kappaB, p53, HIF) or tissue-specific (i.e., PEBP-2, Pax-5 and -8, TTF-1), in controlling different cellular processes such as apoptosis, proliferation, and differentiation. Second, it acts, as an apurinic/apyrimidinic endonuclease, during the second step of the DNA base excision repair pathway, which is responsible for the repair of cellular alkylation and oxidative DNA damages. Third, it controls the intracellular reactive oxygen species production by negatively regulating the activity of the Ras-related GTPase Rac1. Despite these known functions of APE1/Ref-1, information is still scanty about the molecular mechanisms responsible for the coordinated control of its several activities. Some evidence suggests that the expression and subcellular localization of APE1/Ref-1 are finely tuned. APE1/Ref-1 is a ubiquitous protein, but its expression pattern differs according to the different cell types. APE1/Ref-1 subcellular localization is mainly nuclear, but cytoplasmic staining has also been reported, the latter being associated with mitochondria and/or presence within the endoplasmic reticulum. It is not by chance that both expression and subcellular localization are altered in several metabolic and proliferative disorders, such as in tumors and aging. Moreover, a fundamental role played by different posttranslational modifications in modulating APE1/Ref-1 functional activity is becoming evident. In the present review, we tried to put together a growing body of information concerning APE1/Ref-1's different functions, shedding new light on present and future directions to understand fully this unique molecule.