Affinity labeling the DNA polymerase alpha complex. I. Pyridoxal 5′-phosphate inhibition of DNA polymerase and DNA primase activities of the DNA polymerase alpha complex from Drosophila melanogaster embryos

Association of Mutations in Replicative DNA Polymerase Genes with Human Disease: Possible Application of Drosophila Models for Studies

Replicative DNA polymerases, such as DNA polymerase α-primase, δ and ε, are multi-subunit complexes that are responsible for the bulk of nuclear DNA replication during the S phase. Over the last decade, extensive genome-wide association studies and expression profiling studies of the replicative DNA polymerase genes in human patients have revealed a link between the replicative DNA polymerase genes and various human diseases and disorders including cancer, intellectual disability, microcephalic primordial dwarfism and immunodeficiency. These studies suggest the importance of dissecting the mechanisms involved in the functioning of replicative DNA polymerases in understanding and treating a range of human diseases. Previous studies in Drosophila have established this organism as a useful model to understand a variety of human diseases. Here, we review the studies on Drosophila that explored the link between DNA polymerases and human disease. First, we summarize the recent studies linking replicative DNA polymerases to various human diseases and disorders. We then review studies on replicative DNA polymerases in Drosophila. Finally, we suggest the possible use of Drosophila models to study human diseases and disorders associated with replicative DNA polymerases.

Vitamin B6Suppresses Growth of the Feline Mammary Tumor Cell Line FRM

Growth of FRM cells was inhibited by the addition of pyridoxine in a dose-dependent manner. Use of 5 mM pyridoxine caused an almost complete arrest of cell growth. Pyridoxal was as effective as pyridoxine, but pyridoxamine showed weak inhibitory action. Electron-microscopic examination of control cells revealed large nuclei and cellular membranes with villi, but, in pyridoxine-treated cells, condensed or degraded nuclei were observed. Many vacuoles and cholesterol crystals were widely distributed inside the cellular membrane of pyridoxine-treated cells. One of the vacuoles was identified as a lipid droplet. The DNA ladder was observed in the pyridoxine-treated cells. It is suggested that pyridoxine treatment of FRM cells causes cytolysis of cells by apoptosis.

High concentrations of pyridoxal stimulate the expression of IGFBP1 in HepG2 cells through upregulation of the ERK/c‑Jun pathway

Increasing evidence suggests that dietary vitamin B6 is linked to the prevention of cancer and cardiovascular disease. However, the molecular mechanisms involved in this process are not yet understood. Preliminary results in the current study indicated, following DNA microarray analysis and quantitative PCR, that insulin‑like growth factor‑binding protein 1 (IGFBP1) mRNA is upregulated in HT29 colon carcinoma cells exposed to pyridoxal (PL, 500 µM). IGFBP1 is secreted from the liver and is hypothesized to exert a protective role in the development of cancer and cardiovascular disease. Thus, further experiments were performed to investigate the effect of PL on the expression of IGFBP1 in HepG2 hepatocellular carcinoma cells. The addition of PL (500 µM) markedly increased the expression of IGFBP1 mRNA in HepG2 cells at 6, 12 and 24 h (P<0.01), whereas other vitamers (500 µM), including pyridoxal 5'‑phosphate (PLP), pyridoxine (PN) and pyridoxamine (PM), caused no such effect. The expression of the IGFBP1 protein in the cell lysate and culture medium was elevated in the presence of PL. PL elevated expression of the active form of ERK1 protein, p‑ERK1, and the p‑c‑Jun protein, a downstream factor of ERK. Furthermore, IGFBP1 expression, elevated by PL, was suppressed by PD98059, an ERK inhibitor. Higher expression of IGFBP1 protein by PL was suppressed by cycloheximide. These results suggest that PL may induce the expression of IGFBP1 in hepatoma cells via a mechanism involving the ERK/c‑Jun pathway.

Prediagnostic plasma vitamin B6 (pyridoxal 5'-phosphate) and survival in patients with colorectal cancer

PURPOSE

Higher plasma pyridoxal 5'-phosphate (PLP) levels are associated with a decreased incidence of colorectal cancer, but the influence of plasma PLP on survival of patients with colorectal cancer is unknown. We prospectively examined whether prediagnostic plasma PLP levels are associated with mortality among colorectal cancer patients.

METHODS

We included 472 incident cases of colorectal cancer identified in the Nurses' Health Study, the Health Professionals Follow-up Study, and the Physicians' Health Study from 1984 to 2002. The patients provided blood samples two or more years before cancer diagnosis. Stratified Cox proportional hazards models were used to calculate hazard ratios (HR) with 95 % confidence intervals (CI) adjusted for other risk factors for cancer survival.

RESULTS

Higher plasma PLP levels were not associated with a significant reduction in colorectal cancer-specific (169 deaths) or overall mortality (259 deaths). Compared with patients who had less than 45 pmol/ml of plasma PLP (median: 33.6 pmol/ml), those who had 110 pmol/ml or higher levels (median: 158.8 pmol/ml) had multivariable HRs of 0.85 (95 % CI 0.50-1.45, p trend = 0.37) and 0.87 (95 % CI 0.56-1.35, p trend = 0.24) for colorectal cancer-specific and overall mortality. Higher plasma PLP levels, however, seemed to be associated with better survival among patients who had lower circulating 25-hydroxyvitamin D3 levels (<26.5 ng/ml) (p interaction ≤.005).

CONCLUSIONS

Higher prediagnostic plasma PLP levels were not associated with an improvement on colorectal cancer survival overall. Further research is needed to clarify the influence of vitamin B6 on colorectal cancer progression and survival.

Modulation of gene expression by vitamin B6

The physiologically active form of vitamin B6, pyridoxal 5'-phosphate (PLP), is known to function as a cofactor in many enzymic reactions in amino acid metabolism. Recent studies have shown that, apart from its role as a coenzyme, PLP acts as a modulator of steroid hormone receptor-mediated gene expression. Specifically, elevation of intracellular PLP leads to a decreased transcriptional response to glucocorticoid hormones, progesterone, androgens, and oestrogens. For example, the induction of cytosolic aspartate aminotransferase (cAspAT) in rat liver by hydrocortisone is suppressed by the administration of pyridoxine. The suppression of the cAspAT induction by pyridoxine is caused by a decrease in the expression of the cAspAT gene, which is brought about by inactivation of the binding activity of the glucocorticoid receptor to the glucocorticoid-responsive element in the regulatory region of the cAspAT gene. Vitamin B6 has recently been found to modulate gene expression not only for steroid hormone-responsive or PLP-dependent enzymes but also for steroid- and PLP-unrelated proteins such as serum albumin. Albumin gene expression was found to be modulated by vitamin B6 through a novel mechanism that involves inactivation of tissue-specific transcription factors, such as HNF-1 or C/EBP, by direct interaction with PLP in a similar manner to glucocorticoid receptor. Enhancement of albumin gene expression in the liver by an increased supply of amino acids can be explained by elevated binding of HNF-1 and C/EBP to their DNA-binding sites which, in turn, is caused by a decrease in the intracellular level of PLP by the increased amino acid supply. These findings that vitamin B6 acts as a physiological modulator of gene expression add a new dimension to the hitherto recognized function of vitamin B6 as a cofactor of enzyme action.

Prospective study of plasma vitamin B6 and risk of colorectal cancer in men

Vitamin B(6) may lower risk of colorectal cancer by preventing aberrations in one-carbon metabolism or by anti-inflammatory effects. We prospectively evaluated the association between plasma levels of pyridoxal 5'-phosphate (PLP; the active form of vitamin B(6)) and risk of colorectal cancer in a nested case-control study within the Physicians' Health Study. Among 14,916 men who provided blood specimens in 1982 to 1984, we identified 197 incident colorectal cancer cases through 2000 and individually matched them to 371 controls by age and smoking status. Plasma PLP levels were positively correlated with cold cereal intake and plasma levels of folate and vitamin B(12) (age- and smoking-adjusted partial correction r = 0.28-0.48) and slightly inversely correlated with body mass index (r = -0.11) and plasma levels of homocysteine, C-reactive protein, tumor necrosis factor-alpha receptor 2, and interleukin-6 (r = -0.23 to -0.14). With control for these factors and known risk factors for colorectal cancer, plasma PLP levels were significantly inversely associated with risk of colorectal cancer; compared with men in the lowest quartile, those with PLP in quartiles 2 to 4 had relative risks (95% confidence interval) of 0.92 (0.55-1.56), 0.42 (0.23-0.75), and 0.49 (0.26-0.92; P(trend) = 0.01), respectively. In conclusion, vitamin B(6) may protect against colorectal cancer independent of other one-carbon metabolites and inflammatory biomarkers.

Molecular modelling studies on the interactions of human DNA topoisomerase IB with pyridoxal-compounds

Candida guilliermondii and human DNA topoisomerases I are inhibited by PL (pyridoxal), PLP (pyridoxal 5'-phosphate) and PLP-AMP (pyridoxal 5'-diphospho-5'-adenosine) (PL<PLP<PLP-AMP). We have recently shown that PLP acted as a competitive inhibitor of C. guilliermondii topoisomerase I, impeding the formation of the cleavable complex from a selective binding to an active site lysine. The targeted lysine in C. guilliermondii topoisomerase I occupies a position equivalent to that of lysine 532 (K(532)) in human topoisomerase I. K(532) acts as a general acid catalyst and is essential for the enzyme activity. This observation has suggested that, in the cell, PLP could down-regulate topoisomerases IB. We have proposed that PLP could be used as a new lead for anticancer drugs trapping the active site lysine (K(532)) and also as a tool to explore the enzyme dynamics required for catalysis. Now we explore the effects of PL, PLP and PLP-AMP on topoisomerases by a molecular modelling approach using the crystal structure of the human topoisomerase I active site and the conformation of K(39)-PLP moiety in Bacillus subtilis alanine racemase as templates. In the modified topoisomerase I several reactive atoms of the K(532)-PLP moiety are at close distance of the catalytic residues R(488), R(590), H(632) and Y(723,) suggesting that PLP develops disturbing interactions with these important residues. These interactions and the corresponding induced fit in the active site conformation are compared with the ones occurring with PL and PLP-AMP. The results could be useful in the search of topoisomerase I inhibitors related to the pyridoxal family.

MC-1 (pyridoxal 5'-phosphate): novel therapeutic applications to reduce ischaemic injury

Despite the overall efficacy of mechanical reperfusion therapies, such as percutaneous coronary intervention and coronary artery bypass graft surgery, in reducing the morbidity and mortality that is associated with acute ischaemic syndromes, many of the treated patients develop ischaemia-reperfusion injury due to impaired microvascular integrity, embolisation of atherothrombotic debris and/or disrupted end-organ metabolism. MC-1 is an investigational drug from Medicure, Inc. In preclinical models of ischaemia and ischaemia-reperfusion injury, treatment with MC-1 has demonstrated significant cardio- and neuroprotective effects. Although the pharmacological activity of MC-1 may involve multiple mechanisms, research suggests that at least part of the protective effect may be mediated through its actions on purinergic receptors. Early clinical experience with MC-1 also appears to be promising: in a recent Phase II evaluation, treatment with MC-1 was associated with a statistically significant reduction in periprocedural infarct size (as measured by area under the curve creatine kinase-myocardial band) among high-risk patients undergoing elective percutaneous coronary intervention. Based on these findings, larger, randomised trials to confirm the safety and efficacy of MC-1 in the setting of coronary artery revascularisation with coronary artery bypass graft, acute coronary syndromes and stroke are ongoing or in development. These forthcoming evaluations should clarify the safety and efficacy of MC-1 and improve the understanding regarding its potential therapeutic role in a variety of clinical settings and indications.

Vitamin B6-mediated suppression of colon tumorigenesis, cell proliferation, and angiogenesis (review)

This review describes current research on the preventive effect of dietary vitamin B(6) against colon tumorigenesis and its possible mechanisms. Studies in cell culture have demonstrated that high levels of vitamin B(6) suppress growth of some cancer cells. From these studies it has been considered that supraphysiological doses of vitamin B(6) suppress tumor growth and metastasis. However, recent rodent study has indicated that azoxymethane-induced colon tumorigenesis in mice is suppressed by moderate doses of dietary vitamin B(6.) Epidemiological studies also support an inverse relationship between vitamin B(6) intake and colon cancer risk. Potential mechanisms underlying the preventive effect of dietary vitamin B(6) have been suggested to include the suppression of cell proliferation, oxidative stress, nitric oxide (NO) synthesis, and angiogenesis.

Antitumor effect of vitamin B6 and its mechanisms

Epidemiological studies have reported an inverse association between vitamin B(6) intake and colon cancer risk. Our recent study has been conducted to examine the effect of dietary vitamin B(6) on colon tumorigenesis in mice. Mice were fed diets containing 1, 7, 14 or 36 mg/kg pyridoxine for 22 weeks, and given a weekly injection of azoxymethane (AOM) for the initial 10 weeks. Compared with the 1 mg/kg pyridoxine diet, 7, 14 and 35 mg/kg pyridoxine diets significantly suppressed the incidence and number of colon tumors, colon cell proliferation and expressions of c-myc and c-fos proteins. Supplemental vitamin B(6) lowered the levels of colonic 8-hydroxyguanosine (8-OHdG), 4-hydroxy-2-nonenal (4-HNE, oxidative stress markers) and inducible nitric oxide (NO) synthase protein. In an ex vivo serum-free matrix culture model using rat aortic ring, supplemental pyridoxine and pyridoxal 5'-phosphate (PLP) had antiangiogenic effect. The results suggest that dietary vitamin B(6) suppresses colon tumorigenesis by reducing cell proliferation, oxidative stress, NO production and angiogenesis.

Pyridoxal 5'-phosphate inhibits DNA binding of HNF1

An efficient Escherichia coli expression system was constructed for the production of a variant form of HNF1 protein having the additional five amino acid residues (Asp-Arg-Trp-Gly-Ser) at the NH(2)-terminal. The cDNA encoding HNF1 was ligated to 6 x His tag and inserted into an inducible bacterial expression vector pRSET A. After expression in E. coli, the recombinant product was purified by Ni-NTA affinity column chromatography. The purified product showed expected NH(2)-terminal sequence and specific binding to the HNF1 site. The effect of pyridoxal 5'-phosphate and its analogues on the binding activity of recombinant HNF1 was examined and found that only pyridoxal 5'-phosphate effectively inhibited the DNA binding. The concentration of pyridoxal 5'-phosphate that inhibited 50% of DNA binding was around 100 microM. Furthermore, we identified Lys197 of HNF1 molecule as the essential residue of DNA binding. These observations suggest that pyridoxal 5'-phosphate directly interacts with tissue-specific transcription factor HNF1 and modulates the binding to DNA.

Vitamin B-6-supplemented diets compared with a low vitamin B-6 diet suppress azoxymethane-induced colon tumorigenesis in mice by reducing cell proliferation

Male ICR mice were examined for the effect of vitamin B-6 [pyridoxine (PN) HCl] on azoxymethane-induced colon tumorigenesis. Mice were fed the diets containing 1, 7, 14 or 35 mg PN HCl/kg for 22 wk, and given a weekly injection of azoxymethane (5 mg/kg body) for the initial 10 wk. Compared with the 1 mg PN HCl/kg diet, 7, 14 and 35 mg PN HCl/kg diets significantly suppressed the incidence and number of colon tumors, colon cell proliferation and expressions of c-myc and c-fos proteins. For some variables, 14 and 35 mg PN HCl/kg diets were more effective than the 7 mg/kg diet. Supplemental vitamin B-6 had no influence on the number of colon apoptotic cells. The results suggest that elevating dietary vitamin B-6 suppresses colon tumorigenesis by reducing cell proliferation.

Vitamin B6 suppresses growth and expression of albumin gene in a human hepatoma cell line HepG2

The effect of vitamin B6 on the growth of a human hepatoma cell line HepG2 in culture was studied. The growth of HepG2 cells and protein synthesis were almost completely inhibited in medium supplemented with 5 mM pyridoxine. Pyridoxal was as effective as pyridoxine, but pyridoxamine showed no inhibitory action. The growth inhibition of HepG2 cells by pyridoxine was accompanied by a marked inhibition of secretion of plasma proteins, particularly albumin. Northern blot analysis of albumin mRNA showed that pyridoxine caused a rapid decrease in the expression of albumin gene. The electron-microscopic examination of pyridoxine-treated HepG2 cells revealed a smoothing of nuclear membrane, a decrease in the number of nucleoli, and an appearance of aggregated heterochromatin structures. These morphological features are compatible with the depressed transcriptional activity in the pyridoxine-treated cells. The mechanism by which vitamin B6 exerts its inhibitory effect was discussed in terms of our recent finding that vitamin B6 modulates expression of albumin gene by inactivating tissue-specific DNA-binding proteins. Binding of pyridoxal phosphate with tissue-specific transcription factors may reduce the capacity of these factors to interact with the regulatory region of albumin gene, resulting in the inhibition of the gene expression.

Pyridoxal 5'-phosphate inhibition of adenovirus DNA polymerase

Pyridoxal phosphate modification of adenovirus DNA polymerase results in loss of DNA polymerase activity, whereas the 3' --> 5' exonuclease activity is unaffected. Inhibition by pyridoxal phosphate is time-dependent, displays saturation kinetics, and is reversible in the presence of excess primary amine unless the pyridoxal phosphate-enzyme adduct is first reduced with NaBH4. Thus, inhibition is the consequence of Schiff base formation between the aldehyde moiety of pyridoxal phosphate and primary amino groups on the enzyme. In addition to inhibiting DNA polymerase activity, pyridoxal phosphate also inhibited the ability of the enzyme to initiate viral DNA replication, by transfer of dCMP onto the preterminal protein. Neither template-primer nor dNTP protect against pyridoxal phosphate inhibition, but the combination of template-primer and complementary substrate dNTP protected both initiation and DNA polymerase activities. Thus, it is likely that both the dCMP transfer activity required for initiation and DNA polymerase activity are carried out at the same site of the enzyme.

Vitamin B6 deficiency causes activation of RNA polymerase and general enhancement of gene expression in rat liver

The effect of vitamin B6 deficiency on the activity of RNA polymerase and expression of several mRNAs in rat liver was investigated. The activities of RNA polymerase I and II in the liver of vitamin B6-deficient rats were found to be higher than the control rats by 30%. The expression of several mRNAs, including mRNAs for beta-actin and glyceraldehyde-3-phosphate dehydrogenase, and the content of poly(A)+ RNA were also increased in vitamin deficiency. These observations suggest that vitamin B6 influences gene expression in the liver, at least in part, by modulating the activity of RNA polymerase.

Pyridoxal-5'-phosphate inhibits the polymerase activity of a recombinant RNAase H-deficient mutant of HIV-1 reverse transcriptase

We have investigated the ability of pyridoxal-5'-phosphate to inhibit a recombinant deletion mutant of human immunodeficiency virus type 1(HIV-1) reverse transcriptase (RT) which is missing the last 23 amino acids of the C-terminus. This mutant reverse transcriptase is characterized by normal polymerase activity as compared with full-length enzyme; however, it has no RNase H activity. Inhibition studies with pyridoxal-5'-phosphate showed several differences as compared with inhibition of full-length enzyme: (1) Inhibition of mutant reverse transcriptase was independent of divalent cation, (2) Either substrate alone could protect mutant reverse transcriptase from inactivation by pyridoxal-5'-phosphate, and (3) stoichiometry of pyridoxal-5'-phosphate binding to mutant reverse transcriptase was 2 mol/mol under the same conditions in which 1 mol/mol bound to full-length enzyme. Furthermore, in the presence of either substrate alone, the stoichiometry of pyridoxal-5'-phosphate binding to the mutant was reduced to 1 mol/mol. These results indicate that the second binding site for pyridoxal-5'-phosphate seen in the mutant reverse transcriptase is at or near the primer-template binding site of the enzyme. They also suggest that the RNase H domain of HIV RT plays a functional role in substrate binding at the polymerase domain.