Sphingosine inhibits the synthesis of RNA primers by primase in vitro

Sphingoid Base-Upregulated Caspase-14 Expression Involves MAPK

Sphingolipids are putative intracellular signal mediators in cell differentiation, growth inhibition, and apoptosis. Especially, sphingoid base-backbones of sphingolipids (sphingosine, sphinganine, and phytosphingosine) and their metabolites N-acyl-sphingoid bases (ceramides) are highly bioactive. In skin, one of the caspases, caspase-14, is expressed predominantly in cornifying epithelia, and caspase-14 plays an important role in keratinocyte differentiation. As ceramides were surrounding lipids in the keratinocytes and ceramides stimulate keratinocyte differentiation, we therefore examined the upregulation of caspase-14 by various sphingoid bases and ceramide. Sphingosine, sphinganine, phytosphingosine, and C2-ceramide treatment at the doses not damaging cells significantly increased caspase-14 mRNA and protein expression in dose-dependent manner on human keratinocyte HaCaT cells. These results indicated that sphingoid bases and ceramide upregulated caspase-14 mRNA to increase intracellular caspase-14 protein level. We next examined the caspase-14 upregulation mechanism by sphingoid bases. We used the most effective sphingoid base, phytosphingosine, and revealed that specific inhibitors of the mitogen-activated protein kinase, p38 and c-jun N-terminal protein kinase (JNK), blocked caspase-14 expression. This indicates that phytosphingosine upregulation of caspase-14 is involved of p38 and JNK activation. Moreover, phytosphingosine induced caspase-14 upregulation in vivo, suggesting that sphingoid bases were involved in keratinocyte differentiation by affecting caspase-14.

DnaG Primase-A Target for the Development of Novel Antibacterial Agents

The bacterial primase-an essential component in the replisome-is a promising but underexploited target for novel antibiotic drugs. Bacterial primases have a markedly different structure than the human primase. Inhibition of primase activity is expected to selectively halt bacterial DNA replication. Evidence is growing that halting DNA replication has a bacteriocidal effect. Therefore, inhibitors of DNA primase could provide antibiotic agents. Compounds that inhibit bacterial DnaG primase have been developed using different approaches. In this paper, we provide an overview of the current literature on DNA primases as novel drug targets and the methods used to find their inhibitors. Although few inhibitors have been identified, there are still challenges to develop inhibitors that can efficiently halt DNA replication and may be applied in a clinical setting.

Structures to complement the archaeo-eukaryotic primases catalytic cycle description: What's next?

DNA replication is a crucial stage in the transfer of genetic information from parent to daughter cells. This mechanism involves multiple proteins with one key player being the primase. Primases are single-stranded DNA dependent RNA polymerases. On the leading strand, they synthesize the primer once allowing DNA elongation while on the lagging strand primers are generated repeatedly (Okazaki fragments). Primases have the unique ability to create the first phosphodiester bond yielding a dinucleotide which is initially elongated by primases and then by DNA polymerases.

Primase activity has been studied in the last decades but the detailed molecular steps explaining some unique features remain unclear. High-resolution structures of free and bound primases domains have brought significant insights in the understanding of the primase reaction cycle. Here, we give a short review of the structural work conducted in the field of archaeo-eukaryotic primases and we underline the missing “pictures” of the active forms of the enzyme which are of major interest. We organized our analysis with respect to the progression through the catalytic pathway.

Resveratrol and its oligomers: modulation of sphingolipid metabolism and signaling in disease

Resveratrol, a natural compound endowed with multiple health-promoting effects, has received much attention given its potential for the treatment of cardiovascular, inflammatory, neurodegenerative, metabolic and age-related diseases. However, the translational potential of resveratrol has been limited by its specificity, poor bioavailability and uncertain toxicity. In recent years, there has been an accumulation of evidence demonstrating that resveratrol modulates sphingolipid metabolism. Moreover, resveratrol forms higher order oligomers that exhibit better selectivity and potency in modulating sphingolipid metabolism. This review evaluates the evidence supporting the modulation of sphingolipid metabolism and signaling as a mechanism of action underlying the therapeutic efficacy of resveratrol and oligomers in diseases, such as cancer.

Sphingosine, a modulator of human translesion DNA polymerase activity

Translesion (TLS) DNA polymerases are specialized, error-prone enzymes that synthesize DNA across bulky, replication-stalling DNA adducts. In so doing, they facilitate the progression of DNA synthesis and promote cell proliferation. To potentiate the effect of cancer chemotherapeutic regimens, we sought to identify inhibitors of TLS DNA polymerases. We screened five libraries of ∼ 3000 small molecules, including one comprising ∼ 600 nucleoside analogs, for their effect on primer extension activity of DNA polymerase η (Pol η). We serendipitously identified sphingosine, a lipid-signaling molecule that robustly stimulates the activity of Pol η by ∼ 100-fold at low micromolar concentrations but inhibits it at higher concentrations. This effect is specific to the Y-family DNA polymerases, Pols η, κ, and ι. The addition of a single phosphate group on sphingosine completely abrogates this effect. Likewise, the inclusion of other sphingolipids, including ceramide and sphingomyelin to extension reactions does not elicit this response. Sphingosine increases the rate of correct and incorrect nucleotide incorporation while having no effect on polymerase processivity. Endogenous Pol η activity is modulated similarly as the recombinant enzyme. Importantly, sphingosine-treated cells exhibit increased lesion bypass activity, and sphingosine tethered to membrane lipids mimics the effects of free sphingosine. Our studies have uncovered sphingosine as a modulator of TLS DNA polymerase activity; this property of sphingosine may be associated with its known role as a signaling molecule in regulating cell proliferation in response to cellular stress.

Phytosphingosine induced mitochondria-involved apoptosis

Sphingolipids are putative intracellular signal mediators in cell differentiation, growth inhibition, and apoptosis. Sphingosine, sphinganine, and phytosphingosine are structural analogs of sphingolipids and are classified as long-chain sphingoid bases. Sphingosine and sphinganine are known to play important roles in apoptosis. In the present study, we examined the phytosphingosine-induced apoptosis mechanism, focusing on mitochondria in human T-cell lymphoma Jurkat cells. Phytosphingosine significantly induced chromatin DNA fragmentation, which is a hallmark of apoptosis. Enzymatic activity measurements of caspases revealed that caspase-3 and caspase-9 are activated in phytosphingosine-induced apoptosis, but there is little activation of caspase-8 suggesting that phytosphingosine influences mitochondrial functions. In agreement with this hypothesis, a decrease in DeltaPsi(m) and the release of cytochrome c to the cytosol were observed upon phytosphingosine treatment. Furthermore, overexpression of mitochondria-localized anti-apoptotic protein Bcl-2 prevented phytosphingosine apoptotic stimuli. Western blot assays revealed that phytosphingosine decreases phosphorylated Akt and p70S6k. Dephosphorylation of Akt was partially inhibited by protein phosphatase inhibitor OA and OA attenuated phytosphingosine-induced apoptosis. Moreover, using a cell-free system, phytosphingosine directly reduced DeltaPsi(m). These results indicate that phytosphingosine perturbs mitochondria both directly and indirectly to induce apoptosis.

Expression profiles of a human pancreatic cancer cell line upon induction of apoptosis search for modulators in cancer therapy

We analyzed the differential gene expression in the pancreatic cancer cell line NP-18 upon induction of apoptosis caused by cyclin-dependent kinase inhibition triggered by either overexpression of the tumor suppressor gene p16(INK4A)using an adenoviral construction or incubation with the chemical inhibitors, roscovitine or olomoucine. Screening was performed using cDNA arrays from Clontech that allowed the determination of the expression of 1,176 genes specifically related with cancer. The analysis was carried out using the Atlas Image 2.01 (Clontech) and GeneSpring 4.2 (Silicon Genetics) softwares. Among the differentially expressed genes, we chose for further validation histone deacetylase 1 (HDAC1), von Hippel Lindau and decorin as upregulated genes, and Sp1, hypoxia-inducible factor-1 alpha and DNA primase as downregulated genes. The changes in the expression of these genes to mRNA were validated by quantitative RT-PCR and the final translation into protein by Western blot analysis. Inhibition of HDAC activity, Sp1 binding and DNA primase expression led to an increase in the level of apoptosis, both in parental cells and in doxorubicin-resistant cells. Therefore, these proteins could constitute possible targets to develop modulators in cancer chemotherapy that would increase or restore apoptosis.

Inhibition of DNA primase and induction of apoptosis by 3,3’-diethyl-9-methylthia-carbocyanine iodide in hepatocellular carcinoma BEL-7402 cells

AIM: To evaluate the effects of 3,3’-diethyl-9-methylthia-carbocyanine iodide (DMTCCI) on DNA primase activity and on apoptosis of human hepatocellular carcinoma BEL-7402 cells.

METHODS: DNA primase assay was used to investigate DNA primase activity. MTT assay was applied to determine cell proliferation. Flow cytometric analysis, transmission electron microscopy, DNA fragmentation assay were performed to detect DMTCCI-induced apoptosis. Expression levels of p53, Bcl-2, Bcl-xL, Bad, Bax, survivin, Caspase-3 and poly (ADP-ribose) polymerase (PARP) were evaluated by immunoblot analysis. Caspase-3 activity was assessed with ApoAlert Caspase-3 colorimetric assay kit.

RESULTS: DMTCCI had inhibitory effects on eukaryotic DNA primase activity with IC₅₀ value of 162.2 nmol/L. It also inhibited proliferation of human hepatocellular carcinoma BEL-7402 cells with IC₅₀ value of 2.09 μmol/L. Furthermore, DMTCCI-induced BEL-7402 cell apoptosis was confirmed by DNA fragmentation (DNA ladders and sub-G1 formation) and transmission electron microscopy (apoptotic bodies formation). During the induction of apoptosis, expression of Bcl-2, Bcl-xL and survivin was decreased, and that of p53, Bad and Bax was increased. Caspase-3 was activated and poly (ADP-ribose) polymerase (PARP) was cleaved in BEL-7402 cells treated with DMTCCI.

CONCLUSION: The present data suggest that DMTCCI has inhibitory effects on eukaryotic DNA primase and can induce apoptosis of BEL-7402 cells. The modulation of expression of p53 and Bcl-2 family proteins, and activation of Caspase-3 might be involved in the induction of apoptosis.

Nuclear lipid signalling

During the past twenty years, evidence has accumulated for the presence of phospholipids within the nuclei of eukaryotic cells. These phospholipids are distinct from those that are obviously present in the nuclear envelope. The best characterized of the intranuclear lipids are the inositol lipids that form the components of a phosphoinositide-phospholipase C cycle. However, exactly as has been discovered in the cytoplasm, this is just part of a complex picture that involves many other lipids and functions.

Nuclear lipid signaling

Abundant evidence now supports the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope as part of a bilayer membrane, but are actually within the nucleus in the form of proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol (DAG) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet entirely clear. Generation of DAG within the nucleus is believed to recruit protein kinase C (PKC) to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P3 may mobilize Ca2+ from the space between the nuclear membranes and thus increase nucleoplasmic Ca2+. Less well understood are the increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus, (i) two routes of synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]; (ii) two sources of DAG, one from the PI-PLC pathway and the other probably from phosphatidylcholine; (iii) several isoforms of PKC translocating to nuclei; (iv) increases in activity of the PI-PLC pathway at two points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P3, which may have several functions, including a role in the transfer of mRNA out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and, in particular, 3-phosphorylated inositol lipids, which are now emerging as possible major players in nuclear signaling.

DNA primases

DNA primases are enzymes whose continual activity is required at the DNA replication fork. They catalyze the synthesis of short RNA molecules used as primers for DNA polymerases. Primers are synthesized from ribonucleoside triphosphates and are four to fifteen nucleotides long. Most DNA primases can be divided into two classes. The first class contains bacterial and bacteriophage enzymes found associated with replicative DNA helicases. These prokaryotic primases contain three distinct domains: an amino terminal domain with a zinc ribbon motif involved in binding template DNA, a middle RNA polymerase domain, and a carboxyl-terminal region that either is itself a DNA helicase or interacts with a DNA helicase. The second major primase class comprises heterodimeric eukaryotic primases that form a complex with DNA polymerase alpha and its accessory B subunit. The small eukaryotic primase subunit contains the active site for RNA synthesis, and its activity correlates with DNA replication during the cell cycle.

Nuclear localization of neutral sphingomyelinase 1: biochemical and immunocytochemical analyses

To examine the intracellular localization of neutral sphingomyelinase 1 (nSMase 1), a rabbit polyclonal antibody was raised against a recombinant form of the enzyme expressed in E. coli. It has been reported that, in rat liver or in ascites hepatoma AH7974, high activity of neutral sphingomyelinase (SMase) is found at the plasma membrane, with a lesser but significant amount in nucleus and cytoplasm. The biochemical properties, dithiothreitol requirement and high salt concentration dependency, of cloned and expressed nSMase 1 resemble those of previously described nuclear neutral SMase of AH7974. The present study was therefore focused on the nuclear localization of this enzyme. Western blotting of subcellular fractions using anti-rat nSMase 1 antibody revealed most nSMase 1 to be associated with the nuclei and some with microsomes, but not with plasma membranes. Consistently, neutral SMase activity in nuclear extract was immunoprecipitated by the antibody, while that of plasma membranes was not. The results indicate that nSMase 1 mainly resides in the nucleus and may thus differ from neutral SMase in plasma membrane. On gel-filtration column chromatography of nuclear extract, the profile of neutral SMase activity corresponded well with immunoreactive protein bands on western blotting, suggesting that a large part of nuclear neutral SMase may be nSMase 1. Removal of the nuclear envelope by treatment with Triton X-100 did not significantly decrease the amount of nuclear nSMase 1, and western blotting of subnuclear fractions (i.e. nuclear envelope, chromatin, and nuclear matrix) revealed nSMase 1 signal exclusively in the nuclear matrix. Immunocytochemistry with AH7974, as well as rat fibroblast cell line 3Y1, demonstrated nSMase 1 to be localized mainly in the nucleus, with some in the cytoplasm. Moreover, immuno-electron microscopy clearly showed the signal of nSMase 1 to be more dense in the nucleus than in the cytoplasm of AH7974.

Nuclear lipid signaling

There is now abundant evidence for the existence of phospholipids in the nucleus that resist washing of nuclei with detergents. These lipids are apparently not in the nuclear envelope, but are actually within the nucleus, presumably not in a bilayer membrane but instead forming proteolipid complexes with unidentified proteins. This review discusses the experimental evidence that attempts to explain their existence. Among these nuclear lipids are the polyphosphoinositol lipids which, together with the enzymes that synthesize them, form an intranuclear phospholipase C (PI-PLC) signaling system that generates diacylglycerol and inositol-1,4,5-trisphosphate [Ins(1,4,5)P(3)]. The isoforms of PI-PLC that are involved in this signaling system, and how they are regulated, are not yet clear. Generation of diacylglycerol within the nucleus is believed to recruit protein kinase C to the nucleus to phosphorylate intranuclear proteins. Generation of Ins(1,4,5)P(3) may mobilize Ca(2+) from the space between the nuclear membranes and thus increase nucleoplasmic Ca(2+). Less well understood are an increasing number of variations and complications on the "simple" idea of a PI-PLC system. These include, all apparently within the nucleus: (i) two separate routes of synthesis of phosphatidylinositol-4,5-bisphosphate; (ii) two different sources of diacylglycerol, one being from the PI-PLC pathway, and the other probably from phosphatidylcholine; (iii) several different isoforms of PKC translocating to the nuclei; (iv) increases in activity of the PI-PLC pathway at two different points in the cell cycle; (v) a pathway of phosphorylation of Ins(1,4,5)P(3), which may have several functions, including a role in the transfer of messenger RNA (mRNA) out of the nucleus; and (vi) the possible existence of other lipid signaling pathways that may include sphingolipids, phospholipase A2, and 3-phosphorylated inositol lipids.

A possible role of nuclear ceramide and sphingosine in hepatocyte apoptosis in rat liver

BACKGROUND/AIMS

Portal vein branch ligation induces apoptosis of hepatocytes in the ligated lobes in rat liver. Sphingomyelin degradation was studied during the process to evaluate its possible involvement in apoptosis in vivo.

METHODS

DNA scissions were detected by the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) and an agarose gel electrophoresis of DNA. Using both ligated and non-ligated lobes, we measured activities of sphingomyelin degradation enzymes and contents of their products in purified nuclei and plasma membrane.

RESULTS

DNA fragmentation was detectable in the ligated lobes at 90 min after the portal vein branch ligation by gel electrophoresis. At 15 h after the ligation, 27% of hepatocytes became TUNEL-positive. Prior to the onset of apoptosis, the activity of neutral sphingomyelinase increased in the nuclei of hepatocytes in ligated lobes (30 min after the ligation). The increase in sphingomyelinase paralleled its reaction product, ceramide. This was followed by the elevation of ceramidase activity in nuclei (60 min after the ligation) in association with an increase of its reaction product, sphingosine. Activities of these two enzymes and their products increased for at least 90 min. These changes were not observed in nuclei of the non-ligated lobes, or in the plasma membranes from either ligated or non-ligated lobes.

CONCLUSIONS

These results, specific to the liver where apoptosis is being generated, suggest that nuclear sphingomyelin breakdown with an accumulation of ceramide and/or sphingosine in nuclei may induce the apoptosis of hepatocytes in vivo.

Characterization of cell-cycle arrest by fumonisin B1 in CV-1 cells

Fusarium moniliforme is a widespread fungal pathogen which primarily infects corn, but can also infect rice or wheat. Fusarium moniliforme produce several mycotoxins, the most prominent of which is called fumonisin B1 (FB1). Epidemiological studies have indicated that ingestion of fumonisins correlates with a higher incidence of oesophageal cancer in Africa and China. Fumonisins also cause a neurodegenerative disease in horses, induce hepatic cancer in rats, are nephrotoxic in rats, or cause pulmonary oedema in swine. Structurally, fumonisins resemble sphingolipids and can alter sphingolipid biosynthesis. suggesting that sphingolipid alterations play a role in disease and carcinogenesis. Previous studies determined that FB1 blocked cell-cycle progression in CV-1 cells but not COS-7 cells. Herein, we have examined the effects that FB1 treatment has on cell-cycle regulatory proteins. Our studies established that FB1 treatment of CV-1 cells, but not COS-7 cells, leads to dephosphorylation of the retinoblastoma (Rb) protein. Cyclin dependent kinase 2 (CDK2) activity was repressed five- to 10-fold and cyclin E protein levels were lower in CV-1 cells after fumonisin treatment. Two CDK inhibitors, Kip1 and Kip2, were induced within 3 hours after fumonisin treatment of CV-1 cells, suggesting these two proteins mediate cell-cycle arrest induced by FB1. This mycotoxin caused large increases in sphinganine within 3 hours after addition of FB1. As sphingoid bases are known to induce Rb phosphorylation, this increase in sphinganinie might be the stimulus for the suppression of cyclin dependent kinase activities via Kip1 and Kip2. The ability of FB1 to accumulate sphingosine or sphinganine and arrest the cell cycle in some cells but not others may play an important role in carcinogenesis or disease.

Sulfated glycoglycerolipid from archaebacterium inhibits eukaryotic DNA polymerase alpha, beta and retroviral reverse transcriptase and affects methyl methanesulfonate cytotoxicity

A sulfated glycoglycerolipid, 1-O-(6'-sulfo-alpha-D-glucopyranosyl)-2,3-di-O-phytanyl- sn-glycerol (KN-208), a derivative of the polar lipid isolated from an archaebacterium, strongly inhibited DNA polymerase (pol) alpha and pol beta in vitro among 5 eukaryotic DNA polymerases (alpha, beta, gamma, delta, and epsilon). It also inhibited Escherichia coli DNA polymerase I Klenow fragment (E. coli pol I) and human immunodeficiency virus reverse transcriptase (HIV RT). The mode of inhibition of these polymerases was competitive with the DNA template primer and was non-competitive with the substrate dTTP. KN-208 inhibited pol beta most strongly, with a Ki value of 0.05 microM, 10-fold lower than that for pol alpha (0.5 microM) and 60- or 140-fold lower than that for HIV RT (3 microM) or for E. coli pol I (7 microM), respectively. The loss of sulfate on the 6'-position of glucopyranoside of this compound completely abrogated inhibition. However, the hydrophilic part of KN-208, glucose 6-sulfate alone, showed no inhibition. Other sulfated compounds containing different hydrophobic structures, such as dodecyl sulfate and cholesterol sulfate, exhibited a much weaker inhibition. Our results suggest that the whole molecular structure of KN-208 is required for inhibition. KN-208 was shown to be modestly cytotoxic for the human leukemic cell line K562. Interestingly, a subcytotoxic dose of KN-208 increased the sensitivity of the human leukemic cells to an alkylating agent, methyl methanesulfonate, while it did not potentiate the effects of ultraviolet light or of cisplatin.

Induction of apoptosis by fumonisin B1 in HT29 cells is mediated by the accumulation of endogenous free sphingoid bases

Fumonisin B1 (FB1) and aminopentol (AP1) (which is formed by hydrolysis of FB1) are found in corn contaminated with some strains of Fusarium moniliforme. Incubation of HT29 cells (a human colonic cell line) with FB1 or AP1 caused a significant reduction in cell number; AP1 was less potent, with 50 microM AP1 causing the same reduction (ca. 30% after 24 h) as 10 microM FB1. The reduction in cell number reflected increases in DNA fragmentation and the percentage of apoptotic cells. Both FB1 and AP1 caused the accumulation of sphinganine (25- and 35-fold by 10 microM FB1 and 50 microM AP1, respectively); thus, concentrations of FB1 and AP1 that caused comparable reductions in cell number were also similar with respect to elevation of sphinganine, a compound that is growth inhibitory and cytotoxic. Inhibition of the first step of sphingolipid biosynthesis with ISP-1 prevented the elevation in sphinganine, DNA fragmentation, and apoptosis induced by FB1. Therefore, these effects of FB1 on HT29 cells can be attributed to the accumulation of sphinganine. Since consumption of food contaminated with Fusarium moniliforme (Sheldon) exposes colonic cells to these mycotoxins, the possibility that FB1 and AP1 are toxic for intestinal cells in vivo should be evaluated, especially in the light of the recent report (Bhat et al., Clin. Toxicol. 35, 249, 1997) describing intestinal disturbances in humans after consumption of moldy corn and sorghum containing fumonisins.

Changes in composition of newly synthesized sphingolipids of HeLa cells during the cell cycle -- suppression of sphingomyelin and higher-glycosphingolipid synthesis and accumulation of ceramide and glucosylceramide in mitotic cells

Sphingolipid biosynthesis in synchronized HeLa cells was studied by pulse labeling with [14C]Ser or [14C]Gal and a simple TLC method. The major HeLa cell sphingolipids are ceramide (Cer), sphingomyelin, glucosylceramide (GlcCer), lactosylceramide (LacCer), globotriaosylceramide (Gb3Cer), N-acetylneuraminosylgangl iotriaosylceramide (GM2) and sialylparagloboside (G[M1-GlcNAc]). The sphingolipid biosynthetic profiles of HeLa cells in the G1, G1/S boundary, S and G2 phases were similar, but significant changes occurred during M phase, when incorporation of radioactivity into sphingomyelin, Gb3Cer and a mixture of GM2 and G(M1-GlcNAc) decreased, and those of Cer and GlcCer increased. These data indicate that transfer of phosphocholine and galactose to Cer and GlcCer, respectively, decreased in mitotic cells, resulting in accumulation of Cer and GlcCer. Analysis of LacCer synthase activity revealed that GlcCer accumulation was not due to reduced activity of this enzyme. The results suggest that Cer and GlcCer accumulation in mitotic cells resulted from suppression of sphingomyelin and LacCer synthesis, probably caused by vesiculation of membranous organelles, such as the endoplasmic reticulum and Golgi apparatus.

Lack of promotion of colon carcinogenesis by high-oleic safflower oil

BACKGROUND

The nonpromoting effect of olive oil on colon carcinogenesis has been attributed to its high oleic acid content, whereas a positive association of monounsaturated fat in beef tallow with colon tumors has been reported. The effect of constituents other than fatty acids could not be neglected in these experiments. In order to minimize the effects of minor constituents in the oils, the authors compared conventional safflower oil with oil from a mutant strain of safflower that is rich in oleic acid.

METHODS

ICR mice were treated with 1,2-dimethylhydrazine (DMH, 20 mg/kg body weight every week for 12 weeks) and then were fed either a high-fat diet (23.5% by weight), containing safflower oil (HF-LA) or high-oleic safflower oil (HF-OA), or a low-fat diet (5% by weight), containing safflower oil (LF-LA) or high-oleic safflower oil (LF-OA). The test diets were continued until termination of the experiment at 30 weeks after the first administration of DMH. Fatty acid composition of colon phospholipids was determined by gas-liquid chromatography-mass spectrometry.

RESULTS

Tumor multiplicity in animals fed the HF-OA diet was indistinguishable from that in animals fed LF-LA or LF-OA. In contrast, animals fed the HF-LA diet had a significantly higher incidence of colon tumors (mostly adenocarcinomas) than the other groups. Fatty acid profiles of colon phospholipids reflected those of the diet. Animals fed a HF-LA diet showed a marked decrease of nervonic acid (C24:1, n-9) in the colon sphingomyelin.

CONCLUSIONS

These data indicate that oleic acid does not enhance DMH-induced colon carcinogenesis in mice, even when they are fed a high-fat diet.