Curcuma zedoaria 50% methanol extracts increase adiponectin secretion by enhancing PPARγ mRNA expression in 3T3-L1 cells.

Curcuma zedoaria is a characteristic species of its genus that contains little to no curcuminoid. Here, we demonstrate that C. zedoaria extracts with 50% methanol increases adiponectin secretion into the media by enhancing PPARγ mRNA expression in 3T3-L1 cells. These results indicate that C. zedoaria may be useful for preventing/improving lifestyle-related diseases such as diabetes and atherosclerosis.

5-O-Acyl plumbagins inhibit DNA polymerase activity and suppress the inflammatory response.

We previously found that vitamin K3 (menadione, 2-methyl-1,4-naphthoquinone) inhibits the activity of human mitochondrial DNA polymerase (pol) γ. In this study, we focused on plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone), and chemically synthesized novel plumbagins conjugated with C2:0 to C22:6 fatty acids (5-O-acyl plumbagins). These chemically modified plumbagins displayed enhanced mammalian pol inhibition, with plumbagin conjugated to docosahexaenoic acid (C22:6-acyl plumbagin) exhibiting the strongest inhibition of pol λ among the ten 5-O-acyl plumbagins synthesized. C22:6-acyl plumbagin selectively inhibited the activities of mammalian pol species, but did not influence the activities of other pols or DNA metabolic enzymes tested. The inhibition of pol λ, a DNA repair/recombination pol, by these compounds was significantly correlated with both their suppression of lipopolysaccharide (LPS) induced tumor necrosis factor-α (TNF-α) production by mouse RAW264.7 macrophages and the reduction of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inflammation in the mouse ear. These data indicate that 5-O-acyl plumbagins act as anti-inflammatory agents by inhibiting mammalian pol λ. These results further suggest that C22:6-acyl plumbagin is a promising anti-inflammatory candidate and that acylation could be an effective chemical modification to improve the anti-inflammatory activity of vitamin K3 derivatives, such as plumbagin.

Formosusin A, a novel specific inhibitor of mammalian DNA polymerase ß from the fungus Paecilomyces formosus.

Variotin (1) and three novel compounds, formosusin A (2), B (3), and C (4), were isolated from the cultures of the fungus Paecilomyces formosus, and their structures were determined by spectroscopic analyses. Compound 2 is (6Z,8E,10E)-variotin, a new cis-olefin analog of compound 1. Compound 2 selectively inhibited the activity of mammalian DNA polymerase ß (pol ß) in vitro, with an IC50 of 35.6µM. By contrast, compounds 1, 3, and 4 did not influence the activity of pol ß. These four compounds showed no effect on the activities of other 10 mammalian pols (i.e., pols α, γ, δ, ε, η, ι, κ, λ, and µ, and terminal deoxynucleotidyl transferase). These compounds also did not inhibit the activities of fish, insect, plant, and prokaryotic pols and other DNA metabolic enzymes tested. These results suggested that compound 2 could be a selective inhibitor of mammalian pol ß. The compound 2-induced inhibition of rat pol ß activity was competitive and non-competitive with respect to the DNA template-primer substrate and the dNTP substrate, respectively. On the basis of these results, the relationship between the three-dimensional structure and pol ß inhibitory mechanism of compound 2 is discussed.

Bioactive dihydronaphthoquinone derivatives from Fusarium solani.

New dihydronaphthoquinone derivatives, karuquinone A (1), karuquinone B (2), and karuquinone C (3), were isolated from a fungal culture broth of Fusarium solani. The structures were determined by interpretation of spectroscopic data (1D/2D NMR, MS, and IR). Three known compounds, javanicin (4), 2,3-dihydro-5-hydroxy-8-methoxy-2,4-dimethylnaphtho[1,2-b]furan-6,9-dione (5), and 5-hydroxydihydrofusarubin C (6), were also isolated. The six isolated compounds were tested for cytotoxicity against three human cancer cell lines and a human umbilical vein endothelial cell (HUVEC) line. Of these, karuquinone A exhibited the strongest cytotoxic activity. Karuquinone B did not affect the proliferation of the cancer cell lines but did inhibit the proliferation of HUVEC. Additionally, we demonstrated that karuquinone A induces apoptosis in cancer cells through the generation of reactive oxygen species (ROS).

Inhibitory effect of novel somatostatin peptide analogues on human cancer cell growth based on the selective inhibition of DNA polymerase ß.

The present study was designed to investigate the anticancer activity of novel nine small peptides (compounds 1-9) derived from TT-232, a somatostatin structural analogue, by analyzing the inhibition of mammalian DNA polymerase (pol) and human cancer cell growth. Among the compounds tested, compounds 3 [tert-butyloxycarbonyl (Boc)-Tyr-Phe-1-naphthylamide], 4 (Boc-Tyr-Ile-1-naphthylamide), 5 (Boc-Tyr-Leu-1-naphthylamide) and 6 (Boc-Tyr-Val-1-naphthylamide) containing tyrosine (Tyr) but no carboxyl groups, selectively inhibited the activity of rat pol ß, which is a DNA repair-related pol. Compounds 3-6 strongly inhibited the growth of human colon carcinoma HCT116 p53(+/+) cells. The influence of compounds 1-9 on HCT116 p53(-/-) cell growth was similar to that observed for HCT116 p53(+/+) cells. These results suggest that the cancer cell growth suppression induced by these compounds might be related to their inhibition of pol. Compound 4 was the strongest inhibitor of pol ß and cancer cell growth among the nine compounds tested. This compound specifically inhibited rat pol ß activity, but had no effect on the other 10 mammalian pols investigated. Compound 4 combined with methyl methane sulfonate (MMS) treatment synergistically suppressed HCT116 p53(-/-) cell growth compared with MMS alone. This compound also induced apoptosis in HCT116 cells with or without p53. From these results, the influence of compound 4, a specific pol ß inhibitor, on the relationship between DNA repair and cancer cell growth is discussed.

Synthesis, structure, and cytotoxicity studies of some fungal isochromanes.

Ustusorane D and penicisochromans B-D are natural isochromans isolated from Aspergillus ustus 094102 and Penicillium sp. PSU-F40, respectively. Herein, we report the syntheses of (-)-ustusorane D and (+)-penicisochroman B and the structures of penicisochromans C and D. The relative configuration of natural ustusorane D and the absolute configuration of natural penicisochroman B were determined. Two plausible structures for penicisochroman C were evaluated through synthesis, but their ¹H and ¹³C NMR data were not in agreement with those of the natural product. The structural revision and the determination of the absolute configuration of natural penicisochroman D were achieved. Structure-activity relationship studies of the synthetic compounds as well as a series of related isochromans indicated that the enone of the furanone moiety was essential for the cytotoxicity of these compounds toward HCT116 human colon cancer cells. Pseudodeflectusin, the related natural isochroman, suppressed cell growth and induced apoptosis in HCT116 cells.

Pinophilins A and B, inhibitors of mammalian A-, B-, and Y-family DNA polymerases and human cancer cell proliferation.

Pinophilins A (1) and B (2), new hydrogenated azaphilones, and Sch 725680 (3) were isolated from cultures of a fungus (Penicillium pinophilum Hedgcok) derived from a seaweed, and their structures were determined using spectroscopic analyses. These compounds selectively inhibited the activities of mammalian DNA polymerases (pols), A (pol γ), B (pols α, δ, and ε), and Y (pols η, ι, and κ) families, but did not influence the activities of the four X-family pols (pols ß, λ, µ, and terminal deoxynucleotidyl transferase). Compound 1 was the strongest inhibitor, with IC50 values of 48.6 to 55.6 µM. Kinetic analysis showed that compound 1 is a noncompetitive inhibitor of both pol α and κ activities with the DNA template-primer substrate, and a competitive inhibitor with the nucleotide substrate. In contrast, compounds 1-3 showed no effect on the activities of plant and prokaryotic pols or any other DNA metabolic enzymes tested. The compounds suppressed cell proliferation and growth in five human cancer cell lines, but had no effect on the viability of normal human cell lines.

Inhibitory effect of novel 5-O-acyl juglones on mammalian DNA polymerase activity, cancer cell growth and inflammatory response.

We previously found that vitamin K(3) (menadione, 2-methyl-1,4-naphthoquinone) inhibits the activity of human mitochondrial DNA polymerase γ (pol γ). In this study, we focused on juglone (5-hydroxy-1,4-naphthoquinone), which is a 1,4-naphthoquinone derivative, and chemically synthesized novel juglones conjugated with C2:0 to C22:6 fatty acid (5-O-acyl juglones). The chemically modified juglones enhanced mammalian pol inhibition and their cytotoxic and anti-inflammatory activities. The juglone conjugated with oleic acid (C18:1-acyl juglone) showed the strongest inhibition of DNA replicative pol α activity and human colon carcinoma (HCT116) cell growth in 10 synthesized 5-O-acyl juglones. C12:0-Acyl juglone was the strongest inhibitor of DNA repair-related pol λ, as well as the strongest suppression of the production of tumor necrosis factor (TNF)-α production induced by lipopolysaccharide (LPS) in the compounds tested. Moreover, this compound caused the greatest reduction in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced acute inflammation in mouse ears. C12:0- and C18:1-Acyl juglones selectively inhibited the activities of mammalian pol species, but did not influence the activities of other pols and DNA metabolic enzymes tested. These data indicate that the novel 5-O-acyl juglones target anti-cancer and/or anti-inflammatory agents based on mammalian pol inhibition. Moreover, the results suggest that acylation of juglone is an effective chemical modification to improve the anti-cancer and anti-inflammation of vitamin K(3) derivatives, such as juglone.

Effects of intermediates between vitamins K(2) and K(3) on mammalian DNA polymerase inhibition and anti-inflammatory activity.

Previously, we reported that vitamin K(3) (VK(3)), but not VK(1) or VK(2) (=MK-4), inhibits the activity of human DNA polymerase γ (pol γ). In this study, we chemically synthesized three intermediate compounds between VK(2) and VK(3), namely MK-3, MK-2 and MK-1, and investigated the inhibitory effects of all five compounds on the activity of mammalian pols. Among these compounds, MK-2 was the strongest inhibitor of mammalian pols α, κ and λ, which belong to the B, Y and X families of pols, respectively; whereas VK(3) was the strongest inhibitor of human pol γ, an A-family pol. MK-2 potently inhibited the activity of all animal species of pol tested, and its inhibitory effect on pol λ activity was the strongest with an IC(50) value of 24.6 µM. However, MK-2 did not affect the activity of plant or prokaryotic pols, or that of other DNA metabolic enzymes such as primase of pol α, RNA polymerase, polynucleotide kinase or deoxyribonuclease I. Because we previously found a positive relationship between pol λ inhibition and anti-inflammatory action, we examined whether these compounds could inhibit inflammatory responses. Among the five compounds tested, MK-2 caused the greatest reduction in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced acute inflammation in mouse ear. In addition, in a cell culture system using mouse macrophages, MK-2 displayed the strongest suppression of the production of tumor necrosis factor (TNF)-α induced by lipopolysaccharide (LPS). Moreover, MK-2 was found to inhibit the action of nuclear factor (NF)-κB. In an in vivo mouse model of LPS-evoked acute inflammation, intraperitoneal injection of MK-2 in mice led to suppression of TNF-α production in serum. In conclusion, this study has identified VK(2) and VK(3) intermediates, such as MK-2, that are promising anti-inflammatory candidates.

Novel terpenoids, trichoderonic acids A and B isolated from Trichoderma virens, are selective inhibitors of family X DNA polymerases.

Trichoderonic acids A (1) and B (2), novel terpenoids, and (+)-heptelidic acid (3) isolated from cultures of a fungus, Trichoderma virens, and their structures were identified by spectroscopic analysis. These compounds selectively and competitively inhibited the activities of mammalian DNA polymerases beta, lambda (pols beta, lambda), and terminal deoxynucleotidyl transferase (TdT) in family X of pols, and compound 2 was a stronger inhibitor than compound 1 or 3. On the other hand, compounds 1-3 did not influence the activities of the other families (A-, B-, and the Y-families) of the mammalian pols tested, and showed no effect on the activities of plant pol alpha, fish pol delta, prokaryotic pol, or the other DNA metabolic enzymes tested. Compound 2 suppressed the growth of two human cancer cell lines, cervix carcinoma cells (HeLa) and breast carcinoma cells (MCF-7). The results suggest that these compounds identified inhibition among the families of mammalian pols.

Synthesis and structure-activity relationships of dehydroaltenusin derivatives as selective DNA polymerase alpha inhibitors.

Herein, we describe the synthesis and structure-activity relationships of dehydroaltenusin derivatives as inhibitors of a mammalian DNA polymerase alpha. We have newly synthesized nine dehydroaltenusin derivatives modified at the side chains or benzoquinone moiety. We also achieved the first synthesis of desmethylaltenusin and desmethyldehydroaltenusin, metabolites of Alternaria sp. or Talaromyces flavus, respectively. Among all synthesized derivatives, demethoxydehydroaltenusin was the most selective inhibitor of DNA polymerase alpha. The o-hydroxy-p-benzoquinone (2-hydroxycyclohexa-2,5-dienone) moiety is essential for the inhibition of DNA polymerases. Substitution at the 5-position of dehydroaltenusin is important for the inhibitory potency. Because dehydroaltenusin is conjugated with N-acetylcysteine methyl ester at the o-hydroxy-p-benzoquinone moiety, one or more cysteine residues of DNA polymerase alpha may act as a target for this compound.

The specific inhibitory effect of demethoxydehydroaltenusin, a derivative of dehydroaltenusin, on mammalian DNA polymerase alpha.

In the screening of selective inhibitors of eukaryotic DNA polymerases (pols), dehydroaltenusin from the fungus Acremonium sp. was found to be an inhibitor of pol alpha. The present study succeeded in chemically synthesizing dehydroaltenusin, and the compound strongly inhibited calf pol alpha activity and weakly suppressed rat pol beta activity, with IC50 values of 0.68 and 64 microM, respectively. We purified or synthesized various slightly modified derivatives of dehydroaltenusin, and using these, investigated the relationship between chemical structure and the inhibitory effects. These results suggest that the ketone group at the 5'-position in dehydroaltenusin is essential for pol inhibitory activity, and the group at the 5-position is important for the specificity of pol alpha inhibition. Demethoxydehydroaltenusin was found to be the most specific pol alpha inhibitor among the prepared derivatives, and the IC50 values for pols alpha and beta were 0.24 and 89 microM, respectively. This compound did not influence the activities of other replicative pols such as pols delta and epsilon, and also demonstrated no effect on pol alpha activity from another vertebrate, fish and a plant species. Demethoxydehydroaltenusin also had no influence on the other pols and DNA metabolic enzymes tested. Therefore, demethoxydehydroaltenusin is of interest as a mammalian pol alpha-selective inhibitor as a 'chemical knockout agent' in vitro and in vivo.

Inhibitory effects of cholesterol derivatives on DNA polymerase and topoisomerase activities, and human cancer cell growth.

This paper describes the inhibitory activities of cholesterol derivatives such as cholesterol, sodium cholesteryl sulfate, cholesteryl-5alpha, 6alpha-epoxide, cholesteryl chloride, cholesteryl bromide, and cholesteryl hemisuccinate (compounds 1-6, respectively) against DNA polymerase (pol), DNA topoisomerase (topo), and human cancer cell growth. Among the compounds tested, compounds 2 and 6 revealed themselves to be potent inhibitors of animal pols, and the IC50 values for pols were 0.84-11.6 and 2.9-148 microM, respectively. Compounds 2, 3 and 6 inhibited the activity of human topo II, with IC50 values of 5.0, 12.5 and 120 microM, respectively. Compounds 2, 3 and 6 also suppressed human cancer cell (promyelocytic leukemia cell line, HL-60) growth, and LD50 values were 8.8, 20.2 and 72.3 microM, respectively, suggesting that cell growth inhibition had the same tendency as the inhibition of topos rather than pols. Compounds 2 and 6 arrested the cells in S and G2/M phases, compound 3 arrested the cells in the G2/M phase, and these compounds also increased sub-G1 phase in the cell cycle. These results suggested that the effect of cell cycle arrest might be effective on both pols and topos activities. From these findings, the action mode of cholesterol derivatives as anti-cancer compounds is discussed.

Anti-tumor effect of orally administered spinach glycolipid fraction on implanted cancer cells, colon-26, in mice.

We succeeded in purifying a major glycolipid fraction from a green vegetable, spinach. This fraction consists mainly of three glycolipids: monogalactosyl diacylglycerol (MGDG), digalactosyl diacylglycerol (DGDG), and sulfoquinovosyl diacylglycerol (SQDG). In a previous study, we found that the glycolipid fraction inhibited DNA polymerase activity, cancer cell growth and tumor growth with subcutaneous injection. We aimed to clarify oral administration of the glycolipid fraction, suppressing colon adenocarcinoma (colon-26) tumor growth in mice. A tumor graft study showed that oral administration of 20 mg/kg glycolipid fraction for 2 weeks induced a 56.1% decrease in the solid tumor volume (P < 0.05) without any side-effects, such as loss of body weight or major organ failure, in mice. The glycolipid fraction induced the suppression of colon-26 tumor growth with inhibition of angiogenesis and the expression of cell proliferation marker proteins such as Ki-67, proliferating cell nuclear antigen (PCNA), and Cyclin E in the tumor tissue. These results suggest that the orally administered glycolipid fraction from spinach could suppress colon tumor growth in mice by inhibiting the activities of neovascularization and cancer cellular proliferation in tumor tissue.

Diallyl sulfides: Selective inhibitors of family X DNA polymerases from garlic (Allium sativum L.).

Diallyl sulfides, organosulfur compounds isolated from garlic (Allium sativum L.), selectively inhibit the activities of mammalian family X DNA polymerases (pols), such as pol ß, pol λ and terminal deoxynucleotidyl transferase (TdT), in vitro. The purified fraction (i.e., Sample-A) consisted of diallyl trisulfide, diallyl tetrasulfide and diallyl pentasulfide (molecular ratio: 5.3:3:1). Commercially purchased diallyl sulfides also inhibited the activities of family X pols, and the order of their effect was as follows: Sample-A>diallyl trisulfide>diallyl disulfide>diallyl monosulfide, suggesting that the number of sulfur atoms in the compounds might play an important structural role in enzyme inhibition. The suppression of human cancer cell (promyelocytic leukaemia cell line, HL-60) growth had the same tendency as the inhibition of pol X family among the compounds. Diallyl sulfides were suggested to bind to the pol ß-like region of family X pols.

Effect of dehydroaltenusin-C12 derivative, a selective DNA polymerase alpha inhibitor, on DNA replication in cultured cells.

Dehydroaltenusin is a selective inhibitor of mammalian DNA polymerase alpha (pol alpha) from a fungus (Alternaria tennuis). We have designed, synthesized, and characterized a derivative of dehydroaltenusin conjugated with a C12-alkyl side chain (dehydroaltenusin-C12 [C12]). C12 was the strongest pol alpha inhibitor in vitro. We introduced C12 into NIH3T3 cells with the help of a hypotonic shift, that is, a transient exposure of cultured cells in hypotonic buffer with small molecules which can not penetrate cells. The cells that took in C12 by hypotonic shift showed cell growth inhibition. At a low concentration (5 microM), DNA replication was inhibited and several large replication protein A (RPA) foci, which is different from dUTP foci. Furthermore, when C12 was incubated with aphidicolin, RPA foci were not observed in cells. Finally, these findings suggest that C12 inhibited DNA replication through pol alpha inhibition, and generated single-stranded DNA, resulted in uncoupling of the leading strand and lagging strand synthesis. These findings suggest that C12 could be more interesting as a molecule probe or anticancer agent than aphidicolin. C12 might provide novel markers for the development of antiproliferative drugs.

Beta-sitosterol-3-O-beta-D-glucopyranoside: a eukaryotic DNA polymerase lambda inhibitor.

Beta-sitosterol-3-O-beta-D-glucopyranoside (compound 1), a steroidal glycoside isolated from onion (Allium cepa L.) selectively inhibited the activity of mammalian DNA polymerase lambda (pol lambda) in vitro. The compound did not influence the activities of replicative DNA polymerases such as alpha, delta and epsilon, but also showed no effect even on the activity of pol beta which is thought to have a very similar three-dimensional structure to the pol beta-like region of pol lambda. Since parts of compound 1 such as beta-sitosterol (compound 2) and D-glucose (compound 3) did not influence the activities of any enzymes tested, the converted structure of compounds 2 and 3 might be important for pol lambda inhibition. The inhibitory effect of compound 1 on both intact pol lambda (i.e. residues 1-575) and a truncated pol lambda lacking the N-terminal BRCA1 C-terminus (BRCT) domain (133-575, del-1 pol lambda) was dose-dependent, and 50% inhibition was observed at a concentration of 9.1 and 5.4 microM, respectively. The compound 1-induced inhibition of del-1 pol lambda activity was non-competitive with respect to both the DNA template-primer and the dNTP substrate. On the basis of these results, the pol lambda inhibitory mechanism of compound 1 is discussed.

The effects of dehydroaltenusin, a novel mammalian DNA polymerase alpha inhibitor, on cell proliferation and cell cycle progression.

As described previously, a natural product isolated from fungus (Acremonium sp.), dehydroaltenusin, is an inhibitor of mammalian DNA polymerase alpha in vitro [Y. Mizushina, S. Kamisuki, T. Mizuno, M. Takemura, H. Asahara, S. Linn, T. Yamaguchi, A. Matsukage, F. Hanaoka, S. Yoshida, M. Saneyoshi, F. Sugawara, K. Sakaguchi, Dehydroaltenusin, a mammalian DNA polymerase alpha inhibitor, J. Biol. Chem. 275 (2000) 33957_33961]. In this study, we investigated the interaction of dehydroaltenusin with lipid bilayers using an in vitro liposome system, which is a model of the cell membrane, and found that approximately 4% of dehydroaltenusin was incorporated into liposomes. We also investigated the influence of dehydroaltenusin on cultured cancer cells. Dehydroaltenusin inhibited the growth of HeLa cells with an LD50 value of 38 microM, and as expected, S phase accumulation in the cell cycle. The total DNA polymerase activity of the extract of incubated cells with dehydroaltenusin was 23% lower than that of nontreated cells. Dehydroaltenusin increased cyclin E and cyclin A levels. In the analysis of the cell cycle using G1/S synchronized cells by employing hydroxyurea, the compound delayed both entry into the S phase and S phase progression. In a similar analysis using G2/M synchronized cells by employing nocodazole, the compound accumulated the cells at G1/S and inhibited entry into the S phase. Thus, the pharmacological abrogation of cell proliferation by dehydroaltenusin may prove to be an effective chemotherapeutic agent against tumors.

Effects of DNA polymerase inhibitory and antitumor activities of lipase-hydrolyzed glycolipid fractions from spinach.

We succeeded in purifying the major glycolipid fraction in the class of sulfoquinovosyl diacylglycerol, monogalactosyl diacylglycerol and digalactosyl diacylglycerol (DGDG) from a green vegetable, spinach (Spinacia oleracea L.). This glycolipid fraction was an inhibitor of DNA polymerases and a growth inhibitor of NUGC-3 human gastric cancer cells, and, interestingly, the activities were much stronger when the fraction was hydrolyzed by lipase. Glycolipids in the hydrolyzed fraction consisted of sulfoquinovosyl monoacylglycerol (SQMG), monogalactosyl monoacylglycerol (MGMG) and DGDG. In the in vivo antitumor assay using Greene's melanoma, the fraction containing SQMG, MGMG and DGDG showed to be a promising suppressor of solid tumors. Spinach glycolipid fraction might be a potent antitumor compound if directly injected into a tumor-carrying body, and this fraction may be a healthy food material that has antitumor activity.

Inhibitory effects of glycolipids fraction from spinach on mammalian DNA polymerase activity and human cancer cell proliferation.

We succeeded in purifying the fraction containing the major glycolipids in monogalactosyl diacylglycerol, digalactosyl diacylglycerol and sulfoquinovosyl diacylglycerol (SQDG) from dried vegetables. This glycolipids fraction was an inhibitor of DNA polymerase alpha (pol alpha) in vitro and also the proliferation of human cancer cells. In this study, eight common vegetables were investigated in terms of the glycolipids fraction, the amounts of major glycolipids, mammalian DNA polymerase inhibitory activity and antiproliferative activity toward human cancer cells. Green tea possessed the largest amount of glycolipids overall. Spinach contained the largest amount of SQDG, followed by parsley, green onion, chive, sweet pepper, green tea, carrot and garlic. Spinach had the strongest inhibitory effect on pol alpha activity and human cancer cell proliferation. A significant correlation was found between SQDG content and inhibition of DNA polymerase. Therefore, the inhibition of pol alpha activity by SQDG may lead to cell growth suppression. Of the six subspecies of spinach (Spinacia oleracea) tested, "Anna" had the largest amount of SQDG, strongest inhibitory activity toward DNA polymerase and greatest effect on human cancer cell proliferation. Based on these results, the glycolipids fraction from spinach is potentially a source of food material for a novel anticancer activity.