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Cancer chemopreventive activity of “rosin” constituents of Pinus spez. and their derivatives in two-stage mouse skin carcinogenesis test
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Phytomedicine 15 (2008) 985–992 www.elsevier.de/phymed
Cancer chemopreventive activity of ‘‘rosin’’ constituents of Pinus spez. and their derivatives in two-stage mouse skin carcinogenesis test Reiko Tanakaa,, Harukuni Tokudab, Yoichiro Ezakic a Department of Medicinal Chemistry, Osaka University of Pharmaceutical Science, 4-20-1 Nasahara, Takatsuki, Osaka 569-1094, Japan b Department of Molecular Biochemistry, Kyoto Prefectural University of Medicine, Kamigyo-ku, Kyoto 602-8566, Japan c Arakawa Chemical Industries Ltd., Tsukuba, Ibaraki 300-2611, Japan
Abstract Natural resin acids present in rosin of Pinus spez., including isopimaric acid (1), mercusic acid (2), neoabietic acid (3), dehydroabietic acid (4), and podocarpic acid (8), as well as resin acid derivatives 8b,9a,13a-H-tetrahydroabietic acid (5), 8a,9a,13a-H-tetrahydroabietic acid (6), 13a-H-D8-dihydroabietic acid (7), maleopimaric acid (9), and fumaropimaric acid (10), were studied for their possible inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). Compounds 1, 3, 4, 7, and 10 (IC50: 352, 330, 311, 340, and 349, respectively) exhibited strong inhibitory effects compared to the other compounds. Among these, 1, 4, and 7 were selected to examine their effects on in vivo two-stage mouse skin carcinogenesis induced by 7,12-dimethylbenz[a]anthracene (DMBA) as initiator and TPA as promoter. Treatment with compounds 4 and 7 (85 nmol) along with DMBA/TPA inhibited papilloma formation up to week 8 and the percentage of papilloma bearers in these two groups was approximately 80% at week 20. The average number of papillomas formed per mouse was 4.4 and 4.2 even at week 20 (p40.05). Compounds 4 and 7 exhibited high activity in the in vivo anti-tumor-promoting test. In addition, rosin was examined in vivo for its chemopreventive effect. Treatment with rosin (50 mmol) along with DMBA (100 mg)/TPA (1 mg) inhibited papilloma formation up to week 8 and the percentage of papilloma bearers in this group was less than 80% at week 20. The average number of papillomas formed per mouse in the rosin-treated group was 3.8 even at week 20 (p40.05). The in vivo two-stage mouse skin carcinogenesis test revealed that rosin possessed a pronounced anticarcinogenetic effect, and its high activity is due to the synergism of the diterpenes contained in it. r 2008 Elsevier GmbH. All rights reserved. Keywords: Rosin; Resin acid; Pinaceae; Isopimaric acid; Dehydroabietic acid; 13a-H-D8-dihydroabietic acid; EBV-EA activation; Two-stage mouse skin carcinogenesis test
Introduction Rosin is a widely used natural product that may still have unknown potential. Commercially produced by removing turpentine from the oleoresin of Pinus species, global rosin production amounts to approximately 1 Corresponding author.
E-mail address: [email protected] (R. Tanaka). 0944-7113/$ - see front matter r 2008 Elsevier GmbH. All rights reserved. doi:10.1016/j.phymed.2008.02.020
million metric tons per year. Rosin is used industrially as an agent in paper sizing, printing inks, soup, cosmetics, adhesives, emulsifiers, and medical supplies. It possesses hypocholesteromic activity (Fujita et al., 1991), antigastric ulcer activity (Ito et al., 1993), blood glucose lowering effect (Aicheret et al., 1999), and macrocyte clumping activity (Cheung et al., 1994). Rosin is mainly composed of five resin acids: abietic acid, neoabietic acid, palustric acid, pimaric acid, and isopimaric acid
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Fig. 1. Structures of resin acids contained in rosin.
(Fig. 1). The components of rosin were analyzed by gas chromatography and identified by comparison with authentic samples. It is known that some resin acids and their derivatives have various biological activities (Feliciano et al., 1993). In recent years, the search for more effective and safer agents for human cancer chemoprevention has gained momentum, and natural products from plants and their synthetic derivatives have been explored in the hope of creating new and better chemopreventive agents (Hong and Sporn, 1997; Sporn, 1999). In particular, there is an urgent need for chemopreventive agents that target the promotion stage of carcinogenesis indicated in the twoor multi-stage theory (Berenblum, 1941) because tumor initiation in human is difficult to avoid. The screening methods employed were a convenient primary in vitro assay for the inhibitory effect on Epstein-Barr virus early antigen (EBV-EA) activation induced by a wellknown tumor promoter, 12-O-tetradecanoylphorbol-13acetate (TPA) (Ito et al., 1981), and an in vivo two-stage mouse skin carcinogenesis test using 7,12-dimethylbenz[a]anthracene (DMBA) as an initiator and TPA as a tumor promoter (Tanaka et al., 2001). Many compounds that exhibited positive results in the in vitro assay have been shown to inhibit tumor promotion in the in vivo two-stage carcinogenesis test (Tanaka et al., 2003). Several biologically active terpenoids from natural sources have been reported to show significant anti-tumor-promoting activity in an in vivo two-stage mouse skin carcinogenesis test: abiesenonic acid methyl,
a chemical derivative of abieslactone isolated from the stem bark of Abies species (Pinaceae) (Takayasu et al., 1990); 15,16-bisnor-13-oxolabda-8(17),11E-dien19-oic acid (Tanaka et al., 2000) and 15-oxolabda8(17),11Z,13E-trien-19-oic acid (Iwamoto et al., 2001), which are labdane-type diterpenoids isolated from the stem bark of Thuja standishii (Cupressaceae); and 13a,14a-epoxy-3b-methoxyserratan- 21b-ol, 21a-hydroxy-3b-methoxyserrat-14-en-29-al (Tanaka et al., 2001), and serrat-14-en-3b,21b-diol (Tanaka et al., 2003), which are serratane-type triterpenoids isolated from the bark of Picea jezoensis (Sieb. et Zucc) Carr. var. jezoensis (Pinaceae) and Picea jezoensis (Sieb. et Zucc) Carr. var. hondoensis (Mayr) Rehder (Pinaceae). Rosin is commercially produced by removing turpentine from the oleoresin of Pinus species. We considered that rosin itself, resin acids, and their derivatives should be evaluated for their cancer chemoprotective activities. Some resin acids (e.g., abietic acid) are very unstable in air (Tanaka et al., 1997). Therefore, we chose five natural resin acids (1–4, 8) and their derivatives (5–7, 9, 10), all of which are relatively stable in air except for 3 (Fig. 2). Rosin, isopimaric acid (1), neoabietic acid (3), and dehydroabietic acid (4) were obtained from Pinus massonia collected in China. Mercusic acid (2) was obtained from rosin of Pinus merkusii (common name: Sumatran pine) collected in China and podocarpic acid (8) was obtained from Podocarpus dacrydioides in China. In this study, we used commercially available compounds. Other compounds used are resin acid
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Fig. 2. Structure of test compounds 1–10.
derivatives: compounds 5–7 were obtained from hydrogenated rosin, and 9 and 10 are maleic anhydride modified rosin and fumaric acid modified rosin, respectively. We present herein the results of in vitro and in vivo studies of the anti-tumor-promoting activities of these resin acids and their derivatives, as well as rosin itself. Inhibition of EBV-EA activation was examined in an in vitro assay and an in vivo two-stage mouse skin carcinogenesis test was performed using DMBA as initiator and TPA as promoter (Tanaka et al., 2001).
Materials and methods Chemicals TPA and DMBA were obtained from Sigma Chemical Co. (St. Louis, USA). Curcumin and b-carotene were
obtained from Wako Pure Chemical Industries (Osaka, Japan), and cell culture reagent, n-butanoic acid, and other reagents for bioassay were purchased from Nacalai Tesque, Inc. (Kyoto, Japan).
Test compounds Isopimaric acid (1), mercusic acid (2), neoabietic acid (3), and dehydroabietic acid (4) are natural resin acids. Compounds 1, 3, and 4 were obtained from Pinus massonia and compound 2 was obtained from the rosin of Pinus merkusii. Podocarpic acid (8) was purchased from Koch-Light Ltd. (Haverhill Suffolk, England). Resin acid derivatives, 8b,9a,13a-H-tetrahydroabietic acid (5), 8a,9a,13a-H-tetrahydroabietic acid (6), and 13a-H-D8-dihydroabietic acid (7) were obtained from hydrogenated rosin; and maleopimaric acid (9) and fumaropimaric acid (10) are maleic anhydride and fumaric acid modified rosins, respectively.
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Assay for inhibition of EBV-EA activation EBV-EA-positive serum from a patient with nasopharyngeal carcinoma (NPC) was a gift from the Department of Biochemistry, Oita Medical University. EBV-genome-carrying lymphoblastoid cells (Raji cells derived from Burkitt lymphoma) were cultured in 10% fetal bovine serum (FBS) in RPMI-1640 medium (Nissui). Inhibition of EBV-EA activation was assayed using Raji cells (virus nonproducer type), an EBV-genome-carrying human lymphoblastoid cell line, as described previously (Ito et al., 1981). Indicator cells (1 106/ml) were incubated at 37 1C for 48 h in medium (1 ml) containing n-butyric acid (4 mM) as trigger, TPA [32 pM ¼ 20 ng in 2 ml of dimethylsulfoxide (DMSO)] as inducer, and various amounts of test compounds dissolved in 5 ml of DMSO. Smears were made from the cell suspension. EBV-EA inducing cells were stained with high-titer EBV-EA-positive serum from the NPC patient and detected by an indirect immunofluorescence technique (Henle and Henle, 1966). In each assay, at least 500 cells were counted, and the number of stained cells (positive cells) was recorded. Triplicate assays were performed for each data point. Average EBV-EA induction of the test compound was expressed as a ratio to the positive control experiment (100%), which was carried out with n-butyric acid (4 mM) plus TPA (32 pM). In the experiments, EBV-EA induction was ordinarily around 35%, and this value was taken as the positive control (100%). n-Butyric acid (4 mM) alone induced less than 0.1% EA-positive cells. The viability of treated Raji cells was assayed by the Trypan blue staining method. Cell viability of TPA positive control was higher than 80%. Therefore, only compounds that induced less than 80% (% of control) of the EBVactivated cells (those with cell viability higher than 60%) were considered to be able to inhibit the activation caused by promoter substances. Student’s t-test was used for statistical analyses.
Animals Specific-pathogen-free (SPF) female ICR mice (6 weeks old, body weight: approx. 30 g) were obtained from Japan SLC Inc., Shizuoka, Japan. They were kept in three groups of five animals per polycarbonated cage in a temperature-controlled room at 2472 1C. Animals were fed an MP solid diet (Oriental Yeast Ltd., Chiba, Japan) and water ad libitum.
Two-stage mouse skin carcinogenesis test The back (2 8 cm2) of each mouse was shaved with surgical clippers and treated topically with DMBA (100 mg, 390 nmol) in acetone (0.1 ml) as an initiation
treatment. For group 1 (positive control group), 1 week after the initiation with DMBA, papilloma formation was promoted twice weekly by the application of TPA (1 mg, 1.7 nmol) in acetone (0.1 ml) on the skin. Groups II, III, IV, and V received a topical application of test samples, isopimaric acid (1), dehydroabietic acid (4), and 13a-H-D8-dihydroabietic acid (7) (85 nmol each), and rosin extract (50 mg) in acetone (0.1 ml) 1 h before the promotion treatment. The percentage of papilloma bearers and the average number of papillomas per mouse were observed weekly for 20 weeks. A pathologist checked the tumor type by histological examination. Statistical significance was determined using Student’s t-test.
Results and discussion Rosin is amber-colored, transparent, glass-shaped resin, and its softening point is 70–80 1C. Rosin contains approximately 80% abietane-type diterpenoids and 20% pimarane-type diterpenoids, and its main constituent is abietic acid. However, abietic acid is very unstable in air (Tanaka et al., 1997). The 10 natural products and their derivatives selected for this study are relatively stable in air except for 3. Isopimaric acid (1), mercusic acid (2), neoabietic acid (3), dehydroabietic acid (4), 8b,9a13a-H-tetrahydroabietic acid (5), 8a,9a13a-H-tetrahydroabietic acid (6), 13a-H-D8dihydroabietic acid (7), maleopimaric acid (9), and fumaropimaric acid (10) were prepared by known methods (Baldwin et al., 1958; Weissman, 1974; Leoblich and Lawrence, 1956; Halbrook and Lawrence, 1966; Huffman et al., 1966; Murai et al., 1976; Lawrence and Eckhardt, 1953). Podocarpic acid (8) was purchased from Koch-Light Ltd. (Haverhill, Suffolk, England). Their structures were determined by optical rotation measurement, elemental analyses, analyses of 1H and 13 C NMR spectra, and EIMS. Compounds 1–10 were evaluated for their in vitro inhibitory effect on EBV-EA activation induced by TPA. Their effects on the viability of Raji cells, and their 50% inhibitory concentration (IC50) values are shown in Table 1, together with data for curcumin and b-carotene. All the compounds tested displayed inhibitory effects with IC50 values of 311–459 mol ratio/32 pmol TPA. The IC50 values of 1 (IC50 352), 3 (IC50 330), 4 (IC50 311), 7 (IC50 340), and 10 (IC50 349) were comparable to those of reference compounds, curcumin (IC50 341), which has been studied extensively for cancer chemoprevention in animal models (Kawamori et al., 1999), and b-carotene (IC50 397), a vitamin A precursor that has been intensively examined in in vitro, in vivo, and epidemiological tests for possible use in cancer chemoprevention (Murakami et al., 1996). Most of the abietane-type diterpenoids (compounds 3–7, 9, 10) showed strong
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Table 1.
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Relative ratioa of EBV-EA activation with respect to positive control (100%) in the presence of compounds 1–10
Compounds
% to control (% viability) Concentration (mol ratio/TPA) 1000 mol ratio/TPAb
1 2 3 4 5 6 7 8 9 10 Curcumin b-Carotene
0 4.1 0 2.9 0 0 0 0 0 0 0 8.6
(70)c (60) (70) (70) (70) (70) (70) (70) (60) (60) (60) (70)
IC50 (mol ratio/32 pmol TPA) 500
100
10
36.9 47.0 34.1 32.7 39.8 38.9 35.3 39.8 37.6 36.2 22.8 34.2
74.8 78.2 71.6 69.8 77.4 77.9 72.5 78.5 75.0 74.0 81.7 82.1
95.9 98 91.7 97.9 96.8 97 95.9 98.1 96.8 95.3 100 100
352 459 330 311 383 375 340 382 360 349 341 397
a
Values represent percentages relative to the positive control value (100%). TPA concentration was 20 ng/ml (32 pmol/ml). c Values in parentheses are the percentage viability of Raji cells. b
inhibitory effects in vitro EBV-EA assay. Pimarane-type diterpenoid, isopimaric acid (1) showed strong inhibitory effect (IC50 352), while the labdane-type diterpenoid, mercusic acid (2) showed weaker effect than abietane- or pimarane-type diterpenoids (IC50 459). Labdane-type diterpenoids showed strong inhibitory effects on EBV-EA activation in our previous study (Tanaka et al., 2000; Iwamoto et al., 2001). The two carboxylic acids in 2 may have weakened its activity. On the other hand, compounds 5 (IC50 383) and 6 (IC50 375) are isomers in terms of the configuration of B/C rings; 5 has B/C trans configuration, while 6 has B/C cis configuration. Despite this structural difference, there was little difference in the activity of those two compounds. Compound 7 had an 8:9 double bond that was absent in compounds 5 and 6, and displayed stronger inhibitory effect relative to 5 and 6. The inhibitory effect was preserved even in compounds 9 (IC50 360) and 10 (IC50 349), both of which had a bulky substituent in the abietane skeleton. The relative ratios of compounds 4 and 3 with respect to TPA (100%) were 2.9, 32.7, 69.8, and 97.9 and 0, 34.1, 71.6, and 91.7% at concentrations of 1000, 500, 100, and 10 mol ratio/TPA, respectively (Table 1), meaning 97.1%, 67.3%, 30.2%, and 2.1% (compound 4), and 100%, 65.9%, 28.4%, and 8.3% (compound 3) inhibition of TPA-induced EBVEA activation. The percentage viability of Raji cells treated with the test compounds was 60–70% at the highest concentration of 1000 mol ratio/TPA, suggesting that the test compounds showed moderate cytotoxicities against in vitro cell lines (Table 1). Because compound 4 is a representative compound of resin acids and its in vitro inhibitory effect is the strongest among the compounds examined, we chose it for use in in vivo
experiments. Although compound 3 showed the highest activity, it could not tolerate long-term in vivo test due to instability. Compound 7 was also chosen for the in vivo test, because it showed strong inhibitory effect in the in vitro assay and a large amount could be easily synthesized. As compound 1 is the only pimaranetype diterpenoid among the 10 compounds examined, we selected it for comparison with abietane-type diterpenoids. At present, the mechanism of the tumor-promoterinduced activation of EBV-EA is still not clear. One possible mechanism would involve the activation of protein kinase C (PKC). TPA can activate PKC and the cellular receptor could be PKC itself (Nishizuka, 1986; Bomser et al., 2000). Our findings and those of other groups have demonstrated that the results of EBV-EA assay well correlate with that of the in vivo anti-tumor test. Based on the results, we selected compounds 1, 4, and 7 to examine their activities in the in vivo two-stage mouse skin carcinogenesis test with DMBA and TPA. The protocol is shown in Fig. 3. Dehydroabietic acid (4) is the main constituent of such coniferous trees as Pinus, Picea, Larix, and Abies, and therefore, the in vivo test of 4 is considered to be meaningful and important. During the in vivo test, body-weight gain of the mice was not influenced by the treatment with the test compounds and toxic effects, such as lesional damage and inflammation on mouse skin areas topically treated with the test compounds, were not observed (Fig. 4). Fig. 4 demonstrates the results of papilloma formation in the skin of mice treated with compounds 1, 4 and 7. As shown in Fig. 4(A), papilloma appeared as early as at week 6 in the positive control group treated with
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Fig. 3. Protocol to two-stage mouse skin carcinogenesis test.
Fig. 4. Inhibition effects of rosin in DMBA-TPA mouse skin carcinogenesis test. Tumor formation in all mice was initiated with DMBA (390 nmol) and promoted with TPA (1.7 nmol) twice weekly, starting from week 1 after initiation. (A) Percentage of mice with papillomas. (B) Average number of papillomas per mouse. K control (TPA alone); B TPA+85 nmol of 1; n TPA+85 nmol of 4; E TPA+85 nmol of 7. a,bStatistically different from control group (P40.05).
DMBA (390 nmol) and TPA (1.7 nmol, twice/week), and the percentage of papilloma bearers increased rapidly to reach 100% after week 10. On the other hand, treatment with compounds 4 and 7 (85 nmol) along with DMBA/ TPA inhibited papilloma formation up to week 8 and the percentage of papilloma bearers in these two groups was approximately 80% at week 20. As shown in Fig. 4(B), in the positive control group treated with DMBA/TPA, the average number of papillomas formed per mouse increased rapidly after week 6 to reach 9.3 papillomas/mouse at week 20, whereas mice treated with
compounds 7 and 4 bore only 4.2 and 4.4 papillomas, respectively, even at week 20 (p40.05). Konishi et al. (1998) reported the inhibitory effect of a beyeren-type diterpene, ent-3b-hydroxy-15-beyeren-2-one, isolated from Excoecaria agallocha in an in vivo test under conditions similar to those described above. They showed that on treatment with the diterpenoid, the percentage of papilloma bearers was approximately 70% and the number of papillomas formed per mouse was approximately 4.0 at week 18 (Konishi et al., 1998). Our results for compounds 7 and 4 were almost the same
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Fig. 5. Inhibition effects of rosin in DMBA-TPA mouse skin carcinogenesis test. Tumor formation in all mice was initiated with DMBA (100 mg) and promoted with TPA (1 mg) twice weekly, starting from week 1 after initiation. (A) Percentage of mice with papillomas. (B) Average number of papillomas per mouse. K control (TPA alone); & TPA+50 mg of rosin. cStatistically different from control group (P40.05).
as those of ent-3b-hydroxy-15-beyeren-2-one and the positive control, curcumin (73.3% and 4.4 papillomas at week 20) (Kawamori et al., 1999). However, these compounds showed slightly lower activity than 15,16bisnor-13-oxolabda-8(17),11E-dien-19-oic acid (67% and 1.9 papillomas at week 20) and 15-oxolabda8(17),11E,13E-trien-19-oic acid (73% and 2.4 papillomas at week 20) from Thuja standishii (Tanaka et al., 2000; Iwamoto et al., 2001). Meanwhile, the inhibitory activity of compound 1 was low. Our study demonstrated that abietane-type diterpenoids, including compounds 4 and 7, may be appropriate lead compounds with anti-tumor-promoting agents. Rosin itself was found to be active in the in vivo twostage mouse skin carcinogenesis test with DMBA and TPA. Fig. 5 demonstrates the results of papilloma formation in the skin of mice treated with 50 mg of rosin in comparison with the positive control group. Treatment with rosin (50 mg/0.1 ml) along with DMBA (100 mg) /TPA (1 mg) inhibited the formation of papillomas up to week 8 and the percentage of papilloma bearers in this group was less than 80% at week 20 (Fig. 5(A)). As shown in Fig. 5B, the number of papillomas formed per mouse in the rosin-treated group was 3.8 even at week 20 (p40.05). These results suggest that rosin itself showed a strong inhibitory effect. Its high activity is due to synergism of the diterpenes contained in it. The fact that rosin is not toxic and is found ubiquitously in nature, and that its annual
commercial production is 1 million metric tons, makes it worthy of study as a cancer chemopreventive agent. Resin acids present in rosin are easy to obtain and highly affordable. Pine trees are attracting attention for their possible benefits as health food and supplement, one example being French pine that produces pycnogenol.
Acknowledgments This study was supported by a Grant-in-Aid for High Technology from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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Phytomedicine 15 (2008) 985–992 www.elsevier.de/phymed
Cancer chemopreventive activity of ‘‘rosin’’ constituents of Pinus spez. and their derivatives in two-stage mouse skin carcinogenesis test Reiko Tanakaa,, Harukuni Tokudab, Yoichiro Ezakic a Department of Medicinal Chemistry, Osaka University of Pharmaceutical Science, 4-20-1 Nasahara, Takatsuki, Osaka 569-1094, Japan b Department of Molecular Biochemistry, Kyoto Prefectural University of Medicine, Kamigyo-ku, Kyoto 602-8566, Japan c Arakawa Chemical Industries Ltd., Tsukuba, Ibaraki 300-2611, Japan
Abstract Natural resin acids present in rosin of Pinus spez., including isopimaric acid (1), mercusic acid (2), neoabietic acid (3), dehydroabietic acid (4), and podocarpic acid (8), as well as resin acid derivatives 8b,9a,13a-H-tetrahydroabietic acid (5), 8a,9a,13a-H-tetrahydroabietic acid (6), 13a-H-D8-dihydroabietic acid (7), maleopimaric acid (9), and fumaropimaric acid (10), were studied for their possible inhibitory effects on Epstein-Barr virus early antigen (EBV-EA) activation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA). Compounds 1, 3, 4, 7, and 10 (IC50: 352, 330, 311, 340, and 349, respectively) exhibited strong inhibitory effects compared to the other compounds. Among these, 1, 4, and 7 were selected to examine their effects on in vivo two-stage mouse skin carcinogenesis induced by 7,12-dimethylbenz[a]anthracene (DMBA) as initiator and TPA as promoter. Treatment with compounds 4 and 7 (85 nmol) along with DMBA/TPA inhibited papilloma formation up to week 8 and the percentage of papilloma bearers in these two groups was approximately 80% at week 20. The average number of papillomas formed per mouse was 4.4 and 4.2 even at week 20 (p40.05). Compounds 4 and 7 exhibited high activity in the in vivo anti-tumor-promoting test. In addition, rosin was examined in vivo for its chemopreventive effect. Treatment with rosin (50 mmol) along with DMBA (100 mg)/TPA (1 mg) inhibited papilloma formation up to week 8 and the percentage of papilloma bearers in this group was less than 80% at week 20. The average number of papillomas formed per mouse in the rosin-treated group was 3.8 even at week 20 (p40.05). The in vivo two-stage mouse skin carcinogenesis test revealed that rosin possessed a pronounced anticarcinogenetic effect, and its high activity is due to the synergism of the diterpenes contained in it. r 2008 Elsevier GmbH. All rights reserved. Keywords: Rosin; Resin acid; Pinaceae; Isopimaric acid; Dehydroabietic acid; 13a-H-D8-dihydroabietic acid; EBV-EA activation; Two-stage mouse skin carcinogenesis test
Introduction Rosin is a widely used natural product that may still have unknown potential. Commercially produced by removing turpentine from the oleoresin of Pinus species, global rosin production amounts to approximately 1 Corresponding author.
E-mail address: [email protected] (R. Tanaka). 0944-7113/$ - see front matter r 2008 Elsevier GmbH. All rights reserved. doi:10.1016/j.phymed.2008.02.020
million metric tons per year. Rosin is used industrially as an agent in paper sizing, printing inks, soup, cosmetics, adhesives, emulsifiers, and medical supplies. It possesses hypocholesteromic activity (Fujita et al., 1991), antigastric ulcer activity (Ito et al., 1993), blood glucose lowering effect (Aicheret et al., 1999), and macrocyte clumping activity (Cheung et al., 1994). Rosin is mainly composed of five resin acids: abietic acid, neoabietic acid, palustric acid, pimaric acid, and isopimaric acid
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Fig. 1. Structures of resin acids contained in rosin.
(Fig. 1). The components of rosin were analyzed by gas chromatography and identified by comparison with authentic samples. It is known that some resin acids and their derivatives have various biological activities (Feliciano et al., 1993). In recent years, the search for more effective and safer agents for human cancer chemoprevention has gained momentum, and natural products from plants and their synthetic derivatives have been explored in the hope of creating new and better chemopreventive agents (Hong and Sporn, 1997; Sporn, 1999). In particular, there is an urgent need for chemopreventive agents that target the promotion stage of carcinogenesis indicated in the twoor multi-stage theory (Berenblum, 1941) because tumor initiation in human is difficult to avoid. The screening methods employed were a convenient primary in vitro assay for the inhibitory effect on Epstein-Barr virus early antigen (EBV-EA) activation induced by a wellknown tumor promoter, 12-O-tetradecanoylphorbol-13acetate (TPA) (Ito et al., 1981), and an in vivo two-stage mouse skin carcinogenesis test using 7,12-dimethylbenz[a]anthracene (DMBA) as an initiator and TPA as a tumor promoter (Tanaka et al., 2001). Many compounds that exhibited positive results in the in vitro assay have been shown to inhibit tumor promotion in the in vivo two-stage carcinogenesis test (Tanaka et al., 2003). Several biologically active terpenoids from natural sources have been reported to show significant anti-tumor-promoting activity in an in vivo two-stage mouse skin carcinogenesis test: abiesenonic acid methyl,
a chemical derivative of abieslactone isolated from the stem bark of Abies species (Pinaceae) (Takayasu et al., 1990); 15,16-bisnor-13-oxolabda-8(17),11E-dien19-oic acid (Tanaka et al., 2000) and 15-oxolabda8(17),11Z,13E-trien-19-oic acid (Iwamoto et al., 2001), which are labdane-type diterpenoids isolated from the stem bark of Thuja standishii (Cupressaceae); and 13a,14a-epoxy-3b-methoxyserratan- 21b-ol, 21a-hydroxy-3b-methoxyserrat-14-en-29-al (Tanaka et al., 2001), and serrat-14-en-3b,21b-diol (Tanaka et al., 2003), which are serratane-type triterpenoids isolated from the bark of Picea jezoensis (Sieb. et Zucc) Carr. var. jezoensis (Pinaceae) and Picea jezoensis (Sieb. et Zucc) Carr. var. hondoensis (Mayr) Rehder (Pinaceae). Rosin is commercially produced by removing turpentine from the oleoresin of Pinus species. We considered that rosin itself, resin acids, and their derivatives should be evaluated for their cancer chemoprotective activities. Some resin acids (e.g., abietic acid) are very unstable in air (Tanaka et al., 1997). Therefore, we chose five natural resin acids (1–4, 8) and their derivatives (5–7, 9, 10), all of which are relatively stable in air except for 3 (Fig. 2). Rosin, isopimaric acid (1), neoabietic acid (3), and dehydroabietic acid (4) were obtained from Pinus massonia collected in China. Mercusic acid (2) was obtained from rosin of Pinus merkusii (common name: Sumatran pine) collected in China and podocarpic acid (8) was obtained from Podocarpus dacrydioides in China. In this study, we used commercially available compounds. Other compounds used are resin acid
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Fig. 2. Structure of test compounds 1–10.
derivatives: compounds 5–7 were obtained from hydrogenated rosin, and 9 and 10 are maleic anhydride modified rosin and fumaric acid modified rosin, respectively. We present herein the results of in vitro and in vivo studies of the anti-tumor-promoting activities of these resin acids and their derivatives, as well as rosin itself. Inhibition of EBV-EA activation was examined in an in vitro assay and an in vivo two-stage mouse skin carcinogenesis test was performed using DMBA as initiator and TPA as promoter (Tanaka et al., 2001).
Materials and methods Chemicals TPA and DMBA were obtained from Sigma Chemical Co. (St. Louis, USA). Curcumin and b-carotene were
obtained from Wako Pure Chemical Industries (Osaka, Japan), and cell culture reagent, n-butanoic acid, and other reagents for bioassay were purchased from Nacalai Tesque, Inc. (Kyoto, Japan).
Test compounds Isopimaric acid (1), mercusic acid (2), neoabietic acid (3), and dehydroabietic acid (4) are natural resin acids. Compounds 1, 3, and 4 were obtained from Pinus massonia and compound 2 was obtained from the rosin of Pinus merkusii. Podocarpic acid (8) was purchased from Koch-Light Ltd. (Haverhill Suffolk, England). Resin acid derivatives, 8b,9a,13a-H-tetrahydroabietic acid (5), 8a,9a,13a-H-tetrahydroabietic acid (6), and 13a-H-D8-dihydroabietic acid (7) were obtained from hydrogenated rosin; and maleopimaric acid (9) and fumaropimaric acid (10) are maleic anhydride and fumaric acid modified rosins, respectively.
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Assay for inhibition of EBV-EA activation EBV-EA-positive serum from a patient with nasopharyngeal carcinoma (NPC) was a gift from the Department of Biochemistry, Oita Medical University. EBV-genome-carrying lymphoblastoid cells (Raji cells derived from Burkitt lymphoma) were cultured in 10% fetal bovine serum (FBS) in RPMI-1640 medium (Nissui). Inhibition of EBV-EA activation was assayed using Raji cells (virus nonproducer type), an EBV-genome-carrying human lymphoblastoid cell line, as described previously (Ito et al., 1981). Indicator cells (1 106/ml) were incubated at 37 1C for 48 h in medium (1 ml) containing n-butyric acid (4 mM) as trigger, TPA [32 pM ¼ 20 ng in 2 ml of dimethylsulfoxide (DMSO)] as inducer, and various amounts of test compounds dissolved in 5 ml of DMSO. Smears were made from the cell suspension. EBV-EA inducing cells were stained with high-titer EBV-EA-positive serum from the NPC patient and detected by an indirect immunofluorescence technique (Henle and Henle, 1966). In each assay, at least 500 cells were counted, and the number of stained cells (positive cells) was recorded. Triplicate assays were performed for each data point. Average EBV-EA induction of the test compound was expressed as a ratio to the positive control experiment (100%), which was carried out with n-butyric acid (4 mM) plus TPA (32 pM). In the experiments, EBV-EA induction was ordinarily around 35%, and this value was taken as the positive control (100%). n-Butyric acid (4 mM) alone induced less than 0.1% EA-positive cells. The viability of treated Raji cells was assayed by the Trypan blue staining method. Cell viability of TPA positive control was higher than 80%. Therefore, only compounds that induced less than 80% (% of control) of the EBVactivated cells (those with cell viability higher than 60%) were considered to be able to inhibit the activation caused by promoter substances. Student’s t-test was used for statistical analyses.
Animals Specific-pathogen-free (SPF) female ICR mice (6 weeks old, body weight: approx. 30 g) were obtained from Japan SLC Inc., Shizuoka, Japan. They were kept in three groups of five animals per polycarbonated cage in a temperature-controlled room at 2472 1C. Animals were fed an MP solid diet (Oriental Yeast Ltd., Chiba, Japan) and water ad libitum.
Two-stage mouse skin carcinogenesis test The back (2 8 cm2) of each mouse was shaved with surgical clippers and treated topically with DMBA (100 mg, 390 nmol) in acetone (0.1 ml) as an initiation
treatment. For group 1 (positive control group), 1 week after the initiation with DMBA, papilloma formation was promoted twice weekly by the application of TPA (1 mg, 1.7 nmol) in acetone (0.1 ml) on the skin. Groups II, III, IV, and V received a topical application of test samples, isopimaric acid (1), dehydroabietic acid (4), and 13a-H-D8-dihydroabietic acid (7) (85 nmol each), and rosin extract (50 mg) in acetone (0.1 ml) 1 h before the promotion treatment. The percentage of papilloma bearers and the average number of papillomas per mouse were observed weekly for 20 weeks. A pathologist checked the tumor type by histological examination. Statistical significance was determined using Student’s t-test.
Results and discussion Rosin is amber-colored, transparent, glass-shaped resin, and its softening point is 70–80 1C. Rosin contains approximately 80% abietane-type diterpenoids and 20% pimarane-type diterpenoids, and its main constituent is abietic acid. However, abietic acid is very unstable in air (Tanaka et al., 1997). The 10 natural products and their derivatives selected for this study are relatively stable in air except for 3. Isopimaric acid (1), mercusic acid (2), neoabietic acid (3), dehydroabietic acid (4), 8b,9a13a-H-tetrahydroabietic acid (5), 8a,9a13a-H-tetrahydroabietic acid (6), 13a-H-D8dihydroabietic acid (7), maleopimaric acid (9), and fumaropimaric acid (10) were prepared by known methods (Baldwin et al., 1958; Weissman, 1974; Leoblich and Lawrence, 1956; Halbrook and Lawrence, 1966; Huffman et al., 1966; Murai et al., 1976; Lawrence and Eckhardt, 1953). Podocarpic acid (8) was purchased from Koch-Light Ltd. (Haverhill, Suffolk, England). Their structures were determined by optical rotation measurement, elemental analyses, analyses of 1H and 13 C NMR spectra, and EIMS. Compounds 1–10 were evaluated for their in vitro inhibitory effect on EBV-EA activation induced by TPA. Their effects on the viability of Raji cells, and their 50% inhibitory concentration (IC50) values are shown in Table 1, together with data for curcumin and b-carotene. All the compounds tested displayed inhibitory effects with IC50 values of 311–459 mol ratio/32 pmol TPA. The IC50 values of 1 (IC50 352), 3 (IC50 330), 4 (IC50 311), 7 (IC50 340), and 10 (IC50 349) were comparable to those of reference compounds, curcumin (IC50 341), which has been studied extensively for cancer chemoprevention in animal models (Kawamori et al., 1999), and b-carotene (IC50 397), a vitamin A precursor that has been intensively examined in in vitro, in vivo, and epidemiological tests for possible use in cancer chemoprevention (Murakami et al., 1996). Most of the abietane-type diterpenoids (compounds 3–7, 9, 10) showed strong
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Table 1.
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Relative ratioa of EBV-EA activation with respect to positive control (100%) in the presence of compounds 1–10
Compounds
% to control (% viability) Concentration (mol ratio/TPA) 1000 mol ratio/TPAb
1 2 3 4 5 6 7 8 9 10 Curcumin b-Carotene
0 4.1 0 2.9 0 0 0 0 0 0 0 8.6
(70)c (60) (70) (70) (70) (70) (70) (70) (60) (60) (60) (70)
IC50 (mol ratio/32 pmol TPA) 500
100
10
36.9 47.0 34.1 32.7 39.8 38.9 35.3 39.8 37.6 36.2 22.8 34.2
74.8 78.2 71.6 69.8 77.4 77.9 72.5 78.5 75.0 74.0 81.7 82.1
95.9 98 91.7 97.9 96.8 97 95.9 98.1 96.8 95.3 100 100
352 459 330 311 383 375 340 382 360 349 341 397
a
Values represent percentages relative to the positive control value (100%). TPA concentration was 20 ng/ml (32 pmol/ml). c Values in parentheses are the percentage viability of Raji cells. b
inhibitory effects in vitro EBV-EA assay. Pimarane-type diterpenoid, isopimaric acid (1) showed strong inhibitory effect (IC50 352), while the labdane-type diterpenoid, mercusic acid (2) showed weaker effect than abietane- or pimarane-type diterpenoids (IC50 459). Labdane-type diterpenoids showed strong inhibitory effects on EBV-EA activation in our previous study (Tanaka et al., 2000; Iwamoto et al., 2001). The two carboxylic acids in 2 may have weakened its activity. On the other hand, compounds 5 (IC50 383) and 6 (IC50 375) are isomers in terms of the configuration of B/C rings; 5 has B/C trans configuration, while 6 has B/C cis configuration. Despite this structural difference, there was little difference in the activity of those two compounds. Compound 7 had an 8:9 double bond that was absent in compounds 5 and 6, and displayed stronger inhibitory effect relative to 5 and 6. The inhibitory effect was preserved even in compounds 9 (IC50 360) and 10 (IC50 349), both of which had a bulky substituent in the abietane skeleton. The relative ratios of compounds 4 and 3 with respect to TPA (100%) were 2.9, 32.7, 69.8, and 97.9 and 0, 34.1, 71.6, and 91.7% at concentrations of 1000, 500, 100, and 10 mol ratio/TPA, respectively (Table 1), meaning 97.1%, 67.3%, 30.2%, and 2.1% (compound 4), and 100%, 65.9%, 28.4%, and 8.3% (compound 3) inhibition of TPA-induced EBVEA activation. The percentage viability of Raji cells treated with the test compounds was 60–70% at the highest concentration of 1000 mol ratio/TPA, suggesting that the test compounds showed moderate cytotoxicities against in vitro cell lines (Table 1). Because compound 4 is a representative compound of resin acids and its in vitro inhibitory effect is the strongest among the compounds examined, we chose it for use in in vivo
experiments. Although compound 3 showed the highest activity, it could not tolerate long-term in vivo test due to instability. Compound 7 was also chosen for the in vivo test, because it showed strong inhibitory effect in the in vitro assay and a large amount could be easily synthesized. As compound 1 is the only pimaranetype diterpenoid among the 10 compounds examined, we selected it for comparison with abietane-type diterpenoids. At present, the mechanism of the tumor-promoterinduced activation of EBV-EA is still not clear. One possible mechanism would involve the activation of protein kinase C (PKC). TPA can activate PKC and the cellular receptor could be PKC itself (Nishizuka, 1986; Bomser et al., 2000). Our findings and those of other groups have demonstrated that the results of EBV-EA assay well correlate with that of the in vivo anti-tumor test. Based on the results, we selected compounds 1, 4, and 7 to examine their activities in the in vivo two-stage mouse skin carcinogenesis test with DMBA and TPA. The protocol is shown in Fig. 3. Dehydroabietic acid (4) is the main constituent of such coniferous trees as Pinus, Picea, Larix, and Abies, and therefore, the in vivo test of 4 is considered to be meaningful and important. During the in vivo test, body-weight gain of the mice was not influenced by the treatment with the test compounds and toxic effects, such as lesional damage and inflammation on mouse skin areas topically treated with the test compounds, were not observed (Fig. 4). Fig. 4 demonstrates the results of papilloma formation in the skin of mice treated with compounds 1, 4 and 7. As shown in Fig. 4(A), papilloma appeared as early as at week 6 in the positive control group treated with
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Fig. 3. Protocol to two-stage mouse skin carcinogenesis test.
Fig. 4. Inhibition effects of rosin in DMBA-TPA mouse skin carcinogenesis test. Tumor formation in all mice was initiated with DMBA (390 nmol) and promoted with TPA (1.7 nmol) twice weekly, starting from week 1 after initiation. (A) Percentage of mice with papillomas. (B) Average number of papillomas per mouse. K control (TPA alone); B TPA+85 nmol of 1; n TPA+85 nmol of 4; E TPA+85 nmol of 7. a,bStatistically different from control group (P40.05).
DMBA (390 nmol) and TPA (1.7 nmol, twice/week), and the percentage of papilloma bearers increased rapidly to reach 100% after week 10. On the other hand, treatment with compounds 4 and 7 (85 nmol) along with DMBA/ TPA inhibited papilloma formation up to week 8 and the percentage of papilloma bearers in these two groups was approximately 80% at week 20. As shown in Fig. 4(B), in the positive control group treated with DMBA/TPA, the average number of papillomas formed per mouse increased rapidly after week 6 to reach 9.3 papillomas/mouse at week 20, whereas mice treated with
compounds 7 and 4 bore only 4.2 and 4.4 papillomas, respectively, even at week 20 (p40.05). Konishi et al. (1998) reported the inhibitory effect of a beyeren-type diterpene, ent-3b-hydroxy-15-beyeren-2-one, isolated from Excoecaria agallocha in an in vivo test under conditions similar to those described above. They showed that on treatment with the diterpenoid, the percentage of papilloma bearers was approximately 70% and the number of papillomas formed per mouse was approximately 4.0 at week 18 (Konishi et al., 1998). Our results for compounds 7 and 4 were almost the same
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Fig. 5. Inhibition effects of rosin in DMBA-TPA mouse skin carcinogenesis test. Tumor formation in all mice was initiated with DMBA (100 mg) and promoted with TPA (1 mg) twice weekly, starting from week 1 after initiation. (A) Percentage of mice with papillomas. (B) Average number of papillomas per mouse. K control (TPA alone); & TPA+50 mg of rosin. cStatistically different from control group (P40.05).
as those of ent-3b-hydroxy-15-beyeren-2-one and the positive control, curcumin (73.3% and 4.4 papillomas at week 20) (Kawamori et al., 1999). However, these compounds showed slightly lower activity than 15,16bisnor-13-oxolabda-8(17),11E-dien-19-oic acid (67% and 1.9 papillomas at week 20) and 15-oxolabda8(17),11E,13E-trien-19-oic acid (73% and 2.4 papillomas at week 20) from Thuja standishii (Tanaka et al., 2000; Iwamoto et al., 2001). Meanwhile, the inhibitory activity of compound 1 was low. Our study demonstrated that abietane-type diterpenoids, including compounds 4 and 7, may be appropriate lead compounds with anti-tumor-promoting agents. Rosin itself was found to be active in the in vivo twostage mouse skin carcinogenesis test with DMBA and TPA. Fig. 5 demonstrates the results of papilloma formation in the skin of mice treated with 50 mg of rosin in comparison with the positive control group. Treatment with rosin (50 mg/0.1 ml) along with DMBA (100 mg) /TPA (1 mg) inhibited the formation of papillomas up to week 8 and the percentage of papilloma bearers in this group was less than 80% at week 20 (Fig. 5(A)). As shown in Fig. 5B, the number of papillomas formed per mouse in the rosin-treated group was 3.8 even at week 20 (p40.05). These results suggest that rosin itself showed a strong inhibitory effect. Its high activity is due to synergism of the diterpenes contained in it. The fact that rosin is not toxic and is found ubiquitously in nature, and that its annual
commercial production is 1 million metric tons, makes it worthy of study as a cancer chemopreventive agent. Resin acids present in rosin are easy to obtain and highly affordable. Pine trees are attracting attention for their possible benefits as health food and supplement, one example being French pine that produces pycnogenol.
Acknowledgments This study was supported by a Grant-in-Aid for High Technology from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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