Chapter 17
Jasmonates: Plant Stress Hormones as Anticancer Agents Downloaded by CORNELL UNIV on May 16, 2012 | http://pubs.acs.org Publication Date (Web): March 6, 2012 | doi: 10.1021/bk-2012-1093.ch017
Dorit Reischer-Pelech* and Eliezer Flescher Department of Clinical Microbiology and Immunology, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel *E-mail:
[email protected] Jasmonates are a group of plant stress hormones which are among the most potent regulators of defense-related mechanisms in plants. In recent years, several groups have reported that jasmonates are endowed with anti-cancer activities both in vitro and in vivo. Jasmonates were shown to induce death and inhibit the proliferation and the migration of various cancer cell types, including drug-resistant cells. Moreover, jasmonates were shown to impair the angiogenic process, which is essential for tumor progression. In line with their anti-cancer activities in vitro, jasmonates were shown to increase the survival of lymphoma-bearing mice and to inhibit the development of lung metastases in a mouse melanoma model. Importantly, jasmonates are highly selective towards cancer cells and have little or no effect on normal cells, creating a wide therapeutic window. Recently, a first-in-man study demonstrated that methyl jasmonate has a beneficial effect in treating human pre-cancerous and cancerous skin lesions. Several mechanisms were shown to mediate the anti-cancer activities of jasmonates. These include: direct perturbation of mitochondria, production of reactive oxygen species, induction of cellular differentiation, inhibition of aldo-keto reductases, upregulation of several pro-apoptotic proteins and downregulation of anti-apoptotic proteins. A number of research groups have taken the natural jasmonate compounds as a starting point to prepare and evaluate a wide variety of synthetic jasmonate derivatives. Several of these © 2012 American Chemical Society In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
derivatives exhibited enhanced anti-cancer activities in vitro and in vivo. While the vast majority of studies on jasmonates as potential drugs have been performed in the cancer arena, these compounds have also been evaluated as anti-parasitic and anti-inflammatory agents. In conclusion, jasmonates present a unique class of anti-cancer compounds which deserves continued research at the basic, pharmaceutical and clinical levels in order to yield novel chemotherapeutic agents against a range of neoplastic diseases.
Downloaded by CORNELL UNIV on May 16, 2012 | http://pubs.acs.org Publication Date (Web): March 6, 2012 | doi: 10.1021/bk-2012-1093.ch017
The Anticancer Activity of Jasmonates Cancer is a leading cause of death worldwide (1). According to the World Health Organization (WHO) cancer is accountable for 7.6 million deaths each year. Despite extensive developments in the fields of oncology and translational research, the discovery of novel treatments that substantially improve the survival of cancer patients remains frustratingly low (2). Two major obstacles for successful cancer treatment are the resistance of cancer cells to existing chemotherapeutics and the high level of toxicity induced by many chemotherapeutic drugs (1). As the primary tumor can often be surgically removed, the main challenge of adjuvant therapy is to eliminate the spread of metastases and prevent their development in vital organs. Indeed, the leading cause of cancer mortality is the development of metastases that are resistant to conventional chemotherapy (3). Therefore novel anti-cancer agents that may overcome drug-resistance and inhibit the spread of metastases while exhibiting high selectivity towards cancer cells are in constant demand. The jasmonate group of plant stress hormones fulfills all of these criteria and holds great promise as a potential source of anti-cancer agents. Jasmonates are a group of plant stress hormones which are among the most potent regulators of defense-related mechanisms in plants. Naturally occurring jamonates include numerous compounds the most prominent of which are jasmonic acid (JA, Figure 1) and its methyl ester, methyl jasmonate (MJ, Figure 1) (4). Jasmonates are induced in plants in response to various types of stresses and mediate the defense response to mechanical and infectious insults. Jasmonates regulate plant gene expression including defense related genes and induce the production of various phytochemicals. These phytochemicals can interact directly with plant invaders, such as herbivores, bacteria and fungi, to bring about their neutralization. Although a considerable body of knowledge has been accumulated regarding jasmonate-regulated gene expression, relatively little is known about the signaling pathways mediating these actions of jasmonates. Interestingly, jasmonates can also regulate gene expression in herbivorous insects, suggesting that jasmonate signaling pathways may be conserved between plants and animals. In addition to their role in defense-related mechanisms, jasmonates also regulate additional functions in plant physiology including growth, development and reproduction (4). As a part of their role in plant development and defense, jasmonates were shown to induce programmed cell death in plant cells (5). 304 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
Downloaded by CORNELL UNIV on May 16, 2012 | http://pubs.acs.org Publication Date (Web): March 6, 2012 | doi: 10.1021/bk-2012-1093.ch017
Figure 1. Chemical structures of natural and synthetic jasmonates.
Originally found as major constituents in the etheric oil of jasmine, jasmonates are expressed throughout the plant kingdom (4). However, several plants express relatively high levels of jasmonates. These include: jasminum, olive, ginger, rosemary, honeysuckle and more (6). Interestingly, several studies have demonstrated the anti-cancer activities of jasmonate-rich plants (for review see (6)). Structurally and biosynthetically, jasmonates are cyclopentanones that belong to the family of oxygenated fatty acid derivatives, collectively called oxylipins, which are produced via the oxidative metabolism of polyunsaturated fatty acids. Jasmonate biosynthesis in plants is analogous to eicosanoid biosynthesis in animal cells. Animal eicosanoinds (C20) are lipid bioregulators that are synthesized from arachidonic acid and function as regulators of cell immune response, differentiation and homeostasis. In plants, jasmonates are derived from C18 α-linolenic acid (18:3) and control similar activities (4). Indeed, the structure of jasmonates resembles that of certain prostaglandins (7). Several groups have reported in recent years that jasmonates and some of their synthetic analogs, exhibit anti-cancer activity in vitro and in vivo. Our initial report, published in 2002 (8), revealed that JA inhibited the proliferation of several human cancer cell lines including breast, prostate, and melanoma cells and induced death in human lymphoblastic leukemia cells. MJ induced death in various human and mouse cancer cell lines including: breast, prostate, melanoma, lymphoma and lymphoblastic leukemia cells (8). Subsequent reports demonstrated that jasmonates can induce death or inhibit the proliferation of various other cancer cell lines including lung carcinoma (9–11), colon carcinoma (10), melanoma (12), cervical carcinoma (13), sarcoma (14), glioma (15), myeloid leukemia (16–18) and neuroblastoma (19, 20) cells. Furthermore jasmonates 305 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
Downloaded by CORNELL UNIV on May 16, 2012 | http://pubs.acs.org Publication Date (Web): March 6, 2012 | doi: 10.1021/bk-2012-1093.ch017
increased the life span of T-cell lymphoma-bearing mice, thus demonstrating an anti-cancer effect in vivo (8). Recently, a first-in-man study demonstrated that MJ has a beneficial effect in treating human pre-cancerous and cancerous skin lesions (21). As most chemotherapeutic drugs are designed to induce cell death, toxicity to normal tissue is difficult to avoid during treatment (1). Thus, selectivity towards cancer cells is a highly desirable characteristic for novel anti-cancer agents. Jasmonates induce death selectively in cancer cells while non-transformed cells are relatively unaffected by jasmonate treatment. While MJ at 1mM induced death in human lymphoblastic leukemia cells, normal lymphocyes remained relatively unaffected (8). In another study, MJ at 3 mM decreased the viability of four different cervical cancer cell lines by over 50% while normal foreskin primary keratinocytes were significantly more resistant to MJ treatment, with reduction of only ~15% in cell viability (13). Finally, an ultimate experiment that demonstrated the selectivity of MJ was performed on blood samples obtained from chronic lymphocytic leukemia (CLL) patients and contained a mixed population of normal and leukemic lymphocytes. MJ selectively killed the leukemic cells in the samples, gradually decreasing the percentage of leukemic cells and increasing the percentage of normal lymphocytes, in a dose dependent manner (22). In line with these results, a positive correlation was found between the percentage of leukemic cells in blood samples of CLL patients and the level of cytotoxicity induced by MJ (23). Most cancer-related deaths are not attributed to the development of the primary tumor, which can often be surgically removed, but rather to the dissemination of metastases and their development in vital organs (3). Therefore, agents that may block the spread and development of metastases may prove to be potent anti-cancer agents. Cell migration is an essential component of the metastatic process (24). The ability of MJ to impair cell migration was evaluated in B16-F10 mouse metastatic melanoma cell line. MJ, at non-cytotoxic concentrations, inhibited the migration of the cells in a dose-dependent manner (12). Moreover, MJ exhibited an anti-metastatic effect in vivo, inhibiting the development of melanoma metastases in the lungs of mice (12). These results demonstrate the ability of MJ to interfere with the metastatic process, plausibly by inhibiting cell migration. Another cause of cancer related mortality is the failure of treatment due to drug resistance. Since drug resistance plays a crucial role in the clinical outcome of cancer treatment (1), it was interesting to evaluate the effect of jasmonates on drug-resistant cancer cells. Two different mechanisms of drug-resistance were studied: p53-mutation (25) and P-glycoprotein over-expression (12). p53 is a tumor suppressor gene. Its tumor-suppressive activity involves the induction of cell cycle arrest or apoptosis (a form of programmed cell death which will be further elucidated in the next section). Mutation in p53 occurs in more than 50% of human cancers (26). Various tumors consisting of mutant p53-expressing cells exhibit high resistance to radiation and chemotherapeutic drugs. Circumventing this abnormal resistance is a major challenge in cancer therapy (27). The effect of jasmonates on drug-resistant cells harboring mutated p53 was evaluated using two clones of B-lymphoma cells. One clone expressed wild-type p53 and the other expressed mutated p53. While the mutant 306 In Emerging Trends in Dietary Components for Preventing and Combating Disease; Patil, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2012.
Downloaded by CORNELL UNIV on May 16, 2012 | http://pubs.acs.org Publication Date (Web): March 6, 2012 | doi: 10.1021/bk-2012-1093.ch017
p53-expressing cells where resistant to the radio-mimetic neocazinostatin (NCS) and to the chemotherapeutic agent bleomycin, compared to their wild-type p53-expressing counterparts, they did not exhibit resistance to jasmonate treatment. JA and MJ were both equally cytotoxic to both clones (25). In another study, the cytotoxicity of MJ was evaluated in several cervical carcinoma cell lines expressing either wild-type or mutant p53. MJ induced death in the different cervical carcinoma cell lines regardless of their p53 status (13). Thus, jasmonates can overcome drug resistance induced by p53-mutations. Another model of drug resistance is the P-glycoprotein over-expression mechanism. P-glycoprotein is an efflux pump that is normally expressed in intestine, kidney and liver tissues as well as in endothelial cells of the blood-brain barrier. It protects normal tissue by excreting various cytotoxins out of the cells. P-glycoproein is highly expressed in many tumor types. It excretes many chemically unrelated drugs out of the cancer cell endowing the cell with a multi-drug resistant phenotype. Drugs that are P-glycoprotein substrates include taxanes, anthracyclins, vinca alakaloids and epipodophylotoxins (28). The effects of jasmonates were evaluated on two B16-F10 melanoma clones, one expressing low levels of P-glycoprotein and the other expressing high levels of P-glycoprotein (12). While the high P-glycoprotein expressing clone exhibited resistance to the chemotherapeutics doxorubicin, vinblastine and colchicine, both clones were equally sensitive to the cytotoxic activity of jasmonates (12). Inhibition of P-glycoprotein with a selective inhibitor sensitized the resistant cells to chemotherapeutic agents but did not affect their sensitivity to jasmonates (Flescher, personal communication). Furthermore, the high P-glycoprotein expressing melanoma cells were also shown to be highly metastatic and exhibit increased spread to the lungs as well as higher cell motility rates (29). In addition to its ability to kill these melanoma cells, MJ, at non cytotoxic concentrations, inhibited the migration of these highly metastatic and drug-resistant cells (12). These results indicate that unlike many conventional chemotherapeutic agents, jasmonates are not substrates of the P-glycoprotein efflux pump and thus have the potential of eliminating drug-resistant cancer cells. In a recently published article, Pereira-Lopes et al. (30) demonstrated that not only do jasmonates directly affect tumor cells, they may also have an effect on the development of tumor vasculature. Angiogenesis, the formation of new capillaries from pre-existing vessels, is a crucial step in tumor progression and is therefore an attractive target for therapeutic intervention (31). Pereira-Lopes et al. discovered that MJ impaired vascular growth in a Chorioallantoic Membrane of the Chicken Embryo (CAM) model of angiogenesis. The effect of MJ seemed to depend on the applied dosage. While, millimolar concentrations of MJ significantly reduced capillary growth, an effect which is most likely associated with direct cytotoxicity of MJ towards endothelial cells, lower concentrations (
