A redox-silent analogue of tocotrienol acts as a potential cytotoxic agent against human mesothelioma cells
Introduction
Malignant mesothelioma (MM) from the serosal membranes of the body cavities, is a particular aggressive cancer which is characterised by rapid progression, late metastases, and poor prognosis (Carbone et al. 2002). Although surgery, radiotherapy, chemotherapy, and/or their combinations have been used as therapeutic modalities, median patient survival is 8–18 months (Nowak et al. 2002). MM cells exhibit resistance to many chemotherapeutic agents, including doxorubicin and cisplatin, which are nevertheless widely used to treat MM (Tomek et al. 2003). A recent report of a phase III study showed that the combination of pemetrexed and cisplatin is more effective than cisplatin alone with differences in response rate of 41.3 versus 16.3% (Vogelzang et al. 2003). However, most of the patients relapsed within a year after starting the treatment. Therefore, new therapeutic approaches are urgently needed for MM patients.
Vitamin E, a fat soluble vitamin, is well known for its cellular antioxidant and lipid lowering properties (Packer and Landvik 1989). It is now been increasingly accepted as a generic term for tocotrienols and tocopherols. Although both classes of compounds share the same aromatic chromanol ‘head’, the two isoforms of vitamin E differ in their side chains. Tocopherols posses saturated phytyl side chain while tocotrienols have an unsaturated isoprenoid chain along their ‘tails’. Due to the structural difference of the two compounds, tocotrienols have more potential physiological functions than tocopherols (Pearce et al. 1992). In previous studies including those of our group, α-tocopherol (T), a representative tocopherol, is inactive for the negative growth control of cancer cells. In contrast an ether derivative of T, 6-O-carboxypropyl-α-tocopherol (TE), a redox-silent analogue of T, is a potent growth inhibitor against cancer cells, both in vitro and in vivo (Yano et al., 2001, Nishikawa et al., 2003). Similarly, an ether derivative of α-tocotrienol (T3), 6-O-carboxypropyl-α-tocotorienol (T3E), has cytotoxicity against a lung adenocarcinoma cell line (A549) in vitro much stronger than that of T3 or TE (Yano et al. 2005). In previous studies, a redox-silent analogue of T, α-tocopheryl succinate (TS) effectively suppressed MM cell growth in vitro as well as in vivo (Kogure et al., 2002, Kogure et al., 2003). From our previous report, it is clear that the redox-silent analogue of T3 is much stronger that that of T as an anti-cancer agent (Yano et al. 2005), thus we hypothesized that T3E may be a potential anti-mesothelioma agent.
In this context, we evaluated the effect of T3E on MM cell growth and its possible mechanism. Human MM cell line, H28 was chosen to evaluate the inhibitory effect of T3E on MM cell growth, due to its resistance to cisplatin (CDDP), a representative agent to treat MM clinically.
Section snippets
Reagents
All culture and chemicals were purchased from Gibco BRL (Tokyo, Japan) and Sigma (St. Louis, MO, USA), unless otherwise indicated. PP2 (a Src inhibitor), PP3 (a negative control of PP2), AG490 (a signal transducer and activator of transcription (Stat)3 inhibitor) and AG1478 (a epidermal growth factor receptor (EGFR) inhibitor) were purchased from Calbiochem–Novabiochem (La Jolla, CA, USA). T3 was purchased from Tama Biochemicals (Tokyo, Japan). All antibodies were obtained from Cell Signaling
Effect of T3E on growth of H28 and Met-5A cells
To compare effect of T3E on H28 cell growth with that of CDDP and T3, we first examined time-dependent change in cell viability after each agent exposure for 48 h. As shown in Fig. 1A, only T3E decreased cell viability in a dose-dependent manner (0–20 μM). However, T3 and CDDP did not influence the cell viability within the doses (Fig. 1A). Furthermore, we evaluated the toxic effect of T3E on non-tumorigenic mesothelial cells (Met-5A cell) to estimate selective toxicity of T3E against MM cells.
Discussion
MM is an aggressive malignancy, the incidence of which is expected to increase due to its association with asbestos exposure. A number of chemotherapeutic agents have been used, either alone or in combination, to treat MM with the latter multi-agent regimes generally having the highest response rates (Pistolesi and Rusthoven 2004). Nonetheless, despite the current therapies, the prognosis for many MM patients is very poor. Several signal molecules related to growth and survival are
Conclusion
In conclusion, we demonstrated that T3E could effectively suppress growth of human MM cells (H28 cells) having chemoresistance to CDDP in pharmacological doses, based on simultaneous inactivation of Src and Stat3, and that T3E had no influence on non-tumorigenic mesothelial cells (Met5-A cells). These findings suggest that T3E is a promising and potential anti-MM agent compared with conventional chemotherapeutic agents.
Acknowledgment
This study was supported by a research grant for Health Sciences Focusing on Drug Innovation from the Japan Health Sciences Foundation (KHC1023).
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