Basic nutritional investigationω-3 Polyunsaturated fatty acids and ionizing radiation: Combined cytotoxicity on human colorectal adenocarcinoma cells
Introduction
Colorectal cancer, with about 1 million cases worldwide diagnosed per year, is one of the most prevalent malignancies and the second cause of cancers in industrialized countries [1], [2], [3]. The main curative strategy remains complete surgical resection of the tumor associated with radiotherapy and/or chemotherapy. Unfortunately, colorectal cancer remains often refractory to these classic therapies and 85% of patients relapse within the first 2 y after surgery and display local recurrence or metastasis [4]. In consequence, the search for new therapeutic modalities with minimal toxicity is of particular interest in colon cancer management.
Recent findings have suggested that ω-3 polyunsaturated fatty acids (PUFAs) might be of use as adjuvant for cancer therapy. In addition to inhibiting the progression of several prevalent cancers [5], [6], ω-3 PUFAs have been found to enhance the sensitivity of tumor cells to several anticancer drugs, such as mitomycin C, cyclophosphamide, and doxorubicin [7], [8], [9]. Regarding colorectal cancer, combination of ω-3 PUFA-containing triacylglycerol emulsion with 5-fluorouracil resulted in an additive growth inhibitory effect on the Caco-2 cell line [10].
To date the mechanism by which ω-3 PUFAs modulate tumor sensitivity to anticancer drugs is not completely elucidated. The biological effects of ω-3 PUFAs are frequently attributed to competitive inhibition of eicosanoid production from ω-6 PUFA precursors [5]. This may be of particular value in colorectal cancer, where ectopic overexpression of cyclo-oxygenase-2 (COX-2) has been reported to increase prostaglandin E2 synthesis and to suppress apoptosis through Bcl-2 induction [11]. However, accumulation of ω-3 PUFAs in the cell membrane may act at numerous levels, including membrane fluidity, receptor binding, signal transduction, and enzyme activity [5]. It has been suggested that drug transport in tumor cells is correlated with the unsaturation index in membrane phospholipids [12]. In addition, an increase in the unsaturation index may result in greater susceptibility of the tumor to oxidative stress, which may explain why ω-3 PUFAs enhance tumor sensitivity to anticancer drugs that generate reactive oxygen species (ROS) [9]. This latter hypothesis is supported by recent findings demonstrating that ω-3 PUFAs also enhanced the antitumor effect of ionizing radiation in animal models of head and neck [13] and mammary [14] tumors.
Combination of ω-3 PUFAs with radiotherapy may be of particular interest, because the enhanced antitumor effect has been also associated with a reduction of mucosal/epidermal radiotoxicity and a decrease in proinflammatory COX-2 expression in HEP-2 human carcinoma xenografted mice [13]. To our knowledge, there is no study investigating the combined effect of ω-3 PUFAs with radiotherapy on colorectal cancer, despite its favorable location being susceptible to dietary intervention.
Therefore, the present study evaluated the interaction between ω-3 PUFAs and X-rays on the growth and survival of three human colorectal adenocarcinoma cell lines, which display different radiosensitivities and enterocytic properties. To discriminate between COX-2 pathway modulation and oxidative stress induction, we compared the effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) with those of arachidonic acid (AA) and challenged them with vitamin E.
Our investigations provide new insight into understanding how ω-3 PUFAs mediate their suppressive action on tumor cells and determine whether they could enhance the efficacy of human colorectal cancer radiotherapy.
Section snippets
Cell lines and culture
Three human colorectal adenocarcinoma cell lines, which differ in genetic pattern expression, doubling time, and enterocytic differentiation, were chosen according to the classification of Chantret et al. [15]. The poorly differentiated LS174T and the well-differentiated Caco-2 cell lines were purchased from the American Type Culture Collection (Rockville, MD, USA). Radiosensitive LS174T and Caco-2 cell lines were compared with the radioresistant CO112 cell line (from our laboratory) [16]. They
Cell growth after EPA supplementation
The growth of LS174T (Fig. 1A), CO112 (Fig. 1B), and Caco-2 (Fig. 1C) cells was measured during an incubation of 4 d in culture medium supplemented with EPA. In control condition with EPA-free medium, the most rapidly growing cell line was LS174T (PDT 19 h), whereas CO112 (PDT 23 h) and Caco-2 (PDT 25 h) grew more slowly. A time- and dose-dependent inhibition of cell growth was observed for all cell lines at EPA concentrations ranging from 30 to 100 μmol/L, whereas the effect was less marked at
Discussion
Previous studies have suggested that PUFAs inhibit the development of several prevalent cancers [5], [17], [18], [19] and might be used as adjuvants for cancer therapy [8], [9], [10], [13], [14]. Our results demonstrated that ω-3 PUFAs are able to inhibit the proliferation of the three human colonic cell lines studied. These observations confirm previous findings showing a PUFA inhibitory effect on colorectal tumor cell growth in vitro [20], [21] and in vivo in murine and human xenografted
Conclusion
Our observations strongly suggest that ω-3 PUFAs are potential candidates for optimizing human colorectal cancer therapy. However, further in vivo experiments are needed to determine the optimal timing of ω-3 PUFA supplementation before radiotherapy and to evaluate side effects on healthy tissues and potential induction of angiogenesis and inflammation.
Acknowledgments
The authors gratefully acknowledge the technical assistance of Véronique Karsegard and the availability of the Nuclear Medicine Laboratory team.
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Cited by (0)
This study was supported by a grant of the European Society for Clinical Nutrition and Metabolism (ESPEN-ABBOTT 2004 research grant) and the Nutrition 2000Plus Foundation.
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Gaëlle Benais-Pont, Ph.D., and Yves M. Dupertuis, Ph.D., contributed equally to this study.