Beta-carotene antagonizes the effects of eicosapentaenoic acid on cell growth and lipid peroxidation in WiDr adenocarcinoma cells

Abstract

The effects of combinations between eicosapentaenoic acid (EPA) and β-carotene on cell growth and lipid peroxidation were investigated in human WiDr colon adenocarcinoma cells. EPA alone was able to inhibit the growth of WiDr cells in a dose- and time-dependent manner. Such an inhibition involved fatty acid peroxidation, as shown by the remarkable increase in the levels of Malondialdehyde (MDA) in EPA-treated cells. Beta-carotene was capable of reducing the growth inhibitory effects of EPA and the levels of MDA in a dose- and a time-dependent manner. In addition, EPA increased β-carotene consumption in WiDr cells. This study provides evidence that β-carotene can antagonize the effects of EPA on colon cancer cell growth and lipid peroxidation.

Introduction

Diets enriched in n-3 polyunsaturated fatty acids (PUFA) have been shown to delay the development and growth of experimentally induced tumors [1], [2], [3], [4], [5] and to normalize altered proliferative patterns in colonic mucosa of patients at high risk for colon cancer [6]. Growth inhibition by n-3 PUFA has been also reported in several tumor cell lines [7], [8], [9]. The potential growth inhibition of n-3 PUFA appears to be related but not directly proportional to the number of their double bonds. Fatty acids containing five double bonds, such as eicosapentaenoic acid, have been found to be more effective in inhibiting cell growth than fatty acids containing six double bonds, such as docosahexaenoic acid [7].

Although the mechanism of n-3 PUFA-induced cell growth inhibition is still unknown, it has become increasingly clear that oxidation products of PUFA contribute to this inhibition [10], [11], [12], [13]. The substitution of membrane fatty acids with potentially unstable highly unsaturated fatty acids potentiates lipid peroxidation of cellular membrane, producing metabolites, such as malondialdehyde (MDA), 2-alkenals, and dioxyalkenals, which are hypothesized to alter cell growth [14]. Consistent with this, products of lipid peroxidation added to growth media inhibit cell growth and/or induce cell death in several different cell lines [9], [15]. Moreover, deficiencies in endogenous cellular antioxidants are shown to strongly enhance the growth inhibitory effects of n-3 PUFA, whereas added antioxidants, such as vitamin E, and antioxidant enzymes, such as superoxide dismutase and glutathione peroxidase, are shown to reduce such effects [16], [17], [18].

Recently, the action of β-carotene as an antioxidant has attracted widespread attention. This compound is able to prevent peroxidation of fatty acids induced by singlet oxygen or by radical initiators [19], [20], and to inhibit free radical–induced peroxidation in lipid homogeneous solutions [21], liposomes [22], isolated membranes [23], and intact cells [24]. Previously, we reported that β-carotene is able to inhibit PUFA oxidation induced by the radical initiator azo-bisisobutyronitrile (AIBN) in homogeneous hexane solutions and that this inhibition increases with the degree of fatty acid unsaturation [25].

In this study, in an attempt to investigate the role of lipid peroxidation on tumor cell growth, we analyzed the effect of the highly unsaturated eicosapentaenoic acid (EPA) (20:5, n-3), alone and in combination with β-carotene, on cell growth and lipid peroxidation in WiDr human colon adenocarcinoma cell line.

The results of this study suggest that EPA-induced growth inhibition is a free radical–dependent process and that it can be antagonized by β-carotene.