Effects of beta-carotene on antioxidant enzyme activity, intracellular reactive oxygen and membrane integrity within...

As encountered with a plethora of other natural products, the antioxidant activity of beta-carotene has been proposed as one of the mechanisms by which diets rich in this pro-vitamin A active carotenoid apparently afford chemoprevention. Here, we report the ability of beta-carotene to alter endogenous reactive oxygen levels and antioxidant defences within non-stressed 'differentiated' monolayers of an intestinal epithelial cell line (Caco-2) and to subsequently effect resistance to general oxidative insult. The differentiated monolayers efficiently absorbed beta-carotene. Between 3 and 8 days post confluence, cultures exhibited a progressive increase in antioxidant enzyme activity and a corresponding reduction to intracellular ROS levels. The profile for antioxidant enzyme activity was unaffected by sustained daily supplementation with beta-carotene. However, after two daily treatments with 50 microM beta-carotene intracellular ROS levels were significantly reduced and there was a trend towards reduced intracellular ROS within monolayers subject to five daily treatments with 0.5 and 5 microM beta-carotene. Prolonged supplementation with 0.1 and 0.5 microM beta-carotene or short supplementation periods with 5 and 50 microM beta-carotene did not alter susceptibility to H(2)O(2). However, cultures treated daily between 3 and 8 days post confluence with 5 or 50 microM beta-carotene exhibited enhanced LDH release, increased non-adherence and enhanced Trypan blue staining when challenged with 10 mM H(2)O(2). In the absence of H(2)O(2), the beta-carotene treatments were not overtly toxic to the monolayers. These results indicate that beta-carotene does not enhance antioxidant defences within Caco-2 monolayers. The enhancement of H(2)O(2) toxicity by persistent, high doses of beta-carotene may contribute to the failure of this carotenoid to protect high risk individuals from certain degenerative conditions.