Reduced nitrogen supply enhances the cellular antioxidant potential of phenolic extracts through alteration of the phenolic composition in lettuce (Lactuca sativa L.)

Abstract

BACKGROUND Nitrogen availability is an important environmental factor that determines the production of phenolic compounds in vegetables, but the relationship between low nitrogen‐induced alterations of phenolic compounds in vegetable crops and the cellular antioxidant activities of these compounds remains unclear. This study investigated the effect of reduced nitrogen supply (0.05 mmol L−1 nitrate) on phenolic metabolism in lettuce and the protective role of phenolic extracts against H2O2‐induced oxidative stress in Caco‐2 cells by determining cell damage, reactive oxygen species (ROS) content and antioxidant enzyme activities. RESULTS Reduced nitrogen supply significantly improved the accumulation of phenolic compounds in lettuce, which was partially correlated with the upregulation of genes related to the phenolic synthesis pathway. Phenolic extracts from lettuce cultivated in low‐nitrogen medium exhibited a better protective effect against H2O2‐induced oxidative damage in Caco‐2 cells than those from lettuce cultivated with adequate nitrogen. These extracts act by increasing the activities of antioxidant enzymes and, subsequently, by inhibiting ROS overproduction, which leads to a decrease in mitochondrial membrane and DNA damage. The results of HPLC and correlation analyses implied that the improvement in the protective capacity of lettuce extracts after low‐nitrogen treatment may be related, not only to the increased content of phenolic compounds, but also to the increased percentage contribution of chlorogenic acid and quercetin derivatives to the total phenolic content. CONCLUSION Reduction in nitrogen supply can be a powerful strategy to modify phenolic metabolism and composition in lettuce and, consequently, to improve their antioxidant capacity. © 2019 Society of Chemical Industry

Role of sucrose in modulating the low‐nitrogen‐induced accumulation of phenolic compounds in lettuce (Lactuca sativa L.)

“…Consumers have become increasingly aware of the role of plantderived antioxidants, especially phenolic compounds, in reducing the risk of cancer, cardiovascular disease and accelerated aging. 5,7,15,26 Therefore, in recent years, many studies have been conducted with the aim of increasing phenolic accumulation in plants. Reducing the application of nitrogen during crop production can effectively improve the phenolic content in plants, although the mechanisms involved in this process remain unclear.…”

“…The mobile phase was 1% formic acid (solvent A) and 100% methanol (solvent B) with a flow rate of 0.7 mL min −1 . The gradient used was as follows: 0- 15 Quantitative and real-time PCR analysis Total RNA was extracted using the plant RNA purification kit. Firststrand cDNA was synthesized using HiScript II Q RT SuperMix in a 10 μL reaction volume containing 500 ng of purified RNA.…”

“…Quantitative real-time PCR analyses were performed as previously described. 15 Determination of Suc content Suc concentration was determined according to the method of Filip et al, 30 with minor modifications. Briefly, fresh lettuce leaves (0.5 g) were homogenized in 80% (v/v) ethanol and centrifuged at 10 000 × g at 4°C for 10 min.…”

“…8,12,13 We have previously shown that cultivation under low nitrogen conditions increases the transcriptional levels of genes or enzyme activities related to the phenolic synthesis pathway in lettuce, resulting in higher phenolic accumulation. [14][15][16] Interestingly, reduction of nitrogen supply may also have human and environmental benefits through decreasing the nitrate accumulation in vegetables and limiting nitrogen loss in the environment, respectively. 13,14 However, the underlying mechanisms of low-nitrogen-induced phenolic biosynthesis in vegetables remain unknown.…”

Alteration of Phenolic Composition in Lettuce (Lactuca sativa L.) by Reducing Nitrogen Supply Enhances its Anti-Proliferative Effects on Colorectal Cancer Cells

“…Nitrogen availability is an extremely important factor affecting the production of phenolic compounds in plants, with a reduction in nitrogen availability generally increasing their levels [12,13]. Our previous studies also found that proper nitrogen limitation could lead to increased accumulation of phenolic compounds in lettuce [14,15]. From an environmental standpoint, such a reduction promotes optimal efficiency in the use of nitrogen by plants, thereby limiting its loss in the environment.…”