Anti-Leukemic Effects on a U937 Cell Line of Fresh and Steamed Chinese Kale Juice and Their Pro-Apoptotic Effects via a Caspase-Dependent Pathway

Abstract

Chinese kale is a vegetable belonging to the family Brassicaceae in which members of this family produce unique metabolites called glucosinolates and isothiocyanates. These substances have been found to exhibit many benefits to human health. This study aimed to investigate and compare the contents of glucosinolates and isothiocyanates, and the anti-leukemic activity of fresh and steamed Chinese kale juice (CKJ). Cell viability and proliferation activity of U937 cells treated with CKJ were determined. Cell apoptosis and alterations of apoptosis-related protein expression were studied. Results showed that CKJ significantly decreased the viability of leukemic cells and inhibited cell proliferation in a dose- and time-dependent manner. After treatment with 5% v/v fresh and steamed CKJ for 24 h, the percentage of apoptotic cells increased to 53% and 36%, respectively. Increased amounts of cleaved caspase-3 in U937 cells treated with CKJ were observed, indicating that CKJ can trigger apoptotic cell death through a caspase-dependent pathway. Fresh CKJ was found to be more effective than steamed CKJ in suppressing cell survival and inducing cell apoptosis. The results suggest that Chinese kale possesses an anti-leukemic potential and could be further developed for cancer therapy and prevention. However, thermal cooking could reduce its beneficial function.

Keywords: Brassica; apoptosis; cancer; glucosinolate; isothiocyanate; kale; leukemia.

Figure 1

Effects of CKJ on U937 cell survival (a) and cell growth (b). Cells were exposed to different concentrations of fresh and steamed CKJ for 24, 48, and 72 h. The significance level of the tested sets was compared with a control (normal saline) set (* p = 0.05, ** p = 0.01 and *** p < 0.001) and fresh CKJ-treated cells were compared with steamed CKJ-treated cells (### p < 0.001). Data were representative of three independent experiments.

Figure 2

The distribution of U937 cells by cell cycle phase after treatment with fresh or steamed CKJ for 72 h. The flow cytometry profiles are shown in histograms (a) and the percentage of cells that were treated with various concentrations of fresh CKJ (b) and steamed CKJ (c) in the Sub G1, G0/G1, S, and G2/M phases are presented. The significance level of the tested sets was compared with a control (normal saline) set (* p = 0.05, ** p = 0.01, and *** p < 0.001) and fresh CKJ-treated cells were compared with steamed CKJ-treated cells (## p = 0.01 and ### p < 0.001). Data are representative of at least three independent experiments.

Figure 3

Induction of U937 cell apoptosis by fresh and steamed CKJ. The positive cells were quantified using a flow cytometer (lower left: live, lower right: early apoptosis, upper right: late apoptosis, upper left: necrosis) (a) The percentage of cells in each group was calculated (b,c). The significance level of the tested sets was compared with a control (normal saline) set (* p = 0.05, ** p = 0.01, and *** p < 0.001) and fresh CKJ-treated cells were compared with steamed CKJ-treated cells (## p = 0.01 and ### p < 0.001). Data are representative of at least three independent experiments.

Figure 4

Western blot analysis for cleaved caspase-3, Bcl-2, and Bax using lysates from U937 cells exposed to fresh and steamed CKJ for 72 h (left). The graphs show the densitometric analysis, normalized against β-actin (right). (* p = 0.05 and ** p = 0.01). Data are representative of at least three independent experiments.