In Vitro and In Silico Study of Analogs of Plant Product Plastoquinone to Be Effective in Colorectal Cancer Treatment

Abstract

Plants have paved the way for the attainment of molecules with a wide-range of biological activities. However, plant products occasionally show low biological activities and/or poor pharmacokinetic properties. In that case, development of their derivatives as drugs from the plant world has been actively performed. As plant products, plastoquinones (PQs) have been of high importance in anticancer drug design and discovery; we have previously evaluated and reported the potential cytotoxic effects of a series of PQ analogs. Among these analogs, PQ2, PQ3 and PQ10 were selected for National Cancer Institute (NCI) for in vitro screening of anticancer activity against a wide range of cancer cell lines. The apparent superior anticancer potency of PQ2 on the HCT-116 colorectal cancer cell line than that of PQ3 and PQ10 compared to other tested cell lines has encouraged us to perform further mechanistic studies to enlighten the mode of anti-colorectal cancer action of PQ2. For this purpose, its apoptotic effects on the HCT-116 cell line, DNA binding capacity and several crucial pharmacokinetic properties were investigated. Initially, MTT assay was conducted for PQ2 at different concentrations against HCT-116 cells. Results indicated that PQ2 exhibited significant cytotoxicity in HCT-116 cells with an IC₅₀ value of 4.97 ± 1.93 μM compared to cisplatin (IC₅₀ = 26.65 ± 7.85 μM). Moreover, apoptotic effects of PQ2 on HCT-116 cells were investigated by the annexin V/ethidium homodimer III staining method and PQ2 significantly induced apoptosis in HCT-116 cells compared to cisplatin. Based on the potent DNA cleavage capacity of PQ2, molecular docking studies were conducted in the minor groove of the double helix of DNA and PQ2 presented a key hydrogen bonding through its methoxy moiety. Overall, both in vitro and in silico studies indicated that effective, orally bioavailable drug-like PQ2 attracted attention for colorectal cancer treatment. The most important point to emerge from this study is that appropriate derivatization of a plant product leads to unique biologically active compounds.

Keywords: DNA cleavage; apoptosis; colorectal cancer; cytotoxicity; molecular docking; pharmacokinetic properties; plastoquinones.

Figure 1

Quinone-based plant products with anti-colorectal cancer properties.

Figure 2

Mitomycin C, mitoxantrone, and doxorubicin, potential anticancer drugs the containing 1,4-quinone moiety.

Figure 3

Design concept of PQ analogs based on our previous studies.

Figure 4

Structures of selected PQ analogs involved in the exploration.

Figure 5

One-dose (10 µM) mean graph of PQ2 against different cancer cell lines. All experiments were repeated one time.

Figure 6

One-dose (10 µM) mean graph of PQ3 against different cancer cell lines. All experiments were repeated one time.

Figure 7

One-dose (10 µM) mean graph of PQ10 against different cancer cell lines. All experiments were repeated one time.

Figure 8

The anticancer effects of PQ2 and cisplatin at varying concentrations on HCT-116 cells. Data are representative of the mean of three separate experiments and are reported at the ±SD and p values were determined using Student’s t test (* p < 0.05, ** p < 0.01, ns: not statistically significant).

Figure 9

Alteration of HCT-116 cells following exposure to the IC₅₀ concentration of the control (DMSO), PQ2, and cisplatin (A) for 12 h. The percentage of apoptotic (green), late apoptotic or necrotic (yellow) and necrotic (red) cells (B) was determined by analyzing 100 randomly chosen stained cells in each experiment. Data from three independent experiments are shown as means ± SD, and p values were determined using Student’s t test (ns: not statistically significant).

Figure 9

Alteration of HCT-116 cells following exposure to the IC₅₀ concentration of the control (DMSO), PQ2, and cisplatin (A) for 12 h. The percentage of apoptotic (green), late apoptotic or necrotic (yellow) and necrotic (red) cells (B) was determined by analyzing 100 randomly chosen stained cells in each experiment. Data from three independent experiments are shown as means ± SD, and p values were determined using Student’s t test (ns: not statistically significant).

Figure 10

Docking poses (A) and interactions (B) of PQ2, PQ3 and PQ10 (ligands are highlighted in dark green-, orange- and turquoise-colored sticks) in the minor groove of the double helix of DNA (PDB ID: 2GWA).

Figure 10

Docking poses (A) and interactions (B) of PQ2, PQ3 and PQ10 (ligands are highlighted in dark green-, orange- and turquoise-colored sticks) in the minor groove of the double helix of DNA (PDB ID: 2GWA).

Figure 11

Thymoquinone and betulin analogs as anti-colorectal cancer agents.