Lignans from Opuntia ficus-indica seeds protect rat primary hepatocytes and HepG2 cells against ethanol-induced oxidative stress

Abstract

Bioactivity-guided isolation of Opuntia ficus-indica (Cactaceae) seeds against ethanol-treated primary rat hepatocytes yielded six lignan compounds. Among the isolates, furofuran lignans 4−6, significantly protected rat hepatocytes against ethanol-induced oxidative stress by reducing intracellular reactive oxygen species levels, preserving antioxidative defense enzyme activities, and maintaining the glutathione content. Moreover, 4 dose-dependently induced the heme oxygenase-1 expression in HepG2 cells.

Key words:

• Opuntia ficus-indica
• lignin
• hepatoprotective activity
• heme oxygenase-1

Opuntia ficus-indica (L.) Mill. (Cactaceae) is a perennial plant known as prickly pear or nopal. This cactus is considered to be native to Mexico, but is currently cultivated in Mediterranean countries, Latin America, Southern Asia, and several oceanic islands with warm climates.¹⁾ Previous studies on the chemical composition of O. ficus-indica revealed the presence of alkaloids, flavonoids, terpenoids, polysaccharides, and organic acids.^2–4) Recent studies have reported that the extract of O. ficus-indica showed antioxidant, antiulcer, neuroprotective, and hepatoprotective activities.^5–8) To the best of our knowledge, however, the hepatoprotective chemical constituents in this plant against ethanol-induced hepatotoxicity have not been previously reported. Thus, in this study, we attempted to investigate the hepatoprotective constituents of O. ficus-indica using primary rat hepatocytes and HepG2 cells as screening tools.

Reactive oxygen species (ROS) generated from the ethanol metabolism process induces oxidative stress in hepatocytes and eventually cause liver damage.⁹⁾ Any substance possessing antioxidant properties against ethanol-induced liver toxicity is considered to be a potential therapeutic agent for preventing the progression of alcoholic liver disease (ALD). In the course of searching for antioxidant agents from natural products using ethanol-treated rat primary hepatocytes, 80% ethanol extract of O. ficus-indica seeds showed hepatoprotective activity. Bioassay-guided isolation of O. ficus-indica seeds resulted in the isolation of six lignan compounds that included 7S,8S-isoamericanin A (1), 7R,8R-isoamericanoic acid A (2), 7R,8R-9′-O-methylisoamericanol A (3), 3,3′-bisdemethylpinoresinol (4), 7S,8S-isoprincepin (5), and 7S,8S-princepin (6) (Fig. 1).^10–15) Among the isolates, 2 was reported for the first time from this plant. The presence of 1 and 3−6 in this plant had been previously reported in a Korean domestic patent; however, their absolute configurations of the structures were first identified in this study by comparison of their NMR and CD spectroscopic data with literature values (Table S1, S2, and Fig. S1).^10–15)

Fig. 1. The structures of the isolated compounds 1–6 from Opuntia ficus-indica seeds.

All isolates (1−6) were evaluated for their hepatoprotective activities against ethanol-treated primary cultured rat hepatocytes (Table S3). Quercetin was used as a positive control because it is a well-known hepatoprotective phytochemical.¹⁶⁾ The isolated compounds 1−6 showed higher protection than that of the positive control. Especially, 4–6, furofuran-type lignans consisting of the phenyl propanoid dimer linked by a C8-C8’ covalent bond, exhibited significant protective activities against ethanol-induced hepatotoxicity. The furofuran lignans 4–6 showed almost the same protective activity (EC₅₀; 13.7 μM, 18.6 μM, and 22.2 μM, respectively). We used the most abundant compound 4 that is a little stronger than those of compounds 5 and 6 for the further investigation.

Because ethanol-induced hepatotoxicity is caused by oxidative stress generated during ethanol metabolism, we examined the anti-oxidative activities of compounds 4–6. First, the effects of 4–6 on the content of intracellular peroxides were measured with a fluorimeter using 2,7-DCF-DA fluorescent dye (Fig. 2(A)).⁹⁾ The ROS levels increased by ethanol insult were decreased by the pretreatment of 4–6. Additionally, to confirm the ROS scavenging activity of 4, the intracellular peroxidation of HepG2 cells was assessed by flow cytometry (Fig. 2(B)).¹⁷⁾ Our results showed that the increased intracellular ROS production by ethanol treatment (18.5%) compared to the control group (4.7%) was effectively reduced by the pretreatment of 4 (10.0%). Moreover, the effects of 4–6 on antioxidant enzyme activities and GSH content in ethanol-treated primary rat hepatocytes were evaluated (Table 1).⁹⁾ GR and GSH-P_X activities, reduced by ethanol insult, were recovered by the treatment with 4–6. The reduced content of GSH and increased ratio of GSSG/total GSH induced by ethanol were also reversed by the treatment with 4–6. These results suggest that 4–6 effectively prevented ethanol-induced GSH depletion by restoring the activities of the antioxidant enzymes GSH-P_X and GR. Especially, 4, which exhibited the most remarkable change in reducing the cellular peroxide levels and restoring the antioxidant defense enzymes and total GSH content, has been suggested as the most powerful contributor to the anti-oxidative properties of O. ficus-indica seeds.

Fig. 2. Effects of compounds 4–6 on intracellular ROS production and the relationship between compound 4 and HO-1 expression.

Notes: (A) Rat primary hepatocytes were pretreated with 4–6 for 2 h before exposure to 200 mM ethanol and then maintained for 24 h. The intracellular peroxide content was determined with the fluorescent dye 2,7-DCF-DA. (B) HepG2 cells were pretreated with 4 (50 μM) for 2 h before the treatment with 350 mM ethanol and then maintained for 24 h. Cells were stained with DCF-DA fluorescent dye, and the degree of ROS formation was measured by flow cytometry. (C) HepG2 cells were treated with 4 for 24 h, and HO-1 protein levels were determined by western blotting. (D) The effect of the HO-1 inhibitor, SnPP, on the ROS-scavenging activity of 4 in ethanol-treated HepG2 cells was measured with the 2,7-DCF-DA fluorescent dye. (E) Effect of the HO-1 inhibitor on the hepatoprotective activity of 4 against ethanol-induced cytotoxicity in HepG2 cells. The data represent the mean ± SD. ^###p < 0.001 versus control group; ^*p < 0.05, ^**p < 0.01, and ^***p < 0.001, versus ethanol group; ^≠≠≠p < 0.001 versus 4-treated group (n = 3).

Table 1. The effects of 4–6 on the activities of antioxidant enzymes and the glutathione level in ethanol-treated primary rat hepatocytes.

	Conc. (μM)	GPx (mU/mg protein)	GR (mU/mg protein)	GSH (mU/mg protein)	GSSG/total GSH (ratio)
Ctl		0.08 ± 0.00	117.09 ± 3.50	52.35 ± 6.05	0.29 ± 0.06
EtOH		0.07 ± 0.00^###	85.45 ± 5.11^###	26.08 ± 3.06^###	0.52 ± 0.04^###
4	10	0.07 ± 0.01	93.89 ± 6.92^*	27.68 ± 3.34	0.54 ± 0.16
	25	0.08 ± 0.01	98.85 ± 4.24^***	30.87 ± 3.74	0.40 ± 0.09^**
	50	0.08 ± 0.01^*	100.04 ± 4.83^***	35.89 ± 1.13^*	0.23 ± 0.02^***
5	10	0.08 ± 0.01^**	88.79 ± 5.67	26.91 ± 3.99	0.39 ± 0.05^**
	25	0.09 ± 0.01^***	94.70 ± 4.25^**	28.66 ± 1.81	0.39 ± 0.05^**
	50	0.09 ± 0.01^***	96.10 ± 4.06^**	33.32 ± 3.28^**	0.30 ± 0.08^***
6	10	0.08 ± 0.00^***	88.85 ± 3.56	25.83 ± 0.76^**	0.48 ± 0.09
	25	0.08 ± 0.01^***	89.41 ± 4.29	28.94 ± 1.79	0.39 ± 0.06^**
	50	0.09 ± 0.00^***	97.20 ± 6.63^**	31.32 ± 1.73^*	0.36 ± 0.07^**

Notes: The results differ significantly from those of the control group.

^### p < 0.001, and from the ethanol-treated group.

^* p < 0.05

^** p < 0.01

^*** p < 0.001.

To determine whether compound 4, the substance showing the most potent hepatoprotective and antioxidant properties, regulates HO-1 to protect cells against oxidative damages, we performed western blot analysis (Fig. 2(C)). HO-1, a rate-limiting enzyme in heme catabolism, is known as a stress-response protein which has a fundamental role against oxidative stress by degrading heme into metabolites.¹⁸⁾ We observed that 4 significantly induced HO-1 expression in HepG2 cells in a concentration-dependent manner. In addition, our data showed that the inhibition of the HO-1 activity by HO-1 inhibitor, SnPP, reversed the antioxidative (Fig. 2(D)) and hepatoprotective (Fig. 2(E)) effect of 4 against ethanol-treated HepG2 cells. These results indicate that the antioxidative and hepatoprotective effect of 4, at least partly, is mediated through the expression of HO-1. This is the first report showing that a furofuran lignan exhibits antioxidative and hepatoprotective effects by inducing HO-1 protein expression in HepG2 human liver cells. In-depth study is needed to fully elucidate the hepatoprotecive mechanism by furofuran lignan and further in vivo evaluations are demanded.

Author contributions

Jung Wha Kim carried out the experiments with assistance from Hyeon Woo Kim. Jung Wha Kim prepared the manuscript. Heejung Yang and Hong Pyo Kim contributed analysis and discussion. Jung Wha Kim and Sang Hyun Sung designed the project and reviewed the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), which was funded by the Ministry of Science, ICT and Future Planning [NRF-2015M3A9A5030733].

Supplemental data

The supplemental data for this paper is available online at http://dx.doi.org/10.1080/09168451.2016.1234930.