S-15176 difumarate salt, a derivative of the anti-ischemic metabolic drug trimetazidine, has been intensively studied for its impact on cellular metabolism in animal models of ischemia-reperfusion injury of the liver, heart, spinal cord, and other organs. Despite evidence of some reduction in oxidative damage to cells, the results of therapy with S-15176 have been mostly disappointing, possibly because of the lack of data on its underlying mechanisms. The role of complexes I-IV of the electron transport chain and membrane permeability transition in mitochondrial toxicity associated with S-15176 is investigated.
1. Introduction
Trimetazidine (TMZ), the derivative of piperazine, is an anti-ischemic metabolic agent of a potent class of drugs called partial inhibitors of fatty acid oxidation (pFOX). Meta-analyses of clinical studies have proved the therapeutic effect of TMZ against stable angina. Nowadays, TMZ is included in the guidelines of the European Society of Cardiology for the management of stable angina pectoris and stable coronary artery disease
[1,2,3][1][2][3]. Some studies have demonstrated that TMZ has a cytoprotective effect in models of ischemic damage to the kidney and liver
[4,5][4][5], as well as in several metabolic pathologies, including diabetes mellitus
[6,7,8][6][7][8]. The beneficial effect of TMZ on ischemic tissues is mainly attributed to its inhibitory action on the long-chain 3-ketoacyl-coenzyme A thiolase, resulting in inhibition of the beta-oxidation pathway of free fatty acids
[9,10,11][9][10][11]. TMZ can also increase the activity of pyruvate dehydrogenase, which is the rate-limiting enzyme of glucose oxidation under aerobic conditions
[10,11,12][10][11][12]. This, by switching cellular metabolism from the oxidation of free fatty acids towards the utilization of carbohydrates, would improve the efficiency of ATP synthesis for a given amount of oxygen molecules consumed, thereby maintaining the energy supply in hypoxia
[12]. Taken together, TMZ can shift the energy substrate metabolism, enhancing glucose metabolism and decreasing oxygen consumption. This may be accompanied by decreased ROS production, a limited increase in intracellular acidosis, and reduced accumulation of cytosolic calcium. All of these events are related to the mitochondria, which are likely to have several targets for this drug. Recent studies suggest that TMZ inhibits the opening of the mitochondrial permeability transition (MPT) pore, contributing to protection against dysfunction of mitochondria and cell death in response to ischemia
[13,14,15][13][14][15].
S-15176 (N-{(3,5-di-
tert-butyl4-hydroxy-1-thiophenyl)}-3-propyl-N′-(2,3,4-trimethoxybenzyl) piperazine) difumarate salt is a derivative of TMZ
(Figure S1), but its therapeutic efficacy and mechanisms of action have not yet been fully understood. The drug can act on several targets in the mitochondria. In particular, S-15176 has been reported to inhibit the rate-limiting enzyme of the beta-oxidation cycle carnitine palmitoyltransferase I (CPT-1)
[16], suppress the formation of the MPT pore and the cyclosporin A-insensitive mitochondrial pore, and prevent free radical-induced toxicity
[17,18][17][18]. These effects are suggested to underlie the protective action of S-15176 against mitochondrial and cellular dysfunction in ischemia–reperfusion injury of liver and myocardium tissues, traumatic spinal cord injury, and experimental diabetes mellitus
[19,20,21,22,23][19][20][21][22][23].
Some studies on isolated mitochondria revealed that, in addition to the above effects, S-15176 could induce mitochondrial uncoupling and reverse the activity of the ATP synthase, leading to the hydrolysis of ATP molecules
[17,24][17][24]. However, the mechanism of these processes remains unknown. It was suggested that the S-15176-induced decrease in the mitochondrial membrane potential proceed through a disruption of the function of the respiratory chain complexes. One can assume that treatment with S-15176 may lead to mitochondrial dysfunction, impaired energy metabolism, ion dyshomeostasis, and, ultimately, cell death in tissues that are most vulnerable to the toxic action of chemicals or exposed to higher doses of the agent. Therefore, a study of the role of the mitochondrial membrane potential and mitochondrial enzyme complexes of the respiratory chain in S-15176-induced toxicity is needed, as well as a study of the involvement of mitochondrial ROS production and permeability transition, which could potentially provide possibilities for interventions on the adverse effects caused by this agent.
Based on these considerations, wresearchers used rat thymocytes and isolated rat liver mitochondria as model objects to examine the effect of S-15176 at different concentrations on the mitochondrial membrane potential and mitochondrial respiratory capacities when using different combinations of respiratory substrates. To determine which complex(es) of the electron transfer system is (are) affected by S-15176, weresearchers estimated the enzymatic activity of complexes I-IV in the presence of this agent. WeResearchers then evaluated its effects on the generation of mitochondrial ROS and Ca2+-induced permeability transition as possible mechanisms of the drug-associated mitotoxicity. OuResearchers' results indicated that acute exposure to S-15176 resulted in a higher dissipation of the membrane potential of the organelles energized with succinate or malate/glutamate, but not ascorbate/TMPD. In addition, S-15176 difumarate salt inhibited complex I- and complex II-linked ADP-stimulated respiration but had no effect on CIV-linked ADP-stimulated respiration, which could be due to its direct inhibitory action on enzyme complex III of the mitochondrial electron transfer system. WeResearchers also found that low doses of S-15176 diminished the production of H2O2 and increased the calcium retention capacity index, while at concentrations above 30 μM, S-15176 contributed to mitochondrial membrane permeabilization.
2. S-15176 Difumarate Salt Promotes Mitochondrial Depolarization in Rat Thymocytes
Increasing evidence suggests that S-15176 difumarate salt acts on several targets in the mitochondria
[16,17,18,24][16][17][18][24]. In this
sent
udy, wery, researchers evaluated whether this drug affects the key indicators of mitochondrial activity. The mitochondrial membrane potential is known to be the main index of mitochondrial function since it reflects the processes of electron transfer and oxidative phosphorylation.
OurResearchers' results demonstrated that S-15176 difumarate salt could initiate mitochondrial depolarization in rat thymocytes. As shown in
Figure 1, preincubation of rat thymocytes with S-15176 at concentrations of 10 and 30 µM for 30 min resulted in a decline in the mitochondrial membrane potential in a dose-dependent manner. Importantly, the addition of S-15176 at a concentration of 30 µM led to complete mitochondrial depolarization in the whole cell population.
Figure 1. S-15176 difumarate salt induces the depolarization of the mitochondrial membrane in rat thymocytes. Mitochondrial membrane potential (MitoPotential) was assessed with the Muse Cell Analyzer using the Muse MitoPotential kit. Cells were treated with 0 μM S-15176 (a), 10 μM S-15176 (b), or 30 μM S-15176 (c) for 30 min. Typical profile plots are presented. Panel (d) shows the ratio (%) of living, depolarized, and dead rat thymocytes in the presence of S-15176 at different concentrations. In control experiments (CTR, 0 µM S-15176), an equivalent volume of the solvent (0.1% DMSO) was used. Data represent the mean ± SEM (n = 4).
3. S-15176 Inhibits ADP- and 2,4-Dinitrophenol- Stimulated Mitochondrial Respiration Due to Suppression of the Enzymatic Activity of the Respiratory Complex III
To elucidate the mechanism of mitochondrial depolarization caused by S-15176,
wresearche
rs investigated the effect of the agent on the efficiency of oxidative phosphorylation (OXPHOS) and the enzymatic activity of the individual OXPHOS complexes I–IV in isolated mitochondria.
The influence of S-15176 on the mitochondrial bioenergetics was assessed by the rate of oxygen consumption by rat liver mitochondria in the main metabolic states using different combinations of respiration substrates.
Table 1 demonstrates the effect of S-15176 on the respiration rates of mitochondria oxidizing the substrates of complex I (2.5 mM glutamate and 2.5 mM malate) or complex II (5 mM succinate in the presence of 1 μM rotenone). One can see that when using both of these combinations of substrates, S-15176 difumarate salt stimulated mitochondrial respiration under resting conditions (States 4 and 2) in a dose-dependent manner (
Table 1). Moreover, S-15176 dose-dependently suppressed the rates of mitochondrial respiration in the presence of ADP (State 3) or the uncoupler 2,4-dinitrophenol (DNP) (State 3U
DNP). In parallel, the respiratory control ratio (RCR) (State 3/State 4), which is directly related to the OXPHOS coupling efficiency, was reduced by 2.1 and 1.8 times in the presence of 30 µM S-15176 when using the substrates of complex I and complex II, respectively. Furthermore, the successive addition of 10 μM S-15176 pulses to the mitochondrial suspension resulted in a gradual decrease in the membrane potential of mitochondria energized by complex I- or complex II-linked respiratory substrates (
Figure 2). The latter is consistent with the literature data that S-15176 can display an uncoupling activity in rat liver mitochondria
[24].
Figure 2. Sequential addition of 10 μM S-15176 gradually decreases the membrane potential of isolated rat liver mitochondria energized by the complex I substrates (2.5 mM potassium glutamate + 2.5 mM malate) (a), or the complex II substrate (5 mM potassium succinate in the presence of 1 µM rotenone) (b), but not by the complex IV substrates (5 mM ascorbic acid + 0.2 mM TMPD) (c). Mitochondrial membrane potential was estimated by the distribution of tetraphenylphosphonium bromide (TPP+) with an ion-sensitive electrode. Additions: 10 µM S-15176 (five pulse additions), 50 µM DNP. Typical traces are shown (n = 6).
Table 1.
Effects of S-15176 difumarate salt on the respiration of rat liver mitochondria.
S-15176, μM |
V Respiration, nmol O2 × min−1 × mg−1 Protein |
State 2 |
State 3 |
State 4 |
State 3UDNP |
RCR |
Glutamate + malate |
0 |
3.1 ± 0.2 |
21.5 ± 1.1 |
3.5 ± 0.1 |
21.8 ± 1.0 |
5.9 ± 0.1 |
10 |
4.0 ± 0.3 * |
18.1 ± 0.8 * |
3.9 ± 0.2 |
17.9 ± 0.9 * |
4.7 ± 0.1 *** |
30 |
4.5 ± 0.3 ** |
16.0 ± 0.2 ** |
5.9 ± 0.1 ** |
15.9 ± 0.2 ** |
2.7 ± 0.1 *** |
Succinate |
0 |
6.0 ± 0.3 |
32.8 ± 0.6 |
6.9 ± 0.2 |
47.2 ± 1.2 |
4.8 ± 0.1 |
10 |
7.7 ± 0.2 ** |
30.7 ± 0.6 * |
8.4 ± 0.3 ** |
44.1 ± 0.8 |
3.7 ± 0.2 ** |
30 |
8.9 ± 0.2 ** |
29.3 ± 0.9 * |
10.8 ± 0.3 ** |
40.9 ± 1.4 * |
2.7 ± 0.1 *** |
Ascorbate + TMPD |