Erlotinib mesylate, imatinib mesylate, lapatinib, and sunitinib were purchased from Sequoia research products (Pangbourne, United Kingdom). We prepared stock solutions in DMSO and stored them at -20°C. All other chemicals were supplied by Sigma–Aldrich (Buchs, Switzerland), except where indicated.

The human hepatocellular carcinoma cell line HepG2 was provided by American type culture collection (ATCC, Manassas, VA, United States). HepG2 cells were cultured under two different conditions – low glucose and galactose.

HepG2 cells under low glucose conditions were cultured in Dulbecco’s Modified Eagle Medium (DMEM containing 1 g/l [5.55 mM] glucose, 4 mM L-glutamine, and 1 mM pyruvate from Invitrogen, Basel, Switzerland) supplemented with 10% (v/v) heat-inactivated fetal bovine serum, 2 mM GlutaMax, 10 mM HEPES buffer, 10 mM non-essential amino acids, 100 units/ml penicillin, and 100 μg/ml streptomycin.

HepG2 cells under galactose conditions were cultured in Dulbecco’s Modified Eagle Medium (DMEM, containing no glucose but 4 mM L-glutamine) from Invitrogen (Basel, Switzerland) supplemented with 10% (v/v) heat-inactivated fetal bovine serum, 10 mM galactose, 10 mM HEPES buffer, 1 mM sodium pyruvate, 100 units/ml penicillin, and 100 μg/ml streptomycin.

The HepaRG cell line was provided by Biopredic International (Saint-Gregoire, France). Cells were cultured and differentiated as described earlier (Gripon et al., 2002). Induction of CYP3A4 was achieved by preincubation of differentiated HepaRG cells with 20 μM rifampicin for 72 h, with medium change every 24 h.

All cells were kept at 37°C in a humidified 5% CO₂ cell culture incubator and passaged using trypsin. The cell number was determined using a Neubauer hemacytometer and viability was checked using the trypan blue exclusion method.

The experiments were performed in accordance with the institutional guidelines for the care and use of laboratory animals. Male C57BL/6 mice (n = 12, age 7–10 weeks) were purchased from Charles River Laboratories (Sulzfeld, Germany) and housed in a standard facility with 12 h light–dark cycles and controlled temperature (21–22°C). The mice were fed a standard pellet chow and water ad libitum. The mice did not receive any treatment and were sacrificed by cervical dislocation.

Liver mitochondria were isolated by differential centrifugation as described before (Hoppel et al., 1979). The mitochondrial protein content was determined using the Pierce BCA protein assay kit from Merck (Zug, Switzerland).

Cytotoxicity, a marker for plasma membrane integrity, was assessed by using the ToxiLight assay from Lonza (Basel, Switzerland) as described previously (Felser et al., 2013). The intracellular ATP content, a marker for metabolic cell activity and cell viability, was determined using a CellTiter Glo kit from Promega (Wallisellen, Switzerland) as described before (Felser et al., 2013).

Incubations with culture medium containing 0.1% DMSO were used as negative and incubations containing 0.5% Triton X as positive controls.

The glycolytic flux was determined via the conversion of [³H]-glucose to ³H₂O as described before (Vander Heiden et al., 2001). HepG2 cells were seeded in 6-well plates (500,000 cells/well) and treated with drugs for 48 h. The positive control was 20 mM 2-deoxy-D-glucose. After treatment, HepG2 cells were resuspended in 1 ml Krebs buffer (115 mM sodium chloride, 2 mM potassium chloride, 25 mM sodium bicarbonate, 1 mM magnesium chloride, 2 mM calcium chloride, 0.25% FBS, pH 7.4) and incubated for 30 min at 37°C. After centrifugation, cell pellets were resuspended in 0.5 ml Krebs buffer containing 10 mM glucose and 0.5 μl D-³H(U) glucose (60 Ci/mmol, 0.5 μCi/assay, Perkin Elmer, Schwerzenbach, Switzerland) and incubated for 1 h at 37°C. After centrifugation, 50 μl supernatant were transferred to tubes containing 50 μl 0.2 N HCl. Tubes were transferred to scintillation vials containing 0.5 ml water and sealed. ³H₂O was allowed to evaporate from the tube and to condense in the 0.5 ml water for 1 week. Afterward, the tube was removed and the radioactivity of the water was measured using a Packard 1900 TR liquid scintillation analyzer. The data were normalized to the protein content.

The Δψ_m in freshly isolated mouse-liver mitochondria was determined using the [phenyl-³H]-tetra-phenylphosphonium bromide uptake assay as described previously (Felser et al., 2013). Mitochondria were treated with test compounds for 15 min at 37°C. The radioactivity of the samples was measured on a Packard 1900 TR liquid scintillation analyzer.

Mitochondrial membrane potential in HepG2 cells was determined using TMRM (Invitrogen, Basel, Switzerland). In brief, HepG2 cells were seeded in 24-well plates (200,000 cells/well) and treated with drugs for 48 h. Cells were washed with DPBS and suspended in PBS with 100 nM TMRM. After 15 min incubation in the dark, cells were centrifuged and resuspended in PBS for analyzing them with flow cytometry using a FACSCalibur (BD Bioscience, Allschwil, Switzerland). Data were analyzed using CellQuest Pro 6.0 software (BD Bioscience, Allschwil, Switzerland).

Cellular respiration in intact cells was measured with a Seahorse XF24 analyzer (Seahorse Biosciences, North Billerica, MA, United States) as described before (Felser et al., 2013). Cellular respiration in intact cells was measured with a Seahorse XF24 analyzer (Seahorse Biosciences, North Billerica, MA, United States). HepG2 cells were seeded in Seahorse XF 24-well culture plates at 100,000 cells/well in DMEM growth medium and allow to adhere overnight. Cells were treated for 48 h with drugs. Before assessing the cellular respiration, the medium was replaced with 750 μl unbuffered DMEM medium (4 mM L-glutamate, 1 mM pyruvate, 1 g/l glucose, 63.3 mM sodium chloride, pH 7.4) and equilibrated at 37°C in a CO₂-free incubator for at least 30 min. Afterward, plates were transferred to the XF24 analyzer. Basal OCR was determined in the presence of glutamate/pyruvate (4 and 1 mM, respectively). The oxidative leak (a marker for uncoupling) was determined after inhibition of the mitochondrial phosphorylation by adding 1 μM oligomycin. The mitochondrial electron transport chain was stimulated maximally by the addition of 2 μM FCCP. Finally, the extramitochondrial respiration was determined after the addition of complex I inhibitor rotenone (1 μM). For the determination of the basal respiration, the oxidative leak, and the maximum respiration, the extramitochondrial respiration was subtracted. Respiration was expressed as OCR per minute.

The activity of specific enzyme complexes of the respiratory chain was analyzed using an Oxygraph-2k high-resolution respirometer equipped with DataLab software (Oroboros instruments, Innsbruck, Austria). HepG2 cells were treated for 48 h with drugs. Afterward, they were suspended in MiR05 (mitochondrial respiration medium containing 0.5 mM EGTA, 3 mM magnesium chloride, 20 mM taurine, 10 mM potassium dihydrogen phosphate, 20 mM HEPES, 110 mM sucrose, 1 g/l fatty-acid free bovine serum albumin, and 60 mM lactobionic acid, pH 7.1) and transferred to the pre-calibrated Oxygraph chamber (Pesta and Gnaiger, 2012).

Respiratory capacities through complexes I, II, III, and IV were assessed in HepG2 cells permeabilized with digitonin (10 μg/1 million cells). Complexes I and III were analyzed using L-glutamate/malate (10 and 2 mM, respectively) as substrates followed by the addition of adenosine-diphosphate (ADP; 2.5 mM) and rotenone (0.5 μM) as inhibitor of complex I. Afterward, duroquinol (500 μM) was added to investigate complex III activity.

Complexes II and IV were analyzed using succinate/rotenone (10 mM and 0.5 μM, respectively) as substrates followed by the addition of ADP (2.5 mM) and the complex III inhibitor antimycin A (2.5 μM). N,N,N′,N′-tetramethyl-1,4-phenylendiamine/ascorbate (0.5 and 2 mM, respectively) were added to investigate complex IV.

The integrity of the outer mitochondrial membrane was confirmed by the absence of a stimulatory effect of exogenous cytochrome c (10 μM) on respiration. Respiration was expressed as oxygen consumption per mg protein. Protein concentrations were determined using the Pierce BCA protein assay kit from Merck (Zug, Switzerland).

Freshly isolated mouse liver mitochondria were suspended in MiR06 (mitochondrial respiration medium containing 0.5 mM EGTA, 3 mM magnesium chloride, 20 mM taurine, 10 mM potassium dihydrogen phosphate, 20 mM HEPES, 110 mM sucrose, 1 g/l fatty-acid free bovine serum albumin, 60 mM lactobionic acid, and 280 units/ml catalase, pH 7.1) and 250 μg mitochondria were transferred to the pre-calibrated Oxygraph chamber and treated for 15 min with drugs. Respiratory capacities through complexes I, II, III, and IV were assessed with the same protocol as for HepG2 cells, except the permeabilization step in the beginning.

Metabolism of [1-¹⁴C] palmitic acid (60 mCi/mmol, PerkinElmer, Schwerzenbach, Switzerland) was assessed via the formation of ¹⁴C-acid-soluble β-oxidation products (Felser et al., 2013). HepG2 cells were seeded in 6-well plates (500,000 cells/well) and treated with drugs for 48 h. The positive control was 5 μM etomoxir. After treatment, HepG2 cells were permeabilized with digitonin (10 μg/1 million cells) in 225 μl assay buffer (70 mM sucrose, 43 mM potassium chloride, 3.6 mM magnesium chloride, 7.2 mM potassium dihydrogen phosphate, 36 mM TRIS, 0.2 mM ATP, 50 μM L-carnitine, 15 μM coenzyme A, 5 mM acetoacetate, pH 7.4) and incubated for 10 min at 37°C. Afterward, 25 μl [1-¹⁴C] palmitic acid (200 μM final concentration, 10 μCi/assay) was added to each sample and incubated at 37°C. The reaction was stopped after 15 min by adding 250 μl 6% perchloric acid. The samples were precipitated for 5 min on ice before centrifugation. Radioactivity was measured in the supernatant using a Packard 1900 TR liquid scintillation analyzer.

Generation of ROS was assessed using the ROS-Glo H₂O₂ Assay (Promega, Wallisellen, Switzerland). Briefly, cells were grown in 96-well plates and exposed to a range of TKIs for 48 h. The assay was performed according to manufacturer’s manual and the luminescence was measured using a Tecan M200 Pro Infinity plate reader (Männedorf, Switzerland).

Generation of mitochondrial ROS was assessed using MitoSOX Red (Invitrogen, Basel, Switzerland). HepG2 cells were seeded into black costar 96-well plates and exposed to a range of TKIs. The positive control was 100 μM amiodarone. After 48 h, cell culture medium was removed and 2.5 μM MitoSOX dissolved in 100 μl DPBS was added. After incubation for 10 min at 37°C in the dark, fluorescence was measured (excitation 510 nm, emission 580 nm) using a Tecan M200 Pro Infinity plate reader (Männedorf, Switzerland).

The reduced form of glutathione (GSH) content was determined using the luminescent GSH-Glo Glutathione assay (Promega, Wallisellen, Switzerland). In brief, cells were grown in 96-well plates and exposed to a range of TKIs for 48 h. The positive control was 100 μM BSO. The assay was performed according to manufacturer’s manual and the luminescence was measured after 15 min in the dark using a Tecan M200 Pro Infinity plate reader (Männedorf, Switzerland).

HepG2 cells were treated with TKIs for 48 h. The mRNA expression of SOD1 and SOD2 were assessed as described previously (Felser et al., 2013). Briefly, the mRNA expression was assessed using real-time PCR. RNA was extracted and purified using the Qiagen RNeasy mini extraction kit (Qiagen, Hombrechtikon, Switzerland). The quantity and purity of RNA were measured with NanoDrop 2000 (Thermo Scientific, Wohlen, Switzerland). cDNA was synthesized from 10 μg RNA using the Qiagen omniscript system. The real-time PCR was performed using SYBR Green (Roche Diagnostics, Rotkreuz, Switzerland). We used primers for SOD1 (forward: 5′-TGGCCGATGTGTCTATTGAA-3′, reverse: 5′-ACCTTTGCCCAAGTCATCTG-3′) and SOD2 (forward: 5′-GGTTGTTCACGTAGGCCG-3′, reverse:5′-CAGCAGGCAGCTGGCT-3′) and calculated relative quantities of specifically amplified cDNA with the comparative-threshold cycle method. GAPDH was used as endogenous reference (forward: 5′-CATGGCCTTCCGTGTTCCTA-3′; reverse: 5′-CCT- GCTTCACCACCTTCTTGA-3′).

For the quantification of cytochrome c, cytoplasm and mitochondria were separated using a Mitochondrial/Cytosol Fractionation Kit (ab65320, Abcam, Cambridge, United Kingdom). Afterward, the cytochrome c content in the mitochondrial and cytosolic fraction was quantified by western blotting using Anti-cytochrome C antibody (ab133504, Abcam, Cambridge, United Kingdom). The purity of the fractions was checked by the determination of TOMM20 (a protein of the outer mitochondrial membrane) and α-tubulin (a major constituent of microtubules in the cytoplasm) by western blotting (Supplementary Figure S1). The antibodies used for western blotting were ab78547 (Abcam, Cambridge, United Kingdom) for TOMM20 and ab76290 (Abcam, Cambridge, United Kingdom) for alpha-tubulin. Western blots were performed as described in the following section.

After treatment for 48 h, HepG2 cells were lysed with RIPA buffer containing complete Mini protease inhibitor cocktail (Roche Diagnostics, Mannheim, Germany). After centrifugation, the supernatant was collected and stored at -80°C. Proteins were resolved by SDS–PAGE using commercially available 4–12% NuPAGE Bis-Tris gels (Invitrogen, Basel, Switzerland) and transferred using the Trans-Blot Turbo Blotting System (Bio-Rad, Cressier, Switzerland). The membranes were incubated with PARP (46D11) rabbit mAb (Cell Signaling Technology, Danvers, MA, United States), Anti-active Caspase-3 antibody (ab32042, Abcam, Cambridge, United Kingdom), Anti-pro Caspase 3 antibody (ab32150, Abcam, Cambridge, United Kingdom), Anti-SOD1 (ab20926, Abcam, Cambridge, United Kingdom) or Anti-SOD2 (ab16956, Abcam, Cambridge, United Kingdom) antibodies. After washing, membranes were exposed to secondary antibodies (Santa Cruz Biotechnology, Dallas, TX, United States). Immunoblots were developed using enhanced chemiluminescence (GE Healthcare, Little Chalfont, United Kingdom). Band intensities of the scanned images were quantified using the National Institutes of Health Image J program, version 1.48.

Caspase 3/7 activity was determined using the luminescent Caspase-Glo 3/7 assay (Promega, Wallisellen, Switzerland). Cells were grown in 96-well plates and exposed to a range of TKIs for 48 h. The luminescence was measured using a Tecan M200 Pro Infinity plate reader (Männedorf, Switzerland).

Data are given as the mean ± SEM of at least three independent experiments. Statistical analyses including calculation of EC₅₀ values were performed using the GraphPad Prism 6 (GraphPad Software, La Jolla, CA, United States). For the comparison of more than two groups, one-way ANOVA was used, followed by Dunnett’s post-test procedure. Differences between experiments with multiple conditions were compared using two-way ANOVA followed by Bonferroni’s post hoc test. ^∗P-values < 0.05, ^∗∗P-values < 0.01, or ^∗∗∗P-values < 0.001 were considered significant.

Release of AK was determined as a marker of cytotoxicity, and the cellular ATP content as a measure for mitochondrial function and cellular integrity. As shown in Figures 1A–C, erlotinib (investigated up to 20 μM) and imatinib were only slightly toxic in HepG2 cells at the highest concentrations and after 24 to 48 h of incubation. While erlotinib did not affect the cellular ATP content (Figures 1D–F), imatinib decreased the ATP content in a time- and concentration-dependent fashion (starting at 5 μM after 24 h of incubation). In comparison, lapatinib was cytotoxic starting at 10–20 μM depending on the incubation time (Figures 1A–C). Lapatinib also decreased the cellular ATP content, starting at similar concentrations as cytotoxicity (Figures 1D–F). Sunitinib was cytotoxic starting at 5–10 μM after 24–48 h of incubation (Figures 1A–C) and decreased the cellular ATP content starting at 5 μM after 24 or 48 h of incubation (Figures 1D–F). The corresponding EC₅₀ values are shown in Table 1. For imatinib, sunitinib and erlotinib, similar results were obtained in cells cultured with galactose instead of glucose (Supplementary Figure S2), whereas as lapatinib was less toxic under these conditions.

Cytotoxicity and ATP content were also investigated in HepaRG cells (Figures 2A–H). HepaRG cells contain inducible cytochrome P450 enzymes (CYPs) (Berger et al., 2016), allowing to test the possible contribution of metabolites for toxicity. After 48 h of treatment, erlotinib started to be cytotoxic at 5 μM and to decrease the intracellular ATP at 10 μM (Figures 2A,E). CYP induction was associated with less toxicity, suggesting that the parent compound is more toxic than the metabolites. For imatinib, cytotoxicity and reduction of intracellular ATP started at 50 μM (Figures 2B,F). Since cytotoxicity was significantly more accentuated after CYP induction, metabolites may have played a role. Lapatinib started to be cytotoxic at 20 μM, but caused no significant decrease of intracellular ATP (Figures 2C,G). At the highest concentration studied (50 μM), cytotoxicity was increased by CYP induction. Sunitinib started to be toxic at 10 μM and to decrease the ATP content starting at 50 μM (Figures 2D,H). Similar to imatinib and lapatinib, cytotoxicity was increased by CYP induction at the highest concentration studied (50 μM). The corresponding EC₅₀ values are shown in Table 1.

CYP induction slightly increased cytotoxicity at high concentrations of imatinib, lapatinib and sunitinib. Since the parent compounds were also toxic themselves, we decided to continue our investigations with the parent compounds using HepG2 cells and isolated liver mitochondria.

To investigate possible reasons for the observed decrease in the cellular ATP content, we determined the effect of the TKIs investigated on glycolysis and on the Δψ_m in HepG2 cells and in mouse liver mitochondria.

We determined the glycolytic flux by quantifying the conversion of [³H]glucose to ³H₂O. As shown in Figure 3A, imatinib, lapatinib, and sunitinib reduced the glycolytic flux in HepG2 cells starting at 50, 20, and 20 μM, respectively. Erlotinib did not impair glycolysis. The corresponding EC₅₀ values are given in Table 1. The positive control 2-deoxy-D-glucose reduced glycolysis by 75%.

Next, we quantified the effect on Δψ_m in HepG2 cells treated for 48 h with the TKIs (Figure 3B). Imatinib, lapatinib and sunitinib dissipated Δψ_m starting at 50, 20, and 1 μM, respectively. In contrast, erlotinib did not decrease Δψ_m up to 20 μM. The uncoupler CCCP (50 μM) reduced Δψ_m by 62% (data not shown).

As shown in Figure 3C, imatinib and sunitinib reduced Δψ_m of isolated mouse liver mitochondria after exposure for 15 min starting at 50 and 10 μM, respectively, while erlotinib and lapatinib had no significant effect. Non-radioactive tetra-phenylphosphonium (positive control) apparently reduced Δψ_m by 38% (data not shown). The corresponding EC₅₀ values are given in Table 1.

The observed decrease in intracellular ATP and mitochondrial membrane potential could be caused by impairment of the function and/or uncoupling of the respiratory chain (Kaufmann et al., 2005). We therefore assessed the effect of TKIs on oxygen consumption by HepG2 cells using a XF24 analyzer. Cellular oxygen consumption mainly reflects mitochondrial metabolism (Felser et al., 2013). As shown in Figures 4A–C, exposure with erlotinib, imatinib, and lapatinib between 1 and 20 μM for 48 h did not significantly change the cellular oxygen consumption by HepG2 cells. Sunitinib decreased the maximal respiration rate (after addition of FCCP) in a concentration-dependent fashion, reaching 28% at 10 μM. Sunitinib (1–10 μM) did not increase the leak respiration after addition of oligomycin, excluding an uncoupling effect (Figure 4D).

In order to find out the mechanism of decreased maximal respiration, the respiratory capacities through the complexes of the electron transport chain were analyzed using a high-resolution respirometry system. Irrespective of the culture medium (low glucose or galactose), sunitinib impaired the activity of complex I in HepG2 cells exposed for 48 h in a concentration-dependent fashion, reaching significance at 10 μM for galactose (Figure 5A and Supplementary Figure S3). Imatinib inhibited complex III in a concentration-dependent fashion, reaching significance at 50 μM (low glucose, Supplementary Figure S3) or 10 μM (galactose, Figure 5B).

In isolated mouse liver mitochondria exposed for 15 min, sunitinib and imatinib were less toxic. Under these conditions, 100 μM sunitinib and 100 and 200 μM imatinib significantly decreased the respiratory capacity through complex I (Figure 5C).

As shown in Supplementary Figure S4, erlotinib and lapatinib did not affect palmitate metabolism in HepG2 cells up to 20 and 50 μM, respectively. Sunitinib and imatinib decreased palmitate oxidation starting at 50 and 100 μM, respectively. Since β-oxidation was inhibited at higher concentrations than glycolysis and the function of the electron transport chain, we did not assess the mechanism of this inhibition.

Toxicants inhibiting complex I and III can stimulate superoxide production in mitochondria (Drose and Brandt, 2012). Accordingly, specific mitochondrial superoxide accumulation in HepG2 cells exposed for 48 h started at 50 μM for imatinib and 20 μM for lapatinib, but not for erlotinib up to 20 μM (Figure 6A). Sunitinib could not be investigated due to self-fluorescence interacting with the assay.

We therefore determined also cellular production of H₂O₂ by HepG2 cells exposed to TKIs for 48 h (Figure 6B). All TKIs investigated started to increase the cellular production of H₂O₂ at the lowest concentration investigated (2 μM for sunitinib and 5 μM for imatinib, lapatinib and erlotinib). For sunitinib and imatinib, there was a sharp increase in H₂O₂ production at 20 and 50 μM, respectively. The positive control 50 μM amiodarone increased the H₂O₂ accumulation 11-fold (data not shown).

Superoxide dismutases (SOD) are important for ROS defense and are located in the mitochondrial matrix (SOD2) or in the cytoplasm (SOD1) (Bresciani et al., 2015). In HepG2 cells exposed for 48 h to imatinib, mRNA expression of SOD1 and SOD2 was increased at 50 μM, reaching statistical significance for SOD1 (Figure 6C). Lapatinib suppressed mRNA expression of SOD2, but did not affect SOD1, whereas sunitinib increased SOD1 expression without affecting SOD2. With the exception of erlotinib, TCIs generally increased the protein expression of SOD1 and SOD2 in a concentration-dependent fashion, reaching statistical significance for sunitinib (Figure 6D).

Accumulating ROS can be degraded by the glutathione antioxidant system, which is an effective scavenger of free radicals (Schafer and Buettner, 2001; Fernandez-Checa and Kaplowitz, 2005). In HepG2 cells exposed for 48 h, imatinib started to decrease the GSH content at 10 μM, reaching statistical significance at 50 μM (Figure 6E). In cells exposed to sunitinib, the cellular GSH content was not affected at 10 μM but not different from zero at 20 μM. Cytotoxicity, which was in a range of 30% for 20 μM sunitinib, has likely contributed to this finding. For erlotinib and lapatinib, no significant decrease in GSH was found. 100 μM BSO, which served as positive control, decreased the GSH content by more than 90% (data not shown).

Mitochondrial toxicity associated with imatinib and sunitinib suggested involvement of mitochondria in cell death. Release of cytochrome c from mitochondria into the cytoplasm is an initial trigger for apoptosis (Green and Reed, 1998). As shown in Figure 7A, all TKIs investigated were associated with an increase of cytochrome c in the cytoplasm. This was associated with a concentration-dependent increase in the activity of caspases 3/7 for imatinib, lapatinib, and sunitinib starting at 50, 20, and 20 μM, respectively, but not for erlotinib (Figure 7B). The positive control 200 nM staurosporine increased the caspases 3/7 activity sixfold (data not shown).

As shown in Figure 7C, the ratio of active-to-pro caspase 3 protein level was increased significantly for lapatinib and sunitinib starting at 50 and 20 μM, respectively. For imatinib there was also a threefold increase at 50 μM but without reaching statistical significance, whereas erlotinib did not affect the ratio of active-to-pro caspase 3 protein levels (Figure 7C). Accordingly, the protein expression of the full-length PARP was reduced for imatinib and sunitinib starting for both at 10 μM, but not for erlotinib and lapatinib (Figure 7D).