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Abstract

Rosmarinic acid (RA) is a natural polyphenolic compound derived from many common herbal plants. Although it is known that RA has many important biological activities, its effect on proteasome inhibitor-induced changes in cancer treatment or its effects on any experimental proteasome inhibition model is unknown. The aim of the study was to investigate the effect of RA on MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells. HepG2 cells were treated with 10, 100, and 1000 µM RA in the presence of MG132 for 24 h; 10 and 100 µM RA did not affect but 1000 µM RA decreased cell viability in HepG2 cells. MG132 caused a significant decrease in cell viability and phosphorylation of mammalian target of rapamycin and a significant increase in levels of polyubiquitinated protein, microtubule-associated proteins 1A/1B light chain 3B-II (LC3B-II), heat shock protein 70 (HSP70), binding immunoglobulin protein (BiP), activating transcription factor 4 (ATF4), protein carbonyl, and cleaved poly(adenosine diphosphate-ribose) polymerase 1 (PARP1); 10 and 100 µM RA did not significantly change these effects of MG132 in HepG2 cells; 1000 µM RA caused a significant decrease in cell viability and a significant increase in polyubiquitinated protein, LC3B-II, HSP70, BiP, ATF4, protein carbonyl, and cleaved PARP1 levels in MG132-treated cells. Our study showed that only 1000 µM RA increased MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells. According to our results, cytotoxic concentration of RA can potentiate the effects of MG132 in hepatocellular carcinoma treatment.

Keywords

Rosmarinic acid proteasome inhibitor hepatocellular carcinoma autophagy oxidative stress apoptosis

Introduction

The control of synthesis, folding, trafficking, and degradation of proteins are important for protein homeostasis, which is essential for maintaining cell viability and growth.¹ The ubiquitin-proteasome system (UPS) and autophagy are two major pathways for intracellular protein degradation.² In the UPS, ubiquitin-tagged proteins are recognized and degraded by 26S proteasomes while autophagy includes autophagosome formation, the fusion of the autophagosome with the lysosome, and then degradation of cargo proteins.³

Carbobenzoxy- l -z- l-leucyl-l-leucinal (MG132) is a peptide aldehyde which inhibits 26S proteasome activity. MG132 induces apoptosis through increasing reactive oxygen species, increasing trimerization of heat shock factor-1, inhibiting nuclear factor kappa B, and inducing the pro-apoptotic protein Noxa via a p53-independent mechanism in tumor cells.⁴

HepG2 was derived from a liver hepatocellular carcinoma of a Caucasian male.⁵ Researchers have used MG132 on HepG2 cells to investigate its potential therapeutic role on hepatocellular carcinoma.^6,7 It was reported that proteasome inhibitor MG132 induces cytotoxicity,⁸ autophagy,⁹ apoptosis,^6,7 and cellular stresses such as endoplasmic reticulum (ER) stress,¹⁰ oxidative stress,¹¹ and heat shock protein response¹² in HepG2 cells.

On the other hand, HepG2 cells retain many biological characteristics of primary hepatocytes¹³ and protein degradation is important for cellular processes, such as cell cycle, antigen processing, apoptosis, and lipid metabolism in the liver.¹⁴ Thus, researchers also have used MG132 on HepG2 cells to study the effect of proteasome inhibition on cellular liver models.^15
–17

Rosmarinic acid (RA), an ester of caffeic acid, is a natural polyphenolic compound that is found especially in medicinal herb plants. RA has many important biological activities such as anti-inflammatory, antimicrobial, and antioxidant activities, and also its anticarcinogenic effect is well-known.^18,19 In the literature, both the potential therapeutic role of RA on hepatocellular carcinoma^20,21 and its hepatoprotective effect in experimental toxicology studies^22
–24 were examined using different concentrations of RA on HepG2 cells.

To the best of our knowledge, there is no study investigating the effect of RA on proteasome inhibitor-induced changes in cancer treatment or its effects on any experimental proteasome inhibition model. The aim of the present study was to investigate the effect of RA on MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells.

Materials and methods

Chemicals

HepG2 cells (HB-8065, Lot number: 61983117) were purchased from American Type Culture Collection (Middlesex, UK), stored in liquid nitrogen at early passages (≤5), thawed, and used for experiments between passages 7 and 12. The minimum essential medium was purchased from Wisent Inc. (St-Bruno, QC, Canada). Fetal bovine serum, antibiotic-antimycotic, trypsin-ethylenediaminetetraacetic acid (EDTA), chemiluminescent substrate, and poly(adenosine diphosphate-ribose) polymerase 1 (PARP1) antibody were purchased from Thermo Fisher (Waltham, Massachusetts, USA). Anti-dinitrophenol, heat shock protein 70 (HSP70), α-tubulin and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) antibodies, and horseradish peroxidase (HRP)-conjugated secondary antibodies were purchased from Abcam (Cambridge, UK). Microtubule-associated proteins 1A/1B light chain 3B (LC3B), activating transcription factor 4 (ATF4), mammalian target of rapamycin (mTOR), and phosphorylated mTOR (p-mTOR) (Ser2448) antibodies were purchased from Cell Signaling Technology (Beverly, Massachusetts, USA). Binding immunoglobulin protein (BiP) antibody was purchased from Lifespan Biosciences (Seattle, Washington, USA). Ubiquitin antibody was purchased from Santa Cruz (Heidelberg, Germany). Polyvinylidene fluoride (PVDF) membrane was purchased from Roche Diagnostics (Mannheim, Germany). RA, MG132, and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) were purchased from Sigma–Aldrich Co. (St Louis, Missouri, USA). Other chemicals were purchased from Sigma–Aldrich Co. (St Louis, Missouri, USA) or Merck (Darmstadt, Germany). All reagents were of analytical grade.

Cell culture and experimental design

HepG2 cells were cultured in minimum essential medium containing 10% fetal bovine serum and 1% antibiotic-antimycotic (10,000 units/mL penicillin, 10,000 μg/mL streptomycin, and 25 µg/mL of Gibco Amphotericin B) in a humidified environment at 37°C and 5% carbon dioxide (CO₂) atmosphere.

RA was dissolved in medium and applied at different concentrations (10, 100, and 1000 μM) for 24 h. Cells were also treated with MG132 at 1 μM concentration which was shown to inhibit proteasome activity in previous reports.^17,25,26 MG132 (10 mM in dimethyl sulfoxide (DMSO)) was readymade solution and DMSO concentration was 0.01% in all experimental mediums.

Cell viability assays

Cell viability was evaluated by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay²⁷; 10⁴ cells were seeded into 96 well plates. Cells were treated with different RA concentrations (10, 100, and 1000 μM) in the absence or presence of 1 μM MG132 for 24 h. At the end of treatment, mediums were removed and 10 µL of MTT (5 mg/mL) solved in phosphate-buffered saline (PBS) and 100 µL of medium without phenol red were added to each well. Then, the cells were incubated for 4 h in a humidified environment at 37°C and 5% CO₂ atmosphere. After 4 h, the MTT-containing medium was removed and formazan crystals were dissolved by adding 200 µL DMSO and 25 µL Sorensen buffer (0.1 M glycine and 0.1 M sodium chloride equilibrated to pH 10.5 with 0.1 M NaOH). The optical density of plates was measured using a microplate reader at 570/630 nm.²⁸ The optical density of each sample was then compared with the mean optical density value of the control.

Immunoblotting

Cells were seeded into a 90 mm culture dish. After the cells reached 70–80% confluence, they were treated with RA (0, 10, 100, and 1000 μM) and 1 μM MG132 for 24 h. At the end of treatment, cells were washed twice with ice-cold PBS and lysed with hot (90°C) 10 mM Tris–HCl buffer (pH:8) containing 1% sodium dodecyl sulfate (SDS), 1 mM EDTA, 50 mM NaF, 10 mM N-ethylmaleimide, and 50 mM iodoacetamide. Protein concentrations were measured according to the study by Lowry et al.²⁹ using bovine serum albumin (BSA) as standard; 20 μg of total protein was separated by 4–8% or 4–12% SDS-polyacrylamide gel electrophoresis and transferred to PVDF membrane using a semidry blotting system. Membranes were blocked with 5% skim milk powder or BSA (for p-mTOR) for 1 h at room temperature. After blocking, membranes were incubated with primary antibodies (ubiquitin = 1:500, LC3B = 1:1000, mTOR = 1:2000, p-mTOR = 1:2000, HSP70 = 1:10,000, BiP = 1:10,000, ATF4 = 1:2000, PARP1 = 1:5000, α-tubulin = 1:10,000, and GAPDH = 1:10,000) overnight at 4°C and later with HRP-conjugated secondary antibodies (1:10,000) at room temperature for 1 h. Protein bands were visualized using the electrochemiluminescence detection system and were quantified by Image J (version ImageJ 1.51j8, Wayne Rasband, National Institutes of Health, USA).³⁰ Results were calculated relative to α-tubulin or GAPDH (for polyubiquitinated proteins) as a loading control and expressed as fold change relative to control for each blot.

Derivatization and immunodetection of protein carbonyls

Cells were seeded and treated, cell lysates were prepared, and their protein concentrations were measured as described above in immunoblotting. All sample volumes were adjusted to the same volume with lysis buffer; 8 μg of total protein from each sample was mixed respectively with 1 volume (same amount with sample) 12% SDS and 2 volumes 20 mM 2,4 dinitrophenylhydrazine solution in 10% trifluoroacetic acid, and the mixture was incubated for 20 min. Then, 1.5 volumes neutralization solution (2 M Tris/30% glycerol/19% 2-mercaptoethanol) was added and mixture loaded onto 4–8% SDS-polyacrylamide gel.³¹ After separation with electrophoresis, the immunoblotting procedure was performed using the anti-dinitrophenol antibody (1:10,000) as mentioned before.

Statistical analysis

Results were given as mean ± standard deviation for three results from three independent experimental procedures for all parameters. The one-way analysis of variance test was used for comparison of biochemical parameters among the groups, and then, Tukey’s post-hoc test was used for multiple comparisons when the significant difference obtained. SPSS 20.0 (IBM SPSS Inc., Chicago, Illinois, USA) statistical software was used for statistical analysis. The p-value <0.05 was considered as statistical significant.

Results

Effect of RA on the viability of HepG2 cells in the absence or presence of MG132

The mean percentage of cell viabilities were 99% for 10 µM RA-treated cells, 103% for 100 µM RA-treated cells, 44% for 1000 µM RA-treated cells, 40% for MG132-treated cells, 42% for MG132+10 µM RA-treated cells, 40% for MG132+100 µM RA-treated cells, and 28% for MG132+1000 µM RA-treated cells; 1000 µM RA caused a significant decrease in cell viability as compared with control (p < 0.05). All cells treated with MG132 had significantly lower cell viability when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly lower cell viability than 1000 µM RA-treated cells and MG132-treated cells (p < 0.05 for both; Figure 1).

Figure 1.

Effect of RA on the viability of HepG2 cells in the absence or presence of MG132. Data are expressed as the mean ± SD of three results from three independent experiments. Data were analyzed by one-way ANOVA and Tukey’s post-hoc test was performed for multiple comparisons: ^a p < 0.05 versus control, ^b p < 0.05 versus 1000 µM RA-treated cells, and ^c p < 0.05 versus MG132-treated cells. RA: rosmarinic acid; ANOVA: analysis of variance; SD: standard deviation.

Effect of RA on polyubiquitinated protein levels, LC3B-II levels, and mTOR phosphorylation in MG132-treated HepG2 cells

Polyubiquitinated protein levels of MG132-treated cells were 5.80-fold, MG132+10 µM RA-treated cells were 6.88-fold, MG132+100 µM RA-treated cells were 7.78-fold, and MG132+1000 µM RA-treated cells were 8.40-fold of control. All cells treated with MG132 had significantly higher polyubiquitinated protein levels when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly higher polyubiquitinated protein levels than MG132-treated cells and MG132+10 µM RA-treated cells (p < 0.05 for both; Figure 2(a)).

Figure 2.

Effect of RA on protein degradation pathways in MG132-treated HepG2 cells. Effect of RA concentrations on (a) ubiquitinated protein levels, (b) LC3B-II levels, and (c) p-mTOR/mTOR ratios in MG132-treated cells for 24 h. Data are expressed as the mean ± SD of three results from three independent experiments. Data were analyzed by one-way ANOVA and Tukey’s post-hoc test was performed for multiple comparisons: ^a p < 0.05 versus control, ^b p < 0.05 versus MG132-treated cells, ^c p < 0.05 versus MG132+10 µM RA-treated cells, and ^d p < 0.05 versus MG132+100 µM RA-treated cells. RA: rosmarinic acid; LC3B-II: proteins 1A/1B light chain 3B-II; mTOR: mammalian target of rapamycin; p-mTOR: phosphorylated mTOR; ANOVA: analysis of variance; SD: standard deviation.

LC3B-II levels of MG132-treated cells were 3.67-fold, MG132+10 µM RA-treated cells were 3.73-fold, MG132+100 µM RA-treated cells were 4.22-fold, and MG132+1000 µM RA-treated cells were 5.14-fold of control. All cells treated with MG132 had significantly higher LC3B-II levels when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly higher LC3B-II levels than MG132-treated cells, MG132+10 µM RA-treated cells, and MG132+100 µM RA-treated cells (p < 0.05 for all; Figure 2(b)).

The ratios of p-mTOR/mTOR were 0.86-fold in MG132-treated cells, 0.86-fold in MG132+10 µM RA-treated cells, 0.85-fold in MG132+100 µM RA-treated cells, and 0.81-fold in MG132+1000 µM RA-treated cells. All cells treated with MG132 had significantly lower p-mTOR/mTOR ratios when compared with the control group (p < 0.05 for all; Figure 2(c)).

Effect of RA on HSP70, BiP, and ATF4 levels in MG132-treated HepG2 cells

HSP70 levels of MG132-treated cells were 5.67-fold, MG132+10 µM RA-treated cells were 5.67-fold, MG132+100 µM RA-treated cells were 7.80-fold, and MG132+1000 µM RA-treated cells were 9.21-fold of control. All cells treated with MG132 had significantly higher HSP70 levels when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly higher HSP70 levels than MG132-treated cells and MG132+10 µM RA-treated cells (p < 0.05 for both; Figure 3(a)).

Figure 3.

Effect of RA on heat shock response and endoplasmic reticulum stress in MG132-treated HepG2 cells. Effect of RA concentrations on (a) HSP70, (b) BiP, and (c) ATF4 protein levels in MG132-treated cells for 24 h. Data are expressed as the mean ± SD of three results from three independent experiments. Data were analyzed by one-way ANOVA and Tukey’s post-hoc test was performed for multiple comparisons: ^a p < 0.05 versus control, ^b p < 0.05 versus MG132-treated cells, ^c p < 0.05 versus MG132+10 µM RA-treated cells, and ^d p < 0.05 versus MG132+100 µM RA-treated cells. RA: rosmarinic acid; HSP70: heat shock protein 70; BiP: binding immunoglobulin protein; ATF4: activating transcription factor 4; ANOVA: analysis of variance; SD: standard deviation.

BiP levels of MG132-treated cells were 1.36-fold, MG132+10 µM RA-treated cells were 1.42-fold, MG132+100 µM RA-treated cells were 1.33-fold, and MG132+1000 µM RA-treated cells were 1.84-fold of control. All cells treated with MG132 had significantly higher BiP levels when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly higher BiP levels than MG132-treated cells, MG132+10 µM RA-treated cells, and MG132+100 µM RA-treated cells (p < 0.05 for all; Figure 3(b)).

ATF4 levels of MG132-treated cells were 5.92-fold, MG132+10 µM RA-treated cells were 10.21-fold, MG132+100 µM RA-treated cells were 9.37-fold, and MG132+1000 µM RA-treated cells were 14.57-fold of control. All cells treated with MG132 had significantly higher ATF4 levels when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly higher ATF4 levels than MG132-treated cells and MG132+100 µM RA-treated cells (p < 0.05 for both; Figure 3(c)).

Effect of RA on protein carbonyl and cleaved PARP1 levels in MG132-treated HepG2 cells

Protein carbonyl levels of MG132-treated cells were 1.56-fold, MG132+10 µM RA-treated cells were 1.40-fold, MG132+100 µM RA-treated cells were 1.45-fold, and MG132+1000 µM RA-treated cells were 2.34-fold of control. MG132-treated cells and MG132+1000 µM RA-treated cells had significantly higher protein carbonyl levels when compared with the control group (p < 0.05 for both). MG132+1000 µM RA-treated cells had significantly higher protein carbonyl levels than MG132-treated cells, MG132+10 µM RA-treated cells, and MG132+100 µM RA-treated cells (p < 0.05 for all; Figure 4(a)).

Figure 4.

Effect of RA on protein oxidation and apoptosis in MG132-treated HepG2 cells. Effect of RA concentrations on (a) protein carbonyl and (b) cleaved PARP1 levels in MG132-treated cells for 24 h. Data are expressed as the mean ± SD of three results from three independent experiments. Data were analyzed by one-way ANOVA and Tukey’s post-hoc test was performed for multiple comparisons: ^a p < 0.05 versus control, ^b p < 0.05 versus MG132-treated cells, ^c p < 0.05 versus MG132+10 µM RA-treated cells, and ^d p < 0.05 versus MG132+100 µM RA-treated cells. RA: rosmarinic acid; PARP1: protein carbonyl, and cleaved poly(adenosine diphosphate-ribose) polymerase 1; ANOVA: analysis of variance; SD: standard deviation.

Cleaved PARP1 levels of MG132-treated cells were 3.32-fold, MG132+10 µM RA-treated cells were 3.13-fold, MG132+100 µM RA-treated cells were 5.15-fold, and MG132+1000 µM RA-treated cells were 21.03-fold of control. All cells treated with MG132 had significantly higher cleaved PARP1 levels when compared with the control group (p < 0.05 for all). MG132+1000 µM RA-treated cells had significantly higher cleaved PARP1 levels than MG132-treated cells, MG132+10 µM RA-treated cells, and MG132+100 µM RA-treated cells (p < 0.05 for all; Figure 4(b)).

Discussion

MG132, a proteasome inhibitor, is used on HepG2 cells to investigate the potential role in cancer treatment^6,7 and the effect of proteasome inhibition in experimental models.^15
–17 RA is a natural polyphenolic compound derived from many common herbal plants. Although it is known that RA has many important biological activities, its effect on proteasome inhibitor-induced changes in cancer treatment or its effects on any experimental proteasome inhibition model is unknown.^18,19 The aim of the present study was to investigate the effect of RA on MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells.

In the literature, HepG2 cells were treated with different RA concentrations^32,33 up to 2800 µM.³⁴ To investigate the effect of RA in MG132-induced changes in a wide range, we used three logarithmic increasing concentrations of RA (10, 100, and 1000 µM) in our study. We found that 10 and 100 µM RA have no significant effect on cell viability. Similar to our results, Wu et al.³³ had reported that no significant change in cell viability was observed in HepG2 cells when cells were treated with RA at 0–160 µM for 24 and 48 h. On the other hand, previous studies reported that RA causes cytotoxicity at higher concentrations in HepG2 cells.^32,34 In keeping with these studies,^32,34 highest RA concentration (1000 µM) caused remarkable cytotoxicity (56%) in HepG2 cells after 24 h treatment in our study.

MG132 is cytotoxic to HepG2 cells and it has treatment potential for hepatocellular carcinoma.^7,10 In our study, MG132 caused a significant decrease in cell viability. We also found that RA at 10 and 100 µM concentrations does not make a difference but 1000 µM RA rises MG132-induced cytotoxicity. Our findings indicate that a combination of RA at the cytotoxic concentration (1000 µM) with MG132 has more power to decrease the viability of HepG2 cells than RA or MG132 alone and also non-cytotoxic concentrations of RA (10 and 100 µM) have no protective effect against MG132-induced cytotoxicity.

Proteasome inhibition causes the accumulation of polyubiquitinated proteins in cells.³⁵ Increasing polyubiquitinated proteins in HepG2 cells by MG132 confirms the proteasome inhibition in our study. The inhibition of proteasome causes compensatory activation of autophagy.² Compatible with a previous study,⁹ we also found MG132 caused a significant increase in LC3B-II level which is widely used as a marker to monitor autophagy.³⁶ mTOR has been known as a key regulator for autophagy and activation of mTOR inhibits autophagy.³⁷ For the first time, we showed that MG132 has a decreasing effect on the phosphorylation of mTOR in HepG2 cells. As a chaperone, HSP70 has essential roles on protein folding, disaggregation, and degradation³⁸ and its is also one of the markers of heat shock protein response.³⁹ We found that MG132 increases HSP70 levels, and this result was compatible with the literature.¹² Proteasome inhibition causes ER stress and unfolded protein response.² In our study, MG132 caused an increase in BiP and ATF4 levels which are the markers of ER stress⁴⁰ and our results support a previous report¹⁰ which reported that MG132 increases ER stress in HepG2 cells.

When we co-treated HepG2 cells with MG132 and RA, non-cytotoxic concentrations of RA (10 and 100 µM) did not change but cytotoxic concentration of RA (1000 µM) increased MG132-induced proteasome inhibition, autophagy, HSP70 level, and ER stress. Also, RA concentrations also did not change the phosphorylation of mTOR in MG132-treated cells. Our mTOR phosphorylation results showed that 1000 µM RA increases MG132-induced autophagy independent from the mTOR pathway. In our study, 1000 µM RA treatment results were consistent for proteasome inhibition, autophagy, ER stress, and HSP70 levels and similar additive effects of 1000 µM RA on MG132 treatment were found. Thus, the effect of 1000 µM RA may not be specific for these pathways.

Proteasome plays an important role in degrading oxidatively modified proteins and proteasomal dysfunction can result in the accumulation of oxidized proteins.^41,42 MG132 increases oxidative stress¹¹ and apoptosis⁷ in HepG2 cells. In keeping with previous studies,^7,11 we found that MG132 causes a significant increase on levels of protein carbonyls which is the marker of protein oxidation³¹ and cleaved PARP1 which is an apoptotic fragment of PARP1.⁴³

In the present study, non-cytotoxic concentrations of RA had no significant effect on protein carbonyl and cleaved PARP1 levels in MG132-treated cells. Also, when MG132-treated cells were treated with non-cytotoxic concentrations of RA, there was no significant difference in protein carbonyl levels between these cells and the control group. Non-cytotoxic concentrations of RA were insufficient to decrease protein carbonyl levels in MG132-treated cells. Carbonylation is an irreversible post-translational modification of proteins and proteasome inhibition causes increased protein carbonyl levels by preventing their degradation.⁴² In our study, non-cytotoxic concentrations of RA also had no effect on MG132-induced proteasome inhibition and this can be responsible for the insufficient effect of these RA concentrations on MG132-induced protein oxidation. In keeping with our study, Kolettas et al.⁴⁴ had reported that RA failed to suppress hydrogen peroxide-mediated apoptosis and possessed no antioxidant properties in Jurkat cells.

It was reported that RA induces apoptosis and causes cytotoxicity in HepG2 cells.^20,32 Huang et al.²¹ found that RA combined with adriamycin induces apoptosis in HepG2 cells. In our study, the cytotoxic concentration of RA (1000 µM) increased MG132-induced apoptosis and this result supports the previous reports.^20,21,32 In addition to its antioxidant activities, RA has also pro-oxidant activities.⁴⁵ Murakami et al.⁴⁶ reported that cytotoxicity of RA may be related to its pro-oxidant action and also Hur et al.⁴⁷ reported that RA increases reactive oxygen species and induces apoptosis in Jurkat and peripheral T cells via the mitochondrial pathway. Compatible with these literatures,^46,47 we found an increase in protein oxidation, apoptosis, and cytotoxicity in MG132-treated HepG2 cells when the cytotoxic concentration of RA was applied. In our study, 1000 µM RA may increase apoptosis and cytotoxicity via its pro-oxidant action. Despite the UPS is responsible for the removal of oxidatively damaged proteins, high levels of oxidative stress inhibit UPS.⁴⁸ Thus, UPS inhibition by 1000 µM RA may be associated with increased protein oxidation in MG132-treated cells.

Our study showed that 10 and 100 µM RA have no effect but 1000 µM RA increases MG132-induced cytotoxicity, proteasome inhibition, autophagy, cellular stresses, and apoptosis in HepG2 cells. We can report that non-cytotoxic RA concentrations are not protective against MG132 but RA at cytotoxic concentration increases the effect of MG132. According to our results, cytotoxic concentration of RA can potentiate the effects of MG132 in hepatocellular carcinoma treatment. Thus, combined treatment with MG132 and cytotoxic concentrations of RA may be more effective than the use of MG132 alone for hepatocellular carcinoma treatment. Nevertheless, in the present study, we used only HepG2 cell line to investigate the effects of RA, and therefore, these effects of RA also needs to be supported by future in vitro studies which also use healthy cell lines and by future in vivo studies.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

GS Ozgun

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