Salvianolic acid B inhibits mitochondrial dysfunction by up-regulating mortalin
Salvianolic acid B inhibits mitochondrial dysfunction by up-regulating mortalin
OPEN Salvianolic acid B is an antioxidative ingredient derived from Radix Salviae miltiorrhizae that has been widely used to treat liver diseases. However, the therapeutic mechanism underlying Salvianolic acid B has remained largely unknown. Our studies verified that Salvianolic acid B efficiently blocked mitochondrial deformation and dysfunction induced by H2O2 in the human hepatocyte cell line HL7702. Mortalin, a mitochondrial molecular chaperone, maintains mitochondrial morphology stabilization and function integrity. Previous results showed that mortalin overexpression has been observed in hematoma carcinoma cells and that mortalin maintains mitochondrial homeostasis and antagonizes oxidative stress damage. We found that Salvianolic acid B significantly up-regulated mortalin protein expression levels. In addition, Salvianolic acid B lost the function of preventing mitochondrial deformation and dysfunction induced by oxidative stress under mortalin knockdown conditions. We further found that mortalin overexpression increases the mRNA expression of mitofusin-related factor Mfn1 and mitofission-related factor hFis1. In conclusion, Salvianolic acid B maintains the mitochondrial structure stabilization and functional integrity by up-regulating mortalin, which may be associated with increased mitofusin factor Mfn1 and reduced mitofission factor hFis1.
Hepatic disease is a major global health problem with significant morbidity and increased mortality that affects
one hundred million people worldwide. Hepatic disease is characterized by additional collagen and accumulation
of extracellular matrix in response to hepatocellular damage1. Clinical and experimental evidence suggest that
oxidative stress and mitochondrial dysfunction mediated the pathological progression of hepatocellular
damage2,3. Numerous studies have found that reactive oxygen species (ROS) play a critical role in producing liver
damage and hepatic disease by stimulating the generation of profibrogenic mediators from Kupffer cells and
directly stimulating hepatic satellite cells (HSC)4. Therefore, exploring novel antioxidants that prevent hepatic
fibrogenesis is one of the most important strategies in treating liver diseases.
Salvianolic acid B (SalB, C36H30O16, molecular weight = 718.6138 g/mol) is the aqueous bioactive component
from Salvia miltiorrhiza bunge. SalB is a polyphenlic compound found in abundance in this plant5. SalB is among
the most effective natural antioxidants and has significant scavenging effects on oxygen free radicals and
protecting the liver against injury and fibrosis6?8. Previous studies reported that SalB is effective in inhibiting
hepatocellular apoptosis by regulating apoptotic-related factors in the mitochondrial death pathway. SalB is also involved
in ameliorating mitochondrial energy metabolism9,10. In living cells, mitochondria are relevant sources of reactive
oxygen species. Werner J.H Koopman found that redox homeostasis is closely associated with mitochondrial
morphology and function11. Mitochondria are dynamic organelles that are present in virtually every mammalian
cell and display a continuous cycle of fission and fusion12,13. Mitochondrial fission is guided by Dynamic-related
protein 1(Drp1) and human fission protein1 (hFis1), whereas mitochondrial fusion is executed by two mitofusins
(Mfn1/Mfn2) and optic atrophy 1 protein (Opa1)14,15. However, the mechanisms of action for SalB on hepatocyte
apoptosis have not been thoroughly elucidated, especially those associated with mitochondrial morphology and
Mortalin (mtHsp70/Hsp75/Grp75/PBP74) is a heat un-inducible protein that is a heat shock protein 7
(Hsp70) family member. Mortalin is located predominantly in mitochondria, where it functions to maintain
mitochondrial homeostasis and quality control16,17. As an important mitochondrial chaperone, mortalin drives
nuclear-encoded proteins across the mitochondrial membrane into the matrix by an ATP-dependent mechanism
via the interaction of other chaperones18,19. Previous studies show that mortalin overexpression inhibits cell
apoptosis by attenuating/reducing accumulation of ROS during various forms of stresses20,21. Mortalin knockdown is
also effective in mitochondrial dynamics. Mortalin induces mitochondrial fission by regulating the balance of
Therefore, this study primarily focuses on investigating the effects of SalB on mitochondrial homeostasis and
the balance of mitochondrial dynamics and testing whether mortalin can protect against mitochondrial
dysfunction by regulating the balance of mitochondrial dynamics under H2O2-induced oxidative stress. Additionally, we
studied the possible mechanisms of SalB mitochondrial protection by assessing the correlation with mortalin.
Materials and Methods
Materials. HL-7702 Cells were purchased from the Institute of Biochemistry and Cell Biology, China
Academy of Science, Shanghai. Salvianolic acid B, which was obtained from Traditional Chinese Medicine of
Shanghai University, was dissolved in sterile distilled water. In addition,
3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was purchased from Sigma-Aldrich (St. Louis, MO, USA). Dimethylsulfoxide
(DMSO) was obtained from Shenggong Biology (Shanghai, China). Mitochondrial membrane potential detection
kit, ATP detection kit and Hoechst 33258 were purchased from Beyotime (Jiangsu, China). Dulbecco?s modified
Eagle?s medium (DMEM) supplement was obtained from Gibco Invitrogen Co. (Gaithersburg, MD, USA). The
fluorescent dye 2?,7?-dichlorodihydro-fluorescein diacetate (H2DCF-DA) and MitoTrackerGreen (MTG) were
purchased from Invitrogen (Gaithersburg, MD, USA). Antibodies against cytochrome c, caspase 3 and
mortalin were obtained from Cell Signaling Technology (Beverly, MA, USA) and Epitomics (Burlingame, CA, USA).
Antibodies against Bcl-2 and Bax were purchased from Epitomics (Burlingame, CA, USA), and antibodies against
Mfn2 and hFis1were acquired from Proteintech (Proteintech, Rosemont, IL, USA). All the other chemicals were
of the highest grade of purity available commercially.
Cell culture and treatment. HL-7702 cells were routinely cultured in DMEM (Invitrogen, Gaithersburg,
MD, USA) containing 10% fetal calf serum (FBS; Biowest, Caille, France), 100 U/ml penicillin and 100 mg/ml
streptomycin and maintained at 37 ?C with 5% CO2. Cells were incubated with 400 ?M H2O2 for 2 h to induce cell
apoptosis and pre-incubated with various concentrations of SalB for 2 h.
Preparation of SalB stock solution. The stock solution of SalB was prepared by dissolving 10mg SalB
using 1.39156 ml sterile distilled water to generate a 10 mM solution that was stored at ?70 ?C for future use.
Plasmid construction. To construct the mortalin plasmid, the mortalin target sequence was prepared
from HL-7702 cellular cDNA and subcloned into pcDNA3.1(+). The construct was verified by sequencing.
The primers used in the experiments were 5?-GGTAC CTTTATCCGCCATGATAAGTGCCAG-3? (sense) and
5?-GGATCCTCAGGAAGT CTCTTCACTCCTAAG-3? (anti-sense).
Lentivirus-meditated RNA interference. Lentivirus-meditated RNA interference was performed
as previously described [
]. HL-7702 cells were infected with a lentivirus expressing shRNA targeting the
CGTGCTCAATTTGAAGGGATT sequence of mortalin. This sequence was assessed against the BLAST database
to confirm specificity. A scrambled shRNA sequence was used as a control.
Determination of cell viability. Cell viability was measured by conventional MTT reduction assay. The
cultured cells were seeded at an initial density of 5 ? 104cells/ml in a 96-well plate for 24 h and pre-incubated with
50, 100 and 200 ?M SalB for 2 h and exposed to 400 ?M H2O2 for 2 h. Following incubation, a 20-?l MTT stock
solution (5 mg/ml) was added to each well at a final concentration of 0.5 mg/ml for an additional 4 h. The resulting
formazan was dissolved in 150 ?l DMSO and measured with a microtiter plate reader (Thermo Fisher Scientific,
Waltham, MA, USA) at a wavelength of 492 nm.
Intracellular ROS detection. Intracellular ROS was detected by H2DCF-DA, an oxidation-sensitive
fluorescent probe. Normal and mortalin knockdown (KD) cells were pre-incubated with SalB for 2 h and exposed
to 400 ?M H2O2 for 2 h. The medium was removed. Cells were washed twice with serum-free medium and
incubated with H2DCF-DA (10 ?M) for 20 min at 37 ?C in the dark. The fluorescence intensity was monitored on an
automatic fluorescence microplate reader with an excitation wavelength of 488 nm and an emission wavelength
of 525 nm and was observed under a DM2000 fluorescence microscope (Leica Microsystems, Wetzlar, Germany)
equipped with a Leica DFC420 camera.
Detection of ATP levels. Intracellular ATP levels were measured by a firefly-luciferase-based ATP detection
kit (Beyotime, Jiangsu, China) according to the manufacturer?s instructions. Briefly, cells were seeded into the
24-well plates and washed with phosphate-buffered saline (PBS) thrice. The supernatant of samples was collected
immediately on ice and measured with an illuminometer. ATP levels were calculated according to an ATP
standard curve. Intracellular ATP levels were analyzed as the percentage of the control group.
MMP measurement. Intracellular MMP was evaluated using the fluorescent, lipophilic and cationic
probe, 5,5?,6,6?-tetrachloro-1,1?,3,3?-iodide (JC-1) (Beyotime, Jiangsu, China) according to the manufacturer?s
instructions. Briefly, following treatment, the cells were loaded with JC-1 staining solution for 20 min at 37 ?C
and washed thrice with JC-1 staining buffer. The fluorescence images were obtained with a DM2000 fluorescence
microscope equipped with a Leica DFC420 camera. The fluorescence intensity was measured using a CytoFluor
multiwell plate reader at 514 nm for excitation and 529 nm for emission of green (monomer form) fluorescence
and 585 nm for excitation and 590 nm for emission for red (aggregate form) fluorescence. The MMP of cells in
each group was evaluated as the fluorescence ratio of red to green and observed using a fluorescence microscope.
Immunofluorescence. Cells or mortalin knockdown cells were placed on cover slips in 24-well plates
and pretreated with SalB for 2 h prior to exposure to 400 ?M H2O2 for 2 h. After washing with PBS, the slides
were fixed in 4% paraformaldehyde for 10 min at room temperature, washed thrice with PBS, permeabilized
with 0.1% saponin, blocked with 10% normal goat serum and incubated overnight at 4 ?C with cytochrome c
antibody. The slides were incubated with FITC-conjugated goat anti-rabbit immunoglobulin (Sigma-Aldrich,
Burlingame, CA, USA) for 2 h. Nuclei were stained with Hoechst 33258 (Beyotime, Jiangsu, China). Cover slips
were observed under a fluorescence microscope (Leica Microsystems, Wetzlar, Germany) equipped with a Leica
Cytoplasmic and mitochondrial protein extraction. Cytoplasmic and mitochondrial proteins were
extracted using a mitochondrial and cytoplasmic protein extraction kit (Beyotime, Jiangsu, China) according to
Quantitative real-time PCR analysis. To measure target mRNA expression levels, total RNA was
isolated from HL-7702 cells with Trizol (Invitrogen, Carlsbad, CA, USA) according to the
manufacturer?s instructions. First-strand cDNA was synthesized using the RevertAid first-strand cDNA synthesis kit
(Fermentas, Vilnius, Lithuania). PCR products were measured by real-time PCR with Ultra-Fast SybrGreen
QPCR Master Mix (Agilent Technologies, Pala Alto, CA). The data were calculated using the following
equation: relative mRNA expression = 2???Ct, where ??Ct = (Ctsample ? Ctcontrol) treatment ?(Ctsample ? Ctcontrol)
normal. The following primers were used: mortalin, 5? -CGCCCCACTTGTTTTGGA-3? (sense) and
5?-CAGGAGTTGGTAGT ACCCAAATC-3? (anti-sense); Mfn1, 5?-GCAACTGAAAAACTGAGGAT-3? (sense)
and 5?-ACTTGTTGGCACAGGCGAGC-3? (anti-sense); Mfn2, 5?-ACTTGTTGGCA CAGGCGAGC-3? (sense)
and 5?-TGTTTGGCCGGGAGAGACGC-3?(anti-sense); Opa1, 5?-GCAGGCTCGTCTCAAGGATA-3?(sense)
and 5?-TCTTCCAGTATAACC ACCTC-3?(anti- sense); hFis1, 5?-AAAGGGAGCAAGGAGGAACA-3?(sense)
and 5?-TGGGGCTCTGTCTGCAGCAA-3?(anti-sense); Drp1, 5?-GATGGGAAGGGTTA TTGGAG-3?(sense)
and 5?-CAGTTACACTCTTCTTGTTG-3?(anti-sense); MTP18, 5?-TCATGTCAGAGCCGCAGCCG-3?(sense)
and 5?-TGGCACAAGAGAGCGGA AAG-3?(anti-sense);GAPDH, 5?-TTGCCATCAATGACCCCTTCA-3?
(sense) and 5?-CGCCCCACTTGATTTTGGA-3?(anti-sense).
Quantification of mitochondrial morphology. Quantitative analysis of mitochondrial shape and
number was performed as previously described [HY, mito-analysis, mito-analysis3]. HL-7702 cells were stained with
mitochondrial tracker green (MTG), a mitochondrial-specific fluorescent probe, and visualized using a
fluorescence microscope equipped with a Leica DFC420 camera. Using ImageJ version 1.44, images were optimized
by adjusting the contrast to span the complete gray range from 0 to 255, subjected to a 7 ? 7 ?top hat? spatial
filter and thresholded. The 8-bit binary images were used for automated image analysis yielding the number of
mitochondrial per cell (Nc), mitochondrial aspect ratio (AR: ratio between major and minor axes of the ellipse
equivalent to the mitochondrion), and mitochondrial form factor (FF: perimeter2/4? * area). AR measured
mitochondrial length, and FF determined mitochondrial length and degree of branching.
Western blot analysis. Cells were lysed in SDS buffer supplemented with a mixture of protease inhibitors,
1 ?g/ml aprotinin and 100 ?g/ml phenylmethylsulfonyl fluorides. The cell suspension was incubated on ice for
30 min then centrifuged at 20,000 ? g for 15 min at 4 ?C. The supernatant was collected for further analysis. The
protein concentrations of the supernatants were determined using the Bradford method. Cell lysates were
denatured for 15 min in 5 ? sample buffer and separated by SDS-polyacrylamide gel electrophoresis. For Western blot
analysis, the gel was transferred onto nitrocellulose membranes using a tank transfer system. Blotted membranes
were placed in a blocking solution of 5% non-fat milk in Tris-buffered saline Tween-20 (TBS-T). For
immunodetection, membranes were incubated for 1 h at room temperature and then incubated overnight at 4 ?C with the
relevant primary antibodies. Then, the membranes were washed with TBST and incubated with the appropriate
horseradish peroxidase-conjugated secondary antibodies. Immunocomplexes were visualized using a
commercially available enhanced chemiluminescence kit with exposure of the transfer membrane to X-ray film. The
following antibodies were used: anti-Bcl-2, anti-Bax, anti-Cyto C, anti-mortalin, anti-caspase 3, anti-? actin, and
anti-glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
Primary hepatocyte isolation. Mouse primary hepatocytes were isolated from C57BL/6 aged 8?12 weeks
by collagenase digestion and subsequent purification centrifugation fresh prepared hepatocytes were seeded in
6-well plates in attachment media. Cell media were cultured in DMEM supplemented with 10% fetal bovine
serum, 100 U/ml penicillin and 100 U/ml streptomycin after 24 h. All animals received humane care and the
experimental protocol was approved by the committee of laboratory animals according to institutional guidelines.
Statistical analysis. Data were expressed as the means ? SEM from at least three independent experiments.
Statistical significance analysis was performed by GraphPad Prism 5.0 software (GraphPad Software, Inc., La
Jolla, CA) using Student?s t-test or ANOVA. Mean values were considered to be statistically significant at P < 0.05
or P < 0.01.
The protective effects of SalB on H2O2-induced mitochondrial deformation. Mitochondrial mor
phology is crucial for proper cell function and physiology. Mitochondria are organized into reticular networks
that undergo frequent shape changes. The mitochondrial shape is tightly controlled by the processes of
mitochondrial fission-fusion23. However, the destruction of mitochondrial fission-fusion balance results in mitochondrial
dysfunction and deformation and is associated with numerous human diseases24,25. To determine the effects of
SalB on mitochondrial morphology, hepatocytes were stained with MTG, a mitochondrial-specific fluorescent
probe, and observed by fluorescence microscopy. Quantitative analyses of the mitochondrial morphology were
reported previously. Important mitochondrial morphological parameters, including AR (aspect ratio) that reflects
mitochondrial length and FF (form factor) and represents mitochondrial length and degree, were analyzed using
Image J Software I. In order to determine the proper working concentrations of H2O2, we measured the cell
viabilities of HL-7702 cells. HL-7702 cells were treated with H2O2 (200, 400, 800, 1200, 1600, 2000, 2400, 2800 or
3200 ?M) respectively for different time points (1 h, 2 h, 3 h or 4 h). Results showed (Supplementary?Figure?1)
thatHL-7702 cells treated with H2O2 (400 ?M) for 1 h or 2 h, the cell viability decreased about 50% compared to
the non-treated group. So we selected 400 ?M and 2 h to perform the following experiments.
Our results revealed that mitochondrial morphology of hepatocytes was mostly tubular in the control group
upon exposure to 400 ?M H2O2 for 2 h. Considerably truncated and smaller mitochondria were prevalent in
hepatocytes, indicating mitochondrial fragmentation. However, pretreatment with 50, 100, and 200 ?M SalB
suppressed the mitochondrial fragmentation and maintained normal mitochondrial tubular structure (Fig.?1A). For
objective quantification of mitochondrial morphology, we analyzed mitochondrial shape by computer-assisted
morphometric analyses that calculate AR and FF. AR and FF values increased as mitochondria elongate. Our
data indicated that most mitochondria from cells treated with 400 ?M H2O2 for 2 h had lower FF and AR values
compared with the control. However, pretreatment with SalB inhibited the reduction in AR and FF caused by
H2O2-induced oxidative stress (Fig.?1B?D).
SalB protects against mitochondrial dysfunction induced by H2O2. Mitochondria are main intra
cellular organelles related to apoptosis. Thus, it is interesting to investigate the potential anti-oxidative effect
of SalB on mitochondrial function26. HL-7702 hepatocytes were used to confirm the protective effects of SalB
on hepatocyte injury induced by oxidative stress. Cell viability was not altered in cells treated with various
concentrations of SalB for 2 h, suggesting that (50?200 ?M) SalB is not cytotoxic to cells (Fig.?2A). Pretreatment
with (50?200 ?M) SalB for 2 h resulted in dose-dependent cytoprotective effects on cell viability against the
damage caused by H2O2 (Fig.?2B). Intracellular ROS were measured with DCFH-DA upon exposure to 400 ?M
H2O2 for 2 h. A significant increase in ROS was noted compared with the control group. Then, pretreatment with
SalB markedly prevented ROS accumulation in a concentration-dependent manner (Fig.?2C,D). MMP levels
were measured using JC-1, which exists as a mitochondrial aggregate with red fluorescence or as cytoplasmic
monomers at low mitochondrial potential resulting in green fluorescence. Thus, the monomer/aggregate ratio
is used to monitor changes in MMP. As shown in Fig.?2E,F, cells exposed to 400 ?M H2O2 for 2 h markedly
downgraded MMP levels, which are related to mitochondrial dysfunction. However, SalB significantly improved
the impairment of MMP. Mitochondrial membrane integrity is essential for ATP production, the disruption
of MMP coupled with ATP depletion, and mitochondrial cytochrome c release, which promote an increase in
mitochondrial apoptosis-related factors27. Therefore, ATP, cytochrome c, Bax, Bcl-2 and caspases were
monitored. Intracellular ATP was detected by luciferase. SalB blocked the decrease of ATP content induced by H2O2
in a dose-independent manner (Fig.?2H). Next, we investigated the effect of SalB on H2O2-induced cytochrome
c release from mitochondria to the cytosol (Fig.?2G). SalB markedly prevented the release of cytochrome c. Bcl-2
family members are major regulators of mitochondrial integrity, mitochondrial-initiated cytochrome c release
and caspase activation28. SalB suppressed caspase activation under H2O2-induced stress conditions and inhibited
the augmentation of Bax protein levels related to the mitochondrial apoptotic pathway (Fig.?2I?O).
SalB mediated the up-regulation of mortalin in hepatocytes. The results suggested that SalB
protects against mitochondrial dysfunction and maintains normal mitochondrial morphology. However, the
mechanism of action of SalB on mitochondrial protection has not been thoroughly elucidated. As a central
mitochondrial molecular chaperone, mortalin plays an important role in maintaining mitochondrial function.
Mortalin prevents mitochondrial damage by eliminating ROS during glucose deprivation, and mortalin
knockdown induces mitochondrial fragmented in HeLa cells20,22. Therefore, we assessed whether the protective effects
of SalB on mitochondrial function and morphology are mediated by mortalin. To assess the effects of SalB on
mortalin protein levels, hepatocytes were pre-incubated with different concentrations of SalB (50?200 ?M) for 2 h.
Pre-treatment with SalB (100 or 200 ?M) significantly increased mortalin protein expression levels (Fig.?3A,B).
Mortalin knockdown inhibits the protective effects of SalB on mitochondria. To further
investigate the effects of mortalin on mitochondrial shape, human mortalin overexpression and knockdown
plasmids were constructed. Hepatocytes were transfected with mortalin overexpression and knockdown plasmids
and assessed by real-time PCR and Western blot analyses. As shown in Fig.?4A, immunoblot analysis revealed
that the overexpression plasmid notably increased mortalin expression, whereas the mortalin knockdown
plasmid resulted in remarkable suppression of mortalin protein levels. The mortalin mRNA expression level in the
group was notably increased (n = 3, ***P < 0.001). Data are normalized as a percentage of the control group
and represented as the means ? SEM. (C) Mitochondria from control and mortalin knockdown 7702 cells
were labeled with the mitochondrial-specific dye MTG for 30 min at 37 ?C and viewed with a fluorescence
microscope. Cells grown in normal medium exhibit long, tubular mitochondria. Cells treated with H2O2 exhibit
short, fragmented mitochondria. Mortalin knockdown promotes mitochondrial truncation and fragmentation
under H2O2 treatment. (D) The number of mitochondria in cells grown in H2O2 is significantly decreased
compared with normal cells; mortalin knockdown decrease Nc with or without H2O2. (E,F) As mitochondrial
morphological parameters, AR and FF were quantified by ImageJ software. Images from total 15 individual
cells were analyzed through three independent experiments. *P < 0.05 versus H2O2-treated control. (G) The
graph depicts FF and AR values for individual mitochondrion. Mitochondria of cells grown in normal medium
have increased FF and AR values corresponding to long, tubular mitochondria. Fragmented mitochondria
have reduced FF and AR values. n = 50 mitochondria. (H) Mortalin-knockdown cells were treated without or
with SalB (50, 100, 200 ?M) for 2 h followed by treatment with 400 ?M H2O2 for an additional 2 h. After this
incubation, cell viability was determined by the MTT assay. Data are expressed as the percentage (%) of control
values, which are means ? SEM (n = 6) of representative experiments. (I) Cellular ROS were measured by a
fluorescence microscope using H2DCF-DA dye. (J) Mitochondrial membrane potential was determined using
JC-1 dye. (K) Immunofluorescence analysis of cellular cytochrome c location.
overexpression group was markedly elevated compared with the control group. In the mortalin knockdown
group, the mortalin mRNA was considerably reduced compared with the control (Fig.?4B).
Our data suggested that SalB mediated the increase in mortalin protein levels, we hypothesized that SalB
prevented hepatocyte injury induced by oxidative stress, which up-regulates mortalin. To test this hypothesis,
we transfected mortalin knockdown plasmids into HL-7702 hepatocytes. First, to determine the effects of
mortalin knockdown on mitochondrial shape and function, we observed mitochondrial morphology using MTG as
visualized by fluorescence microscopy. We found significant truncated and fragmented mitochondria in
mortalin knockdown group under H2O2-induced stress conditions (Fig.?4C,D). Morphometric analysis revealed no
obvious alterations in mitochondrial AR and FF in the control and mortalin-knockdown group. However, after
treatment with 400 ?M H2O2 for 2 h, the mortalin-knockdown group exhibited a significant decrease in
To demonstrate the role of SalB and mortalin knockdown in H2O2-induced mitochondrial dysfunction, cell
viability, ROS, MMP and cytochrome c were monitored. Under normal conditions, the viability of mortalin
knockdown cells and normal cells was approximately the same. Upon exposure to 400 ?M H2O2 for 2 h, the
mortalin-knockdown group had a significant decrease in cell viability. Pretreatment with different concentrations
of SalB in mortalin-knockdown cells had no significant protective effects on cell viability under H2O2-induced
stress conditions (Fig.?4H). We measured intracellular ROS levels and found that intracellular ROS levels
markedly increased after treatment with H2O2 and incubation with SalB. SalB did not eliminate intracellular ROS
accumulation induced by H2O2 (Fig.?4I). Next, upon exposure to 400 ?M H2O2 for 2 h, the mortalin-knockdown group
exhibited a significant decrease in MMP levels, and treatment with SalB did not alter MMP levels (Fig.?4J). In
addition, we investigated the protective role of SalB on cytochrome c release in mortalin-knockdown hepatocytes
induced by H2O2. SalB did not prevent the release of cytochrome c from mitochondria to the cytosol (Fig.?4K).
These results suggested that SalB did not prevent mitochondrial dysfunction induced by H2O2 under conditions
of mortalin knockdown.
The effects of mortalin overexpression on mitochondrial fission and fusion. We examined
whether mortalin overexpression prevented mitochondrial fragmentation induced by H2O2 and how mortalin
regulated mitochondrial morphology changes by mediating mitochondrial fission and fusion related factors.
We investigated the roles of mortalin overexpression on H2O2-induced mitochondrial deformation; hepatocytes
were transfected with mortalin overexpression plasmids. Our studies demonstrated that mortalin overexpression
obviously improved the formation of mitochondrial network structure compared with the control group. After
treatment with 400 ?M H2O2 for 2 h, cells with mortalin overexpression maintained a normal mitochondrial
network structure (Fig.?5A). The two parameters AR and FF were used for objective quantification of mitochondrial
shape. As shown in Fig.?5D,E, upon exposure to 400 ?M H2O2 for 2 h, the mortalin overexpression group
markedly exhibited increased AR and FF values compared with the H2O2-induced group. These results indicated that
mortalin overexpression prevented the changes of mitochondrial deformation induced by H2O2. How mortalin
regulated the alterations of mitochondrial shape remains unknown. Mitochondrial morphology and number
continually change in the living cell and are predominantly maintained by the balance of fission and fusion29,30.
Mitochondrial fission is primarily mediated by dynamic related protein 1 (Drp1), human fission 1 (hFis1) and
mitochondrial protein 18 (MTP18). In contrast, mitochondrial fusion is executed by Mitofusin 1 and 2 (Mfn1
and Mfn2) and Optic atrophy 1 (Opa1)31,32. To determine whether mortalin overexpression affects mRNA levels
of fusion and fission-related factors, real-time PCR analysis was performed. As shown in Fig.?4B, in normal
conditions, mortalin overexpression reduced hFis1 mRNA expression levels but did not affectDrp1, MTP18, Opa1,
Mfn1 and Mfn2 mRNA expression. The results indicated that mortalin overexpression promoted mitochondrial
network structure by down-regulating hFis1 mRNA levels to reduce mitochondrial fission. Upon exposure to
400 ?M H2O2, mortalin overexpression increased Opa1 and Mfn1 mRNA expression but did not affect the mRNA
expression level of other factors (Fig.?5F?K). This result suggested that mortalin inhibit H2O2-induced
mitochondrial division by up-regulating mitofusin-related factors Opa1 and Mfn1 to enhance mitochondrial fusion. At
(C,D) As mitochondrial morphological parameters, AR and FF were quantified by ImageJ software. Images
from 15 individual cells were analyzed inthree independent experiments. *P < 0.05 versus H2O2-treated
control. (E) The graph presents FF and AR values for individual mitochondrion. Mitochondria of cells grown
in normal medium have increased FF and AR values corresponding to long, tubular mitochondria. Fragmented
mitochondria have reduced FF and AR values. n = 50 mitochondria. (F?K) Real time PCR was used to analysis
mitochondrial fusion factors Mfn1, Mfn2, and Opa1 and mitochondrial fission factors hFis1, Drp1 and MTP18.
These factors were normalized to GAPDH mRNA expression. Three independent experiments are presented.
Data are normalized as a percentage of the control group and represent means ? SEM, *P < 0.05 versus control
group, n = 3. (L) Western blot analysis of hFis1, Mfn2, Bax, Bcl-2 levels in HL-7702 cell treatment with different
concentration of SalB, SalB increased hFis1 protein level and inhibited Mfn2 protein expression. Full-length
blots are presented in Supplementary?Figure?4. (M?O) Quantitative results from Western blots. GAPDH was
used as a loading control (*P < 0.05; **P < 0.01).
the same time SalB increased hFis1 protein level (Fig.?5L?O), which indicated mortalin maintain mitochondrial
structure stabilization by up-regulating mitofusin factor Mfn2 and down-regulating mitofission hFis1.
SalB balances mitochondrial fusion-fission through up-regulate mortalin in primary hepatocyte.
Our previous results showed that SalB could preserve the morphological and functional integrity of
mitochondriaby up-regulating mortalin in hepatocyte cell line HL-7702. In order to further explore the effects of SalB on
mortalin expression in primary hepatocyte, we incubated primary hepatocyte with 50, 100, 200 ?M SalB for 2 h.
Western blots showed that SalB significantly increased mortalin in primary hepatocyte (Fig.?6A and B). At the
same time, a decrease in mortalin protein level was observed in H2O2 (400 ?M) treated group, however
pretreatment with 200 ?M SalB restored mortalin expression. We further examined the effects of SalB on mitochondrial
morphology in primary hepatocyte. Confocal fluorescence microscope images showed (Fig.?6C) that
mitochondrial morphology in primary hepatocytes were long and tubular, however in H2O2 treated group, truncated and
smaller mitochondria increased. And truncated and smaller mitochondria indicate mitochondrial fragmentation.
Pretreatment with SalB (50, 100 and 200 ?M) for 2 h prohibited mitochondrial fragmentation. Aspect ratio and
Form factor were two mitochondrial morphological parameters, which reflect mitochondrial status. Our data
(Fig.?6D?F) showed that incubation with 200 ?M H2O2 significantly reduced the values of AR and FF, which
means more fragmented, smaller and truncated mitochondrial generation, primary hepatocytes? mitochondrial
fission more quick than fusion. However, pre-incubation with SalB (50, 100 and 200 ?M) for 2 h increase the
values of AR and FF, appearing more tubular and long structure mitochondrial. From all these results, we suppose
that SalB can preserve the mitochondrial morphology by up-regulating mortalin in primary hepatocytes.
One of the initial causes of hepatic fibrosis is an increased number of hepatocyte apoptosis, which is a main factor
contributing to the development and progression of liver diseases33,34. The first goal of our research was to
determine the effects of SalB on the mitochondrial pathway. Cell apoptosis is mainly transduced through the following
two defined pathways: the mitochondrial apoptotic pathway and death receptor pathway. The mitochondrial
pathway plays a critical role in hepatocyte apoptosis9. Our data showed that H2O2-induced oxidative stress leads
to a significant decrease in cell viability, abnormal accumulation of ROS, MMP collapse, ATP depletion and
mitochondrial cytochrome c release from mitochondria to the cytosol. In addition, oxidative stress activates caspase3
and increases the ratio of Bax/Bcl-2. However, pretreatment with SalB remarkably scavenges free radicals,
maintains mitochondrial membrane permeability, guarantees energy basal metabolism, and inhibits the activation of
apoptotic-related factors, indicating that the antioxidant properties of SalB notably rescue the mitochondrial
dysfunction caused by H2O2-induced oxidative stress, which is consistent with other reports3,35,36. The morphology,
interconnectivity and integrity of mitochondria play a critical role in regulating various aspects of mitochondrial
functions, including ATP generation, mitochondrial DNA inheritance, Ca2+ buffering, free radical homeostasis
and quality control37. Mitochondrial fission and fusion mediates mitochondrial morphology changes, which have
become the focus of attention based on cells participating in apoptosis38?40. Mitochondrial division is a key step in
apoptosis induced by severe oxidative stress41. We found that mitochondria are fragmented and truncated when
subject to 400 ?M H2O2-induced oxidative stress conditions for 0.5, 1, or 2 h via a mitochondrial division process
(data not shown). The phenomenon is consistent with a previous study that reported that H2O2-induced
oxidative stress caused mitochondrial fragmentation in C2C12 myocytes42. SalB prevents mitochondrial dysfunction
by scavenging ROS. Our data demonstrate that pretreatment with SalB in hepatocytes prohibits mitochondrial
fragmentation by decreasing mitochondrial fission under oxidative stress conditions. Thus, we ask the following
question: does SalB maintain normal mitochondrial function by restraining mitochondrial fragmentation? These
results indicate that SalB prevents H2O2-induced oxidative stress by regulating mitochondrial morphology and
Mortalin is involved in the maintenance of mitochondrial homeostasis and serves as a critical molecular
chaperone. As an anti-apoptotic protein, mortalin overexpression protects against apoptosis by preventing the
accumulation of redundant ROS in mitochondria, which are induced by glucose deprivation; chemical reperfusion
in neuronal cells20; and iron chelating in a hepatocyte cell line43. Previous studies demonstrate that SalB and
mortalin provide protective effects against mitochondrial dysfunction induced by oxidative damage. Therefore,
we assume that SalB protects mitochondria via mechanisms mediated by mortalin. In our study, we found that
pretreatment with (100 or 200 ?M) SalB, significantly increased mortalin protein levels comparedwith the control
group. The result indicates that SalB attenuates mitochondrial dysfunction by up-regulating mortalin.
Although previous reports demonstrate that mortalin provides effective protection against mitochondrial
dysfunction, its role in relation to mitochondrial shape must be addressed. In our research, we reported that the
effects of mortalin overexpression and knockdown on mitochondrial shape are altered in hepatocytes. Under
normal conditions, mortalin overexpression strengthens mitochondrial network structure compared with the
control group, but mortalin knockdown had no significant change in mitochondrial morphology compared with
untransfected cells. The result is consistent with a recent study performed under normal conditions. Knockdown
of endogenous mortalin did not cause a significant change in mitochondrial morphology22. Under oxidative stress
conditions, mortalin overexpression inhibited mitochondrial fragmentation. However, mortalin knockdown
failed to rescue mitochondria, which is similar to previous reports22. Our study suggests that mortalin
overexpression protects against mitochondrial division induced by oxidative damage. In addition, in mortalin-knockdown
hepatocytes, SalB did not prevent mitochondrial dysfunction induced by H2O2. Specifically, cell viability, ROS
accumulation, MMP collapse and cytochrome c release were not altered. Therefore, it is reasonable to conclude
that SalB maintains mitochondrial morphology and function by up-regulating mortalin in hepatocytes. Thus, we
ask, how is mortalin involved in regulating mitochondrial structure? Mitochondrial shape change is mediated
by mitochondrial fission and fusion. Mitochondrial fission is controlled by Drp1, hFis1 and MTP18, whereas
mitochondrial fusion is mediated by Opa1, Mfn1 and Mfn223,32. To investigate how mortalin regulates
mitochondrial morphology, we studied mortalin interactions with fusion- and fusion-related factors. Under normal
conditions, mortalin overexpression decrease hFis1 mRNA levels, which is involved in mitochondrial fission.
Under oxidative stress conditions, mortalin overexpression increased Opa1 and Mfn1 mRNA levels, which are
involved in mitochondrial fusion. These results suggest that under normal conditions, mortalin overexpression
inhibit mitochondrial fragmentation by lowering mitochondrial fission, which is mediated by hFis1 protein. In
contrast, mortalin overexpression prevents oxidative damage induced by the generation of numerous smaller
and truncated mitochondria by enhancing mitochondrial fusion and reducing mitochondrial division, which is
mediated by Opa1 and Mfn1. These data are similar with previous reports (Fig.?5F,G).
We also examined the effects of SalB on mortalin in primary hepatocytes, and the similar results were
obtained. Pretreatment with SalB suppressed the mitochondrial deformation induced by H2O2 in primary
hepatocytes. At the same time, mortalin was up-regulated by SalB in primary hepatocytes.
In conclusion, our data demonstrated that SalB prevents mitochondrial dysfunction and mitochondrial
fragmentation from H2O2-induced oxidative damage by up-regulating mortalin. Importantly, we demonstrated that
mortalin overexpression mediates the protein levels of fission-related factor of hFis1 and the mRNA levels of
fusion-related factors of Mfn1 and Opa1 to maintain the dynamic balance of fission and fusion. Based on our
data that SalB protects against hepatocyte apoptosis by up-regulating mortalin, mortalin can be a novel target to
develop more traditional Chinese medicines for liver disease therapy.
However, our experiments were limited to the mechanism of action for SalB. Further studies are needed
to confirm the interaction of SalB and mortalin and determine how SalB mediates mitochondrial fission and
fusion-related factors to protect mitochondrial morphology in vivo experiments.
This work was supported by the Basic Research Project of Shanghai Science and Technology Commission
Yunxia Liu and Yingying Hu were involved in the design, acquisition of data, analysis, interpretation of data and
drafting the manuscript. Qiukai E was involved in acquisition of data. Ji Zuo revised the manuscript for important
intellectual content. Ling Yang and Wen Liu supervised the entire study and were involved in design, acquisition
of data, data analysis and interpretation and drafting the manuscript. All authors reviewed and approved the
Supplementary information accompanies this paper at http://www.nature.com/srep
Competing Interests: The authors declare no competing financial interests.
How to cite this article: Liu, Y. et al. Salvianolic acid B inhibits mitochondrial dysfunction by up-regulating
mortalin. Sci. Rep. 7, 43097; doi: 10.1038/srep43097 (2017).
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