Background
Homocysteine (Hcy) is a non-essential sulfhydryl-containing amino acid that is derived from methionine metabolism. Liver is a major organ for Hcy metabolism [
1]. Recently, epidemiological and experimental studies linked Hcy to a wide range of impaired liver function [
2]. Meanwhile, hepatocyte apoptosis plays an important role in liver injury, which correlates with severity of liver disease and participates in the progress of hepatic fibrosis [
3]. Therefore, exploring the molecular mechanisms of Hcy-induced hepatocyte apoptosis is crucial for the treatment of liver disease.
Many studies have indicated that Hcy promotes liver dysfunction through oxidative stress, endoplasmic reticulum (ER) stress activation and inflammatory response [
4,
5]. ER stress often occurs in the cells demanding a high rate of protein synthesis and secretion as well as harmful stresses, including hypoxia, infection and exposure to a toxic substance. It is generally accepted that the unfolded protein reaction (UPR) signaling pathway plays an independent role in ER stress, of which may present opportunities for targeted therapies [
6].
Meanwhile, the glucose-regulated protein 78 (GRP78) also plays a critical role in sensing ER stress, triggering UPR, and leading to cell apoptosis [
7]. In addition, oncoprotein proteasome 26S subunit non-ATPase 10 (PSMD10) is reported to promote ER stress by up-regulating GRP78 and enhancing the activation of the UPR pathway in HCC cells [
8]. Our previous study confirmed that dramatic increase in GRP78 expression induced by Hcy could accelerate ER stress-mediated liver injury [
9]. However, the exact mechanism underlying the regulation of GRP78 in hepatocytes apoptosis induced by Hcy remains unclear.
MicroRNAs (miRNAs) are essential small endogenous non-coding RNAs that negatively regulate gene expression by binding to the 3′-untranslated regions (3′UTR) of target genes, and thereby take part in cell proliferation, differentiation and apoptosis [
10,
11]. Among them, miR-212 located at chromosome 17p13.3 is up-regulated in cancers such as oral carcinoma and lung cancer, while it is down-regulated in colorectal cancer, prostate cancer and hepatocellular carcinoma. In addition, the circulating or liver miR-212 also plays a role in the biology of non-alcoholic fatty liver disease [
12]. Although miR-212 has showed its important role in cancer and hepatic disease, whether it can mediate Hcy-induced liver injury is currently unknown.
In the present study, we explore the possible role of miR-212-5p in regulating ER stress-mediated hepatocytes apoptosis which is induced by Hcy treatment. The results show that miR-212-5p suppresses Hcy-induced ER stress in liver by targeting PSMD10 and thereby destroying the interaction between PSMD10 and GRP78. These findings demonstrate that PSMD10 is involved in the regulation of Hcy-induced liver injury, which may provide a novel PSMD10-based therapeutic target for liver injury.
Discussion
Homocysteine (Hcy) in the liver is methylated to methionine by methionine synthase (requiring folate) and betaine-homocysteine methyltransferase, or converted to cystathionine by cystathionine synthase (CBS) [
17]. Dysfunction of liver alters methionine metabolism, resulting in elevated Hcy that is released into the plasma; on the other hand, Hcy can influence the status of liver as well [
18]. Our previous study proposed that ER stress might contribute to the pathogenic effects of Hcy on liver injury, while its molecular mechanism remains obscure [
19]. In this study, we investigated the function of PSMD10 in liver injury caused by Hcy-induced ER stress, which is an alternative mechanism for the pro-apoptotic effect of Hcy. This was further confirmed by activation of PERK, p-PERK, IRE1α, p-IRE1Α and ATF6, the three major mediators of the ER stress response, as well as eIF2α and p-eIF2α. Activation of PERK causes phosphorylation of the eukaryotic translation initiation factor 2a and then inhibits protein synthesis. Activated IRE1 catalyzes the removal of a small intron from the XBP-1 mRNA, which triggers the induction of ER chaperones and other genes involved in ER-associated protein degradation [
20,
21]. ATF6 and XBP-1 may combine to the ER stress response element and the UPR element, leading to GRP78 expression. Additionally, induction of CHOP, indicative of prolonged ER stress and pro-apoptotic signaling, further supported a Hcy-induced ER stress response linked to apoptosis [
22]. GRP78 is the typical molecule that binds to ATF6, PERK, and IRE1 in normal condition [
23]. When the cells are under stress, these proteins will be separated from GRP78 and activate downstream signal pathway to initiate UPR [
24]. We found that Hcy remarkably up-regulates GRP78 both in vivo and in vitro, and GRP78 promotes ER stress-mediated apoptosis induced by Hcy, which is consistent with previous reports. Interestingly, the recent discovery that cannabidiol causes activated hepatic satellite cell death through a mechanism of ER stress-induced apoptosis [
25], further confirms our results that Hcy promotes ER stress-mediated apoptosis to induce liver injury.
PSMD10 is involved in diverse biological processes, such as cell growth, proliferation, apoptosis and invasion, and contributes to oval cell-mediated liver regeneration and cell cycle progression [
26]. In addition to normal biological functions, emerging evidences support that PSMD10 functions as an oncogene in many cancers including hepatocellular carcinomas, gliomas lung cancer, and colon, and so on [
27]. Moreover, the abnormal expression of PSMD10 is related to the clinicopathological parameters of the disease, elucidating the important function of PSMD10 as a potential biomarker for diagnosis of different disease and a therapeutic target for disease treatment. Consistent over-expression of PSMD10 promotes tumor growth and inhibits apoptosis in hepatocellular carcinomas cells by enhancing the UPR and up-regulating GRP78 expression. As expected, we found that PSMD10 promotes ER stress-mediated hepatocytes apoptosis induced by Hcy, which agrees with previous report that PSMD10 augments CCl4-mediated chronic hepatic injury, inflammation and compensatory proliferation via the Rac1/JNK pathway [
28]. However, what contradicts with us is that PSMD10 protects hepatocellular carcinoma cells from ER stress-induced apoptosis through the enhancement of UPR signaling. A recent study reported that PSMD10 interacts with the IL-1β/IRAK-1 inflammatory signaling pathway, and thereby induces the binding of the nuclear factor (NF-Y) complex to the PSMD10 promoter, facilitates the recruitment of the E1A-binding protein p300 and CREB-binding protein, and finally increases PSMD10 expression; in addition, PSMD10 promotes Rb phosphorylation and inactivation through interaction with cyclin-dependent kinase 4 and retinoblastoma protein (Rb), and activates the E2F transcription factor, leading to cell cycle progression [
29]. Considering the interaction between PSMD10 and proteins, we also investigated if PSMD10 and GRP78 interact with each other. We firstly found that PSMD10 interacts with GRP78, leading to up-regulation of GRP78 and thereby acceleration of ER stress-mediated hepatic apoptosis induced by Hcy.
Additionally, PSMD10 could be regulated by miRNA at the posttranscriptional level. Recent report provided evidence that miR-605 significantly represses intrahepatic cholangiocarcinoma (ICC) cell proliferation and invasion by down-regulating PSMD10 through binding to the 3′UTR of PSMD10 [
30]. Another study showed that ectopic expression of miR-214 inhibits myeloma cell growth and induces apoptosis by inhibiting PSMD10 [
15]. Therefore, repressing PSMD10 activity may be a potential anticancer strategy. Previous studies have revealed many functions of miR-212-5p, such as tumor-promoting properties in NSCLC and proliferation-inhibition properties in gastric cancer [
31,
32]. And many studies reported that miR-212-5p directly targets SIRT2 and prostaglandin endoperoxide synthase-2 to suppress the proliferation of colorectal cancer cells and protect against ferroptosis-mediated neuronal death [
33,
34]. Consistent with these findings, our study indicated for the first time that miR-212-5p attenuates Hcy-induced hepatocytes apoptosis by targeting PSMD10.
Methods
Animals
All animal experiments were approved and carried out in accordance with the Institutional Animal Care and Use Committee of the University of Ningxia Medical University. Eight to ten weeks old cystathionine beta-synthase (CBS) heterozygous knockout mice (cbs+/−) were obtained from Jackson Laboratory (Bar Harbor, ME) and maintained in the animal facility center at Ningxia Medical University. The mice were fed with regular diet plus 2.0% methionine chow and water ad libitum. Mice genotypes were determined by PCR.
Cell culture, infection and transfection
The human HL-7702 hepatocyte cells were purchased from the BioWING Biotechnology (Shanghai, China) and cultured in RPMI-1640 medium (Thermo, USA) containing 10% fetal bovine serum (FBS) (Hyclone, USA) and 1% penicillin (Thermo, Waltham, MA, USA). Cells were incubated in a humidified incubator with 5% CO2 at 37 °C.
The cells were infected with adenoviruses encoding PSMD10 (Ad-PSMD10) when they were 80% confluent. The control cells were infected with Ad-GFP. Hepatocytes were transfected with non-silencing small interfering RNA (NC, 5′-UUCUCCGAACGUGUCACGUTT-3′), PSMD10 siRNA (5′-GCCGGGAUGAGAUUGUAAATT-3′), GRP78 siRNA (5′-GGGCAAAGAUGUCAGGA AATT-3′), miR-212-5p mimics (5′-ACCUUGGCUCUAGACUGCUUACU-3′) and miR-212-5p inhibitor (5′-AGUAAGCAGUCUAGAGCCAAGGU-3′) using Lipofectamine 2000 according to the manufacturer’s instructions (Invitrogen, Carlsbad, CA, USA).
Liver tissue preparation and morphologic observation
All mice were fasted but supplied with water for 24 h followed by peritoneally injection with 3% pentobarbital sodium in a dose of 2 mL/kg. Abdominal cavity of anesthetic mouse was incised, and then inferior vena cava blood was collected using 10 mL syringe. After standing for 3 h at 4 °C, blood samples were centrifuged at 5000g for 15 min. Supernatant was stored at − 80 °C until future use. Liver was incised and two segments (1.0 cm × 1.0 cm × 0.3 cm) were cut from right lobe of the liver. Two liver segments were fixed in 10% neutral formalin and embedded in optimum cutting temperature compound (OCT). To observe morphologic changes in liver tissues, liver segments was HE stained.
TUNEL assay
Liver tissues were stained using a TUNEL Staining Kit (Roche Inc., Basel, Switzerland) and the TUNEL-positive cells were symbolized by fluorescein-dUTP with dNTP according to the manufacturer’s protocol of the in-situ apoptosis Detection Kit (Roche Inc, Basel, Switzerland). Cells with nuclear condensation/fragmentation and apoptotic bodies in the absence of cytoplasmic TUNEL reactivity (green staining of nuclei) were considered as apoptotic cells. Cell nuclei was counterstained with DAPI and visualized by fluorescent microscopy.
Western blot
Western blot analysis was performed as previously described [
11]. Liver tissues and HL-7702 cells were lysed in a lysis buffer (KeyGEN, China) containing the protease inhibitor phenylmethanesulfonylfluoride (PMSF, KeyGEN, China) at 4 °C for 30 min followed by centrifugation to remove cell debris. Protein concentration was measured using BCA protein assay kit (Beyotime Institute of Biotechnology). The protein was separated by SDS-PAGE and transferred to PVDF membranes followed by examination with antibodies against PSMD10, cleaved caspase-3, cleaved caspase-12, Bax, Bcl-2, ATF6, p-PERK, PERK, eIF2α, p-eIF2α, IRE1α, p-IRE1α, CHOP and β-actin (all from Abcam Inc., Cambridge, MA, USA) respectively. Signal intensity was analyzed with Bio-Rad image analysis (Bio-Rad, Hercules, CA, USA).
Real-time PCR (qRT-PCR)
Total RNA was isolated from livers or cultured cells using the miRNA isolation kit (Thermo Scientific, Waltham, MA, USA), and reverse transcribed using TaKaRa Master Mix (Dalian, China). Real-time PCR examination of PSMD10 and GAPDH were performed using Maxima SYBR Green/ROX qPCR Master Mix assay (Thermo Scientific, Waltham, MA, USA). The expression levels of mRNA and miRNA were normalized using GAPDH or U6 as a reference gene. qRT-PCR primer sequences are listed in Table
1.
Table 1
Primer sequences for qRT-PCR analyses
GAPDH | NM_002814.4 | F: GGTGAAGGTCGGTGTGAACG |
R: CTCGCTCCTGGAAGATGGTG |
PSMD10 | NM_001256799.3 | F: GAACTGACCAGGACAGCAGAACTG |
R: AGCAGAAGCCGCAATATGAAGAGG |
U6 | | GCTTCGGCAGCACATATACTAAAAT |
miR-212-5p | | ACCTTGGCTCTAGACTGCTTACTG |
GRP78 | NM_001163434.1 | F:ATGATGAAGTTCACTGTGGTGG |
R:CTGATCGTTGGCTATGATCTCC |
Si-PSMD10 | | F:CCAGAUGCUAAGGACCAUUTT |
R:AAUGGUCCUUAGCAUCUGGTT |
Si-GRP78 | | F:GGCCACUAAUGGAGAUACUTT |
R:AGUAUCUCCAUUAGUGGCCTT |
For miRNA analysis, the expression of miR-212-5p was evaluated with the TaqMan miRNA-assay (Applied Biosystems, Foster City, CA, USA). U6 was selected as an internal control. The primers for miR-212-5p were synthesized by Ribo Biotechnology (Guangzhou, China). The primer sequences were listed in Table
1. The universal reverse primer was provided in the Kit. The PCR cycling conditions were as follows: pre-denaturation at 95 °C for 2 min; 35 cycles of 95 °C for 15 s, 60 °C for 30 s, and 72 °C for 10 s; and a final extension at 72 °C for 2 min. The 2
−ΔΔCt method was utilized to quantify the relative expression of each gene.
Immunofluorescence staining
The paraffin embedded mouse liver tissue sections were permeabilized in PBS containing 0.1% Triton X-100 for 5 min, and then incubated with blocking solution (5% goat serum in PBS) at room temperature for 30 min, followed by incubation with primary antibody (PSMD10-1:50, KDEL receptor-1:50, CK18-1:50, GRP78-1:50) overnight at 4 °C. After washing with PBS for three times, tissues were incubated with fluorescein conjugated secondary antibodies (goat anti-mouse or goat anti-rabbit) at RT for 1 h. After washing with PBS for three times, they were stained with DAPI for 5 min. Sections were then assessed using fluorescence microscopy (OLMPUS FV1000 confocal laser scanning microscope, Tokyo, Japan).
Live/dead staining assay
Cell viability was monitored under laser scanning confocal microscope (LSM710) using cell viability Imaging Kit (Roche Diagnostics, 06432379001) according to the manufacturer’s instruction.
Cell apoptosis
Hepatocytes apoptosis was detected using the FITC-Annexin V/PI staining kit. After Hcy treatment, the cells were washed with ice-cold PBS, and incubated with fluoresce in conjugated Annexin V and PI for 15 min, cell apoptosis was analyzed using a FACS flow cytometer equipped with the FACS talion data management system and Cell Quest software (Becton Dickinson, San Jose, CA, USA).
Dual luciferase assays
Hepatocytes in 6-well plates were co-transfected with 200 ng of either pMIR-PSMD10-wt (5′-GAG AGTGGAAGAAGCAAAACTGCTGGTGTCCCAAGGAGCAAGTATTTACATTGAGAATAA-3′) or pMIR-PSMD10 mut (5′-GAGAGTGGAAGAA GCAAAACTGCTGGTGTCGGTTCCAGCAA GTATTTACATTGAGAATAA-3′) together with 100 nM of miR-212-5p mimics or non-target miRNA mimics using Lipofectamine 2000. In 48 h after transfection, firefly and renilla luciferase activities were measured using the Dual-Glo luciferase assay kit (Gibco Life Technologies).
Co-immunoprecipitation assays
Cells were lysed in a lysis buffer containing protease inhibitor on ice. After centrifugation, the supernatant was incubated with an anti-PSMD10, anti-GRP78 or normal rabbit IgG respectively at 4 °C overnight followed by incubation with Dynabeads Protein G. Immunocomplex was separated by SDS-PAGE and proceeded for western blot analysis.
Statistical analysis
Results are expressed as the mean ± SD from at least three independent experiments. The data were analyzed using one-way ANOVA and additional analysis using the Student Newman-Keuls test for multiple comparisons within treatment groups or t-test for two groups. P < 0.05 was considered to be statistically significant.
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