RETRACTED ARTICLE: Mst1 overexpression combined with Yap knockdown augments thyroid carcinoma apoptosis via promoting MIEF1-related mitochondrial fission and activating the JNK pathway

Human thyroid carcinoma MDA-T32 (ATCC^® CRL-3351™) and MDA-T68 (ATCC^® CRL-3353™) cell lines was purchased from American Type Culture Collection (ATCC) (Manassas, VA, USA). To overexpress the Mst1, adenovirus-Mst1 (ad-Mst1) was transfected into MDA-T32 cells. In brief, pDC315-Mst1 vector was obtained from Shanghai Gene-Pharma Co. (Shanghai, China) and then HEK293 cells (ATCC^® CRL-3216™, American Type Culture Collection, Manassas, VA, USA) were infected with pDC315-Mst1 vector to obtain the Ad-Mst1 according to a previous study [33]. In brief, a total of 1 × 10⁵ cells/well were infected with 50 multiplicity of infection (MOI) adenovirus (Ad)-Mst1 in serum-free RPMI-1640 for 12 h at 37 °C. To silence the expression of Yap, shRNA against Yap (sh-Yap) was used to transfect into MDA-T32 cells. The sh-Yap was purchased from Shanghai Gene-Pharma Co. (Shanghai, China) and transfection was performed with the help of Opti-MEM medium and Lipofectamine 2000 (Thermo Fisher Scientific, Inc.) according to the manufacturer’s protocol [34]. GFP-labelled adenovirus was used as a preliminary study to observe the infection efficiency (Additional file 1: Figure S1A). To prevent the activation of JNK pathway, SP600125 (25 μM, Selleck Chemicals, Houston, TX, USA) was added into the medium of MDA-T32 cells [35].

Cellular proliferation was evaluated via EdU assay. Cells were seeded onto a 6-well plate, and the Cell-Light™ EdU Apollo^®567 In Vitro Imaging Kit (Thermo Fisher Scientific Inc., Waltham, MA, USA; Catalogue No. A10044) was used to observe the EdU-positive cells according to the manufacturer’s instructions [36]. MTT assay was used to observe the cellular viability. Cells were seeded onto a 96-well plate, and the MTT was then added to the medium (2 mg/mL; Sigma-Aldrich) [37]. Subsequently, the cells were cultured in the dark for 4 h, and DMSO was added to the medium. The OD of each well was observed at A490 nm via a spectrophotometer (Epoch 2; BioTek Instruments, Inc., Winooski, VT, USA) [38].

Cells were plated on glass slides in a 6-well plate at a density of 1 × 10⁶ cells per well. Subsequently, cells were fixed in ice-cold 4% paraformaldehyde for 30 min, permeabilized with 0.1% Triton X-100, and blocked with 2% gelatine in PBS at room temperature. The cells were then incubated with the primary antibodies: [cyt-c (1:1000; Abcam; #ab90529), Tom20 (mitochondrial marker, 1:1000, Abcam, #ab186735), LAMP1 (lysosome marker, 1:1000, Abcam, #ab24170), MIEF1 (1:1000, Abcam, #ab89944)] [39].

Mitochondrial membrane potential was measured with JC-1 assays (Thermo Fisher Scientific Inc., Waltham, MA, USA; Catalogue No. M34152). Cells were treated with 5 mM JC-1 and then cultured in the dark for 30 min at 37 °C [40]. Subsequently, cold PBS was used to remove the free JC-1, and DAPI was used to stain the nucleus in the dark for 3 min at 37 °C. The mitochondrial membrane potential was observed under a digital microscope (IX81, Olympus) [41]. Cellular ATP content was measured according to a previous report via ELISA assay. Cells were washed with PBS and then collected at room temperature. Subsequently, a luciferase-based ATP assay kit (Celltiter-Glo Luminescent Cell Viability assay; Promega, Madison, WI, USA; Catalogue No. A22066) was used according to the instructions.

Total protein was extracted by RIPA (R0010, Solarbio Science and Technology, Beijing, China), and the protein concentration of each sample was detected with a bicinchoninic acid (BCA) kit (20201ES76, Yeasen Biotech Co., Ltd, Shanghai, China) [42]. Deionized water was added to generate 30-µg protein samples for each lane. A 10% sodium dodecyl sulphate (SDS) separation gel and concentration gel were prepared [43]. The following diluted primary antibodies were added to the membrane and incubated overnight: JNK (1:1000; Cell Signaling Technology, #4672), p-JNK (1:1000; Cell Signaling Technology, #9251), Mst1 (1:1000, Cell Signaling Technology, #3682), Yap (1:1000; Cell Signaling Technology, #14,074), Mfn2 (1:1000, Abcam, #ab56889), Bcl2 (1:1000, Cell Signaling Technology, #3498), Bax (1:1000, Cell Signaling Technology, #2772), Cyclin D (1:1000, Abcam, #ab134175), CDK4 (1:1000, Abcam, #ab137675), Drp1 (1:1000, Abcam, #ab56788), Fis1 (1:1000, Abcam, #ab71498), Opa1 (1:1000, Abcam, #ab42364), Mff (1:1000, Cell Signaling Technology, #86668).

TRIzol reagent (Invitrogen; Thermo Fisher Scientific, Inc.) was used to isolate total RNA from cells. Subsequently, the Reverse Transcription kit (Kaneka Eurogentec S.A., Seraing, Belgium) was applied to transcribe RNA (1 μg in each group) into cDNA at room temperature (~ 25 °C) for 30 min. The qPCR was performed with primers using SYBR™ Green PCR Master Mix (Thermo Fisher Scientific, Inc. Cat. No. 4309155). The following were primers used in the present study: ROCK-1 (Forward: 5′-ACCTGTAACCCAAGGAGATGTG-3′, Reverse 5′-CACAATTGGCAGGAAAGTGG-3′), and Rac1 (Forward 5′-ATGCAGGCCATCAAGTGTGTGG-3′, Reverse: 5′-TTACAACAGCAGGCATTTTCTC-3′), and GAPDH, (Forward: 5′-AAGTTGTGFATTAGTCA-3′, Reverse 5′-AGAATAGTCCTATAATCA-3′).

The Caspase 9 Activity Assay Kit (Beyotime, China, Cat. No: C1158) was used to measure the activity of caspase-9 according to the manufacturer’s instructions. The concentrations of GSH, SOD and GPX were evaluated using commercial kits (Cellular Glutathione Peroxidase Assay Kit, Beyotime, China, Cat. No: S0056; Glutathione Reductase Assay Kit, Beyotime, China, Cat. No: S0055; Total Superoxide Dismutase Assay Kit, Beyotime, China, Cat. No: S0101, respectively). ATP production was measured using a luciferase-based ATP assay kit (Beyotime Institute of Biotechnology) with a microplate reader [44].

Cellular proliferation was evaluated via EdU assay. Cells were seeded onto a 6-well plate, and the Cell-Light™ EdU Apollo^®567 In Vitro Imaging Kit (Thermo Fisher Scientific Inc., Waltham, MA, USA; Catalogue No. A10044) was used to observe the EdU-positive cells according to the manufacturer’s instructions. Cellular lactate production in the medium was measured via a lactate assay kit (#K607-100; BioVision, Milpitas, CA, USA) according to a previous study [45].

For Transwell migration assays, the upper chambers of 24-well Transwell assay plates were seeded with 2 × 10³ cells in 200 μL serum-free medium per well. The lower chambers were filled with 600 μL medium containing 0.5% FBS. After a 24-h incubation in a humidified incubator at 37 °C, 5% CO₂, cells that had migrated to the underside of the membranes were fixed and stained with 0.1% crystal violet. After washing with distilled water, pictures of each chamber were randomly taken using a 200× microscope field, and these images were used to quantify the total number of migrated cells.

SPSS 21.0 software (IBM Corp., Armonk, New York, USA) was applied for data analysis. All experiments were repeated 3 times in each group. The mean value of the measurement data was expressed as the mean and SEM. Comparisons among groups were by one-way analysis of variance (ANOVA), and multiple comparisons between the average number of samples were performed by LSD analysis. p < 0.05 indicated that the difference was statistically significant.

In the present study, Mst1 adenovirus and/or Yap shRNA were transfected into MDA-T32 cells. Then, the knockdown and overexpression efficiencies were confirmed via western blotting. As shown in Fig. 1a, c, compared to the control group, Ms1 adenovirus (ad-Mst1) enhanced Mst1 expression, whereas Yap shRNA transfection decreased the Yap level in MDAT-32 cells. To observe the influence of Mst1 overexpression and Yap knockdown on cell viability, an MTT assay was used. Both Mst1 overexpression and Yap knockdown reduced cell viability compared to that of the control group (Fig. 1d). Interestingly, Mst1 overexpression in combination with Yap knockdown further suppressed cell viability in MDA-T32 cells. This finding was consistent with the result of the LDH release assay. As shown in Fig. 1e, both Mst1 overexpression and Yap knockdown significantly increased the LDH content in the medium. However, LDH release was further enhanced by Mst1 overexpression in combination with Yap knockdown. The above data indicate that Mst1 overexpression cooperates with Yap knockdown to further promote thyroid cancer cell death in vitro. This finding was further validated by counting the TUNEL-positive cells. As shown in Fig. 1f, g, the number of TUNEL⁺ MDA-T32 cells was rapidly elevated after Yap was silenced and/or Mst1 was overexpressed compared to that in the control group. However, in cells with simultaneous Yap knockdown and Mst1 overexpression, the number of TUNEL⁺ cells was further increased compared to that in cells with Yap inhibition or Mst1 overexpression alone. At the molecular level, caspase activation is a marker of cell apoptosis; therefore, ELISA was used to observe the alteration in caspase-3 activation in response to Mst1 overexpression and/or Yap knockdown. As shown in Fig. 1h, the activity of caspase-3 was rapidly increased in Mst1-overexpressing cells and/or Yap-silenced cells compared to that in the control group. However, Mst1 overexpression in combination with Yap knockdown further elevated caspase-3 activity in MDA-T32 cells. Similar results were noted in MDA-T68 cells (Additional file 1: Figure S1B). Altogether, our data illustrate that Mst1 overexpression and Yap knockdown have synergistic effects that further reduce cell death in MDA-T32 cells.

Next, experiments were performed to analyze the influence of Mst1 overexpression and Yap knockdown on cell proliferation and migration. Western blot analysis demonstrated that the levels of cell cycle proteins such as Cyclin D1 and CDK4 were markedly reduced in cells transfected with Mst1 adenovirus and Yap shRNA (Fig. 2a–c), indicating the inhibitory effects of Mst1 activation and Yap inhibition on cell proliferation. This finding was further quantified via EdU staining. The number of EdU-positive cells indicated the ratio of proliferating cells in the cell cycle. As shown in Fig. 2d, e, both Mst1 overexpression and Yap knockdown repressed the ratio of proliferating MDA-T32 cells compared to that in the control group. However, cell proliferation was further suppressed by Mst1 overexpression in combination with Yap knockdown. In addition to cell proliferation assays, experiments were conducted to assess cell migration. Transwell assays showed that the number of migrated cells was rapidly reduced in response to either Mst1 overexpression or Yap knockdown (Fig. 2g, h). Interestingly, combined treatment with Mst1 adenovirus and Yap shRNA further suppressed MDA-T32 cell motility, indicating the essential roles played by Mst1 and Yap in modulating MDA-T32 cell motility. This finding was further supported by analyzing the transcription of prometastatic genes. As shown in Fig. 2i–g, the transcription levels of prometastatic genes such as Rac1 and ROCK1 were markedly reduced in response to either Mst1 overexpression or Yap knockdown. Interestingly, in cells with combined Mst1 overexpression and Yap knockdown, the transcription levels of Rac1 and ROCK1 were further reduced. These results were also noted in MDA-T68 cells (Additional file 1: Figure S1C, D). Therefore, the above data indicate that Mst1 and Yap differentially regulate cancer cell proliferation and migration.

Based on the results of previous studies, the mitochondrion is the potential target of Mst1/Yap, and mitochondrial dysfunction has been found to be associated with cancer cell death. In the present study, we sought to determine whether Mst1 overexpression cooperated with Yap knockdown to further affect mitochondrial function. First, a JC-1 probe was used to stain mitochondria to assess the mitochondrial membrane potential. Red fluorescence indicated a normal mitochondrial membrane potential, whereas green fluorescence reflected a decreased mitochondrial membrane potential. As shown in Fig. 3a, b, either Yap deletion or Mst1 overexpression significantly reduced the mitochondrial membrane potential compared to that in the control group, as evidenced by the decreased ratio of red to green fluorescence. Interestingly, Mst1 overexpression combined with Yap knockdown further decreased the mitochondrial membrane potential. At the molecular level, a reduction in the mitochondrial membrane potential is an early feature of mitochondrial apoptosis, and the consequence of mitochondrial membrane potential dissipation is the opening of the mitochondrial permeability transition pore (mPTP). Via ELISA, we found that the mPTP opening rate was rapidly increased in cells treated with either Mst1 adenovirus or Yap shRNA (Fig. 3c). Interestingly, Mst1 overexpression in combination with Yap knockdown further promoted mPTP opening (Fig. 3c).

Excessive mPTP opening increases the likelihood of nuclear translocation of the mitochondrial proapoptotic protein cyt-c. As shown in Fig. 3d, e, the nuclear expression of cyt-c was significantly elevated in treated cells compared to that in control cells, indicating the release of mitochondrial cyt-c to the nucleus. Interestingly, this effect was further enhanced via the combination of Mst1 overexpression and Yap knockdown. In addition to cyt-c leakage, the production of mitochondrial ROS was markedly increased in cells transfected with either Mst1 adenovirus or Yap shRNA (Fig. 3f, g). Interestingly, mitochondrial oxidative stress was further augmented via combined Mst1 overexpression and Yap knockdown (Fig. 3f, g). Finally, western blot analysis demonstrated that the levels of mitochondrial proapoptotic proteins were significantly elevated by Mst1 overexpression in combination with Yap deletion (Fig. 3h–l). In contrast, the levels of antiapoptotic factors were markedly reduced by combined Mst1 activation and Yap inhibition (Fig. 3h–l). These results were also observed in MDA-T68 cells (Additional file 1: Figure S1E, F). Taken together, the above data indicate that mitochondrial function was highly modulated by Yap and Mst1. Combined treatment via Mst1 upregulation and Yap downregulation further augmented mitochondrial dysfunction in MDA-T32 cells.

In addition to identifying mitochondrial malfunction, we also sought to determine whether mitochondrial structure was affected by Mst1/Yap. Recent studies reported that mitochondrial function is modulated by mitochondrial structural alterations and especially mitochondrial dynamics. Based on this knowledge, we explored whether mitochondrial dynamics were controlled by Mst1 and Yap in MDA-T32 cells. Immunofluorescence assays showed that mitochondria in normal cells exhibited an intact shuttle structure. However, loss of Yap or overexpression of Mst1 promoted mitochondrial division into several fragments (Fig. 4a, b). This alteration was more prominent in cells with both Yap knockdown and Mst1 overexpression. Subsequently, mitochondrial length was measured to indicate mitochondrial dynamics. As shown in Fig. 4b, the mitochondrial length was ~ 9.3 μm in normal cells and was reduced to ~ 4.2 μm after either knockdown of Yap or overexpression of Mst1. Notably, combined treatment via Mst1 upregulation and Yap downregulation further decreased the mitochondrial length to ~ 2.6 μm. Accordingly, the above results confirmed that mitochondrial dynamics were affected by Mst1 and Yap.

Subsequently, western blotting was performed to analyze the expression of proteins related to mitochondrial dynamics. Drp1/Mff are the factors involved in mitochondrial division. However, Mfn2/Opa1 are the proteins that positively contribute to the fusion of mitochondrial fragments. As shown in Fig. 4c–g, the levels of Drp1 and Mff were rapidly increased in response to Mst1 overexpression compared to those in the control group, and a similar effect was observed in Yap-silenced cells. Notably, Mst1 overexpression combined with Yap knockdown further promoted the upregulation of Mff/Drp1. In contrast, the expression of Mfn2 and Opa1 was significantly downregulated in response to Mst1 overexpression (Fig. 4c–g), consistent with the effect of Yap silencing. Notably, the combination of Mst1 overexpression and Yap knockdown further inhibited Mfn2/Opa1 expression. Taken together, the above data indicate that mitochondrial structural homeostasis was highly affected by Mst1 and Yap.

Next, experiments were performed to determine the downstream effectors of Mst1/Yap in MDA-T32 cells. Previous studies have reported that the JNK pathway could be affected by Mst1 and Yap in different disease states, such as postinfarction myocardial injury and renal ischemia reperfusion stress [46]. Regarding the central role played by JNK in initiating mitochondrial damage, we asked whether JNK was simultaneously modulated by Mst1 and Yap in MDA-T32 cells. Western blot analysis showed that the level of p-JNK was increased ~ 2.5-fold in both Mst1-overexpressing cells and Yap-silenced cells (Fig. 5a–c). Interestingly, Mst1 overexpression in combination with Yap knockdown further elevated the level of p-JNK. This finding indicated that the combination of Mst1 activation and Yap inhibition could further upregulate the activity of the JNK pathway.

Recently, MIEF1 was found to be a novel mediator of mitochondrial function via the modulation of mitochondrial fission and mitochondrial apoptosis [47, 48]. Via western blot analysis, we found that the expression of MIEF1 was slightly increased in response to either Mst1 overexpression or Yap deletion (Fig. 5a–c). Interestingly, cotreatment with Mst1 adenovirus and Yap shRNA further increased the expression of MIEF1 in MDA-T32 cells (Fig. 5a–c). Subsequently, to verify whether MIEF1 was the downstream effector of the JNK pathway, a pathway blocker was used to prevent JNK activation, and the expression of MIEF1 was monitored. As shown in Fig. 5a–c, after treatment with SP600125, the level of p-JNK was reversed to normal levels despite cotreatment with Mst1 adenovirus and Yap shRNA. Interestingly, JNK inhibition also abolished the upregulation of MIEF1 expression induced by Mst1 overexpression and Yap knockdown. Therefore, the above data confirmed that the JNK-MIEF1 pathway was activated by Mst1/Yap in MDA-T31 cells.

This result was further supported via immunofluorescence. As shown in Fig. 5d, e, the fluorescence intensity of MIEF1 was rapidly increased compared to that in the control group after Mst1 overexpression and Yap inhibition. However, blockade of JNK suppressed MIEF1 expression despite cotreatment with Mst1 adenovirus and Yap shRNA. In addition, a coimmunofluorescence assay demonstrated that the average mitochondrial length was reduced in response to Mst1 overexpression and Yap knockdown. However, SP600125 treatment reversed the mitochondrial length in MDA-T32 cells (Fig. 5d, e). Taken together, the above data indicated that the combination of Mst1 upregulation and Yap deletion was connected with the activation of the JNK-MIEF1 pathway.

To demonstrate whether the JNK pathway was also involved in mitochondrial stress and MDA-T32 cell death, we assessed mitochondrial function and cell viability using SP600125 in the presence of Mst1 overexpression/Yap knockdown. First, ATP production was downregulated in response to combined Mst1 overexpression and Yap knockdown (Fig. 6a). Interestingly, this effect was reversed by SP600125, indicating that the JNK pathway was involved in Mst1/Yap-mediated mitochondrial energy metabolism. In addition, an immunofluorescence assay showed that the Mst1/Yap modification-mediated release of cyt-c to the nucleus was reversed by SP600125 (Fig. 6b, c). In addition to cyt-c translocation, mitochondrial oxidative stress was evaluated by measuring the levels of cellular antioxidants. As shown in Fig. 6d–f, the SOD, GPX and GSH levels were significantly decreased in cells cotreated with Mst1 adenovirus and Yap shRNA. However, after treatment with SP600125, the levels of SOD, GSH and GPX were significantly increased to near-normal levels, indicating that mitochondrial oxidative stress was also affected by Mst1/Yap via the JNK pathway. Next, cell viability was evaluated in response to JNK inhibition in the presence of Mst1/Yap modification. As shown in Fig. 6g, cell viability was markedly repressed by Mst1 overexpression in combination with Yap deletion; this effect was reversed by SP600125. Similar results were also obtained in the LDH release assay, which showed that Mst1/Yap modification-mediated LDH release was inhibited by SP600125, highlighting the essential role played by the JNK pathway in mediating cell viability. Taken together, our results indicated that the JNK pathway was required for Mst1/Yap modification-mediated mitochondrial stress and cell death in MDA-T32 cells.