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Key Laboratory of Systems Biomedicine (Ministry of Education) of Rui-Jin Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China
Shanghai-MOST Key Laboratory for Disease and Health Genomics, Chinese National Human Genome Center at Shanghai, Shanghai, China
Shanghai Center for Systems Biomedicine, Shanghai Jiao Tong University, Shanghai, China
Address reprint requests to: Ze-Guang Han, M.D., or Qing Deng, Ph.D., Shanghai-MOST Key Laboratory for Disease and Health Genomics, Chinese National Human Genome Center at Shanghai, 351 Guo Shou-Jing Road, Shanghai 201203, China. E-mail: email@example.com or firstname.lastname@example.org; fax: +86-21-50800402.
Potential conflict of interest: Nothing to report.
Supported by grants from Chinese National Key Program on Basic Research (2010CB529200 and 2012CB722308), National Natural Science Foundation of China (81101875, 81071722, and 81272271), China National Key Projects for Infectious Disease (2012ZX10002012–008 and 2013ZX10002010–006), Shanghai Natural Science Foundation (11ZR1425300), and the Shanghai Commission for Science and Technology.
Cancer/testis (CT) antigens have been considered therapeutic targets for treating cancers. However, a central question is whether their expression contributes to tumorigenesis or if they are functionally irrelevant by-products derived from the process of cellular transformation. In any case, these CT antigens are essential for cancer cell survival and may serve as potential therapeutic targets. Recently, the cell-based RNA interference (RNAi) screen has proven to be a powerful approach for identifying potential therapeutic targets. In this study we sought to identify new CT antigens as potential therapeutic targets for human hepatocellular carcinoma (HCC), and 179 potential CT genes on the X chromosome were screened through a bioinformatics analysis of gene expression profiles. Then an RNAi screen against these potential CT genes identified nine that were required for sustaining the survival of Focus and PLC/PRF/5 cells. Among the nine genes, the physiologically testis-restricted dual specificity phosphatase 21 (DUSP21) encoding a dual specificity phosphatase was up-regulated in 39 (33%) of 118 human HCC specimens. Ectopic DUSP21 had no obvious impact on proliferation and colony formation in HCC cells. However, DUSP21 silencing significantly suppressed cell proliferation, colony formation, and in vivo tumorigenicity in HCC cells. The administration of adenovirus-mediated RNAi and an atelocollagen/siRNA mixture against endogenous DUSP21 significantly suppressed xenograft HCC tumors in mice. Further investigations showed that DUSP21 knockdown led to arrest of the cell cycle in G1 phase, cell senescence, and expression changes of some factors with functions in the cell cycle and/or senescence. Furthermore, the antiproliferative role of DUSP21 knockdown is through activation of p38 mitogen-activated protein kinase in HCC. Conclusion: DUSP21 plays an important role in sustaining HCC cell proliferation and may thus act as a potential therapeutic target in HCC treatment. (Hepatology 2014;59:518–530)
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recombinant adenoviral vector harboring an shRNA against DUSP21
cancer/testis genes on X-chromosome
dual specificity phosphatases
dual specificity phosphatase 21
expressed sequence tag
gene expression omnibus
mitogen-activated protein kinase
massively parallel signature sequencing
reverse transcription-polymerase chain reaction
short-hair pin RNA
small interference RNA
a database of known and predicted protein interactions.
Generally, cancer/testis (CT) genes are defined by a unique expression pattern in which they are expressed exclusively in gametes and trophoblasts in normal tissues and are also found in a number of malignancies, although varying definitions of CT genes exist in the different literature.[1-5] CT genes can be divided into two groups: CT genes that are on the X chromosome (CT-X genes) and CT genes that are not (non-X CT genes). Intriguingly, ∼10% of genes on the human X chromosome are CT-X genes.[1, 6] Most significant, some CT-X antigens are used in therapeutic vaccines against cancers. For example, phase II trials conducted postoperatively in early-stage, nonsmall-cell lung cancer patients demonstrated that the MAGE-A3 vaccine markedly reduced the recurrence rate. Furthermore, NY-ESO-1 (CTAG1B) has also been successfully targeted in trials of NY-ESO-1-specific adoptive T-cell therapy for melanoma.
Hepatocellular carcinoma (HCC) is one of the most prevalent malignancies, ranking as the sixth most common cancer worldwide and the third most common cause of cancer mortality. The prognosis of HCC patients remains very poor, so novel therapeutic approaches against drug targets are urgently needed. Some CT-X genes, such as MAGE, SSX, and GAGE, are detected in tumor tissues or the peripheral blood of HCC patients,[10, 11] indicating that certain CT genes are deregulated in HCC.
Although CT-X antigens are the core focus in the development and clinical testing of experimental cancer vaccines, their biological functions in both germ cells and tumors have remained obscure. Recently, the cell-based RNA interference (RNAi) screen has proven to be a powerful approach for characterizing biological features by targeting genes of interest. In this study, to identify new CT antigens as potential therapeutic targets for HCC, we initially identified potential CT-X genes by comparing gene expression profiles and then performed a cell-based RNAi screen against these candidate CT-X genes to determine those genes that contribute to maintaining cell proliferation and survival in HCC cells. Of 179 CT-X genes, nine were crucial to the viability of HCC cells. Among these genes, dual specificity phosphatase 21 (DUSP21) encodes a member of the atypical dual specificity phosphatases (DUSPs) that are implicated as major modulators of critical signaling pathways and therapeutic targets. To our knowledge, we demonstrate for the first time in this study that DUSP21 is a potential therapeutic target for HCC.
Materials and Methods
In Silico Analysis of Gene Expression Profiling
Cancer/testis genes were retrieved from the CT database of the Ludwig Institute for Cancer Research, and massively parallel signature sequencing (MPSS) data of 31 human tissues extracted from the Gene Expression Omnibus database were compared to identify testis-enriched genes on the X-chromosome. In addition, expressed sequence tag (EST) profiles (NCBI Unigene) were analyzed to recognize testis-enriched genes on the X-chromosome. The following formula was used to calculate testis-enriched genes as previously described: . Here, S is the enrichment, E1 to En are the expression levels across all tissues, and Eh is the expression value observed in testis for a given gene. S values >2 would be considered testis-enriched genes.
RNA Interference Screen
Briefly, 3,000 cells per well were seeded in a 96-well culture plate. Likewise, small interfering RNA (siRNA) duplexes were placed into a 96-well culture plate and delivered into Focus and PLC/PRF/5 cells by Lipofectamine 2000 (Invitrogen) transfection according to the manufacturer's instructions. Cell viability was measured using the Cell Counting Kit-8 (Dojindo Laboratories). These experiments were repeated three times independently. Raw data from each plate were normalized and analyzed according to the described method. Phenotypes were scored for their statistical significance.
Colony Formation Assay
HCC cells, 5-10 × 104, were cultured in 10-cm plates in triplicate, then G418 (Life Technologies, Grand Island, NY) was added to the medium at a final concentration of 0.6-1 mg/mL. After 3 or 4 weeks, the remaining colonies were washed twice with phosphate-buffered saline (PBS) and stained with crystal violet. The colonies were counted according to the defined colony size.
Cell Senescence Assay
Senescence-associated β-galactosidase (SA-β-Gal) staining was performed to evaluate senescence of Focus cells 24 hours after doxorubicin (1 μM) treatment as described. Using a magnification of 100×, SA-β-Gal staining-positive cells were counted in at least six random microscopic fields.
Tumor Therapy Using a Xenograft Model
PLC/PRF/5 cells (5 × 106 in 200 μL of PBS) and MHCC-97H cells (2 × 106 in 100 μL of PBS) were injected subcutaneously near the scapula of 6-week-old nude mice to establish an HCC xenograft model for antitumor assays. The adenoviral- and nonviral atelocollagen- (Koken, Tokyo, Japan) based delivery methods were used for antitumor therapy. After tumor xenografts reached ∼50 mm3, either Ad-shDUSP21 or the atelocollagen/siRNA complex was injected intratumorally every 3 days for a total of five times. The tumor formation kinetics were estimated by measuring the tumor size and volume at 3-day intervals.
Additional methods and descriptions are provided in more detail in the Supporting Information.
RNAi Screen of Candidate CT Genes Responsible for Sustaining Cell Viability
To date, there are 33 CT-X gene families that contain 126 members in the CT database from the Ludwig Institute for Cancer Research. In this study, to define whether more genes on the human X chromosome could be designated as CT-X genes, we characterized the expression profile of 1,336 known genes localized on the X chromosome through an in silico analysis of the transcriptomic data deposited in the public database. The analysis of MPSS data obtained from the GEO database from 31 human tissues revealed 33 testis-enriched genes (Supporting Fig. 1A), and the analysis of EST data from 49 human tissues suggested 68 testis-enriched genes (Supporting Fig. 1B). Of these testis-enriched genes, only 19 overlapped in the two datasets. In addition, four testis-specific genes were manually selected based on the published literature[1, 19] (Supporting Fig. 1C). A total of 86 testis-enriched genes, including 53 potential CT-X genes not deposited in the CT database, could be considered CT genes (Fig. 1A; Supporting Table 1).
Of the integrated 179 genes, including 126 known CT-X genes from 33 families and 53 candidates, 128 genes lacking high homology with each other were further examined by reverse-transcription polymerase chain reaction (RT-PCR) using specific primers in 14 normal human tissues and a panel of 19 HCC cell lines. Among the 113 genes successfully examined, 87 (77.0%) were detected in two or more HCC cell lines (Supporting Fig. 1D, Supporting Table 2), of which 79 (69.9%) significantly exhibited relatively high expression in Focus and PLC/PRF/5 cells (P < 0.001).
To investigate the role of these potential CT-X genes in HCC cell proliferation, we established a cell-based RNAi screen to identify the candidate genes responsible for sustaining cell proliferation by evaluating cell viability (Fig. 1B; Supporting Fig. 2). In this screen, 228 siRNA duplexes were designed against these 179 genes, where some CT-X gene families shared the same siRNAs (Supporting Table 3), and were then applied to both Focus and PLC/PRF/5 cells. Based on the RNAi screen, 19 siRNAs were considered to statistically significantly suppress cell proliferation at the cutoff value of Z < −1 and P < 0.05 (Fig. 1B). Subsequently, we performed a quantitative RT-PCR assessment of the RNAi knockdown efficiency of these 19 siRNAs against 11 genes in Focus and PLC/PRF/5 cells. The resulting data indicated that the messenger RNA (mRNA) levels of nine genes were efficiently and significantly decreased to less than 60% by their corresponding siRNAs (Fig. 1C). An independent evaluation of the effect of these siRNAs on cell viability was also performed and confirmed that the knockdown of these nine genes inhibited cell viability (Fig. 1D). These collective data suggested that the nine candidate CT genes screened by the RNAi strategy could be required for sustaining HCC cell survival.
Tissue Expression Patterns and Role of Candidate CT Genes in HCC Cells
The nine genes include known CT genes, such as CT45, MAGEA9, IL13RA1, and MAGEB6, and new candidates, including DUSP21, ZCCHC13, PNMA5, PIH1D3, and MPC1L (Table 1). To determine whether these genes could be assigned to the CT genes expressed in HCC, we first evaluated their expression patterns in human tissues using RT-PCR. The results showed that only DUSP21, ZCCHC13, CT45, and MAGEB6 were exclusively expressed in testis; MAGEA9, PNMA5, PIH1D3, and MPC1L were also expressed in a few extra tissues. In contrast, IL13RA1 was ubiquitously expressed in the 14 tissues tested (Fig. 2A). The data were generally consistent with the in silico analysis of the transcriptomic data deposited in public databases, including MPSS data from 31 normal human tissues and ESTs of 49 human tissues (Fig. 2B).
Table 1. Expression Pattern of Candidate Cancer/Testis Genes That are Required for HCC Cell Survival
CT identifier is the nomenclature included in the CT database of the Ludwig Institute for Cancer Research (http://www.cta.lncc.br), and “—” denotes no inclusion in the CT database.
The expression pattern of candidate genes in 14 human tissues, including brain, heart, lung, spleen, kidney, stomach, esophagus, small intestine, ovary, breast, prostate, pancreas, liver, and testis, were examined by RT-PCR; “testis-restricted” indicates candidate genes expressed exclusively in testis; “testis-inclusive” denotes these genes expressed in testis and a few extra tissues.
The numbers of HCC patients with an up-regulation of a given gene in 24 human HCCs examined, where the mRNA levels were detected by RT-PCR.
dual specificity phosphatase 21
cancer/testis antigen family 45
zinc finger, CCHC domain containing 13
PIH1 domain containing 3
paraneoplastic Ma antigen family member 5
mitochondrial pyruvate carrier 1-like
melanoma antigen family A, 9
interleukin 13 receptor, alpha 1
melanoma antigen family B, 6
We next evaluated the expression of these nine genes in HCC specimens through a bioinformatics analysis of human HCC sample data from three public microarray databases (49 samples from GSE4024, 91 from GSE1898, and 238 from GSE5975) and through detection in our HCC samples by RT-PCR. Here, the bioinformatics analysis indicated that DUSP21, MAGEA9, PNMA5, and PIH1D3 were significantly elevated in HCC specimens, which was supported by at least two of the three datasets (Fig. 2C). In addition, our RT-PCR data revealed that six genes, DUSP21, CT45, MPC1L, PNMA5, PIH1D3, and ZCCHC13, were noticeably up-regulated in four or more of the 24 HCC samples compared with corresponding normal liver tissues (Table 1; Supporting Fig. 3).
To further confirm the role of these six CT genes in sustaining HCC cell proliferation, we performed an independent evaluation of the growth of Focus and PLC/PRF/5 cells by knocking them down by way of synthesized siRNAs. As expected, the knockdown of these genes resulted in the decreased growth of Focus and PLC/PRF/5 cells (Fig. 2D; Supporting Fig. 4A). Moreover, we evaluated colony formation by way of recombinant pSUPER constructs carrying shRNAs against DUSP21 and ZCCHC13, which are two novel testis-restricted CT-X genes that are up-regulated in HCCs. The pSUPER-mediated RNAi against DUSP21 and ZCCHC13 significantly inhibited colony formation in Focus and PLC/PRF/5 cells (Fig. 2E; Supporting Fig. 4B,C). These observations suggested that these six CT genes exert a significant role in sustaining HCC cell growth and survival.
DUSP21 Is a Novel CT Gene Frequently Up-regulated in Human HCCs
Among the six CT genes we found up-regulated in HCC, only DUSP21, CT45, and ZCCHC13 were exclusively expressed in testis. CT45 is a member of a known CT gene family,[20, 21] whereas both DUSP21 and ZCCHC13 are considered novel CT genes. ZCCHC13, with six CCHC-type zinc fingers, functions as a transcription factor, and interestingly, DUSP21 encodes a member of the DUSP family, which serves as a potential therapeutic targets for cancer.[22, 23] Because it could be a potential target for cancer therapy, we thus focused on the role of DUSP21 in HCC.
To determine the frequency of DUSP21 up-regulation in HCC samples, we also evaluated DUSP21 expression in 40 additional paired HCC samples by qualitative RT-PCR analysis. The data showed that DUSP21 was up-regulated in 15 (37.5%) of the 40 HCC specimens compared with the corresponding normal liver tissues (Fig. 3A). Furthermore, we performed immunohistochemical staining on a tissue array containing 78 pairs of HCC specimens with an anti-DUSP21 antibody. As expected, DUSP21 levels were up-regulated in 24 (30.8%) of the 78 HCC specimens (Fig. 3B; Supporting Fig. 5).
In addition to PLC/PRF/5 and Focus cells, DUSP21 was detected in nearly half of the human HCC cell lines examined by RT-PCR (Fig. 3C). An immunofluorescence assay showed that DUSP21 is mainly localized to the cytoplasm of HCC cells (Fig. 3D; Supporting Fig. 6A), consistent with the results obtained in a previous study. DUSP21 is also localized to the mitochondria in rat testis. We thus examined the subcellular localization of human DUSP21 by transfecting a vector carrying C-terminal FLAG-tagged DUSP21 into Hep3B and Huh-7 cells; however, examination by immunofluorescence showed that DUSP21 did not colocalize with MitoTracker Red, a mitochondria-selective fluorescent probe (Supporting Fig. 6B). This result implied that the subcellular localization of ectopic DUSP21 in HCC cells was distinct from that in testis.
DUSP21 Knockdown Plays an Antiproliferative Role in HCC Cells
To investigate the effect of DUSP21 on HCC cells, we examined growth and colony formation in HCC cell lines, including MHCC-97L and Huh-7 cells with low DUSP21 expression, as well as Focus and PLC/PRF/5 cells with relatively high DUSP21 expression, after transfection of recombinant pcDNA3.1-DUSP21. The results showed that ectopic DUSP21 had no significant impact on proliferation and colony formation in these HCC cells (Supporting Fig. 7A,B).
To further confirm the inhibitory effect of DUSP21 knockdown on HCC cells, we used two additional siRNAs (si-3 and si-4) to silence endogenous DUSP21 in Focus and PLC/PRF/5 cells. The siRNAs also significantly inhibited the growth of Focus and PLC/PRF/5 cells (Supporting Fig. 7C). Furthermore, we performed the colony formation assay on additional HCC cell lines by transfecting recombinant pSUPER-shDUSP21 carrying shRNA against DUSP21 into these cells. The results demonstrated that the pSUPER-mediated DUSP21 knockdown significantly inhibited anchorage-dependent colony formation of the HCC cell lines Hep3B, MHCC-97H, Bel-7404, and QGY-7703 (Fig. 4A; Supporting Fig. 7D) and restrained anchorage-independent colony formation of PLC/PRF/5 cells cultured in soft agar medium (Fig. 4B). To explore the cellular events involved in the inhibitory effects of DUSP21 knockdown on HCC cells, we next analyzed cell apoptosis, senescence, and cell cycle progression when DUSP21 was knocked down. Interestingly, upon DUSP21 knockdown, SA-β-gal staining-positive cells and the proportion of cell population in the G1 phase significantly increased (Fig. 4C,D). However, DUSP21 knockdown had no significant impact on apoptosis in Focus and PLC/PRF/5 cells (Supporting Fig. 7E,F). To further confirm the phenotypic changes, we examined some key molecules, including p53, p27, p16, cyclin D1, and cyclin E, which are known to be responsible for the regulation of the cell cycle and/or cell senescence. Interestingly, the resulting data showed that DUSP21 knockdown led to an increase in p53 and p16 as well as to a decrease in cyclin D1 and E (Fig. 4E), supporting the observation of DUSP21 knockdown-mediated G1 arrest and cell senescence.
DUSP21 Knockdown Reduces Tumorigenicity and Tumor Burden in a Xenograft Mouse Model
To investigate the effect of DUSP21 silencing on tumorigenicity in vivo, a recombinant adenoviral vector harboring an shRNA against DUSP21 (Ad-shDUSP21) was constructed and then transfected into Focus, PLC/PRF/5, and Hep3B cells (Supporting Fig. 8A). Subsequently, these Focus, PLC/PRF/5, and Hep3B cells were subcutaneously inoculated into the flanks of nude mice, whereas cells infected with the adenoviral vector carrying sh-NC (Ad-shNC) were inoculated into the opposite flank of the same mice as a control. Apart from Focus cells that lacked tumor formation, Ad-shDUSP21 significantly suppressed the tumorigenesis of Hep3B cells, with the size and weight of these xenograft tumors reduced compared to tumors formed from cells infected with Ad-shNC (Fig. 5A,B). More significant, Ad-shDUSP21 reduced the occurrence of visible tumors from PLC/PRF/5 cells over 2 months, with only 1 in 10 mice bearing tumors from these cells, while the Ad-shNC infected cells produced tumors in 9 of the same 10 mice. Kaplan-Meier tumor-free survival analysis showed that DUSP21 silencing significantly reduced tumorigenesis (P = 0.033, Fig. 5C).
To further evaluate the antitumor action of Ad-shDUSP21, a xenograft mouse model with 50-60 mm3 visible tumors, established by the subcutaneous implantation of PLC/PRF/5 cells, was subjected to intratumoral injection with Ad-shDUSP21. Interestingly, Ad-shDUSP21 significantly reduced tumor volume in five mice compared with the control Ad-shNC (Fig. 5D,E).
We sought to evaluate the therapeutic efficacy of siRNA against DUSP21 through an atelocollagen-based nonviral delivery method.[25, 26] A xenograft mouse model with visible tumors derived from MHCC-97H cells predisposed to metastasis was subjected to intratumoral injection every 3 days for a total of five injections using a mixture of atelocollagen and DUSP21 siRNA. We removed all tumors from the 10 tested mice to assess the relationship between DUSP21 silencing and therapeutic efficacy. The administration of siRNA significantly knocked down the endogenous DUSP21 levels in these xenograft tumors in 5 of the 10 mice tested (Supporting Fig. 8B). Interestingly, the sizes and weights of the tumors with lower DUSP21 levels were significantly reduced compared with those injected with control si-NC (P = 0.006, Fig. 5F). These collective data suggest that DUSP21 plays a role in HCC cell survival, thereby serving as a potential therapeutic target for HCC treatment.
Vanadate Inhibits DUSP21 Phosphatase Activity and HCC Cell Proliferation
To determine whether DUSP21 has phosphatase activity, DUSP21 was expressed and purified using a prokaryotic expression system, and its phosphatase activity was validated in vitro using pNPP as a substrate (Fig. 6A). We further evaluated the effects of 18 known phosphatase inhibitors, including 14 specific and four nonspecific phosphatase inhibitors, on DUSP21 phosphatase activity. The results showed that only vanadate (orthovanadate), a nonspecific phosphatase inhibitor, significantly inhibited DUSP21 phosphatase activity with an IC50 value of 62.5 μM (Fig. 6B,C; Supporting Fig. 9A), which is a relatively high concentration. This result suggested that high-affinity specific inhibitors for DUSP21 phosphatase activity should be further screened and validated.
To evaluate whether vanadate also has an inhibitory effect on HCC cells similar to the antiproliferative result of DUSP21 silencing, we next examined the effects of vanadate on HCC cell proliferation. Expectedly, vanadate significantly inhibited cell viability and colony formation in HCC cells in a dose-dependent fashion, whereas neither sodium fluoride nor sodium pyrophosphate (no more than 500 μM), which were shown to have no inhibitory effects on DUSP21 phosphatase activity (Fig. 6B), had significant effects on cell viability and colony formation of Focus, PLC/PRF/5, and BEL-7404 cells (Fig. 6D,E; Supporting Fig. 9B,C). Collectively, these data suggested that vanadate could suppress HCC cell proliferation possibly through inhibiting DUSP21 phosphatase activity.
DUSP21 Knockdown Plays an Antiproliferative Role Through Activation of p38 Mitogen-Activated Protein Kinase (MAPK) in HCC
Previous studies demonstrated that DUSPs are involved in MAPK signaling cascades. Here, we proposed that DUSP21 as an atypical DUSP could also regulate the MAPKs pathway in HCCs, based on data found in STRING, a database tool showing protein interactions (Supporting Fig. 10A). To validate this hypothesis, we first performed a coimmunoprecipitation (Co-IP) assay to determine whether DUSP21 binds to the three classical MAPKs, i.e., p38, JNK, and ERK, in Huh7 cells stably expressing FLAG-tagged DUSP21. However, p38, JNK, and ERK were not immunoprecipitated by the FLAG-tagged DUSP21 (Supporting Fig. 10B). Subsequently, we examined the phosphorylation levels of the three MAPKs in HCC cells by western blot assay upon ectopic DUSP21 overexpression, endogenous DUSP21 knockdown, or vanadate treatment. Overexpression of DUSP21 had no obvious impact on the phosphorylation levels of the three MAPKs in Focus and PLC/PRF/5 cells (Supporting Fig. 10C), consistent with a previous report. However, upon the vanadate-mediated inhibition of phosphatase activity including DUSP21, the phosphorylation levels of p38, JNK, and ERK were markedly increased whereas the expression of cyclin D1 and cyclin E were significantly decreased in Focus cells in a dose-dependent manner (Supporting Fig. 10D). Intriguingly, upon DUSP21 knockdown, the phosphorylation level of p38, not of ERK or JNK, was observably increased in Focus and PLC/PRF/5 cells (Fig. 7A), and even in Focus cells treated with vanadate or epidermal growth factor (EGF) (Supporting Fig. 10E). Collectively, these observations suggest that DUSP21 knockdown may lead to the activation of the p38 MAPK pathway in HCC cells.
To explore the molecular mechanism by which DUSP21 plays a role in the proliferation and survival of HCC cells, we investigated the role of p38 MAPK in HCC proliferation and survival. Our results demonstrated that p38α, a common member of the p38 MAPK family, significantly inhibited colony formation in Focus and PLC/PRF/5 cells (Fig. 7B), consistent with the previous report that p38α MAPK acts as a suppressor of cell proliferation and tumorigenesis.[27, 28] To further determine whether the activation of p38 MAPK contributes to the antiproliferative effect of DUSP21 knockdown in HCC cells, we evaluated cell viability and colony formation when both DUSP21 and p38 MAPK were simultaneously silenced by way of RNAi. Interestingly, RNAi against p38 abolished the inhibitory effect of DUSP21 knockdown on viability and colony formation in Focus, PLC/PRF/5, and MHCC-97H cells (Fig. 7C,D; Supporting Fig. 11A). To further confirm this observation, the chemical SB203580, a known p38 MAPK inhibitor, was employed in the same experiments. Expectedly, the inhibitory effect of DUSP21 knockdown on cell viability and colony formation was significantly blocked by the p38 inhibitor SB203580 in Hep3B, Focus, and MHCC-97H cells (Fig. 7E,F; Supporting Fig. 11B). These collective data suggest that the antiproliferative role of DUSP21 knockdown in HCC is through the activation of the p38 pathway.
CT antigens have been considered therapeutic targets for many cancers. However, which CT antigens could serve as therapeutic targets remains obscure. It is reasonable that these CT antigens essential for cancer cell survival may serve as potential therapeutic targets. The cell-based RNAi screen has proven to be a powerful approach for characterizing biological features, including cell survival. In this study, we employed an RNAi screen against 179 potential CT-X genes to screen for CT genes required for sustaining cell survival. Significantly, we found nine candidates responsible for sustaining cell viability, implying that these genes and protein products could be considered potential therapeutic targets for HCC treatment.
Among these candidates, four are known CT genes. IL13RA1 encodes a subunit of the interleukin 13 receptor, which is a potent therapeutic target for tumors, and MAGEA9 and MAGEB6 are members of the MAGE family and are potential biomarkers in HCC.[30, 31] CT45 is frequently expressed in lung cancer, Hodgkin's lymphoma, myeloma, and breast cancers. In addition to the above four genes, the remaining five genes are considered novel CT-X genes and potential biomarkers in HCC. Apart from DUSP21, ZCCHC13 contains six CCHC-type zinc fingers and functions as a transcription factor. PIH1D3, also known as NY-SAR-97, contains a PIH1 domain, which is involved in pre-rRNA processing. PNMA5 is a member of the paraneoplastic Ma antigen protein family, whose proteins are implicated in the development of paraneoplastic disorders. MPC1L is also termed the mitochondrial pyruvate carrier 1-like protein. Whether these genes could serve as potential therapeutic targets for HCC treatment should be investigated further.
In this study, we focused on DUSP21, a member of the atypical DUSP family. The DUSPs, a heterogeneous group of protein phosphatases that can dephosphorylate both phosphotyrosine and phosphoserine/phosphothreonine residues within one substrate, are major modulators of some critical signaling pathways that are deregulated in cancer, obesity, diabetes, inflammation, and Alzheimer's disease.[13, 36] Thus, DUSPs have increasingly become promising therapeutic targets in drug discovery by way of high-throughput screening and compound-library development efforts. Here, our data indicated that the testis-restricted DUSP21 was up-regulated in ∼30% of HCC specimens. Significantly, the DUSP21 reexpressed in HCC cells could be indispensable for sustaining cell proliferation and survival, possibly through nononcogene addiction, which is a promising therapeutic target for tumors.
As for the molecular mechanisms involved in the functions of the atypical DUSP family, it has been reported that different atypical DUSP members have varied substrate specificity. In some cases, the DUSPs (e.g., DUSP19, DUSP22, and DUSP23) may act as scaffold proteins that contribute to the interaction of signaling proteins.[13, 38] Our data revealed that DUSP21 knockdown resulted in the increased phosphorylation level of p38 in HCC cells, implying that DUSP21 knockdown may trigger p38 activity, even though we detected no interaction between p38 and DUSP21 in our Co-IP assay. However, it is possible that DUSP21 may act as scaffold in its role in the activation of the p38 MAPK pathway in HCC cells. Taken together, based on in vitro and in vivo experiments, we propose that DUSP21 could be a potential therapeutic target for HCC treatment.
We thank Dr. Da-Li Zheng and Jian Huang for help with data analysis, and Dr. Angel R. Nebreda (Institute for Research in Biomedicine, Spain) for providing us with p38α plasmid.