TNFAIP8 overexpression could serve as an indicator of mesenchymal subtype
Significant higher expression of TNFAIP8 was discovered in GBM compared with lower grade gliomas (LGG) according to the Cancer Genome Atlas (TCGA) and Chinese Glioma Genome Atlas (CGGA) database (LGG VS GBM, all P<0.0001, Fig. 1A, left two panels). Immunohistochemical (IHC) staining (Fig. 1C-D) for clinical samples showed that TNFAIP8 was predominantly expressed in nucleus in glioma cells. Immune reactive score (IRS) analysis for IHC verified obvious higher expression of TNFAIP8 in GBM (mean IRS = 9.200, n=30) than in grade 2 (mean IRS = 3.917, n=24, P= 0.0001) and 3 (mean IRS = 6.472, n=36, P=0.0004) gliomas, which was in line with bioinformatic findings. In addition, western blot assay also illustrated that TNFAIP8 expression was markedly upregulated in GBM (Fig. 1E).
In the meantime, an elevation of TNFAIP8 level was observed in IDH1 wild type GBM compared with their mutant type counterparts, though the differences in TCGA failed to reach the statistical significance (TCGA P=0.0535, CGGA P<0.0001, Fig. 1A, middle panels). Besides, TNFAIP8 mRNA level among proneural, neural, classical and mesenchymal subtype was compared and significantly increased TNFAIP8 was found in mesenchymal glioma (proneural, neural and classical VS mesenchymal, all P<0.001, Fig. 1A, right panels). These results established an association between high expression of TNFAIP8 and mesenchymal subtype. The specificity and sensitivity of TNFAIP8 in indicating mesenchymal subtype were evaluated. As expected, the area under curve (AUC) was 0.955 and 0.888 in CGGA and TCGA, respectively, suggesting that TNFAIP8 could serve as an indicator for mesenchymal glioma (Fig. 1B). In general, TNFAIP8 played a vital role in tumorigenesis and development of glioma. TNFAIP8 was highly expressed in GBM and could be used as an indicator of mesenchymal subtype.
TNFAIP8 could predict adverse prognosis for glioma patients
Glioma patients in TCGA or CGGA cohorts were divided into two subgroups according to TNFAIP8 mRNA abundance and higher expression of TNFAIP8 was correlated with shortened survival in both GBM and LGG glioma patients (TCGA LGG P<0.0001, GBM P<0.05, CGGA LGG P<0.001, GBM P<0.05, Fig. 2A). In addition, similar association was found when astrocytoma and oligodendroglioma was analyzed separately (TCGA astrocytoma P<0.001, oligodendroglioma P<0.001, CGGA astrocytoma P<0.05, oligodendroglioma P<0.05, Fig. 2A). Multivariate (HR = 1.81, P = 0.007) and univariate (HR = 4.41, P < 0.0001) Cox regression analysis was performed using the patient clinical characteristics including age, gender, WHO grade, IDH status and therapeutic regimen (chemotherapy and radiotherapy, Table1). The prognostic nomogram with a risk classification system for 1-, 3- and 5-year survival rates of glioma was established based on TCGA database (n = 545, Fig. 2B). We found that TNFAIP8 was a robust and independent prognostic biomarker for evaluating patient outcomes (Fig. 2B). The calibration curves showed optimal conformity between the prediction by nomogram and actual observation in TCGA database (Fig. 2C). To sum up, TNFAIP8 was confirmed to serve as an unfavorable predictor for survival in glioma patients.
TNFAIP8 could enhance viability and motility in GBM cells
To explore the function of TNFAIP8 on glioma in vitro, loss/gain of function experiments were employed in GBM cell lines. We searched Cancer Cell Line Encyclopedia derived from Betastasis (https://betastasis.com/) for TNFAIP8 mRNA expression in different GBM cell lines. Discrepant TNFAIP8 mRNA level was found in various GBM cell lines, among which U87, U138, U118 and T98 cells highly expressed TNFAIP8, whereas U251 GBM cells exhibited lowest abundance of TNFAIP8 (data not shown). We thereafter utilized U87 and U251 cell lines for TNFAIP8 knockdown and overexpression, respectively (Fig. 3A). Results for CCK8, ATP production and colony formation assays uniformly demonstrated that overexpression of TNFAIP8 enhanced cell proliferation in GBM cells and vice versa, suggesting a proliferation-promoting role for TNFAIP8 (all P<0.01, Fig. 3B-D). The elevated migration and invasion ability is one of the most significant biological features of GBM(24). Herein, TNFAIP8-knockdown in GBM cells significantly hampered migration and invasion while TNFAIP8-overexpression boosted cell motility (migration assay, all P<0.001, invasion assay, all P<0.01, Fig. 3E). Moreover, TNFAIP8-overexpression elicited upregulation of N-cadherin, vimentin and decreased the expression of E-cadherin, implying an occurrence of epithelial-to-mesenchymal transition process (Fig. 3F). In summary, we demonstrated that TNFAIP8 promoted proliferation, migration and invasion in GBM cells.
Knockdown of TNFAIP8 could affect nucleotide metabolic process and TCA cycle
RNA sequencing was performed for U87 GBM cells transfected by sh-TNFAIP8. KEGG pathway enrichment analysis for differential expressed genes (DEGs; 3098 upregulated DEGs and 4493 downregulated DEGs) in U87-shTNFAIP8 cells illustrated that TNFAIP8 was involved in multiple biological pathways related to metabolism, cell cycle, transcriptional regulation, cell polarity and cell junction, most of which were potentially associated with tumor progression (Fig. 4A-B). Since metabolic process accounted for a considerable proportion among top twenty altered pathways, mass spectrum was performed to examine metabolite changes after TNFAIP8 knockdown. In line with the results for RNA sequencing, metabolites relevant to purine (IMP, xanthine, allantoin) and pyrimidine (orotate, UMP, cytidine) metabolism and TCA cycle (citrate/isocitrate, orotate, malate) were drastically affected by TNFAIP8 (all P<0.05, Fig. 4C-D), implying that aberrant nucleotide metabolism and dysfunction of TCA cycle potentially contributed to TNFAIP8-associated poor outcomes in glioma patients.
TNFAIP8-knockdown could impair GBM growth in vivo
Xenografted models were established orthotopically in nude mice to further validate tumor promoting effect of TNFAIP8. U87 cells knocking down TNFAIP8 were injected intracranially. Consistent with the above findings, tumor burden of mice in shTNFAIP8 group was much smaller than that in control group (P<0.01, Fig. 5A-B). In the meantime, TNFAIP8 knockdown significantly prolonged the life span of nude mice (median survival time, vector group=33 days, sh-TNFAIP8-1 group=52 days, P<0.0005, Fig. 5C). Additionally, HE staining for xenografted GBM samples displayed smaller tumor volume in TNFAIP8-knockdown groups (Fig. 5D). Altogether, these findings suggested that TNFAIP8-knockdown impaired GBM growth in vivo.