Screening of immune-related metabolic genes
We obtained 1041 immune-related genes and 1613 metabolic-related genes from the Online website. Then we got 346 immune-related metabolic genes by Spearman correlation calculation.
GO and KEGG Enrichment Analysis and PPI Network Construction of immune-related metabolic genes
The GO enrichment and KEGG pathway analyses of the 346 immune-related metabolic genes were conducted using “clusterProfiler” package in the R environment. As mentioned earlier, the GO analysis results consisted of three parts: BP, CC, and MF. The results indicated that the Immune-related metabolic genes were significantly enriched in the BP-associated organic acid biosynthetic process, carboxylic acid biosynthetic process and monocarboxylic acid biosynthetic process. For the CC, the immune-related metabolic genes were mainly enriched in the Golgi lumen, lysosomal lumen, and vacuolar lumen. Furthermore, through the MF analysis, it was found that the immune-related metabolic genes were notably enriched in cofactor binding, oxidoreductase activity, acting on the CH−OH group of donors and carboxylic acid-binding. KEGG was used to analyze the signaling pathways that immune-related metabolic gene enrichment. Immune-related metabolic genes were enriched in Arachidonic acid metabolism, PPAR signaling pathway, and Biosynthesis of amino acids (Figure 1A-1B). To further explore its potential mechanism, we use Cytoscape software [National Institute of General Medical Sciences (NIGMS)] to build a PPI network (confidence level = 0.9) (Figure 1C) based on the STRING database. Then we identified the core genes of top10 through the Cytoscape plug-in cytoHubba: SDC2, GPC3, GPC1, HSPG2, AGRN, GPC2, GPC5, GPC4, GPC6, VCAN (Figure 1D). Then we further analyzed the differences between immune-related metabolic genes in cancer and para-cancerous tissues. As shown in Supplementary Figure 1, the heat map clearly distinguishes the LUAD samples from adjacent tissue samples (Figure S1A). A total of 141 DEGs were identified (| log2fold change |> 1, P < 0.05), including 72 upregulated and 69 downregulated genes (Figure S1B). Then, we performed GO and KEGG enrichment analysis on 141 differential genes (Figure S1C-S1F). The function enrichment of DEGs is mainly concentrated in: fast acid metabolic process; organic hydroxy compound metabolic process; small molecular metabolic process.
Part 1: Immune characteristics and Molecular characteristics of immune-related metabolic genes.
Consistent clustering of immune-related metabolic genes
We already know that immune metabolism genes regulate different biological pathways through the previous research results, speculating that various tumors will have different metabolic methods and show other biological characteristics. We clustered the immune-related metabolic genes in a consistent cluster to explore the metabolic patterns of tumor cells. We can divide tumor samples into different clusters according to different immune infiltration results through cell clustering. Plot the cumulative distribution function CDF to identify the number of optimal clusters (Figure 2A-2B). And then, we identified three different clusters and drew heat maps to compare the expression of immune-related metabolic genes among the various clusters (Figure 2C). The results showed that cluster2 had the best immune cell infiltration, followed by cluster3 and cluster1. In the end, we evaluate the survival status of the three clusters. By comparing progression-free survival (PFS), it can be found that cluster2 has a better survival rate (Figure 2D).
Immune characteristics between clusters-expression of immune-related molecules
We further analyzed the expression of immune-related molecules among these clusters. We analyze the molecules correlated with antigen‐presentation (B2M, HLA-A, HLA-B, HLA-C, HLA-DPA1, HLA-DQA1, TAP1, TAP2), chemokine-related genes (CCL4, CCL5, CXCL10, CXCL13, CXCL9), immune checkpoint molecules (CD226, CD274, CD276, CD40, CTLA4, HAVCR2, LAG3, PDCD1) and cytokines (GZMB, GZMH, IFNG, IL2, PRF1, TNF) expressions by drawing a box plot for comparison (Figure 3A-3D). We found that different clusters have different immune functions. There are apparent differences between them, and there is no complete consistency. In Antigen, we found that HLA-DPA1 and HLA-DQA1 have the highest expression levels in cluster2. Chemokine found that CXCL13 is a higher expression level in cluster2. The immune checkpoints showed that CD226 and CTLA4 had higher expression levels, and IL2 also increased significantly. The differences in these immune-related genes may help explain the better survival status of patients in cluster2.
Immune characteristics between clusters-expression of infiltrating immune cells and clinicopathological characteristics
We use the four reported methods (ssGSEA, MCP-counter, CIBERSORT, and Xcell) to evaluate these three clusters' immune cell infiltration level. We explored two aspects: immune effector cells (Figure4A-4D) and immunosuppressive cells (Figure4E-G). It can be seen that in general, cluster1 had the least infiltration of immune effector cells and immunosuppressive cells, suggesting that cluster1 might be the immunologically-cold tumors, while cluster 2 and cluster 3 were enriched by both immune effector cells and immunosuppressive cells, which might suggest that both cluster 2 and cluster 3 had immunologically-hot tumor immune microenvironments. In cluster2, the content of activated B cells, DC, and monocytes was significantly increased. Finally, we compared three different immune cell infiltrating clusters and reached their immune score (Figure4H). The results show that cluster2 has a higher immune score and stromal score. Besides, we compare the different clinicopathological parameters between the clusters, including tumor stage, TNM classification and drug response. As shown in Figure S2, we found that different clusters are significantly related to the TNM staging of diabetes. In cluster 2, the patients with stage III and IV is significantly lower than that in stage I and II (A-C). In terms of lymph node metastasis, the proportion of non-metastatic patients is highest in cluster2 (D-F), and the lowest in cluster1; also, in terms of distant metastasis of tumor, tumor stage and drug response, patients in cluster2 have better performance (G-O).
Part 2: Validation of prognostic prediction based on models of immune-related metabolic genes
LASSO COX regression analysis
Next, we explore whether these immune metabolism genes have the function of predicting survival. To achieve this goal, we randomly divide the LUAD matrix into a training set and a test set (70% of the training set and 30% of the test set). First, perform single factor analysis, select a total of 80 genes with p<0.05, These significant genes entered into LASSO COX regression analysis, and the regression coefficient was computed (Figure 5A-B). After selecting the best combination, use multivariate regression, select p<0.05 as the gene for constructing predictive models. The distributions of risk score of LUAD patients and the relationships between risk score and survival time were visualized in Figure 5C-F. Finally, nine genes were identified: TK1, TCN1, CAV1, ACMSD, HS3ST2, HS3ST5, AMN, ADRA2C, ACOXL.
Construct prognostic prediction models of immune-related metabolic genes
First, we perform single-factor COX analysis based on the hub genes (Figure 6A). In the training set and test set, patients were divided into high-risk and low-risk groups. According to the two groups of patient's clinical information, a survival curve was drawn (Figure 6B&6D). The results in the training set and test Focus on high score patients had a worse Overall Survival (OS) than those of low score patients (p <0.0001). The area under the ROC curves (Area Under Curve: AUC) of the predictive model for LUAD has the same performance in the first year, third year and fifth year (Test set: AUC at one year: 0.68, AUC at three years: 0.76, AUC at five years: 0.61; Training set: AUC at one year: 0.83, AUC at three years: 0.72, AUC at five years: 0.71) (Figure 6C&6E).
Part 3: Identify potential metabolic checkpoints
Finally, we discussed that immune-related metabolic molecules could be used as a metabolic checkpoint to provide the theoretical basis for future treatment. To achieve this goal, we first selected high expression immune metabolic genes (logFC > 1.5, FDR < 0.05) in the tumor site because the high level of genes in the tumor may be a potential factor to promote tumor growth. In addition, we obtained five potential targets, HMMR, PFKP, RRM2, TCN1 and TK1(supplementary figure S3), by taking the cross points of immune metabolism genes negatively correlated with survival rate (these are the genes in the survival model training set). By comparing the survival rate of five hub genes in Pan-cancer and their relationship with immunity, it was confirmed that the RRM2 gene was highly correlated with immunity (supplementary figure S4&S5). Finally, we analyzed the correlation between the RRM2 gene and the CDK family, and the results show that these genes are highly correlated (Fig. 7C). These were also consistent with the results analyzed in the tumor samples by qRT-PCR (Figure 7D-G).