ATF3 expression in patients with HCC
We analyzed the ATF3 mRNA levels in tumor and corresponding normal tissues of different tumors types using ONCOMINE and TIMER. The results revealed that ATF3 expression was lower in bladder urothelial carcinoma (BLCA), breast invasive carcinoma (BRCA), cholangiocarcinoma (CHOL), kidney chromophobe (KICH), and liver hepatocellular carcinoma (LIHC) cancer patients (Supplementary Figure 1). Next, we evaluated ATF3 transcription levels in multiple HCC studies from the HCCDB database, which includes 12 HCC cohorts. The expression of ATF3 mRNA was significantly lower in HCC tissues than in normal adjacent tissues (Figure 1A). Furthermore, ONCOMINE revealed that ATF3 expression levels and DNA copy number variations (CNVs) were considerably lower in HCC tissues than in normal tissues. ATF3 exhibited lower expression levels in HCC tissues than in normal samples in the Chen liver dataset (fold change = -2.066, P = 5.51E-16, Figure 1B), Wurmbach liver dataset (fold change = -2.906, P = 3.45E-4, Figure 1C), and Roesser liver dataset (Figure 1D, 1E). In addition, we also detected substantially lower ATF3 mRNA in the livers of patients with HCC tumor tissues than in normal tissues using data from the Human Protein Atlas (Figure 1F). Furthermore, we detected the expression of ATF3 in HCC cell lines, and results showed that ATF3 was significantly down-regulated in HCC cell lines compared to normal human hepatocyte (Figure 1G). Finally, the expression of ATF3 was compared between HCC and adjacent samples in four GEO datasets and we found that significantly downregulated in HCC consistently (Table 1 and Supplementary Figure 2). Taken together, our results demonstrated that ATF3 expression was lower in patients with HCC than in healthy controls at both the mRNA and protein levels.
ATF3 expression in subgroups of patients with HCC stratified by various criteria
Sub-group analyses based on multiple clinicopathological features in 371 LIHC samples in TCGA consistently indicated lower transcription levels of ATF3 in HCC patients than in healthy controls (Figure 2A). ATF3 levels were substantially lower in patients with HCC than in healthy individuals of subgroups analyzed based on gender (Figure 2B), age (Figure 2C), ethnicity (Figure 2D), disease stage (Figure 2E), tumor grade (Figure 2F), weight (Figure 2G), and nodal metastasis (Figure 2H). Thus, ATF3 expression can be used as a diagnostic indicator in patients with HCC.
Prognostic value of ATF3 expression in patients with HCC
Prognostic value of ATF3 mRNA expression in patients with liver cancer was determined using Kaplan-Meier plot. As shown in Figure 3A, low levels of ATF3 were associated with a shorter overall survival (OS) (HR=0.66, P=0.023). Recurrence-free survival (RFS) and progression-free survival (PFS) were not associated with ATF3 mRNA expression levels in HCC (Figure 3B-C). More importantly, low mRNA expression of ATF3 was associated with a worse prognosis based on disease-specific survival (DSS) (Figure 3D, HR =0.55 and P=0.01). These results indicated that ATF3 expression might be an effective prognostic biomarker in HCC. We then used Kaplan-Meier plotter data to study the relationship between ATF3 expression and clinical characteristics of HCC patients. Low expression of ATF was associated with lower OS and PFS in male patients as well as in patients of Asian ethnicity (P< 0.05). Specifically, low ATF3 mRNA expression was associated with shorter OS in stage 1/3/4 HCC, and was substantially associated with OS and PFS of grade 2 HCC (Table 2). Furthermore, low ATF3 expression was associated with shorter OS in stage 1/2/3 of TNM category T (Table 2). More importantly, we found that low ATF3 mRNA expression was correlated with lower OS and PFS after sorafenib treatment (Table 2), suggesting that ATF3 is a novel predictive biomarker that could be used for evaluation of therapeutic HCC treatment. Low ATF3 mRNA expression was also associated with lower OS and PFS after alcohol consumption and in patients without viral hepatitis infection (Table 2). Furthermore, ATF3 expression had a good performance in discriminating HCC across above four datasets (GSE14520: AUC = 0.6935, P < 0.0001; GSE25097: AUC = 0.8339, P < 0.0001; GSE76427: AUC = 0.7218, P < 0.0001 and GSE121248: AUC = 8506, P < 0.0001) (Figure 3E-H). These results suggest that ATF3 expression level substantially affects the progression and prognosis of HCC patients.
Associations between ATF3 mutations and survival in HCC
We used cBioPortal to evaluate genetic alterations in ATF3 in HCC based on sequencing data of LIHC patients. ATF3 was altered in 39 of 372 (10.4%) patients with LIHC (Figure 4A), including various mutation types such as missense mutations and amplifications. However, Kaplan-Meier plot and log-rank tests revealed that genetic mutations in ATF3 were not significantly associated with a lower OS (Figure 4B, P =0.665) or disease-free survival (DFS) (Figure 4C, P=0.479) in patients with HCC.
Enrichment analysis of ATF3 functional networks in HCC
The Function module of LinkedOmics was used to analyze mRNA sequencing data from 371 patients with LIHC in TCGA. As shown in the volcano chart (Figure 5A) and heat map (Figure 5B, 5C), 50 gene sets were substantially positively or negatively correlated with ATF3. The results suggest that ATF3 has a substantial effect on the transcriptome. ATF3 expression showed strong positive association with expression of CSRNP1 (Pearson correlation coefficient = 0.6729, P= 2.948e-50), JUN (Pearson correlation coefficient = 0.6167, P= 3.081e-40), and NR4A3 (Pearson correlation coefficient = 0.5822, P = 4.767e-35), reflecting changes in the suppression of tumor growth (Figure 5D-F). GO terms identified in gene set enrichment analysis (GSEA) revealed that differentially expressed genes correlated with ATF3 were mainly involved in acute inflammatory response and macrophage activation (Figure 5G). KEGG pathway analysis showed enrichment of complement and coagulation cascade pathways and cytokine-cytokine receptor interactions, which regulate cancer-related signaling pathways (Figure 5H).
ATF3 networks involve kinases, miRNAs, and transcription factors in HCC
To reveal the targets networks of ATF3 in HCC, we analyzed positively correlated gene sets generated by GSEA, such as kinases, miRNAs, and transcription factor. The top 5 most significant kinase networks were related to mitogen-activated protein kinase, inhibitor of nuclear factor kappa B kinase subunit beta (Kinase_IKBKB), cyclin-dependent kinase 3 (Kinase_CDK3), glycogen synthase kinase 3 beta (Kinase_GSK3B), and FER tyrosine kinase (Kinase_FER) (Table 3). The miRNA-target network was associated with (ATTACAT) MIR-380-3P, (TGAATGT) MIR-181A, (TATTATA) MIR-374, (ACTTTAT) MIR-142-5P, and (TAATGTG) MIR-323 (Table 3). Transcription factor-target network was mainly related to CREB family members, including ATF3_Q6, CREBP1_Q2, ATF_01, CREB_Q2_01, and CREB_Q2 (Table 3). In fact, low expression of these kinase genes and transcription factors was considerably associated with worse OS prognosis in patients with HCC (Supplementary Figure 3).
ATF3 is correlated with tumor purity and immune infiltration level in HCC
Tumor infiltrating lymphocytes (TIL) are independent predictors of cancer survival [26]. Therefore, we used TIMER to study whether the expression of ATF3 is related to the level of infiltrating lymphocytes in liver cancer. ATF3 expression exhibited significant negative correlation with tumor purity (r = -0.145, P = 6.92E-03) and slight association with dominant macrophage levels (Figure 6A). Specifically, ATF3 CNV was significantly associated with infiltration levels of CD8+ T cells, macrophages, neutrophils, and dendritic cells (Figure 6B). In addition, in order to expand the understanding of the interaction between ATF3 and immune markers, we analyzed the correlation between ATF3 expression and various immune markers, including immune marker genes for tumor infiltrating lymphocytes (TILs) and immune suppression and immune checkpoint gene sets (Figure 7A-P). After adjusting for tumor purity, the results showed that ATF3 expression was significantly correlated with macrophage subpopulations, including tumor-associated macrophages (TAMs; CCL2, r = 0.245, P =4.16e-06), M1 (NOS2, r = 0.137, P =0.08e-02), and M2 (CD163, r = 0.141, P =8.90e-03) (Figure 7E-G). The results suggested that expression of ATF3 is related to the macrophage subset of immune infiltration in HCC.