Biological functions are regulated by the set of upstream coding genes of proteins. The activity of the biological function in a certain sample could be determined by evaluating the overall expression level of the gene sets corresponding to each biological function. Based on this, we calculated the macrophage-related function score. TAM plays an important role in shaping TME. In this study, the samples were divided into C1 and C2 by evaluating functional differences related to macrophages. Lasso regression screening of 12 MRRGS has a significant effect on the survival, indicating that macrophages play an important role in the prognosis of HCC. Notably, Almost all the paracancerous tissues contained in TCGA and normal tissues in ICGC are clustered in C2, which means that C2 has a level of macrophage response similar to that of normal tissue. Therefore, we can use normal tissue as a reference to distinguish C2 subtypes in HCC, which can effectively simplify the clinical application. Meanwhile, C2 also had a better prognosis, which means that normal macrophage function is important for patient survival. In addition, the results showed that C2 had higher immune levels,, which may be related to the shaping of TME by macrophage function. Previous studies have also shown that TAM in HCC inhibits the overall immune level of TME by expressing cell surface proteins or releasing soluble factors related to immunosuppressive function, such as arginase 1, IL-10, TGF-β [49–51]. TAM could recruit regulatory T cells to mediate immune depletion of CD8 + T cells in TME [49].
Functional analysis showed that the main functions of C1 were mainly concentrated on cell cycle, cell proliferation and protein genesis, which were considered to be more likely to promote the occurrence and deterioration of HCC. Recent studies have shown[52] that high levels of DNA repair are associated with proliferation of HCC cell, poor prognosis, heterogeneity and mutation. E2F is the encoding gene of a set of transcription factors family, related to cell cycle and DNA synthesis, and has been confirmed to be involved in regulating the proliferation and progression of HCC in a number of researches[53–56]. G2M checkpoint ensures adequate repair of DNA during G2M transformation and plays a key role in the proliferation, apoptosis and invasion of HCC [57]. Myc is a transcription factor encoded by protooncogene, which is closely related to the occurrence and deterioration of liver cancer and drug resistance[58]. UPR and Myc have a synergistic effect on cancer and could jointly promote tumor progression[59]. These evidence suggest that abnormal cell proliferation and metabolism are important factors leading to poor prognosis of C1. On the other hand, C2, has relatively stronger level of immunity and apoptosis, such as tumor necrosis factor (TNF) signal and interferon (INF) signal, which have been confirmed to be related to the formation of anti-HCC tumor immunity in literature studies[60–62]. It is worth noting that C2 is accompanied by activation of HCC signaling and immune negative feedback related functions, including IL2/STAT5 signaling, cell proliferation, angiogenesis and tumor migration. Among them, IL2/STAT5 is an important maintenance signal of regulatory T cells (Tregs). Cytokines participate in the depletion of T cells in TME and promote the immune escape of tumor cells [63]. This suggests that the reason why C2 has a better survival than C1 is its strong immune level. As a diagnosed HCC, it also has some malignant characteristics of cancer, and immunosuppression caused by high-level immune response may be an important factor in the occurrence of this subtypes.Therefore, for C1 and C2 patients, using different therapies respectively will have a better effect. The former is more suitable for targeting TAM reprogramming and inhibiting cell proliferation, while the latter may be more suitable for immune checkpoint therapy and anti-proliferative metastasis drugs.
Immune infiltration analysis showed that C2 has stronger infiltration levels of CD8 + T cells, CD4 + T cells, NK cells and M2 macrophages compared with C1, which is consistent with our previous analysis results and more strongly proved that TAM had a strong shaping ability to the overall immunity level of TME. At the same time, the gene expression of 42 immune checkpoints were significantly different between C1 and C2. The different expression of these immune checkpoints will have a great impact on the therapeutic efficacy of patients, which is one of the important reasons for tumor heterogeneity and drug resistance. Interestingly, the inhibitory checkpoint molecules such as IDO1, IDO2, PD1, BTLA, CTLA4, LAG3 and TIGIT are upregulated in C2 patients, which are all markers of T cell exhaustion. The highly expressed immunosuppressive components suggest that C2 may have a good response to immune checkpoint inhibitors. Previous studies [64] have shown that some cancer patients have high levels of CD8 + T cell infiltration, but these infiltrating cytotoxic T cells (CTL) do not seem to give full play to their anticancer effects and are in a states of dysfunction. At the same time, high infiltration level of CD8 + T cells can promote the activity of immunosuppressive components such as PD-L1, IDO and Tregs in TME [65], which is strikingly consistent with the phenotype of C2 patients. TIDE analysis confirmed our conclusion that C2 patients had a higher T cell dysfunction score and ICI response, while C1 patients had a higher level of TAM infiltration and tumor immune exclusion score. The higher TAM infiltration level is an important shaping factor of immune rejection in C1 patients, which leads to lower TME immune cell infiltration level. Therefore, the design of personalized treatment for HCC patients with different subtypes of macrophage function will help to save drug resources and improve the therapeutic effect. For the C2 patients with high PD1 expression levels, the use of ICI such as PD1/PDL1 seems to reactivate the saturated functional impaired CTL in their TME and improve the patient's condition. For C1 patients with low PD1 expression level, ICI seems to be difficult to achieve good efficacy. It may be a better choice to use targeted TAM to regulate immune exclusion and inhibit tumor cell proliferation. Our research also showed that two subtypes with different macrophages response have appeared in the HCC patients. The first subtype has similar macrophage function states to normal and often behave a higher immune cell infiltration levels, but with a certain degree of immunosuppression, while the other subtype displays higher tumor proliferation characteristics and lower immune cell infiltration levels. The former subtype is pretty similar to the "hot tumor" proposed in previous studies[66] and is sensitive to ICI treatment. The latter subtype seems to be similar to the "cold tumor". At the same time, these results suggest that constructing macrophage classification model has great clinical value in evaluating the response potential of patients receiving immunotherapy. Moreover, the classifier constructed by macrophages can also be used to evaluate the response of HCC to TACE therapy. Due to the large number of genes involved in the 12 MRRGS, in order to facilitate clinical use, machine learning method has mined the characteristic gene SASH3 between C1 and C2. Evaluated by ROC in three cohorts, it has a very reliable diagnostic performance. At present, there are few studies on this gene, and a small number of studies have shown that SASH3 is very important for the survival and proliferation of T cells, which is related to immune deficiency and disorder in patients. These evidences show that SASH3 has further research and application value.
The mutation data showed that the TMB of C1 was higher than that of C2, which further showed a significant heterogeneity between the two subtypes of patients. Studies have shown that somatic mutations are important factors driving tumorigenesis[67]. This study found that the mutation level of TP53 in C1 was significantly increased, and had a significant impact on the prognosis of patients. Previous studies have shown that TP53 mutation exists in a large number of HCC, and the p53 protein encoded downstream is a key molecule in the p53 signal pathway, which plays an important role in regulating cell proliferation and immune level, and is an important molecular marker and therapeutic target. The higher TP53 mutation level of C1 than C2 is an important upstream basis affecting the difference of macrophage function and a key factor leading to different immune types of patients. At the same time, it shows that C1 patients with high TP53 mutation are more suitable to be treated with related molecular therapy. In addition, C1 patients generally had high mRNAsi scores, which confirmed that C1 patients had a more active states of cell proliferation.
Most of the genes extracted from MRRGS were confirmed to be related to the occurrence of tumors and the level of immunity through WGCNA analysis. Subsequent KEGG enrichment analysis of five gene modules revealed that these genes involved PD1-PDL1 signal, NF-κB signal, Ras signal pathway, TNF signal pathway, IL17 signal, viral infection pathway, reaction to chemical substances, immune cell-associated receptor pathway, cell cycle and proliferation, amino acid synthesis and decomposition, protein processing, DNA shearing and repair, substance metabolism, complement system, and coagulation cascade reactions and C-type lectin receptor pathways etc. are mutually confirmed with our previous analysis, and most of them have been mentioned in the literatures related to the progression of liver cancer[68–72]. These function differences between C1 and C2 are important components of tumor heterogeneity. Drug design by targeting these pathways and their core molecule will have a better chance to improve patient survival. Single-cell analysis further revealed that the recruited TAM plays a crucial role in the shaping of TME. The identification of patients with TAM response states will be of great benefit to the choice of drugs and prognosis of HCC.
Finally, we identified potential therapeutic for each subtype based on the patient expression profile. And we found that HDACs inhibitors seem to be more suitable for C1 patients, while COX inhibitors may be more effective for C2 patients. The result will help us better understand the therapeutic mechanism of HCC and be used for subsequent drug combination and exploration. HDACs inhibitors in a number of clinical studies were used for the treatment of various cancers [73–76]. Studies have shown that inhibition of HDACS leads to increased acetylation of relevant tumor suppressive genomic proteins and non-histones regulated by chromatin modification, resulting in transcriptional activation and recovery of downstream cell proliferation inhibition and apoptosis [77, 78]. At the same time, HDACs inhibitors have been shown to target to reshape the epigenetic state of TAM in TME, which may have a good therapeutic effect on C1 patients with high TAM infiltration [79, 80]. COX inhibitors can not only inhibit the growth, invasion and migration of tumor cells [81], but also effectively inhibit the expression of IDO and the accumulation of Tregs in TME [82]. COX inhibitors can be used in cancer-related inflammation with overactive IDO1 to help regulate the immune response in TME [82]. A study shows that the combination of HADC inhibitors and COX inhibitors has no obvious side effects on patients, but has synergistic anticancer effect [83]. These results suggest that a combination of these drug may have a therapeutic effect on most patients. At the same time, drug sensitivity analysis showed that there were significant differences in the response of HCC with different subtypes to most chemotherapeutic drugs, which further emphasized the importance of accurate medical treatment.
TAM plays an important role in the shaping of TME and the formation of tumor heterogeneity. Our research shows that HCC can be divided into two subtypes according to the functional response states of macrophages. Among them, patients with high levels of immune cell infiltration are usually accompanied with lower TMB, less M2 TAM infiltration and stronger immunosuppression, and the potential negative immune feedback mechanism is the primary driving force of tumor deterioration. However, patients with low levels of immune cell infiltration are usually accompanied with higher TMB, more M2 TAM immune infiltration and stronger cell proliferation, and immune exclusion and extensive mutations are the primary driving forces of tumor occurrence and deterioration in this subtype. For the former, it seems to be necessary to curb the aggravation of negative immune regulation, and targeted suppression of high expression PD1, IDO1, IDO2, BTLA, CTLA4, LAG3 and TIGIT immune checkpoints is a better choice. For the latter, it seems to be necessary to improve the proportion of immune cell infiltration and inhibit the active proliferation of tumor cells. Immune-related pathways (e.g., PD1-PDL1 signaling), mutations, cell proliferation, viral infection, various metabolic responses, chemicals (e.g., alcohol) are important upstream bases for the formation of patient heterogeneity. Through the classification of MRRGS, we can well predict the response of patients to immunotherapy, TACE and various drugs, which will save resources more effectively and help to achieve accurate treatment. It is noteworthy that our study further emphasizes that the TME shaped by macrophages is an important factor in the formation of tumor heterogeneity and an important research breakthrough for tumor immunotherapy in the future. In addition, two potential therapeutics like COX inhibitors and HDACs inhibitors are worthy of further clinical trials.
Nonetheless, this research still has some limitations. First, although our results used more than 1000 HCC samples and were validated in two independent cohorts, this needs to be further incorporated into a larger clinical cohort for study. Secondly, other epigenetic methods such as non-coding RNA may have an impact on the production of subtypes, which is worthy of further analysis. Finally, the mechanism of 12 MRRGS and SASH3 on tumor heterogeneity is also worthy of further exploration. We believe that macrophages play an important role in the formation of HCC tumor heterogeneity, and the evaluation of HCC immunity through macrophage function will provide great help for the realization of accurate medicine.