The increasingly reported crucial role of the TME in malignancies has shifted the focus of cancer research from the tumor itself to the surrounding microenvironment,[20, 21], which represents a major TME component and mediates tumor progression, poor prognosis, and therapy resistance in various malignancies.[22] TAMs are involved in promoting TNBC aggressiveness.[23] Moreover, TAMs subpopulations change dynamically during tumor development and correlate with immunotherapy efficacy.[22] Here, we comprehensively investigated the characteristics and classification of TAMs in TNBC using scRNA-seq data. We identified four distinct TAM clusters and examined their correlation with prognosis The TAM-0 cluster showed a unique prognostic value, indicating that specific TAM clusters may have potential prognostic prediction and help identify new prognostic subtyping patterns for TNBC.
Based on the DEGs identified within the TAMs clusters that were significantly associated with TNBC prognosis, we developed a risk signature that comprises six risk genes (C7, EDNRB, GPR34, SDS, STOM, and VSTM4) and two protective genes (ADGRF5 and SAMD1). High C7 expression in breast cancer indicates poor prognosis and decreases sensitivity to taxane-anthracycline,[24] while STOM is involved in breast cancer [25] and GPR34, a member of the G-protein coupled receptor superfamily, participates in cellular physiological functions such as cell growth, differentiation, and motility.[26] GPR34 also promotes malignancy in certain cancers, including cervical cancer, gastric adenocarcinoma, and colorectal cancer,[26, 27] showing a high expression in cell lines of cervical and colorectal cancers.[28, 29] Consistent with this, GPR34 was highly expressed in TNBC tissues and MDA-MB-231 cells in our study. Additionally, our findings demonstrated a stronger immune association between GPR34 and TNBC than that of the other seven genes. The significance of GPR34 in immune responses emphasizes its role in stimulating paracrine signaling in malignant B cells.[27, 30] Therefore, we hypothesized that GPR34 could be involved in the immune-related malignant characteristics of TNBC and serve as a promising immunotherapy target. However, this hypothesis warrants further investigation. Additionally, we examined the association between the risk signature and TNBC prognosis, revealing that the high-risk group had a poorer prognosis. Our findings suggest that our risk signature is a reliable tool for precise prognostic prediction in patients with TNBC. Moreover, almost all genes had SNV mutations, and three oncogenic pathways (PI3K, TP53, and RTK-RAS) were selected. The PI3K pathway is a prominent and extensively studied target for therapeutic intervention.[31] PI3K is a kinase involved in cellular transformation and is associated with polyoma middle T antigens; its activity in mitogenesis and oncogenesis plays an important role in human cancers.[32] PI3K regulates essential cellular functions including protein synthesis, glucose metabolism, apoptosis, and survival, namely through the generation of phosphatidylinositol 3,4,5-trisphosphate, which acts as a potent secondary messenger and recruits specific kinases, such as AKT and PDPK1, to the plasma membrane.[32] Furthermore, cancer genome sequencing has shed light on PI3K mutations in various human cancers, such as breast, colorectal, gastric, and lung tumors.[33] Several drugs targeting the PI3K pathway have gained regular or accelerated approval from the FDA for the treatment of PIK3CA-mutant and estrogen receptor-positive advanced-stage breast cancers.[32] TNBC harbors PI3K mutations that may trigger disease progression and therapy resistance.[34, 35] Activation of the PI3K/AKT pathway has been reported to induce chemotherapeutic resistance in TNBC. Moreover, our low-risk group showed higher TMBs. These findings indicate that our risk signature may play an important role in the development and therapeutic response of TNBC, while enabling exploring new TNBC therapeutic targets. Developments in immunotherapy have emphasized the importance of understanding the immune landscape within the TME.[36] TAMs are intimately involved in immunosuppression and can influence immunotherapy in many cancers; consequently, targeting TAM markers could enhance cancer immunotherapy.[37] Our study mainly analyzed the immune characteristics of the risk signature in TNBC, showing that immune-related pathways were highly enriched in the high-risk group. Furthermore, the high-expression group had significantly higher immune scores. The ssGSEA and CIBERSORT methods revealed significant differences in the immune cell scores and correlations between the two risk groups. Additionally, the high-risk group exhibited higher expression levels of TRS, CYT, and IFN-γ, representing a more immunoreactive microenvironment. Subsequently, the immune checkpoint genes, including PD1, PD-L1, and CTLA4, were more expressed in the high-risk group. Our results highlight the important immunological value of our risk signature. Therefore, we explored potential immunotherapeutic drugs based on this signature and identified 9 CTRP- and 21 PRISM-derived agents. CTRP-derived GDC-0941 is a PI3K inhibitor. PI3K is an important component that is aberrant in signaling pathways related to various cellular biological functions in several malignancies, including breast cancer.[38] Despite previous findings indicating the restricted therapeutic efficacy of GDC-0941 in TNBC, recent in vitro investigations have proposed that its co-administration with third-generation retinoid adapalene could enhance TNBC responsiveness to GDC, suppressing tumor growth, and decreasing treatment resistance.[38, 39] Our risk signature may provide novel strategies for the improvement of existing immunotherapies and enable exploration TNBC immunotherapies.
Our study has some limitations. The risk signature was constructed using mainly retrospective data; prospective studies need to be conducted. Our experiments focused on histological and cellular aspects; animal-level experiments should be conducted in the future. Finally, the mechanism underlying the contribution of the risk signature to prognosis and immunity was not elucidated; this shall be further investigated using both bioinformatics and experiments.
Collectively, this study is an important attempt to propose TAM cluster-related risk signatures in TNBC, providing evidence for identifying independent prognostic factors and effective therapeutic targets and updating immunotherapy methods.