AXL-initiated paracrine activation of pSTAT3 enhances mesenchymal and vasculogenic supportive features of tumor-associated macrophages

Prospects of current AXL-targeting therapies in early phase cancer trials

INTRODUCTION

AXL, a member of the TAM (TYRO3, AXL, and MERTK) family of receptor tyrosine kinases, controls pro-tumorigenic signaling cascades and cancer-immunological functions, and promotes drug resistance. Due to AXL's multifaceted role and therapeutic activity in preclinical studies, a variety of AXL inhibitors are being developed and tested in clinical trials for cancer treatment. Some clinical studies are showing promising results for AXL inhibitors as monotherapy and in combination with standard of care therapeutics. Currently, no selective AXL-targeting therapy has reached FDA-approval, but several compounds have entered phase II and III studies.

AREA COVERED

We elaborate on the role of AXL in cancer progression and suppressing anti-cancer immunity at both the molecular level and immune cell interaction level. Additionally, we review pre-clinical and clinical data of AXL-targeting agents.

EXPERT OPINION

Preclinical and several early clinical trials demonstrated the safety of AXL-targeting monotherapies with some evidence of efficacy. Additionally, multiple novel combination regimens including AXL-targeting agents to overcome resistance mechanisms are being actively examined with some promising results. However, patient selection and companion biomarkers may be critical for the success of AXL-targeting therapies.

AXL promotes inflammatory breast cancer progression by regulating immunosuppressive macrophage polarization

BACKGROUND

Tumor-associated macrophages (TAMs) are key promoters of inflammatory breast cancer (IBC), the most aggressive form of breast cancer. The receptor tyrosine kinase AXL is highly expressed in various cancer types, including IBC, but its role in TAMs remains unexplored.

METHODS

We examined the effects of AXL inhibitor TP-0903 on tumor growth and tumor microenvironment (TME) component M2 macrophages (CD206+) in IBC and triple-negative breast cancer mouse models using flow cytometry and immunohistochemical staining. Additionally, we knocked out AXL expression in human THP-1 monocytes and evaluated the effect of AXL signaling on immunosuppressive M2 macrophage polarization and IBC cell growth and migration. We then investigated the underlying mechanisms through RNA sequencing analysis. Last, we performed CIBERSORT deconvolution to analyze the association between AXL expression and tumor-infiltrating immune cell types in tumor samples from the Inflammatory Breast Cancer International Consortium.

RESULTS

We found that inhibiting the AXL pathway significantly reduced IBC tumor growth and decreased CD206+ macrophage populations within tumors. Mechanistically, our in vitro data showed that AXL promoted M2 macrophage polarization and enhanced the secretion of immunosuppressive chemokines, including CCL20, CCL26, and epiregulin, via the transcription factor STAT6 and thereby accelerated IBC cell growth and migration. RNA sequencing analysis further indicated that AXL signaling in immunosuppressive M2 macrophages regulated the expression of molecules and cytokines, contributing to an immunosuppressive TME in IBC. Moreover, high AXL expression was correlated with larger populations of immunosuppressive immune cells but smaller populations of immunoactive immune cells in tissues from patients with IBC.

CONCLUSIONS

AXL signaling promotes IBC growth by inducing M2 macrophage polarization and driving the secretion of immunosuppressive molecules and cytokines via STAT6 signaling, thereby contributing to an immunosuppressive TME. Collectively, these findings highlight the potential of targeting AXL signaling as a novel therapeutic approach for IBC that warrants further investigation in clinical trials.

Proteolysis of TAM receptors in autoimmune diseases and cancer: what does it say to us?

Proteolytic processing of Receptor Tyrosine Kinases (RTKs) leads to the release of ectodomains in the extracellular space. These soluble ectodomains often retain the ligand binding activity and dampen canonical pathways by acting as decoy receptors. On the other hand, shedding the ectodomains may initiate new molecular events and diversification of signalling. Members of the TAM (TYRO3, AXL, MER) family of RTKs undergo proteolytic cleavage, and their soluble forms are present in the extracellular space and biological fluids. TAM receptors are expressed in professional phagocytes, mediating apoptotic cell clearance, and suppressing innate immunity. Enhanced shedding of TAM ectodomains is documented in autoimmune and some inflammatory conditions. Also, soluble TAM receptors are present at high levels in the biological fluids of cancer patients and are associated with poor survival. We outline the biology of TAM receptors and discuss how their proteolytic processing impacts autoimmunity and tumorigenesis. In autoimmune diseases, proteolysis of TAM receptors likely reflects reduced canonical signalling in professional phagocytes. In cancer, TAM receptors are expressed in the immune cells of the tumour microenvironment, where they control pathways facilitating immune evasion. In tumour cells, ectodomain shedding activates non-canonical TAM pathways, leading to epithelial-mesenchymal transition, metastasis, and drug resistance.

AXL: shapers of tumor progression and immunosuppressive microenvironments

As research progresses, our understanding of the tumor microenvironment (TME) has undergone profound changes. The TME evolves with the developmental stages of cancer and the implementation of therapeutic interventions, transitioning from an immune-promoting to an immunosuppressive microenvironment. Consequently, we focus intently on the significant role of the TME in tumor proliferation, metastasis, and the development of drug resistance. AXL is highly associated with tumor progression; however, previous studies on AXL have been limited to its impact on the biological behavior of cancer cells. An increasing body of research now demonstrates that AXL can influence the function and differentiation of immune cells, mediating immune suppression and thereby fostering tumor growth. A comprehensive analysis to identify and overcome the causes of immunosuppressive microenvironments represents a novel approach to conquering cancer. In this review, we focus on elucidating the role of AXL within the immunosuppressive microenvironments, discussing and analyzing the effects of AXL on tumor cells, T cells, macrophages, natural killer (NK) cells, fibroblasts, and other immune-stromal cells. We aim to clarify the contributions of AXL to the progression and drug resistance of cancer from its functional role in the immune microenvironment.

Research advances on signaling pathways regulating the polarization of tumor-associated macrophages in lung cancer microenvironment

Lung cancer (LC) is one of the most common cancer worldwide. Tumor-associated macrophages (TAMs) are important component of the tumor microenvironment (TME) and are closely related to the stages of tumor occurrence, development, and metastasis. Macrophages are plastic and can differentiate into different phenotypes and functions under the influence of different signaling pathways in TME. The classically activated (M1-like) and alternatively activated (M2-like) represent the two polarization states of macrophages. M1 macrophages exhibit anti-tumor functions, while M2 macrophages are considered to support tumor cell survival and metastasis. Macrophage polarization involves complex signaling pathways, and blocking or regulating these signaling pathways to enhance macrophages' anti-tumor effects has become a research hotspot in recent years. At the same time, there have been new discoveries regarding the modulation of TAMs towards an anti-tumor phenotype by synthetic and natural drug components. Nanotechnology can better achieve combination therapy and targeted delivery of drugs, maximizing the efficacy of the drugs while minimizing side effects. Up to now, nanomedicines targeting the delivery of various active substances for reprogramming TAMs have made significant progress. In this review, we primarily provided a comprehensive overview of the signaling crosstalk between TAMs and various cells in the LC microenvironment. Additionally, the latest advancements in novel drugs and nano-based drug delivery systems (NDDSs) that target macrophages were also reviewed. Finally, we discussed the prospects of macrophages as therapeutic targets and the barriers to clinical translation.

Targeting TAM-secreted S100A9 effectively enhances the tumor-suppressive effect of metformin in treating lung adenocarcinoma

Metformin's effect on tumor treatment was complex, because it significantly reduced cancer cell proliferation in vitro, but made no difference in prognosis in several clinical cohorts. Our transcriptome sequencing results revealed that tumor-associated macrophage (TAM) infiltration significantly increased in active lung adenocarcinoma (LUAD) patients with long-term metformin use. We further identified that the tumor suppressive effect of metformin was more significant in mice after the depletion of macrophages, suggesting that TAMs might play an important role in metformin's effects in LUAD. Combining 10X Genomics single-cell sequencing of tumor samples, transcriptome sequencing of metformin-treated TAMs, and the ChIP-Seq data of the Encode database, we identified and validated that metformin significantly increased the expression and secretion of S100A9 of TAMs through AMPK-CEBP/β pathway. For the downstream, S100A9 binds to RAGE receptors on the surface of LUAD cells, and then activates the NF-κB pathway to promote EMT and progression of LUAD, counteracting the inhibitory effect of metformin on LUAD cells. In cell-derived xenograft models (CDX) and patient-derived xenograft models (PDX) models, our results showed that neutralizing antibodies targeting TAM-secreted S100A9 effectively enhanced the tumor suppressive effect of metformin in treating LUAD. Our results will enable us to better comprehend the complex role of metformin in LUAD, and advance its clinical application in cancer treatment.