Lipid Droplet Accumulation Mediates Macrophage Survival and Treg Recruitment in Human Hepatocellular Carcinoma via the CCL20/CCR6 Axis

Background

Macrophages are a major component of the tumor microenvironment (TME) and regulate the entire tumor process. In the TME, macrophages exhibit dynamic heterogeneity and plasticity, which affects their role in tumor progression and treatment resistance. Existing research shows that metabolic reprogramming of macrophages can redraw their transformation towards an anti-tumor phenotype and has become an emerging macrophage-targeted intervention pathway. However, the metabolic regulation and function of macrophages in tumors are not fully understood.

Lipids are important components of cell membranes and key factors in signal transduction, energy storage, and metabolism. Lipid metabolism imbalance may lead to excessive fat accumulation, resulting in abnormal macrophage function and promoting disease development. For example, macrophage foam cells in atherosclerotic plaques are a typical case. In the tumor context, studies have shown that lipid droplets (LDs), especially in LD-laden macrophages (LLMs) rich in triglycerides and cholesterol esters, are abundant and support tumor progression through multiple mechanisms.

Paper Source

This paper was written by Yongchun Wang, Weibai Chen, Shuang Qiao, Hao Zou, Xing-Juan Yu, Yanyan Yang, Zhixiong Li, Junfeng Wang, Min-Shan Chen, Jing Xu, and Limin Zheng. Wang and Chen are co-first authors, and Xu and Zheng are corresponding authors. The paper was published in 2024 in the journal “Cellular & Molecular Immunology”.

Research Process

The main objective of this study was to explore the phenotype, contribution, and regulatory mechanisms of LD-laden macrophages in hepatocellular carcinoma (HCC). The specific research process is as follows:

Relationship between macrophage accumulation and disease progression in HCC patients

First, researchers detected LD levels using the neutral lipid dye BODIPY493/503 in CD45+CD14+ cells freshly isolated from HCC tissues. The study found that the number of LD-laden macrophages in tumor tissues was significantly higher than in non-tumor tissues. In HCC patients, higher LLM levels were associated with more evident disease progression.

Lipid metabolism reprogramming of macrophages in TME

To explore the mechanism of LLM formation in macrophages, the research team established an in vitro model using conditioned medium (TSN) from tumor cells to treat human peripheral blood monocytes. Results showed that TSN treatment significantly increased LD formation in monocytes and upregulated PLIN2 expression.

Contribution of tumor lipid metabolism reprogramming to LLM formation

The research team further explored the sources of fatty acids, including cell membrane lipid reorganization and tumor cell lipid uptake. By inhibiting phospholipase A2 (PLA2) or using fluorescently labeled fatty acids to monitor lipid uptake, it was found that both tumor lipid reorganization and uptake significantly contributed to LLM formation.

Lipid droplets prolong macrophage survival and promote CCL20 secretion

Investigation revealed that accumulated lipid droplets prolonged macrophage survival and induced CCL20 secretion, which further attracted CCR6+ Treg cells into tumor tissues.

Programming LD metabolism to inhibit tumor progression

The research team found that inhibiting DGAT1 and DGAT2 (enzymes catalyzing triglyceride synthesis) significantly reduced Treg cell recruitment and delayed tumor growth in a liver cancer mouse model.

Main Results

The main findings of this study include:

1. Accumulation of LD-laden macrophages (LLMs) positively correlates with disease progression in HCC patients: Using fluorescence microscopy and flow cytometry, researchers found that the number of LLMs in HCC tissues was significantly higher than in non-tumor tissues, and their levels negatively correlated with tumor size, TNM stage, and patients’ recurrence-free survival.

2. LD formation mechanism and its impact on macrophage function: It was discovered that LD formation mainly occurs through tumor-induced lipid reorganization and TNFα-mediated tumor fatty acid uptake. The accumulation of LDs in macrophages not only prolonged their survival but also triggered CCL20 secretion, attracting CCR6+ Treg cells into tumor tissues.

3. Inhibiting DGAT reduces LD accumulation and delays tumor development: By inhibiting DGAT1 and DGAT2, researchers successfully reduced LLM formation in mouse tumor models, decreased Treg cell infiltration in tumor tissues, and significantly delayed tumor growth.

Conclusion and Significance

This study reveals the inhibitory phenotype of LLMs in HCC and their relationship with disease progression, suggesting that targeting lipid droplet accumulation in macrophages could be a new strategy for HCC treatment. This research not only provides new insights for HCC treatment but also indicates the potential application of lipid metabolism reprogramming in tumor immune intervention.

Research Highlights

1. Lipid metabolism reprogramming: For the first time, the study deeply explores the important role of lipid metabolism reprogramming in liver cancer macrophages and reveals its mechanism in regulating tumor progression.
2. Therapeutic potential: Targeting DGAT1 and DGAT2 to inhibit LLM formation provides a potential HCC treatment strategy.
3. Immunosuppressive phenomenon: The study extends the understanding of LLM’s role in immunosuppressive phenomena in the tumor microenvironment, providing reference for future immunotherapy strategies.

Other Information

This paper not only reveals the phenotype of LLMs and their role in HCC but also provides in-depth molecular mechanism discussions. Future research can further explore the broad impact of specific metabolic pathways on the tumor microenvironment and tumor progression.