Metastasis, or cancer spreading, is complex and still largely mysterious. As a tumor grows, it interacts constantly with the surrounding environment, including blood vessels, signaling molecules, and immune cells. While in some cases the immune system recognizes cancer cells as foreign and works to eliminate them, in other contexts immune cells can actually promote tumors. Cancer cells travel from their original home in the primary tumor to new homes throughout the body. They release microparticles to instruct resident cells, including macrophages, to create a niche that supports metastatic growth. In a previous article, Dr. Mark Headley, an assistant professor in the Translational Science and Therapeutics Division at Fred Hutch, demonstrated that macrophages were the predominant immune cell population that engulfed the microparticles in the early metastatic lung. In a recent study published in Cell Reports, Dr. Headley’s team demonstrated that tumor-derived microparticles engulfed by macrophages induced mTORC1 activation, driving a metabolic switch characterized by enhanced oxidative phosphorylation that supports anti-metastatic function in the early metastatic lung. “This work sheds new light on how the immune system interfaces with cancer cells during the process of metastasis to the lung, a major cause of death in cancer patients,” said Dr. Headley. “Our study builds off of a prior discovery made in my group that revealed widespread interaction between the lung immune system and small packets of cellular information (in the form of microparticles) produced by tumor cells.” He added, “We found that immune cells called macrophages rapidly ‘eat’ these particles through a process termed phagocytosis and become dramatically altered by this event.”
To study this metabolic switch, the authors injected B16F10 cells tagged with a fluorescent marker (ZsGreen) intravenously into mice. Most of the tumor-derived particles were phagocytosed by macrophages (ZsGreen positive). After establishing this system, the authors examined changes in proteomics and transcriptomics in the macrophages following microparticle engulfment. The proteomics analysis revealed an increase in cell surface markers’ expression in ZsGreen+ macrophages compared with ZsGreen- macrophages. RNA sequencing and Gene Set Enrichment Analysis (GSEA) showed that mitochondrial respiration and cell surface signature pathways are enriched in ZsGreen+ macrophages, suggesting mitochondrial reprogramming following the macrophages' engulfment of tumor-derived microparticles. In vitro and in vivo experiments demonstrated that ZsGreen+ macrophages displayed significantly increased mitochondrial mass, potential, oxygen consumption, and ATP production. The authors also conducted hallmark pathway analyses using transcriptional profiles of ZsGreen+ and ZsGreen- macrophages. In the ZsGreen+ macrophages, oxidative phosphorylation pathways were enriched. These results suggested that tumor-derived microparticles induce macrophage metabolic reprogramming.
To pinpoint the cellular pathways driving ZsGreen+ macrophage reprogramming, the authors focused on the mammalian target of rapamycin (mTOR), a central regulator of cellular metabolism. Mitochondrial biogenesis, oxidative phosphorylation, and mTORC1 signaling were among the enriched pathways in ZsGreen+ versus ZsGreen- macrophages. “Ingestion of this tumor material engages a metabolic switch in these macrophages by triggering a key metabolism regulating protein, mTOR,” Dr. Headley said. To test whether ZsGreen+ macrophage reprograming is dependent on mTOR, mTOR inhibitors were administered to both B16F10 ZsGreen and control mice. ZsGreen+ macrophages were unable to express adhesion molecules after mTOR inhibition, suggesting that mTOR signaling is crucial for reprograming ZsGreen+ macrophages. To validate their findings, the authors took a genetic approach. The authors generated mice with deletions of mTOR1 or mTOR2, the catalytic subunits of two different mTOR complexes, in macrophages. Although the deletion of mTOR1 or mTOR2 did not affect the ability of the macrophages to ingest the tumor-derived microparticles, the deletion of mTOR1 decreased the mitochondrial reprograming in ZsGreen+ macrophages in comparison with controls or deletion of mTOR2. The authors then examined mitochondrial glucose metabolism and ATP production. ZsGreen+ macrophages lacking mTOR1 reduced mitochondrial glucose metabolism and ATP production compared with wild-type (WT) mTOR1 ZsGreen+ macrophages.
Lastly, the authors tested the relevance of mTOR in early metastasis in clinical outcomes. They used WT and KO mTOR1 and mTOR2 mice that received B16F10 ZsGreen and allowed lung metastasis to develop. They found that the loss of mTOR1 decreased the growth of metastasis lesions compared with the WT ZsGreen+ mice after 3 weeks. “Intriguingly, this 'switch' appears to render these immune cells a better able to block metastasis - however - the effect is short-lived and eventually the cancer wins,” Dr. Headley said. “We are currently working to understand how this protective program is subverted and how we might design new therapeutics to support the protective function of these immune cells while countering roles they play in promoting metastasis,” Dr. Headley concluded.
This research was supported by funding from the Netherlands Organization for Scientific Research, a Parker Scholar Award from the Parker Institute for Cancer Immunotherapy, the National Institute of Health, and Metavivor grants.
Fred Hutch/University of Washington/Seattle Children's Cancer Consortium member Mark B. Headley contributed to this work.
Kersten K, You R, Liang S, Tharp KM, Pollack J, Weaver VM, Krummel MF, Headley MB. 2023. Uptake of tumor-derived microparticles induces metabolic reprogramming of macrophages in the early metastatic lung. Cell Rep. 42(6):112582.