Irreversible repolarization of tumour‐associated macrophages by low‐Pi stress inhibits the progression of hepatocellular carcinoma

Abstract

Numerous studies have shown the positive correlation between high levels of Pi and tumour progression. A critical goal of macrophage‐based cancer therapeutics is to reduce anti‐inflammatory macrophages (M2) and increase proinflammatory antitumour macrophages (M1). This study aimed to investigate the relationship between macrophage polarization and low‐Pi stress. First, the spatial populations of M2 and M1 macrophages in 22 HCC patient specimens were quantified and correlated with the local Pi concentration. The levels of M2 and M1 macrophage markers expressed in the peritumour area were higher than the intratumour levels, and the expression of M2 markers was positively correlated with Pi concentration. Next, monocytes differentiated from THP‐1 cells were polarized against different Pi concentrations to investigate the activation or silencing of the expression of p65, IκB‐α and STAT3 as well as their phosphorylation. Results showed that low‐Pi stress irreversibly repolarizes tumour‐associated macrophages (TAMs) towards the M1 phenotype by silencing stat6 and activating p65. Moreover, HepG‐2 and SMCC‐7721 cells were cultured in conditioned medium to investigate the innate anticancer immune effects on tumour progression. Both cancer cell lines showed reduced proliferation, migration and invasion, as epithelial–mesenchymal transition (EMT) was inactivated. In vivo therapeutic effect on the innate and adaptive immune processes was validated in a subcutaneous liver cancer model by the intratumoural injection of sevelamer. Tumour growth was significantly inhibited by the partial deprivation of intratumoural Pi as the tumour microenvironment under low‐Pi stress is more immunostimulatory. The anticancer immune response, activated by low‐Pi stress, suggests a new macrophage‐based immunotherapeutic modality.

Macrophage colony-stimulating factor and its role in the tumor microenvironment: novel therapeutic avenues and mechanistic insights

“…In certain diseases such as cancer and autoimmune disorders, M-CSF modulates T cell activation and function through its regulation of macrophage functionality (14, 18,20,21). This regulation is partially mediated by M2-type macrophages induced by M-CSF, which can secrete antiinflammatory factors like interleukin (IL)-10, thereby suppressing effector T cell activation and modulating immune responses (13,22,23). Research indicates that M-CSF also impacts the functionality of other immune cells, such as dendritic cells (18,24).…”

“…TAMs primarily originate from CD14 + monocytes from the bone marrow and differentiate into macrophages upon entering the tumor microenvironment (11,13,14,16,23), where they subsequently undergo local proliferation (13). TAMs dominate the tumor microenvironment (17,18,21,34), particularly in the peritumoral regions, outnumbering intratumoral macrophages (23) and accounting for approximately 40% of total tumor cell population (35).…”

“…TAMs are typically categorized into two subtypes, known as the classically activated phenotype (M1 or M1-like phenotype) and alternatively activated phenotype (M2 or M2-like phenotype) (12,14,22,23,35,(37)(38)(39). Importantly, these M1 and M2 macrophages represent dynamically interconvertible subpopulations, and their phenotypes can reversibly switch (11,12,40).…”

“…In their tumor-suppressive role, TAMs engulf tumor cells and apoptotic cells while producing cytotoxic oxygen and nitrogen species, cytokines, and enzymes to restrain tumor growth (40,41). Conversely, more recent research findings have highlighted TAMs' pro-tumorigenic effects (12,34), as they support tumor growth and metastasis through various mechanisms (36,42), including promoting angiogenesis, suppressing anti-tumor immune responses (13,22,23), facilitating tumor cell invasion and metastasis (11,21), and sustaining the survival of tumor stem cells (21).…”

“…Secondly, M-CSF modulates TAM cytokine secretion, including IL-2, VEGF, EGF, TGF-b, and CCL22 (13,21,68), thereby impacting the behavior of tumor cells. These cytokines play diverse roles in tumor development, including stimulating angiogenesis, suppressing T cell-mediated immune responses, and promoting tumor cell survival and proliferation (13,23,36). Particularly under M-CSF regulation, TAMs promote angiogenesis and tumor cell proliferation by secreting VEGF and EGF, creating a favorable microenvironment for tumor progression (22).…”