Deregulation of signal transduction pathways in macrophages by arachidonic acid in the ovarian cancer microenvironment
Malignant ascites contributes to all hallmarks of ovarian cancer (OC). It occurs as an accumulation of fluid in the peritoneal cavity at advanced stages of the disease. Ascites is associated with a poor prognosis along with a deteriorated quality of life due to abdominal distension. It is now well e...
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|Malignant ascites contributes to all hallmarks of ovarian cancer (OC). It occurs as an accumulation of fluid in the peritoneal cavity at advanced stages of the disease. Ascites is associated with a poor prognosis along with a deteriorated quality of life due to abdominal distension. It is now well established that malignant ascites is implicated in promoting different features of OC through its cellular and soluble components. The cellular component of ascites comprises mainly tumor and immune cells. In the tumor microenvironment (TME), immune cells are reprogrammed to support the
progression of the tumor rather than exert tumoricidal activity. Tumor-associated macrophages (TAMs) are the main immune cells in ascites and are involved in tumor growth, migration, metastasis, and angiogenesis. Furthermore, TAMs are also immunosuppressive, impeding the elimination of tumor cells through multiple mechanisms. Therefore, they are frequently linked to poor prognosis in different types of cancer.
Cancer and host cells interact with each other primarily through cytokines, growth factors, extracellular vesicles, and bioactive lipids. Ascites is rich with bioactive lipids, which are utilized as an important source of energy and regulate both tumor and tumor-related immune cells. They include lysophosphatidic acids (LPAs) and polyunsaturated fatty acids (PUFAs) as well as their metabolites. Arachidonic acid (AA) is a particularly important PUFA because it is abundant in ascites and associated with a short relapse free survival (RFS) of OC.
Immune suppression in the TME is a major challenge in optimizing the clinical benefits of immunotherapies. Hence, a deep understanding of the dampened anti-cancer immune response is required. The purpose of the present thesis was to elucidate the mechanisms of the AA-induced dysfunction of macrophages in OC. Our findings demonstrate that AA interferes with fundamental functions of macrophages and impairs the macrophages-mediated anti-tumor immune responses by perturbing lipid raft structures.
To delineate the signaling pathways stimulated by AA, we performed a comprehensive phosphoproteomic analysis of macrophages and identified multiple phosphoproteins modulated by AA. We defined the Ca2+ - CAMK II - ASK1 - p38δ/α (MAPK13/14) - HSP27 axis as a central signaling pathway in macrophages induced by AA. Furthermore, our data show that many of the AA-regulated phosphoproteins are involved in cytoskeletal organization and GTPase signaling. Consistent with this result, AA perturbed actin filament formation and impaired actin-dependent macropinocytosis.
Finally, transcriptomic profiling showed that AA suppresses the expression of genes induced by interferons (IFNs) or IL-6. Importantly, many of these genes were also repressed in TAMs from patients with a short RFS. To elucidate the underlying molecular mechanisms, we investigated the signaling pathways induced by IL-6, IFNβ, and IFNγ in more detail. We demonstrated that AA accumulates in lipid rafts and interferes with the JAK-STAT signaling by perturbing lipid rafts structure. Intriguingly, these effects could be at least partially reversed by restoring the structure of the lipid
rafts by the addition of exogenous water-soluble cholesterol. These findings point to a central role of AA in impairing macrophage-dependent anti-tumor surveillance in the OC microenvironment.