The contribution of inflammatory cells to the progression of prostate cancer, 2016

In recent years, the causal relationship between inflammation and cancer has gained wider acknowledgement and acceptance. While various types of immune cells are involved in the process of inflammation, macrophages represent the major inflammatory component of many tumors. Derived from circulating monocytes, these cells migrate to tumor sites in response to molecular cues present within the tumor microenvironment. Once there, interactions with neoplastic cells shape the differentiation and functional orientation of macrophages into two phenotypically distinct subsets: the classically activated M1 macrophages and the alternatively activated M2 macrophages. The preeminent paradigm in macrophage-related cancer research is that within the tumor stoma, macrophages acquire an M2 phenotype characterized by production of pro-angiogenic factors, ECM degrading enzymes and up-regulation of anti-inflammatory responses, thereby promoting tumor progression. M1 macrophages, on the other hand are thought to exert anti-tumorigenic effects due to their production of pro-inflammatory cytokines, and reactive oxygen species (ROS). While the generation of ROS during immune responses is an important aspect of immune regulation and host defense, excessive ROS production has been implicated in the pathogenesis of various degenerative diseases, including cancer. Yet, despite the well-established role of M1 macrophages in generating high levels of ROS via NADPH oxidase (NOX), M1 macrophages are still largely viewed as anti-tumorigenic. Hence, this study reevaluates the complex interaction between prostate cancer (PCa) cells and tumor-associated macrophages (TAMs), and operates on the premise that PCa cells promote a pro-tumor microenvironment, denoted by increased inflammation and oxidative stress, in part, through M1 macrophage-mediated, NOX-derived ROS production. Accordingly, immunofluorescent analysis of prostate tissue microarrays demonstrated an influx of M1 macrophages in prostate carcinoma. Immature monocytes co-cultured with the poorly tumorigenic prostate cell line, LNCaP, demonstrated changes in morphology and protein expression consistent with M1 macrophage polarization. PCa cells co-cultured with M1 macrophages displayed significantly higher intracellular ROS levels. Furthermore, M1-mediated ROS generation through NOXs increased prostate cell invasiveness and anchorage-independent growth. Taken together, results from this study suggest a potentially novel pro-tumorigenic function of M1 macrophages in early PCa progression, and aid in understanding the complex role of inflammation in cancer. KEY TERMS: prostate cancer, inflammation, macrophages, NOX, reactive oxygen species, Biology, Laboratory and Basic Science Researc