Colony‐stimulating factor 1 should be considered when studying the miR‐28‐5p‐IL‐34‐macrophage feedback loop in hepatocellular carcinoma
Authors: Yuankang Zou, Guoyin Li, Lei Wang
Journal: Hepatology (2016)

Potential conflict of interest: Nothing to report. Author names in bold designate shared co‐first authorship. To the Editor: The study from Dr. Zhou's group described an interesting phenomenon that microRNAs also shape the interactions between hepatocellular carcinoma (HCC) cells and their associated stroma.1 They reported that miR‐28‐5p deficiency led to the up‐regulation of interleukin‐34 (IL‐34) and consequent tumor growth and metastasis through recruitment and infiltration of tumor‐associated macrophages (TAMs). They also found that TAMs induced by miR‐28‐5p‐IL‐34 signaling inhibit miR‐28‐5p expression in HCC cells through transforming growth factor‐β1, thus yielding an miR‐28‐5p‐IL‐34‐macrophage–positive feedback loop.1 This is the first study to demonstrate that the miR‐28‐5p‐IL‐34‐macrophage feedback loop modulates HCC metastasis and serves as a novel prognostic factor and as a therapeutic target for HCC. Yet, we still have several questions about this study. First, is a xenograft model suitable for studying the tumor microenvironment? Zhou et al. used BALB/c nude mice to investigate the interplay between HCC cells and TAMs. Obviously, the HCC cells are derived from human, while TAMs are derived from mouse. Homology analysis indicates that the amino acid sequence of human IL‐34 shares only 68.2% homology with mouse IL‐34. And this also holds true for the receptor of IL‐34, colony‐stimulating factor 1 receptor (CSF1R). So, would the IL‐34 secreted by human HCC cells be effectively recognized by the mouse CSF1R on the cell surface of macrophages?2 Besides, it is well established that T cells and macrophages can regulate each other and play important roles in tumorigenesis.3 However, nude mice are immunodeficient and lack T cells. Therefore, the crosstalk between TAMs and T cells could be missing in Zhou et al.'s study. In our opinion, a spontaneous tumor model would be better for dissecting the mechanisms underlying tumor‐stroma coevolution. Second, does colony‐stimulating factor 1 (CSF1) have any effect on the miR‐28‐5p‐IL‐34‐macrophage feedback loop? As we know, CSF1R has two ligands across vertebrates, one is IL‐34 and the other is CSF1.2 CSF1 is widely expressed in both normal and malignant tissues and mainly functions as a regulator of macrophage activation.4 Zhou et al. focused on the IL‐34/CSF1R axis but overlooked that CSF1 secrected by human HCC cells might also have a function. We suggest that if enforced expression of miR‐28‐5p in metastatic HCC cells did not disturb the CSF1 level, the observation that miR‐28‐5p overexpression in HCC cells inhibits macrophage chemotactic migration and proliferation should be reassessed carefully. Third, are the TAMs recruited by IL‐34 located in perivascular areas to promote liver cancer metastasis? Accumulating evidence demonstrates that macrophages in different tumor microenvironments have distinct roles. The distinct microenvironments where TAMs act include areas of invasion where TAMs promote cancer cell motility, stromal and perivascular areas where TAMs promote metastasis, and avascular and perinecrotic areas where TAMs stimulate angiogenesis.5 Therefore, we suggest that additional CD31 staining of vascular endothelial cells together with TAM labeling would better clarify the specific role played by TAMs in the HCC microenvironment.