Introduction:
Macrophages are the primary cells of the mononuclear system. Studies have demonstrated that macrophages play an active role in the pathophysiology of cancers due to their remarkable adaptation capacities. The objective of this investigation was to examine the impact of apelin on macrophage polarization in the colon cancer microenvironment.

Material and methods:
In this study, the colon adenocarcinoma cell line SW480 and mouse macro- phage cells RAW264.7 were used. Using shRNA, expression of the apelin gene was suppressed in SW480 cells. RAW264.7 cells were co-cultured with SW480 cells with the apelin gene silenced and no shRNA introduced. qRT-PCR and protein expression analysis were applied to assess the effect of apelin on inflammation-related genes and proteins, respectively, in co-cultured RAW264.7 cells.

Results:
In comparison to the control, apelin knockdown led to significantly lower apelin expression in SW480 cells and a significant greater pro-inflammatory response in co-cultured macrophages. The ex- pression levels of tumor necrosis factor α (TNF-α), interleukin (IL)-6, and IL-1 genes were significantly elevated, while the amount of IL-10, which is known as an anti-inflammatory cytokine, was dramatically decreased. Pro-inflammatory genes, namely IL-1, IL-2 and TNF-α, were found to be downregulated, and anti-inflammatory genes, including IL-10 and transforming growth factor b (TGF-β), were found to be upregulated in the apelin-knockdown group compared to the control. Remarkably, the expression levels of IL-1, IL-6, and TNF-α proteins, which are involved in macrophage polarization, were in agreement with the qRT-PCR data.

Conclusions:
These results indicate that the apelin peptide may be associated with the dense presence of M2-type macrophages in the cancer microenvironment, suggesting it as a therapeutic target for cancer cells.