SMAD4 Inhibits CXCL1/CXCR2 to Trigger Antitumor Immunity in Colorectal Cancer

Abstract Background SMAD4 plays a crucial role in the TGF-β signaling pathway and acts as a tumor suppressor in colorectal cancer (CRC), where it is frequently downregulated and associated with an unfavorable prognosis. However, its role in tumor immunity is unknown. Methods SMAD4 overexpression and knockdown based on the cell experiments and mice model were performed. Utilizing a SMAD4-deficient CRC model, we investigated the role of SMAD4 on myeloid-derived suppressor cells (MDSCs) within the immune microenvironment of MSI-H CRC. Results It was observed that knockdown of SMAD4 promoted cell invasion. Conversely, overexpression of SMAD4 inhibited cell invasion and migration. In a murine xenograft tumor model, loss of SMAD4 promoted tumor growth. Analysis of mouse serum samples demonstrated that the loss of SMAD4 caused a significant increase in CXCL1 and CXCR2 levels, while SMAD4 overexpression caused a decrease in their levels. Further investigations indicated that SMAD4 deletion notably decreased the levels of inflammatory cytokines IL-2 and IFN-γ, and the levels of CD4+ T cells and CD8+ T cells. Flow cytometry analysis of the relationship between MDSCs and SMAD4 revealed that loss of SMAD4 facilitated MDSCs recruitment via activation of the CXCL1-CXCR2 axis. Conclusions Our results provide evidence that SMAD4 deficiency promotes CXCL1/CXCR2-induced MDSC accumulation and reduces the infiltration of IFN-γ, CD4 + T, CD8 + T cells, thus facilitating immune escape and CRC progression. Targeting MDSCs through the regulation of SMAD4 could be an innovative strategy for treating CRC patients.