Abstract
Background: Midkine (MDK), a secreted heparin-binding growth factor, is involved in tumor progression and metastasis. While serum MDK is widely recognized as a potential prognostic biomarker for colorectal cancer (CRC), its specific functional role and underlying mechanisms in CRC development are not fully understood. Methods: The four publicly available CRC microarray datasets—GSE41258, GSE44076, GSE81558, and GSE117606—along with TCGA-COAD and TCGA-READ datasets and their associated clinical data were obtained. MDK expression was measured at both the mRNA and protein levels using quantitative real-time PCR (qRT-PCR) and Western blotting. To investigate its oncogenic functions, a comprehensive set of assays was performed: transwell and wound healing assays for invasion and migration; CCK-8 and colony formation assays for proliferation; and tail vein/spleen injection models combined with xenograft models to study metastasis and tumor growth in vivo. To uncover underlying mechanisms, Western blotting was used to examine the involvement of epithelial–mesenchymal transition (EMT) and the PI3K/AKT signaling pathway. Results: MDK is significantly overexpressed in CRC tissues and cells compared to normal tissues and cells. Notably, patients with high MDK levels show poorer overall survival (OS). Overexpression of MDK increases CRC invasion, migration, proliferation, and metastasis both in vivo and in vitro, while its knockdown reverses these effects. Mechanistically, MDK activates the PI3K/AKT pathway, leading to increased AP2A1 expression and promotion of EMT in CRC. Conclusions: MDK promotes invasion, migration, proliferation, metastasis, and EMT in CRC cells through the PI3K/AKT pathway by inducing AP2A1 expression, which could serve as a diagnostic marker. The PI3K inhibitor LY294002 significantly reduces AP2A1 levels and inhibits MDK-induced malignant behaviors. Targeting MDK-related signaling pathways may offer new strategies for CRC treatment.