Effect of Algal Lectin Siye on Proliferation and Apoptosis of Breast and Colon Cancer Cells

Lectins are carbohydrate-binding proteins, some of which exhibit significant anti-tumor activity. Siye is a lectin derived from the red alga Kappaphycus alvarezii that was previously discovered using an artificial intelligence-guided genome mining strategy and shown to exert cytotoxic effects against several human cancer cell lines, including breast adenocarcinoma HCC1937. Based on the presence of shared glycopatterns between breast and colon cancers, we hypothesized that Siye may also exhibit anti-tumor activity against colon cancer cells. The cytotoxic effect of Siye on human colon cancer HCT116 cells was evaluated using the CCK-8 assay. Apoptosis was assessed by flow cytometry with Annexin V-FITC/PI staining. Expression levels of apoptosis-related genes (Bax, Bcl-2, Casp3, Casp8, Casp9, and TP53) were determined by qRT-PCR. Competitive inhibition assays using mannan were performed to assess the role of cell surface glycan binding. Siye significantly reduced the viability of HCT116 cells in a dose-dependent manner, with an IC₅₀ value of 14.065 μg/mL (=0.488 μM). Flow cytometry revealed that Siye promoted both early and late apoptosis in HCT116 cells, whereas in HCC1937 cells, the effect was primarily on early apoptosis. Mechanistically, Siye significantly upregulated the expression of the pro-apoptotic genes Bax (p < 0.05) and Casp9 (p < 0.001) in HCT116 cells, while in HCC1937 cells, Casp9 expression was significantly increased (p < 0.001). Morphological changes, including cell rounding and agglutination, were observed within 4 h of Siye treatment in both cell lines and were attenuated by co-treatment with mannan, suggesting that Siye-induced morphological changes are associated with binding to cell surface glycans. This study suggests that the red algal lectin Siye exerts anti-tumor effects against colon cancer HCT116 cells by inducing caspase-associated apoptosis. The differential apoptotic response between HCC1937 and HCT116 cells suggests cell-type-specific mechanisms. These findings extend the known anti-tumor activity spectrum of AI-discovered red algal lectin Siye and provide a basis for further investigation of its glycan-associated cellular effects and marine drug discovery potential.