Sialylation of the Fas Death Receptor by ST6Gal-I Provides Protection against Fas-mediated Apoptosis in Colon Carcinoma Cells

Abstract

The glycosyltransferase, ST6Gal-I, adds sialic acid in an ␣2-6 linkage to the N-glycans of membrane and secreted glycoproteins. Up-regulation of ST6Gal-I occurs in many cancers, including colon carcinoma, and correlates with metastasis and poor prognosis. However, mechanisms by which ST6Gal-I facilitates tumor progression remain poorly understood due to limited knowledge of enzyme substrates. Herein we identify the death receptor, Fas (CD95), as an ST6Gal-I substrate, and show that ␣2-6 sialylation of Fas confers protection against Fas-mediated apoptosis. Intriguingly, differences in ST6Gal-I activity do not affect the function of DR4 or DR5 death receptors upon treatment with TRAIL, implicating a selective effect of ST6Gal-I on the Fas receptor. Using ST6Gal-I knockdown and forced overexpression colon carcinoma cell models, we find that ␣2-6 sialylation of Fas prevents apoptosis stimulated by FasL as well as the Fas-activating antibody, CH11, as evidenced by decreased activation of caspases 8 and 3. We also show that ␣2-6 sialylation of Fas does not alter the binding of CH11, but rather inhibits the capacity of Fas to induce apoptosis by blocking the association of FADD with Fas cytoplasmic tails, an event that initiates death-inducing signaling complex formation. Furthermore, ␣2-6 sialylation of Fas inhibits Fas internalization, which is required for apoptotic signaling. Although dysregulated Fas activity is a well known mechanism through which tumors evade apoptosis, the current study is the first to link Fas insensitivity to the actions of a specific sialyltransferase. This finding establishes a new paradigm by which death receptor function is impaired for the self-protection of tumors against apoptosis.

Remodeling of Marrow Hematopoietic Stem and Progenitor Cells by Non-self ST6Gal-1 Sialyltransferase

“…The role of terminal glycosylation in cell-cell interactions and in signal transduction is well documented, particularly in immune cells, e.g. during selectin-mediated homing of leukocytes to lymph nodes and inflamed sites (23,24), in galectin-mediated apoptotic selection during lymphocyte maturation (25,26), and in altered integrin and death receptor signaling by ␣2,6-sialylation (27)(28)(29)(30). Lactosaminyl glycan-based epitopes and their selectin receptors are also important for the homing of stem cells to bone marrow niches (31,32), and recent indications suggest that differential glycan recognition may serve as a checkpoint between hematopoietic stem cell (HSC) quiescence and proliferation (33).…”

Targeting Aberrant Sialylation in Cancer Cells Using a Fluorinated Sialic Acid Analog Impairs Adhesion, Migration, and In Vivo Tumor Growth

“…The high expression of sialic acids can protect cancer cells from apoptosis, promote metastasis, and has been suggested to confer resistance to therapy (4)(5)(6)(7)(8). Despite increasing amounts of evidence showing the involvement of sialyltransferases and aberrant sialylation in cancer progression, therapeutic strategies to reduce aberrant sialylation lag behind.…”

“…These sialylated structures are a hallmark of many cancer cells and have been shown to contribute to numerous processes in cancer growth (15)(16)(17)(18)(19). For example, overexpression of ST6Gal-I and increased a2,6-sialylation of the Fas receptor mediates resistance to Fas-induced apoptosis (4). Hypersialylation (a2,6) of a1b1 integrin enhances binding to collagen type I and favors cancer cell migration (20).…”

Identification of proteins with the CDw75 epitope in human colorectal cancer

“…In CRC, ST6Gal I activity affects the adherence (13) and invasion (14) ability of tumor cells. ST6Gal I activity is also associated with the acquisition of the neoplastic phenotype (15), as well as the resistance to irinotecan (16) and Fas-mediated apoptosis (17). Furthermore, in CRC patients, ST6Gal I activity is greater in tumor than in healthy tissue (18,19), particularly in metastatic tumors (20).…”