MUC1 protein induces urokinase-type plasminogen activator (uPA) by forming a complex with NF-κB p65 transcription factor and binding to the uPA promoter, leading to enhanced invasiveness of cancer cells.

Mucin 1 (MUC1) is overexpressed in various human malignant tumors and its expression is correlated with a poor prognosis. MUC1 engages in signal transduction by interacting with receptors for growth and differentiation factors, which contributes to the growth and survival of cancer cells. However, the mechanism by which MUC1 promotes cancer cell invasion remains unclear.

Negative regulation of Toll-like receptor-4 signaling through the binding of glycosylphosphatidylinositol-anchored glycoprotein, CD14, with the sialic acid-binding lectin, CD33.

When monocyte-derived immature dendritic cells (imDCs) were stimulated with LPS in the presence of anti-CD33/Siglec-3 mAb, the production of IL-12 and phosphorylation of NF-κB decreased significantly. The cell surface proteins of imDCs were chemically cross-linked, and CD33-linked proteins were analyzed by SDS-PAGE and immunoblotting. It was CD14 that was found to be cross-linked with CD33.

CA125/MUC16 interacts with Src family kinases, and over-expression of its C-terminal fragment in human epithelial cancer cells reduces cell-cell adhesion.

MUC16/CA125 is over-expressed in human epithelial tumors including ovarian, breast and some other carcinomas. The purpose of this study is to investigate how cell surface MUC16 is functionally involved in tumor progression, with a special focus on the role of its cytoplasmic tail. Forced expression of C-terminal MUC16 fragment (MUC16C) in epithelial cancer cells increased cell migration.

Binding of the sialic acid-binding lectin, Siglec-9, to the membrane mucin, MUC1, induces recruitment of β-catenin and subsequent cell growth.

Because MUC1 carries a variety of sialoglycans that are possibly recognized by the siglec family, we examined MUC1-binding siglecs and found that Siglec-9 prominently bound to MUC1. An immunochemical study showed that Siglec-9-positive immune cells were associated with MUC1-positive cells in human colon, pancreas, and breast tumor tissues. We investigated whether or not this interaction has any functional implications for MUC1-expressing cells.

Prohibitins function as endogenous ligands for Siglec-9 and negatively regulate TCR signaling upon ligation.

Previously we demonstrated that prohibitin-1 and -2 (prohibitins) were expressed on the surface of T cell leukemia cell lines and activated T lymphocytes. In the present study, we found that prohibitins play a role as counter receptors for Siglec-9 expressed on macrophages and dendritic cells. Siglec-9 bound to prohibitins in a sialic acid-independent manner.

Expression of prohibitins on the surface of activated T cells.

Prohibitins (prohibitin-1 and -2) comprise a family of highly conserved proteins that are mainly localized to mitochondria. Recent studies showed that prohibitins are up-regulated upon T cell activation and play an essential role in maintaining mitochondrial homeostasis. In the present study, we found that a considerable proportion of prohibitin-1 and -2 induced in response to T cell activation was expressed on the surface of activated T cells.