Crumbs3: Expression and biological significance in normal and neoplastic tissues

Cellular polarity plays a fundamental role in tissue organization and homeostasis, and its disruption is closely linked to tumorigenesis. Crumbs3 (CRB3), a conserved polarity protein, is essential for epithelial morphogenesis, tight-junction formation, and barrier function. This review summarizes current knowledge regarding CRB3 expression in normal and malignant human tissues and its dual roles in cancer progression. Systematic immunohistochemical analyses revealed strong CRB3 expression in non-neoplastic glandular epithelia of the gastrointestinal, hepato-pancreato-biliary, renal, and respiratory tracts, as well as in fetal tissues, suggesting its importance in organ development and maintenance. In neoplastic tissues-represented by colorectal adenocarcinoma and oral squamous cell carcinoma-CRB3 expression is preserved or even enhanced compared with normal tissues, which promotes tumor cell migration, triggering invasion/metastasis as well as cellular proliferation through signaling pathways involving FGFR and RhoA activation. Conversely, previous studies reported that CRB3 functions as a tumor suppressor, based on findings that CRB3 expression induces loss of epithelial-mesenchymal transition, whereas loss of CRB3 expression attenuates the integrity of tight junctions, resulting in significantly poorer prognosis in certain cancers. Current data thus suggest that the biological role of CRB3 in tumors is complex. Whether CRB3 acts as a tumor accelerator or suppressor may depend on the individual-specific, unique characteristics of tumor cells. Understanding these dual functions may contribute to the development of novel polarity-targeted therapeutic strategies for cancers of differing origin.