TROP2 is a major determinant of growth and metastatic spreading of human cancer

Background: The TROP2 gene encodes a transmembrane calcium signal transducer, involved in the regulation of cell- cell adhesion. Methods: To identify the role of Trop-2 in transformed cells, we studied its expression pattern and function by DNA array and SAGE analysis, Northern and Western blotting, flow cytometry, confocal microscopy and IHC in experimental systems and in man. Overexpression or down-regulation of Trop-2 and directed mutagenesis were used to identify its role in tumor growth and metastasis in vivo. Results: DNA microarray, EST, SAGE, RT-PCR and Northern blot analysis of human tumors revealed expression of the TROP2 gene in most cancers. IHC analysis of human tumors (1755 cases) revealed a corresponding overexpression of Trop-2 protein. Trop-2 potently stimulated the growth of tumor cells, whereas TROP2 siRNA inhibited it. Deletion of the cytoplasmic region of Trop-2 abolished the growth stimulatory capacity, as did mutagenesis of the S303 PKC phosphorylation site. Proteomic analysis showed that multiple PKC isoforms partecipate to the Trop-2 signaling network. In vivo imaging showed dynamic colocalization of PKCs and Trop-2 in vivo in membrane ruffles and podosomes. ON PKCs and siRNA abolished Trop-2-induced growth. Strikingly, comparative global gene expression analysis revealed that TROP2 was the only gene up-regulated across different metastatic models, tumor types and animal species. IHC analysis revealed a dramatic upregulation in metastases from colon, stomach, breast and ovary tumors in man. To assess if Trop-2 may play a causal role in metastatic spreading, TROP2-transfected KM12SM colon cancer cells were orthotopically injected in nude mice. TROP2-overexpressing transfectants demonstrated increased metastatic potential to the liver. Deletion of the HIKE domain of Trop-2 severely diminished, whereas that of the whole cytoplasmic region vastly increased metastatic diffusion, indicating the existence of both metastatic enhancers and silencers in the Trop-2 cytoplasmic tail. Conclusions: Our findings demonstrate that Trop- 2 is a novel, widespread, stimulator of human cancer growth and a unique marker and causal factor of metastatic cancer, and candidate Trop-2 as a target of novel diagnostic and therapeutic procedures.