Trop-2 is a general cancer growth stimulator through ubiquitous tetraspanin platforms

Background: Trop-2 is a transmembrane calcium signal transducer, first identified in trophoblast and subsequently found in diverse transformed cells. The expression of Trop-2 has been associated with biological aggressiveness and poor prognosis of pancreatic, gastric, oral, ovarian and colorectal cancers, suggesting a potential role in tumour progression. However, a comprehensive scenario of Trop-2 expression is still missing and the function of Trop 2 is as yet unknown. Material and methods: Human tumors and normal tissues were analysed for the expression of the TROP2 gene by DNA microarray, EST, SAGE, RT-PCR, Northern blot analysis. Expression of the Trop-2 protein and of downstream signal transducers was investigated by immunohistochemistry, flow cytometry, high-throughput Western blotting, proteomic array, 2D gels / mass spectrometry and dynamic multi-color imaging. Induction of expression and somatic knock-down of relevant targets was utilized for function analysis, together with protein-protein, protein-gene network interaction analysis. Results: Trop-2 was demonstrated to be overexpressed by most human cancers, but not on non-epithelial malignancies, suggesting strong selective pressure for a conserved function. Trop-2 was then demonstrated to be necessary and sufficient to stimulate cancer growth, with a linear relationship between growth rates and Trop-2 expression levels. Cell growth stimulation was shown to be conserved across cell-types and species. These findings indicated impingement on a ubiquitous downstream signal-transduction module. Trop-2 was demonstrated to bind multiple tetraspanins, triggering their growth-promoting ability via a feed-forward activation loop of CD9-recruited PKCα and phosphorylation of the Trop-2 cytoplasmic tail. We demonstrated that both CD9 and PKCα stimulate growth in a Trop-2-restricted manner and that these signaling structures are coordinately transported in recurrent waves to membrane ruffles and podosomes. Trop-2 induction was shown to activate the ERK pathway, to up-regulate NF-κB, and to modulate apoptotic factors, including p53 and Rb. Key members of the Trop-2 signaling pathway were shown to be coordinately upregulated in large human cancer case series, indicating functional relevance of this growth stimulatory mechanism in tumors in man. Conclusions: These findings reveal the existence of a unique, strikingly widespread mechanism of stimulation of cancer growth. This is quantitatively driven by overexpressed, but otherwise wild-type, Trop-2 and acts upon ready-to-signal, but otherwise silent, ubiquitous signal-transduction platforms.