Trop-2 is a novel PKC-dependent growth-stimulator trans-membrane molecule and a metastasis marker in human cancer

Trop-2 is a 323 amino acid-long monomeric trans-membrane molecule involved in cell-cell adhesion, that transduces a calcium signal upon cross-linking (Ripani et al. Int. J. Cancer 76: 671,1998; Fornaro et al. Int. J. Cancer 62: 610, 1995). Global gene expression analysis of human cancers by SAGE and DNA arrays demonstrates that the TROP2 mRNA is overexpressed by cancers of the lung, stomach, colon, pancreas, kidney, liver and ovary. A large scale analysis of the expression of the Trop-2 protein was conducted in over 1500 cases human breast, colon, lung and ovary tumors and demonstrated that Trop-2 is overexpressed by a large fraction (42-85%) of the tumors analysed. These findings indicated a strong selective pressure for Trop-2 expression in cancer cells. Thus, we investigated a role of Trop-2 in transformed cell growth. Trop-2 expression significantly increased the growth rate of both fibroblastic / sarcomatous and epithelioid / cancerous cells at different stages of tumor progression both in man and in mouse (L, MTE, Igrov-1, KM12SM, 293) in vitro and in vivo. A deletion of the cytoplasmic region of Trop-2 abolished the Trop-2-dependent growth stimulation effect. Trop-2 is phosphorylated by PKC in Ser303 (Basu et al. Int. J. Cancer 62: 472, 1995). Mutagenesis of the Trop-2 cytoplasmic PKC phosphorylation site abolished the growth stimulatory role of Trop-2, showing dependence of PKC signaling. Yeast two hybrid, proteomic and phosphoproteomic analysis identified CREB and a novel homeodomain molecule as Trop-2 targets, starting unraveling the Trop-2 signaling pathway. Molecular modeling and N-terminal sequencing of the thyroglobulin domain of Trop-2 demonstrated a single proteolytic cleavage site. between Arg87 and Thr88. Mutagenesis of the cleavage site abolished growth stimulation, indicating this as an activatory switch for Trop-2. Thus, Trop-2 is a novel, widespread, growth-stimulatory cell-surface molecule in human cancer. Strikingly, comparative gene expression profiling of metastatic cells indicated that TROP2 was the only gene up-regulated across different models and species. Consistently, most human colon, breast and ovary tumor metastases were verified to overexpress the Trop-2 protein, identifying Trop-2 as a unique marker of metastasis. Taken together, our findings identify Trop-2 as a novel key player in human tumor growth and as a unique marker of metastatic cancer. These results also candidate Trop-2 for novel diagnostic and therapeutic procedures.