IntroductionInterleukin-like epithelial-to-mesenchymal transition inducer (ILEI) is an essential cytokine in tumor progression that is upregulated in several cancers, and its altered subcellular localization is a predictor of poor survival in human breast cancer. However, the regulation of ILEI activity and the molecular meaning of its altered localization remain elusive.MethodsThe influence of serum withdrawal, broad-specificity protease inhibitors, different serine proteases and plasminogen depletion on the size and amount of the secreted ILEI protein was investigated by Western blot analysis of EpRas cells. Proteases with ILEI-processing capacity were identified by carrying out an in vitro cleavage assay. Murine mammary tumor and metastasis models of EpC40 and 4T1 cells overexpressing different mutant forms of ILEI were used—extended with in vivo aprotinin treatment for the inhibition of ILEI-processing proteases—to test the in vivo relevance of proteolytic cleavage. Stable knockdown of urokinase plasminogen activator receptor (uPAR) in EpRas cells was performed to investigate the involvement of uPAR in ILEI secretion. The subcellular localization of the ILEI protein in tumor cell lines was analyzed by immunofluorescence. Immunohistochemistry for ILEI localization and uPAR expression was performed on two human breast cancer arrays, and ILEI and uPAR scores were correlated with the metastasis-free survival of patients.ResultsWe demonstrate that secreted ILEI requires site-specific proteolytic maturation into its short form for its tumor-promoting function, which is executed by serine proteases, most efficiently by plasmin. Noncleaved ILEI is tethered to fibronectin-containing fibers of the extracellular matrix through a propeptide-dependent interaction. In addition to ILEI processing, plasmin rapidly increases ILEI secretion by mobilizing its intracellular protein pool in a uPAR-dependent manner. Elevated ILEI secretion correlates with an altered subcellular localization of the protein, most likely representing a shift into secretory vesicles. Moreover, altered subcellular ILEI localization strongly correlates with high tumor cell–associated uPAR protein expression, as well as with poor survival, in human breast cancer.ConclusionsOur findings point out extracellular serine proteases, in particular plasmin, and uPAR as valuable therapeutic targets against ILEI-driven tumor progression and emphasize the prognostic relevance of ILEI localization and a combined ILEI-uPAR marker analysis in human breast cancer.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-014-0433-7) contains supplementary material, which is available to authorized users.

The interleukin-like epithelial-to-mesenchymal transition (EMT) inducer (ILEI)/FAM3C is a member of the highly homologous FAM3 family and is essential for EMT and metastasis formation. It is upregulated in several cancers, and its altered subcellular localization strongly correlates with poor survival. However, the mechanism of ILEI action, including the structural requirements for ILEI activity, remains elusive. Here, we show that ILEI formed both monomers and covalent dimers in cancer cell lines and in tumors. Using mutational analysis and pulse-chase experiments, we found that the four ILEI cysteines, conserved throughout the FAM3 family and involved in disulfide bond formation were essential for extracellular ILEI accumulation in cultured cells. Modification of a fifth cysteine (C185), unique for ILEI, did not alter protein secretion, but completely inhibited ILEI dimerization. Wild-type ILEI monomers, but not C185A mutants, could be converted into covalent dimers extracellularly upon overexpression by intramolecular-to-intermolecular disulfide bond isomerization. Incubation of purified ILEI with cell culture medium showed that dimerization was triggered by bovine serum in a dose- and time-dependent manner. Purified ILEI dimers induced EMT and trans-well invasion of cancer cells in vitro. In contrast, ILEI monomers and the dimerization-defective C185A mutant affected only cell motility as detected by scratch assays and cell tracking via time-lapse microscopy. Importantly, tumor cells overexpressing wild-type ILEI caused large tumors and lung metastases in nude mice, while cells overexpressing the dimerization-defective C185A mutant behaved similar to control cells. These data show that covalent ILEI self-assembly is essential for EMT induction, elevated tumor growth, and metastasis.