E-cadherin cell-cell adhesion in ewing tumor cells mediates suppression of anoikis through activation of the ErbB4 tyrosine kinase.

AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA 3094 We demonstrated that human melanoma cells characterized by a high tumorigenic and metastatic phenotype overexpressed cathepsin L, a cysteine protease and much more highly secreted the procathepsin L form (Frade et al., Cancer Research, 1998). Recently, we also demonstrated that expression of cathepsin L gene in human tumor cells is under the control of distinct regulatory mechanisms (Jean et al., Oncogene, 2006). The involvement of procathepsin L secretion in tumor progression was demonstrated by preparing and using an anti-cathepsin L ScFv. Indeed, stable intracellular expression of this anti-cathepsin L ScFv strongly inhibits procathepsin L secretion but also the tumorigenic and metastatic phenotype of human melanoma cells in nude mice (Rousselet et al., Cancer Research, 2004). In addition, inhibition of procathepsin L secretion in human melanoma cells decreased neovascularization during in vivo tumor growth. These data led us to analyze cathepsin L expression and procathepsin L secretion in human melanoma cells grown under hypoxic conditions. We herein demonstrated that hypoxia (1% O2) increased intracellular expression of cathepsin L and secretion of procathepsin L in the highly tumorigenic and metastatic A375SM human melanoma cells. However, under the same hypoxic conditions, cathepsin L mRNA level and promoter activity were inhibited, suggesting that up-regulation of cathepsin L by hypoxia was not induced at the transcriptionnal level. Analysis of molecular mechanisms involved in regulation of cathepsin L revealed that the 5'-untranslated region (5'-UTR) of cathepsin L mRNA is essential for its expression. We demonstrated that cathepsin L 5'-UTR contains a cryptic promoter and an internal ribosome entry site (IRES). It was shown by others that under hypoxia conditions, IRES contributes to maintain expression of several genes involved in angiogenesis, as vascular endothelial growth factor (VEGF), hypoxia-inducible factor (HIF-1) and angiopoietin receptor tyrosine kinase Tie2. All these results strongly suggest that presence of an IRES allow efficient cathepsin L mRNA translation under hypoxia, leading to the increase of procathepsin L secretion which favorises angiogenesis, tumor growth and metastasis.