No abstract

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) mediates biosynthetic sorting and endocytosis of various factors that impinge on the proliferation, migration and invasiveness of tumour cells. The gene encoding M6P/IGF2R is frequently lost or mutated in a wide range of malignant tumours including squamous cell carcinomas. We have previously shown that M6P/IGF2R-deficient SCC-VII murine squamous cell carcinoma cells secrete large amounts of pro-invasive lysosomal proteinases. Furthermore, the formation of mature lysosomes is impaired in SCC-VII cells. To assess the link between M6P/ IGF2R status and tumour invasion, we have now generated SCC-VII lines stably transfected with human M6P/IGF2R cDNA. Reconstitution of functional M6P/IGF2R expression in SCC-VII cells strongly improves the intracellular retention of lysosomal proteinases and restores the formation of mature lysosomes. In addition, the presence of heterologous M6P/IGF2R compromises the growth of SCC-VII cells both in vitro and in vivo. Remarkably, M6P/IGF2R expression also reduces the invasive capacity of SCC-VII cells in response to various chemoattractants. These results indicate that the M6P/IGF2R status influences the metastatic propensity of squamous cell carcinomas. ' 2008 Wiley-Liss, Inc.Key words: cathepsin; lysosome; proteolysis; squamous cell carcinoma; tumour invasion A key requirement for metastatic cancer cell invasion is the penetration of extracellular matrix (ECM) barriers. This process involves the degradation of different ECM proteins and proteoglycans. 1,2 Various proteinases have been implicated in ECM degradation associated with tumour invasion and metastasis, including urokinase-type plasminogen activator, matrix metalloproteinases and cathepsins. [3][4][5] The involvement of the latter enzymes in ECM proteolysis is perplexing, since cathepsins are normally localized in lysosomes. However, tumour cells often secrete significant amounts of these proteinases into the pericellular fluid, as first observed for cathepsin B. 6 As typical for lysosomal enzymes, the N-glycan moieties of cathepsins are modified during their biosynthesis with mannose 6-phosphate (M6P) residues which permit interaction with the main lysosomal sorting receptors, the 300-kDa mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) and the 46-kDa mannose 6-phosphate receptor (MPR46). 7 Evidence has been provided that excessive secretion of cathepsins by tumour cells may be due to transformation-induced changes to the M6P receptor pathway. [8][9][10] Lysosomal sorting via M6P/IGF2R is generally far more efficient than by MPR46, demonstrating that the former is the main lysosomal targeting receptor in mammalian cells. 11,12 However, M6P/IGF2R also binds a variety of other factors that impinge on the proliferation, migration and invasiveness of tumour cells, including insulin-like growth factor II (IGF-II), 13 transforming growth factor b, 14 urokinase-type plasminogen activator receptor 15 and plasminogen. 16 Hence, it is o...

In mammalian cells, the mannose 6-phosphate receptor pathway accounts for the transport of most soluble acid hydrolases to lysosomes. It is believed that dissociation of mannose 6-phosphate receptors and their ligands is entirely driven by the acidic environment in endosomal compartments. Indeed, pH-perturbing substances such as ammonium chloride and monensin have been shown to inhibit lysosomal enzyme targeting in cells that express both known mannose 6-phosphate receptors. We now demonstrate that ammonium chloride and monensin exert modest effects on the intracellular retention of lysosomal hydrolases in murine cells that synthesize only the 46-kDa mannose 6-phosphate receptor. Neither ammonium chloride nor monensin induces changes to the subcellular localization of lysosomal hydrolases and the 46-kDa mannose 6-phosphate receptor in these cells. This suggests that endosomal dissociation of the receptor and its ligands still occurs in the presence of these agents. We conclude that the murine 46-kDa mannose 6-phosphate receptor has the capacity to deliver its cargo proteins to lysosomes even in the absence of endosomal acidification.

The M6P (mannose 6-phosphate)/IGF2R (insulin-like growth factor II receptor) interacts with a variety of factors that impinge on tumour invasion and metastasis. It has been shown that expression of wild-type M6P/IGF2R reduces the tumorigenic and invasive properties of receptor-deficient SCC-VII squamous cell carcinoma cells. We have now used mutant forms of M6P/IGF2R to assess the relevance of the different ligand-binding sites of the receptor for its biological activities in this cellular system. The results of the present study demonstrate that M6P/IGF2R does not require a functional binding site for insulin-like growth factor II for inhibition of anchorage-independent growth and matrix invasion by SCC-VII cells. In contrast, the simultaneous mutation of both M6P-binding sites is sufficient to impair all cellular functions of the receptor tested. These findings highlight that the interaction between M6P/IGF2R and M6P-modified ligands is not only important for intracellular accumulation of lysosomal enzymes and formation of dense lysosomes, but is also crucial for the ability of the receptor to suppress SCC-VII growth and invasion. The present study also shows that some of the biological activities of M6P/IGF2R in SCC-VII cells strongly depend on a functional M6P-binding site within domain 3, thus providing further evidence for the non-redundant cellular functions of the individual carbohydrate-binding domains of the receptor.