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...