Invasive properties of murine squamous carcinoma cells: Secretion of matrix-degrading cathepsins is attributable to a deficiency in the mannose 6-phosphate/insulin-like growth factor II receptor

Penetration of basement membrane layers is a hallmark feature of metastatic tumor cells. The invasive propensity of murine SCC-VII squamous carcinoma cells is in part attributable to the extracellular action of the lysosomal cysteine proteinase cathepsin B. Although most noncancerous cells store thi s enzyme in the lysosomes, we found that SCC-VII cells release a large frac tion (42%) of their newly synthesized procathepsin B into the culture mediu m. Procathepsins D and L, the precursors of other major lysosomal proteinas es, are also secreted in significant amounts (24 and 75%, respectively). In contrast, normal murine 3T3-L1 fibroblasts exocytose only minor amounts of their newly synthesized procathepsins B (10%), D (<1%), and L (16%). Weste rn blotting analysis revealed that SCC-VII cells are deficient in the 300 k Da mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF2R), a tumor suppressor with a central role in the intracellular transport of ly sosomal enzymes. Consistent with the absence of M6P/IGF2R, SCC-VII cells la ck dense lysosomes, with the bulk of intracellular acid hydrolases residing in late endosomes/prelysosomes. On the other hand, the synthesis of the M6 P recognition marker on lysosomal enzymes is not impaired in SCC-VII cells, because [P-33]P-i is incorporated into the carbohydrate moieties of procat hepsins B, D, and L. Furthermore, 69% of the phosphorylated N-linked oligos accharides synthesized by SCC-VII cells carry phosphomonoester groups and a s such constitute high-affinity ligands for M6P receptors. SCC-VII cells ex press the 46 kDa cation-dependent M6P receptor (MPR46), but intracellular r etention of procathepsins B, D, and L is not affected by ammonium chloride and chloroquine, agents known to perturb the M6P receptor system. Our resul ts indicate that failure to express the 300 kDa M6P/IGF2R may enhance the m etastatic capacity of tumor cells by inducing the secretion of procathepsin s B, D, and L.