Cathepsin D maturation and its stimulatory effect on metastasis are prevented by addition of KDEL retention signal.

Cathepsin D is overexpressed in most primary breast cancers where its concentration is correlated with increased metastatic potential. To investigate the possible role and mechanism of this lysosomal protease in metastasis, we transfected low-metastatic rat tumor cells with wild-type human cathepsin D, or mutated forms obtained by insertion of a KDEL peptide signal responsible for ER retention, or a control KDAS peptide. The overexpressed pro-cathepsin D in wild-type and KDAS clones was normally sorted and maturated in lysosomes. In KDEL clones, pro-cathepsin D was mostly retained in the ER or partially secreted by high-producer clones but was not maturated. While overexpressed cathepsin D increased experimental metastasis in athymic mice, the pro-cathepsin/D-KDEL was totally ineffective. Moreover, the effect of cathepsin D on metastasis did not seem to be due to saturation of the mannose-6-phosphate receptor since the secretion of two other rat lysosomal enzymes was unaffected by cathepsin D overexpression. We conclude that pro-cathepsin D overexpression facilitates tumor metastasis only when maturated into an active enzyme.