Decreased secretion of Cathepsin D in breast cancer in vivo by tamoxifen: mediated by the mannose-6-phosphate/IGF-II receptor?

The lysosomal protease Cathepsin D (Cath D) is associated with increased invasiveness and metastasis in breast cancer. Both estrogen and tamoxifen have been reported to increase Cath D, which seems to contradict the efficacy of tamoxifen as an adjuvant for estrogen dependent breast cancer. Cath D is bioactive in the extracellular space but very little is known about hormonal regulation of secreted Cath D in vivo. In this study we used microdialysis to sample the extracellular fluid in estrogen receptor positive MCF-7 tumors in nude mice. We show that tamoxifen in combination with estradiol decreased secreted Cath D compared with estradiol treatment only in solid tumors in situ. Cell culture of MCF-7 cells revealed that estradiol and tamoxifen increased intracellular proteolytic activity of Cath D in a similar fashion whereas secretion of Cath D was increased by estradiol and inhibited by tamoxifen. Immunofluorescence showed that estradiol located Cath D to the cell surface, while tamoxifen accumulated Cath D to dense lysosomes in perinuclear regions. Moreover, tamoxifen increased the intracellular transporter of Cath D, the mannose 6-phosphate/IGF-II receptor (M6P/IGF2R). In contrast, estradiol decreased the levels of this receptor. Thus, secretion of Cath D is hormone dependent and may be mediated by altered expression of the M6P/IGF2R. Our results highlight the importance of measurements of proteins in all compartments where they are biological active and show that microdialysis is a viable technique for sampling of Cath D in vivo.