Background: The cysteine peptidase cathepsin B is important in thyroid physiology by being involved in prohormone processing initiated in the follicle lumen and completed in endo-lysosomal compartments. However, cathepsin B has also been localized to the extrafollicular space in thyroid cancer tissue, and is therefore suggested to promote invasiveness and metastasis in thyroid carcinomas through e.g. extracellular matrix degradation. Methods. Transport of cathepsin B in normal thyroid epithelial and carcinoma cells was investigated through immunolocalization of endogenous cathepsin B in combination with probing protease activity. Transport analyses of cathepsin B-eGFP and its active-site mutant counterpart cathepsin B-C29A-eGFP were used to test whether intrinsic sequences of a protease influence its trafficking. Results: Our approach employing activity based probes, which distinguish between active and inactive cysteine proteases, demonstrated that both eGFP-tagged normal and active-site mutated cathepsin B chimeras reached the endo-lysosomal compartments of thyroid epithelial cells, thereby ruling out alterations of sorting signals by mutagenesis of the active-site cysteine. Analysis of chimeric protein trafficking further showed that GFP-tagged cathepsin B was transported to the expected compartments, i.e. endoplasmic reticulum, Golgi apparatus and endo-lysosomes of normal and thyroid carcinoma cell lines. However, the active-site mutated cathepsin B chimera was mostly retained in the endoplasmic reticulum and Golgi of KTC-1 and HTh7 cells. Hence the latter, as the least polarized of the three carcinoma cell lines analyzed, exhibited severe transport defects in that it retained chimeras in pre-endolysosomal compartments. Furthermore, secretion of endogenous cathepsin B and of other cysteine peptidases, which occurs at the apical pole of normal thyroid epithelial cells, was most prominent and occurred in a non-directed fashion in thyroid carcinoma cells. Conclusions: Transport of endogenous and eGFP-tagged active and inactive cathepsin B in the cultured thyroid carcinoma cells reflected the distribution patterns of this protease in thyroid carcinoma tissue. Hence, our studies showed that sub-cellular localization of proteolysis is a crucial step in regulation of tissue homeostasis. We conclude that any interference with protease trafficking resulting in altered regulation of proteolytic events leads to, or is a consequence of the onset and progression of thyroid cancer.
Bibliographical noteFunding Information:
The authors would like to thank Ruth Hunegnaw, Martin Linke and Maren Rehders for excellent technical contributions and Nils-Erik Heldin as well as Junichi Kurebayashi for providing the thyroid carcinoma cell lines. This work was supported by Jacobs University Bremen, Foundation Blanceflor and The Royal Society of Arts and Sciences in Göteborg. This article has been published as part of Thyroid Research Volume 4 Supplement 1, 2011: New aspects of thyroid hormone synthesis and action. The full contents of the supplement are available online at http://www. thyroidresearchjournal.com/supplements/4/S1 1School of Engineering and Science, Research Center for Molecular Life Science, Jacobs University Bremen, 28759 Bremen, Germany. 2School of Pharmacy, Faculty of Medicine, The Hebrew University, 91120 Jerusalem, Israel. 3Departments of Pathology and Microbiology and Immunology, Stanford University School of Medicine, Stanford, California 94305-5324, USA. 4Universitätsklinikum Leipzig Medizinische Klinik III, 04103 Leipzig, Germany; as of June 2011: Klinik für Endokrinologie, Zentrum für Innere Medizin, Bereich Forschung und Lehre im Zentrallabor, 45147 Essen, Germany.