Can nuclear cathepsin variants promote tumor progression in the thyroid?

Introduction: Cathepsins are proteolytic enzymes that are crucial for thyroid physiology through their processing of the prohormone thyroglobulin followed by thyroid hormone liberation. This processing is initiated in the thyroid follicle lumen and continued in endo-lysosomes. In pathological conditions, such as thyroid carcinoma, cathepsin B is secreted from tumor cells in a non-polarized way, and has been suggested to promote metastasis through degradation of extracellular matrix constituents. Interestingly, nuclear variants of cathepsins have been identified in thyroid carcinoma, with histones being plausible candidates for nuclear substrates.

Methods: Here, by overexpressing chimeras of cathepsin variants fused to green fluorescent protein (GFP) in thyroid carcinoma cells, we monitored the subcellular trafficking pathways and destinations of these proteases.

Results: We show that variants of cysteine cathepsins B and V are localized in the nucleus of tumor and tumor-associated cells in situ. A variant of cathepsin V was also shown to be associated with microtubules in M-phase cells in vitro. The GFP-tagged cathepsin variants were N-terminally truncated, and therefore lacking the signal peptide, in order to enable a destination different from endo-lysosomes. We show that a 51 amino acid N-terminal truncation of cathepsin B, lacking the signal peptide and part of the propeptide, caused its targeting to the mitochondria. On the other hand, N-terminally truncated cathepsin V at amino acid 56 displayed a nuclear localization. This construct is thus suitable to use for the analysis of the impact of nuclear cathepsin variants on cell functions associated with thyroid malignancy.

Conclusion: We conclude that variants of cathepsins are destined to novel locations in thyroid tumor cells and might promote cancer onset and progression through new mechanisms involving e.g. chromatin condensation and transcriptional regulation.