Procathepsin V Is Secreted in a TSH Regulated Manner from Human Thyroid Epithelial Cells and Is Accessible to an Activity-Based Probe

Abstract

The significance of cysteine cathepsins for the liberation of thyroid hormones from the precursor thyroglobulin was previously shown by in vivo and in vitro studies. Cathepsin L is most important for thyroglobulin processing in mice. The present study aims at specifying the possible contribution of its closest relative, cysteine cathepsin L2/V, to thyroid function. Immunofluorescence analysis on normal human thyroid tissue revealed its predominant localization at the apical plasma membrane of thyrocytes and within the follicle lumen, indicating the secretion of cathepsin V and extracellular tasks rather than its acting within endo-lysosomes. To explore the trafficking pathways of cathepsin V in more detail, a chimeric protein consisting of human cathepsin V tagged with green fluorescent protein (GFP) was stably expressed in the Nthy-ori 3-1 thyroid epithelial cell line. Colocalization studies with compartment-specific markers and analyses of post-translational modifications revealed that the chimeric protein was sorted into the lumen of the endoplasmic reticulum and subsequently transported to the Golgi apparatus, while being N-glycosylated. Immunoblotting showed that the chimeric protein reached endo-lysosomes and it became secreted from the transduced cells. Astonishingly, thyroid stimulating hormone (TSH)-induced secretion of GFP-tagged cathepsin V occurred as the proform, suggesting that TSH upregulates its transport to the plasma membrane before it reaches endo-lysosomes for maturation. The proform of cathepsin V was found to be reactive with the activity-based probe DCG-04, suggesting that it possesses catalytic activity. We propose that TSH-stimulated secretion of procathepsin V is the default pathway in the thyroid to enable its contribution to thyroglobulin processing by extracellular means.

New inhibitors of cathepsin V impair tumor cell proliferation and elastin degradation and increase immune cell cytotoxicity

“…Elevated cathepsin V levels have been associated with various pathological processes, including myasthenia gravis [9] , atherosclerosis [8] , vascular inflammation [14] , type 1 diabetes [15] , mucopolysaccharidoses [16] , neurological diseases [12] , and cancer [3] . In cancer, high cathepsin V expression was first detected in colorectal and breast carcinomas [3] , and later also in ovarian, endometrial, renal, squamous cell, thymic epithelial [17] , hepatocellular, and thyroid carcinomas [18] , [19] . Higher cathepsin V levels were associated with increasing tumor stage [20] , [21] , distant metastases [22] , [23] , [24] , and lower patient survival [24] .…”