The lysosomal cysteine protease cathepsin B is involved in proteolytic processes both inside and outside of cells. Apart from the lysosomal activity of cathepsin B, its extracellular activity in the lumen of thyroid follicles is a prerequisite for the solubilization of thyroglobulin and the liberation of thyroid hormones. Therefore, the secretion of lysosomal enzymes from within thyroid epithelial cells is of relevance for proper thyroid function.
Recently, we have shown that the secretion of cathepsin B into the extracellular space is regulated by thyroid stimulating hormone (TSH) in a time- and dose-dependent manner. Furthermore, the re-internalization of the secreted cathepsin B from within the extracellular space back into thyroid epithelial cells occurred about 2 h after its secretion, and was also TSH-dependent. We proposed that a long-term effect of TSH might be the up-regulation of the expression of a receptor functioning in the clearance of secreted lysosomal enzymes. However, up until now nothing is known about the putative cathepsin B receptor of thyroid epithelial cells. The identification of the putative receptor will be part of future work, and will help to understand the timing of extracellular proteolysis in the thyroid.
To further elucidate the molecular mechanisms underlying the secretion of cathepsin B, a series of future experiments will address its transport pathways within thyroid epithelial cells. In order to track its trafficking we will use chimeras of cathepsin B with green fluorescent protein (GFP). The transport of normal, proteolytically active cathepsin B from rat will be compared with the trafficking of a proteolytically inactive variant (cysteine to alanine active-site-mutant) and a proteolytically active cathepsin B derived from a human cancer cell line. Preliminary results indicated that the active-site-mutant was retained within the endoplasmic reticulum, whereas the human cancer-derived cathepsin B was transported to compartments differing from those reached by the normal rat cathepsin B. This led us to the conclusion that intrinsic signals of cathepsin B affect its intra- and extracellular distribution.
Thursday, 27 February 2003, 18:00 Research II - Seminar Room II
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