The cytoplasmic domains of EGF-like ligands have important biological functions. Stable transfectants of the human follicular thyroid carcinoma cell line FTC-133 over-expressing the cytoplasmic domain of proEGF (FTC-133-proEGFcyt) demonstrated a transcriptional up-regulation of the lysosomal hydrolases cathepsin- (cath-) B and -D and alterations in the processing of cath-L protein. Cath-L has strong elastinolytic activity and was the only of the three cathepsins to be secreted by FTC-133 transfectants. FTC-133-proEGFcyt clones displayed a markedly reduced ability to migrate through elastin matrices when compared with mock transfectants (empty plasmid) or a natural proEGFsplice mutant construct. Decreased migration through elastin matrix coincided with a reduction of cath-L secretion in FTC-133-proEGFcyt clones. When incubated with a specific cath-L inhibitor, FTC-133-proEGFsplice and mock transfectants showed a similar reduction in elastinolytic activity implicating cath-L to be largely responsible for the elastinolytic activity in our elastin micration assays. Down-regulation of cath-L in FTC-133-proEGFcyt coincided with an upregulation of the t-SNARE component SNAP25 as determined by specific siRNA knock-down of SNAP25. Incubation of FTC-133-proEGFcyt with soluble EGF reduced SNAP25 protein levels in proEGFcyt transfectants and, thus, reversed the inhibitory actions of proEGFcyt on elastinolytic migration. This antagonistic EGF action was mediated by the EGFR. In summary, we have identified proEGFcyt as a novel regulator of the function of the t-SNARE membrane-vesicle fusion complex involved in the exocytosis/secretion of elastinolytic cath-L. This mechanism facilitates the suppressive role of proEGFcyt domain on thyroid carcinoma cell elastinolytic activity and invasiveness. Partly supported by Krebshilfe.
03 - 07 May 2008
European Society of Endocrinology