The intracellular domains of the membrane-anchoring regions of some precursors of epidermal growth factor (EGF) family members have intrinsic biologic activities. We have determined the role of the human proEGF cytoplasmic domain (proEGFcyt) as part of the proEGF transmembrane-anchored region (proEGFctF) in the regulation of motility and elastinolytic invasion in human thyroid cancer cells. We found proEGFctF to act as a negative regulator of motility and elastin matrix penetration and the presence of proEGFcyt or proEGF22.23 resulted in a similar reduction in motility and elastinolytic migration. This activity was counteracted by EGF-induced activation of EGF receptor signaling. Decreased elastinolytic migratory activity in the presence of proEGFctF and proEGFcyt/proEGF22.23 coincided with decreased secretion of elastinolytic procathepsin L. The presence of proEGFctF and proEGFcyt/proEGF22.23 coincided with the specific transcriptional up-regulation of t-SNARE member SNAP25. Treatment with siRNA-SNAP25 resulted in motility and elastin migration being restored to normal levels. Epidermal growth factor treatment down-regulated SNAP25 protein by activating EGF receptor-mediated proteasomal degradation of SNAP25. These data provide first evidence for an important function of the cytoplasmic domain of the human proEGF transmembrane region as a novel suppressor of motility and cathepsin L-mediated elastinolytic invasion in human thyroid carcinoma cells and suggest important clinical implications for EGF-expressing tumors.

We studied the distribution of transcripts encoding the cytoplasmic domain of the membrane-anchored precursor epidermal growth factor (proEGFcyt) and a novel cytoplasmic proEGF splice isoform with a deleted exon 23 and an out-offrame fusion of exon 24 (proEGFdel23) in human normal and neoplastic thyroid tissues. In papillary thyroid carcinoma (PTC), coexpression of transcripts encoding for both proEGFcyt and proEGFdel23 correlated with poor differentiation of PTC. To determine potential roles of the cytoplasmic proEGF domain in human thyroid cells, we generated stable transfectants of the human follicular thyroid carcinoma cell line FTC-133 overexpressing the normal cytoplasmic domain proEGFcyt, a truncated proEGFcyt composed of the peptide sequence encoded by exons 22 and 23 (proEGF22.23) and proEGFdel23. The proEGFcyt and proEGF22.23 transfectants displayed significantly reduced proliferation rates, an enlarged cellular phenotype, and alterations in the distribution and post-translational modification of the microtubular system. These transfectants also displayed increased production of microtubule-associated proteins 1b and 2c, which was absent in FTC-133-proEGFdel23 or FTC-133-empty plasmid transfectants. This is the first evidence of an involvement of proEGF cytoplasmic domain in microtubular stability in the human thyroid carcinoma cell line FTC-133 and may suggest a specific role for the cytoplasmic domain of membraneanchored proEGF, particularly exon 23, in thyroid carcinoma. The up-regulation of proEGFdel23 in poorly differentiated PTC and the exclusive detection of both proEGF isoforms in undifferentiated thyroid carcinoma may indicate an involvement of this novel truncated proEGFdel23 cytoplasmic domain during dedifferentiation processes of human thyroid cells. (Cancer Res 2005; 65(4): 1343-51)

No abstract

No abstract