propylthiouracil and Cell-Transformation--Neoplastic

RET/PTC1 is a rearranged form of the RET proto-oncogene detected in human papillary thyroid carcinomas. We previously showed that thyroid-targeted expression of RET/PTC1 leads to thyroid tumor formation in Tg-PTC1 transgenic mice. Signal transduction pathways mediated by phosphotyrosine 294, 404, or 451 in RET/PTC1 have been shown to be critical for RET-induced transforming activity in vitro. To investigate the contribution of these signaling pathways in RET/PTC1-induced thyroid tumor formation in vivo, we generated and characterized transgenic mice expressing thyroid-targeted RET/PTC1 mutants carrying a site-directed mutation changing tyrosine (Y) to phenylalanine (F) at the residue 294, 404, or 451. In contrast to the 100% tumor formation rate in Tg-PTC1 transgenic mice, tumor formation rates were significantly decreased in Tg-PTC1-Y294F (6%), Tg-PTC1-Y404F (41%), and Tg-PTC1-Y451F (30%) transgenic mice. This indicates that signaling pathways mediated by pY294, pY404, and pY451 do play a role in RET/PTC1-induced tumor formation. However, as tumors are still able to form in some mice within these three mutant transgenic groups, it indicates that none of the signaling pathways mediated by pY294, pY404, or pY451, are solely essential for RET/PTC1-induced tumor formation.

Topics: Amino Acid Substitution; Animals; Antithyroid Agents; Carcinoma, Papillary; Cell Transformation, Neoplastic; Diet; Humans; Iodine; MAP Kinase Signaling System; Mice; Mice, Transgenic; Mutagenesis, Site-Directed; Oncogene Proteins; Oncogene Proteins, Fusion; Phosphorylation; Phosphotyrosine; Propylthiouracil; Protein Processing, Post-Translational; Protein-Tyrosine Kinases; Proto-Oncogene Mas; Proto-Oncogene Proteins c-ret; Receptor Protein-Tyrosine Kinases; Signal Transduction; Thyroid Neoplasms; Tubulin

We have recently shown that the rat hepatic lectin (RHL)-1 subunit of the asialoglycoprotein receptor (ASGPr) is expressed in the PC C13 differentiated thyroid cell line. To investigate in vivo the expression of RHL-1 and the ability of thyrotropin (TSH) to modulate its expression, reverse-transcriptase polymerase chain reaction (RT-PCR) and Western blot assays have been performed on thyroid extracts from rats treated with thyroxine (T4) or propylthiouracil (PTU), each of which modulates TSH levels. It is shown that RHL-1 expression is down-regulated by T4 (which decreases serum TSH) and upregulated by PTU (which increases serum TSH), at both mRNA and protein levels. The sensitivity of RHL-1 to neoplastic transformation of thyroid cells has been investigated. The RHL-1 expression pattern has been studied in PC C13 thyroid cells transformed by several oncogenes that induce different degrees of malignancy and dedifferentiation. RT-PCR and Western blot assays show that RHL-1 expression progressively decreases as PC C13 cells acquire a more transformed phenotype. Expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA, a housekeeping gene used as internal control to normalize RHL-1 mRNA content, exhibits no variations in the different PC C13 cell lines used. In addition, we show that both native and asialo-thyroglobulin (Tg) bind RHL-1 in vitro, and native Tg binds RHL-1 on the surface of PC C13 cells. After thyroid cells transformation, the surface expression of RHL-1 is inhibited in a measure that correlates with the mRNA and protein levels. Therefore, the RHL-1 inhibition at the mRNA, protein and plasma membrane expression follows a gradient that parallels the progressive acquisition of the fully transformed phenotype in the PC C13 system. The results reported in the present article, together with our previous data, suggest that RHL-1 expression could be regulated, at least in part, by the same transcription factors involved in the expression of the other molecules characteristic of the thyroid differentiated state.

Topics: Animals; Asialoglycoprotein Receptor; Cell Line; Cell Transformation, Neoplastic; Down-Regulation; Male; Propylthiouracil; Protein Biosynthesis; Rats; Rats, Wistar; Receptors, Cell Surface; Reverse Transcriptase Polymerase Chain Reaction; Thyroid Gland; Thyrotropin; Thyroxine; Transcription, Genetic; Up-Regulation

Topics: Adenoma; Animals; Cell Transformation, Neoplastic; Iodine Isotopes; Male; Mitosis; Neoplasms, Experimental; Neoplasms, Radiation-Induced; Propylthiouracil; Rats; Thyroid Neoplasms; Thyrotropin