title: Reversible inhibition of a thyroid-specific trans-acting factor by Ras.
abstract: Exposure of rat thyroid cells for 1 week to a temperature-sensitive variant of Kirsten murine sarcoma virus (KiMSV) Ras inactivated the thyroglobulin promoter (pTg). Cellular dedifferentiation was paralleled by the loss of the thyroid-specific trans-acting factor, TgTF1, which binds to pTg. When Ras was denatured by shifting cells to 39 degrees C, TgTF1 binding and pTg function recovered rapidly without the synthesis of new protein. TgTF1 could be reactivated in vitro by treating nuclear extracts with protein kinase A. After 4 weeks of exposure to the oncogene, denaturation of Ras no longer restored TgTF1 binding or reactivated pTg. Incubation of nuclear extracts with protein kinase A likewise did not reactivate TgTF1. Cells chronically exposed to Ras did, however, yield differentiated clones after treatment with 5-azacytidine. We suggest that Ras induces dedifferentiation in two sequential steps: (1) Ras reduces PKA activity; TgTF1 (or an auxiliary protein) becomes dephosphorylated, and binding to pTg is abolished. (2) The effects of Ras become imprinted by methylation, possibly of the TgTF1 gene.
authors.affiliation: Institute of Cancer Research, Columbia University, New York, New York 10032.
authors.forename: V E
authors.initials: VE
authors.lastname: Avvedimento
authors.forename: A M
authors.initials: AM
authors.lastname: Musti
authors.forename: M
authors.initials: M
authors.lastname: Ueffing
authors.forename: S
authors.initials: S
authors.lastname: Obici
authors.forename: A
authors.initials: A
authors.lastname: Gallo
authors.forename: M
authors.initials: M
authors.lastname: Sanchez
authors.forename: D
authors.initials: D
authors.lastname: DeBrasi
authors.forename: M E
authors.initials: ME
authors.lastname: Gottesman
mesh_terms: D000818:Animals; D001483:Base Sequence; D002472:Cell Transformation, Viral / Q000235:genetics; D004247:DNA / Q000378:metabolism; D011905:Genes, ras; D007708:Kirsten murine sarcoma virus / Q000235:genetics; D008745:Methylation; D008969:Molecular Sequence Data; D015345:Oligonucleotide Probes; D011401:Promoter Regions, Genetic; D051381:Rats; D013954:Thyroglobulin / Q000235:genetics*; D013961:Thyroid Gland / Q000378:metabolism*; D015534:Trans-Activators / Q000037:antagonists & inhibitors*; D014158:Transcription, Genetic
chemical_list: D015345:Oligonucleotide Probes; D015534:Trans-Activators; D004247:DNA; D013954:Thyroglobulin