Northern blot analysis of total RNA from the mouse C3H/10T½ cell line indicated that the erbAα gene transcribed three mRNA species of similar sizes (2.6, 5.5, 6.6 kilobases) as found in rodents. The 2.6-kilobase mRNA (erbA-α2) was approximately 7- to 8-fold more abundant than either the 5.5- (erbA-α1) or 6.6-kilobase species. The expression of the erbA-α2 transcript increased 3- to 30-fold when “normal” mouse or rat cells were growth arrested by confluence. Triiodothyronine, at a concentration of 1 nm, had no effect on the levels of the erbA-α mRNA species in confluent cells nor on the levels of erbA-α2 in proliferative normal or transformed C3H/10T½ cells. In log-phase growing cells there was a 2.5- to 5-fold increase in the relative expression of erbA-α2 mRNA in transformed mouse C3H/10T½ cells, transformed cloned rat embryo fibroblasts (CREF), transformed rat embryo fibroblasts (REF), and a transformed temperature-sensitive rat mutant cell line (ts7E) when compared with their non-transformed counterparts. In contrast to the elevation of erbA-α2 in transformed cells, erbA-α1 and erbA-β1 mRNAs decreased in transformed mouse and rat cell lines. In conclusion, it is suggested that the increased levels of the erbA-α2 transcript and the decreased levels of erbA-α1 and erbA-β1 in neoplastic cells may account for the loss of thyroid hormone regulation of inducible pathways and decreased nuclear triiodothyronine binding as previously reported.

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This work was supported by the National Cancer Institute of Canada (Grant 1618), the Dalhousie Medical Research Foundation, and the Dalhousie University Faculty of Medicine.

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