Increased messenger RNA levels of the antagonist thyroid hormone receptor erbA-alpha 2 and decreased levels of erbA-alpha 1 and erbA-beta 1 receptor messenger RNAs in neoplastic rodent cells.

Nothern blot analysis of total RNA from the mouse C3H/10T1/2 cell line indicated that the erbA alpha 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-alpha 2) was approximately 7- to 8-fold more abundant than either the 5.5- (erbA-alpha 1) or 6.6-kilobase species. The expression of the erbA-alpha 2 transcript increased 3- to 30-fold when "normal" mouse or rat cells were growth arrested by concluence. Triiodothyronine, at a concentration of 1 nM, had no effect on the levels of the erbA-alpha mRNA species in confluent cells nor on the levels of erbA-alpha 2 in proliferative normal or transformed C3H/10T1/2 cells. In log-phase growing cells there was a 2.5- to 5-fold increase in the relative expression of erbA-alpha 2 mRNA in transformed mouse C3H/10T1/2 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-alpha 2 in transformed cells, erbA-alpha 1 and erbA-beta 1 mRNAs decreased in transformed mouse and rat cell lines. In conclusion, it is suggested that the increased levels of the erbA-alpha 2 transcript and the decreased levels of erbA-alpha 1 and erbA-beta 1 in neoplastic cells may account for the loss of thyroid hormone regulation of inducible pathways and decreased nuclear triiodothyronine binding as previously reported.