The c-erbA gene encodes two thyroid hormone receptors, TR 1 and TR 2, that arise from alternative splicing of the TR pre-mRNA. TR 2 is not able to bind triiodothyronine (T 3 ) and acts as a weak antagonist of TRs. It has been suggested that the balance of TR 1 to TR 2 is important in maintaining homeostasis. Here, we study the effect of thyroid hormone on the splicing of TR under various conditions in HepG2 cells. First, T 3 was added to HepG2 cells that endogenously express TR . This resulted in a decrease in the TR 1:TR 2 mRNA ratio after the addition of 10 8 M or 10 7 M T 3 . Then, HepG2 cells were incubated with sera from hypothyroid or hyperthyroid patients. Sera from hyperthyroid patients (n=6) decreased the TR 1:TR 2 ratio compared with HepG2 cells incubated with sera from euthyroid patients (n=8). Sera from hypothyroid patients (n=6) had no effect on the TR 1:TR 2 ratio but supplementation with T 3 caused a decrease in the ratio. Finally, we tested sera from patients with nonthyroidal illness (NTI; n=17) which showed no effect on TR splicing when compared with controls. Free thyroxine levels in sera from hypo-, eu-, and hyperthyroid patients, but not that of NTI patients, were negatively correlated (P,0·01) to the TR 1:TR 2 ratio. We next studied the expression of the splicing factors hnRNP A1 and ASF/SF2 (SF2) in relation to the splicing of the TR gene. In HepG2 cells incubated with NTI sera a negative relationship was found between the ratio of hnRNP A1:SF2 and the TR 1:TR 2 ratio. A high hnRNP A1:SF2 ratio is associated with the use of the distal 5 -splice site. The splicing direction should then change towards TR 2, which is indeed the case. Rev-ErbA, which is partly complementary to TR 2 and could therefore interfere in the splicing process, did not relate to the TR 1:TR 2 ratio.In conclusion, high T 3 levels induce a low TR 1:TR 2 ratio which could protect the cell from excessive T 3 -induced gene expression. In vivo, this might be a mechanism to keep tissues relatively euthyroid during high serum T 3 levels.

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