Cloning and identification of a novel thyroid hormone receptor β isoform expressed in the pituitary gland

We have previously identified a novel Trβ isoform (TrβΔ) in the rat, in which a novel exon N (108 bps) was found between exon 3 and exon 4 of TrβΔ, which represents the only difference between TrβΔ and Trβ1. In this study, we searched for an elongated Trβ2-like subtype with one additional exon N. We successfully isolated the entire mRNA/cDNA of a novel elongated Trβ2 isoform via PCR in the rat pituitary gland. The mRNA/cDNA was only 108 bps (exon N) longer than that Trβ2, and the extension of the sequence was between exon 3 and 4 of Trβ. The whole sequence of this novel Trβ isoform has been published in NCBI GenBank (HM043807.1); it is named TRbeta2Delta (Trβ2Δ). In adult rat pituitary tissue, quantitative real-time RT-PCR analysis showed that the mRNA levels of Trβ2Δ and Trβ2 were roughly equal (P > 0.05). We cloned, expressed, and purified the His-Trβ2Δ protein [recombinant TRβ2Δ (rTRβ2Δ)]. SDS-PAGE and western blotting revealed that the molecular weight of rTRβ2Δ was 58.2 kDa. Using a radioligand binding assay and an electrophoretic mobility shift assay, rTRβ2Δ-bound T3 with high affinity and recognized thyroid hormone response element (TRE) binding sites. Finally, in vitro transfection experiments further confirmed that rTRβ2Δ binding T3 significantly promotes the transcription of target genes via the TRE. Here, we have provided evidence suggesting that rTRβ2Δ is a novel functional TR isoform.