A novel thyroid stimulating hormone beta-subunit isoform in human pituitary, peripheral blood leukocytes, and thyroid

Abstract

Thyroid stimulating hormone (TSH) is produced by the anterior pituitary and is used to regulate thyroid hormone output, which in turn controls metabolic activity. Currently, the pituitary is believed to be the only source of TSH used by the thyroid. Recent studies in mice from our laboratory have identified a TSHbeta isoform that is expressed in the pituitary, in peripheral blood leukocytes (PBL), and in the thyroid. To determine whether a human TSHbeta splice variant exists that is analogous to the mouse TSHbeta splice variant, and whether the pattern of expression of the splice variant is similar to that observed in mice, PCR amplification of RNAs from pituitary, thyroid, PBL, and bone marrow was done by reverse-transcriptase PCR and quantitative realtime PCR. Human pituitary expressed a TSHbeta isoform that is analogous to the mouse TSHbeta splice variant, consisting of a 27 nucleotide portion of intron 2 and all of exon 3, coding for 71.2% of the native human TSHbeta polypeptide. Of particular interest, the TSHbeta splice variant was expressed at significantly higher levels than the native form or TSHbeta in PBL and the thyroid. The TSHalpha gene also was expressed in the pituitary, thyroid, and PBL, but not the BM, suggesting that the TSHbeta polypeptide in the thyroid and PBL may exist as a dimer with TSHalpha. These findings identify an unknown splice variant of human TSHbeta. They also have implications for immune-endocrine interactions in the thyroid and for understanding autoimmune thyroid disease from a new perspective.

Fig. 1

(A) Agarose gel analysis and qRT-PCR of transcript expression of human pituitary, thyroid, PBL, and bone marrow RNAs amplified with native and novel primer sets. PCR-amplified products of both native and novel TSH were detected in pituitary RNA; however, only novel TSH was detected in thyroid and PBL RNA. Neither native nor novel TSH products were detected in BM RNA. (B) The TSH gene was expressed in the pituitary, the thyroid, and PBL, but not BM. (C) 18s gene expression levels were equivalent in all four samples.

Fig. 2

(A) qRT-PCR analysis revealed high levels of both native and novel TSH transcripts in pituitary RNA. In contrast, only the novel TSH isoform was detected in thyroid and PBL RNAs. Neither native nor novel forms of TSH were detected in BM RNA. (B) qRT-PCR analysis revealed high expression of TSH in the pituitary, modest levels in the thyroid, and low levels in PBL. No TSH gene expression was detected in BM. Data are mean values ± SEM of three replicate samples.

Fig. 3

Sequence analysis of a PCR-amplified product of novel TSH from thyroid RNA. The sequence was compared by aligning with human TSH. Black nucleotides correspond to exon 3 down to the TAA stop codon. Red nucleotides are the twenty-seven nucleotides in intron 2 that are in-frame with exon 3, and which begin with an ATG start codon. The three green nucleotides correspond to the first three nucleotides of the upstream novel TSH primer.

Fig. 4

(A) Nucleotide sequence alignment of the human (top rows) and mouse (bottom rows) TSH splice variant. Green nucleotides are the putative intron-coded leader sequences. Red nucleotides represent exon 3 and exon 5 of the human and mouse TSH gene, respectively. Underlined black nucleotides show the difference of the mouse relative to the nucleotide in the corresponding human gene. (B) Comparison of amino acids in the putative leader sequence of the human and mouse TSH splice variant, indicating species differences at amino acid positions three and four.