Comment on "Thyrotropin Receptor Expression in Cardiac Muscle Tissue"

Donald F. Sellitti, Sonia Q. Doi, Takashi Akamizu and Hiroyuki Koshiyama

Uniformed Services University of the Health Sciences, (D.F.S., S.Q.D.), Bethesda, Maryland; Kyoto University Graduate School of Medicine (T.A.), Kyoto, Japan; and Hyogo Prefecutural Amagasaki Hospital (H.K.), Hyogo, Japan

Address correspondence to: Donald F. Sellitti, Ph.D., Department of Medicine, Division of Endocrinology, Uniformed Services University of the Health Sciences, Room A 3060, 4301 Jones Bridge Road, Bethesda, Maryland 20814-4799.

To the editor:

Busuttil and Frauman (1) have recently studied the expression of the TSH receptor (TSH-R) in all four chambers of the human heart, and have failed to confirm using RT-PCR earlier findings of TSH-R transcript in this organ (2, 3). We suggest that this report, rather than confirming an absence of TSH-R in the heart, is better interpreted as supporting a scarcity of the transcript in cardiac muscle per se. In our earlier report of TSH-R expression in the porcine heart (4), we did detect TSH-R transcript by RT-PCR with primers spanning exons 9 and 10 of the full-length receptor, but expression levels varied regionally. Although essentially undetectable by single-round PCR in the left ventricular myocardium and in purified ventricular cardiocytes (in support of the study by Busuttil and Frauman), definitive TSH-R expression was observed in the right atrium and in purified atrial cardiocytes. However, the highest expression levels were observed in isolated coronary arteries and in epicardial adipose tissue, neither of which was examined in the study of human cardiac tissues (1). All PCR products in our study were confirmed as TSH-R by DNA sequencing. The differential pattern of expression of TSH-R in porcine cardiac tissues and its reproducibility in two different pig hearts argues strongly against "illegitimate transcription" (4) as an explanation of our results, as does the recent detection of TSH-R transcript in the salmon heart using a similar PCR technique (5).

We suggest that our differences from the study by Busuttil and Frauman (1) regarding TSH-R transcript in cardiac muscle tissue reflect the difficulties inherent in detecting transcripts of low abundance, namely the requirement for high-quality RNA prepared from fresh tissue, and for optimized primers and optimal PCR amplification conditions. In this context, we note that our RT-PCR studies were performed with poly A+ RNA (not total RNA) extracted from porcine cardiac tissues that were flash frozen in liquid nitrogen.

It remains to be determined whether TSH-R transcripts in the heart may have functional significance. Some evidence to this effect may be found in our subsequent studies with TSH-R expression and TSH activation of cAMP in human coronary artery smooth muscle cells in culture (6). However, a complete picture of TSH-R function in cardiac physiology will require additional studies to elucidate which specific cell types express this receptor, and how its expression might be altered in thyroid or cardiac diseases, as earlier suggested (7).

Received July 30, 2001.

References

  1. Busuttil BE, Frauman AG. 2001 Extrathyroidal manifestations of Graves’ disease: the thyrotropin receptor is expressed in extraocular, but not cardiac, muscle tissues. J Clin Endocrinol Metab 86:2315–2319.[Abstract/Free Full Text]
  2. Drvota V, Janson A, Norman C, Sylven C, Haggblad J, Bronnegard M, Marcus C. 1995 Evidence for the presence of functional thyrotropin receptor in cardiac muscle. Biochem Biophys Res Commun 211:426–431.[CrossRef][Medline]
  3. Koshiyama H, Sellitti DF, Akamizu T, Doi SQ, Takeuchi Y, Inoue D, Sakaguchi H, Takemura G, Sato Y, Takatsu Y, Nakao K. 1996 Cardiomyopathy associated with Graves’ disease. Clin Endocrinol (Oxf) 45:111–116.[Medline]
  4. Sellitti DF, Hill R, Doi SQ, Akamizu T, Czaja J, Tao S, Koshiyama H. 1997 Differential expression of thyrotropin receptor mRNA in the porcine heart. Thyroid 7:641–646.[Medline]
  5. Kumar RS, Ijiri S, Kight K, Swanson P, Dittman A, Alok D, Zohar Y, Trant JM. 2000 Cloning and functional expression of a thyrotropin receptor from the gonads of a vertebrate (bony fish): potential thyroid-independent role for thyrotropin in reproduction. Mol Cell Endocrinol 167:1–9.[CrossRef][Medline]
  6. Sellitti DF, Dennison D, Akamizu T, Doi SQ, Kohn LD, Koshiyama H. 2000 Thyrotropin regulation of cyclic adenosine monophosphate production in human coronary artery smooth muscle cells. Thyroid 10:219–225.[Medline]
  7. Wall J. 1995 Extrathyroidal manifestation of Graves’ disease. J Clin Endocrinol Metab 80:3427–3429.[Medline]