Presence of thyrotropin receptor in hepatocytes: not a case of illegitimate transcription.

The function of thyrotropin (TSH) in the thyroid gland is mediated by thyrotropin receptor (TSHR). In addition to the thyroid, TSHR expression has been described in some non-thyroidal tissues, although it is uncertain whether TSHR is present in hepatocytes. One study has reported hepatic expression of TSHR mRNA, but this was considered to be because of illegitimate transcription, and there has not been a study investigating its protein expression and function in hepatocytes. Here, we examined the expression of TSHR in human and rat liver tissues, as well as human normal hepatocyte cell line L-02. Our results demonstrated that hepatic TSHR mRNA could be detected and had the same sequence as that of thyroid-derived mRNA. TSHR protein was also expressed and mainly located in the hepatocyte cell membrane. Moreover, bovine TSH and immunoglobulin from sera of patients with Graves' disease stimulated cAMP production in these cells. Taken together, these data show that TSHR is present and functional in hepatocytes, and this expression is not a case of illegitimate transcription. Given the pivotal role of the liver in body metabolism and many human diseases, our findings provide important implications for a potentially novel physiopathological role of TSH via acting on the TSHR in hepatocytes besides its classical role in regulating the thyroid function.