TSH Elicits Cell-Autonomous, Biphasic Responses: A Mechanism Inhibiting Hyperstimulation Free

Negative feedback inhibition is a well-appreciated mechanism by which thyroid gland function is regulated to maintain the euthyroid state. TSH from the pituitary stimulates thyroid hormone production/secretion from the thyroid gland and circulating thyroid hormones feed back on the pituitary thyrotroph to inhibit TSH production/secretion. This type of regulation, therefore, is dependent on 2 cell types: the thyroid cell and the thyrotroph. A type of thyroid cell independent regulation of thyroid hormone production/secretion has been reported previously that involves feedback inhibition by thyroglobulin (1). In some cell systems, biphasic dose responses are generated that exhibit escalation with increasing low doses of stimulus followed by progressive inhibition at higher stimulus doses in a single cell type, that is, are cell-autonomous (2). These responses are termed inverted U-shaped dose response curves (IUDRCs) that appear to function to prevent hyperstimulation of the physiologic response system. Cell-autonomous IUDRCs may be initiated by G protein-coupled receptors (GPCRs). In 1 type of IUDRC, there are 2 or more GPCR subtypes expressed in a single cell type that are activated by different doses of agonist leading to the integration of 2 or more monophasic dose response curves that occur through different signal transduction pathways affecting a common endpoint with opposing effects. In a second type, a single GPCR is activated at different doses by a single agonist to signal via more than 1 signaling pathway. TSH receptor (TSHR) is a GPCR that has been shown to signal via all 4 classes of G proteins with different TSH dose-dependencies for their regulation of cellular responses (3).

Cell-autonomous, biphasic regulation of thyroid cells, generating IUDRCs by different doses of TSH, has been shown for activation of TSHRs (4-7). Morgan et al (7) were the first to demonstrate that TSH generated an IUDRC when regulating the expression of a thyroid-specific gene, the sodium-iodide symporter, in normal human thyroid cells in primary cultures. Jang et al (5) confirmed these IUDRCs in regulation of other thyroid-specific genes including thyroglobulin, thyroid peroxidase, iodothyronine deiodinase 2, and TSHR. However, because TSHR signals by coupling to G proteins of all 4 classes and to beta-arrestins (8), and there are multiple steps between TSHR activation and regulation of gene expression, the signaling pathways that mediate the IUDRCs involving gene expression in human thyrocytes have not yet been identified.

In contrast to regulation of gene expression, the initial steps in TSH regulation of cAMP production involves only TSHR, Gs and Gi proteins, and adenylyl cyclase. Therefore, it was likely to be simpler to identify the pathways involved in both phases of the IUDRC for regulation of cAMP production. Laurent et al (6) were the first to demonstrate that increasing doses of TSH generated an IUDRC for cAMP production in normal human thyroid tissue slices in vitro. TSHR levels in these fresh tissue slices were likely similar to those present in vivo. Boutin et al (4) reported that TSHR levels in thyroid tissue are 100-fold higher than in cell culture, and most importantly, in vivo receptor expression levels, achieved through adenovirus-mediated overexpression of human TSHR, is required for an IUDRC for cAMP production in vitro. Moreover, this study showed that the two phases of the IUDRC were mediated by 2 different G proteins—the upward slope (low TSH doses/cAMP increase) was mediated by Gs and the downward slope (high TSH doses/cAMP inhibition) by Gi/o.

In conclusion, a thyroid cell-autonomous, biphasic dose response to TSH has been shown to regulate cAMP production and transcription of genes involved in thyroid hormone biosynthesis in normal human thyrocytes in primary culture. This biphasic response to TSH may be another fine-tuning regulatory mechanism, in addition to the hypothalamic-pituitary-thyroid axis negative feedback and inhibition by thyroglobulin, to limit hyperstimulation of thyroid gland function. We believe these findings that TSH regulates thyroid gland function by an alternative cell-autonomous mechanism in primary cultures of human thyrocytes further support the idea that this regulation may be important in humans.

Acknowledgments

Financial Support: This research was supported by the Intramural Research Program of the National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health (Z01 DK011006).

Additional Information

Disclosure Summary: The authors have nothing to disclose.

Data Availability: Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

References