Pituitary TSH controls bile salt synthesis
Authors: Peter L.M. Jansen, Frank G. Schaap
Journal: Journal of Hepatology (2015)

Thyroid-stimulating hormone regulates hepatic bile acid homeostasis via SREBP-2/HNF-4α/CYP7A1 axisJournal of HepatologyVol. 62Issue 5PreviewBile acids (BAs) play a crucial role in dietary fat digestion and in the regulation of lipid, glucose, and energy metabolism. Thyroid-stimulating hormone (TSH) is a hormone produced by the anterior pituitary gland that directly regulates several metabolic pathways. However, the impact of TSH on BA homeostasis remains largely unknown. Full-Text PDF Binding of thyroid-stimulating hormone (TSH, thyrotropin) to its receptor (TSHR) on the surface of thyroid follicular cells stimulates the formation and release of the active thyroid hormone (TH) triiodothyronine (T3), and its precursor, the prohormone thyroxine (T4) [1Brent G.A. Mechanisms of thyroid hormone action.J Clin Invest. 2012; 122: 3035-3043Crossref PubMed Scopus (622) Google Scholar, 2Davies T.F. Ando T. Lin R.Y. Tomer Y. Latif R. Thyrotropin receptor-associated diseases: from adenomata to Graves disease.J Clin Invest. 2005; 115: 1972-1983Crossref PubMed Scopus (211) Google Scholar]. THs affect numerous biological processes through genomic as well as nongenomic actions involving nuclear TH receptors of the NR1A subfamily and specific members of the integrin family of plasma membrane receptors, respectively [[1]Brent G.A. Mechanisms of thyroid hormone action.J Clin Invest. 2012; 122: 3035-3043Crossref PubMed Scopus (622) Google Scholar]. The liver is an important but somewhat undervalued target organ for TH action. Although most abundantly expressed in the thyroid gland, TSHR protein expression is detected amongst others in (pre)adipocytes, bone and kidney and this has led several groups to explore extrathyroidal actions of TSH [[2]Davies T.F. Ando T. Lin R.Y. Tomer Y. Latif R. Thyrotropin receptor-associated diseases: from adenomata to Graves disease.J Clin Invest. 2005; 115: 1972-1983Crossref PubMed Scopus (211) Google Scholar]. In this issue, the group of Jiajun Zhao, which first reported on functional TSHR signaling in the liver [3Zhang W. Tian L.M. Han Y. Ma H.Y. Wang L.C. Guo J. et al.Presence of thyrotropin receptor in hepatocytes: not a case of illegitimate transcription.J Cell Mol Med. 2009; 13: 4636-4642Crossref PubMed Scopus (85) Google Scholar, 4Tian L. Song Y. Xing M. Zhang W. Ning G. Li X. et al.A novel role for thyroid-stimulating hormone: up-regulation of hepatic 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase expression through the cyclic adenosine monophosphate/protein kinase A/cyclic adenosine monophosphate-responsive element binding protein pathway.Hepatology. 2010; 52: 1401-1409Crossref PubMed Scopus (72) Google Scholar, 5Yan F. Wang Q. Lu M. Chen W. Song Y. Jing F. et al.Thyrotropin increases hepatic triglyceride content through upregulation of SREBP-1c activity.J Hepatol. 2014; 61: 1358-1364Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar], describes a direct effect of TSH on bile salt synthesis [[6]Song Y. Xu C. Shao S. Liu J. Xing W. Xu J. et al.Thyroid-stimulating hormone (TSH) regulates hepatic bile acid homeostasis via SREBP-2/HNF-4alpha/CYP7A1 axis.J Hepatol. 2015; 62: 1171-1179Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar]. Bile salts act as emulsifiers of lipids, and as such are crucial for solubilization of biliary cholesterol and effective processing of diet-derived lipids. Levels of these biological detergents must be tightly controlled to prevent detrimental effects outside the gastrointestinal tract. In the liver, cholesterol is converted into bile salt conjugates by sequential action of at least 16 enzymes, and this pathway provides the major route for disposal of excess cholesterol [[7]Russell D.W. The enzymes, regulation, and genetics of bile acid synthesis.Annu Rev Biochem. 2003; 72: 137-174Crossref PubMed Scopus (1427) Google Scholar]. Bile salt synthesis proceeds via either two routes, the classical pathway initiated by cholesterol-7α-hydroxylase (encoded by the CYP7A1 gene), and the acidic pathway initiated by enzymes that hydroxylate the side chain of cholesterol [[7]Russell D.W. The enzymes, regulation, and genetics of bile acid synthesis.Annu Rev Biochem. 2003; 72: 137-174Crossref PubMed Scopus (1427) Google Scholar]. The classical pathway accounts for the bulk of newly synthesized bile salts in rodents and human, with regulation exerted primarily at the level of CYP7A1 transcription [7Russell D.W. The enzymes, regulation, and genetics of bile acid synthesis.Annu Rev Biochem. 2003; 72: 137-174Crossref PubMed Scopus (1427) Google Scholar, 8Chiang J.Y. Bile acids: regulation of synthesis.J Lipid Res. 2009; 50: 1955-1966Crossref PubMed Scopus (1127) Google Scholar]. The proximal CYP7A1 promoter contains overlapping binding sites for a number of positively and negatively acting transcription factors that together determine the overall transcriptional output. Members of the aforementioned family of nuclear receptors (NRs) that bind in this region include NR1A2/TRβ, which mediates transcriptional regulation of CYP7A1 by THs, and NR2A1/HNF4α, a liver-enriched transcription factor that determines basal expression of CYP7A1 and a large number of other liver-specific genes [[9]Gonzalez F.J. Regulation of hepatocyte nuclear factor 4 alpha-mediated transcription.Drug Metab Pharmacokinet. 2008; 23: 2-7Crossref PubMed Scopus (160) Google Scholar]. It is this latter factor that is targeted by the newly uncovered hepatic action of TSH (Fig. 1). Using complementary approaches, Song et al. provide evidence that TSH is able to repress rodent Cyp7a1 expression in a manner independent of changes in total TH levels [[6]Song Y. Xu C. Shao S. Liu J. Xing W. Xu J. et al.Thyroid-stimulating hormone (TSH) regulates hepatic bile acid homeostasis via SREBP-2/HNF-4alpha/CYP7A1 axis.J Hepatol. 2015; 62: 1171-1179Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar]. Firstly, in thyroidectomized rats with stable systemic levels of exogenously supplied T4, administration of recombinant TSH results in dose-dependent lowering of Cyp7a1 activity and reduction of systemic and hepatic bile salt levels. Secondly, in Tshr−/− mice in which hypothyroidism was corrected by supplementation with TH extract, Cyp7a1 protein is induced. Elevation of total bile salt pool size, enhanced fecal bile salt excretion and enlarged gallbladder volume is consistent with enhanced bile salt synthesis in these mice. Furthermore, liver-specific silencing of Tshr appears to result in elevation of Cyp7a1 protein and serum bile salts. Although an appropriate non-targeting siRNA control was lacking in the latter experiment, it suggests that endogenous TSH levels contribute to the regulation of bile salt synthesis via a direct effect on its hepatic receptor. In an elaborate set of in vitro experiments using human hepatoma cells, Song and colleagues went on to explore the molecular pathways underlying the Cyp7a1 repressive effect of TSH [[6]Song Y. Xu C. Shao S. Liu J. Xing W. Xu J. et al.Thyroid-stimulating hormone (TSH) regulates hepatic bile acid homeostasis via SREBP-2/HNF-4alpha/CYP7A1 axis.J Hepatol. 2015; 62: 1171-1179Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar]. Having established that dysregulated Cyp7a1 expression in Tshr−/− mice was apparently not due to alterations in the important FXR/Fgf15 regulatory axis [10Schaap F.G. Trauner M. Jansen P.L. Bile acid receptors as targets for drug development.Nat Rev Gastroenterol Hepatol. 2014; 11: 55-67Crossref PubMed Scopus (481) Google Scholar, 11Kir S. Kliewer S.A. Mangelsdorf D.J. Roles of FGF19 in liver metabolism.Cold Spring Harb Symp Quant Biol. 2011; 76: 139-144Crossref PubMed Scopus (98) Google Scholar], they observed enhanced hepatic levels of HNF4α and its target genes in knock-out animals, implying that TSH reduces HNF4α protein. This turned out to rely on activation of a PI3K-dependent pathway that led to enhanced expression of the nuclear, transcriptionally active form of SREBP2. It is currently unclear how activation of the PI3K pathway is linked to the two sequential proteolytic cleavages of SREBP2 that are required to yield mature SREBP2 [[12]Brown M.S. Goldstein J.L. The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor.Cell. 1997; 89: 331-340Abstract Full Text Full Text PDF PubMed Scopus (3029) Google Scholar]. Putative binding sites for SREBP2 were predicted in the promoter region of (human) HNF4α, and TSH promoted recruitment of SREBP2 to this region. Moreover, a repressive effect of SREBP2 on human HNF4α promoter activity was observed in reporter assays. The aggregated mechanistic findings suggest that TSH represses Cyp7a1 via SREBP2-mediated interference with HNF4α transcription. Earlier animal studies by this group revealed that hepatic TSH signaling resulted in activation of SREBP1c, a key regulator of lipogenesis [[12]Brown M.S. Goldstein J.L. The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor.Cell. 1997; 89: 331-340Abstract Full Text Full Text PDF PubMed Scopus (3029) Google Scholar]. The concurrent activation of SREBP2, which stimulates cholesterol biosynthesis and interferes with subsequent conversion of cholesterol to bile salts, indicates that hepatic TSHR activation may impact on hepatic and serum lipid homeostasis. Moreover, the targeting of HNF4α, which controls diverse hepatic processes including glucose homeostasis, drug metabolism and cellular differentiation [[9]Gonzalez F.J. Regulation of hepatocyte nuclear factor 4 alpha-mediated transcription.Drug Metab Pharmacokinet. 2008; 23: 2-7Crossref PubMed Scopus (160) Google Scholar], implies that hepatic TSH signaling may have broader consequences. Further explorations are required to assess whether hepatic TSHR activation occurs at (patho)physiological levels of TSH. It will be interesting to learn whether hepatic TSHR expression, which in humans appears at least three orders of magnitude lower than expression in the thyroid gland (see e.g., supplementary materials in Ref. [[13]Uhlen M. Fagerberg L. Hallstrom B.M. Lindskog C. Oksvold P. Mardinoglu A. et al.Proteomics. Tissue-based map of the human proteome.Science. 2015; 347: 1260419Crossref PubMed Scopus (7664) Google Scholar] or http://biogps.org/#goto=genereport&id=7253), is altered in liver disorders. Moreover, the contribution of adipose tissue, where Tshr signaling is engaged in regulation of lipolysis and inflammatory tone [2Davies T.F. Ando T. Lin R.Y. Tomer Y. Latif R. Thyrotropin receptor-associated diseases: from adenomata to Graves disease.J Clin Invest. 2005; 115: 1972-1983Crossref PubMed Scopus (211) Google Scholar, 14Gagnon A. Langille M.L. Chaker S. Antunes T.T. Durand J. Sorisky A. TSH signaling pathways that regulate MCP-1 in human differentiated adipocytes.Metabolism. 2014; 63: 812-821Abstract Full Text Full Text PDF PubMed Scopus (28) Google Scholar], should be considered in hepatic TSH action, as lipolytic products and cytokines are likely to have impact on hepatic processes. Studies employing tissue-specific Tshr−/− mice could be helpful in this respect. Importantly, as well as challenging, is to address the physiological relevance of hepatic TSH action in humans. To assess whether TSH could contribute to regulation of bile salt synthesis in humans, Song and coworkers studied diurnal changes in circulating TSH, TH and bile salts in 15 volunteers receiving standardized meals for breakfast, lunch and dinner [[6]Song Y. Xu C. Shao S. Liu J. Xing W. Xu J. et al.Thyroid-stimulating hormone (TSH) regulates hepatic bile acid homeostasis via SREBP-2/HNF-4alpha/CYP7A1 axis.J Hepatol. 2015; 62: 1171-1179Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar]. Postprandial changes in bile salts, peaking ca. 30–45 min after ingestion of each meal, were mirrored by opposite changes in TSH, with free T4 levels remaining constant throughout the 12 h study period. Levels of a serum marker for bile salt synthesis (C4) started to decline ca. 2 h after the first diurnal peak in TSH. It is reasonable to assume that CYP7A1 protein turnover must occur before transcriptional repression of the gene translates into a decline of serum C4 levels. Thus, the observed time lag appears compatible with, but is no direct evidence for, a role of TSH in repression of bile salt synthesis. Further studies with a more frequent blood sampling scheme, and simultaneous study of the intestinal FXR/FGF19 axis that is considered the major pathway controlling bile salt synthesis in the postprandial state [10Schaap F.G. Trauner M. Jansen P.L. Bile acid receptors as targets for drug development.Nat Rev Gastroenterol Hepatol. 2014; 11: 55-67Crossref PubMed Scopus (481) Google Scholar, 11Kir S. Kliewer S.A. Mangelsdorf D.J. Roles of FGF19 in liver metabolism.Cold Spring Harb Symp Quant Biol. 2011; 76: 139-144Crossref PubMed Scopus (98) Google Scholar], could help to further delineate the respective contributions. The negative association between serum bile salts and TSH uncovered in the present study raises the intriguing question whether bile salts may actually control TSH levels. Pituitary release of TSH is subject to negative feedback control by locally produced T3 [[15]Arrojo E.D.R. Fonseca T.L. Werneck-de-Castro J.P. Bianco A.C. Role of the type 2 iodothyronine deiodinase (D2) in the control of thyroid hormone signaling.Biochim Biophys Acta. 2013; 1830: 3956-3964Crossref PubMed Scopus (106) Google Scholar]. This requires the action of one of three iodothyronine deiodinases encoded by the DIO1–3 genes. These enzymes catalyze activation and inactivation of THs [[2]Davies T.F. Ando T. Lin R.Y. Tomer Y. Latif R. Thyrotropin receptor-associated diseases: from adenomata to Graves disease.J Clin Invest. 2005; 115: 1972-1983Crossref PubMed Scopus (211) Google Scholar]. DIO2 has previously been identified as a bile salt-induced gene in brown adipose tissue and skeletal muscle [[16]Watanabe M. Houten S.M. Mataki C. Christoffolete M.A. Kim B.W. Sato H. et al.Bile acids induce energy expenditure by promoting intracellular thyroid hormone activation.Nature. 2006; 439: 484-489Crossref PubMed Scopus (1617) Google Scholar], and catalyzes formation of active T3. The G-protein coupled receptor TGR5 (a.k.a. GPBAR1) confers induction of DIO2 by bile salts [[16]Watanabe M. Houten S.M. Mataki C. Christoffolete M.A. Kim B.W. Sato H. et al.Bile acids induce energy expenditure by promoting intracellular thyroid hormone activation.Nature. 2006; 439: 484-489Crossref PubMed Scopus (1617) Google Scholar]. This plasma membrane bile salt receptor is broadly expressed, including in the hypothalamus and pituitary [[17]Duboc H. Tache Y. Hofmann A.F. The bile acid TGR5 membrane receptor: from basic research to clinical application.Dig Liver Dis. 2014; 46: 302-312Abstract Full Text Full Text PDF PubMed Scopus (303) Google Scholar]. If bile salt-inducibility of DIO2 is maintained in these tissues, bile salts may regulate the negative feedback control of release of hypothalamic TSH-releasing hormone (TRH) and pituitary TSH [[15]Arrojo E.D.R. Fonseca T.L. Werneck-de-Castro J.P. Bianco A.C. Role of the type 2 iodothyronine deiodinase (D2) in the control of thyroid hormone signaling.Biochim Biophys Acta. 2013; 1830: 3956-3964Crossref PubMed Scopus (106) Google Scholar]. TGR5 may thus link circulating bile salts to peripheral TH/TSH action, with elevation of bile salts resulting in diminished TSH levels and vice versa, as is observed in the human study of Song et al. [[6]Song Y. Xu C. Shao S. Liu J. Xing W. Xu J. et al.Thyroid-stimulating hormone (TSH) regulates hepatic bile acid homeostasis via SREBP-2/HNF-4alpha/CYP7A1 axis.J Hepatol. 2015; 62: 1171-1179Abstract Full Text Full Text PDF PubMed Scopus (73) Google Scholar]. Functional consequences related to this interaction can be expected in clinical conditions that are accompanied by elevation of systemic bile salts. For instance, bile salts are elevated, along with slightly lowered TSH, after Roux-en-Y gastric bypass [18Jansen P.L. van Werven J. Aarts E. Berends F. Janssen I. Stoker J. et al.Alterations of hormonally active fibroblast growth factors after Roux-en-Y gastric bypass surgery.Dig Dis. 2011; 29: 48-51Crossref PubMed Scopus (111) Google Scholar, 19Patti M.E. Houten S.M. Bianco A.C. Bernier R. Larsen P.R. Holst J.J. et al.Serum bile acids are higher in humans with prior gastric bypass: potential contribution to improved glucose and lipid metabolism.Obesity. 2009; 17: 1671-1677Crossref PubMed Scopus (448) Google Scholar]. In cholestatic liver disease, elevated serum bile salts may repress TSH release causing a subclinical reduction of TH synthesis. This may contribute to the enigmatic but devastating symptom of fatigue in patients with cholestatic liver disease (e.g. in primary biliary cirrhosis). In conclusion, direct hepatic TSH action is identified as a novel regulator of bile salt synthesis, although its modus operandi is likely to affect additional metabolic pathways in the liver as well. The importance of hepatic TSHR signaling in the complex network of regulatory inputs (including THs) that determines CYP7A1 expression, and its integration with the bile salt-induced FGF19 signaling pathway that is considered crucial for regulation of bile salt synthesis in the postprandial state, warrants further study. The mutual interaction between bile salts and TSH via a hypothalamic-pituitary circuit is another aspect that deserves further exploration, as well as rethinking the consequences of elevation of bile salts. The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.