Changes in Hepatic TRβ Protein Expression, Lipogenic Gene Expression, and Long-Chain Acylcarnitine Levels During Chronic Hyperthyroidism and Triiodothyronine Withdrawal in a Mouse Model

Abstract

Background: Thyroid hormone (TH) has important roles in regulating hepatic metabolism. It was previously reported that most hepatic genes activated by a single triiodothyronine (T3) injection became desensitized after multiple injections, and that approximately 10% of target genes did not return to basal expression levels after T3 withdrawal, despite normalization of serum TH and thyrotropin (TSH) levels. To determine the possible mechanism(s) for desensitization and incomplete recovery of hepatic target gene transcription and their effects on metabolism, mRNA and/or protein expression levels of key regulators of TH action were measured, as well as metabolomic changes after chronic T3 treatment and withdrawal. Methods: Adult male mice were treated with daily injections of T3 (20 lg/100 g body weight) for 14 days followed by the cessation of T3 for 10 days. Livers were harvested at 6 hours, 24 hours, and 14 days after the first T3 injection, and at 10 days after withdrawal, and then analyzed by quantitative reverse transcription polymerase chain reaction, Western blotting, and metabolomics. Results: Although TH receptor (TRa and TRb) mRNAs decreased slightly after chronic T3 treatment, only TRb protein decreased before returning to basal expression level after withdrawal. The expression of other regulators of TH action was unchanged. TRb protein expression was also decreased in adult male monocarboxylate transporter-8 (Mct8)-knockout mice, an in vivo model of chronic intrahepatic hyperthyroidism. Previously, increased hepatic long-chain acylcarnitine levels were found after acute TH treatment. However, in this study, long-chain acylcarnitine levels were unchanged after chronic T3, and paradoxically increased after T3 withdrawal. Pathway analyses of the previous microarray results showed upregulation of lipogenic genes after acute T3 treatment and withdrawal. Phosphorylation of acetyl-CoA carboxylase also decreased after T3 withdrawal. Conclusions: Decreased hepatic TRb protein expression occurred after chronic T3 exposure in adult male wild-type and Mct8-knockout mice. Gene array pathway and metabolomics analyses showed abnormalities in hepatic lipogenic gene expression and acylcarnitine levels, respectively, after withdrawal, despite normalization of serum TSH and TH levels. These findings may help explain the variable clinical presentations of some patients during hyperthyroidism and recovery, since TRb protein, target gene expression, and metabolic adaptive changes can occur in individual tissues without necessarily being reflected by circulating TH and TSH concentrations.

Effects of fluoride on morphology, growth, development, and thyroid hormone of Chinese toad (Bufo gargarizans) embryos

“…In the present study, fluoride treatment could significantly upregulate TRβ mRNA and significantly downregulate TRα expression on 41.9 and 62.7 mg/L groups. Although numerous studies have determined that the expression TRβ gene was induced by T3 (Fini et al, ; Hinther et al, ; Liu et al, ), Ohba et al () found that T3 treatment decreased the expression TRβ gene in mouse. In addition, TRα knockdown caused an increase expression of TRβ mRNA in Xenopus tropicalis tadpoles (Wen and Shi, ).…”

An Update on the Pathophysiology and Diagnosis of Inappropriate Secretion of Thyroid-Stimulating Hormone

“…Based on these findings, different chromatin modifications of target genes could play an important role in regulating transcription during acute and chronic thyroid hormone treatments. We have also reported that thyroid hormones reduce hepatic TRβ protein expression in a time-dependent manner [ 35 ]. Taken together, time-dependent changes in transcriptional mechanisms may partly explain the hysteresis of thyroid hormone-responsive genes.…”

Novel Transcriptional Mechanisms for Regulating Metabolism by Thyroid Hormone

“…Thus, the TH regulation of transcription may be more complex than originally thought and may undergo temporal changes depending upon its ability to express secondary transcriptional regulators in addition to its own direct transcriptional regulation. In this connection, it is noteworthy that more than 90% of the hepatic genes that were positively- or negatively-regulated by TH were different in mice injected daily with T 3 for 10 days than those seen in mice only 6 hours after one injection with T 3 [ 21 , 77 ]. These findings suggested that there may be a secondary induction of other transcription factors that regulated the transcription of downstream target genes.…”