Lipids in rat liver submitted to acute and chronic hyperthyroidism

Role of Hormones During Gestation and Early Development: Pathways Involved in Developmental Programming

Accumulating evidence suggests that an altered maternal milieu and environmental insults during the intrauterine and perinatal periods of life affect the developing organism, leading to detrimental long-term outcomes and often to adult pathologies through programming effects. Hormones, together with growth factors, play critical roles in the regulation of maternal-fetal and maternal-neonate interfaces, and alterations in any of them may lead to programming effects on the developing organism. In this chapter, we will review the role of sex steroids, thyroid hormones, and insulin-like growth factors, as crucial factors involved in physiological processes during pregnancy and lactation, and their role in developmental programming effects during fetal and early neonatal life. Also, we will consider epidemiological evidence and data from animal models of altered maternal hormonal environments and focus on the role of different tissues in the establishment of maternal and fetus/infant interaction. Finally, we will identify unresolved questions and discuss potential future research directions.

High-normal thyroid function predicts incident non-alcoholic fatty liver disease among middle-aged and elderly euthyroid subjects

BACKGROUND

Thyroid hormones (THs) influence hepatic lipid homeostasis through multiple pathways, suggesting that THs may predict the risk of non-alcoholic fatty liver disease (NAFLD). However, prospective studies on the association between THs levels and incident NAFLD in euthyroid subjects are limited. This prospective cohort study aimed to explore whether THs were associated with the development of NAFLD in middle-aged and elderly euthyroid subjects.

METHODS

A total of 6,462 subjects without baseline NAFLD were included in the cohort study (~6-year follow-up period, median: 4.2 years). Chemiluminescence immunoassay was used to measure serum free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH), and NAFLD was diagnosed by abdominal ultrasonography. Multivariable Cox proportional hazards regression models were used to assess the association between baseline THs, TSH, and the risk of NAFLD.

RESULTS

During the follow-up period, 1,675 subjects developed NAFLD. The incidence rate of NAFLD was 85.0 per 1000 person-years. Compared with the lowest FT3, FT4, and TSH quartiles, the multivariable-adjusted hazard ratios (95% confidence interval) of incident NAFLD for highest quartiles were 1.30 (1.12, 1.51), 1.07 (0.93, 1.23), 0.82 (0.71, 0.95) (P <0.001, =0.56, =0.01, respectively), respectively.

CONCLUSIONS

In middle-aged and elderly euthyroid subjects, high-normal FT3 and low-normal TSH are independently associated with a higher incidence of NAFLD.

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

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 μg/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 (TRα and TRβ) mRNAs decreased slightly after chronic T3 treatment, only TRβ protein decreased before returning to basal expression level after withdrawal. The expression of other regulators of TH action was unchanged. TRβ 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 TRβ 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 TRβ protein, target gene expression, and metabolic adaptive changes can occur in individual tissues without necessarily being reflected by circulating TH and TSH concentrations.

Desensitization and Incomplete Recovery of Hepatic Target Genes After Chronic Thyroid Hormone Treatment and Withdrawal in Male Adult Mice

Clinical symptoms may vary and not necessarily reflect serum thyroid hormone (TH) levels during acute and chronic hyperthyroidism as well as recovery from hyperthyroidism. We thus examined changes in hepatic gene expression and serum TH/TSH levels in adult male mice treated either with a single T3 (20 μg per 100 g body weight) injection (acute T3) or daily injections for 14 days (chronic T3) followed by 10 days of withdrawal. Gene expression arrays from livers harvested at these time points showed that among positively-regulated target genes, 320 were stimulated acutely and 429 chronically by T3. Surprisingly, only 69 of 680 genes (10.1%) were induced during both periods, suggesting desensitization of the majority of acutely stimulated target genes. About 90% of positively regulated target genes returned to baseline expression levels after 10 days of withdrawal; however, 67 of 680 (9.9%) did not return to baseline despite normalization of serum TH/TSH levels. Similar findings also were observed for negatively regulated target genes. Chromatin immunoprecipitation analysis of representative positively regulated target genes suggested that acetylation of H3K9/K14 was associated with acute stimulation, whereas trimethylation of H3K4 was associated with chronic stimulation. In an in vivo model of chronic intrahepatic hyperthyroidism since birth, adult male monocarboxylate transporter-8 knockout mice also demonstrated desensitization of most acutely stimulated target genes that were examined. In summary, we have identified transcriptional desensitization and incomplete recovery of gene expression during chronic hyperthyroidism and recovery. Our findings may be a potential reason for discordance between clinical symptoms and serum TH levels observed in these conditions.

Reduction of mammary and liver lipogenesis and alteration of milk composition during lactation in rats by hypothyroidism

OBJECTIVE

The profound impairment in litter growth produced by untreated maternal hypothyroidism (HypoT) may be a consequence of maternal metabolic dysfunctions affecting lactation. In this work we studied the effects of HypoT on mammary and liver lipid metabolism and its consequences on milk quality.

DESIGN

We studied the effects of prolonged 6-propyl-2-thiouracil (PTU)-induced HypoT (0.01% PTU in drinking water starting 8 days before mating until sacrifice) on milk macronutrient composition, liver and mammary lipid metabolism and content and serum lipid, and glucose and insulin concentrations in rats on days 7, 15 (L15), and 20 (L20) of lactation. Mammary and hepatic mRNA abundances of lipogenic enzymes were measured using semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) on L15 and L20.

MAIN OUTCOME

Milk lactose and triglycerides (TG) were reduced by HypoT, as well as mammary acetyl CoA carboxylase (ACC) activity on L15 and L20, and ACC and lipoprotein lipase (LPL) mRNA on L20. HypoT also decreased hepatic ACC activity on both days, ACC mRNA on L15 and liver [(3)H]H(2)O incorporation to TGs and TG content on L20. HypoT diminished insulinemia, increased serum total lipids, and decreased serum TGs on some or all the days of lactation studied.

CONCLUSION

HypoT produces a drastic decrease in milk TGs; the main cause for this seems to be the decreases in liver TG synthesis and in circulating TGs, which, along with reduced mammary uptake of fatty acids caused by decreased LPL expression and possibly diminished mammary lipogenesis, result in an impaired mammary output of TGs to the milk. Thus, the impaired growth of the litters of HypoT mothers can be largely attributed to the low milk quality along with the impaired milk ejection.

Thyroid hormone regulates the hypotriglyceridemic gene APOA5

The apolipoprotein AV gene (APOA5) is a key determinant of plasma triglyceride levels, a major risk factor for coronary artery disease and a biomarker for the metabolic syndrome. Since thyroid hormones influence very low density lipoprotein triglyceride metabolism and clinical studies have demonstrated an inverse correlation between thyroid status and plasma triglyceride levels, we examined whether APOA5 is regulated by thyroid hormone. Here we report that 3,5,3'-triiodo-L-thyronine (T3) and a synthetic thyroid receptor beta (TRbeta) ligand increase APOA5 mRNA and protein levels in hepatocytes. Our data revealed that T3-activated TR directly regulates APOA5 promoter through a functional direct repeat separated by four nucleotides (DR4). Interestingly, we show that upstream stimulatory factor 1, a transcription factor associated with familial combined hyperlipidemia and elevated triglyceride levels in humans, and upstream stimulatory factor 2 cooperate with TR, resulting in a synergistic activation of APOA5 promoter in a ligand-dependent manner via an adjacent E-box motif. In rats, we observed that apoAV levels declines with thyroid hormone depletion but returned to normal levels upon T3 administration. In addition, treatments with a TRbeta-selective agonist increased apoAV and diminished triglyceride levels. The identification of APOA5 as a T3 target gene provides a new potential mechanism whereby thyroid hormones can influence triglyceride homeostasis. Additionally, these data suggest that TRbeta may be a potential pharmacological target for the treatment of hypertriglyceridemia.

Effects of hypothyroidism on mammary and liver lipid metabolism in virgin and late-pregnant rats

Untreated maternal hypothyroidism (hypoT) has serious consequences in offspring development that may result from the effect on lactation of maternal metabolism dysfunction. We studied the effects of prolonged propylthiouracil (PTU)-induced hypoT (0.1% PTU in drinking water starting 8 days before mating until day 21 of pregnancy or for 30 days in virgin rats) on liver and mammary lipid metabolism and serum lipid concentrations. In virgins, hypoT reduced hepatic mRNAs associated with triglyceride (TG) and cholesterol synthesis (including fatty acid synthase and 3-hydroxy-3-methylglutaryl coenzyme A reductase), and induced lobuloalveolar mammary development. Pregnancy increased hepatic mRNAs associated with TG and cholesterol synthesis and uptake (including LDL receptor) and with lipid oxidation, such as acyl CoA oxidase. HypoT decreased mRNAs and the activity of proteins associated with TG synthesis, and mRNAs associated with cholesterol uptake and lipid oxidation. Pregnancy increased mammary mRNAs related to lipid oxidation and decreased cholesterol synthesis, whereas hypoT decreased mRNAs and activities of proteins associated with TG synthesis and decreased epithelial mammary tissue. Virgin and pregnant hypoT rats had increased circulating VLDL + LDL cholesterol. HypoT decreased circulating TGs in pregnant rats. The observed effects of hypoT may result in decreased mammary lipid availability. This, along with the decreased epithelial mammary tissue during lactogenesis, may contribute to the future lactational deficit of hypoT mothers.

Hyperthyroidism affects lipid metabolism in lactating and suckling rats

Two per thousand pregnant women have hyperthyroidism (HT), and although the symptoms are attenuated during pregnancy, they rebound after delivery, affecting infant development. To examine the effects of hyperthyroidism on lactation, we studied lipid metabolism in maternal mammary glands and livers of hyperthyroid rats and their pups. Thyroxine (10 microg/100 g body weight/d) or vehicle-treated rats were made pregnant 2 wk after commencement of treatment and sacrificed on days 7, 14, and 21 of lactation with the litters. Circulating triiodothyronine and tetraiodothyronine concentrations in the HT mothers were increased on all days. Hepatic esterified cholesterol (EC) and free cholesterol (FC) and triglyceride (TG) concentrations were diminished on days 14 and 21. Lipid synthesis, measured by incorporation of [3H]H2O into EC, FC, and TG, fatty acid synthase, and acetyl CoA carboxylase activities increased at day 14, while incorporation into FC and EC decreased at days 7 and 21, respectively. Mammary FC and TG concentrations were diminished at day 14; incorporation of [3H]H2O into TG decreased at days 7 and 21, and incorporation of [3H]H2O into FC increased at day 14. In the HT pups, growth rate was diminished, tetraiodothyronine concentration rose at days 7 and 14 of lactation, and triiodothyronine increased only at day 14. Liver TG concentrations increased at day 7 and fell at day 14, while FC increased at day 14 and only acetyl CoA carboxylase activity fell at day 14. Thus, hyperthyroidism changed maternal liver and mammary lipid metabolism, with decreased lipid concentration in spite of increased liver rate of synthesis and decreases in mammary synthesis. These changes, along with the mild hyperthyroidism of the litters, may have contributed to their reduced growth rate.