3,3',5-triiodo-l-thyronine inhibits drug-induced liver injury through activation of PPARα as revealed by network pharmacology and biological experimental verification

Drug-induced liver injury (DILI) has increased in recent years, leading to acute liver failure. 3,3',5-triiodo-l-thyronine (T3) has been reported to exert a potent hepatoprotective effect. However, the mechanism and efficacy of T3 on DILI remain undocumented. In this study, an MTT assay was used to detect the effect of T3 on hepatotoxicity of acetaminophen (APAP) in L02 cells. Then, we screened key targets and related biological pathways by network pharmacology. Finally, enzyme-linked immunosorbent assay (ELISA) and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) were used to verify the mechanism and key targets of T3 on DILI. The results of the MTT assay showed that T3 significantly decreased hepatocellular injury induced by APAP. Network pharmacology and bioinformatics analysis showed that 118 intersection targets of T3 and DILI were identified and the mechanism of T3 on DILI was related to cell proliferation and oxidative stress. ELISA results showed that T3 may be an effective treatment for DILI as biomarkers of hepatocellular injury such as AST, ALP were decreased compared to APAP only treated cells, and the mechanism of T3 may be mediated in part through improving redox balance. The topological parameter screening results suggested 12 key targets of T3 for DILI. Among them, PPARα is associated with DILI, and activation of PPARα can reduce oxidative stress and cell necrosis. Therefore, PPARα was identified as a target for verification. qRT-PCR analysis demonstrated that T3 could reverse the down-regulation of PPARα induced by APAP exposure. Taken together, we demonstrated for the first time that T3 could activate PPARα, promote cell proliferation and reduce oxidative stress, and play a vital role in the treatment of DILI, which provides a reference for T3 as a candidate treatment for DILI.

Canonical Thyroid Hormone Receptor β Action Stimulates Hepatocyte Proliferation in Male Mice

CONTEXT

3,5,3'-L-triiodothyronine (T3) is a potent inducer of hepatocyte proliferation via the Wnt/β-catenin signaling pathway. Previous studies suggested the involvement of rapid noncanonical thyroid hormone receptor (TR) β signaling, directly activating hepatic Wnt/β-catenin signaling independent from TRβ DNA binding. However, the mechanism by which T3 increases Wnt/β-catenin signaling in hepatocytes has not yet been determined.

OBJECTIVE

We aimed to determine whether DNA binding of TRβ is required for stimulation of hepatocyte proliferation by T3.

METHODS

Wild-type (WT) mice, TRβ knockout mice (TRβ KO), and TRβ mutant mice with either specifically abrogated DNA binding (TRβ GS) or abrogated direct phosphatidylinositol 3 kinase activation (TRβ 147F) were treated with T3 for 6 hours or 7 days. Hepatocyte proliferation was assessed by Kiel-67 (Ki67) staining and apoptosis by terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. Activation of β-catenin signaling was measured in primary murine hepatocytes. Gene expression was analyzed by microarray, gene set enrichment analysis (GSEA), and quantitative reverse transcription polymerase chain reaction.

RESULTS

T3 induced hepatocyte proliferation with an increased number of Ki67-positive cells in WT and TRβ 147F mice (9.2% ± 6.5% and 10.1% ± 2.9%, respectively) compared to TRβ KO and TRβ GS mice (1.2% ± 1.1% and 1.5% ± 0.9%, respectively). Microarray analysis and GSEA showed that genes of the Wnt/β-catenin pathway-among them, Fzd8 (frizzled receptor 8) and Ctnnb1 (β-catenin)-were positively enriched only in T3-treated WT and TRβ 147F mice while B-cell translocation gene anti-proliferation factor 2 was repressed. Consequently, expression of Ccnd1 (CyclinD1) was induced.

CONCLUSIONS

Instead of directly activating Wnt signaling, T3 and TRβ induce key genes of the Wnt/β-catenin pathway, ultimately stimulating hepatocyte proliferation via CyclinD1. Thus, canonical transcriptional TRβ action is necessary for T3-mediated stimulation of hepatocyte proliferation.

Pregnancy and weaning regulate human maternal liver size and function

During pregnancy, the rodent liver undergoes hepatocyte proliferation and increases in size, followed by weaning-induced involution via hepatocyte cell death and stromal remodeling, creating a prometastatic niche. These data suggest a mechanism for increased liver metastasis in breast cancer patients with recent childbirth. It is unknown whether the human liver changes in size and function during pregnancy and weaning. In this study, abdominal imaging was obtained in healthy women at early and late pregnancy and postwean. During pregnancy time points, glucose production and utilization and circulating bile acids were measured. Independently of weight gain, most women's livers increased in size with pregnancy, then returned to baseline postwean. Putative roles for bile acids in liver growth and regression were observed. Together, the data support the hypothesis that the human liver is regulated by reproductive state with growth during pregnancy and volume loss postwean. These findings have implications for sex-specific liver diseases and for breast cancer outcomes.

Analysis of metabolomics profile in hypothyroid patients before and after thyroid hormone replacement

PURPOSE

The serum metabolic changes occurring during the transition from hypothyroidism to euthyroidism are not known. This study aimed to determine the metabolomic profile in hypothyroid patients before (HypoT₀) and after (HypoT₁) euthyroidism achieved through levothyroxine (L-T4) treatment.

METHODS

Eighteen patients with overt primary hypothyroidism were recruited for the study. All patients were treated with L-T4 to achieve euthyroidism. Thyrotropin (TSH), free thyroxine (FT4), free triiodothyronine (FT3) and metabolomics profiles were measured before and after 3 months of treatment. The euthyroid control group consisted of 28 healthy volunteers. Metabolomics analysis was performed using Nuclear Magnetic Resonance (NMR) spectroscopy.

RESULTS

¹H NMR-based metabolomics profiling of patients with newly diagnosed hypothyroidism (HypoT₀) showed significantly higher levels of citrate, creatinine, glycerol, myo-inositol and serine, and lower levels of proline and taurine compared to controls. Interestingly, some metabolic changes were persistent three months after pharmacological treatments, despite normal serum TSH and thyroid hormone concentrations (HypoT₁). When an Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA) model was built to evaluate possible differences in the metabolic profile between HypoT₀ and HypoT₁, the data obtained were not significantly different.

CONCLUSION

These results suggest that metabolic changes in the patients with hypothyroidism may persist after normalization of serum levels of FT3, FT4, and TSH, which currently represent the gold standard in laboratory testing for diagnosis and evaluation of thyroid pathology. So, the metabolomics approach may contribute to integrate classical hormone assays and to determine the euthyroid status achievement with greater efficacy.

Hormonal Contribution to Liver Regeneration

An understanding of the molecular basis of liver regeneration will open new horizons for the development of novel therapies for chronic liver failure. Such therapies would solve the drawbacks associated with liver transplant, including the shortage of donor organs, long waitlist time, high medical costs, and lifelong use of immunosuppressive agents. Regeneration after partial hepatectomy has been studied in animal models, particularly fumarylacetoacetate hydrolase-deficient (FAH ^-/-) mice and pigs. The process of regeneration is distinctive, complex, and well coordinated, and it depends on the interplay among several signaling pathways (eg, nuclear factor κβ, Notch, Hippo), cytokines (eg, tumor necrosis factor α, interleukin 6), and growth factors (eg, hepatocyte growth factor, epidermal growth factor, vascular endothelial growth factor), and other components. Furthermore, endocrinal hormones (eg, norepinephrine, growth hormone, insulin, thyroid hormones) also can influence the aforementioned pathways and factors. We believe that these endocrinal hormones are important hepatic mitogens that strongly induce and accelerate hepatocyte proliferation (regeneration) by directly and indirectly triggering the activity of the involved signaling pathways, cytokines, growth factors, and transcription factors. The subsequent induction of cyclins and associated cyclin-dependent kinase complexes allow hepatocytes to enter the cell cycle. In this review article, we comprehensively summarize the current knowledge regarding the roles and mechanisms of these hormones in liver regeneration. Articles used for this review were identified by searching MEDLINE and EMBASE databases from inception through June 1, 2019.

Potential role of two novel agonists of thyroid hormone receptor-β on liver regeneration

OBJECTIVES

Although the hepatomitogenic activity of triiodothyronine (T3) is well established, the wide range of harmful effects exerted by this hormone precludes its use in liver regenerative therapy. Selective agonists of the beta isoform of thyroid hormone receptor (TRβ) do not exhibit T3-induced cardiotoxicity and show a good safety profile in patients with NASH. The aim of this study was to investigate whether two novel TRβ agonists, the prodrug TG68 and the active compound IS25 could stimulate hepatocyte proliferation without T3/TRα-dependent side effects.

METHODS

Rats were treated with three different doses (12.5, 25 and 50 μg/100 g body weight) for one week. Hepatocyte proliferation, liver injury and serum biochemical parameters were measured by immunohistochemistry, qRT-PCR and Western blot.

RESULTS

Both drugs increased hepatocyte proliferation as assessed by bromodeoxyuridine incorporation (from 14% to 28% vs 5% of controls) and mitotic activity. Enhanced proliferation occurred in the absence of significant signs of liver injury as shown by lack of increased serum transaminase levels or of apoptosis. No cardiac or renal hypertrophy typically associated with treatment with T3 was observed. Importantly, no proliferation of pancreatic acinar cells, such as that seen after administration of T3 or the TRβ agonist GC1 was detected following either TG68 or IS25, demonstrating the hepato-specificity of these novel TRβ agonists.

CONCLUSIONS

The present study shows that TG68 and IS25 induce massive hepatocyte proliferation without overt toxicity. Hence, these agents may have a significant clinical application for regenerative therapies in liver transplantation or other surgical settings.

Can hypothyroidism be a protective factor for hepatocellular carcinoma in cirrhosis?

Despite many studies, the molecular mechanisms of hepatocellular carcinoma (HCC) development remain unclear. Thyroid hormone (TH) levels may vary in many chronic diseases including cirrhosis. The aim of this study was to evaluate TH status in patients with cirrhosis and HCC and to investigate the relationship between THs and HCC development.Five hundred seventy-seven patients with cirrhosis who applied to Demiroğlu Bilim University, Faculty of Medicine, Gastroenterology Department between 2004 and 2019 were included the study. Three hundred sixty-seven patients who applied to Internal Medicine Unit for general health check-up were included in the study as healthy control group. Demographic, laboratory, and imaging findings of study groups were retrospectively reviewed and recorded from hospital information system.In the cirrhosis group, 252 patients had HCC (43.67%), and 325 patients had non-HCC cirrhosis (56.33%). Free thyroxine (FT4) levels were higher in the control group than in the cirrhotic group but there was no significant difference (P = .501). Thyroid-stimulating hormone (TSH) and FT4 levels were similar between groups, while free triiodothyronine (FT3) levels were significantly different between HCC group, non-HCC cirrhosis group, and control group (P = .299 for TSH, P = .263 for FT4, P < .001 for FT3). FT3 levels were significantly higher in HCC group than non-HCC cirrhosis group, but significantly lower than control group (P < .05).Our study confirmed the presence of hypothyroidism in cirrhosis patients and clearly demonstrated a strong relationship between FT3 levels and HCC development.

Heme oxygenase-1 inhibitor tin-protoporphyrin improves liver regeneration after partial hepatectomy

AIMS

This study investigates the effects of the heme oxygenase-1 (HO-1) inhibitor tin protoporphyrin IX (SnPP), on rat liver regeneration following 2/3 partial hepatectomy (PH) in order to clarify the controversial role of HO-1 in the regulation of cellular growth.

MAIN METHODS

Male Wistar rats received a subcutaneous injection of either SnPP (10 μmoles/kg body weight) or saline 12 h before PH and 0, 12 and 24 h after surgery. Rats were killed from 0.5 to 36 h after PH. Bromodeoxyuridine (BrdU) incorporation was used to analyze cell proliferation. Immunohistochemistry, Western blot analysis and quantitative Real Time-PCR were used to assess molecular and cellular changes after PH.

KEY FINDINGS

Data obtained have shown that administration of SnPP caused an increased entry of hepatocytes into S phase after PH, as demonstrated by labeling (L.I.) and mitotic (M.I.) indexes. Furthermore, enhanced cell cycle entry in PH-animals pre-treated with SnPP was associated with an earlier activation of IL-6 and transcription factors involved in liver regeneration, such as phospho-JNK and phospho-STAT3.

SIGNIFICANCE

Summarizing, data here reported demonstrate that inhibition of HO-1 enhances rat liver regeneration after PH which is associated to a very rapid increase in the levels of inflammatory mediators such as IL-6, phopsho-JNK and phospho-STAT3, suggesting that HO-1 could act as a negative modulator of liver regeneration. Knowledge about the mechanisms of liver regeneration can be applied to clinical problems caused by delayed liver growth, and HO-1 repression may be a mechanism by which cells can faster proliferate in response to tissue damage.

GC-1: A Thyromimetic With Multiple Therapeutic Applications in Liver Disease

Thyroid hormones (THs), namely, 3,5,3'-triiodo-l-thyronine (T3) and 3,5,3',5'-tetraiodo-l-thyronine (thyroxine or T4), influence a variety of physiological processes that have important implications in fetal development, metabolism, cell growth, and proliferation. While THs elicit several beneficial effects on lipid metabolism and improve myocardial contractility, these therapeutically desirable effects are associated to a thyrotoxic state that severely limits the possible use of THs as therapeutic agents. Therefore, several efforts have been made to develop T3 analogs that could retain the beneficial actions (triglyceride, cholesterol, obesity, and body mass lowering) without the adverse TH-dependent side effects. This goal was achieved by the synthesis of TRβ-selective agonists. In this review, we summarize the current knowledge on the effects of one of the best characterized TH analogs, the TRβ1-selective thyromimetic, GC-1. In particular, we review some of the effects of GC-1 on different liver disorders, with reference to its possible clinical application. A brief comment on the possible therapeutic use of GC-1 in extrahepatic disorders is also included.

Thyroid Hormone Receptor-β Agonist GC-1 Inhibits Met-β-Catenin-Driven Hepatocellular Cancer

The thyromimetic agent GC-1 induces hepatocyte proliferation via Wnt/β-catenin signaling and may promote regeneration in both acute and chronic liver insufficiencies. However, β-catenin activation due to mutations in CTNNB1 is seen in a subset of hepatocellular carcinomas (HCC). Thus, it is critical to address any effect of GC-1 on HCC growth and development before its use can be advocated to stimulate regeneration in chronic liver diseases. In this study, we first examined the effect of GC-1 on β-catenin-T cell factor 4 activity in HCC cell lines harboring wild-type or mutated-CTNNB1. Next, we assessed the effect of GC-1 on HCC in FVB mice generated by hydrodynamic tail vein injection of hMet-S45Y-β-catenin, using the sleeping beauty transposon-transposase. Four weeks following injection, mice were fed 5 mg/kg GC-1 or basal diet for 10 or 21 days. GC-1 treatment showed no effect on β-catenin-T cell factor 4 activity in HCC cells, irrespective of CTNNB1 mutations. Treatment with GC-1 for 10 or 21 days led to a significant reduction in tumor burden, associated with decreased tumor cell proliferation and dramatic decreases in phospho-(p-)Met (Y1234/1235), p-extracellular signal-related kinase, and p-STAT3 without affecting β-catenin and its downstream targets. GC-1 exerts a notable antitumoral effect on hMet-S45Y-β-catenin HCC by inactivating Met signaling. GC-1 does not promote β-catenin activation in HCC. Thus, GC-1 may be safe for use in inducing regeneration during chronic hepatic insufficiency.

The Thyromimetic KB2115 (Eprotirome) Induces Rat Hepatocyte Proliferation

Although the hepatomitogenic activity of T3 is well established, the wide range of harmful effects exerted by this hormone precludes its use in regenerative therapy. The aim of this study was to investigate whether an agonist of TRβ, KB2115 (Eprotirome), could exert a mitogenic effect in the liver, without most of the adverse T3/TRα-dependent side effects. F-344 rats treated with KB2115 for 1 week displayed a massive increase in bromodeoxyuridine incorporation (from 20% to 40% vs. 5% of controls), which was associated with increased mitotic activity in the absence of significant signs of liver toxicity. Noteworthy, while cardiac hypertrophy typical of T3 was not observed, beneficial effects, such as lowering blood cholesterol levels, were associated to KB2115 administration. Following a single dose of KB2115, hepatocyte proliferation was evident as early as 18 h, demonstrating its direct mitogenic effect. No increase in serum transaminase levels or apoptosis was observed prior to or concomitantly with the S phase. While KB2115-induced mitogenesis was not associated to enhance expression of c-fos, c-jun, and c-myc, cyclin D1 levels rapidly increased. In conclusion, KB2115 induces hepatocyte proliferation without overt toxicity. Hence, this agent may be useful for regenerative therapies in liver transplantation or other surgical settings.

Thyroid Hormone Receptor β Agonist Induces β-Catenin-Dependent Hepatocyte Proliferation in Mice: Implications in Hepatic Regeneration

Triiodothyronine (T3) induces hepatocyte proliferation in rodents. Recent work has shown molecular mechanism for T3's mitogenic effect to be through activation of β-catenin signaling. Since systemic side effects of T3 may preclude its clinical use, and hepatocytes mostly express T3 hormone receptor β (TRβ), we investigated if selective TRβ agonists like GC-1 may also have β-catenin-dependent hepatocyte mitogenic effects. Here we studied the effect of GC-1 and T3 in conditional knockouts of various Wnt pathway components. We also assessed any regenerative advantage of T3 or GC-1 when given prior to partial hepatectomy in mice. Mice administered GC-1 showed increased pSer675-β-catenin, cyclin D1, BrdU incorporation, and PCNA. No abnormalities in liver function tests were noted. GC-1-injected liver-specific β-catenin knockouts (β-catenin LKO) showed decreased proliferation when compared to wild-type littermates. To address if Wnt signaling was required for T3- or GC-1-mediated hepatocyte proliferation, we used LRP5-6-LKO, which lacks the two redundant Wnt coreceptors. Surprisingly, decreased hepatocyte proliferation was also evident in LRP5-6-LKO in response to T3 and GC-1, despite increased pSer675-β-catenin. Further, increased levels of active β-catenin (hypophosphorylated at Ser33, Ser37, and Thr41) were evident after T3 and GC-1 treatment. Finally, mice pretreated with T3 or GC-1 for 7 days followed by partial hepatectomy showed a significant increase in hepatocyte proliferation both at the time (T0) and 24 h after surgery. In conclusion, like T3, TRβ-selective agonists induce hepatocyte proliferation through β-catenin activation via both PKA- and Wnt-dependent mechanisms and confer a regenerative advantage following surgical resection. Hence, these agents may be useful regenerative therapies in liver transplantation or other surgical settings.

A thyroid hormone receptor/KLF9 axis in human hepatocytes and pluripotent stem cells

Biological processes require close cooperation of multiple transcription factors that integrate different signals. Thyroid hormone receptors (TRs) induce Krüppel-like factor 9 (KLF9) to regulate neurogenesis. Here, we show that triiodothyronine (T3) also works through TR to induce KLF9 in HepG2 liver cells, mouse liver, and mouse and human primary hepatocytes and sought to understand TR/KLF9 network function in the hepatocyte lineage and stem cells. Knockdown experiments reveal that KLF9 regulates hundreds of HepG2 target genes and modulates T3 response. Together, T3 and KLF9 target genes influence pathways implicated in stem cell self-renewal and differentiation, including Notch signaling, and we verify that T3 and KLF9 cooperate to regulate key Notch pathway genes and work independently to regulate others. T3 also induces KLF9 in human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSC) and this effect persists during differentiation to definitive endoderm and hiPSC-derived hepatocytes. Microarray analysis reveals that T3 regulates hundreds of hESC and hiPSC target genes that cluster into many of the same pathways implicated in TR and KLF9 regulation in HepG2 cells. KLF9 knockdown confirms that TR and KLF9 cooperate to regulate Notch pathway genes in hESC and hiPSC, albeit in a partly cell-specific manner. Broader analysis of T3 responsive hESC/hiPSC genes suggests that TRs regulate multiple early steps in ESC differentiation. We propose that TRs cooperate with KLF9 to regulate hepatocyte proliferation and differentiation and early stages of organogenesis and that TRs exert widespread and important influences on ESC biology.

Role and regulation of β-catenin signaling during physiological liver growth

Wnt/β-catenin signaling plays key roles not only during development but also in adult tissue homeostasis. This is also evident in liver biology where many temporal roles of β-catenin have been identified during hepatic development, where, in hepatic progenitors or hepatoblasts, it is a key determinant of proliferation and eventually differentiation to mature hepatocytes, while also playing an important role in bile duct homeostasis. β-Catenin signaling cascade is mostly quiescent in hepatocytes in an adult liver except in the centrizonal region of a hepatic lobule. This small rim of hepatocytes around the central vein show constitutive β-catenin activation that in turn regulates expression of genes whose products play an important role in ammonia and xenobiotic metabolism. Intriguingly, β-catenin can also undergo activation in hepatocytes after acute liver loss secondary to surgical or toxicant insult. Such activation of this progrowth protein is observed as nuclear translocation of β-catenin and formation of its complex with the T-cell factor (TCF) family of transcription factors. Expression of cyclin-D1, a key inducer of transition from the G1 to S phase of cell cycle, is regulated by β-catenin-TCF complex. Thus, β-catenin activation is absolutely critical in the normal regeneration process of the liver as shown by studies in several models across various species. In the current review, the temporal role and regulation of β-catenin in liver development, metabolic zonation in a basal adult liver, and during the liver regeneration process will be discussed. In addition, the probability of therapeutically regulating β-catenin activity as a possible future treatment strategy for liver insufficiency will also be discussed.

Neuron-derived orphan receptor 1 promotes proliferation of quiescent hepatocytes

BACKGROUND & AIMS

Studies of the transcriptional networks that regulate nuclear receptor-mediated proliferation of quiescent hepatocytes could lead to new information about liver growth and hepatoprotective strategies.

METHODS

We used quantitative real-time PCR to analyze expression of neuron-derived orphan receptor 1 (Nor-1) and its target genes during liver regeneration after hepatectomy in mice, and in hepatocellular carcinoma (HCC) samples from patients. We used adenoviral vectors to express Nor-1 in normal liver (Ad/CMV/V5-Nor-1), or reduce its level with small hairpin RNAs (Ad/BLOCK-iT/Nor-1(small hairpin RNA)) after partial hepatectomy.

RESULTS

Levels of Nor-1 messenger RNA and protein, and transcription of Nor-1 target genes (Ccnd1 and Vcam-1), increased during the late priming and proliferative phases of liver regeneration after partial hepatectomy. Levels of NOR-1 messenger RNA and transcription of its target gene CCND1 and of the NOR-1 subfamily member NUR-77 also increased in human HCC samples compared with paired HCC-free tissue. Ad-Nor-1(small hairpin RNA) reduced the hepatocyte proliferation after hepatectomy. Overexpression of Nor-1 in normal livers of mice induced proliferation of quiescent hepatocytes independently of interleukin-6 and tumor necrosis factor-α signaling. In gene expression profile analysis, Nor-1 altered expression of genes involved in the cell cycle, proliferation, and tumorigenesis.

CONCLUSIONS

In mice, the orphan nuclear receptor Nor-1 activates proliferation of quiescent hepatocytes and is required for hepatocyte proliferation after partial hepatectomy. Nor-1 and its gene targets are also up-regulated in human HCC samples. Nor-1 activates a transcriptional program that induces hepatocyte proliferation independently of inflammatory signaling pathways.

3,5,3'-triiodothyronine (T3) stimulates cell proliferation through the activation of the PI3K/Akt pathway and reactive oxygen species (ROS) production in chick embryo hepatocytes

Thyroid hormones (THs) have a wide variety of essential roles in vertebrates, ranging from the regulation of key metabolic processes to cell proliferation and apoptosis. The classical mechanism of action of THs is genomic; 3,5,3'-triiodothyronine (T3) binds to specific nuclear receptors (TRs) and modifies the expression of specific genes. Recently, a new category of mechanisms, termed nongenomic, has been discovered for T3. These mechanisms include, among others, the rapid activation of signal transduction pathways, such as PI3K/Akt and MAPK, which eventually lead to cell proliferation. These effects are mediated in some cell types by a plasma membrane receptor, identified as integrin αvβ3, and in other cell types by cytoplasmic TRβ1. The aim of this work was to analyze the effect of T3 on the cell growth of chick embryo hepatocytes at two different stages of development, 14 and 19 days, and to determine the activation of the signal transduction pathways, focusing on the potential involvement of a plasma membrane receptor and the possible participation of PI3K/Akt and reactive oxygen species (ROS). Our results clearly show that T3 stimulates cell proliferation at both stages of development through the activation of the PI3K/Akt pathway and the production of small amounts of ROS, which operate as effective second messengers. Moreover, we prove that these effects are not initiated at the plasma membrane receptor for T3.

Proteomic characterization of early changes induced by triiodothyronine in rat liver

High doses of T3 are mitogenic in liver, causing hyperplasia that has numerous differences from the compensatory regeneration induced by partial hepatectomy (PH). T3 binds to the thyroid hormone receptor (TR), which directly regulates transcription, while PH acts indirectly through signal transduction pathways. We therefore carried out a proteomic analysis to compare early effects of the two treatments. Transcriptome analysis by DNA microarray also confirmed the observed proteomic changes, demonstrating that they were caused by transcriptional regulation. Among the differentially expressed proteins, many are directly or indirectly involved in energy metabolism and response to oxidative stress. Several enzymes of lipid metabolism (e.g., Acaa2, Acads, Hadh, and Echs1) were differentially regulated by T3. In addition, altered expression levels of several mitochondrial proteins (e.g., Hspa9, Atp5b, Cps1, Glud1, Aldh2, Ak2, Acads) demonstrated the known increase of mitochondrial biogenesis mediated by T3. The present results provide insights in changes in metabolic balance occurring following T3-stimulation and define a basis for dissecting the molecular pathways of hepatocyte hyperplasia.

Yes-associated protein regulation of adaptive liver enlargement and hepatocellular carcinoma development in mice

The Hippo kinase cascade, a growth-suppressive pathway that ultimately antagonizes the transcriptional coactivator Yes-associated protein (YAP), has been shown in transgenic animals to orchestrate organ size regulation. The purpose of this study was to determine whether in non-genetically modified mice (1) the Hippo pathway is involved in the regulation of adaptive liver enlargement caused by the mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), an agonist of constitutive androstane receptor and (2) a dysregulation of this pathway occurs during the development of chemically induced hepatocellular carcinoma (HCC). We show that liver enlargement caused by TCPOBOP was associated with an increase of YAP protein levels that paralleled the increase in 2-bromodeoxyuridine incorporation. Interestingly, when a second dose of TCPOBOP was given to mice with enlarged livers, no further increases in liver mass or YAP protein levels were observed, suggesting that the Hippo pathway prevents further growth of the hyperplastic liver. Viral-mediated exogenous expression of active YAP in mouse livers was able to partially overcome the block of hepatocyte proliferation. We also show that HCCs developed in mice given diethylnitrosamine and then subjected to repeated treatments with TCPOBOP had increased levels of YAP that were associated with down-regulation of microRNA 375, which is known to control YAP expression, and with enhanced levels of alpha-fetoprotein and connective tissue growth factor, two target genes of YAP.

CONCLUSION

These results suggest that the Hippo pathway regulates adaptive liver enlargement and is probably inactivated in initiated cells that escape the suppressive constrain exerted on the surrounding normal tissue, thus allowing clonal expansion to HCC.

Cyclin D1: polymorphism, aberrant splicing and cancer risk

The cyclin D1 proto-oncogene exercises powerful control over the mechanisms that regulate the mitotic cell cycle, and excessive cyclin D1 expression and/or activity is common in human cancers. Although somatic mutations of the cyclin D1 locus are rarely observed, mounting evidence demonstrates that a specific polymorphism of cyclin D1 (G/A870) and a protein product of a potentially related alternate splicing event (cyclin D1b) may influence cancer risk and outcome. Herein, we review the epidemiological and functional literatures that link these alterations of cyclin D1 to human tumor development and progression.

Thyroid hormone-induced oxidative stress in rodents and humans: a comparative view and relation to redox regulation of gene expression

Thyroid hormone (3,3',5-triiodothyronine, T(3)) exerts significant actions on energy metabolism, with mitochondria being the major target for its calorigenic effects. Acceleration of O(2) consumption by T(3) leads to an enhanced generation of reactive oxygen and nitrogen species in target tissues, with a higher consumption of cellular antioxidants and inactivation of antioxidant enzymes, thus inducing oxidative stress. This redox imbalance occurring in rodent liver and extrahepatic tissues with a calorigenic response, as well as in hyperthyroid patients, is further enhanced by an increased respiratory burst activity in Kupffer cells, which may activate redox-sensitive transcription factors such as NF-kappaB thus up-regulating gene expression. T(3) elicits an 80-fold increase in the serum levels of tumor necrosis factor-alpha (TNF-alpha), which is abolished by pretreatment with the antioxidants alpha-tocopherol and N-acetylcysteine, the Kupffer-cell inactivator GdCl(3), or an antisense oligonucleotide against TNF-alpha. In addition, T(3) treatment activates hepatic NF-kappaB, a response that is (i) inhibited by antioxidants and GdCl(3) and (ii) accompanied by induced mRNA expression of the NF-kappaB-responsive genes for TNF-alpha and interleukin (IL)-10. T(3) also increases the hepatic levels of mRNA for IL-1alpha and those of IL-1alpha in serum. Up-regulation of liver iNOS expression is also achieved by T(3), through a cascade initiated by TNF-alpha and involving IkappaB-alpha phosphorylation and NF-kappaB activation. In conclusion, T(3)-induced oxidative stress in the liver enhances the DNA-binding of NF-kappaB and the NF-kappaB-dependent expression of cytokines and iNOS by actions primarily exerted at the Kupffer cell level.

Bombesin regulates cyclin D1 expression through the early growth response protein Egr-1 in prostate cancer cells

Our previous studies indicate that the activation of mitogen-activated protein kinase (MAPK) pathway is involved in bombesin-induced cell proliferation in prostate cancer cells. Cyclin D1 is a critical regulator involved in cell cycle progression through the G1 phase into the S phase, thereby contributing to cell proliferation. Mostly, mitogen-stimulated expression of cyclin D1 is attributed to the extracellular signal-regulated kinase (ERK) activation. Here, we found that bombesin induced human cyclin D1 expression on both mRNA and protein levels in DU-145 prostate cancer cells. Mutational analyses showed that bombesin-enhanced cyclin D1 transcription required the binding of nuclear proteins to the -143 to -105 region of the human cyclin D1 promoter, which contains binding sites for transcription factors Sp-1 and early growth response protein (Egr-1). Do novo protein synthesis was requisite for bombesin-induced cyclin D1 expression. Further studies showed Egr-1 was induced upon bombesin stimulation. The induction of Egr-1 expression and its binding to the cyclin D1 promoter were essential for bombesin-enhanced cyclin D1 transcription. Inhibition of MAPK pathway with either the MEK1 inhibitor PD98059 or a dominant-negative Ras mutant, RasN17, abolished bombesin-induced cyclin D1 activation. Taken together, bombesin-induced cyclin D1 expression in prostate cancer cells is mediated by Egr-1 activation and the interaction of Egr-1 with the Egr-1/Sp1 motif of the cyclin D1 promoter through the activation of MAPK pathway. These findings represent a novel mechanism of bombesin-dependent stimulation of mitogenesis by regulating directly the cell cycle in prostate cancer.

Hypothyroid phenotype is contributed by mitochondrial complex I inactivation due to translocated neuronal nitric-oxide synthase

Although transcriptional effects of thyroid hormones have substantial influence on oxidative metabolism, how thyroid sets basal metabolic rate remains obscure. Compartmental localization of nitric-oxide synthases is important for nitric oxide signaling. We therefore examined liver neuronal nitric-oxide synthase-alpha (nNOS) subcellular distribution as a putative mechanism for thyroid effects on rat metabolic rate. At low 3,3',5-triiodo-L-thyronine levels, nNOS mRNA increased by 3-fold, protein expression by one-fold, and nNOS was selectively translocated to mitochondria without changes in other isoforms. In contrast, under thyroid hormone administration, mRNA level did not change and nNOS remained predominantly localized in cytosol. In hypothyroidism, nNOS translocation resulted in enhanced mitochondrial nitric-oxide synthase activity with low O2 uptake. In this context, NO utilization increased active O2 species and peroxynitrite yields and tyrosine nitration of complex I proteins that reduced complex activity. Hypothyroidism was also associated to high phospho-p38 mitogen-activated protein kinase and decreased phospho-extracellular signal-regulated kinase 1/2 and cyclin D1 levels. Similarly to thyroid hormones, but without changing thyroid status, nitric-oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester increased basal metabolic rate, prevented mitochondrial nitration and complex I derangement, and turned mitogen-activated protein kinase signaling and cyclin D1 expression back to control pattern. We surmise that nNOS spatial confinement in mitochondria is a significant downstream effector of thyroid hormone and hypothyroid phenotype.

Beta-catenin/Tcf-1-mediated transactivation of cyclin D1 promoter is negatively regulated by thyroid hormone

Cyclin D1 is an oncogenic cyclin frequently over-expressed in cancer. To examine the effect of thyroid hormone (T3) and its receptor (TR) on the transcription of cyclin D1 gene, we co-transfected the chloramphenicol acetyl-transferase (CAT) reporter plasmid containing cyclin D1 promoter together with the expression plasmids for TRbeta1 and wild-type or mutant beta-catenin (SA) into 293T cells. In the presence of T3, beta-catenin-dependent transactivation of cyclin D1 promoter was suppressed by co-transfection of TRbeta1. The suppression by T3/TRbeta1 was in a dose-dependent manner. The CAT reporter gene in which Tcf/Lef-1 sites were fused to heterologous promoter was also suppressed by T3/TRbeta1. Furthermore, inhibition of endogenous wild-type beta-catenin by T3/TRbeta1 was observed in SW480 colon carcinoma cells with mutation of the adenomatous polyposis coli gene. These results indicate that the T3-bound TR inhibits the transcription of cyclin D1 through the Tcf/Lef-1 site, which is positively regulated by the Wnt-signaling pathway.

Triiodothyronine and interleukin-6 (IL-6) induce expression of HGF in an immortalized rat hepatic stellate cell line

BACKGROUND/AIMS

Despite its being considered a primary mitogen for hepatocytes, triiodothyronine (T3) has no effect on the proliferation of hepatocytes in vitro, and in our studies, induces significant in vivo hepatocyte proliferation only during liver injury. We hypothesized that T3 may affect hepatocytes proliferation indirectly, by inducing other cells in the liver to secrete hepatic mitogens.

METHODS

In vivo studies: Lipopolysaccharide, T3 and a combination of the two were injected into rats, and hepatocyte proliferation was determined by PCNA staining and mitotic index.

IN VITRO STUDIES

a rat hepatic stellate cell line (HSC-6T) was cultured with T3, IL-6 and a combination of the two, and we assessed the effect of these cytokine/hormone combinations on the cell proliferation and on secretion of IL-6 and HGF, measured by ELISA. Expression of thyroid hormone receptors was assessed by RT-PCR.

RESULTS

In vivo: T3, together with lipopolysaccharide, enhances PCNA staining and the mitotic index of hepatocytes in the treated rats. In vitro: the hepatic stellate cell line expresses thyroid hormone receptor alpha 1, but not beta 1. Proliferation of stellate cells is not affected by T3, with or without IL-6. T3 has no effect on secreted levels of IL-6 in the stellate cell line. Hepatic stellate cells cultured with T3 and IL-6 show significantly increased amounts of secreted HGF after 48 h in culture.

CONCLUSION

T3 may induce hepatocyte proliferation in vivo during injury by turning on expression of HGF in stellate cells and acting together with IL-6.

Loss of cyclin D1 does not inhibit the proliferative response of mouse liver to mitogenic stimuli

Cyclin D1 is considered to play a critical role in the progression from G1 to S phase of the cell cycle, and its overexpression is seen in many human tumors. However, previous studies in cell lines have shown that cyclin D1 is not sufficient to trigger cell replication. To directly test the role of cyclin D1 in the progression of the cell cycle, we have examined the proliferative response of hepatocytes to the hepatomitogen 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) in mice with homozygous disruption of the cyclin D1 gene. We found that 24 hours after administration of TCPOBOP, the number of bromodeoxyuridine (BrdU)-positive hepatocytes was significantly reduced in cyclin D1(-/-) (labeling index was 1.9% in knockout mice vs. 9.7% of heterozygous mice); however, no difference in the number of proliferating hepatocytes was found 36 or 72 hours after treatment (labeling index was 16% and 43% in cyclin D1(-/-) mice vs. 20% and 41% of heterozygous mice), indicating that lack of cyclin D1 may transiently delay entry into S phase but is not sufficient to inhibit the response of hepatocytes to mitogenic stimuli. The results also show that although there was no difference in hepatic protein levels of cyclin D2 and D3 between untreated cyclin D1(-/-) and cyclin D1(+/-) mice, messenger RNA (mRNA) and protein levels of cyclin E were much higher in the former. In conclusion, our results show that cyclin D1 is not essential for liver development and hepatocyte proliferation induced by mitogenic stimuli and suggest that overexpression of cyclin E may compensate for the lack of cyclin D1.

Dysregulation of iodothyronine deiodinase enzyme expression and function in human pituitary tumours

OBJECTIVE

Thyroid hormones (THs) perform essential roles in pituitary function. They regulate anterior pituitary hormone secretion and are also key determinants of pituitary cell proliferation and differentiation. The critical role of deiodinase enzymes, which serve as prereceptor regulators of TH action, remains largely unexplored. Three deiodinase enzymes metabolize active and inactive THs and thereby determine tissue concentrations of the biologically active ligand, tri-iodothyronine (T3). We hypothesized that aberrant expression of deiodinase enzymes and/or altered enzyme activity in pituitary tumours may change tissue concentrations of THs and influence their growth and secretory characteristics.

STUDY DESIGN AND PATIENTS

We studied 105 pituitary tumours and 10 normal pituitaries for expression of deiodinase enzyme mRNAs encoding types 1 (D1), 2 (D2) and 3 (D3) using real-time RT-PCR. Enzyme activity data from 20 pituitary samples were also obtained.

RESULTS

Pituitary tumours expressed significantly increased D3 mRNA (6.5-fold, P < 0.0005) compared with normal pituitaries. D2 mRNA was also increased 2.6-fold (P = 0.005) in pituitary tumours compared with normals. The rare TSH-secreting pituitary tumour subtype expressed a 13.1-fold excess of D3 mRNA and reduced D2 mRNA (0.1-fold of normal pituitaries). D2 mRNA expression in ACTH-secreting tumours was similarly reduced to 0.1-fold that in normal pituitaries.

CONCLUSIONS

Pituitary adenomas express abnormal levels of deiodinase enzymes compared to normal pituitaries. These abnormalities may have functional consequences on pituitary tumour growth. In the case of TSH-secreting pituitary adenomas, the observed pattern of deiodinase mRNA expression may explain the 'resistance' of this tumour type to TH feedback.