The signaling pathways activated by thyroid hormone receptors (THR) are of fundamental importance for organogenesis, growth and differentiation, and significantly influence energy metabolism, lipid ...utilization and glucose homeostasis. Pharmacological control of these pathways would likely impact the treatment of several human diseases characterized by altered metabolism, growth or differentiation. Not surprisingly, biomedical research has been trying for the past decades to pharmacologically target the 3,5,3'-triiodothyronine (T3)/THR axis.
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studies have provided evidence of the potential utility of the activation of the T3-dependent pathways in metabolic syndrome, non-alcoholic fatty liver disease (NAFLD), and in the treatment of hepatocellular carcinoma (HCC). Unfortunately, supra-physiological doses of the THR agonist T3 cause severe thyrotoxicosis thus hampering its therapeutic use. However, the observation that most of the desired beneficial effects of T3 are mediated by the activation of the beta isoform of THR (THRβ) in metabolically active organs has led to the synthesis of a number of THRβ-selective thyromimetics. Among these drugs, GC-1, GC-24, KB141, KB2115, and MB07344 displayed a promising therapeutic strategy for liver diseases. However, although these drugs exhibited encouraging results when tested in the treatment of experimentally-induced obesity, dyslipidemia, and HCC, significant adverse effects limited their use in clinical trials. More recently, evidence has been provided that some metabolites of thyroid hormones (TH), mono and diiodothyronines, could also play a role in the treatment of liver disease. These molecules, for a long time considered inactive byproducts of the metabolism of thyroid hormones, have now been proposed to be able to modulate and control lipid and cell energy metabolism. In this review, we will summarize the current knowledge regarding T3, its metabolites and analogs with reference to their possible clinical application in the treatment of liver disease. In particular, we will focus our attention on NAFLD, non-alcoholic steatohepatitis (NASH) and HCC. In addition, the possible therapeutic use of mono- and diiodothyronines in metabolic and/or neoplastic liver disease will be discussed.
Cancer cells reprogram their metabolic pathways to support bioenergetic and biosynthetic needs and to maintain their redox balance. In several human tumors, the Keap1-Nrf2 system controls ...proliferation and metabolic reprogramming by regulating the pentose phosphate pathway (PPP). However, whether this metabolic reprogramming also occurs in normal proliferating cells is unclear.
To define the metabolic phenotype in normal proliferating hepatocytes, we induced cell proliferation in the liver by 3 distinct stimuli: liver regeneration by partial hepatectomy and hepatic hyperplasia induced by 2 direct mitogens: lead nitrate (LN) or triiodothyronine. Following LN treatment, well-established features of cancer metabolic reprogramming, including enhanced glycolysis, oxidative PPP, nucleic acid synthesis, NAD + /NADH synthesis, and altered amino acid content, as well as downregulated oxidative phosphorylation, occurred in normal proliferating hepatocytes displaying Nrf2 activation. Genetic deletion of Nrf2 blunted LN-induced PPP activation and suppressed hepatocyte proliferation. Moreover, Nrf2 activation and following metabolic reprogramming did not occur when hepatocyte proliferation was induced by partial hepatectomy or triiodothyronine.
Many metabolic changes in cancer cells are shared by proliferating normal hepatocytes in response to a hostile environment. Nrf2 activation is essential for bridging metabolic changes with crucial components of cancer metabolic reprogramming, including the activation of oxidative PPP. Our study demonstrates that matured hepatocytes exposed to LN undergo cancer-like metabolic reprogramming and offers a rapid and useful in vivo model to study the molecular alterations underpinning the differences/similarities of metabolic changes in normal and neoplastic hepatocytes.
Activation of Wnt/β-catenin signaling is frequent in human and rodent hepatocarcinogenesis. Although in mice the tumor-promoting activity of agonists of constitutive androstane receptor (CAR) occurs ...by selection of carcinogen-initiated cells harboring β-catenin mutations, the molecular alterations leading to hepatocellular carcinoma (HCC) development by the CAR agonist 1,4-bis2-(3,5-dichloropyridyloxy)benzene (TCP) in the absence of genotoxic injury are unknown. Here, we show that CAR activation per se induced HCC in mice and that 91% of them carried β-catenin point mutations or large in-frame deletions/exon skipping targeting Ctnnb1 exon 3. Point mutations in HCCs induced by TCP alone displayed different nucleotide substitutions compared with those found in HCCs from mice pretreated with diethylnitrosamine. Moreover, unlike those occurring in HCCs from diethylnitrosamine + TCP mice, they did not result in increased expression of β-catenin target genes, such as Glul, Lgr5, Rgn, Lect2, Tbx3, Axin2, and Ccnd1, or nuclear translocation of β-catenin compared with the control liver. Remarkably, in the nontumoral liver tissue, chronic CAR activation led to down-regulation of these genes and to a partial loss of glutamine synthetase–positive hepatocytes. These results show that, although chronic CAR activation per se induces HCCs carrying β-catenin mutations, it concurrently down-regulates the Wnt/β-catenin pathway in nontumoral liver. They also indicate that the relationship between CAR and β-catenin may be profoundly different between normal and neoplastic hepatocytes.
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‐bis2‐(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 (HEPATOLOGY 2011;)
Summary
Sickle cell disease (SCD) is a widespread genetic disease associated with severe disability and multi‐organ damage, resulting in a reduced life expectancy. None of the existing clinical ...treatments provide a solution for all patients. Gene therapy and fetal haemoglobin (HbF) reactivation through genetic approaches have obtained promising, but early, results in patients. Furthermore, the search for active molecules to increase HbF is still ongoing. The delta‐globin gene produces the delta‐globin of haemoglobin A2 (HbA2). Although expressed at a low level, HbA2 is fully functional and could be a valid anti‐sickling agent in SCD. To evaluate the therapeutic potential of a strategy aimed to over‐express the delta‐globin gene in vivo, we crossed transgenic mice carrying a single copy of the delta‐globin gene, genetically modified to be expressed at a higher level (activated), with a humanised mouse model of SCD. The activated delta‐globin gene gives rise to a consistent production of HbA2, effectively improving the SCD phenotype. For the first time in vivo, these results demonstrate the therapeutic potential of delta‐globin, which could lead to novel approaches to the cure of SCD.
Nonalcoholic fatty liver disease is the most common noninfectious liver disease in clinical practice, and there is an increasing need for new therapeutic approaches for the treatment of this liver ...disease. Here, we examined the effect of the thyroid hormone triiodothyronine (T3) and the agonist of the thyroid hormone receptor β isoform (TRβ), GC-1, on fatty liver and steatohepatitis induced in rodents by a choline-methionine deficient (CMD) diet. Male Fischer 344 rats fed a CMD diet for 1 wk developed a marked fatty liver and mild hepatitis. Concurrent administration of T3 resulted in a complete prevention of the fatty change associated with increased fatty acid mitochondrial and peroxisomal β-oxidation. To investigate whether T3 could also reverse fully established fatty liver, rats were fed a CMD diet for 10 wk and then cofed T3 for 1 wk. Coadministration of T3 resulted in a complete regression of liver steatosis associated with a decrease of lipid peroxidation, cyclooxygenase-2 expression, and activation of phospho-STAT3 and phospho-SAPK/JNK. Finally, additional experiments showed that GC-1, which has no significant side effects on heart rate, prevented and reverted CMD-induced fat accumulation, and ameliorated steatohepatitis. These results indicate that TR agonists have the potential to inhibit or reverse hepatic steatosis induced by a nutritional model.--Perra, A., Simbula, G., Simbula, M., Pibiri, M., Kowalik, M. A., Sulas, P., Cocco, M. T., Ledda-Columbano, G. M., Columbano, A. Thyroid hormone (T3) and TRβ agonist GC-1 inhibit/reverse nonalcoholic fatty liver in rats.
Background & Aims Thyroid hormones elicit many cellular and metabolic effects in various organs. Most of these actions, including mitogenesis, are mediated by the thyroid hormone 3,5,3′-triiodo- l ...-thyronine (T3) nuclear receptors (TRs). They are transcription factors, expressed as different isoforms encoded by the TRα and TRβ genes. Here, experiments were performed to determine whether (i) T3-induces hepatocyte proliferation in mouse liver and pancreas, and, (ii) which TR isoform, is responsible for its mitogenic effect. Methods Cell proliferation was measured by bromodeoxyuridine (BrdU) incorporation after T3 or the TRβ agonist GC-1 in liver and pancreas of CD-1, C57BL, or TRα0/0 mice. Cell cycle-associated proteins were measured by Western blot. Results T3 added to the diet at a concentration of 4 mg/kg caused a striking increase in BrdU incorporation in mouse hepatocytes. Increased BrdU incorporation was associated with enhanced protein levels of cyclin D1 and PCNA and decreased levels of p27. Treatment with GC-1, a selective agonist of the TRβ isoform, also induced a strong mitogenic response of mouse hepatocytes and pancreatic acinar cells which was similar to that elicited by T3. Finally, treatment with T3 of mice TRα0/0 induced a proliferative response in the liver and pancreas, similar to that of their wild type counterpart. Conclusions These results demonstrate that T3 is a powerful inducer of cell proliferation in mouse liver and suggest that the β-isoform is responsible for the hepatomitogenic activity of T3. The same isoform seems to also mediate the proliferation of mouse pancreatic acinar cells.
Background & Aims Mice lacking c-jun in the liver display impaired regeneration after partial hepatectomy (PH), and were reported to be more resistant to chemically-induced hepatocellular carcinoma ...(HCC). We investigated the role of c-jun in normal and preneoplastic hepatocyte proliferation induced by ligands of nuclear receptors, which cause liver hyperplasia in the absence of cell loss/death. Methods The effect of 1,4-bis2-(3,5-dichloropyridyloxy)benzene (TCPOBOP) on hepatocyte proliferation was determined in c-jun conditional knockout ( c-junΔli ) or in mouse liver where c-jun has been silenced. To study the role of c-jun in HCC development, c-junΔli and WT mice were given diethylnitrosamine (DENA) followed by repeated injections of TCPOBOP. Results Hepatocyte proliferation induced by TCPOBOP was associated with a stronger proliferative response and earlier S phase entry in c-junΔli mice, compared to WT animals. Moreover, silencing of c-jun in the liver of CD-1 mice caused increased hepatocyte proliferation. A stronger hepatocyte proliferative response of c-junΔli mice was observed also following treatment with a ligand of thyroid hormone receptor. Finally, loss of c-jun did not inhibit the development of HCC induced by DENA and promoted by TCPOBOP. Conclusions (i) c-jun may, under certain conditions, negatively regulate proliferation of normal hepatocytes, (ii) c-jun is not an absolute requirement for DENA/TCPOBOP-induced HCC formation, suggesting that the therapeutic potential of c-jun/JNK inhibition in liver tumors might be impaired by an increased stimulation of cell growth due to blockade of the c-jun pathway.
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