Analysis of Thyroid Response Element Activity during Retinal Development

Thyroid hormone (TH) signaling components are expressed during retinal development in dynamic spatial and temporal patterns. To probe the competence of retinal cells to mount a transcriptional response to TH, reporters that included thyroid response elements (TREs) were introduced into developing retinal tissue. The TREs were placed upstream of a minimal TATA-box and two reporter genes, green fluorescent protein (GFP) and human placental alkaline phosphatase (PLAP). Six of the seven tested TREs were first tested in vitro where they were shown to drive TH-dependent expression. However, when introduced into the developing retina, the TREs reported in different cell types in both a TH-dependent and TH-independent manner, as well as revealed specific spatial patterns in their expression. The role of the known thyroid receptors (TR), TRα and TRβ, was probed using shRNAs, which were co-electroporated into the retina with the TREs. Some TREs were positively activated by TR+TH in the developing outer nuclear layer (ONL), where photoreceptors reside, as well as in the outer neuroblastic layer (ONBL) where cycling progenitor cells are located. Other TREs were actively repressed by TR+TH in cells of the ONBL. These data demonstrate that non-TRs can activate some TREs in a spatially regulated manner, whereas other TREs respond only to the known TRs, which also read out activity in a spatially regulated manner. The transcriptional response to even simple TREs provides a starting point for understanding the regulation of genes by TH, and highlights the complexity of transcriptional regulation within developing tissue

Hypothyroidism Enhances Tumor Invasiveness and Metastasis Development

11 pages, 9 figures.[Background]: Whereas there is increasing evidence that loss of expression and/or function of the thyroid hormone receptors (TRs) could result in a selective advantage for tumor development, the relationship between thyroid hormone levels and human cancer is a controversial issue. It has been reported that hypothyroidism might be a possible risk factor for liver and breast cancer in humans, but a lower incidence of breast carcinoma has been also reported in hypothyroid patients [Methodology/Principal Findings]: In this work we have analyzed the influence of hypothyroidism on tumor progression and metastasis development using xenografts of parental and TRβ1–expressing human hepatocarcinoma (SK-hep1) and breast cancer cells (MDA-MB-468). In agreement with our previous observations tumor invasiveness and metastasis formation was strongly repressed when TRβ–expressing cells were injected into euthyroid nude mice. Whereas tumor growth was retarded when cells were inoculated into hypothyroid hosts, tumors had a more mesenchymal phenotype, were more invasive and metastatic growth was enhanced. Increased aggressiveness and tumor growth retardation was also observed with parental cells that do not express TRs. [Conclusions/Significance]: These results show that changes in the stromal cells secondary to host hypothyroidism can modulate tumor progression and metastatic growth independently of the presence of TRs on the tumor cells. On the other hand, the finding that hypothyroidism can affect differentially tumor growth and invasiveness can contribute to the explanation of the confounding reports on the influence of thyroidal status in human cancer.This work was supported by grants BFU2007-62402 from MEC; RD06/0020/0036 from FIS and from the EU Project CRESCENDO (FP6-018652.Peer reviewe

Local hyperthyroidism promotes pancreatic acinar cell proliferation during acute pancreatitis

Proliferation of pancreatic acinar cells is a critical process in the pathophysiology of pancreatic diseases, because limited or defective proliferation is associated with organ dysfunction and patient morbidity. In this context, elucidating the signalling pathways that trigger and sustain acinar proliferation is pivotal to develop therapeutic interventions promoting the regenerative process of the organ.In this study we used genetic and pharmacological approaches to manipulate both local and systemic levels of thyroid hormones to elucidate their role in acinar proliferation following caerulein‐mediated acute pancreatitis in mice. In addition, molecular mechanisms mediating the effects of thyroid hormones were identified by genetic and pharmacological inactivation of selected signalling pathways.In this study we demonstrated that levels of the thyroid hormone 3,3’,5‐triodo‐L‐thyronine (T3) transiently increased in the pancreas during acute pancreatitis. Moreover, by using genetic and pharmacological approaches to manipulate both local and systemic levels of thyroid hormones, we showed that T3 was required to promote proliferation of pancreatic acinar cells, without affecting the extent of tissue damage or inflammatory infiltration.Finally, upon genetic and pharmacological inactivation of selected signalling pathways, we demonstrated that T3 exerted its mitogenic effect on acinar cells via a tightly controlled action on different molecular effectors, including histone deacetylase, AKT, and TGFβ signalling.In conclusion, our data suggest that local availability of T3 in the pancreas is required to promote acinar cell proliferation and provide the rationale to exploit thyroid hormone signalling to enhance pancreatic regeneration

Repair-related activation of hedgehog signaling in stromal cells promotes intrahepatic hypothyroidism

Thyroid hormone (TH) is important for tissue repair because it regulates cellular differentiation. Intrahepatic TH activity is controlled by both serum TH levels and hepatic deiodinases. TH substrate (T4) is converted into active hormone (T3) by deiodinase 1 (D1) but into inactive hormone (rT3) by deiodinase 3 (D3). Although the relative expressions of D1 and D3 are known to change during liver injury, the cell types and signaling mechanisms involved are unclear. We evaluated the hypothesis that changes in hepatic deiodinases result from repair-related activation of the Hedgehog pathway in stromal cells. We localized deiodinase expression, assessed changes during injury, and determined how targeted manipulation of Hedgehog signaling in stromal cells impacted hepatic deiodinase expression, TH content, and TH action in rodents. Humans with chronic liver disease were also studied. In healthy liver, hepatocytes strongly expressed D1 and stromal cells weakly expressed D3. During injury, hepatocyte expression of D1 decreased, whereas stromal expression of D3 increased, particularly in myofibroblasts. Conditionally disrupting Hedgehog signaling in myofibroblasts normalized deiodinase expression. Repair-related changes in deiodinases were accompanied by reduced hepatic TH content and TH-regulated gene expression. In patients, this was reflected by increased serum rT3. Moreover, the decreases in the free T3 to rT3 and free T4 to rT3 ratios distinguished advanced from mild fibrosis, even in individuals with similar serum levels of TSH and free T4. In conclusion, the Hedgehog-dependent changes in liver stromal cells drive repair-related changes in hepatic deiodinase expression that promote intrahepatic hypothyroidism, thereby limiting exposure to T3, an important factor for cellular differentiation

Defects in adaptive energy metabolism with CNS-linked hyperactivity in PGC-1alpha null mice

PGC-1alpha is a coactivator of nuclear receptors and other transcription factors that regulates several metabolic processes, including mitochondrial biogenesis and respiration, hepatic gluconeogenesis, and muscle fiber-type switching. We show here that, while hepatocytes lacking PGC-1alpha are defective in the program of hormone-stimulated gluconeogenesis, the mice have constitutively activated gluconeogenic gene expression that is completely insensitive to normal feeding controls. C/EBPbeta is elevated in the livers of these mice and activates the gluconeogenic genes in a PGC-1alpha-independent manner. Despite having reduced mitochondrial function, PGC-1alpha null mice are paradoxically lean and resistant to diet-induced obesity. This is largely due to a profound hyperactivity displayed by the null animals and is associated with lesions in the striatal region of the brain that controls movement. These data illustrate a central role for PGC-1alpha in the control of energy metabolism but also reveal novel systemic compensatory mechanisms and pathogenic effects of impaired energy homeostasis