Effect of thyroid hormone responsive protein (THRP) expression on PC12 cell survival

Pathophysiology and Clinical Features of Neuropsychiatric Manifestations of Thyroid Disease

Thyroid hormones (TH) have a cardinal role in the development of the central nervous system during embryogenesis and early infancy. However, the TH-responsive genes in the developing brain cease to respond to TH in adulthood. Nevertheless, thyroid dysfunction in adults is commonly associated with a host of cognitive and psychiatric problems. Cognitive decline, dysphoria, and depression are common manifestations of overt hypothyroidism while hyperthyroidism can cause agitation, acute psychosis, and apathy, especially in older people. Whereas levothyroxine treatment can reverse dementia in the setting of hypothyroidism, the effect of levothyroxine on depressive symptoms in subjects with subclinical hypothyroidism is controversial. The use of supraphysiologic doses of TH to treat depression refractory to antidepressant remains a viable therapeutic tool with the caveat that excessive doses of thyroid hormone to treat depression may have potentially damaging effects on other organ systems. The present communication describes the pathophysiology of neuropsychiatric manifestations of thyroid disease, including changes in neurotransmission, alterations in neuronal or glial cell gene expression, blood-brain barrier dysfunction, increased risk of cerebrovascular disease, and occasionally cerebral inflammatory disease in the context of autoimmune thyroid disease. Elucidating the molecular mechanisms of TH effect on cerebral tissue will help identify novel therapeutic targets for managing people with neuropsychiatric disorders.

Statins Prevent Dextrose-Induced Endoplasmic Reticulum Stress and Oxidative Stress in Endothelial and HepG2 Cells

Statins have favorable effects on endothelial function partly because of their capacity to reduce oxidative stress. However, antioxidant vitamins, unlike statins, are not as cardioprotective, and this paradox has been explained by failure of vitamin antioxidants to ameliorate endoplasmic reticulum (ER) stress. To determine whether statins prevent dextrose-induced ER stress in addition to their antioxidative effects, human umbilical vein endothelial cells and HepG2 hepatocytes were treated with 27.5 mM dextrose in the presence of simvastatin (lipophilic statin that is a prodrug) and pravastatin (water-soluble active drug), and oxidative stress, ER stress, and cell death were measured. Superoxide generation was measured using 2-methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[1,2-A]pyrazin-3-one hydrochloride. ER stress was measured using the placental alkaline phosphatase assay and Western blot of glucose-regulated protein 75, c-jun-N-terminal kinase, phospho-JNK, eukaryotic initiating factor 2α and phospho-eIF2α, and X-box binding protein 1 mRNA splicing. Cell viability was measured by propidium iodide staining. Superoxide anion production, ER stress, and cell death induced by 27.5 mM dextrose were inhibited by therapeutic concentrations of simvastatin and pravastatin. The salutary effects of statins on endothelial cells in reducing both ER stress and oxidative stress observed with pravastatin and the prodrug simvastatin suggest that the effects may be independent of cholesterol-lowering activity.

The potential neurotoxicity of emerging tetrabromobisphenol A derivatives based on rat pheochromocytoma cells

Tetrabromobisphenol A (TBBPA) can cause diverse adverse effects including neurotoxicity. Emerging TBBPA derivatives, with high structure similarity to the parent compound, are now being concerned. In this study, the potential neurotoxicities of four TBBPA derivatives and their parent compound were studied by cell viability inhibition in rat pheochromocytoma cells (PC12) and the corresponding molecular mechanisms were investigated. The cellular toxicity was correlated with the chemical hydrophobicity. Tetrabromobisphenol A bis(2-hydroxyethyl ether) (TBBPA-BHEE) exhibited the highest cellular toxicity to PC12 due to its lowest hydrophobicity among these 5 tested compounds. Further experiments showed that TBBPA-BHEE disturbed dopamine (DA) secretion and altered acetylcholinesterase (AChE) enzymatic activity in PC12 cells. The molecular mechanism study indicated that TBBPA-BHEE induced cellular toxicity to PC12 cells through ROS-mediated caspase activation to a large extent, which was partially attenuated by the anti-oxidation of Vitamin E. Moreover, in contrast to TBBPA, the occurrence of TBBPA-BHEE toxicity to PC12 was not attributed to activation of mitogen-activated protein kinases (MAPKs) or thyroid hormone (TH) signaling pathway. These findings suggest TBBPA derivatives, especially TBBPA-BHEE, as potential neurotoxins need urgent attention.

Thyroid hormones differentially regulate phosphorylation of ERK and Akt via integrin αvβ3 receptor in undifferentiated and differentiated PC-12 cells

The effects of 3,5,3'-triiodo-l-thyronine (T3) and l-thyroxine (T4) on the integrin αvβ3 receptor of thyroid hormones (TH) were investigated in pheochromocytoma PC-12 cells. Differentiation was induced by treatment of PC-12 cells with fisetin and the levels of phosphorylated extracellular signal-regulated kinase (ERK) and Akt in cytoplasm, as well as the content of FoxO6 transcription factor in nuclei was analysed in undifferentiated and differentiated conditions. We have found that in undifferentiated PC-12 cells, tetraiodothyroacetic acid (TETRAC), a known inhibitor of binding of T4 and T3 to plasma membrane integrin αvβ3 receptor inhibits T4-dependent phosphorylation of ERK, whereas in differentiated PC-12 cells, TETRAC abolishes the effect of T3. In undifferentiated PC-12 cells, both TH increase the level of p-Akt, and this enhancement is not sensitive to TETRAC. In differentiated PC-12 cells, both TH increase the level of p-Akt; however, only T3-dependent activation of Akt is sensitive to the TETRAC. Furthermore, our results have shown that in differentiated PC-12 cells, the expression of FoxO6 was higher than in undifferentiated PC-12 cells, and this elevation has not changed under the action of TH. Only in undifferentiated PC-12 cells the T3-dependent expression of FoxO6 was sensitive to the TETRAC. We propose that PC-12 cells contain integrin αvβ3 receptor, which T3 and T3/T4 sites are differentially regulated by TH in undifferentiated and differentiated conditions.

The thyroid hormone responsive protein (THRP) has a critical role in the embryogenesis of Xenopus laevis

The human and mouse homologs of the rat thyroid hormone responsive protein (THRP), c-abl-interacting protein 2 (Abi-2), are critically involved in neurological development. The Abi-2 gene is evolutionarily conserved in vertebrates, and is also found in Xenopus laevis and Drosophila melanogaster. The THRP gene is one of the few genes regulated by thyroid hormone in adult animals. Sequence analysis of the 5'-flanking region of the THRP gene identified a putative thyroid hormone response element (TRE) that is conserved between rat and human. To determine whether or not THRP regulates neural growth and development, THRP was constitutively expressed in transgenic X. laevis. Growth of most animals was halted in early neurulation while the few animals that survived the process developed into grossly malformed tadpoles. In contrast, control animals reached late embryonic stage 25. These observations suggest that THRP over-expression in early development is not compatible with completion of early embryogenesis and that a different strategy needs to be employed to investigate THRP function in this model.

Thyroid hormone responsive protein (THRP) mediates thyroid hormone-induced cytotoxicity in primary neuronal cultures

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone (TH) in the cerebral cortex of adult rats. To study the effects of THRP on neuronal cell survival, primary neurons cultured from rats at embryonic day 19 were treated with either 10(-7) mol L(-1) 3,5,3'-triiodothyronine (T(3)), or 10(-7) mol L(-1) L: -thyroxine (T(4)). This resulted in decreasing neuronal cell number starting 48 h after treatment. T(3) -related cytotoxicity was also documented by measurement of lactate dehydrogenase release into the medium and by propidium iodide staining. Treatment of cells with 10(-7) mol L(-1) T(3) resulted in a significant increase in THRP mRNA levels as early as 24 h of treatment in a concentration-dependent manner. T(3) treatment did not alter glyceraldehyde 3-phosphate dehydrogenase (G3PDH) mRNA levels. Exogenous expression of THRP by transfecting cells with a THRP expression construct (pSVL-THRP) was associated with a significant increase in cell death as measured by the increased number of propidium iodide staining cells (18.0+/-2.1 cells per field) compared with mock-transfected cells (3.3+/-0.2), P<0.002. To further document THRP-induced cytotoxicity, the cells were either transfected with pSVL (empty vector)+pSV2neo (neomycin resistance vector for cell labeling), pSVL-THRP+pSV2neo, or pSVL-THRP+pc-Abl (cAbl tyrosine kinase expressing vector)+pSV2neo. After 24 h the cells were treated with 500 microg mL(-1) G418 (a congener of neomycin) to eliminate the non-transfected cells. Transfection with pSVL-THRP reduced neuronal survival relative to cells transfected with pSVL (356+/-15.6 compared with 145+/-16.9, P<0.05). Co-transfection of THRP with wild-type c-Abl did not alter the effect of THRP on cell survival. It is concluded that THRP is an important factor in TH-induced neuronal cell death.