Protein half-lives of two subunits of an NMDA receptor-like complex, the 71-kDa glutamate-binding and the 80-kDa CPP-binding protein

Thyroid hormone resistance: Mechanisms and therapeutic development

As an essential primary hormone, thyroid hormone (TH) is indispensable for human growth, development and metabolism. Impairment of TH function in several aspects, including TH synthesis, activation, transportation and receptor-dependent transactivation, can eventually lead to thyroid hormone resistance syndrome (RTH). RTH is a rare syndrome that manifests as a reduced target cell response to TH signaling. The majority of RTH cases are related to thyroid hormone receptor β (TRβ) mutations, and only a few RTH cases are associated with thyroid hormone receptor α (TRα) mutations or other causes. Patients with RTH suffer from goiter, mental retardation, short stature and bradycardia or tachycardia. To date, approximately 170 mutated TRβ variants and more than 20 mutated TRα variants at the amino acid level have been reported in RTH patients. In addition to these mutated proteins, some TR isoforms can also reduce TH function by competing with primary TRs for TRE and RXR binding. Fortunately, different treatments for RTH have been explored with structure-activity relationship (SAR) studies and drug design, and among these treatments. With thyromimetic potency but biochemical properties that differ from those of primary TH (T3 and T4), these TH analogs can bypass specific defective transporters or reactive mutant TRs. However, these compounds must be carefully applied to avoid over activating TRα, which is associated with more severe heart impairment. The structural mechanisms of mutation-induced RTH in the TR ligand-binding domain are summarized in this review. Furthermore, strategies to overcome this resistance for therapeutic development are also discussed.

Molecular Functions of Thyroid Hormone Signaling in Regulation of Cancer Progression and Anti-Apoptosis

Several physiological processes, including cellular growth, embryonic development, differentiation, metabolism and proliferation, are modulated by genomic and nongenomic actions of thyroid hormones (TH). Several intracellular and extracellular candidate proteins are regulated by THs. 3,3,5-Triiodo-L-thyronine (T₃) can interact with nuclear thyroid hormone receptors (TR) to modulate transcriptional activities via thyroid hormone response elements (TRE) in the regulatory regions of target genes or bind receptor molecules showing no structural homology to TRs, such as the cell surface receptor site on integrin αvβ3. Additionally, L-thyroxine (T₄) binding to integrin αvβ3 is reported to induce gene expression through initiating non-genomic actions, further influencing angiogenesis and cell proliferation. Notably, thyroid hormones not only regulate the physiological processes of normal cells but also stimulate cancer cell proliferation via dysregulation of molecular and signaling pathways. Clinical hypothyroidism is associated with delayed cancer growth. Conversely, hyperthyroidism is correlated with cancer prevalence in various tumor types, including breast, thyroid, lung, brain, liver and colorectal cancer. In specific types of cancer, both nuclear thyroid hormone receptor isoforms and those on the extracellular domain of integrin αvβ3 are high risk factors and considered potential therapeutic targets. In addition, thyroid hormone analogs showing substantial thyromimetic activity, including triiodothyroacetic acid (Triac), an acetic acid metabolite of T₃, and tetraiodothyroacetic acid (Tetrac), a derivative of T₄, have been shown to reduce risk of cancer progression, enhance therapeutic effects and suppress cancer recurrence. Here, we have reviewed recent studies focusing on the roles of THs and TRs in five cancer types and further discussed the potential therapeutic applications and underlying molecular mechanisms of THs.

The connexin turnover, an important modulating factor of the level of cell-to-cell junctional communication: comparison with other integral membrane proteins

The constituent proteins of gap junctions, called "connexins" (Cxs) in chordates, are generally renewed several times a day, in approximately the same rate range as many other integral plasma membrane proteins and the proteins of other channels, other intercellular junctions or different membrane receptors. This permanent renewal turns on a fine-tuned balance among various processes, such as gene transcription, mRNA stability and processing, protein synthesis and oligomerization, posttranslational modifications, transport to the plasma membrane, anchoring to the cytoskeleton, connexon aggregation and docking, regulation of endocytosis and controlled degradations of the proteins. Subtle changes at one or some of these steps would represent an exquisite level of regulation that extends beyond the rapid channel opening and closure events associated with channel gating; membrane channels and receptors are constantly able to answer to physiological requirements to either up- or downregulate their activity. The Cx turnover rate thereby appears to be a key component in the regulation of any protein, particularly of gap junctional proteins. However, the physiological stimuli that control the assembly of Cxs into gap junctions and their degradation remain poorly understood.

Post-training N-methyl-d-aspartate receptor blockade offers protection from retrograde interference but does not affect consolidation of weak or strong memory traces in the water maze

Memory consolidation is the process where labile memory traces become long-lasting, stable memories. Previous work has demonstrated that spatial memory consolidation, several days after training in a water maze had ceased, can be disrupted by a temporary intra hippocampal infusion of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate antagonist LY326325 (Riedel et al., 1999). Such reversible pharmacological techniques offer advantages over the permanent lesion studies that had first suggested a role for the hippocampus in memory consolidation. However, to date the role of N-methyl-D-aspartate receptors in such systems level processes remains controversial with evidence for impairments and augmentation of performance. Here we investigate the role of post-training hippocampal N-methyl-D-aspartate receptor blockade in rats and mice on the consolidation of weak and strong memory traces using an Atlantis water maze protocol. A hidden Atlantis platform was employed and rats (experiments 1 and 2) and mice (experiment 3) were required to dwell within 20 cm of the trained location to activate and subsequently reveal the escape platform. In experiments 1 and 3 a strong memory trace was established by training rats or mice for several days in the water maze. In experiment 2 a significantly weaker trace was instituted by reducing the training period. N-methyl-D-aspartate receptor blockade was induced after the last training trial and continued for seven days. Reliable memory for the trained platform location in a retention test 15 days after the last training day demonstrated that N-methyl-D-aspartate receptor blockade did not affect memory consolidation in rats or mice. Our results also show that post-training N-methyl-D-aspartate receptor blockade can lead to better performance in further retention tests conducted after the consolidation and drug administration period. Those data suggest that specific post-training N-methyl-D-aspartate receptor blockade does not impair memory consolidation and it may also offer a memory trace mild protection from retrograde interference.

Chronic ethanol exposure increases gene transcription of subunits of an N-methyl-D-aspartate receptor-like complex in cortical neurons in culture

Chronic exposure to ethanol leads to adaptive responses in neurons, including enhanced activity of N-methyl-D-aspartate (NMDA) receptors. Chronic treatment of neurons with ethanol also leads to increases in the protein levels of three subunits of an NMDA receptor-like complex. In the present study, we explored whether the increases in subunit protein levels are the result of either increased transcription or diminished protein turnover. We found that a 72-h exposure of cortical neurons in culture to 100 mM ethanol caused enhanced transcription of the genes for the glutamate-binding (GBP) and glycine-binding protein subunits of the receptor-like complex. This treatment had no effect on protein turnover of either GBP or of the NMDA receptor subunit NR1, suggesting that the increases in protein expression are the result of increased DNA transcription.

Calcium influx through NMDA receptors, chronic receptor inhibition by ethanol and 2-amino-5-phosponopentanoic acid, and receptor protein expression

Chronic treatment of neurons with either ethanol or competitive and noncompetitive antagonists of NMDA receptors leads to enhanced expression of NMDA receptor density and function in neurons. The signal transduction pathways for such receptor up-regulation are not known. The focus of the present study was on the role of Ca2+ entry into neurons, either through receptor or voltage-gated channels, in the expression of the NMDA receptor subunit NR1 and the 71-kDa glutamate-binding protein (GBP) of a glutamate/NMDA receptor-like complex. Chronic inhibition of NMDA receptors in cortical neurons in primary cultures by either 100 mM ethanol or 100 microM 2-amino-5-phosphonopentanoic acid (2-AP5) increased the expression of NR1 and GBP. The effect of 2-AP5 on the expression of the two proteins was not additive with that of ethanol when neuronal cultures were treated with both agents at the same time. However, the effects of ethanol on NR1 and GBP expression were blocked by the simultaneous treatment with NMDA (50 microM). Activation or inhibition of other glutamate ionotropic receptors had no effect on the expression of NR1 and GBP. The inhibition of L- or N-type voltage-sensitive Ca2+ channels and voltage-gated Na+ channels also had little effect on the expression of either protein; neither did exposure of neurons to elevated extracellular Ca2+ concentrations (3 or 5 mM). On the other hand, treatment of neurons for 48 h with the intracellular Ca2+ chelator BAPTA-AM as well as partial chelation of extracellular Ca2+ with EGTA caused an up-regulation in NR1 and GBP expression. The enhanced expression of NR1 in neurons treated for 48 h with either ethanol or EGTA was correlated with increases in the activity of NMDA receptors demonstrated as a doubling of the NMDA-stimulated rise in intracellular free Ca2+ concentration. The effects of chronic administration of EGTA on both NR1 expression as well as NMDA receptor function were probably related to an acute inhibition by EGTA of NMDA-induced Ca2+ influx into neurons. It appears that the expression of both the NR1 subunit of NMDA receptors and the GBP of a receptor-like complex is regulated by intracellular Ca2+, especially that entering through NMDA receptor ion channels.