The role of inflammasomes in human diseases and their potential as therapeutic targets

Inflammasomes are large protein complexes that play a major role in sensing inflammatory signals and triggering the innate immune response. Each inflammasome complex has three major components: an upstream sensor molecule that is connected to a downstream effector protein such as caspase-1 through the adapter protein ASC. Inflammasome formation typically occurs in response to infectious agents or cellular damage. The active inflammasome then triggers caspase-1 activation, followed by the secretion of pro-inflammatory cytokines and pyroptotic cell death. Aberrant inflammasome activation and activity contribute to the development of diabetes, cancer, and several cardiovascular and neurodegenerative disorders. As a result, recent research has increasingly focused on investigating the mechanisms that regulate inflammasome assembly and activation, as well as the potential of targeting inflammasomes to treat various diseases. Multiple clinical trials are currently underway to evaluate the therapeutic potential of several distinct inflammasome-targeting therapies. Therefore, understanding how different inflammasomes contribute to disease pathology may have significant implications for developing novel therapeutic strategies. In this article, we provide a summary of the biological and pathological roles of inflammasomes in health and disease. We also highlight key evidence that suggests targeting inflammasomes could be a novel strategy for developing new disease-modifying therapies that may be effective in several conditions.

Amyloid β-based therapy for Alzheimer's disease: challenges, successes and future

Amyloid β protein (Aβ) is the main component of neuritic plaques in Alzheimer's disease (AD), and its accumulation has been considered as the molecular driver of Alzheimer's pathogenesis and progression. Aβ has been the prime target for the development of AD therapy. However, the repeated failures of Aβ-targeted clinical trials have cast considerable doubt on the amyloid cascade hypothesis and whether the development of Alzheimer's drug has followed the correct course. However, the recent successes of Aβ targeted trials have assuaged those doubts. In this review, we discussed the evolution of the amyloid cascade hypothesis over the last 30 years and summarized its application in Alzheimer's diagnosis and modification. In particular, we extensively discussed the pitfalls, promises and important unanswered questions regarding the current anti-Aβ therapy, as well as strategies for further study and development of more feasible Aβ-targeted approaches in the optimization of AD prevention and treatment.

CNTNAP2 Protein Is Degraded by the Ubiquitin-Proteasome System and the Macroautophagy-Lysosome Pathway

Contactin-associated protein-like 2 (CNTNAP2) gene, located on chromosome 7q35, is one of the largest genes in the human genome. CNTNAP2 protein is a type-I transmembrane protein specifically expressed in the nervous system, with versatile roles in the axonal organization, synaptic functions, neuronal migration, and functional connectivity. CNTNAP2 has been widely investigated as a risk gene for autism spectrum disorder (ASD), and recent studies also implicated CNTNAP2 in Alzheimer's disease (AD). Knowledge of the regulations on CNTNAP2's life cycle is necessary for understanding the related physiological functions and pathological conditions. However, the mechanisms underlying CNTNAP2 protein degradation remain elusive. Therefore, we systematically investigated the half-life and degradation pathway of the human CNTNAP2 protein. We discovered that CNTNAP2 has C-terminal fragments (CTF), which may have essential physiological functions. Our results demonstrated that CNTNAP2 full-length protein and CTF have a short half-life of about 3-4 h. CNTNAP2 proteins are degraded by the ubiquitin-proteasome system and the macroautophagy-lysosome pathway, while the lysosome pathway is more common for CNTNAP2 degradation. This study will provide novel insights and valuable tools for CNTNAP2 functional research in physiological and pathological scenarios.

Brain-derived neurotrophic factor in Alzheimer's disease and its pharmaceutical potential

Synaptic abnormalities are a cardinal feature of Alzheimer's disease (AD) that are known to arise as the disease progresses. A growing body of evidence suggests that pathological alterations to neuronal circuits and synapses may provide a mechanistic link between amyloid β (Aβ) and tau pathology and thus may serve as an obligatory relay of the cognitive impairment in AD. Brain-derived neurotrophic factors (BDNFs) play an important role in maintaining synaptic plasticity in learning and memory. Considering AD as a synaptic disorder, BDNF has attracted increasing attention as a potential diagnostic biomarker and a therapeutical molecule for AD. Although depletion of BDNF has been linked with Aβ accumulation, tau phosphorylation, neuroinflammation and neuronal apoptosis, the exact mechanisms underlying the effect of impaired BDNF signaling on AD are still unknown. Here, we present an overview of how BDNF genomic structure is connected to factors that regulate BDNF signaling. We then discuss the role of BDNF in AD and the potential of BDNF-targeting therapeutics for AD.

Association between menthol-flavoured cigarette smoking and flavoured little cigar and cigarillo use among African-American, Hispanic, and white young and middle-aged adult smokers

OBJECTIVES

Flavour additives in cigarettes and little cigars and cigarillos (LCCs), which influence smokers' risk perceptions, may reinforce dual flavoured tobacco use. We examined the association among mentholated cigarette use, risk perceptions for flavour additives in LCCs and flavoured LCC smoking behaviour.

METHODS

Data from a national probability sample of 964 young and middle-aged adult current cigarette smokers were analysed. Multinomial logistic regression models examined the relationship among mentholated cigarette smoking, risk perceptions and current flavoured LCC use for the analytic sample and gender and race/ethnicity.

RESULTS

Daily menthol cigarette smokers, compared to occasional, non-menthol smokers, had increased odds of flavoured LCC smoking (OR=1.75, 95% CI 1.02 to 2.98). This relationship was found for males, blacks/African-Americans and Hispanics/Latinos (p<0.05). Positive perceptions of menthol-flavoured additives in LCCs was associated with increased odds of flavoured LCC use among the analytic sample, males and blacks/African-Americans (p<0.05). Positive perceptions for clove-flavoured, spice-flavoured and alcohol-flavoured additives were also associated with flavoured LCC use among the analytic sample (p<0.05).

CONCLUSIONS

Use of menthol-flavoured cigarettes and positive perceptions about menthol-flavoured and other flavour additives in LCCs may contribute to dual use with flavoured LCCs among adult cigarette smokers, specifically those from vulnerable populations.

Thyroxine Turnover and Transport in Laennec's Cirrhosis of the Liver

Studies of the metabolism of thyroxine in 14 cases of cirrhosis revealed a variety of deviations from normal. In addition to radiothyroxine turnover studies, determinations were made of the free thyroxine fractions and free thyroxine iodine concentrations in serum (magnesium precipitation method) as well as the maximal binding capacities of thyroxine-binding alpha globulin (TBG) and thyroxine-binding prealbumin (TBPA) by reverse flow paper electrophoresis in a glycine acetate system at pH 8.6.All cases of cirrhosis exhibited diminutions in TBPA capacities but their TBG capacities showed a wide scatter (13.4 to 41.6 mug/100 ml). The free thyroxine fraction was quite variable, with distinct elevations in nine of the 17 sera.The binding proteins appeared to be determinants of the free thyroxine fraction, which in turn, appeared to be a direct determinant of the half-time of turnover. These inferences did not exclude other possible factors including diminished hepatic uptake and metabolism of the hormone in liver disease.Despite considerable alterations in biological half-times, free thyroxine values, and binding proteins, it was remarkable that the absolute hormone disposal was normal in all 14 patients with cirrhosis.

Prostate Specific Antigen Isoforms and Human Glandular Kallikrein 2—Which Offers the Best Screening Performance in a Predominantly Black Population?

PURPOSE

Free prostate specific antigen, complexed PSA and human glandular kallikrein 2 have independently been tested against the gold standard of total PSA for prostate cancer screening in largely white populations. With the incidence of prostate cancer much higher in black men, we sought to evaluate these markers simultaneously in a predominantly black population.

MATERIALS AND METHODS

A total of 138 men, of whom 108 were black, underwent ultrasound guided biopsy of the prostate for tPSA levels greater than 2.5 ng/ml or an abnormal digital rectal examination. Sera were drawn before biopsy and analyzed for tPSA, fPSA, cPSA and hK2 concentrations using standard methods (hK2 assay is for research use only, not for use in diagnostic procedures). The areas under the receiver operator characteristic curves were determined for each marker as well as biomarker combinations. Additionally, each parameter's specificity, positive and negative predictive values, and theoretical screening efficiency were assessed at or above the 95% sensitivity level.

RESULTS

A total of 43 (31.1%) men had prostate cancer by biopsy. While the AUC for %fPSA was statistically the highest (0.822, p <0.001), cPSA offered the highest specificity (31.6%) and positive predictive power (31.7%) of any of the tested biomarkers at comparable sensitivity (greater than 95%). The calculated efficiency of cPSA (51.4%) was also higher than the other markers. Nearly 20% of biopsies would be avoided using cPSA vs standard tPSA screening methods.

CONCLUSIONS

Comparing the major PSA isoforms and hK2, cPSA alone appears to offer superior diagnostic discrimination for cancer detection in a predominantly black population.

Early detection of prostate cancer in African-American men through use of multiple biomarkers: human kallikrein 2 (hK2), prostate-specific antigen (PSA), and free PSA (fPSA)

Recent studies have reported enhanced prostate cancer detection in Caucasians with serum human glandular kallikrein 2 (hK2) in combination with total- (tPSA) and free-prostate-specific antigen (fPSA). The purpose of this study is to validate these findings in an African-American patient cohort. A total of 137 African-American men were found by routine screening to have tPSA levels above 2.5 ng/ml or an abnormal digital rectal examination. Sera were drawn prior to biopsy of the prostate and Hybritech PSA, FPSA and hK2 (for research use only, not for use in diagnostic procedures) concentrations were determined on Beckman Coulter's Access immunoanalyzer. These independent variables and the ratios of percent fPSA (%fPSA), hK2/tPSA, hK2/fPSA, and hK2*tPSA/fPSA were compared between cancer and non-cancer groups. In all, 49 of 137 men had prostate cancer. hK2 and its calculated ratios outperformed tPSA on receiver operator characteristic (ROC) analysis, but %fPSA had statistically the highest area under the curve (AUC) at 0.801. When restricting the analysis to only the tPSA range of 4.0-10 ng/ml, hK2/fPSA yielded the highest AUC (0.721). The ratio of hK2/fPSA was also found to increase the positive predictive value (PPV) of the %fPSA ranges less than 10 and 10-25%. %fPSA offered the best performance and highest specificity in prostate cancer detection in African-American males over the entire range of tPSA. hK2/fPSA may offer modest improvement in the tPSA range of 4.0-10 ng/ml. Furthermore, hK2/fPSA can enhance the PPV of low %fPSA values. Therefore, the use of multiple biomarkers may ultimately increase the specificity of prostate cancer screening in African-American men.

Oct-1 transcription factor is a negative regulator of rat CYP1A1 expression via an octamer sequence in its negative regulatory element

The rat CYP1A1 negative regulatory element (NRE) contains AP-1 and Oct-1 motifs at -808 to -788 bp. The CYP1A1 sequence from -813 to -779 bp and an identical sequence bearing a point mutation in the octamer motif were synthesized. Gel mobility shift assays showed the formation of two complexes with the wild-type CYP1A1 sequence and nuclear extracts from H4IIE and HepG2 hepatoma cells and from rat liver. The formation of the major complex was significantly reduced with the mutant octamer-containing oligomer and was specifically competed by an Oct-1 oligodeoxyribonucleotide. The addition of Oct-1 antibody caused a supershift of the major complex. The presence of the wild-type sequence, but not the mutant octamer sequence, caused a 3-fold decrease in SV40 enhancerless promoter activity in transfected HepG2 cells. Co-transfection of an Oct-1 expression vector with rat CYP1A1 NRE octamer-containing, promoter/reporter gene constructs specifically further decreased promoter activity of the wild-type octamer-containing constructs in HepG2 cells. The results indicate that Oct-1 binds to the rat CYP1A1 promoter NRE and is a negative regulator of rat CYP1A1 expression.

A 24 year follow up of an isolated lymphangioma of the kidney

Renal lymphangioma is a rare benign tumor of the kidney. An example is presented which was initially identified by excretory urography, angiography and biopsy. A 24 year follow up included computed tomography. The radiographic findings, pathogenesis and natural history of this unusual tumor are discussed.

Thyroid hormone action: effect of triiodothyronine on mitochondrial adenine nucleotide translocase in vivo and in vitro

Adenine nucleotide translocase (AdNT) levels were measured as the exchange of extramitochondrial against intramitochondrial adenosine diphosphate (ADP) in liver, spleen, and testes mitochondria isolated from normal and hypothyroid rats using the "back-exchange" and atractyloside-stop method of Pfaff and Klingenberg. The results provide confirmation of previous reports that mitochondria from hypothyroid rats show a markedly diminished AdNT activity, which is restored to normal levels within 72 hours by intraperitoneal injection of 10 to 20 micrograms triiodothyronine (T3)/100 g body weight. The latter dose was found in dose-response studies to result in maximal stimulation of AdNT in liver mitochondria. Qualitatively similar results on AdNT activity were obtained in liver mitochondria within 30 to 60 minutes following intravenous injection into hypothyroid rats of a more physiological dose of T3 (40 ng/100 g body weight). AdNT in mitochondria isolated from spleen and testes (organs that do not exhibit a calorigenic response after administration of thyroid hormone to the whole animal) failed to respond to thyroidectomy and to administration of T3. More recently, we have observed that in vitro replacement of T3 also stimulates AdNT activity in hypothyroid liver mitochondria. The enzyme adenosine triphosphate (ATP) synthase was examined as another possible candidate for direct hormonal stimulation of mitochondria. Simultaneous determinations on the same rats after intraperitoneal injection of T3 (20 micrograms/100 g body weight) showed little or no effect on ATP synthase until after 37 to 85 hours, whereas enhanced activity of the translocator was regularly observed at 17 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of CYP1A1 gene expression in mouse hepatoma cells by benzo[e]pyrene, a ligand of the 4S polycyclic hydrocarbon-binding protein

Hepa 1c1c7 (WT), TAOc1BPrc1 (CI), and BPrc1 (CII) mouse hepatoma cells were exposed to benzo[e]pyrene (B[e]P) or benzo[a]pyrene (B[a]P). B[e]P induced activity of a rat CYP1A1 reporter gene construct (-3015 to +2545 bp) by 1.8- to 2-fold and 5-fold in WT and CI cells, respectively. B[e]P caused a 2-fold induction of a truncated CYP1A1 reporter gene construct (-658 to +2545 bp) in WT cells and induced ethoxyresorufin O-deethylase (EROD) activity by 24- and 13-fold in WT and CI cells. B[a]P also induced CYP1A1 reporter gene and EROD activity in these cells. WT and CII cells had both 8S (Ah) receptor and 4S polycyclic hydrocarbon (PAH)-binding activity, while CI cells exhibited a lower 4S binding activity; 8S binding activity was not detected in CI cells under two separate binding conditions. 8S binding activity in the presence of sodium molybdate was 60-fold greater in WT cells than in CII cells. The absence of sodium molybdate resulted in a dramatic decrease of 8S binding activity in WT cells. The ability of B[e]P to induce CYP1A1 promoter-reporter gene activity and EROD activity in WT and CI cells suggested a role for the 4S PAH-binding protein in the induction of CYP1A1. The lack of detectable 8S binding activity in CI cells was in concert with this role.

Rat CYP1A1 negative regulatory element: biological activity and interaction with a protein from liver and hepatoma cells

Rat CYP1A1 promoter activity was suppressed by the presence of a cis negative regulatory element (NRE) at position -843 to -746 in transiently transfected rat H4IIE and human HepG2 hepatoma cells. Removal of the NRE from the promoter-fusion gene constructs caused an increase in the basal promoter activity of 2-6-fold. Co-transfection of the NRE-containing or non-NRE-containing CYP1A1 promoter-fusion gene constructs with a cloned rat NRE, i.e., pNRE, into HepG2 cells caused a 2-fold or greater reduction in constitutive and induced promoter activities. 2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced expression of the endogenous human CYPA1 was also inhibited by transfection of pNRE into HepG2 cells. Deletion of the sequence from base pairs (bp) -658 to -269 in the NRE-containing construct caused a dramatic decrease of constitutive expression in transiently transfected HepG2 cells, compared with an identical construct that lacked the NRE. Deletion of the sequences between bp -658 and -158 in the CYP1A1 promoter did not affect reporter gene activity, indicating a second site of interaction. At least three different rat liver nuclear proteins bound to the rat NRE, as determined by gel mobility shift and DNase I footprinting assays. A 32-bp sequence within the rat NRE, with significant sequence identity to the 26-bp c-myc, fos/jun-octamer-binding, NRE, was protected from DNAse I cleavage by rat liver nuclear extracts. These data suggested a role for this region in the negative regulation of rat CYP1A1.

Thyroid hormone action: identification of the mitochondrial thyroid hormone receptor as adenine nucleotide translocase

A preliminary report from our laboratory suggested that the thyroid hormone triiodothyronine (T3) is bound with an association constant (Ka) approximating 2 x 10(11) M-1 by adenine nucleotide translocase (AdNT) purified from beef heart mitochondria. We now report that [125I]T3 is capable of photoaffinity labeling not only purified AdNT but also the carrier in intact beef heart mitochondria. Photoaffinity labeling in intact mitochondria was appreciably greater than that observed with purified AdNT. The covalently labeled AdNT was identified by 2-dimensional electrophoresis with pI of 10 on electrofocusing and M(r) of 31,000 on SDS gel. Identification of the covalently labeled protein as authentic AdNT was substantiated by its interaction with a specific monoclonal antibody preparation.

Thyroid hormone action: early calorigenic effect on dispersed rat liver cells in the absence of protein synthesis

Isolated dispersed rat liver cells were prepared by hypothyroid Sprague-Dawley rats. The cells were incubated under 95% O2/5% CO2 in Krebs-Ringer-bicarbonate buffer at pH 7.3-7.4 at 37 degrees C. The medium had been enriched with 2% bovine serum albumin (previously stripped of thyroid hormone) and 5-10 mM alanine as substrate. Two hour incubations were carried out with or without added triiodothyronine (T3) at 3 nM or 300-1,000 nM concentrations. Oxygen consumption determined at the end of the period of incubation with the Clark oxygen electrode showed stimulation above control values in the hormone treated flasks; parallel studies in which cycloheximide (100 microM) had been added to cells to block protein synthesis also showed enhanced oxygen consumption in response to T3. The results indicated a response to the hormone not dependent on new protein formation.

Tissue-specific gene expression results from a purine- and pyrimidine-free diet and 6-mercaptopurine in the rat small intestine and colon

Dietary purines and pyrimidines are not considered to have a nutritional role, much less a direct effect on the functioning of the gastrointestinal tract. We found that a dramatic decrease in adult rat small intestinal and colonic total ribonucleic acid (RNA) results from the removal of dietary purines and pyrimidines or the administration of 6-mercaptopurine. Ribonucleic acid hybridization analysis indicated specific decrease of the messenger ribonucleic acid (mRNA) for the purine salvage enzymes hypoxanthine-guanine phosphoribosyl transferase and adenine phosphoribosyl transferase in the small intestine and proximal colon but not in the liver of animals fed a diet lacking purines and pyrimidines. Levels of intestinal and hepatic beta-actin mRNA transcripts were generally not depressed by either diet or by the administration of 6-mercaptopurine. Liver hypoxanthine-guanine phosphoribosyl transferase and adenine phosphoribosyl transferase mRNAs were unaffected by the change in diet but were lowered by the administration of 6-mercaptopurine. These data indicate that nutrition and 6-mercaptopurine affect both total RNA, and individual mRNA concentrations at specific sites in the gastrointestinal tract. These findings are of potentially great significance because the regulation of intestinal total RNA levels and purine salvage mRNAs by both 6-mercaptopurine and a purine- and pyrimidine-free diet suggests a potential mechanism by which dietary components differentially control specific proteins synthesized in the body. These findings may be related to the efficacy of 6-mercaptopurine as well as so-called elemental diets as therapeutic agents in chronic inflammatory bowel disease (i.e., Crohn's disease).

Direct thyroid hormone activation of mitochondria: the role of adenine nucleotide translocase

A presumptive mitochondrial T3 receptor previously reported from this and other laboratories appears capable of accounting for the activation of liver mitochondrial oxidative phosphorylation within 30 min after iv bolus injection of nanogram doses of T3 into hypothyroid rats. The inner mitochondrial membrane carrier adenine nucleotide translocase (AdNT) catalyzes the exchange between the extra- and intramitochondrial ADP and ATP, and has been shown by measurements of flux control coefficients to exert a significant measure of control over the rate of mitochondrial oxidative phosphorylation. The activity of this carrier had been reported to be depressed below normal in hypothyroid rats and restored to normal by hormone replacement. Preparations of AdNT from beef heart mitochondria were found to exhibit high affinity, low capacity binding of [125I]T3. The findings make the mitochondrial carrier AdNT a strong candidate for the initiating site for thyroid hormone stimulation in mammalian species.

Mitochondrial binding of triiodothyronine (T3). Demonstration by electron-microscopic radioautography of dispersed liver cells

To assess the distribution of the thyroid hormone triiodothyronine (T3) within intact living cells, freshly prepared dispersed rat hepatocytes were incubated with [125I]-T3 for periods of 5 min and 30 min. Light- and electron-microscopic (EM) radioautography was carried out to determine the distribution of grains over the isolated cells. Both procedures showed the grains distributed almost entirely over the cytoplasmic matrix rather than the nucleus. Grain counts under the EM were compared with expectation based on established quantitative methods. Only the mitochondria showed obvious and statistically significant grain counts, whereas the nucleus failed to accumulate grains in excess of expectations by chance alone based on area. The findings support the existence of mitochondrial binding of T3, presumably a prerequisite for its action in direct stimulation of the mitochondria.

Purification of the mitochondrial triiodothyronine (T3) receptor from rat liver

The thyroid hormone receptor of the inner membrane of rat liver mitochondria was purified by osmotic and freeze-thaw lyses followed by partial purification on Sephadex G-200, and then by affinity chromatography with T3-Sepharose 4B. A single predominant protein band demonstrable on sodium dodecylsulphate (SDS) polyacrylamide gel electrophoresis was present in the first 4 mM NaOH elution peak of affinity chromatography. This was collected from affinity peaks from about 30 rat livers followed by preparative polyacrylamide gel electrophoresis. A single absorbance peak was observed by high pressure liquid chromatography (HPLC). The purified protein was analyzed for binding constants, amino acid composition, and characterized by analytical ultracentrifugation. The association constant (KA) exceeded 10(11) M-1. The sedimentation coefficient (S20,W) was 2.2S, partial specific volume (v) 0.72, frictional coefficient (f/fo)s M 1.68 and the molecular weight was estimated at 28 000. The amino acid composition was obtained.

Acquired refractoriness to thyroid hormone action in treated toxic nodular goiter

A case of apparent target organ refractoriness to thyroid hormones has been observed during the recovery phase of toxic nodular goiter after 131I therapy. The serum PBI was normal before the onset, but elevated serum PBI and T4 concentrations were observed during the toxic phase, and after recovery, with elevated T3 as well. The case differed in important respects from the congenital, hereditary disorder known as the Refetoff syndrome.

Rapid effect of triiodothyronine on the mitochondrial pathway in rat liver in vivo

Intravenous infections of minute doses of triiodothyronine were administered to thyroidectomized rats 30 minutes before they were killed. Hepatic mitochondria were isolated rapidly and formation of adenosine triphosphate and consumption of oxygen were assessed by a 2-minute incubation. Hormone injection enhanced formation of adenosine triphosphate 114 to 217 percent over control values, with a proportionate increase in consumption of oxygen. The ratio of phosphate to oxygen was about 2.0, signifying tightly coupled oxidative phosphorylation. Stimulation was not abolished by injection of cycloheximide, puromycin, actinomycin D, or chloramphenicol 1 hour before the rats were killed. This signifies direct mitochondrial stimulation by triiodothyronine in the absence of protein synthesis.

Mitochondrial thyroid hormone receptor: localization and physiological significance

Binding studies of thyroid hormone to submitochondrial fractions from rat liver suggest that the component responsible for high-affinity, low-capacity (saturable) binding of hormones arises from the inner mitochondrial membrane. The partially purified component, approximately 150,000 daltons, appears to be half protein and half lipid, largely phospholipids, tentatively identified as lecithin, phosphatidyl ethanolamine, and cardiolipin. A similar hormone-binding macromolecule was found in mitochondria from rabbit kidney, from human liver and kidney, and from rat kidney, myocardium, skeletal muscle, intestinal mucosa, whole small intestine, adipose tissue, and lung. It was absent from mitochondria of adult rat brain, spleen, and testis, organs calorigenically unresponsive to thyroid hormones injected in vivo, but was present in mitochondria from brains of rats 12 days old and younger. The organ distribution of the hormone-binding protein and its presence in neonatal brain mitochondria supports the biological relevance of the mitochondrial component as a thyroid hormone receptor.

Thyroid hormone action: the mitochondrial pathway

The subcellular compartments have been investigated to compare proteins capable of binding triiodothyronine and thyroxine; specific binders have been found in cytosol, nuclei, and mitochondria from rat liver and kidney. The binding protein from the inner mitochondrial membrane had the highest association constant (greater than 10(11) liters per mole), suggesting possible direct hormone action on the mitochondria. Binding of hormone analogs was found to be related to known physiological potency, and stereospecific discrimination between L- and D-thyroxine was observed. The saturable receptor was found in the mitochondrial membranes of rat liver, kidney, myocardium, and skeletal muscle but not in mitochondria from the unresponsive tissues: brain, spleen, and testis. Oxidative phosphorylation by mitochondrial vesicles from hypothyroid rats increased after the addition of physiological concentrations of triiodothyronine, which corroborated direct hormone action on mitochondria.

The mitochondrial route of thyroid hormone action

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Thyroid hormone binding by a component of mitochondrial membrane

The thyroid hormone, triiodothyronine, has been shown to be bound by the intranuclear chromatin protein associated with active DNA, where it is believed to stimulate transcription. Evidence exists that the thyroid hormones have direct action not only on nuclei, but also on mitochondria. Threfore, specific proteins that bind thyroid hormones in the mitochondria should be demonstrable. Mitochondria were isolated from homogenized rat livers by sedimentation through 0.25 M sucrose solution, followed by washing four times to free them of microsomes. Strong binding of thyroid hormones was observed in mitochondrial fractions prepared from both the membranes and the matrix. After incubation in an ice bath with increasing amonts of triiodothyronine with added tracer [125I]triiodothyronine, the matrix infrequently contained specific saturable receptor sites, but usually exhibited strong "nonspecific" interaction...

Serum triiodothyronine concentration in thyroid storm

Serum triiodothyronine levels were elevated in 6 patients with thyroid storm (769 plus or minus 181 ng/100 ml) but the values observed were not significantly different from those found in uncomplicated thyrotoxicosis (752 plus or minus 282 ng/100 ml). This observation suggests that the pathogenesis of thyroid storm resides in mechanisms other than a simple increase in serum triiodothyronine concentration.

Effects of replacement doses of sodium L-thyroxine on the peripheral metabolism of thyroxine and triiodothyronine in man

Studies of the effect of L-thyroxine administration (0.3 mg daily for 7-9 wk) on the peripheral metabolism of (131)I-labeled triiodothyronine (T(3)) and (125)I-labeled thyroxine (T(4)) and on the concentration and binding of T(4) and T(3) in serum were carried out in 11 euthyroid female subjects. Administration of L-thyroxine led to consistent increases in serum T(3) concentration (137 vs. 197 ng/100 ml), T(3) distribution space (39.3 vs. 51.7 liters), T(3) clearance rate (22.9 vs. 30.6 liters/day) and absolute T(3) disposal rate (30 vs. 58 mug/day), but no change in apparent fractional turnover rate (60.3 vs. 60.6%/day). The proportion and absolute concentration of free T(3) also increased during L-thyroxine administration. Increases in serum total T(4) concentration (7.3 vs. 12.8 mug/100 ml) and in both the proportion and absolute concentration of free thyroxine also occurred. In five of the subjects, the kinetics of peripheral T(4) turnover were simultaneously determined and a consistent increase in fractional turnover rate (9.7 vs. 14.2%/day), clearance rate (0.84 vs. 1.37 liters/day), and absolute disposal rate (64.2 vs. 185.0 mug/day) occurred during L-thyroxine administration. Despite these increases in the serum concentration and daily disposal rate of both T(4) and T(3), the patients were not clinically thyrotoxic. However, basal metabolic rate (BMR) values were marginally elevated and, as in frank thyrotoxicosis, T(4)-binding capacities of thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA) reduced, suggesting that subclinical thyrotoxicosis was present. Thus, the often recommended replacement dose of 0.3 mg L-thyroxine daily may be greater than that required to achieve the euthyroid state. The studies have also provided additional evidence of the peripheral conversion of T(4) to T(3) in man and have permitted the calculation that approximately one-third of exogenously administered T(4) underwent deiodination to form T(3). To the extent that a similar fractional conversion occurs in the normal state, it can be calculated that a major fraction of the T(3) in serum derives from the peripheral deiodination of T(4) and that only a lesser fraction derives from direct secretion by the thyroid gland.

Conversion of thyroxine to triiodothyronine by cultured human cells

Human liver and kidney cells convert 6 to 10 percent of added thyroxine to triiodothyronine in vitro at 37 degrees C. This extent of conversion is ten times greater than that in control studies with killed cells. Conversion is evident within 10 minutes and appears to be maximal within 1 hour. Greater net triiodothyronine formation results if greater amounts of exogenous thyroxine are added to the system, with no plateau evident even at very high thyroxine concentrations. The addition of high concentrations of nonradioactive triiodothyronine resulted in no evident inhibition of the conversion.