School practices to promote social distancing in K-12 schools: review of influenza pandemic policies and practices

BACKGROUND

During an evolving influenza pandemic, community mitigation strategies, such as social distancing, can slow down virus transmission in schools and surrounding communities. To date, research on school practices to promote social distancing in primary and secondary schools has focused on prolonged school closure, with little attention paid to the identification and feasibility of other more sustainable interventions. To develop a list and typology of school practices that have been proposed and/or implemented in an influenza pandemic and to uncover any barriers identified, lessons learned from their use, and documented impacts.

METHODS

We conducted a review of the peer-reviewed and grey literature on social distancing interventions in schools other than school closure. We also collected state government guidance documents directed to local education agencies or schools to assess state policies regarding social distancing. We collected standardized information from each document using an abstraction form and generated descriptive statistics on common plan elements.

RESULTS

The document review revealed limited literature on school practices to promote social distancing, as well as limited incorporation of school practices to promote social distancing into state government guidance documents. Among the 38 states that had guidance documents that met inclusion criteria, fewer than half (42%) mentioned a single school practice to promote social distancing, and none provided any substantive detail about the policies or practices needed to enact them. The most frequently identified school practices were cancelling or postponing after-school activities, canceling classes or activities with a high rate of mixing/contact that occur within the school day, and reducing mixing during transport.

CONCLUSION

Little information is available to schools to develop policies and procedures on social distancing. Additional research and guidance are needed to assess the feasibility and effectiveness of school practices to promote social distancing.

Health implications of social networks for children living in public housing

This study sought to examine whether: (1) the health composition of the social networks of children living in subsidized housing within market rate developments (among higher-income neighbors) differs from the social network composition of children living in public housing developments (among lower-income neighbors); and (2) children's social network composition is associated with children's own health. We found no significant differences in the health characteristics of the social networks of children living in these different types of public housing. However, social network composition was significantly associated with several aspects of children's own health, suggesting the potential importance of social networks for the health of vulnerable populations.

Distinct antigenic features of linear epitopes at the N-terminus and C-terminus of 65 kDa glutamic acid decarboxylase (GAD65): implications for autoantigen modification during pathogenesis

Autoantibodies to 65 kDa glutamic acid decarboxylase (GAD65) are produced in many patients with autoimmune polyendocrine syndrome type II (APS-II) or stiff-man syndrome (SMS) and are heterogeneous in their epitope specificities, recognizing both conformational and linear determinants. Major linear epitopes of GAD, which are recognized by autoantibodies in a minority of these patients, occur in the N-terminal and C-terminal regions. We have investigated antibody recognition of the N- and C-termini of GAD65 in relation to their structural features as an approach to understanding what modifications to the native GAD structure may occur that facilitate the generation of antibodies specific to linear epitopes in these regions during the autoimmune pathogenesis. A monoclonal antibody specific to the N-terminus of GAD65 bound both native and denatured GAD in ELISA, whereas monoclonal and polyclonal antibodies specific to the C-terminus of GAD bound only denatured GAD. These antibodies were epitope mapped using random peptide phage-display libraries and the epitopes related to a previously proposed structural model of GAD65. This has led us to propose that the alpha-helical secondary structure of the C-terminus of GAD65 must be denatured to generate linear epitopes. In contrast, the N-terminus is both surface exposed and linear in the native structure, but may be masked by membrane interactions, which must be broken to facilitate recognition by B cells.

Quantitative assessment of pituitary resistance to thyroid hormone from plots of the logarithm of thyrotropin versus serum free thyroxine index

Previous studies have shown that, in patients with primary alterations in thyroid hormone secretion, the level of the natural logarithm of serum TSH (lnTSH) is negatively related to the level of free T4. Because such patients can generally be assumed to exhibit normal tissue responsivity to thyroid hormone, we were interested in determining whether the lnTSH/free T4 index (FTI) relationship in patients with established thyroid hormone resistance (THR) exhibit a lower slope than patients with normal tissue sensitivity to thyroid hormone. We have therefore analyzed the relationship between the lnTSH and the FTI in members of three families with documented THR. In these patients, a given dose of T4 was maintained for a 1- to 2-month period, to achieve hormonal equilibration. Two of the families, though not related, exhibited the same mutation, E460K. The third was identified as A317T. As anticipated, the slope of the lnTSH/FTI ratio was significantly lower in the patients with THR than in T4-treated patients who were presumed to have normal sensitivity to thyroid hormone. The slope of the lnTSH/FTI relationship seemed to be characteristic of the specific mutation involved in the three genotypes (wild-type and two mutations) examined. Further, the in vivo slope of the lnTSH/FTI relationship seemed to be linearly related to the T3 association constant of the in vitro translated receptor. These findings support the potential usefulness of measuring the slope of lnTSH, as a function of the FTI, in quantitating pituitary THR.

High-resolution autoreactive epitope mapping and structural modeling of the 65 kDa form of human glutamic acid decarboxylase

The smaller isoform of the GABA-synthesizing enzyme, glutamic acid decarboxylase 65 (GAD65), is unusually susceptible to becoming a target of autoimmunity affecting its major sites of expression, GABA-ergic neurons and pancreatic beta-cells. In contrast, a highly homologous isoform, GAD67, is not an autoantigen. We used homolog-scanning mutagenesis to identify GAD65-specific amino acid residues which form autoreactive B-cell epitopes in this molecule. Detailed mapping of 13 conformational epitopes, recognized by human monoclonal antibodies derived from patients, together with two and three-dimensional structure prediction led to a model of the GAD65 dimer. GAD65 has structural similarities to ornithine decarboxylase in the pyridoxal-5'-phosphate-binding middle domain (residues 201-460) and to dialkylglycine decarboxylase in the C-terminal domain (residues 461-585). Six distinct conformational and one linear epitopes cluster on the hydrophilic face of three amphipathic alpha-helices in exons 14-16 in the C-terminal domain. Two of those epitopes also require amino acids in exon 4 in the N-terminal domain. Two distinct epitopes reside entirely in the N-terminal domain. In the middle domain, four distinct conformational epitopes cluster on a charged patch formed by amino acids from three alpha-helices away from the active site, and a fifth epitope resides at the back of the pyridoxal 5'-phosphate binding site and involves amino acid residues in exons 6 and 11-12. The epitopes localize to multiple hydrophilic patches, several of which also harbor DR*0401-restricted T-cell epitopes, and cover most of the surface of the protein. The results reveal a remarkable spectrum of human autoreactivity to GAD65, targeting almost the entire surface, and suggest that native folded GAD65 is the immunogen for autoreactive B-cells.

Chicken ovalbumin upstream promoter-transcription factor (COUP-TF) modulates expression of the Purkinje cell protein-2 gene. A potential role for COUP-TF in repressing premature thyroid hormone action in the developing brain

The cerebellar Purkinje cell-specific PCP-2 gene is transcriptionally activated by thyroid hormone during the 2nd and 3rd weeks of postnatal life in the rat. In contrast, thyroid hormone has no detectable effects on PCP-2 expression in the fetal rat. We now present data that suggest that the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor (COUP-TF) represses triiodothyronine (T3)-dependent transcriptional activation of PCP-2 in the immature Purkinje cell. Gel shift assays show that the PCP-2 A1TRE and adjoining sequences (-295/-199 region) bind to rat and mouse brain nucleoproteins in a developmentally regulated fashion and that one of these nucleoproteins could be the orphan nucleoprotein COUP-TF. In support of this hypothesis, in vitro translated COUP-TF binds to the -295/-199 region and COUP-TF represses T3-dependent activation of the PCP-2 promoter in transient transfection analyses. Finally, immunohistochemical studies reveal that COUP-TF is specifically expressed in the immature fetal and early neonatal Purkinje cell and that this expression diminishes coincident with thyroid hormone induction of PCP-2 expression. Our findings are consistent with the hypothesis that the presence or absence of inhibitory proteins bound to the thyroid hormone response element of T3-responsive genes governs the responsivity of these genes to thyroid hormone during brain development.

Beta receptor isoforms are not essential for thyroid hormone-dependent acceleration of PCP-2 and myelin basic protein gene expression in the developing brains of neonatal mice

In rat pups, thyroid hormone dependent brain development coincides with the appearance of the thyroid hormone receptor (TR)beta1 isoform. This finding led to the suggestion that TRbeta1 plays an essential role in brain development. The recent availability of a mouse TRbeta knockout strain allowed us to test this possibility by determining whether TRbeta is essential for the normal developmental pattern of expression of two thyroid hormone regulated brain genes, myelin basic protein (MBP), and Purkinje cell protein 2 (Pcp-2). Northern analysis of total mRNA from the brains of wild-type mice established that, as in the rat pup, the initial rate of rise of the MBP and Pcp-2 mRNA is slowed in the hypothyroid state. Supporting the effectiveness of TRbeta gene deletion was the finding that the thiiodothyronine (T3) nuclear binding capacity in the livers and brains of knockout animals was consistent with the fractional contribution of TRbeta1 to total binding capacity in the wild-type tissues. Further, no TRbeta1 could be detected by isoform-specific immunoprecipitation of nuclear receptor extracts. However, deletion of the functional TRbeta in the TRbeta knockout mice did not affect the normal ontogeny of expression of the Pcp-2 and MBP genes in the postnatal pup. We conclude that TRbeta is not essential for the normal developmental expression of these T3 dependent brain genes.

Lack of effect of thyroid hormone on late fetal rat brain development

Studies were undertaken to test whether alterations in fetal brain thyroid hormone levels during the final week of gestation can prematurely induce gene expression in brain or affect cerebellar morphogenesis. Pregnant dams were treated either by administration of 0.025% methimazole (MMI) in the drinking water from day 14 post conception (PC14) or administration of 2.5 mg T4/100 g BW on PC15. On PC21, treatment with MMI resulted in a 53% fall in fetal brain T3 levels and excess T4 resulted in a 2- to 3-fold increase to concentrations observed in adult brains. Neither excess nor reduced levels of T3 caused alterations in the expression of the myelin basic protein, Pcp-2 or calmodulin kinase IV genes. Cerebella of control brains showed early evidence of foliation and the presence of a several cell thick Purkinje cell layer and an external granule layer. No treatment induced effects were evident. Thus, at the late fetal stage in the rat, the developing brain appears to be unresponsive to thyroid hormone despite the presence of thyroid hormone receptors. We infer the presence of as yet unidentified factors that suppress precocious response to thyroid hormone or the absence of cofactors essential for such a response.

Purkinje cell protein-2 cis-elements mediate repression of T3-dependent transcriptional activation

Previous studies in our laboratory show that triiodothyronine upregulates expression of the cerebellar Purkinje cell-specific gene Pcp-2 during the first 2 weeks of rat neonatal life. A specific thyroid hormone response element, the A1 TRE, mediates this regulation. The finding that the contiguous 68 bases (-267/ -199) of the Pcp-2 promoter 3' to the A1 TRE repressed T3 response in transactivation studies suggested that this sequence could play a role in preventing premature T3-dependent activation of Pcp-2 in the fetus. We now show that deletion of this region resulted in enhanced T3-dependent activation of the native Pcp-2 promoter. The sequence is not a generalized silencer since it does not alter basal activity of mouse mammary tumor virus (MMTV) or thymidine kinase (TK) promoters. Deletion and linker scanning studies indicate that the 5' 30 bases of the -267/ -199 region mediate most of the response silencing activity. The -267/ -199 region also attenuates T3-induced transactivation mediated by other TREs. Gel shift analysis reveals that nuclear proteins from fetal but not adult brains complex with the -267/ -199 region, supporting the hypothesis that this region binds proteins that suppress Pcp-2 expression early in brain development.

Transient stimulation of myelin basic protein gene expression in differentiating cultured oligodendrocytes: a model for 3,5,3'-triiodothyronine-induced brain development

We compared the regulation of myelin basic protein (MBP) gene expression by T3 in differentiating oligodendrocytes in culture with that previously observed by us in the neonatal rat brain. As in intact brain, expression of the T3R alpha gene preceded that of the T3R beta gene. Although the absence of T3 retarded the rate of accumulation of MBP messenger RNA, the level ultimately attained was similar to that reached in the presence of T3. This relationship mirrored the pattern observed in the neonatal brain. Transient transfection experiments showed that T3 regulates MBP expression at the transcriptional level, but only for a limited period during differentiation. These observations imply that the early rise of MBP messenger RNA is T3 dependent, whereas the terminal levels are maintained independently of T3. Both the T3-dependent and, surprisingly, the T3-independent expression of MBP require the presence of an intact T3 response element. T3 receptor may regulate MBP expression in a ligand-independent manner, or a nuclear factor other than T3 receptor may bind to the T3 response element of MBP to regulate terminal gene expression. These findings support the use of differentiating oligodendrocytes as a model of T3-induced brain development.

Isoform-specific 3,5,3'-triiodothyronine receptor binding capacity and messenger ribonucleic acid content in rat adenohypophysis: effect of thyroidal state and comparison with extrapituitary tissues

Although the role of the three functional thyroid hormone receptor isoforms (TR beta 1, TR beta 2, and TR alpha 1) remains unclear, studies by Hodin and Lazar et al. have suggested that restriction of TR beta 2 messenger RNA (mRNA) to rat pituitary could reflect a specific regulatory role in the pituitary. Supporting their hypothesis was a significant fall in pituitary TR beta 2 mRNA after T3 administration. These observations prompted us to assess the effect of thyroidal state on the level of TR beta 2 protein, as inferred by immunoprecipitation of TR beta 2 nuclear binding activity. In contrast to the behavior of the mRNA, we noted surprising stability in the levels of total nuclear TR binding capacity and TR isoform distribution in the transition from hypo- to hyperthyroid states. Calculations based on these and previous data from this laboratory (7) show that the average cellular content of TR beta 2 mRNA in pituitary is 0.6 molecules, whereas the content of TR beta 2 mRNA molecules in extrapituitary tissues is less than 0.007 molecule/cell. A high TR beta 2 protein/mRNA ratio in extrapituitary tissues thus could reflect a rapid turnover of TR beta 2 mRNA compared to TR beta 2 protein. This would explain the widespread distribution of TR beta 2 protein and the scarcity of mRNA in extrapituitary tissues.

Tissue-specific regulation of malic enzyme by thyroid hormone in the neonatal rat

Two recent studies have claimed that thyroid hormone administration accelerates malic enzyme gene expression in the neonatal brain in contrast to the well-documented lack of effect of triiodothyronine on malic enzyme gene expression in the adult brain. Since these observations conflict with earlier observations in our laboratory, we reinvestigated the effect of thyroid hormone status on the ontogeny of malic enzyme gene expression in the neonatal rat. Neither hypothyroidism nor hyperthyroidism influenced the ontogenesis of malic enzyme activity in neonatal brain whereas the patterns of gene expression and enzyme activity in liver were markedly affected. Our results suggest that tissue-specific factors in brain block thyroid hormone-induced gene expression by thyroid hormone.

Thyroid hormone receptor isoform content in cultured type 1 and type 2 astrocytes

Immunohistochemical studies previously reported from this laboratory showed that astrocytes in adult rat brain appear devoid of all thyroid hormone receptor (TR) isoforms. These findings, however, contrast with reports of measurable nuclear T3 binding in astrocytes in cell culture. To address this discrepancy, TR protein and messenger RNA (mRNA) content of type 1 and type 2 astrocytes in culture were assayed. Type 1 cells represent astrocytes present in brain in vivo. Type 2 astrocytes differentiate in culture from bipotential progenitor O-2A cells in the presence of serum. Under serum-free conditions, these progenitor cells differentiate into oligodendroglia. Total nuclear T3 binding capacity in both type 1 and type 2 astrocytes was approximately 3000 sites/cell. Northern blots showed the presence of mRNA for TRbeta1, TRalpha1, and TRalpha2 in type 2 cells but failed to reveal the presence of these mRNAs in type 1 astrocytes. Moreover, Northern blots also failed to reveal TRbeta2 mRNA in both type 1 and type 2 astrocytes. These findings, therefore, raised a question as to which receptor isoform was responsible for the nuclear binding capacity observed in type 1 astrocytes. As anticipated, immunocytochemical analysis demonstrated prominent nuclear signals for TRbeta1, TRalpha1, and TRalpha2 mRNA in type 2 astrocytes but failed to demonstrate TRbeta1, TRalpha1, or TRalpha2 in type 2 astrocytes. Application of RT-PCR, however, revealed the presence of low levels of TRbeta2 mRNA in type 1 astrocytes. When stained with a specific anti-TRbeta2 antiserum, both type 1 and type 2 astrocytes showed a strong fluorescent signal concentrated in the nucleus. These data indicate that under the special conditions of cell culture, expression of the TRbeta2 isoform in type 1 accounts for the measured nuclear T3 binding capacity.

Immunofluorescent localization of thyroid hormone receptor isoforms in glial cells of rat brain

The three currently recognized T3 binding thyroid hormone receptor (TR) isoforms, TR alpha 1, TR beta 1, and TR beta 2, arise from two distinct genes (alpha and beta), whereas two closely related non-T3-binding receptor variants, collectively designated TR alpha 2, arise from alternate splicing of the alpha gene transcript. Using a panel of specific antisera to these isoforms we have assessed the presence or absence of TRs in oligodendrocytes and astrocytes of rat cerebrum and cerebellum. Inferences as to colocalization of the receptor isoforms and cell-specific marker proteins were based on immunohistochemical analysis of the differential emissions of paired immunofluorescent probes. Antisera against myelin basic protein (MBP) identified oligodendroglia, and glial fibrillary acidic protein identified astrocytes. MBP-positive oligodendrocytes displayed positive fluorescent signals with each of the three TR isoform-specific antisera and the antiserum to the receptor variants. These findings are consistent with the concept that the MBP gene is a direct target for thyroid hormone action. TR immunoreactivity appeared to localize primarily to the nuclei of these cells. In contrast, we observed no immunofluorescent signals for any of the TR isoforms in glial fibrillary acidic protein-positive astrocytes. These findings raise the possibility that any effect of thyroid hormone on astrocyte function and structure is mediated indirectly as a result of interaction of thyroid hormone with receptors situated in nonastrocyte cells or as a result of nonnuclear mechanisms.

Widespread distribution of immunoreactive thyroid hormone beta 2 receptor (TR beta 2) in the nuclei of extrapituitary rat tissues

Messenger RNA for thyroid hormone receptor (TR) isoforms alpha 1 and beta 1 are widely distributed in rat tissues. Until recently, TR beta 2 mRNA was believed to be limited to the pituitary and the assumption was made that TR beta 2 protein was similarly restricted. We determined the distribution of TR beta 2 protein in selected adult and fetal rat tissues using three anti-TR beta 2 antisera directed to different amino acid sequences of the distinctive A/B domain of TR beta 2. The proportion of total nuclear binding capacity cleared by each antiserum was determined by saturation analysis. 10-20% of total binding capacity in adult brain, liver, kidney, and heart was immunoprecipitated by each antiserum. Use of specific antibodies to TR beta 1 and TR alpha 1 showed these isoforms accounted for the remainder of total T3 binding. Fetal liver and brain, however, contained only TR alpha 1. Immunohistochemical analysis of the adult tissues showed TR beta 2 present in nuclei. Reverse transcription polymerase chain reaction detected low levels of TR beta 2 mRNA in the adult tissues. We infer that TR beta 2 accounts for a significant fraction of TR in adult rat tissues despite the low levels of its mRNA.

Gene transcription in differentiating immature T cell receptor(neg) thymocytes resembles antigen-activated mature T cells

Early in ontogeny thymocytes have a surface marker phenotype that resembles activated mature T cells but they lack expression of the T cell receptor (TCR) complex. We have made preparations of day 14/15 triple negative fetal thymocytes that exhibit the activated T lymphocyte markers CD25, intercellular adhesion molecule 1, Ly-6A/E, CD44, and heat stable antigen and are rapidly proliferating as evidenced by flow cytometric examination of BrdU incorporation. We found that binding activities of the gene regulators nuclear factor (NF)-kappa B, the NF-kappa B p50 homodimer complex, nuclear factor of activated T cells (NF-AT), oct-1, oct-2, activator protein 1 (AP-1), and serum response factor (SRF), are all present in these early thymocytes. Whereas the octamer factors and SRF persist during ontogeny, NF-kappa B, NF-AT, and AP-1 decrease and are undetectable in the adult thymus. Transfection of disaggregated thymocytes by electroporation or intact thymic lobes by gold-particle bombardment revealed that reporter constructs for NF-kappa B, NF-AT, AP-1, octamer factors and, to a small extent, the TCR-alpha enhancer were active in early thymocyte development. We rigorously eliminated the possibility that these transcriptional events were due to minor populations of TCR+ cells by showing that these reporter constructs were also active in recombinase activating gene (RAG)-/- thymocytes that are incapable of completing TCR gene rearrangement, and predominantly contain cells that have an activated phenotype. Thus, transcriptional events that are usually triggered by antigen stimulation in mature T cells take place early in thymic ontogeny in the absence of the TCR. Our analysis suggests that there are striking regulatory similarities but also important differences between the activation processes that take place in antigen-stimulated mature T cells and thymic progenitor cells.

Molecular mechanisms of thyroid hormone action. A physiologic perspective

At present, it appears abundantly clear that thyroid hormone exerts its major action at the nuclear level by regulating the level of mRNAs of specific genes. There are at least three TR isoforms that mediate hormonal effects at the tissue level. Characterization of the functional domains of these receptor isoforms is as yet incomplete, and the possibility that these receptors could have ligand-independent functions is a matter under current investigation. TRs are now recognized as members of a large superfamily of transactivating proteins involved in the regulation of gene expression. Recent studies have shown an unexpected degree of complexity in the nature of the association of the T3 receptors and the DNA of target genes. They have vividly pointed out the multiple interactions possible between the T3-receptor complex and other proteins participating in the process of gene regulation. These insights have provided a solid base for understanding differences in the gradation of thyroid hormone effect from one tissue to another. The microdissection of the molecular process that has occurred in the past 20 years has proceeded in part through the application of relatively artificial in vitro systems and assays. Whereas such approaches have undoubtedly reaped rich rewards in pointing out potential or possible mechanisms, they do not define the actual workings in the animal. Additional studies designed to examine at the molecular level the operation in vivo of physiologic networks influenced by thyroid hormones appear as an essential next step in understanding the biology of the hormone system. The application of transgenic models should materially assist such efforts.

Immunocytochemical delineation of thyroid hormone receptor beta 2-like immunoreactivity in the rat central nervous system

The thyroid hormone receptors (TR) are nuclear proteins that include TR alpha and TR beta subtypes, each encoded by a separate gene. Both TR alpha and TR beta give rise to several isoforms of which three, TR alpha 1, TR beta 1, and TR beta 2 bind T3 and mediate the action of thyroid hormone. Although TR beta 2 was initially thought to be confined to the anterior pituitary, we recently observed small quantities of TR beta 2 messenger RNA (mRNA) by polymerase chain reaction analysis of discrete hypothalamic regions. To further examine the distribution of TR beta 2 in the brain, we performed immunocytochemical studies using a highly specific antiserum to TR beta 2, raised against a unique amino acid sequence (TR beta 2[131-145]) that is not present in the other known TRs. This antiserum immunoprecipitated TR beta 2 but not TR alpha 1 or TR beta 1. Immunoreactive TR beta 2 was widely distributed throughout the brain and primarily localized to the cell nucleus. Particularly intense immunostaining was present in the cerebral cortex, cerebellum, and hypothalamus, including regions where TR beta 2 mRNA had not previously been identified. In addition, immunoprecipitation of nuclear extracts with anti-TR beta 2 reduced total T3 binding capacity by approximately 20%, suggesting that immunoreactive TR beta 2 comprises a substantial portion of the total content of nuclear thyroid hormone binding proteins. These studies demonstrate that immunoreactive TR beta 2 is more widely represented in the central nervous system than previously suspected and may play an important role in mediating the action of T3 in many different regions of the brain. The finding of TR beta 2-like material could be due to a disproportionately high ratio of the TR beta 2 translation product and its mRNA in certain regions of the brain, or could indicate the existence of a novel TR beta 2-related protein that is important for T3 binding.

Ontogeny of hepatic nuclear triiodothyronine receptor isoforms in the rat

We have determined the contribution of the thyroid hormone receptor (TR) isoforms TR alpha 1 and TR beta 1 to the postnatal rise in rat hepatic nuclear T3-binding capacity. In agreement with previous studies, total hepatic nuclear binding capacity rose by about 8-fold from the 19th day of gestation to young adulthood at 2 months of age (0.10 +/- 0.03 to 0.86 +/- 0.17 pmol/mg DNA). The levels of specific TR species were measured by immunoprecipitation of T3-binding activity from hepatic extracts using a panel of antisera directed against specific regions of the TR isoforms. The difference between receptor immunoprecipitated with antibody against TR beta 1 and that precipitated with an antibody against an identical region in both TR beta 1 and TR alpha 1 was tentatively assumed to represent TR alpha 1. TR alpha 1 accounted for virtually all T3-binding activity in fetal liver on gestational day 19 (G19), increased by 2-fold shortly after birth, and remained constant thereafter. TR alpha 1 mRNA, on the other hand, was highest in concentration on G16 and fell by 50-75% in the adult. TR beta 1 was undetectable by immunoprecipitation of hepatic extracts from fetuses on G19. However, Northern analysis showed the presence of TR beta 1 mRNA in the fetal liver, which rose in concentration by 3- to 4-fold in late gestation and then remained constant. The contribution of TR beta 1 to total binding capacity rose to 33% and 40% on postnatal days 15 and 30, respectively, and to 80% in the adult liver. Immunohistochemical analyses of hepatic sections confirmed the presence of very low levels of TR beta 1 in fetal liver as early as G16 and G19, and a sharp rise in TR beta 1 protein concentration in the postnatal period. This indicated that the increase in TR beta 1-binding capacity results from increased TR beta 1 mass. The increase in TR beta 1-binding capacity, thus, is due to increased translational efficiency of the beta 1 mRNA or stabilization of the TR beta 1 protein. The prominence of TR alpha 1 in both rat fetal liver and fetal brain, as previously demonstrated in our laboratory, raises the possibility that this receptor isoform may carry out specialized functions in the fetus and that TR beta 1 subserves still other functions at later stages of development.(ABSTRACT TRUNCATED AT 400 WORDS)

The elusive cure: health care reform

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Quantitation of rat tissue thyroid hormone binding receptor isoforms by immunoprecipitation of nuclear triiodothyronine binding capacity

A panel of anti-thyroid hormone receptor (TR) antisera were generated to allow direct assay of the concentrations of the alpha 1 and beta 1 receptor isoforms in nuclear extracts from adult rat liver, kidney, brain and heart, and fetal brain. An antiserum, immunoglobulin G (IgG)-beta 1, raised against amino acid sequence 62-92 of the rat TR-beta 1 specifically precipitated only TR-beta 1 in vitro translation products. A second antiserum, IgG-alpha 1/beta, generated against a sequence that is identical in the ligand binding region of rat TR-alpha 1 and TR-beta isoforms immunoprecipitated both TR-alpha 1 and -beta 1 translation products. These IgG preparations were used to specifically immunoprecipitate thyroid hormone receptor binding activity from nuclear extracts. IgG-beta 1 cleared almost 80%, and the IgG-alpha 1/beta immunoprecipitated nearly all binding from hepatic nuclear extracts. This distribution of TR protein, 80% beta 1 and 20% alpha 1, is the same as previously reported for their respective mRNAs in liver. In heart, kidney, and brain IgG-beta 1 cleared 45, 43, and 28% of total binding, respectively, and IgG-alpha 1/beta cleared all T3 binding activity from these tissues. In agreement with an earlier study, marked variations in specific protein/mRNA ratios were noted among these tissues. Consistent with our earlier report of the presence of only very low levels of TR-beta 1 mRNA in fetal brain, IgG-beta 1 cleared just 5% of binding in this tissue. Studies using an antiserum (IgG-ch) generated against homologous segments of the hinge region in both TR-alpha 1 and -beta 1 yielded results which contrasted sharply with those of IgG-alpha 1/beta. Whereas IgG-ch could also immunoprecipitate virtually all binding from hepatic extracts it cleared only 40-50% of binding from the other tissues, including fetal brain in which TR-alpha 1 accounts for greater than 90% of binding protein. The data suggest the presence of posttranslational modification of the TR-alpha 1 protein in the hinge region, consistent with the presence in this segment of potential phosphorylation sites.

Prolonged fasting reduces rat hepatic beta 1 thyroid hormone receptor protein without changing the level of its messenger ribonucleic acid

The level of hepatic nuclear T3-binding capacity falls in rats subjected to fasting. To define the mechanism underlying these changes, we have assayed in liver the concentration of the mRNA coding for the beta 1-receptor (beta 1-TR) isoform, the total nuclear T3-binding capacity, and the fraction of the total binding capacity that can be specifically immunoprecipitated with an anti-beta 1-TR immunoglobulin G preparation. Although no changes in beta 1-TR mRNA concentration were noted, we observed a 60% fall in total binding capacity. beta 1-TR mRNA levels were preserved despite a 50% fall in total poly(A)+ RNA. The fall in beta 1-TR protein, however, was consistent with a generalized decrease in total hepatic protein content. This study provides yet another instance in which measurement of receptor mRNA is not consonant with the behavior of the nuclear T3 receptor protein.

Immunofluorescence localization of thyroid hormone receptor protein beta 1 and variant alpha 2 in selected tissues: cerebellar Purkinje cells as a model for beta 1 receptor-mediated developmental effects of thyroid hormone in brain

Rat c-erbA beta 1 mRNA rises in cerebrum during the first 10 days of life, coincident with an increase in tissue triiodothyronine (T3) levels and T3-dependent brain development. These data suggest that the beta 1 receptor may mediate the T3 effect. However, in cerebellum c-erbA beta 1 mRNA levels were very low. Since cerebellar development, including dendritic arborization of Purkinje cells, is a T3-sensitive process, we assessed the levels of the beta 1 receptor protein in cerebellum during development. Antisera to unique peptide regions of beta 1 were raised. Their specificity was demonstrated by specific immunoprecipitation of the in vitro translated product, 85% immunoprecipitation of the T3 binding activity in hepatic nuclear extracts, and Western blot analysis of tissue extracts. Immunohistochemical studies using anti-beta 1 antiserum stained liver nuclei but not testis nuclei, which contain no T3 binding activity or beta 1 mRNA. In cerebellar Purkinje cells, an immunofluorescent signal, localized to the nucleus and more intense than that seen in the liver, was observed. A positive but weaker signal was also present in the granule cells. Thus, we may infer that the cerebellum contains significant concentrations of beta 1 receptor protein despite the low beta 1 mRNA content. Both the intensity of staining in Purkinje cell nuclei and immunoprecipitable beta 1 receptor binding capacity rose in the neonatal period. Antiserum to the non-T3 binding alpha 2 variant protein was also prepared and a distinctive pattern of fluorescence was observed. Strong fluorescence was seen in the nuclei of granule cells, but none was seen in Purkinje cells. The alpha 2 fluorescence in testis was high, consistent with the high levels of alpha 2 mRNA in this tissue. The fluorescent signal appeared to originate primarily in dividing spermatogonia. Our findings support the concept that the beta 1 receptor plays a central role in T3-induced brain development and strongly suggest that the Purkinje cell is a direct target for T3.

Functional relationship of thyroid hormone-induced lipogenesis, lipolysis, and thermogenesis in the rat

Metabolic balance studies were carried out to determine the interrelationships of thyroid hormone-induced lipogenesis, lipolysis, and energy balance in the free-living rat. Intraperitoneal doses of 15 micrograms triiodothyronine (T3)/100 g body wt per d caused an increase in caloric intake from 26.5 +/- 1.7 (mean +/- SEM) kcal/100 g per d to 38.1 +/- 1.5 kcal/100 g per d. Food intake, however, rose only after 4-6 d of treatment and was maximal by the 8th day. In contrast, total body basal oxygen consumption rose by 24 h and reached a maximum by 4 d. Since total urinary nitrogen excretion and hepatic phosphoenolpyruvate carboxykinase mRNA did not rise, gluconeogenesis from protein sources did not supply the needed substrate for the early increase in calorigenesis. Total body fat stores fell approximately 50% by the 6th day of treatment and could account for the entire increase in caloric expenditure during the initial period of T3 treatment. Total body lipogenesis increased within 1 d and reached a plateau 4-5 d after the start of T3 treatment. 15-19% of the increased caloric intake was channeled through lipogenesis, assuming glucose to be the sole substrate for lipogenesis. The metabolic cost of the increased lipogenesis, however, accounted for only 3-4% of the T3-induced increase in calorigenesis. These results suggest that fatty acids derived from adipose tissue are the primary source of substrate for thyroid hormone-induced calorigenesis and that the early increase in lipogenesis serves simply to maintain fat stores. Since the mRNAs coding for lipogenic enzymes rise many hours before oxygen consumption and lipolysis, these results suggest that T3 acts at least in part by an early coordinate induction of the genes responsible for these processes.

Relationship of c-erbA mRNA content to tissue triiodothyronine nuclear binding capacity and function in developing and adult rats

We have quantitated in adult and developing rat tissues the molar concentrations of c-erbA alpha 1- and beta 1-mRNAs, which code for nuclear T3-binding proteins, and c-erbA alpha 2-mRNA, which is generated by alternate splicing of the alpha gene transcript and codes for a receptor variant that does not bind T3. Comparison of the concentrations of c-erbA alpha 1-mRNA, beta 1-mRNA, or their sum to the T3 nuclear binding capacity per mg of DNA in adult liver, kidney, heart, cerebrum, and cerebellum and during the ontogeny of liver and brain shows that the T3 binding capacity/c-erbA mRNA ratio is tissue-specific and related to developmental state. Administration of T3 resulted in a 40-50% fall in the alpha 1 signal of adult liver, kidney, and heart without changing either the beta 1 signal or T3 binding capacity. A 40-fold increase in rat brain beta 1-mRNA occurred in the transition between the 19-day gestational fetus and the 10-day-old neonate. This corresponds to the period during which the T3 content rises in brain and during which T3 is known to influence central nervous system development. Our findings indicate that important translational or post-translational factors influence nuclear binding capacity and raise the possibility that c-erbA beta 1 may play a primary role in mediating T3 effects in developing and adult animals.

Binding of 3,5,3'-triiodothyronine (T3) and its analogs to the in vitro translational products of c-erbA protooncogenes: differences in the affinity of the alpha- and beta-forms for the acetic acid analog and failure of the human testis and kidney alpha-2 products to bind T3

We have compared the affinities for T3 and the T3 analog binding characteristics of the in vitro translational products of seven c-erbA cDNAs (chicken c-erbA alpha; human placental c-erbA beta; rat c-erbA beta-1; rat c-erbA alpha-1; rat c-erbA alpha-2; human testis c-erbA alpha-2; and human kidney c-erbA alpha-2). Four of these (chicken c-erbA alpha, human placental c-erbA beta, rat c-erbA beta-1, rat c-erbA alpha-1) bound T3 with high affinity as previously described. When compared under identical conditions of synthesis and [125I]T3 binding, there was no significant difference between the affinity of the chicken c-erb A alpha-1 and the human c-erbA beta but in a more limited series the affinity of rat c-erbA beta-1 for T3 was 4.6-fold higher than that of the rat c-erbA alpha-1. In vitro translational products of the beta-probes showed a characteristic 2.2-fold higher triiodothyroacetic acid/T3 ratio than did the products of the alpha-probes, regardless of the species of origin of the probe. As previously established, the rat c-erbA alpha-2 product did not bind T3. However, in contrast to two published reports, the human testis and kidney alpha-2 probe products also failed to bind T3. These findings indicate that highly conserved C-terminal 37-40 residues are important for high affinity T3 binding by proteins encoded by the c-erb A family of genes.

The regulation of lipogenesis by thyroid hormone and its contribution to thermogenesis

We have used the tritiated water method to quantitate the effects of thyroid hormone on lipogenesis in the rat and then determined the contribution of this process to thyroid hormone-induced thermogenesis. After thyroid hormone administration to hypothyroid animals, fatty acid synthesis rose after a lag time of 12-16 h and reached a plateau after 4-5 days. This is consistent with the kinetics of an increase in oxygen consumption measured by others in similar animals. A diurnal variation was maintained in all thyroid states, with the peak value in the middle of the dark period being 3-fold higher than the nadir. Fatty acid synthesis in the livers of hyperthyroid animals was 3- to 4-fold higher than that in euthyroid rats, which, in turn, was 3- to 5-fold higher than the rate observed in hypothyroid rats. Slightly smaller but similar fold increases were measured in epididymal fat. A stimulation of fatty acid synthesis by thyroid hormone was also measured in the rest of the carcass, with hyperthyroid rates being twice those in hypothyroid animals. The contribution of the liver was much greater in hyperthyroid rats (34% of total fatty acid synthesis) than in hypothyroid animals (5%). The energy costs of this synthesis were calculated and compared to published values for total oxygen consumption in different thyroid states. Thus, 6-10% of the total increment in oxygen consumption between hyperthyroid and hypothyroid animals could be attributed to lipogenesis, depending on which published figures were used. About 3% of this increment was due to the liver alone.

Time course of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and messenger ribonucleic acid, biliary lipid secretion, and hepatic cholesterol content in methimazole-treated hypothyroid and hypophysectomized rats after triiodothyronine administration: possible linkage of cholesterol synthesis to biliary secretion

In an effort to define the mechanism by which thyroid hormone increases the synthesis of hepatic cholesterol, we have investigated both in hypophysectomized and methimazole-treated hypothyroid rats the time course of T3 effects on plasma cholesterol concentration, total hepatic cholesterol, the rate of biliary secretion of cholesterol, bile acids, and phospholipids, and the activity and mRNA levels of 3-hydroxy-3-methylglutaryl coenzyme-A (HMG-CoA) reductase, the rate-limiting enzyme in the hepatic synthesis of cholesterol. A single dose of 200 micrograms T3 was estimated to maintain at least 90% nuclear occupancy for the ensuing 54 h of the experiment. In both preparations the relative rise in biliary secretion of cholesterol exceeded that of other biliary constituents and preceded by 12 h an increase in HMG-CoA reductase enzyme activity and its mRNA. The level of total hepatic cholesterol remained constant throughout the experiment. We interpret these findings to suggest that T3-stimulated cholesterol synthesis is mediated by an antecedent T3-induced rise in biliary cholesterol secretion. We postulate that biliary cholesterol secretion is augmented by an intrahepatic shift of cholesterol and depletion of the hepatic sampling center responsible for the feedback regulation of cholesterol synthesis. The level of HMG CoA reductase mRNA appeared to govern enzyme activity in both preparations, but the ratio of mRNA to hepatic enzyme activity was substantially greater in the methimazole-treated compared with the hyphophysectomized animals.

Thyroid hormone-, carbohydrate, and age-dependent regulation of a methylation site in the hepatic S14 gene

The rat hepatic S14 gene has served as a model of thyroid hormone regulation of gene expression. Earlier studies of the S14-containing chromatin region demonstrated that a cytosine residue at position 625 (C-625) in the 3' untranslated exon was hypermethylated in hepatic DNA derived from hypothyroid animals. This observation was consistent with the markedly reduced level of expression of the S14 gene in these rats. The current studies have extended these observations to groups of rats in various thyroidal states. By using the restriction enzyme Hhal, the percent demethylation of this site was quantitated (hypothyroid, 9.3%; euthyroid, 19.2%; hyperthyroid, 66.6%). Moreover, the level of methylation was shown to be reversible as the thyroidal state was altered. Our data also indicate that these changes are probably independent of de novo DNA synthesis. Kinetic studies of the demethylation of this cytosine residue after T3 administration showed no change for at least 1 day and maximal change after about 4 days. This contrasts with the significant rise in S14 mRNA evident within 30 min and suggests that demethylation plays no role in the acute induction of this gene by T3. Carbohydrate feeding, another stimulus of S14 expression, similarly caused the demethylation of this cytosine residue. Earlier studies had demonstrated that mRNA S14 expression was not detectable in rat pups before about 20 days of age and continued to rise through the first year of life. Consistent with those findings, S-14 C-625 was fully methylated up to 15 days of age. Progressive demethylation then occurred up to 12 months of age. These results indicate that increased demethylation of a specific site in the 3' untranslated region of the S14 gene, possibly resulting from augmented excision repair processes, is correlated with increased expression of the gene.

Thyroid hormone and circadian regulation of the binding activity of a liver-specific protein associated with the 5'-flanking region of the S14 gene

Recent studies have described a DNase I hypersensitive site in the 5'-flanking region of the rat hepatic S14 gene that is closely associated with its expression. A 111-base pair subfragment (-389 to -279) of this region interacts specifically in a gel shift assay with a protein present in hepatic nuclear protein extracts. This protein, designated P1, was not present in extracts of other tissues, even those in which the gene is expressed and hormonally regulated. The binding activity of P1 is exceedingly low in extracts from hypothyroid rats and is markedly increased by administration of thyroid hormone. However, the slow accumulation of P1 after thyroid hormone administration indicates that increased levels of P1 are not necessary for the acute hormonal induction of S14 gene expression. The level of P1 binding activity increases in the evening, synchronous with circadian variation of hepatic mRNA S14. Since neither P1 binding activity nor circadian variation in mRNA-S14 levels are observed in the other tissues expressing the S14 gene, P1 may function to modulate the circadian rhythm observed in hepatic S14 gene expression. DNase I footprinting analysis revealed that P1 binds to a defined nucleotide sequence, 5'-AAAAGAGCTATTGATTGCCTGCA-3', located between -310 and -288 in the S14 gene.

Triiodothyronine rapidly reverses inhibition of S14 gene transcription by glucagon

The rapid response of hepatic mRNA-S14 to T3 has made this sequence an important model for studying the mechanism of hormonal induction of gene expression. In previous studies we showed, in the intact rat, that glucagon administration during the peak of the mRNA S14 diurnal rhythm causes a monoexponential fall in the level of mRNA-S14, and that T3 reverses this effect. We have now defined more precisely the mechanism governing this interaction. Measurement of in vitro nuclear transcriptional rates shows that T3 can reverse the glucagon-induced reduction of mRNA-S14 transcription. Reversal can be demonstrated within 5 min after the iv injection of T3. Further, the reversal appears to be related to the occupation of specific nuclear receptors, as inferred from the calculated nuclear occupancy and the effects of various iodothyronine analogs of T3. These results suggest that the effects of T3 are mediated by varying rates of production of the nuclear precursor and not by its stabilization, as previously proposed. Ancillary evidence supporting this conclusion came from the demonstration that the apparent t1/2 of the 4.5-kilobase precursor was not prolonged by T3.

Triiodothyronine nuclear binding capacity in rat tissues correlates with a 6.0 kilobase (kb) and not a 2.6 kb messenger ribonucleic acid hybridization signal generated by a human c-erbA probe

Recent studies have raised the possibility of multiple structurally distinctive tissue-specific nuclear T3 receptors, all exhibiting homology with the v-erbA oncogene and represented by mRNAs of various sizes. We have assayed the level of mRNAs recognized by a 32P-labeled cRNA derived from human plancetal c-erbA-A beta cDNA by solution hybridization and by Northern transfer in different rat tissues, as well as human liver and placenta. Two related mRNAs were demonstrated in the rat tissues analyzed, one measuring 6.0 and the other 2.6 kilobases (kb). In human liver and placenta a 6.0 kb mRNA was seen, but not a 2.6 kb mRNA. Only the 6.0 kb sequence correlated with the receptor concentration determined by 125I-T3 displacement analysis.

Rat hepatic mRNA-S14 and lipogenic enzymes during weaning: role of S14 in lipogenesis

The rapid and marked response of hepatic mRNA-S14 sequence to both triiodothyronine and carbohydrate intake has made this sequence an attractive model for studying the action of hormonal and dietary factors. Because it is highly expressed and regulated only in lipogenic tissues, we have suggested that it plays a role in some aspect of lipid synthesis, transport, or metabolism. To provide more precise information regarding the function of S14 we have measured lipogenesis, lipogenic enzymes, beta-oxidation, and mRNA-S14 levels in spontaneously weaning neonatal rats and in rats prematurely weaned to a laboratory diet on postnatal day 17. After birth, the levels of lipogenesis, mRNA-S14, and the lipogenic enzymes malic enzyme (ME) and fatty acid synthase (FAS) were almost undetectable but increased with the onset of spontaneous weaning. Coincident with these changes, beta-oxidation decreased. Premature weaning beginning on day 17 resulted in an earlier and even more marked increase in lipogenesis, ME, FAS, and mRNA-S14. On day 19, ME and FAS activities were 6- to 19-fold more than activities in control suckling pups, whereas mRNA-S14 levels had risen to greater than 100 times the control values. Thus directional shifts in mRNA-S14 corresponded with indices of lipogenesis and were opposite to indices reflecting beta-oxidation. The response of mRNA-S14 therefore suggests that it may be related to the synthesis of fatty acids. On the other hand, the level of lipogenesis in the fetus was high despite the fact that the levels of both mRNA-S14 and ME were low. This dissociation raises the possibility that the S14 protein participates in lipogenesis in the neonate and adult but not in the fetus.

Interaction of dietary carbohydrate and glucagon in regulation of rat hepatic messenger ribonucleic acid S14 expression: role of circadian factors and 3',5'-cyclic adenosine monophosphate

The mRNA of the rat hepatic S14 gene accumulates rapidly after administration of T3 and carbohydrate, making it an excellent model for studies of the effects of dietary and hormonal stimuli at the hepatocellular level. We undertook studies to assess circadian changes in responsivity of this sequence to intragastric sucrose administration combined with insulin injection, and evaluated the capacity of glucagon to reverse these effects. As in the case of T3, the response of mRNA-S14 to carbohydrate in the morning was brisk whereas there was no significant increment when the stimulus was applied in the evening. In confirmation of previous studies, glucagon markedly lowered levels of mRNA-S14 in the evening but exerted no effect in the morning. These results support the concept that the rate of hepatic production of mRNA-S14 in unmanipulated rats is maximal in the evening, thus allowing no further induction by carbohydrate or T3 but permitting reduction by glucagon. Conversely, the rate of production is minimal in the morning, permitting induction by carbohydrate or T3 but allowing no further reduction by glucagon. A major difference between the effects of carbohydrate and those of T3 was the observed failure of carbohydrate to reverse the effect of glucagon in the evening. The effect of glucagon was stimulated by (Bu)2cAMP, and this was reversed by T3. However, T3 did not modify the glucagon-induced increase in hepatic cAMP levels. We therefore conclude that the capacity of T3 to abolish the glucagon effect is mediated at a step distal to the generation of cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Cycloheximide inhibits S-14 gene transcription and abolishes DNase I hypersensitive S-14 sites in the livers of euthyroid but not hypothyroid rats

Earlier studies from our laboratory have demonstrated that cycloheximide administration to hypothyroid rats inhibited the induction of the hepatic mRNA-S14 by T3. These results suggested a role of short-lived proteins in the hormonal regulation of this gene. To define the possible mechanism of the cycloheximide effect, we examined the influence of cycloheximide on the in vitro transcription rate of the gene and its chromatin structure. Forty-five minutes after injection of cycloheximide to euthyroid rats, the in vitro transcriptional rate fell by 60% and this effect persisted for 4 h. In the same euthyroid rats, cycloheximide caused the disappearance of all four DNase I-hypersensitive sites situated in the 5'-flanking region of the gene. However, cycloheximide given to hypothyroid rats affected neither the basal transcription rate nor the chromatin structure. When cycloheximide was administered 30 min after an acute injection of T3 (200 micrograms/100 g BW) to hypothyroid animals, it completely blocked the hormone induction of the transcriptional rate. These results suggest that one or more labile proteins are required for maintenance of S14 chromatin structure in a configuration which permits hormonal regulation of gene expression. The ability of cycloheximide to block mRNA-S14 induction by T3 appears to be mediated at least in part by an inhibition of T3-stimulated transcription.

Triiodothyronine regulation of multiple rat hepatic genes: requirement for ongoing protein synthesis

We have examined the role of rapidly turning over proteins in the T3 regulation of multiple rat hepatic genes. T3 induction of the rapidly responsive mRNA-S14 was markedly inhibited by cycloheximide (1 mg/100 g BW) or emetine (3 mg/100 g) injected ip 30 min before T3 (mRNA-S14 concentration was only 35% of that in T3-treated controls 8.5 h after administration of either protein synthesis inhibitor, P less than 0.01). Cycloheximide exhibited a similar effect on each of five other more slowly responsive T3 regulated genes. When cycloheximide was given 10 h after T3, the expected T3-induced rise of mRNA-S7 activity was completely prevented, and for mRNA-S4 activity the anticipated rise was blunted to 40% of T3-treated control (P less than 0.05). Cycloheximide caused sharp declines in the activity of two other mRNAs, S6 and S8, which because of shorter lag times of response to T3, had already risen when the drug was given. Values for both these mRNAs returned to the baseline hypothyroid level within 6 h of injection of the drug and remained low for a further 8 h (P less than 0.05). The expected deinduction of mRNA-S10 by T3 was also markedly modified. T3 lowered this mRNA to 11% of the hypothyroid control after 8 h, whereas cycloheximide given 30 min before the hormone blunted this fall to only 72% of control (P less than 0.01). Thus there appeared to be a 70% reduction in the rate of T3 induced fall of mRNA-S10. We did not find that cycloheximide caused a generalized decrease in poly (A)+ RNA mass.(ABSTRACT TRUNCATED AT 250 WORDS)

Diurnal variation in hepatic expression of the rat S14 gene is synchronized by the photoperiod

We have analyzed the factors responsible for the circadian variation in rat hepatic mRNA-S14. Regulation of this sequence, which is found in lipogenic tissues and encodes a protein (S14) believed to be associated with fatty acid synthesis, is an excellent model of the interaction of thyroid hormone and dietary factors at the hepatocellular level. The mRNA exhibits a 3-fold diurnal variation (peak, approximately 2000 h; nadir, 0800 h) in ad libitum feeding rats on a 12-h light, 12-h dark photoschedule. We studied the effects of the photoschedule, periodic food intake, hypophysectomy, and induction by thyroid hormone (T3) on the mRNA-S14 rhythm. Adaptation to feeding restricted to either light or dark periods for 15 days did not greatly affect the diurnal rhythm. Photoreversal resulted in a 180 degrees phase shift, whereas the rhythm persisted in the presence of constant light. Oscillation continued around a higher baseline after a receptor-saturating dose of T3 in both normal and hypophysectomized rats. Our results indicate primary entrainment of the mRNA-S14 diurnal rhythm to the photoperiod, rather than to periodic food intake. Moreover, the circadian regulatory signal, which probably originates in the central nervous system, appears capable of antagonizing a maximal T3-inductive stimulus and does not originate in the pituitary gland. Persistence of the oscillation in constant light rules out circulating melatonin as the mediator. Synchronization of the rhythm by the photoschedule suggests that neuroendocrine factors are important determinants of rhythmic changes in hepatic gene expression.

Replacement dose, metabolism, and bioavailability of levothyroxine in the treatment of hypothyroidism. Role of triiodothyronine in pituitary feedback in humans

A change in the formulation of the levothyroxine preparation Synthroid (Flint) in 1982 prompted us to reevaluate the replacement dose of this drug in 19 patients with hypothyroidism. The dose was titrated monthly until thyrotropin levels became normal. The mean replacement dose (+/- SD) was 112 +/- 19 micrograms per day, significantly less (P less than 0.001) than the dose of an earlier formulation--169 +/- 66 micrograms per day--used in a similar study (Stock JM, et al. N Engl J Med 1974; 290:529-33). The fractional gastrointestinal absorption of a tablet of the current formulation is 81 percent, considerably higher than the earlier estimate of 48 percent. Using high-performance liquid chromatographic analysis, we found that the current tablet contains the amount of thyroxine stated by the manufacturer. By measuring the bioavailability of the earlier type of tablet in five patients, we inferred that the strength of the previous tablet had been overestimated. In the present study, the thyrotropin levels of patients on replacement therapy returned to normal when serum triiodothyronine concentrations were not significantly different from those of controls (122 vs. 115 ng per deciliter [1.87 vs. 1.77 nmol per liter]), but when serum thyroxine levels were significantly above those of controls (11.3 vs. 8.7 micrograms per deciliter [145 vs. 112 nmol per liter], P less than 0.001). These findings suggest the possibility that in humans, serum triiodothyronine may play a more important part than serum thyroxine in regulating the serum thyrotropin concentration.

An analysis of the interrelationship of nuclear and plasma triiodothyronine in the sea lamprey, lake trout, and rat: evolutionary considerations

We have studied the interrelationship of plasma and hepatic nuclear T3 in two lower vertebrates, lake trout and sea lamprey. Specific nuclear T3-binding sites were observed in liver of both species. Binding affinities for T3 and several analogs in lamprey and trout were similar to those in rat liver. The findings of identical sedimentation coefficients, Stokes radii, and mol wt suggest a high degree of homology of the receptor molecule among these species. In lamprey ammocoete, both total (approximately 26 nM) and free (approximately 0.12 nM) T3 concentrations were 7- to 36-fold higher than in the trout or rat. The concentration of free T4 was 4 times greater than that in rat plasma despite somewhat lower total T4 levels. This resulted from a 6 times lesser binding of T4 by ammocoete plasma. Total and free T3 and T4 levels fell sharply during transformation to the adult form. In trout, a 27 times greater concentration of free T3 in hepatic nuclei than in cytosol, previously documented for rat liver, gave evidence of the early evolution of an active transport process between these compartments. Analysis of [125I]T3 interchange in trout indicated the existence of two kinetically distinct compartments within the liver, an outer compartment in rapid equilibrium with plasma, and a slowly equilibrating compartment decaying with a t1/2 of 3.7 h. T3 associated with the nuclear receptor appeared to be in equilibrium with the slower hepatic compartment. Despite the apparent resemblance of T3 receptors in these species to those in the rat, administration of T3 (200 micrograms/100 g BW for 4 days) to trout caused no increase in hepatic oxygen consumption or the activities of malic enzyme or alpha-glycerophosphate dehydrogenase. Further, such treatment produced no consistent changes in hepatic mRNA activity profiles analyzed by two-dimensional electrophoresis of in vitro translational products. As suggested by the high free T3 concentration in the larval lamprey ammocoete, T3 in early vertebrates may have a primarily developmental function, manifested over a restricted time frame.

Opposing effects of glucagon and triiodothyronine on the hepatic levels of messenger ribonucleic acid S14 and the dependence of such effects on circadian factors

We have studied the effect of glucagon on the expression of a triiodothyronine (T3) and carbohydrate-inducible mRNA sequence (mRNA-S14) in rat liver that undergoes a threefold diurnal variation (peak, 2200 h; nadir, 0800 h). Glucagon injection into euthyroid rats (25 micrograms/100 g body wt i.p., three doses at 15-min intervals) during the nocturnal plateau of mRNA-S14 caused a monoexponential disappearance of this sequence (t1/2, 90 min) accompanied by a 90% reduction in the transcriptional rate in a nuclear run-off assay, indicative of a near total reduction of synthesis. This effect was markedly attenuated in rats treated with T3 (200 micrograms/100 g body wt i.p.) 24 h before glucagon injection. When T3 was given 15 min after glucagon, the glucagon-initiated decline in mRNA-S14 was reversed within 90 min, suggesting a rapid interaction between the two hormones in the evening. Curiously, administration of T3 alone at this hour did not affect a significant increase in mRNA-S14. At 0800 h, however, T3 caused the expected brisk induction of this sequence, whereas glucagon was without effect. In essence, glucagon affected mRNA-S14 synthesis only in the evening, while T3 increased levels of this sequence above the baseline only in the morning. T3, however, reversed the effect of prior glucagon injection at night. The observed alterations in hormonal responsivity could underly the diurnal variation of mRNA-S14 expression. Moreover, the data suggest the hypothesis that T3 may act on S14 gene expression by antagonizing factors that inhibit its transcription.

Rapid synergistic interaction between thyroid hormone and carbohydrate on mRNAS14 induction

Recent studies have shown that hepatic mRNAS14 responds rapidly to thyroid hormone administration. Moreover, this mRNA is known to increase in mass with the administration of a high carbohydrate fat-free diet. Therefore, it appears to share many of the same properties of the known hepatic lipogenic enzymes. Because the lipogenic enzymes display a synergistic interaction between thyroid hormones and carbohydrates, we investigated the kinetics of response of mRNAS14 to carbohydrate feeding, as well as its interaction with triiodothyronine (T3). We found that mRNAS14 responds rapidly to the dietary administration of sucrose in euthyroid rats, with a 2-fold increase within 30 min, and a 25-fold increase by 4 h. On the other hand, when given to hypothyroid rats, sucrose ultimately lead to only a 2-3-fold increase in the level of mRNAS14, attaining a level less than that found in starved euthyroid rats. The diminished response of mRNAS14 to sucrose in hypothyroidism could not be enhanced by insulin administration. However, administration of replacement doses of T3 (400 ng/100 g of body weight) immediately restored the rapid response to sucrose feeding. The response of sucrose and T3 was synergistic. Dose-response studies with T3 indicated that the rapid interaction between T3 and sucrose was limited by the occupancy of the T3 nuclear receptor. A similar synergistic response to T3 and glucose was noted in primary hepatocyte cultures, thus indicating that the synergism between these two stimuli is not due to changes in extrahepatic hormones or metabolites. Our data are most consistent with the hypothesis that the T3-nuclear receptor complex multiplies a signal generated by carbohydrate metabolism to induce hepatic mRNAS14. The interaction does not appear to require the preliminary induction of carbohydrate-metabolizing enzymes and their mRNAs.

Effect of dichloroacetic acid on rat hepatic messenger RNA activity profiles

We have previously suggested that the ability of glucose to induce rat hepatic lipogenic enzymes is mediated by the mitochondrial oxidation of pyruvate. In part, this hypothesis is supported by the finding that an activator of pyruvate dehydrogenase, dichloroacetic acid (DCA), is capable of inducing malic enzyme in hepatocyte cultures. In order to further test this hypothesis, we compared the mRNA responses induced by carbohydrate feeding in vivo and by glucose administration to hepatocytes in culture with those mRNA responses induced in DCA both in vivo and in culture. DCA administration to rats resulted in a significant increase in liver:body weight ratio. It was, in addition, a potent inducer of malic enzyme. Hepatic mRNA activity profiles were examined by two-dimensional gel electrophoretic analysis of in vitro translation products. Six of the seven mRNAs altered by carbohydrate feeding were similarly altered by DCA feeding in vivo. In cultured hepatocytes 10 mmol/L DCA significantly increased four of six glucose-induced mRNAs. The mRNA for malic enzyme was among those mRNA sequences induced both in vivo and in culture. Increasing glucose concentrations in the culture medium resulted in an expected rise in pyruvate levels, whereas DCA caused a significant decrease in the concentration of this intermediate. It is likely, therefore, that augmentation of the flux of pyruvate through pyruvate dehydrogenase rather than alterations in pyruvate levels per se, is a proximal event leading to the induction of multiple mRNAs. The marked overlap in mRNA response to both carbohydrate and DCA indicates that the signal regulating the content of the carbohydrate responsive mRNAs is derived from mitochondrial pyruvate oxidation.

Stereospecific transport of triiodothyronine to cytoplasm and nucleus in GH1 cells

We have recently demonstrated substantial stereospecific nuclear/cytosolic free triiodothyronine (T3) gradients within T3 responsive rat tissues in situ. These studies have now been extended to examine T3 transport in a rat pituitary tumor cell line, GH1. L-T3 had a 7.6-fold higher affinity for the nuclear receptor when assayed in whole cell incubations in comparison to isolated nuclei, though D-T3 affinity was not altered under these conditions. An apparently higher number of receptors for D-T3 was explained by racemic contamination of the isotopes used. Measurement of free hormone concentration ratios for both enantiomers revealed a small step up from medium to cytosol for L-T3 (1.65) but a reverse ratio for D-T3 (0.46). The nuclei were able to concentrate both enantiomers, though stereospecificity was maintained (nucleus/cytosol, L-T3, 4.5, D-T3 1.7). Transport of L-T3 at both boundaries could be inhibited by monodansylcadaverine. Thus, stereospecific transport functions are found within GH1 cells, though the magnitude of the free nucleus/cytosol gradient is reduced from those seen in rat tissues in situ.

Transcellular and transnuclear transport of 3,5,3'-triiodothyronine in isolated hepatocytes

We have recently reported evidence for the presence of stereospecific energy-dependent transport processes for T3 in rat tissues. These processes were responsible for maintenance of concentration differences of free L- and D-T3 across the cellular plasma and nuclear membranes. In rat liver, the free L-T3 concentration in cytosol was almost 3 times higher than that in plasma, and nuclear free L-T3 was 58-fold that in cytosol. In the present studies, freshly isolated hepatocytes were used to study these processes in vitro. Kinetic experiments demonstrated that equilibrium of [125I]T3 between cells and medium was rapid and complete within 5 min. Neither the rate of cellular accumulation nor the equilibrium distribution of T3 between cells and medium was influenced by the addition of up to 2 X 10(-7) M T3. Equilibrium of T3 between the nuclear and extranuclear fractions of the hepatocytes was reached more slowly, only after 45-60 min of incubation. The nuclear free T3 concentration was calculated from mass action principles with knowledge of the association constant (Ka) of the nuclear T3-binding sites under in vitro conditions and the fractional occupancy of the sites. Cytosolic free T3 was determined from measurements of the fraction of cellular [125I]T3 associated with cytosol (pc), and the binding power of cytosol was determined by equilibrium dialysis (bc). The cytosol to plasma free T3 ratio in these cells was near unity, suggesting an absence of the concentration difference previously observed in liver in situ. The nuclear to cytosol free T3 ratio was 7.9, approximately 7 times less than that in vivo. The addition of 2 mM KCN caused a further 23% reduction in the nuclear to cytosol ratio. As previously reported for liver in situ, uptake of T3 by hepatocytes is stereospecific. Cellular uptake of D-T3 was greater than that for L-T3. However, nuclear transport favored L-T3. The nuclear to cell ratio for L-T3 was almost 4 times greater than that for D-T3 (mean +/- SEM, 0.020 +/- 0.0005 vs. 0.0085 +/- 0.0005; P less than 0.001). Our studies indicate the presence in the isolated hepatocyte of a nuclear transport process for T3 similar to that observed in vivo, but operating with a markedly reduced efficiency.