Thyroid hormone regulation of transmembrane signalling in neonatal rat ventricular myocytes by selective alteration of the expression and coupling of G-protein alpha-subunits

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Molecular cloning and expression of the gene for G protein alpha subunit induced by bisphenol A in marine polychaete Perinereis aibuhitensis

A G protein alpha subunit gene named Pa Gα was isolated from the marine polychaete Perinereis aibuhitensis. The full-length cDNA of Pa Gα was 1832 bp and contained a 205 bp 5' untranslated region (5'UTR), a 565 bp 3' UTR and a 1062 bp open reading frame encoding 353 amino acid residues. The deduced protein sequence of Pa Gα showed 73% homology with the Gα protein of Dipolydora quadrilobata. Tissue-specific expression induced by exposure to bisphenol A (BPA) in P. aibuhitensis was detected by real-time PCR, which showed BPA induced expression of the Pa Gα gene, and the level of transcription was related positively to the concentration of BPA and the length of exposure time. With increasing concentration of BPA and length of exposure time, the level of mRNA transcription was raised gradually, but the level of increasing expression of Pa Gα mRNA induced by exposure to BPA varied significantly among different tissues.

Differentiation of human T cells alters their repertoire of G protein alpha-subunits

Because T cell differentiation leads to an expanded repertoire of chemokine receptors, a subgroup of G protein-coupled receptors, we hypothesized that the repertoire of G proteins might be altered in parallel. We analyzed the abundance of mRNA and/or protein of six G protein α-subunits in human CD4(+) and CD8(+) T cell subsets from blood. Although most G protein α-subunits were similarly expressed in all subsets, the abundance of Gα(o), a protein not previously described in hematopoietic cells, was much higher in memory versus naive cells. Consistent with these data, activation of naive CD4(+) T cells in vitro significantly increased the abundance of Gα(o) in cells stimulated under nonpolarizing or T(H)17 (but not T(H)1 or T(H)2)-polarizing conditions. In functional studies, the use of a chimeric G protein α-subunit, Gα(qo5), demonstrated that chemokine receptors could couple to Gα(o)-containing G proteins. We also found that Gα(i1), another α-subunit not described previously in leukocytes, was expressed in naive T cells but virtually absent from memory subsets. Corresponding to their patterns of expression, siRNA-mediated knockdown of Gα(o) in memory (but not naive) and Gα(i1) in naive (but not memory) CD4(+) T cells inhibited chemokine-dependent migration. Moreover, although even in Gα(o)- and Gα(i1)-expressing cells mRNAs of these α-subunits were much less abundant than Gα(i2) or Gα(i3), knockdown of any of these subunits impaired chemokine receptor-mediated migration similarly. Together, our data reveal a change in the repertoire of Gα(i/o) subunits during T cell differentiation and suggest functional equivalence among Gα(i/o) subunits irrespective of their relative abundance.

Thyroid hormones and cardiac arrhythmias

Thyroid hormone plays an important role in cardiac electrophysiology and Ca2+ handling through both genomic and nongenomic mechanisms of action, while both actions can interfere. Chronic changes in the amount of circulating thyroid hormone due to thyroid dysfunction or systemic disease result in structural, electrophysiological and Ca2+ handling remodeling, while acute changes may affect basal activity of cardiac cells membrane systems. Consequently, long-term or rapid modulation of sarcolemmal ion channels, Ca2+ cycling proteins and intercellular communicating channels by thyroid hormone may affect heart function as well as susceptibility of the heart to arrhythmias. This aspect including pro- and anti-arrhythmic potential of thyroid hormone is highlighted in this review.

AKAP79-mediated targeting of the cyclic AMP-dependent protein kinase to the beta1-adrenergic receptor promotes recycling and functional resensitization of the receptor

Resensitization of G protein-coupled receptors (GPCR) following prolonged agonist exposure is critical for restoring the responsiveness of the receptor to subsequent challenges by agonist. The 3'-5' cyclic AMP-dependent protein kinase (PKA) and serine 312 in the third intracellular loop of the human beta(1)-adrenergic receptor (beta(1)-AR) were both necessary for efficient recycling and resensitization of the agonist-internalized beta(1)-AR (Gardner, L. A., Delos Santos, N. M., Matta, S. G., Whitt, M. A., and Bahouth, S. W. (2004) J. Biol. Chem. 279, 21135-21143). Because PKA is compartmentalized near target substrates by interacting with protein kinase A anchoring proteins (AKAPs), the present study was undertaken to identify the AKAP involved in PKA-mediated phosphorylation of the beta(1)-AR and in its recycling and resensitization. Here, we report that Ht-31 peptide-mediated disruption of PKA/AKAP interactions prevented the recycling and functional resensitization of heterologously expressed beta(1)-AR in HEK-293 cells and endogenously expressed beta(1)-AR in SK-N-MC cells and neonatal rat cortical neurons. Whereas several endogenous AKAPs were identified in HEK-293 cells, small interfering RNA-mediated down-regulation of AKAP79 prevented the recycling of the beta(1)-AR in this cell line. Co-immunoprecipitations and fluorescence resonance energy transfer (FRET) microscopy experiments in HEK-293 cells revealed that the beta(1)-AR, AKAP79, and PKA form a ternary complex at the carboxyl terminus of the beta(1)-AR. This complex was involved in PKA-mediated phosphorylation of the third intracellular loop of the beta(1)-AR because disruption of PKA/AKAP interactions or small interfering RNA-mediated down-regulation of AKAP79 both inhibited this response. Thus, AKAP79 provides PKA to phosphorylate the beta(1)-AR and thereby dictate the recycling and resensitization itineraries of the beta(1)-AR.

Characterization of the residues in helix 8 of the human beta1-adrenergic receptor that are involved in coupling the receptor to G proteins

Several key amino acids within amphipathic helix 8 of the human beta1-adrenergic receptor (beta1-AR) were mutagenized to characterize their role in signaling by G protein-coupled receptors. Mutagenesis of phenylalanine at position 383 in the hydrophobic interface to histidine (F383H) prevented the biosynthesis of the receptor, indicating that the orientation of helix 8 is important for receptor biosynthesis. Mutagenesis of aspartic acid at position 382 in the hydrophilic interface to leucine (D382L) reduced the binding and uncoupled the receptor from G protein activation. Mutagenesis of the basic arginine residue at position 384 to glutamine (R384Q) or to glutamic acid (R384E) increased basal and agonist-stimulated adenylyl cyclase activities. R384Q and R384E displayed features associated with constitutively active receptors because inverse agonists markedly reduced their elevated basal adenylyl cyclase activities. Isoproterenol increased the phosphorylation and promoted the desensitization of the Gly389 or Arg389 allelic variants of the wild type beta1-AR but failed to produce these effects in R384Q and R384E, because these receptors were maximally phosphorylated and desensitized under basal conditions. In contrast to the membranous distribution of the wild type beta1-AR, R384Q and R384E were localized mostly within intracellular punctate structures. Inverse agonists restored the membranous distribution of R384Q and R384E, indicating that they recycled normally when their constitutive internalization was blocked by inverse agonists. These data combined with computer modeling of the putative three-dimensional organization of helix 8 indicated that the amphipathic character of helix 8 and side chain projections of Asp382 and Arg384 within the hydrophilic interface might serve as a tethering site for the G protein.

Enhanced non-esterified fatty acids and corticosterone in blood plasma of chickens treated with insulin are significantly depleted by reverse T: minor changes in hypoglycaemia

Previously, it has been observed that dexamethasone or adrenaline-induced hyperlipaemia in blood of chicken was significantly reduced after administration of reverse triiodothyronine (rT3). The present experiment was performed on chicken to determine the altered circulating non-esterified fatty acids (NEFA) induced by physiologically enhanced endogenous corticosterone and catecholamines may also be influenced by rT3. Rise of both hormones were induced by insulin administration. Changes in circulating glucose, corticosterone and catecholamines were additionally measured. Following insulin injection blood glucose fell on the average by 32.7% below control at 2 h of the experiment. Additional treatment with rT3 (rT3 + insulin group) gradually attenuated this decrease and at 4 and 6 h of the experiment it was 17.1% and 12.9% below control, respectively, suggesting on slight inhibition by rT3 of insulin-stimulated glucose utilization. Exposure to insulin significantly increased NEFA levels to about 670% above control group. Additional treatment with rT3 reduced this increase to 309% of control, suggesting inhibition of lipolysis by rT3. Similar alterations were observed in plasma corticosterone levels. Insulin treatment peaked the corticosterone levels maximally by 507.6% above control. Additional treatment with rT3 abolished this rise in the averages to 194.2% above control, possibly by interaction of rT3 with hypothalamo-adrenal axis. Insulin injection increased plasma catecholamines on the average by 21.5% and 53.4% for adrenaline and noradrenaline respectively. Supplementary treatment with rT3 intensified this rise by 55.6% and 71.6% respectively. The obtained results suggest on inhibitory effect of rT3 on hypoglycaemia, hyperlipaemia and plasma corticosterone concentrations in chickens treated with insulin. Contrary to this, rT3 enhanced the rise of plasma catecholamines due to insulin treatment. The obtained data favour the assumption that hypometabolic properties of rT3 depends mainly upon reduced supply of NEFA as a result of restricted lipolysis and to a lesser extent upon the supply of glucose.

Thyroid hormone-induced morphological differentiation and maturation of astrocytes involves activation of protein kinase A and ERK signalling pathway

Thyroid hormone (TH) has a profound effect on astrocyte differentiation and maturation. Astrocytes cultured under TH-deficient conditions fail to transform from flat polygonal morphology to mature, process-bearing, stellate cells. Supplementation of physiological concentrations of TH initiate gradual transformation of the cells and the process takes approximately 48 h to complete. The signal transduction pathways associated with TH-mediated maturation of astrocytes have been investigated. TH treatment caused an initial activation of protein kinase A (PKA), with a peak activity at 2 h which fell back to basal level there after. Although there was no visible change in morphology of the cells during the observed activation of PKA, it was sufficient to drive the process of transformation to completion, suggesting the involvement of downstream regulators of PKA. PKA inhibitors as well as the MEK inhibitor PD098059 attenuated the TH-induced morphological transformation. Further studies showed that TH treatment resulted in a biphasic response on the cellular phospho-MAP kinase (p-MAPK or p-ERK) level: an initial decline in the p-ERK level followed by an induction at 18-24 h, both of which could be blocked by a PKA inhibitor. Such sustained activation of p-ERK levels by TH at this later stage coincided with initiation of morphological differentiation of the astrocytes and appeared to be critical for the transformation of astrocytes. The nitric oxide synthase (NOS) inhibitor 7-NI inhibited this induction of p-ERK activity. Moreover, the induction was accompanied by a parallel increase in phospho-CREB activity which, however, persisted at the end of the transformation of the astroglial cells.

Heterologous desensitization of insulin secretion by GIP (glucose-dependent insulinotropic peptide) in INS-1 cells: the significance of Galphai2 and investigations on the mechanism involved

Heterologous desensitization is a term that describes the observation that chronic exposure of a cell to an agonist attenuates its response to other agonists. To characterize the cellular mechanisms that might be responsible for heterologous desensitization in an insulin secretory cell system (INS-1), we investigated the link between G-protein alphai2 level and insulin secretion as the biological effect after prolonged incubation with glucose-dependent insulinotropic polypeptide (GIP). Persistent activation (8 h) of the GIP signalling pathway decreased the GLP (glucagon-like peptide)-1 dependent insulin secretion (specific radioimmunoassay) accompanied by an upregulation of G-protein alphai2 protein level to about 126% whereas G-protein alphai3 and alphas protein levels remained unchanged (assessed by Western blots using specific antibodies). This was accompanied by similar changes in Galphai2 mRNA. By using either the CaM kinase II inhibitor KN-62, the calcineurin inhibitor FK 506 or the protein kinase A (PKA) inhibitor Rp-8-Br-cAMPS, the GIP-mediated Galphai2 mRNA increase was fully reversed. Heterologous desensitization of GLP-1-dependent insulin secretion by pretreatment with GIP, however, was not inhibited by calcium/calmodulin-dependent enzymes (using KN-62 and FK 506), but only by suppressing the cAMP/PKA signalling pathway using Rp-8-Br-cAMPS. The outcome is not disturbed by effects initiated by these compounds per se since an 8-h preincubation of cells did not affect glucose-induced insulin secretion. We, therefore, suggest that heterologous desensitization in INS-1 cells may be mediated by Galphai2 changes but depend on the cAMP/PKA signalling pathway probably distant form the Galphai2 protein.

Thyroid hormone regulates Galphai1 gene expression in the rat cerebellar cortex during post-natal development

Thyroid hormone regulates the expression of G protein in tissues such as fat and heart. In the brain, very little information is available relative to the regulation by thyroid hormone of G proteins. Here, we show that the expression of the Galphai1 gene is induced by thyroid hormones in the rat cerebellum during development. Hence, the levels of Galphai1 transcripts and protein were decreased in the cerebellum of hypothyroid neonates. In situ hybridization studies showed that the neurons of the cerebellar cortex, particularly Purkinje cells, were affected. Surprisingly, and in contrast with the in vivo stimulatory effect described above, thyroid hormone repressed the activity of the rat Galphai1 promoter in vitro, suggesting that the effect of this hormone in the cerebellum is indirect. In this regard, we present data suggesting that the transcription factor C/EBPbeta could be implicated. First, there are active CEBP binding sites in the Galphai1 promoter. Second, we have found a diminished DNA binding activity of hypothyroid nuclear proteins to a Galphai1 promoter sequence containing a C/EBP binding site. Third, this complex is likely to contain C/EBPbeta protein as it is displaced by specific anti-C/EBPbeta antibodies. Finally, there is a significant decrease in the C/EBPbeta protein content in the hypothyroid cerebellar cortex.

Role of the Cyclic AMP-dependent Protein Kinase in Homologous Resensitization of the β1-Adrenergic Receptor

A fundamental question in biology is how the various motifs in G protein-coupled receptors participate in the divergent functions orchestrated by these molecules. Here we describe a fundamental role for a serine residue at position 312 in the third intracellular loop of the human beta(1)-adrenergic receptor (beta(1)-AR) in endocytic recycling of the agonist-internalized receptor. In receptor recycling experiments that were monitored by confocal microscopy, the agonist-internalized wild-type (WT) beta(1)-AR recycled with a t(0.5) of 14 +/- 3 min. Mutagenesis of Ser(312) to alanine (Ser(312) --> Ala beta(1)-AR) or to the phosphoserine mimic aspartic acid (Ser(312) --> Asp beta(1)-AR) resulted in beta(1)-AR constructs that were pharmacologically indistinguishable from the WT beta(1)-AR. The internalized Ser(312) --> Asp beta(1)-AR recycled efficiently with a t(0.5) of 11 +/- 3 min, whereas the internalized Ser(312) --> Ala beta(1)-AR was not recycled or functionally resensitized through the endosomal pathway. Because this serine is a putative residue for phosphorylation by the cyclic AMP-dependent protein kinase (PKA), we examined the role of this kinase in recycling of the internalized beta(1)-AR. Inhibition of PKA biochemically or genetically using a dominant negative PKA construct blocked the recycling of the internalized WT beta(1)-AR. Phosphorylation studies revealed that the beta(1)-AR is partially phosphorylated by PKA and that phosphorylation of the beta(1)-AR by the catalytic subunit of PKA occurs exclusively at Ser(312). Our results identify a new signaling paradigm in which homologous activation of a kinase provides a reversible modification that shifts the itinerary of the internalized receptor toward recycling and resensitization. Therefore, PKA-mediated phosphorylation of G protein-coupled receptors might result in motif-dependent desensitization or resensitization.

Membrane-Bound and cytosolic forms of heterotrimeric G proteins in young and adult rat myocardium: influence of neonatal hypo- and hyperthyroidism

Membrane and cytosolic fractions prepared from ventricular myocardium of young (21-day-old) hypo- or hyperthyroid rats and adult (84-day-old) previously hypo- or hyperthyroid rats were analyzed by immunoblotting with specific anti-G-protein antibodies for the relative content of Gs alpha, Gi alpha/Go alpha, Gq alpha/G11 alpha, and G beta. All tested G protein subunits were present not only in myocardial membranes but were at least partially distributed in the cytosol, except for Go alpha2, and G11 alpha. Cytosolic forms of the individual G proteins represented about 5-60% of total cellular amounts of these proteins. The long (Gs alpha-L) isoform of Gs alpha prevailed over the short (Gs alpha-S) isoform in both crude myocardial membranes and cytosol. The Gs alpha-L/Gs alpha-S ratio in membranes as well as in cytosol increased during maturation due to a substantial increase in Gs alpha-L. Interestingly, whereas the amount of membrane-bound Gi alpha/Go alpha and Gq alpha/G11 alpha proteins tend to lower during postnatal development, cytosolic forms of these G proteins mostly rise. Neonatal hypothyroidism reduced the amount of myocardial Gs alpha and increased that of Gi alpha/Go alpha proteins. By contrast, neonatal hyperthyroidism increased expression of Gs alpha and decreased that of Gi alpha and G11 alpha in young myocardium. Changes in G protein content induced by neonatal hypo- and hyperthyroidism in young rat myocardium were restored in adulthood. Alterations in the membrane-cytosol balance of G protein subunits associated with maturation or induced by altered thyroid status indicate physiological importance of cytosolic forms of these proteins in the rat myocardium.

Mechanisms of regulation of G(11)alpha protein by dexamethasone in osteoblastic UMR 106-01 cells

We have previously demonstrated that glucocorticoids increased G(q/11)alpha protein expression and phospholipase C activity in the rat osteosarcoma cell line UMR 106-01. In this study, we demonstrated that G(11)alpha is the primary G(q)-subtype family member expressed in UMR cells. Dexamethasone treatment increased the expression of G(11)alpha protein in both a time- and a dose-dependent manner. Glucocorticoid treatment significantly increased the half-life of G(11)alpha protein from 20.3 to 63 h. Steady-state G(11)alpha mRNA level was also increased by glucocorticoid treatment by approximately 70%. This change was not the result of changes in RNA stability but rather the result of increased transcription, because the glucocorticoid-mediated upregulation of G(11)alpha mRNA was blocked by the transcription inhibitor actinomycin D. The dexamethasone induction of G(11)alpha mRNA occurred after a time lag of 12-24 h and was blocked by the protein synthesis inhibitor cycloheximide. These results suggest that the dexamethasone-induced rise in G(11)alpha protein results primarily from changes in the degradation rate of the protein, whereas changes in G(11)alpha mRNA play a smaller role and require de novo synthesis of regulatory protein(s).

Identification of dopamine responsive mRNAs in glial cells by suppression subtractive hybridization

Recent studies have established that glial cells are important targets of the neurotransmitter dopamine (DA), but the regulatory effects of DA on glial cells have not been extensively studied. In the present study, we have investigated the influence of DA on gene transcription in glial cells. Two-directional (forward and backward) suppression subtraction hybridization (SSH) was performed on astrocytes cultured from rat cerebral tissues in standard media or in culture media treated with DA. PCR-select differential screening was used to further verify the differentially expressed cDNA clones, positive clones were sequenced, and the mRNAs were re-examined on Northern blots. Fourteen sequences were identified of which eleven are homologous to known genes, three are homologous to expressed sequence tags (ESTs). Three novel full-length cDNAs were isolated using the EST fragments as probes to screen a cDNA library constructed from human brain. Analysis of these sequences suggested that complex intracellular signaling pathways, involving crosstalk with growth factor pathways, steroid hormone pathways, and an interferon-regulated 2-5 A pathway, are responsive to DA in astrocytes. The responsive proteins downstream from the signaling pathways were found to fall into at least three groups, including a series of metabolic enzymes, stress proteins, transfer proteins, etc. In addition, several of them have established their relationships with specific neurodegenerative diseases, showing that there is overlap in the pathogenic mechanisms of different diseases. Our results have provided a foundation for better understanding of the molecular basis of glial cell functions in dopaminergic transmission and an approach to find possible medication for the related disorders.

Role of stimulatory guanine nucleotide binding protein (GSalpha) in proliferation of PC-3M prostate cancer cells

Previous studies have shown that calcitonin-like immunoreactive substances are secreted by primary prostate cells. Furthermore, exogenously added calcitonin stimulates proliferation of androgen-responsive LnCaP cells. To examine the possible effect of calcitonin on growth of invasive prostate cancer cells, we tested its effects on proliferation of PC-3M cells. Calcitonin stimulated DNA synthesis of PC-3M cells in a dose-dependent fashion, and also stimulated adenylyl cyclase and protein kinase C activities. To further delineate the role of these signaling cascades in proliferation of PC-3M prostate cancer cells, we selectively activated these pathways by transfecting cDNAs expressing constitutively active forms of either Gsalpha (Gsalpha-QL) or Gqalpha (Gqalpha-QL). cDNAs expressing wild-type forms of G-proteins (Gsalpha-WT and Gqalpha-WT) were used as vehicle controls. Gqalpha-QL transfectants exhibited growth inhibition and terminal differentiation. Those expressing Gsalpha-QL exhibited a dramatic increase in growth rate. Gsalpha-QL transfectants displayed an almost 3-fold increase in [3H]-thymidine incorporation and over a 4-fold increase in growth rate when compared with parental PC-3M cells or those expressing wild-type Gsalpha (Gsalpha-WT). The growth-promoting action of Gsalpha-QL could not be mimicked by either 8-bromo cAMP or forskolin. However, nifedipine, a calcium channel antagonist, potently and selectively inhibited DNA synthesis in Gsalpha-QL transfectants. These results suggest that the growth-promoting actions of Gsalpha on PC-3M cells may be mediated by nifedipine-sensitive proliferative events.

Expression of Plasmodium falciparum trimeric G proteins and their involvement in switching to sexual development

Both cholera and pertussis toxins were used to label and study the expression of heterotrimeric G protein alpha subunits in Plasmodium falciparum extracts. Expression of these proteins is developmentally regulated throughout the erythrocytic cycle with peak expression during early asexual development and in mature sexual stages. Treatment of P. falciparum cultures with cholera toxin causes an increase in conversion to sexual development, and at the same concentration has a marginal inhibitory effect on asexual growth and division. Through precise synchronisation of the parasites' asexual cell cycle, we have defined the period of sensitivity to this induction at around the time of invasion, one cycle before the development of the sexual form. Fluorescent microscopy confirmed that access of the toxin to the parasite is limited to the invasive form--the free merozoite, while further labelling studies revealed expression of a single G protein alpha subunit in these stages. These observations are consistent with the view that a G protein-dependent signal transduction pathway is involved in coupling the parasite's environment to commitment to sexual development (gametocytogenesis). This means of artificially stimulating the pathways leading to sexual development can now be used to biochemically follow the activation of the signalling pathways involved.

TNF-alpha increases transcription of Galpha(i-2) in human airway smooth muscle cells

Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine that has an important role in the regulation of airway smooth muscle tone and reactivity. We have shown previously that TNF-alpha upregulates the expression of Galpha(i-2) protein without significantly increasing G(s)alpha protein and enhances adenylyl cyclase inhibition by carbachol in cultured human airway smooth muscle cells (Hotta K, Emala CW, and Hirshman CA. Am J Physiol Lung Cell Mol Physiol 276: L405-L411, 1999). The present study was designed to investigate the molecular mechanisms by which TNF-alpha upregulates Galpha(i-2) protein in these cells. TNF-alpha pretreatment for 48 h increased the expression of Galpha(i-2) protein without significantly altering the Galpha(i-2) protein half-life (41.0 +/- 8.2 h for control and 46.8 +/- 5.2 h for TNF-alpha-treated cells). Inhibition of new protein synthesis by cycloheximide blocked the increase in Galpha(i-2) protein induced by TNF-alpha. Furthermore, TNF-alpha treatment for 12-24 h increased the steady-state level of Galpha(i-2) mRNA without significantly altering Galpha(i-2) mRNA half-life (9.0 +/- 0.75 h for control and 8.9 +/- 1.1 h for TNF-alpha-treated cells). The transcription inhibitor actinomycin D blocked the increase in Galpha(i-2) mRNA induced by TNF-alpha. These observations indicate that the increase in Galpha(i-2) protein induced by TNF-alpha is due to an increased rate of Galpha(i-2) protein synthesis, most likely as a consequence of the transcriptional increase in the steady-state levels of its mRNA.

Thyroid hormone regulation of hepatic genes in vivo detected by complementary DNA microarray

The liver is an important target organ of thyroid hormone. However, only a limited number of hepatic target genes have been identified, and little is known about the pattern of their regulation by thyroid hormone. We used a quantitative fluorescent cDNA microarray to identify novel hepatic genes regulated by thyroid hormone. Fluorescent-labeled cDNA prepared from hepatic RNA of T3-treated and hypothyroid mice was hybridized to a cDNA microarray, representing 2225 different mouse genes, followed by computer analysis to compare relative changes in gene expression. Fifty five genes, 45 not previously known to be thyroid hormone-responsive genes, were found to be regulated by thyroid hormone. Among them, 14 were positively regulated by thyroid hormone, and unexpectedly, 41 were negatively regulated. The expression of 8 of these genes was confirmed by Northern blot analyses. Thyroid hormone affected gene expression for a diverse range of cellular pathways and functions, including gluconeogenesis, lipogenesis, insulin signaling, adenylate cyclase signaling, cell proliferation, and apoptosis. This is the first application of the microarray technique to study hormonal regulation of gene expression in vivo and should prove to be a powerful tool for future studies of hormone and drug action.

Expression of Plasmodium falciparum trimeric G proteins and their involvement in switching to sexual development

Both cholera and pertussis toxins were used to label and study the expression of heterotrimeric G protein alpha subunits in Plasmodium falciparum extracts. Expression of these proteins is developmentally regulated throughout the erythrocytic cycle with peak expression during early asexual development and in mature sexual stages. Treatment of P. falciparum cultures with cholera toxin causes an increase in conversion to sexual development, and at the same concentration has a marginal inhibitory effect on asexual growth and division. Through precise synchronisation of the parasites' asexual cell cycle, we have defined the period of sensitivity to this induction at around the time of invasion, one cycle before the development of the sexual form. Fluorescent microscopy confirmed that access of the toxin to the parasite is limited to the invasive form - the free merozoite, while further labelling studies revealed expression of a single G protein alpha subunit in these stages. These observations are consistent with the view that a G protein-dependent signal transduction pathway is involved in coupling the parasite's environment to commitment to sexual development (gametocytogenesis). This means of artificially stimulating the pathways leading to sexual development can now be used to biochemically follow the activation of the signalling pathways involved.

Involvement of Gialpha2 in sodium butyrate-induced erythroblastic differentiation of K562 cells

The chronic myelogenous leukaemia cell line K562 can be triggered in culture to differentiate along the erythrocytic pathway in response to a variety of stimulatory agents. In the presence of sodium butyrate, these cells differentiate to erythroblasts and acquire the capability to synthesize haemoglobin. We used this cell system to study alterations in the levels of several G-protein subunits during the cell differentiation programme and to assess the involvement of G(i)alpha2 in this process. Western immunoblot analysis revealed the presence of G(s)alpha1, G(s)alpha2, G(i)alpha2, G(q)alpha, Galpha(12), Gbeta1 and Gbeta2 in K562 cells. G(o)alpha, G(z)alpha, Galpha(13) and Galpha(16) were not detected. Although the levels of several G-protein subunits were altered after treatment with sodium butyrate, the most striking change was the robust increase in the levels of G(i)alpha2, which was accompanied by an increase in the mRNA for G(i)alpha2. Inactivation of G(i)alpha2 by adding Bordetella pertussis toxin to the cultures inhibited erythroblastic differentiation by as much as 62%, as measured by haemoglobin accumulation. Furthermore, the addition of an oligonucleotide anti-sense to G(i)alpha2 inhibited the sodium butyrate-induced robust increase in G(i)alpha2 levels, decreasing it to the basal levels seen in control cells; this treatment decreased the erythroblastic differentiation of the cells (as measured by haemoglobin expression) by 50%. Taken together, these findings imply that increased levels of G(i)alpha2 contribute to the sodium butyrate-induced erythroblastic differentiation of K562 cells.

Changes in adenylyl cyclase isoforms as a mechanism for thyroid hormone modulation of cardiac beta-adrenergic receptor responsiveness

Although thyroid hormones are known to modulate cardiac beta-adrenergic receptor expression, the physiologic implications of these changes in the cardiac manifestations of altered thyroid hormone metabolism have been disputed. This study examined whether thyroid hormone modulates signaling via the cyclic adenosine monophosphate (cAMP) pathway by regulating cardiac adenylyl cyclase (AC) isoform expression. Northern blot analyses and AC enzyme assays were performed on preparations from hypothyroid, euthyroid, and hyperthyroid rat ventricles. Steady-state levels of cardiac AC mRNA types V and VI in hypothyroid ventricles were 173% +/- 8% and 149% +/- 12%, respectively, of the values in euthyroid ventricles (P < .01). This increase in AC mRNA isoforms was accompanied by a 1.5-fold increase (P < .05) in the activation of catalytic AC by forskolin and Mn. In contrast, the relative abundance of transcripts for types V and VI AC was similar in hyperthyroid and euthyroid ventricles, but catalytic AC activation by forskolin and Mn was significantly reduced by 35% in membranes obtained from hyperthyroid ventricles. AC activation through beta-adrenergic receptor stimulation by isoproterenol was not altered by thyroid hormone status. Thus, the effect of thyroid hormone to repress AC catalytic activity would be anticipated to offset the increase in beta-adrenergic receptor expression in hyperthyroidism. These studies identify cardiac AC enzymes as important targets for thyroid hormone-dependent regulation of signaling via the cAMP pathway, and support the finding that cardiac adrenergic responsiveness is unaltered in thyroid disease states.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Galpha(i2)-mRNA and -protein regulation as a mechanism for heterologous sensitization and desensitization of insulin secretion

Prolonged exposure of cells to an agonist of a G-protein-coupled receptor usually results in an attenuation of the cellular response. To elucidate the cellular mechanisms of sensitization or desensitization in an insulin secretory cell system (INS-1 cells), we investigated a regulatory link between G-protein alpha(s)- and alpha(i2)-subunits mRNA, their protein levels and insulin secretion as the biological effect using various compounds. Incubation with epinephrine (50 microM) for 8 h decreased alpha(s)- and alpha(i2)-mRNA levels to 58% and 72%, respectively, which is reversed after a longer incubation. From results using isoprenaline and the alpha2-agonist UK 14,304 epinephrine is shown to mediate its actions via alpha2- but not beta-adrenoceptors. The insulin inhibitory neuropeptide galanin (50 nM) caused a decrease of alpha(s)- and alpha(i2)-mRNA levels, whereas insulinotropic compounds (incretin hormones) such as GIP or GLP-1 (both 10 nM) led to an increase of alpha(s)- and alpha(i2)-mRNA levels. By using the Ca2+ channel blocker verapamil (50 microM) alpha(i2)-mRNA changes clearly depend on Ca2+ influx. The effects on alpha(i2)-mRNA were accompanied by a parallel, albeit weaker effect on the protein level (only GIP and UK 14,304 were investigated). The changes in alpha(i2)-mRNA levels by either compound were paralleled by inverse changes in insulin secretion: preincubation with UK 14,304 for 8 h led to an increased insulin secretion when challenged by either GLP-1, GIP or glucose (8.3 mM). This was similar for galanin, another potent inhibitor of insulin release. On the other hand, exposure to the incretins GIP or GLP-1 for 8 h induced a smaller insulin release when challenged afterwards by either UK 14,304, galanin, GIP, GLP-1, or glucose. Thus the influence on insulin secretion of various compounds is reciprocal to the regulation of alpha(i2)-mRNA levels but not alpha(s)-mRNA levels. There is, therefore, evidence from all the manoeuvres used that alpha(i2)-mRNA regulation may play a role in heterologous sensitization and desensitization of insulin secretion.

Isoprenaline can activate the acetylcholine-induced K+ current in canine atrial myocytes via Gs-derived betagamma subunits

1. G protein betagamma subunits activate the acetylcholine-induced potassium current IK,ACh. There is no evidence of specificity at the level of the betagamma subunits. Therefore all G protein-coupled receptors in atrial myocytes should be able to activate IK,ACh. Paradoxically, it is often stated that isoprenaline does not activate IK,ACh. Rationales to explain this negative result include insufficient concentrations of Gs in the atrium or restricted access of Gs-derived betagamma subunits to the IK,ACh channel. We took advantage of a non-specific increase in Gs that results after infection with adenovirus. 2. Adenoviral infection unmasked a 1 microM isoprenaline-induced IK,ACh which was prevented by propranolol. Isoprenaline occasionally activated IK,ACh in uninfected and freshly dissociated atrial myocytes but the effect was larger and more consistent in infected myocytes. 3. Pertussis toxin pretreatment (100 ng ml-1 overnight) did not block the effect of isoprenaline. The effect of isoprenaline became persistent if cells were pretreated with cholera toxin (200 ng nl-1). 4. Signal transduction events distal to adenylyl cyclase were not involved in isoprenaline-induced IK,ACh. Forskolin (10 microM) did not activate IK,ACh. Inhibition of adenylyl cyclase with cytoplasmic application of 300 microM 2'-deoxyadenosine 3'-monophosphate did not prevent the activation of IK,ACh by isoprenaline. 5. Cytoplasmic application of a betagamma binding peptide derived from the C terminus of beta-adrenergic receptor kinase 1 (50 microM) prevented the effect of isoprenaline on IK,ACh. The peptide did not prevent the stimulation of the L-type calcium current by isoprenaline. 6. The results indicate that beta-adrenoceptors can activate IK,ACh in atrial myocytes through the release of betagamma subunits from Gs.

Specific activation of adenylyl cyclase V by a purinergic agonist

The present study was designed to investigate whether and how the purinergic stimulation of rat ventricular myocytes modulates the cAMP-dependent pathway. Stimulation of cardiomyocytes with ATPgammaS in the presence of the phosphodiesterase inhibitor IBMX increases by 3-fold intracellular cAMP content. In contrast to beta-adrenergic stimulation, the purinergic stimulation of adenylyl cyclase was not inhibited by activation or enhanced by inhibition of a Gi protein. Forskolin did not potentiate the effect of extracellular ATPgammaS on intracellular cAMP content but the effect of isoproterenol did. Like isoproterenol, the purinergic agonist decreased subsequent ADP-ribosylation of a 45 kDa G(alpha s) by cholera toxin. ATPgammaS also increased cAMP content in neonatal rat cardiomyocytes, a cell type that expresses a long form of Gs protein (alpha(s), 52 kDa) in contrast to adult rat cardiomyocytes that express mostly a short form of Gs protein (alpha(s), 45 kDa). Both purinergic and beta-adrenergic agonists increased cAMP in HEK 293 cells expressing type V adenylyl cyclase while cAMP was only increased by beta-adrenergic stimulation of HEK expressing type IV or VI adenylyl cyclases. Thus, we propose that the purinergic and beta-adrenergic stimulations differentially activate adenylyl cyclase isoforms in rat cardiomyocytes and that adenylyl cyclase V is the specific target of the purinergic stimulation.

Mechanisms of thyroid hormone control over sensitivity and maximal contractile responsiveness to beta-adrenergic agonists in atria

This paper discusses the mechanisms of two basic effects of thyroid hormones on atrial responses to beta-adrenergic agonists, i.e. increased inotropic sensitivity and decreased maximal contractile responsiveness. The increased sensitivity of atria to beta-adrenergic agonists under thyroid hormones appears to be related to increases in beta-adrenoceptor density and Gs/Gi protein ratio, leading to activation of Gs-mediated pathway, but suppression of Gi-mediated pathway of adenylate cyclase regulation. Therefore, the i/c concentrations of cAMP and corresponding inotropic responses achieve their maximums at lower doses of beta-adrenergic agonist. Thyroid hormones also decrease the expression of phospholamban, but increase the expression of sarcoplasmic reticulum Ca2+-pump. As a result, the basal activity of sarcoplasmic reticulum Ca2+-pump increases, but its beta-adrenergic activation through phosphorylation of phospholamban decreases. It is suggested that these changes are causal for decreased maximal inotropic and lusitropic responses of atria to beta-adrenergic agonists.

Effects of triiodothyronine administration on the adenylyl cyclase system in brown adipose tissue of rat

This study was undertaken to investigate the effect of triiodothyronine (T3) administration to euthyroid rats on beta 3-adrenoceptor (beta 3-AR) expression and on the different components of the adenylyl cyclase (AC) system in brown adipose tissue (BAT). In rats treated with T3, the beta 3-AR density (assessed by the binding of [3H]CGP-12177) showed a decrease of 50%, as did their mRNA, as analyzed by reverse transcriptase-polymerase chain reaction. In hyperthyroid rats, compared with control rats, there was a 40% increase in G alpha s activity (stimulated by NaF or GTP gamma S) and a fourfold increase in the protein concentration (Western blotting). In contrast, the level of the pertussis toxin substrate Gi declined by 35% in response to T3. Analysis of dose-response curves for isoproterenol and CGP-12177 revealed that neither basal nor stimulated AC activities nor 50% stimulatory concentration for these agonists was changed by T3 administration. In conclusion, these results suggest that downregulation of the beta 3-AR by T3 was counter-balanced by changes in other components of the AC cascade (i.e., Gs and Gi), so no change occurred in the capacity of BAT to generate adenosine 3',5'-cyclic monophosphate.