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S N Nayfeh

Publications and source records attributed to S N Nayfeh.

At least 19 recordsLinked to original sources
PubMed1992

Differential regulation of multiple c-erbA expression by thyrotropin, insulin and insulin-like growth factor I in rat thyroid FRTL-5 cells.

Regulation of multiple c-erbA gene expression was studied in rat thyroid FRTL-5 cells. Two species of erbA alpha (alpha 1 and alpha 2) mRNA and one species of erbA beta (beta 1) mRNA were identified by Northern blot analysis. Withdrawal of thyrotropin (TSH), insulin and serum from the complete medium resulted in an increase in alpha 1 and alpha 2 erbA mRNA levels without altering the level of erbA beta 1 mRNA. Readdition of TSH, N6,2'-O-dibutyryl cAMP or forskolin caused a transient reduction of alpha 1 and alpha 2 mRNA levels (75-90%) at 3-12 h. The alpha 1 and alpha 2 mRNA levels were restored at 24 h. The TSH action was dose-dependent showing the half-maximal effect at around 10(-9) M. Readdition of TSH did not show any effect on beta 1 mRNA level. The action of TSH was not dependent on ongoing protein synthesis but required ongoing transcription. Inhibitors of thyroid hormone biosynthesis, propylthiouracil and methylmercaptoimidazole, did not show any effect on TSH action. Readdition of insulin or insulin-like growth factor I (IGF-I) caused a dose-dependent reduction of alpha 1 and alpha 2 mRNA levels without any effect on beta 1 mRNA level. Their action was slower than TSH and persistent. The actions of insulin and IGF-I were dependent on both ongoing translation and transcription. These results indicate that TSH and insulin/IGF-I reduce levels of c-erbA alpha 1 and alpha 2 mRNA possibly by two distinct mechanisms without altering c-erbA beta 1 mRNA level in FRTL-5 cells.

Animals↗
PubMed1992

Thyroid-specific and hormone-dependent expression of rat thyroglobulin promoter fused with bacterial chloramphenicol acetyltransferase gene in transgenic mice.

The minimal promoter of rat thyroglobulin (TG) gene (168 bp) was fused with bacterial chloramphenicol acetyltransferase (CAT) gene, and transgenic mice carrying the TGCAT gene were produced. The minimal promoter is sufficient for thyroid-specific and hormone-dependent expression of TGCAT in transgenic mice. Deletion of a region between -128 and -92 bp (TGII), which is not required for the expression of TGCAT in transient expression assays but whose sequence is most extensively conserved among different species, appears to decrease frequency of the expression of TGCAT in transgenic mice. However, the same deletion apparently has no significant effect on TG promoter activity in stably transformed rat FRTL-5 cells.

Animals↗
PubMed1991

The deoxyribonucleic acid regions involved in the hormonal regulation of thyroglobulin gene expression.

Transcription of the thyroglobulin (TG) gene is stimulated by TSH via cAMP. We have characterized the sequence elements responsible for the hormone-dependent expression of TG gene in rat thyroid FRTL-5 cells using internal deletion and linker-scanning mutants of the minimal TG promoter (-170 basepairs) fused with the bacterial chloramphenicol acetyltransferase reporter gene. The TG gene is regulated by at least two regions located between -165 and -140 bp (TG-III) and between -95 and -65 bp (TG-I) from the transcription initiation site. The intervening region can be deleted without significant effect on the promoter activity. Either of the two regions alone does not promote hormone-dependent transcription. A DNase footprinting assay showed that TG-I and TG-III are the principal protein-binding sites and that the proteins interacting with these two regions are induced by TSH or cAMP. These results suggest that the hormone-dependent expression of TG gene may be achieved by cooperative interaction of the proteins bound to TG-I and TG-III.

Animals↗
PubMed1990

Similar nuclear factors mediate stimulation of rat thyroglobulin gene transcription by thyrotropin and insulin-like growth factor-I.

Transcription of the thyroglobulin (TG) gene in rat thyroid FRTL-5 cells is stimulated by two hormones, TSH and insulin-like growth factor-I (IGF-I). The effect of TSH is mimicked by cAMP. Promoter regions of the rat TG gene responsible for hormonal action as well as the nuclear regulatory proteins that interact with these regions were characterized. Minimal promoter that responds to both hormones has been found to be up to -171 basepairs from the transcription initiation site. In DNase-I footprinting analysis, nuclear extracts from cells treated with either of these hormones protected the same two major regions within the minimal promoter. Mutations in these two regions abolished basal, TSH-stimulated, as well as IGF-I-stimulated expression of the fused reporter gene chloramphenicol acetyltransferase. DNA mobility shift assay revealed that cAMP and IGF-I induce binding of similar nuclear proteins to these promoter regions. These results suggest that rat TG gene transcription is regulated by the convergent action of two distinct signaling pathways, possibly involving similar DNA-binding nuclear proteins and regulatory sequences of the TG gene promoter.

Animals↗
PubMed1989

Induction of nuclear protein factors specific for hormone-responsive region during activation of thyroglobulin gene by thyrotropin in rat thyroid FRTL-5 cells.

We have investigated the mechanism of stimulation of thyroglobulin gene expression by thyrotropin (TSH) and cAMP in rat thyroid FRTL-5 cells. In contrast to the c-fos gene, induction of the thyroglobulin gene by TSH or cAMP is slow (10 h) and sensitive to cycloheximide treatment. We have identified a TSH and cAMP-responsive region of thyroglobulin gene between - 171 and - 140 base pairs from the transcription initiation site. The hormone-responsive region contains DNA sequence elements similar to the consensus cAMP-responsive element as well as the transcription factor AP-1-binding site but with opposite sequence polarity. Three DNA-protein complexes are formed when the hormone-responsive region is incubated with nuclear extracts of FRTL-5 cells. Formation of these complexes is dependent on TSH or cAMP stimulation, thus suggesting that the factors involved in binding to the hormone-responsive region may be induced by TSH. Although the identity of these factors is not yet known, they do not appear to be related to either cAMP-responsive element-binding protein or AP-1. These results suggest that thyroglobulin gene expression in FRTL-5 cells may be mediated by nuclear factors that are induced by cAMP in contrast to other genes (e.g. c-fos) whose activation involves post-translational modification of the pre-existing proteins specific for cAMP-responsive element.

Animals↗
PubMed1987

Alpha 1-adrenergic regulation of TSH-stimulated cyclic AMP accumulation in rat thyroid cells.

Addition of epinephrine to cultured FRTL-5 rat thyroid cells led to a concentration-dependent reduction of TSH- and forskolin-stimulated cAMP accumulation. Clonidine, which preferentially activates the alpha 2-adrenoreceptor, had no effect on cAMP levels. The reduction of cAMP levels by epinephrine was selectively blocked by prazosin, an alpha 1-adrenoreceptor antagonist, but not by yohimbine, an alpha 2-adrenoreceptor antagonist. Pretreatment of FRTL-5 cells with pertussis toxin failed to abolish the inhibitory effect of epinephrine on cAMP accumulation. The bioactivity of the pertussis toxin preparation in this cell line was verified by its ability to ADP-ribosylate the alpha-subunit of the inhibitory guanine nucleotide regulatory protein, Ni, as well as its ability to abolish the inhibitory effect of N6-[L-2-phenylisopropyl]-adenosine on TSH-stimulated cAMP formation. The inhibitory effect of epinephrine on cAMP levels was dependent on Ca2+ and was reversed by 3-isobutyl-1-methylxanthine. Taken together, these results suggest that epinephrine reduces cAMP levels via alpha 1-adrenoreceptors. The failure of pertussis toxin to abolish this alpha-adrenergic effect is consistent with the conclusion that epinephrine-induced attenuation of cAMP accumulation occurs through activation of a Ca2+-calmodulin-sensitive phosphodiesterase and does not involve Ni or Ni-like proteins.

1-Methyl-3-isobutylxanthine↗
PubMed1987

Further studies on the covalent crosslinking of thyrotropin to its receptor: evidence that both the alpha and beta subunits of thyrotropin are crosslinked to the receptor.

Highly purified alpha- and beta-subunits of thyrotropin were individually radioiodinated and, subsequently, recombined with their unlabeled complementary subunits. This procedure resulted in the formation of [125I]thyrotropin(TSH) hybrid molecules which were labeled on only one hormone subunit. Characterization of the binding properties of these two hybrid molecules demonstrated that both yielded nonlinear Scatchard plots with Kd and Bmax values similar to those obtained with radioiodinated native TSH and that both were capable of interaction with the high- and low-affinity binding components of the TSH receptor. The recombined [125I]TSH molecules were then crosslinked to the TSH receptor using disuccinimidyl suberate. Following electrophoresis and autoradiography, two labeled TSH-receptor complexes with Mr of 68,000 and 80,000 were observed. These two complexes exhibited hormone specificity and electrophoretic mobility identical to those previously observed using native [125I]TSH. Crosslinking with increasing concentrations of disuccinimidyl suberate suggested that the formation of the 68,000 and 80,000 complexes was sequential with the 68,000 appearing before the 80,000. Furthermore, the two bands were labeled regardless of which TSH subunit of the hybrid TSH was radioiodinated. These data strongly suggest that the 68,000 and 80,000 TSH-receptor complexes are the result of crosslinking to the TSH alpha-beta dimer and not to one subunit in the case of the 68,000 complex and to the TSH alpha-beta dimer in the case of the 80,000 complex, as had been hypothesized previously.

Animals↗
PubMed1987

Stimulation of inositol phosphate formation in FRTL-5 rat thyroid cells by catecholamines and its relationship to changes in 45Ca2+ efflux and cyclic AMP accumulation.

Catecholamines specifically stimulated the rapid formation of inositol phosphates, bisphosphates and trisphosphates in a concentration-dependent manner in FRTL-5 thyroid cells. Further analysis by high performance liquid chromatography revealed the presence of two isomers of inositol trisphosphate, 1,4,5- and 1,3,4-trisphosphate, suggesting that the 1,4,5-trisphosphate of inositol is further metabolized to the 1,3,4-trisphosphate isomer. The alpha 1-adrenoreceptor antagonist, prazosin, inhibited the effects of epinephrine, while the alpha 2-adrenoreceptor antagonist, yohimbine, was without effect. Treatment of FRTL-5 cells with pertussis toxin (to inhibit Ni) did not abolish the epinephrine effect on inositol trisphosphate formation. Carbachol, N6-[L-2-phenylisopropyl]-adenosine and forskolin were without effect on phosphoinositide metabolism. Both epinephrine and the calcium ionophore A23187 stimulated 45Ca2+ efflux from 45Ca2+-loaded FRTL-5 cells. The time-course of the epinephrine effect indicates that inositol 1,4,5-trisphosphate formation (t1/2 approximately 1 s) precedes both the efflux of 45Ca2+ (t1/2 approximately 30 s) as well as the reduction of cyclic AMP levels (t1/2 approximately 90 s) in response to epinephrine. These results strongly suggest that inositol 1,4,5-trisphosphate has the appropriate properties to act as a second messenger by which alpha 1-adrenergic hormones, through mobilization of intracellular Ca2+ and activation of cyclic AMP phosphodiesterase, reduce cyclic AMP levels in FRTL-5 cells.

Animals↗
PubMed1987

Characterization of the subunit structure of the thyrotropin receptor in the FRTL-5 rat thyroid cell line.

Radioiodinated TSH was covalently cross-linked to monolayers of FRTL-5 rat thyroid cells using the homobifunctional cross-linking agent disuccinimidyl suberate. Analysis of the cross-linked samples by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions demonstrated the presence of three specifically labeled complexes with apparent mol wt of 68,000, 85,000, and 145,000, in addition to the TSH alpha-beta dimer and its alpha- and beta-subunits. When bound [125I]iodo-TSH was cross-linked with increasing concentrations of disuccinimidyl suberate, the formation of the 68,000 and 85,000 mol wt complexes was sequential, with the 68,000 complex appearing first. These two complexes were also observed after labeling with radioiodinated TSH hybrid molecules (alpha-beta or alpha-beta), in which the label is in only one subunit, or immuno-precipitation with antibodies against either the alpha- or beta-subunit of TSH. Similar complexes (65,000, 82,000, and 145,000 mol wt) were also formed after cross-linking with the alkaline-cleavable cross-linker (bis-[2-(succinimidooxycarbonyloxy)ethyl]sulfone. Again, the appearance of these three complexes was sequential and dependent on the cross-linker concentration. At low concentrations and under reducing conditions, the 65,000 mol wt complex was the major band. However, at high concentrations, especially under nonreducing conditions, most of the radioactivity was present in the 145,000 mol wt complex. Alkaline cleavage of these three complexes followed by electrophoresis in a second dimension resulted in the release of three components with approximate mol wt of 31,000, 17,000, and 63,000 in addition to the TSH alpha-beta dimer and its alpha- and beta-subunits. Reduction by dithiothreitol followed by electrophoresis in a second dimension resulted in the release of only the 17,000 and 63,000 components. Taken together, these results suggest that 1) both the 65,000 and 82,000 complexes are formed after incremental cross-linking of TSH alpha-beta dimer to receptor subunits; and 2) the TSH receptor may be an oligomer composed of three different subunits, 31,000, 17,000, and 63,000, of which only the 31,000 subunit binds TSH.

Animals↗
PubMed1986

Further characterization of the low and high affinity binding components of the thyrotropin receptor.

Following cross-linking with disuccinimidyl suberate and analysis by SDS-PAGE and autoradiography, both the high- and low-affinity TSH binding components exhibited two similar 125I-TSH-labeled bands, with Mr values of 80,000 and 68,000. IgG fractions from patients with Graves' disease inhibited 125I-TSH binding to both components, while normal IgG had no effect. Although not entirely conclusive, these results suggest that the high- and low-affinity components share similar subunit composition and antigenic determinants.

Animals↗
PubMed1986

Characterization of the guinea pig adipocyte thyrotropin receptor.

125I-TSH binding to porcine thyroid and guinea pig fat resulted in curvilinear Scatchard plots with similar dissociation constants for the high and low affinity binding components. Antibodies from the sera of patients with Graves' disease inhibited binding to the high and low affinity binding components of both tissues. Covalent cross-linking of 125I-TSH to membranes from each tissue resulted in the specific labeling of two protein bands. The guinea pig fat receptor subunits have Mr values of 52,000 and 38,000, whereas the porcine thyroid receptor subunits have values of 46,000 & 35,000. The labeling of the receptor subunits was inhibited by preincubation with Graves' autoantibodies. Despite possessing a different subunit composition, the receptors from these tissues exhibit similar affinity for TSH and share similar antigenic determinants for Graves' autoantibodies.

Adipose Tissue↗
PubMed1986

Covalent crosslinking of thyrotropin to thyroid plasma membrane receptors: subunit composition of the thyrotropin receptor.

The subunit composition of the thyrotropin (TSH) receptor has been characterized using the bifunctional crosslinking agent, disuccinimidyl suberate (DSS), to covalently link [125I]TSH to its receptor. Purified thyroid membranes were labeled with [125I]TSH, and the hormone-receptor complex was crosslinked by incubation with 0.1 mM DSS. Analysis of this crosslinked complex by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions indicated the presence of a specifically labeled hormone-receptor complex, corresponding to a Mr of 68,000 +/- 3000 before correction for the relative molecular mass of TSH. When reducing agents were absent during SDS solubilization, the mobility of the band increased slightly, suggesting the presence of intramolecular disulfide bonds. The labeling of the 68,000 band was specifically inhibited by TSH, but not by other glycoprotein hormones. Specific labeling occurred only in thyroid, and not in liver or muscle plasma membranes. Protease-free immunoglobulin G, isolated from sera of patients with Graves' disease and capable of competing with TSH for binding to its receptor, inhibited the labeling of the 68,000 complex. When the hormone-receptor complex was crosslinked with higher concentrations of DSS (greater than 0.3 mM), a second specifically labeled band was observed, with a Mr of 80,000 +/- 5000. This complex exhibited hormone, tissue, and immunologic specificities similar to those of the 68,000 band. Continuous sucrose density gradient analysis indicated that the intact solubilized receptor possessed a sedimentation coefficient of 10.5 S prior to correction for detergent binding. However, this value increased to 16 S when determined under conditions which took into account the change in hydrodynamic properties attributable to bound Triton X-100. These data suggest that the 80,000 and 68,000 bands represent binding components of the TSH receptor and that the receptor molecule most likely contains multiple subunits, linked by noncovalent forces.

Animals↗
PubMed1986

Inhibition of thyrotropin-stimulated adenosine 3',5'-monophosphate formation in rat thyroid cells by an adenosine analog. Evidence that the inhibition is mediated by the putative inhibitory guanine nucleotide regulatory protein.

Addition of N6-(L-2-phenylisopropyl)-adenosine (PIA) to cultured FRTL-5 rat thyroid cells led to a concentration-dependent inhibition of TSH-stimulated cAMP formation. Half-maximal inhibition was attained with approximately 0.5 nM PIA. Forskolin and cholera toxin-stimulated cAMP production were also inhibited by PIA. 3-Isobutyl-methylxanthine inhibited the effect of PIA. These results are consistent with the presence of inhibitory adenosine receptors (Ri). Ri-sites were further demonstrated by the binding of 3H-cyclohexyl-adenosine to FRTL-5 plasma membranes. High (Kd = 0.50 +/- 0.07 nM) and low affinity (Kd = 5.95 +/- 2.33 nM) binding sites were observed. Pretreatment of FRTL-5 cells with pertussis, but not cholera, toxin effectively antagonized the inhibitory effects of PIA on cAMP production. ADP-ribosylation of FRTL-5 membranes with [32P]-NAD in the presence of cholera or pertussis toxin specifically labeled a 45,000 and 41,000 Mr species, respectively, which correspond to the alpha subunit of the stimulatory and inhibitory guanine nucleotide regulatory proteins. These results demonstrate that PIA inhibits TSH-stimulated cAMP production via Ri-sites on FRTL-5 thyroid cells. PIA appears to exert its inhibitory effects through the inhibitory guanine nucleotide regulatory protein.

Adenosine↗
PubMed1985

The role of the carbohydrate moiety in thyrotropin action.

The relative binding affinity of deglycosylated human TSH was 6-fold higher than that of native TSH. Although deglycosylated human TSH significantly stimulated adenylate cyclase, it was less effective than the native hormone. When deglycosylated human TSH was added with bovine TSH, however, a dose-dependent antagonism was observed. In particular, submaximal and maximal concentrations of bovine TSH and deglycosylated human TSH resulted in cAMP values much lower than the sum of activities of the individual hormones. The data suggest that although the effects of TSH deglycosylation are not as dramatic as with the gonadotropins, the carbohydrates of TSH appear to be required for maximal activation of adenylate cyclase by the hormone.

Adenylyl Cyclases↗
PubMed1982

Separation of two thyrotropin binding components from porcine thyroid tissue by affinity chromatography: characterization of high and low affinity sites.

Two distinct thyrotropin (TSH) binding species have been separated from solubilized porcine thyroid membranes. Membranes was solubilized with 1% Triton X-100, and the supernatant was recovered by centrifugation at 105,000 X g. Scatchard analysis of thyrotropin binding to solubilized membranes (SM) yielded a nonlinear plot with Kd values for the high and low affinity components similar to those of intact membranes. Chromatography of the SM preparation on concanavalin A-Sepharose 4B resulted in the retention of 10-20% of the binding activity. Upon elution of the column, a peak of binding material (5-7% of total activity) was eluted at 0.3 M alpha-methyl-D-mannoside. This concanavalin A (Con A) bound fraction exhibited a linear Scatchard plot with a Kd value similar to that of the high affinity component of the SM. The protein fraction that did not bind to Con A (Con A unbound) also exhibited a linear Scatchard plot, but with affinity similar to that of the low affinity component of SM. Discontinuous sucrose density gradient ultracentrifugation revealed the presence of two major binding peaks in the solubilized membrane preparation. The slowly sedimenting peak corresponded to that seen in the Con A bound fraction, whereas the rapidly sedimenting peak corresponded to that of the Con A unbound fraction. Sepharose 6B chromatography indicated that in the case of the Con A unbound fraction, a single peak of specific binding activity was eluted in the void volume, and in the case of the Con A bound fraction, one major peak with an approximate Stokes radius of 67 A and several other minor peaks were eluted. These results demonstrate the physical separation of two distinct TSH binding species from thyroid membranes and provide further support for the model of multiple classes of binding sites.

Animals↗
PubMed1981

Dissociation kinetics of the thyrotropin-receptor complex. Characterization of a slowly dissociable component.

The kinetics of the dissociation of thyrotropin (TSH) from human thyroid plasma membranes were studied in an attempt to further understand the molecular dynamics of the TSH--receptor interaction. Dissociation of bound [125I]TSH from thyroid plasma membranes was a biphasic process consisting of rapidly and slowly dissociable components, RDC and SDC, respectively. The dilution induced dissociation of bound [125I]TSH was enhanced by the addition of excess TSH (DEC). DEC was proportional to the dose of unlabeled TSH and its magnitude increased linearly with temperature. These results are in contrast to those reported for the kinetics of [125I]insulin dissociation. The functional significance of DEC remains largely unexplained. It was found that the fraction of SDC was dependent upon time of association in a temperature-dependent and apparently saturable process. It could not be attributed to alterations in the electrophoretic, immunologic or binding properties of [125I]TSH. Furthermore, no correlation was observed between generation of SDC and change in the Scatchard profile of TSH binding, in contrast to studies on growth hormone. These data suggest that, like some other polypeptide hormones, binding of TSH to its receptor does not proceed according to laws describing simple, rapidly reversible, bimolecular reactions. Furthermore, bound TSH undergoes a receptor-mediated conversion from a rapidly to a slowly dissociable state with time of incubation.

Animals↗