Cell fate following ER stress: just a matter of "quo ante" recovery or death?

The endoplasmic reticulum (ER) is a complex and multifunctional organelle. It is the intracellular compartment of protein folding, a complex task, both facilitated and monitored by ER folding enzymes and molecular chaperones. The ER is also a stress-sensing organelle. It senses stress caused by disequilibrium between ER load and folding capacity and responds by activating signal transduction pathways, known as unfolded protein response (UPR). Three major classes of transducer are known, inositol-requiring protein-1 (IRE1), activating transcription factor-6 (ATF6), and protein kinase RNA (PKR)-like endoplasmic reticulum kinase (PERK), which sense with their endoluminal domain the state of protein folding, although the exact mechanism(s) involved is not entirely clear. Depending on whether the homeostatic response of the UPR is successful in restoring an equilibrium between ER load and protein folding or not, the two possible outcomes of the UPR so far considered have been life or death. Indeed, recent efforts have been devoted to understand the life/death switch mechanisms. However, recent data suggest that what appears to be a pure binary decision may in fact be more complex, and survival may be achieved at the expenses of luxury cell functions, such as expression of differentiation genes.

Increased hexosamine biosynthetic pathway flux dedifferentiates INS-1E cells and murine islets by an extracellular signal-regulated kinase (ERK)1/2-mediated signal transmission pathway

AIMS/HYPOTHESIS

Beta cell failure is caused by loss of cell mass, mostly by apoptosis, but also by simple dysfunction (decline of glucose-stimulated insulin secretion, downregulation of specific gene expression). Apoptosis and dysfunction are caused, at least in part, by lipoglucotoxicity. The mechanisms implicated are oxidative stress, increase in the hexosamine biosynthetic pathway (HBP) flux and endoplasmic reticulum (ER) stress. Oxidative stress plays a role in glucotoxicity-induced beta cell dedifferentiation, while glucotoxicity-induced ER stress has been mostly linked to beta cell apoptosis. We sought to clarify whether ER stress caused by increased HBP flux participates in a dedifferentiating response of beta cells, in the absence of relevant apoptosis.

METHODS

We used INS-1E cells and murine islets. We analysed the unfolded protein response and the expression profile of beta cells by real-time RT-PCR and western blot. The signal transmission pathway elicited by ER stress was investigated by real-time RT-PCR and immunofluorescence.

RESULTS

Glucosamine and high glucose induced ER stress, but did not decrease cell viability in INS-1E cells. ER stress caused dedifferentiation of beta cells, as shown by downregulation of beta cell markers and of the transcription factor, pancreatic and duodenal homeobox 1. Glucose-stimulated insulin secretion was inhibited. These effects were prevented by the chemical chaperone, 4-phenyl butyric acid. The extracellular signal-regulated kinase (ERK) signal transmission pathway was implicated, since its inhibition prevented the effects induced by glucosamine and high glucose.

CONCLUSIONS/INTERPRETATION

Glucotoxic ER stress dedifferentiates beta cells, in the absence of apoptosis, through a transcriptional response. These effects are mediated by the activation of ERK1/2.

ER stress impairs MHC Class I surface expression and increases susceptibility of thyroid cells to NK-mediated cytotoxicity

We recently reported that, in thyroid cells, ER stress triggered by thapsigargin or tunicamycin, two well known ER stressing agents, induced dedifferentiation and loss of the epithelial phenotype in rat thyroid cells. In this study, we sought to evaluate if, in thyroid cells, ER stress could affect MHC class I expression and the possible implications of this effect in the alteration of function of natural killer cells, suggesting a role in thyroid pathology. In both, a human line of fetal thyroid cells (TAD-2 cells) and primary cultures of human thyroid cells, thapsigargin and tunicamicin triggered ER stress evaluated by BiP mRNA levels and XBP-1 splicing. In both cell types, TAD-2 cell line and primary cultures, major histocompatibility complex class I (MHC-I) plasmamembrane expression was significantly reduced by ER stress. This effect was accompanied by signs of natural killer activation. Thus, natural killer cells dramatically increased IFN-γ production and markedly increased their cytotoxicity against thyroid cells. Together, these data indicate that ER stress induces a decrease of MHC class I surface expression in thyroid cells, resulting in reduced natural killer-cell self-tolerance.

Glucosamine-induced endoplasmic reticulum stress affects GLUT4 expression via activating transcription factor 6 in rat and human skeletal muscle cells

AIMS/HYPOTHESIS

Glucosamine, generated during hyperglycaemia, causes insulin resistance in different cells. Here we sought to evaluate the possible role of endoplasmic reticulum (ER) stress in the induction of insulin resistance by glucosamine in skeletal muscle cells.

METHODS

Real-time RT-PCR analysis, 2-deoxy-D: -glucose (2-DG) uptake and western blot analysis were carried out in rat and human muscle cell lines.

RESULTS

In both rat and human myotubes, glucosamine treatment caused a significant increase in the expression of the ER stress markers immunoglobulin heavy chain-binding protein/glucose-regulated protein 78 kDa (BIP/GRP78 [also known as HSPA5]), X-box binding protein-1 (XBP1) and activating transcription factor 6 (ATF6). In addition, glucosamine impaired insulin-stimulated 2-DG uptake in both rat and human myotubes. Interestingly, pretreatment of both rat and human myotubes with the chemical chaperones 4-phenylbutyric acid (PBA) or tauroursodeoxycholic acid (TUDCA), completely prevented the effect of glucosamine on both ER stress induction and insulin-induced glucose uptake. In both rat and human myotubes, glucosamine treatment reduced mRNA and protein levels of the gene encoding GLUT4 and mRNA levels of the main regulators of the gene encoding GLUT4 (myocyte enhancer factor 2 a [MEF2A] and peroxisome proliferator-activated receptor-gamma coactivator 1alpha [PGC1alpha]). Again, PBA or TUDCA pretreatment prevented glucosamine-induced inhibition of GLUT4 (also known as SLC2A4), MEF2A and PGC1alpha (also known as PPARGC1A). Finally, we showed that overproduction of ATF6 is sufficient to inhibit the expression of genes GLUT4, MEF2A and PGC1alpha and that ATF6 silencing with a specific small interfering RNA is sufficient to completely prevent glucosamine-induced inhibition of GLUT4, MEF2A and PGC1alpha in skeletal muscle cells.

CONCLUSIONS/INTERPRETATION

In this work we show that glucosamine-induced ER stress causes insulin resistance in both human and rat myotubes and impairs GLUT4 production and insulin-induced glucose uptake via an ATF6-dependent decrease of the GLUT4 regulators MEF2A and PGC1alpha.

Differential functions of PKC-delta and PKC-zeta in cisplatin response of normal and transformed thyroid cells

We investigated the effects of cisplatin (cisPt) in normal PC Cl3 and in transformed and tumourigenic PC E1Araf cells. cisPt cytotoxicity was higher in PC Cl3 than in PC E1Araf cells. In both cell lines, cisPt provoked the ERK1/2 phosphorylation; this was unaltered by Gö6976, a conventional PKC inhibitor, whilst it was blocked by low doses (0.1 microM) or high doses (10 microM) of GF109203X, an inhibitor of all PKC isozymes, in PC Cl3 and in PC E1Araf cells, respectively. In PC E1Araf, the cisPt-provoked ERK phosphorylation was also blocked by the use of a myristoylated PKC-zeta pseudosubstrate peptide. Conversely, in PC Cl3 the cisPt-provoked ERK phosphorylation was blocked by the use of rottlerin, a PKC-delta inhibitor. Results show that cisPt activates both PKC (the -delta and the -zeta isozymes in PC Cl3 and in PC E1Araf cells, respectively) and ERK in association with prolonged survival of thyroid cell lines.

The expression of the sarco/endoplasmic reticulum Ca2+-ATPases in thyroid and its down-regulation following neoplastic transformation

Maintaining a high Ca(2+) concentration in the lumen of the endoplasmic reticulum (ER), by the action of sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCAs), is important in many cellular processes, such as Ca(2+)-mediated cytosolic signaling in response to extracellular stimuli, cell growth and proliferation, and synthesis, processing and folding of ER-translated proteins. In the thyroid gland, SERCAs have not been studied yet, and there is little information available on general problems such as the expression of SERCAs following neoplastic transformation. In this study we investigated the expression of SERCA2b and SERCA3 in rat thyroid tIssue and, in addition, in normal and transformed rat thyroid cell lines. RT-PCR and Northern blot assays showed that SERCA2b is the SERCA form preferentially expressed in the thyroid. In rat thyroid, SERCA2b mRNA was expressed at a higher level than that of other non-muscle tIssues such as liver or spleen, but at much lower level than in brain. On the other hand, SERCA3 mRNA was not detected in thyroid by Northern blot analysis, or barely detected by RT-PCR assays. We also studied the SERCA2b expression pattern in PC Cl3 thyroid cells transformed by several oncogenes that induce different degrees of malignancy and dedifferentiation. RT-PCR and Northern blot assays showed that SERCA2b mRNA expression dramatically decreased in highly tumorigenic thyroid cells, while expression of glyceraldehyde-3-phosphate dehydrogenase mRNA, a housekeeping gene used as internal control, exhibited no variations. The dramatic down-regulation of SERCA2b expression in fully transformed thyroid cells was also evident by Western blot analysis. Also, following neoplastic transformation of thyroid cells, the enzymatic activity of SERCA2b was reduced in a measure which correlated with the mRNA and protein levels. Therefore, rat thyrocytes expressed intermediate levels of SERCAs, mostly the SERCA2b isoform. This pattern of expression was basically reproduced in fully differentiated thyroid cells in culture and was sensitive to neoplastic transformation.

The rat hepatic lectin-1 subunit of the asialoglycoprotein receptor is upregulated by thyrotropin and downregulated by neoplastic transformation of thyroid cells

We have recently shown that the rat hepatic lectin (RHL)-1 subunit of the asialoglycoprotein receptor (ASGPr) is expressed in the PC C13 differentiated thyroid cell line. To investigate in vivo the expression of RHL-1 and the ability of thyrotropin (TSH) to modulate its expression, reverse-transcriptase polymerase chain reaction (RT-PCR) and Western blot assays have been performed on thyroid extracts from rats treated with thyroxine (T4) or propylthiouracil (PTU), each of which modulates TSH levels. It is shown that RHL-1 expression is down-regulated by T4 (which decreases serum TSH) and upregulated by PTU (which increases serum TSH), at both mRNA and protein levels. The sensitivity of RHL-1 to neoplastic transformation of thyroid cells has been investigated. The RHL-1 expression pattern has been studied in PC C13 thyroid cells transformed by several oncogenes that induce different degrees of malignancy and dedifferentiation. RT-PCR and Western blot assays show that RHL-1 expression progressively decreases as PC C13 cells acquire a more transformed phenotype. Expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA, a housekeeping gene used as internal control to normalize RHL-1 mRNA content, exhibits no variations in the different PC C13 cell lines used. In addition, we show that both native and asialo-thyroglobulin (Tg) bind RHL-1 in vitro, and native Tg binds RHL-1 on the surface of PC C13 cells. After thyroid cells transformation, the surface expression of RHL-1 is inhibited in a measure that correlates with the mRNA and protein levels. Therefore, the RHL-1 inhibition at the mRNA, protein and plasma membrane expression follows a gradient that parallels the progressive acquisition of the fully transformed phenotype in the PC C13 system. The results reported in the present article, together with our previous data, suggest that RHL-1 expression could be regulated, at least in part, by the same transcription factors involved in the expression of the other molecules characteristic of the thyroid differentiated state.

The rat asialoglycoprotein receptor binds the amino-terminal domain of thyroglobulin

We have previously reported that the rat hepatic lectin-1 (RHL-1) subunit of rat asialoglycoprotein receptor (ASGPr), the endocytic receptor found on the basolateral surface of hepatocytes, was expressed in rat thyroid tissue and localized on the apical surface of polarized rat thyroid FRT cells. Here we show that PC Cl3 cells, a differentiated rat thyroid cell line, bound thyroglobulin (Tg) via ASGPr. In fact, both the bacterial recombinant carbohydrate recognition domain of RHL-1 (rCRD(RHL-1)) and the anti-rCRD(RHL-1) antibody markedly inhibited (125)I-Tg binding to the cell surface of PC Cl3 cells. Ligand blot assays with deglycosylated Tg show that the rCRD(RHL-1) was able to interact with Tg even after remotion of sugars. The region of Tg involved in the binding to RHL-1 was investigated by ligand blot assays with biotinylated rCRD(RHL-1) on thermolysin-digested native and desialated rat thyroglobulin. It is shown that the rCRD(RHL-1) specifically recognized a thyroglobulin fragment with an apparent M(r) of 68,000, corresponding to the amino-terminal part of the molecule. To our knowledge, this is the first report that attributes to the amino-terminal portion of Tg molecule, containing its earliest and major hormonogenic site, the function of binding to a cell surface receptor of the thyroid. Moreover, we show that oligosaccharides are not the only molecular signals for binding to RHL-1, but amino acidic determinants could also play a role.

Depletion of divalent cations within the secretory pathway inhibits the terminal glycosylation of complex carbohydrates of thyroglobulin

Newly synthesized thyroglobulin transiting the secretory pathway is posttranslationally modified by addition of oligosaccharides to asparagine N-linked residues. The effect of divalent cation depletion on oligosaccharide processing of Tg was studied in FRTL-5 cells. Treatment with an ionophore, A23187, or thapsigargin, an inhibitor of the sarcoplasmic/endoplasmic reticulum ATPases delayed Tg secretion. These effects were accompanied by a normal distribution of the marker of the endoplasmic reticulum protein disulfide isomerase. Analysis of the thyroglobulin oligosaccharides by Bio-gel P4 chromatography showed that in the presence of A23187 and thapsigargin the addition of peripheral sialic acid and possibly galactose is inhibited. These findings were strengthened by experiments of exoglycosidase digestion and SDS-PAGE analysis of the resulting products. These results reveal a cellular mechanism of production of thyroglobulin with incompletely processed complex chains, i.e., the ligand of the recently described GlcNAc and asialoglycoprotein receptors of the thyroid. Since A23187 and thapsigargin inhibit biosynthetically the addition of peripheral sugars on N-linked oligosaccharides chains, the thyroglobulin molecules secreted in the presence of A23187 and thapsigargin should greatly facilitate studies on the function of the GlcNAc and asialoglycoprotein receptors of the thyroid.

Demonstration of a Ca2+ requirement for thyroglobulin dimerization and export to the golgi complex

We have examined the effects of depleting the endoplasmic reticulum Ca2+ store on the maturation of newly synthesized thyroglobulin molecules, their export to the Golgi complex, and their secretion by FRTL-5 cells. An inhibitor of the endoplasmic reticulum Ca2+ pump, thapsigargin, and the Ca2+ ionophore A23187 depleted the endoplasmic reticulum Ca2+ store and strongly inhibited thyroglobulin secretion in cells chased in medium containing 0.1 mM Ca2+. Inhibition of thyroglobulin secretion was caused by a block in the export of newly synthesized thyroglobulin molecules from the endoplasmic reticulum to the Golgi complex, as shown by cell-fractionation experiments and the intracellular accumulation of endoH-sensitive thyroglobulin. The thyroglobulin molecules retained in the endoplasmic reticulum of cells treated with the drugs were found to assemble more slowly into dimers than thyroglobulin in control cells. Protease-sensitivity experiments demonstrated that thyroglobulin dimers assembled in the presence of thapsigargin had a different conformation with respect to dimers assembled in controls cells.

Inhibition by glass-ionomer cements of protein synthesis by human gingival fibroblasts in continuous culture

The effects of several glass-ionomer cements (Baseline and Chem Fil of De Trey; Ketac Fil and Ketac Bond of ESPE; Vitrebond and Vitremer of 3 M) on the protein synthesis of cultured gingival fibroblasts were studied. The presence of cements in the culture medium inhibited protein synthesis, although the relative effectiveness varied significantly. The cements tested have been ranked in three groups, group A (Chem Fil and Ketac Fil), group B (Ketac Bond and Baseline) and group C (Vitrebond and Vitremer), showing 50, 75 and 100% reduction in protein synthesis, respectively. Incubation of cells with medium previously conditioned by Baseline and Ketac Fil caused only transient inhibition of protein synthesis followed by almost complete recovery. This recovery was not observed when the medium was conditioned with Vitrebond. A characterization of the factors determining the inhibitory effect of one cement in each group was attempted. The cements, irrespective of the type, produced small but reproducible decreases in the pH of the medium but released fluoride ions to a different extent. Both changes in the pH of the medium and in the fluoride concentration inhibited protein synthesis by cultured gingival fibroblasts. The different action of cements can be explained, at least in part, by a differing release of fluoride ions.

Perturbation of cellular calcium delays the secretion and alters the glycosylation of thyroglobulin in FRTL-5 cells

Treatment of FRTL-5 cells with a Ca2+ ionophore, A23187, or a specific inhibitor of the endoplasmic reticulum Ca2+ ATPases, thapsigargin, delayed thyroglobulin secretion. The secreted thyroglobulin showed an increased electrophoretic mobility and a reduced sensitivity to neuraminidase. Only thyroglobulin that was still in the endoplasmic reticulum was sensitive to the Ca(2+)-perturbant drugs as shown by experiments in which the drugs were added at different times during a chase. Analysis of the carbohydrate chains by BioGel P4 showed that thyroglobulin secreted in the presence of the Ca(2+)-perturbants displayed an increased ratio high mannose/complex chains.

Inhibition of c-ras reactivates thyroglobulin synthesis in middle-T transformed thyroid cells

Differentiated thyroid cells expressing polyoma Middle-T became transformed and tumorigenic when injected into syngenic animals. The expression of thyroglobulin was greatly reduced and no longer responsive to thyrotropin (TSH) and to cAMP. Inhibition of endogenous c-ras by the expression of two transdominant negative mutant H-ras genes, Asn17 and Leu61-Ser186, reactivated thyroglobulin synthesis. Reactivation of thyroglobulin synthesis by c-ras inhibition was not observed in the absence of TSH. These findings indicate that MT elicits dedifferentiation of thyroid cells by activating endogenous c-ras and that c-ras interferes with TSH or cAMP signaling.

Presence of dityrosine bridges in thyroglobulin and their relationship with iodination

The presence of dimeric thyroglobulin (19S), after reduction and alkylation, suggests that within thyroglobulin there may be intermolecular cross-links, other than disulphide bridges. However, the nature of these intermolecular cross-links is still unknown. In this study, we show the presence of 3-3' dityrosine bridges in the molecule of bovine thyroglobulin by NMR and fluorescence studies. Also, we evaluated the role of iodination in dityrosine formation in vivo and in vitro.

P2 purinergic agonists and 12-O-tetradecanoylphorbol-13-acetate, as well as protein kinase A activators, stimulate thyroglobulin secretion in FRTL-5 cells

We studied the role of various intracellular pathways in thyroglobulin secretion. The P2 agonists (ATP, ADP, GTP), 12-O-tetradecanoylphorbol-13-acetate (TPA), and protein kinase A activators stimulate thyroglobulin secretion in cells grown without TSH. The effects of these agents are additive. Pertussis toxin partially inhibits the effect of ATP but has no effect on the action of GTP. ATP and GTP increase cytosolic calcium (279 +/- 16% and 302 +/- 22%, respectively) while TPA and TSH (1 mU/ml) do not. Thus, both the protein kinase A and kinase C pathways regulate thyroglobulin secretion in FRTL-5 cells.

TSH-induced galactose incorporation at the NH2 terminus of thyroglobulin secreted by FRTL-5 cells

FRTL-5 cells were cultured in media containing standard growth factors with or without TSH, plus labeled precursors of N-linked oligosaccharide chains. The thyroglobulin secreted in the medium was purified and fragmented with CNBr. Three peptides were identified by NH2-terminal sequencing, that were labeled mainly with D-[2-3H]mannose, independent of TSH. One of them, corresponding to the NH2-terminus of thyroglobulin, incorporated both more D-[2-3H]mannose and more D-[1-3H]galactose upon TSH addition. These data likely reflect a TSH-induced increment of N-linked glycosylation at the NH2-terminus of thyroglobulin, mostly with the maturation of high-mannose to complex chains.

Modulation of the carbohydrate moiety of thyroglobulin by thyrotropin and calcium in Fisher rat thyroid line-5 cells

Thyroglobulin secreted in the medium by Fisher rat thyroid line-5 (FRTL-5) cells cultured in the presence of thyroid stimulating hormone (TSH) shows a slower electrophoretic mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis and a higher density position in a CsCl gradient than thyroglobulin secreted by FRTL-5 cells cultured in the absence of TSH for 5-7 days. Such a TSH effect is much less or not evident when secreted thyroglobulin is digested with peptide N-glycohydrolase F or when intracellular thyroglobulin is compared. Intracellular thyroglobulin migrates faster than thyroglobulin secreted either in the presence or in the absence of TSH. Evaluation of the mannose and galactose content of thyroglobulin demonstrates that intracellular thyroglobulin has more mannose and less galactose than extracellular thyroglobulin; it also shows that TSH decreases the mannose content of thyroglobulin while increasing its galactose content. Bio-Gel P6 chromatography shows that TSH increases the complex type carbohydrate chains while decreasing the high mannose chains in the secreted thyroglobulin. High mannose type oligosaccharides were characterized by fast atom bombardment-mass spectrometry analysis. Treatment with the calcium ionophore A23187 (5 microM) of FRTL-5 cells cultured with or without TSH causes the appearance of a "fast" migrating form of thyroglobulinin in the culture medium. Bio-Gel P6 chromatography shows that A23187 causes a dramatic decrease of the complex carbohydrate chains of the secreted thyroglobulin.

Calcium interaction with bovine thyroglobulin: stoichiometry and structural consequences of calcium binding

Gel filtration studies show that the thyroglobulin (Tg) molecule (dimer) binds from 18 to 50 Ca2+ ions. At pH 7.5 Tg binds 18 Ca2+ ions with a Kd of 1.3 x 10(-5) M, and 50 Ca2+ ions with a Kd of 5.5 x 10(-4) M. The binding of calcium to bovine thyroglobulin increases the absorption band of iodoamino acid residues at 315 nm. In the presence of Ca2+, the fluorescence intensity of 1-anilino-8-naphthalene sulfonate (ANS) is increased about 5-fold by Tg, with a shift in the fluorescence emission maximum from 505 to 490 nm. Thus, thyroglobulin possesses two classes of calcium binding sites with different affinities. The data reported indicate, also, that Ca2+ binding to Tg increases the hydrophobicity of the surface of the molecule.

Hormonal regulation of thyroid peroxidase in normal and transformed rat thyroid cells

The hormonal induction of thyroid peroxidase (TPO) mRNA is studied in the functional rat thyroid cell line FRTL-5 and compared to the induction of thyroglobulin (TG) mRNA and I- uptake. TPO and TG mRNAs are regulated by TSH and by insulin-like growth factor I (IGF-I) and/or insulin. However, while TPO is more sensitive to TSH regulation (5- to 6-fold increase vs. 2- to 3-fold increase by IGF-I), TSH and IGF-I are equally potent in increasing TG mRNA levels (3- to 4-fold). Regulation of I- uptake appears to be different: thus TSH greatly (15-fold) increases I- uptake, while IGF-I or insulin are completely ineffective. TPO and TG mRNAs and I- transport display different sensitivity to transformation of rat thyroid cells. Thus, when another differentiated rat thyroid cell line, the PC cells, are transformed by human c-myc (PC myc), TPO and TG mRNAs are both present at normal levels, while I- uptake is slightly decreased; in the PC cells transformed by polyomavirus middle-T-antigen (PC PyMLV) TPO mRNA is undetectable and I- uptake is greatly decreased, while TG mRNA is present at normal levels. All three differentiated functions are switched off in PC cells transformed by the cooperation of c-myc and polyomavirus middle-T-antigen (PC myc + PyMLV).

The promoter region of the human transferrin receptor gene

Genomic DNA fragments corresponding to the promoter region of the human transferrin receptor were linked to either the full-length receptor cDNA or to the bacterial enzyme chloramphenicol acetyltransferase. These constructs were transfected into mouse and human cells, respectively. Gene expression was monitored 40-48 hours after transfection. Bal31 exonuclease was employed to produce 5' to 3' deletions of the promoter region. Deletion of DNA between -86 and -70 upstream of the receptor's mRNA start site resulted in a greater than 80% reduction in apparent promoter activity. DNA sequencing of the 150 bp upstream of the start site revealed that the promoter region contained several sequence elements more than 90% homologous to the consensus sequence for binding of the transcription factor Sp1. In addition, an 11 bp sequence identical to a segment of the enhancers of polyoma virus and adenovirus was located between -80 and -70. Internal deletions confirmed that this enhancer homologue was critical for full promoter activity. A 66 bp fragment encompassing the -80/-70 element augmented gene expression when the fragment was placed in either orientation upstream of the remainder of the transferrin receptor promoter.

Two genetic loci participate in the regulation by iron of the gene for the human transferrin receptor

Iron regulation of the human transferrin receptor gene was examined in murine cells transformed with chimeric constructs containing the human transferrin receptor gene's promoter and either the structural gene for bacterial chloramphenicol acetyltransferase or the human transferrin receptor cDNA. The activity of the transferrin receptor gene's promoter with the heterologous indicator gene was found to be approximately equal to 3-fold higher in cells treated with the iron chelator desferrioxamine than in cells treated with the iron source, hemin. A higher degree of iron regulation was seen in the expression of the human transferrin receptor cDNA driven by its own promoter. The receptor cDNA under the control of the simian virus 40 early promoter was also iron-regulated. Several human transferrin receptor transcripts differing in their 3' end were produced in the murine cells regardless of the promoter used, with the shorter transcripts being relatively unregulated by iron. Deletion of cDNA corresponding to most of the 3' untranslated portion of the mRNA for the receptor ablated the iron regulation. We conclude that at least two genetic elements exist for the regulation of the transferrin receptor gene by iron. One has its locus in the DNA upstream of the transferrin receptor gene's transcription start site, and the other is dependent upon the integrity of the sequences in the 3' end of the gene.

Deletional analysis of the promoter region of the human transferrin receptor gene

Fragments of human genomic DNA corresponding to the promoter region of the gene for the transferrin receptor have been cloned upstream of the bacterial gene for chloramphenicol acetyltransferase and these constructs used to assess promoter activity following transfection into a human rhabdomyosarcoma cell line. Progressive 5' deletions as well as internal linker-substitution constructs support a critical role in gene expression of a sequence element approximately 70 bp upstream of the mRNA start site. In this region, the receptor gene was found to contain 11bp that are identical to a segment of the enhancers of polyoma virus and adenovirus. A fragment encompassing this element was shown to increase gene expression when the fragment was placed in either orientation upstream of the remainder of the transferrin receptor promoter but the same fragment did not activate an enhancer-less SV40 promoter. Removal from within the receptor promoter of three potential binding sites for the transcription factor Sp1 did not decrease the promoter's activity.

Cross-linking with dimethylsuberimidate to study thyroglobulin conformation

The present investigation demonstrates that the cross-linking agent, dimethylsuberimidate, is an usefull tool to study thyroglobulin structure. In fact, while reproducible and discrete polymerization products are obtained in strictly controlled conditions, valuable information on the native assemblage of thyroglobulin subunits and the effects of its major post-translational modification (iodination) on its structure, are reported.

Calcium-induced changes in thyroglobulin conformation

Polyethylene glycol solutions (10% w/v) were used to detect the effect of mono- and divalent cations on some properties of thyroglobulin. It is shown that in presence of 10% w/v polyethylene glycol in 0.01 M Tris-HCl, pH 7.5, calcium (less than 0.05 M) modifies the solubility, the sedimentation rate, and the Stokes' radius of thyroglobulin, while monovalent cations up to 0.6 M do not effect any of these properties. These findings can be explained by an increase in molecular compactness of thyroglobulin. Furthermore, it was shown that a synthetic polymer, polyethylene glycol, could be used to detect conformational changes.