Regulation of the cellular transport and compactation of thyroglobulin

The cellular transport ot thyroglobulin in thyrocytes is a TSH-regulated process and characterized by a bidirectional export-, storage- and recapture pathway. The regulation of single steps in this pathway is unclear, but a possible basis for this transport appeared to be the presence of the lysosomal targetting, signal mannose-6-phosphate (M6P), on porcine thyroglobulin and the detection of the cation-dependent M6P-receptor on the apical cell surface and the Golgi-complex in thyrocytes. The question arose, why thyroglobulin is not directed on an intracellular pathway to lysosomes thereby following the route characteristic of lysosomal enzymes. Recent observations have shown that the affinity of thyroglobulin to the M6P-receptor is very low and the receptor is presumably not effective in thyrocytes to direct thyroglobulin to lysosomes. Instead, a low affinity binding site for thyroglobulin has been found to operate on the surface of thyrocytes. This receptor is specific for thyroglobulin and is saturable at 8-13 mg thyroglobulin per ml (5-9 microM). The highest affinity of thyroglobulin was that to itself which is the basis for the positive cooperation observed during binding of thyroglobulin to thyrocyte membranes. The receptor has a relative molecular mass of 46 kDa but is not identical with the cation-dependent M6P-receptor. Because of the high-thyroglobulin-concentration in the follicle lumen (100-400 mg/ml) it has always been argued that there is no need for the presence of a thyroglobulin receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

The Effect of Nitric Oxide Inhibition and Temporal Expression Patterns of the mRNA and Protein Products of Nitric Oxide Synthase Genes During In Vitro Development of Bovine Pre-implantation Embryos

This study was conducted to determine the effect of Nitric oxide (NO) inhibition in bovine in vitro development and expression analysis of the three Nitric oxide synthase (NOS) isoforms: endothelial (eNOS), neuronal (nNOS) and inducible (iNOS), mRNA and protein in bovine oocytes and embryos. Selective inhibitor of NOS, N-omega-nitro-l-arginine methyl ester (l-NAME) was applied at different doses (0, 0.1, 1 and 10 mm) in maturation (experiment 1A), culture medium (experiment 1B) and in both maturation and culture media (experiment 1C). No significant differences were observed in cleavage and blastocyst rates when oocytes were matured in the presence of l-NAME as long as the inhibitor was omitted during fertilization and culture. However, significantly lower blastocyst rates were observed when l-NAME was present at higher level (10 mm) in culture medium alone and in both maturation and culture media. In experiment 2, mRNA isolated from triplicate pools of oocytes and embryos (n = 15-20) was subjected to quantitative real time reverse transcription polymerase chain reaction to investigate the expression of eNOS, iNOS and nNOS mRNA in normal IVP bovine oocytes and embryos. While eNOS and iNOS transcripts were detected at higher level in oocytes (immature and mature), two-cell and four-cell stage embryos, the nNOS was detected only in immature oocyte, two-cell and morula stages. In experiment 3, eNOS and iNOS protein expression analysis was performed in IVP oocytes and embryos and both proteins were detected in the cytoplasm and the nuclei (weak) of oocytes and embryos. These data provide the first evidence for the role of NO production and the presence of mRNA and protein products of NOS isoforms during bovine embryogenesis.

Structure and microtubule-nucleation activity of isolated Drosophila embryo centrosomes characterized by whole mount scanning and transmission electron microscopy

Experimental approaches in Drosophila melanogaster over the last 20 years have played a fundamental role in elucidating the function, structure and molecular composition of the centrosome. However, quantitative data on the structure and function of the Drosophila centrosome are still lacking. This study uses, for the first time, whole mount electron microscopy in combination with negative staining on isolated centrosomes from the early Drosophila embryos to analyze its dimensions, structure and capacity to nucleate microtubules in vitro. We show that these organelles are on average 0.75 microm in diameter and have abundant pericentriolar material which often appears fibrillar and with bulbous protrusions. Corresponding to the abundant pericentriolar material, extensive microtubule nucleation occurs. Quantification of the number of microtubules nucleated showed that 50-300 active nucleation sites are present. We examined via electron microscopy immunogold labeling the distribution of gamma-tubulin, CNN, Asp and the MPM-2 epitopes that are phosphorylated through Polo and the Cdk1 kinase. The distribution of these proteins is homogeneous, with the MPM-2 epitopes exhibiting the highest density. In contrast, centrosomal subdomains are identified using a centriole marker to relate centrosome size to the centriole number by electron microscopy. In conclusion, we present a clear-cut technique assaying and quantifying the microtubule nucleation capacity and antigen distribution complementing molecular studies on centrosome protein complexes, cell organelle assembly and protein composition.

Self-reported eye symptoms and related diagnostic findings--comparison of risk factor profiles

Profiles of factors affecting self-reported eye symptoms and objectively determined tear film characteristics have been examined and compared using data from 814 office workers. Multiple logistic regression analysis reveals significantly increased risks of subjective perception of eye symptoms for female gender, use of contact lenses and persons with health problems. A reduced break-up time (BUT) depends on personal criteria such as a history of eye disease and female gender. Age under 40 years, high relative humidity and formaldehyde exposure exceeding the 90th percentile are protective regarding BUT. For a thin lipid layer (as an indication of dry eyes) significantly increased risks are detected for high particle load, high endotoxin concentration and female gender. A thick lipid layer (as an indication of inflamed eyes) is significantly associated with a low educational level. The estimation of foam in the medial eye canthus seems to be unsuitable for evaluating indoor problems. The risk factor profiles agree on a few points only. The objectively examined thin lipid layer is the best eye-related indicator of the indoor environment. We therefore conclude that there is a need for the development and application of objective clinical methods for field monitoring in parallel with questioning.

PRACTICAL IMPLICATIONS

Self-reported eye symptoms in conjunction with indoor environmental problems should be validated by objective medical examinations such as semi-quantitative estimation of the superficial lipid layer, measurement of the break-up time or assessment of conjunctival epithelial damage. For unbiased proof of environmental impact, personal factors such as acute illness or low job satisfaction should be excluded. As a minimum requirement, measurements of particles, NO(2) and relative humidity (and if possible endotoxin) should be carried out to detect any indoor environmental reason for eye symptoms.

Migration of epidermal keratinocytes: mechanisms, regulation, and biological significance

Keratinocytes are the prevalent cell type of the epidermis, a multilayered cornified epithelium which provides the cellular basis of the outermost barrier between the organism and its environment. By this barrier function the epidermis protects the organism against a variety of environmental hazards such as dehydration and mechanical stress. Under normal conditions, keratinocytes of all layers are interconnected by desmosomes and anchored by hemidesmosomes to a specialised type of extracellular matrix, the basement membrane. When the epidermis is injured, a vitally important response is initiated with the aim to restore the protective function of the epithelium. A fast but provisional sealing is achieved by the deposition of the fibrin clot before within 24 h after wounding keratinocytes from the wound margins begin to migrate into the wound bed, where they start to proliferate and to form the new epithelium. The development of new high-resolution assays for the study of cell migration and motility has potentiated major progress in our understanding of keratinocyte migration in vitro and in situ. The data reviewed here point to a sophisticated cooperation between soluble motogenic growth factors, cell-matrix interactions, and cell-to-cell communications as major parts of the machinery regulating keratinocyte migration.

Comparison of risk factor profiles concerning self-reported skin complaints and objectively determined skin symptoms in German office workers

The correspondence between impact factor profiles of self-reported skin sensation and of objectively determined skin symptoms was examined using data from the ProKlimA project (1994-1999). A sub-sample of 925 office workers participated in measurements of skin hydration and sebum content and responded to a questionnaire assessing sensory perception. The calculation of multiple logistic regression models revealed a significant increased risk for female sex [Odds ratio (OR): 2.3; confidence interval (CI): 1.4-3.6], poor software (OR: 2.2; CI: 1.3-4.0), unfavorable job characteristics (OR: 1.8; CI: 1.1-2.8), allergic illness (OR: 1.5; CI: 1.1-2.2) and the use of skin cream (OR:2.6; CI: 1.6-4.4) on the subjective perception of skin sensation. Regarding the objective medical examination of the skin humidity a significant increased risk was detected for a high concentration of Total Volatile Organic Compounds (OR: 2.5; CI: 1.3-4.8) and a low relative humidity (OR:1.9; CI: 1.1-3.4). The likewise objectively measured low sebum content is not associated with environmental variables. The impact profiles on subjective vs. objective outcome variables differ in a clear and typical way. Skin related sensory perception is mainly influenced by job-related and personal impacts. Indoor environmental characteristics affect skin hydration. We conclude the need to develop, to adapt and to use objective clinical methods applicable for field monitoring parallel to questioning.

IgA class antineutrophil cytoplasmic antibodies in primary sclerosing cholangitis and autoimmune hepatitis

Antineutrophil cytoplasmic antibodies (ANCA) of IgG class have been described at high prevalence in autoimmune hepatitis (AIH) and primary sclerosing cholangitis (PSC). Data on IgA class ANCA in these diseases are limited. The aim of this study was to determine the prevalence and fluorescence patterns of IgA class ANCA in AIH and PSC and to examine a relationship between the presence of IgA ANCA and clinical characteristics in these patients. Sera from 35 patients with PSC (21 with concomitant inflammatory bowel disease), 40 patients with AIH and 10 healthy controls were studied. ANCA were detected on ethanol-fixed neutrophils using an indirect immunofluorescence technique. ANCA of the IgA class were found in 20% of sera from patients with PSC and in 50% of AIH sera. The majority of AIH patients with IgA class ANCA showed a 'classical' perinuclear staining pattern, whereas the 'classical' and 'atypical' perinuclear fluorescence patterns were distributed equally in PSC. In sera containing IgG and IgA class ANCA simultaneously, IgG class ANCA showed an 'atypical' pANCA fluorescence pattern whereas IgA class ANCA produced a 'classical' perinuclear staining. The presence of IgA class ANCA was not associated with disease-specific clinical characteristics. IgA class ANCA are more frequently detected in sera of patients with AIH than PSC. The diversity of fluorescence patterns points to different target antigens of IgA class ANCA with distinct subcellular localizations.

Regulation of expression of cytokines and growth factors in osteoarthritic cartilage explants

Osteoarthritis (OA) is a degenerative joint disease characterised by the breakdown of the extracellular matrix of chondrocytes in the affected joints. Cytokines and growth factors which are known to play a role in the synthesis and degradation of cartilage matrix have been shown to be upregulated in osteoarthritic cartilage. This upregulation resulted in two different phenotypes, overexpressing either TNF-alpha and IL-6 or IL-1beta, TGFbeta1, IL-4 and IL-10. To investigate the hierarchy among growth factors and cytokines involved in cartilage metabolism, we analysed osteoarthritic cartilage explants for their responses to human recombinant (rh) cytokines and growth factors. The cytokine expression patterns of the explants before and after in vitro culture were compared by immunohistological staining of cartilage sections. We found a coordinate expression of TNF-alpha and IL-6 on the one hand, and of IL-1beta, TGFbeta1, IL-4 and IL-10 on the other. Although TNF-alpha and IL-6 stimulated each other's expression, they downregulated TGF beta1, IL-4 and IL-10 or IL-1beta, TGF beta1 and IFNdelta, respectively. IL upregulated the expression of TGF beta1, IL-4 and IL-10, and jointly these four cytokines and growth factors downregulated IL-6. Both of the expression patterns described for OA cartilage can be explained by these regulatory mechanisms. Interestingly, no cytokine efficiently downregulated TNF-alpha, and even though IL-1beta is upregulated in one of the OA phenotypes, none of the growth factors and cytokines tested--except for IL-1beta itself--seemed capable of mediating this upregulation. This unresponsiveness to cytokine stimulation might hint at a genetic cause for the elevated expression in the respective phenotypes.

The biological role of the Alzheimer amyloid precursor protein in epithelial cells

Proteolytic processing of the Alzheimer amyloid precursor protein (APP) results in the generation of at least two distinct classes of biologically relevant peptides: (1) the amyloid beta peptides which are believed to be involved in the pathogenesis of Alzheimer's disease and (2) the soluble N-terminal ectodomain (sAPP) which exhibits a protective but as yet ill-defined effect on neurons and epithelial cells. In this report we present an overview on the functions of sAPP as an epithelial growth factor. This function involves specific binding of sAPP to membrane rafts and results in signal transduction and various physiological effects in epithelial cells as different as keratinocytes and thyrocytes. At nanomolar concentrations sAPP induces a two to fourfold increase in the rate of cell proliferation and cell migration. Specific inhibition of APP expression by antisense techniques results in decreased sAPP release and in reduced proliferative and motogenic activities. Proliferation and migration are known to be part of complex processes such as wound healing which, therefore, might be facilitated by the growth factor function of sAPP.

Differentiation of antineutrophil nuclear antibodies in inflammatory bowel and autoimmune liver diseases from antineutrophil cytoplasmic antibodies (p-ANCA) using immunofluorescence microscopy

Perinuclear antineutrophil cytoplasmic antibodies (p-ANCA) directed against cytoplasmic proteins of neutrophils have been studied extensively in patients with systemic vasculitides. Recent data indicate that antineutrophil antibodies in sera from patients with chronic inflammatory bowel diseases (IBD) or autoimmune liver disorders, currently called 'atypical p-ANCA', recognize a nuclear target antigen, rendering the term 'ANCA' inaccurate. Specific microscopic criteria to distinguish atypical p-ANCA from p-ANCA are lacking. We used planar and confocal laser scanning indirect immunofluorescence microscopy to examine the labelling characteristics of ethanol-, methanol- and formaldehyde-fixed neutrophils by antineutrophil antibodies in 153 serum samples from patients with IBD, autoimmune liver disorders, systemic vasculitides or healthy blood donors. On ethanol- or methanol-fixed neutrophils, multiple intranuclear fluorescent foci together with either a rim-like peripheral nuclear staining ('type A') or a combined cytoplasmic and peripheral nuclear staining ('type B') was noted exclusively with atypical p-ANCA in sera from patients with IBD or autoimmune liver disorders. Intranuclear foci, which probably corresponded to invaginations of the nuclear envelope, were not labelled by p-ANCA from patients with microscopic polyangiitis or cytoplasmic ANCA (c-ANCA) from patients with Wegener's granulomatosis. On formaldehyde-fixed neutrophils, atypical p-ANCA gave a fine rim-like staining of the nuclear periphery, whereas ANCA diffusely labelled the cytoplasm. To distinguish reliably between the patterns produced by atypical p-ANCA or p-ANCA, particularly p-ANCA, careful indirect immunofluorescence microscopy on ethanol- as well as on formaldehyde-fixed neutrophils is necessary, with particular emphasis on the presence of multiple intranuclear fluorescent foci.

Cysteine proteinases mediate extracellular prohormone processing in the thyroid

Thyroglobulin, the precursor of thyroid hormones, is extracellularly stored in a highly condensed and covalently cross-linked form. Solublization of thyroglobulin is facilitated by cysteine proteinases like cathepsins B and K which are proteolytically active at the surface of thyroid epithelial cells. The cysteine proteinases mediate the processing of thyroglobulin by limited extracellular proteolysis at the apical plasma membrane, thereby rapidly liberating thyroxine. The trafficking of cysteine proteinases in thyroid epithelial cells includes their targeting to lysosomes where they become maturated before being transported to the apical plasma membrane and, thus, into the extracellular follicle lumen. We propose that thyroid stimulating hormone regulates extracellular proteolysis of thyroglobulin in that it enhances the rate of exocytosis of lysosomal proteins at the apical plasma membrane. Later, thyroid stimulating hormone upregulates thyroglobulin synthesis and its secretion into the follicle lumen for subsequent compaction by covalent cross-linking. Hence, cycles of thyroglobulin proteolysis and thyroglobulin deposition might result in the regulation of the size of the luminal content of thyroid follicles. We conclude that the biological significance of extracellularly acting cysteine proteinases of the thyroid is the rapid utilization of thyroglobulin for the maintenance of constant thyroid hormone levels in vertebrate organisms.

Cathepsin K in thyroid epithelial cells: sequence, localization and possible function in extracellular proteolysis of thyroglobulin

Extracellular proteolysis of thyroglobulin at the apical surface of thyroid epithelial cells results in liberation of thyroxine, and is mediated by lysosomal cysteine proteases such as cathepsins B and L. Here, we report on the expression of the cysteine protease cathepsin K in thyroid epithelial cells. The cDNA for porcine thyroid cathepsin K showed homologies ranging from 71% to 94% to the cDNA of cathepsin K from various species and cell types. The deduced amino acid sequence of porcine thyroid cathepsin K predicted a 37 kDa preproenzyme, with the active site residues Cys-140, His-277 and Asn-297, and one potential N-glycosylation site. The localization of cathepsin K was not restricted to lysosomes. Rather, secreted cathepsin K was predominantly found within the follicular lumen and in association with the apical plasma membrane of thyroid epithelial cells. Enzyme cytochemistry showed that cell-surface associated cathepsin K was proteolytically active at neutral pH. In vitro, recombinant cathepsin K liberated thyroxine from thyroglobulin by limited proteolysis at neutral pH. We postulate that its localization enables cathepsin K to contribute to the extracellular proteolysis of thyroglobulin, i.e. thyroid hormone liberation, at the apical surface of thyroid epithelial cells in situ.

Association of genotypes affecting the expression of interleukin-1beta or interleukin-1 receptor antagonist with osteoarthritis

OBJECTIVE

The majority of cytokines and growth factors known to be involved in cartilage metabolism are synthesized by the chondrocytes themselves. They are up-regulated in osteoarthritic (OA) cartilage, resulting in 2 opposite phenotypes, TNFalpha(high) and TNFalpha(low), that are characterized by an elevated number of tumor necrosis factor alpha (TNFalpha)-positive and interleukin-1beta (IL-1beta)-positive chondrocytes, respectively. To establish a hierarchy among the cytokines and growth factors expressed in articular chondrocytes, this study investigated cytokine genes for known polymorphisms that may contribute to the deregulated expression in OA cartilage.

METHODS

Polymerase chain reaction techniques were performed either in a thermal cycler using standard methods or in a light cycler to analyze the frequencies of the TNFalpha (-308), IL-1 receptor antagonist (IL-1Ra) (intron 2), IL-1beta (exon 5), and IL-6 (-174) polymorphisms in 61 OA patients and 254 randomly chosen controls.

RESULTS

For the TNFalpha(low) phenotype, a statistically significant association was found with the less frequent allele of IL-1beta, which carries a single-basepair substitution in exon 5 and may contribute to the characteristic increase in IL-beta-positive chondrocytes. In contrast, the TNFalpha(high) phenotype was significantly associated with the less frequent allele of IL-1Ra, which carries two 86-bp repeats in the second intron and is assumed to lead to an elevated expression of the antagonist.

CONCLUSION

These results point to an association between the IL-1beta polymorphism and the TNFalpha(high) phenotype and between the IL-1Ra polymorphism and the TNFalpha(low) phenotype found in OA. Both associations suggest that IL-1beta may be more important than TNFalpha for the regulation of cytokine and growth factor expression in articular chondrocytes.

A possible role for the Alzheimer amyloid precursor protein in the regulation of epidermal basal cell proliferation

The regulation of epidermal growth involves a number of ions, growth factors and cytokines and possibly additional but as yet unknown factors. Here we report on the potential role of the secretory N-terminal domain (sAPP) of the Alzheimer amyloid precursor protein (APP) in the regulation of keratinocyte proliferation. In human skin APP was detectable predominantly in the basal cell layer of the epidermis whereas the immunocytochemical signal in the underlying mesenchymal tissue was very low. Cultured normal human keratinocytes expressed the three APP isoforms 695, 751 and 770 with highest values for the isoforms 751 and 770. HaCaT cells, a spontaneously immortalized human keratinocyte cell line, exhibited almost identical patterns in the expression of the APP isoforms and in the release of endogenous sAPP. In HaCaT cells, recombinant sAPP (sAPPrec) was found to compete with endogenous sAPP for the same binding sites. Binding of sAPPrec was specific and occurred in microdomains of approximately 0.1 to approximately 0.3 microm in diameter. At 10 nM, sAPPrec binding induced a 2- to 4-fold increase in the rate of cell growth. sAPP concentrations in the conditioned media were found to reach 5-20 nM which is in the mitogenic range of sAPPrec. The proliferative effect of sAPP was inhibited by approximately 50% when antisense oligonucleotides directed against the APP mRNA were applied. The predominant expression of

Thyroglobulin type-I-like domains in invariant chain fusion proteins mediate resistance to cathepsin L digestion

The MHCII associated invariant chain isoform Ii41 shows homology to a repeat in thyroglobulin (TgR). We show that the Ii31 isoform, which lacks the TgR-like domain, is sensitive to cathepsin L treatment whereas Ii41 displays substantial resistance. The TgR-like sequence of Ii41 was exchanged for thyroglobulin type-IA and -IB repeats, that contain six or four cysteine residues. Resistance to cathepsin L digestion was maintained upon substitution of the Ii41 TgR for homologous sequences from TgR type-IA. Mutation of a conserved cysteine in the TgR domain of an Ii fusion protein strongly reduced resistance to cathepsin L digestion.

Association of genotypes affecting the expression of interleukin-1beta or interleukin-1 receptor antagonist with osteoarthritis

OBJECTIVE

The majority of cytokines and growth factors known to be involved in cartilage metabolism are synthesized by the chondrocytes themselves. They are up-regulated in osteoarthritic (OA) cartilage, resulting in 2 opposite phenotypes, TNFalpha(high) and TNFalpha(low), that are characterized by an elevated number of tumor necrosis factor alpha (TNFalpha)-positive and interleukin-1beta (IL-1beta)-positive chondrocytes, respectively. To establish a hierarchy among the cytokines and growth factors expressed in articular chondrocytes, this study investigated cytokine genes for known polymorphisms that may contribute to the deregulated expression in OA cartilage.

METHODS

Polymerase chain reaction techniques were performed either in a thermal cycler using standard methods or in a light cycler to analyze the frequencies of the TNFalpha (-308), IL-1 receptor antagonist (IL-1Ra) (intron 2), IL-1beta (exon 5), and IL-6 (-174) polymorphisms in 61 OA patients and 254 randomly chosen controls.

RESULTS

For the TNFalpha(low) phenotype, a statistically significant association was found with the less frequent allele of IL-1beta, which carries a single-basepair substitution in exon 5 and may contribute to the characteristic increase in IL-beta-positive chondrocytes. In contrast, the TNFalpha(high) phenotype was significantly associated with the less frequent allele of IL-1Ra, which carries two 86-bp repeats in the second intron and is assumed to lead to an elevated expression of the antagonist.

CONCLUSION

These results point to an association between the IL-1beta polymorphism and the TNFalpha(high) phenotype and between the IL-1Ra polymorphism and the TNFalpha(low) phenotype found in OA. Both associations suggest that IL-1beta may be more important than TNFalpha for the regulation of cytokine and growth factor expression in articular chondrocytes.

Characterization of regulatory elements in the 5'-flanking region of the GM2 activator gene

Lysosomal degradation of the ganglioside GM2 by human beta-hexosaminidase A requires the presence of the GM2 activator protein as an essential cofactor. Here we demonstrate that GM2 activator mRNA is differentially expressed and mainly localized to the apical part of the epithelial cells of distal renal tubules and the collecting duct. In order to understand the mechanism underlying the regulation of the GM2 activator gene, we analyzed the genomic organization upstream exon 2 as well as the 5'-flanking region. The GM2 activator gene spans about 16.8 kb with a first intron of 6.5 kb, and the transcription start is located at position -96 upstream from the ATG. DNA elements responsible for GM2 activator expression were identified in a PCR-based method of long-distance DNA walking. Sequence analysis revealed a 2.9 kb region upstream of the ATG that contained regulatory elements like CAAT boxes, Sp1 binding sites as well as AP1, and AP2 sites. Transfection experiments in COS-1 cells with a series of chimeras of 5'-stepwise deletion mutants of the GM2 activator gene 5'-flanking region and the secretory alkaline phosphatase (SEAP)-reporter gene indicated that a genomic fragment encompassing -323 to +1 bp had significant promoter activity. EMSA experiments showed that Sp1 and other transcription factors like AP1, AP2 and CCAAT-Box binding proteins are involved in GM2 activator gene regulation.

The thioredoxin boxes of thyroglobulin: possible implications for intermolecular disulfide bond formation in the follicle lumen

Multimerization of thyroglobulin (TG) takes place extracellularly in the thyroid follicle lumen and is regarded as a mechanism to store TG at high concentrations. Human thyroglobulin (hTG) has been shown to multimerize mainly by intermolecular disulfide cross-links. We recently noted that TG of various mammalian species contains three highly conserved thioredoxin boxes (CXXC). This sequence is known to underlie the enzymatic activity of protein disulfide isomerase (PDI). As hTG formed intermolecular disulfide bonds in the absence of other proteins depending on the redox conditions and hTG concentration, the CXXC-boxes of TG might provide the structural basis for self-assisted intermolecular cross-linking. To test this hypothesis we prepared a recombinant TG fragment containing the three thioredoxin boxes. This fragment exhibited a redox activity amounting to about 10% of the activity of PDI at redox conditions supposed to be present in the extracellular space. This activity might be supplemented by the oxidizing system of the apical cell surfaces of thyrocytes facing the follicle lumen. Indeed, incubation of hTG with peroxidase and H202 resulted in intermolecular disulfide bridge formation. Our results suggest a combined mechanism of self-assisted and peroxidase-mediated disulfide bond formation leading to the intermolecular cross-linking of lumenal hTG.

Covalent cross-linking of secreted bovine thyroglobulin by transglutaminase

Extracellular storage of thyroglobulin (TG) is a prerequisite for maintaining constant levels of thyroid hormones in vertebrates. Storage of TG within the follicle lumen is achieved by compactation and by the formation of covalent cross-links between TG molecules. In bovine thyroids, approximately 75% of the cross-links are other than disulfide bonds (J. Cell Biol. 180, 1071-1081). We have now shown that polymeric TG contains a large number of N(epsilon)(gamma-glutamyl)lysine cross-links and that only traces of these can be found in the soluble form of TG. Because such isopeptide bridges are generated usually by the action of a transglutaminase, it is reasonable to propose that the covalent polymerization of TG in the globules is under the control of this enzyme. Soluble TG was shown to be a substrate for transglutaminase in vitro; moreover, the presence of transglutaminase was demonstrated by immunofluorescence and by immunoblotting in freshly isolated bovine thyroid globules. With immunoelectron microscopy, transglutaminase was detected in the cytoplasm of thyrocytes, but not in compartments of the secretory pathway. Only one messenger RNA for transglutaminase was found by Northern blotting. Sequencing of the cloned gene failed to reveal a secretory signal, which supports the notion that the thyroid transglutaminase is the cytosolic type. Apparently, the enzyme reaches the lumen of the follicle by an as yet unknown pathway to catalyze the covalent cross-linking of thyroid globules in this extracellular compartment.

Expression pattern of galectin-3 in neural tumor cell lines

Galectin-3 is a member of the galectin family of beta-galactoside-specific animal lectins. Here we show that galectin-3 is constitutively expressed in 15 out of 16 glioma cell lines tested, but not by normal or reactive astrocytes, oligodendrocytes, glial O-2A progenitor cells and the oligodendrocyte precursor cell line Oli-neu. Galectin-3 is also expressed by one oligodendroglioma cell line, but not by primitive neuroectodermal tumor and 4 neuroblastoma cell lines tested so far. In all galectin-3 expressing cell lines, the lectin is predominantly, if not exclusively, localized intracellularly and carries an active carbohydrate recognition domain (shown for C6 rat glioma cells). Moreover, in contrast to primary astrocytes, glioma cells do not or only weakly adhere to substratum-bound galectin-3, probably reflecting an unusual glycosylation pattern. Our findings indicate that the expression of galectin-3 selectively correlates with glial cell transformation in the central nervous system and could thus serve as a marker for glial tumor cell lines and glial tumors.

Cholera toxin is exported from microsomes by the Sec61p complex

After endocytosis cholera toxin is transported to the endoplasmic reticulum (ER), from where its A1 subunit (CTA1) is assumed to be transferred to the cytosol by an as-yet unknown mechanism. Here, export of CTA1 from the ER to the cytosol was investigated in a cell-free assay using either microsomes loaded with CTA1 by in vitro translation or reconstituted microsomes containing CTA1 purified from V. cholerae. Export of CTA1 from the microsomes was time- and adenosine triphosphate-dependent and required lumenal ER proteins. By coimmunoprecipitation CTA1 was shown to be associated during export with the Sec61p complex, which mediates import of proteins into the ER. Export of CTA1 was inhibited when the Sec61p complexes were blocked by nascent polypeptides arrested during import, demonstrating that the export of CTA1 depended on translocation-competent Sec61p complexes. Export of CTA1 from the reconstituted microsomes indicated the de novo insertion of the toxin into the Sec61p complex from the lumenal side. Our results suggest that Sec61p complex-mediated protein export from the ER is not restricted to ER-associated protein degradation but is also used by bacterial toxins, enabling their entry into the cytosol of the target cell.

Hyperproliferative hepatocellular alterations after intraportal transplantation of thyroid follicles

The thyroid hormone 3,5,3'-triiodo-L-thyronine (T3) is a strong direct hepatocyte mitogen in vivo. The effects of T3 resemble those of peroxisome proliferators, which are known to induce hepatocellular tumors in rats. With the aim of studying long-term local effects of thyroid hormones on liver parenchyma, small pieces of thyroid tissue were transplanted via the portal veins into the livers of thyroidectomized male Lewis rats. At 1 week, 3 weeks, 3 months, and 18 months after transplantation, the transplants were found to proliferate, to synthesize thyroglobulin, and to release thyroxine and T3. At 3 and 18 months after transplantation, the hepatocytes of the liver acini downstream of the transplanted follicles showed an increase in cytoplasmic basophilia, a loss of glycogen, an enlargement and hyperchromasia of their nuclei, and a strong increase in cell turnover compared with unaltered liver acini. The altered hepatocytes exhibited an increase in the activities of glucose-6-phosphate dehydrogenase, glucose-6-phosphatase, malic enzyme, mitochondrial glycerol-3-phosphate dehydrogenase, cytochrome-c-oxidase, and acid phosphatase; the activities of glycogen synthase and glycogen phosphorylase were strongly decreased. The hepatocytic alterations downstream of the transplanted follicles could be explained by effects of T3. On the other hand, they resembled alterations characteristic of amphophilic preneoplastic liver foci observed in different models of hepatocarcinogenesis.

Quantifying lamella dynamics of cultured cells by SACED, a new computer-assisted motion analysis

Formation of lamellipodia and the retraction of ruffles are essential activities during motility and migration of eukaryotic cells. We have developed a computer-assisted stroboscopic method for the continuous observation of cell dynamics (stroboscopic analysis of cell dynamics, SACED) that allows one to analyze changes in lamellipodia protrusion and ruffle retraction with high resolution in space and time. To demonstrate the potential of this method we analyzed keratinocytes in culture, unstimulated or stimulated with epidermal growth factor (EGF), which is known to induce cell motility and migration. Keratinocytes stimulated with EGF exhibited a 2.6-fold increase in their migration velocity, which coincided with enhanced ruffle retraction velocity (144%) and increased ruffle frequency (135%) compared to control cells. We also recorded an enhanced frequency of lamellipodia (135%), whereas the velocity of lamellipodia protrusion remained unchanged. These results on ruffle and lamellipodia dynamics in epidermal cells show that SACED is at least equal to established methods in terms of accuracy. SACED is, however, advantageous concerning resolution in time and therefore allows one to analyze the activity of lamellipodia and ruffles in as yet unknown detail. Moreover, SACED offers two opportunities that render this technique superior to established methods: First, several parameters relevant to cell motility can be analyzed simultaneously. Second, a large number of cells can conveniently be examined, which facilitates the compilation of statistically significant data.

Subcellular distribution, secretion, and posttranslational modifications of clusterin in thyrocytes

A prominent secretory glycoprotein was detected in the culture medium of porcine thyrocytes which was identified as clusterin by microsequencing. Treatment of thyrocytes with thyroid stimulating hormone revealed a tight regulation of both synthesis and secretion of clusterin, with a distinct fraction of clusterin being always associated with the cells. At least three N-bound glycans were found on each subunit of clusterin, receiving most of the incorporated [(32)P]phosphate-label. Binding of clusterin to the immobilized cation-independent mannose 6-phosphate (M6P) receptor indicated that part of the phosphate label was contained in M6P moieties. Immunolabeling of cultured thyrocytes and of thyrocytes in situ showed clusterin on the apical cell surfaces where it colocalized with gp330/megalin, which is known to serve as a binding site for clusterin. The association with the apical plasma membrane, which, in thyrocytes, carries the iodinating system, was confirmed by biosynthetic iodination, an as yet unknown posttranslational modification of clusterin. On the basolateral plasma membranes clusterin was found within distinct, bipartite patches, suggesting that it is a constituent of cell-adhesion complexes and that it participates in cell-cell and cell-matrix interactions.

Differential modulation of interleukin-4 and interleukin-13 secretion from human peripheral blood mononuclear cells

Interleukin-4 (IL-4) and IL-13 which have been implicated in the pathogenesis of atopic reactions elicit many of the same biologic responses. Therefore, time- and stimulus-dependent differences in the regulation of IL-4 and IL-13 production could be of relevance to their biological effects. In this study we tested the hypothesis that stimulation of peripheral blood mononuclear cells (PBMCs) with different inducers of cell activation would result in a differential expression of IL-4 and IL-13. For this purpose, PBMCs of nonatopic volunteers were incubated with phytohaemagglutinin (PHA), phorbolester (PMA), calcium ionophore A23187, or IL-3. The effect of these stimuli on IL-4 and IL-13 production were analysed by enzyme-linked immunoassay (ELISA) in supernatants of cultured PBMCs. Incubation of PBMCs with A23167 and PHA induced both a dose- and time-dependent increase in IL-4 and IL-13 release. A23187 induced concentrations of IL-4 were higher than those of IL-13 whereas IL-4 release following stimulation with PHA was considerably higher for IL-13 compared to IL-4. In contrast, there was a selective increase in IL-13 but not IL-4 concentrations following stimulation of PBMCs with PMA and IL-3 in vitro. In conclusion in this study evidence is provided that IL-4 and IL-13 production are regulated differently which might explain their functional redundancy.

Intracellular distribution of a biotin-labeled ganglioside, GM1, by immunoelectron microscopy after endocytosis in fibroblasts

A radioactive and biotin-labeled analogue of GM1 (biotin-GM1) was synthesized which enabled us to analyze its intracellular distribution in the compartments of the endocytic route by electron microscopic immunocytochemistry using thin sections of human skin fibroblasts labeled with gold-conjugated antibiotin antibodies. Metabolic studies with the biotin-GM1 showed its partial degradation to the corresponding GM2 and GM3 derivatives. Further degradation was inhibited by the biotin residue. The distribution of biotin-GM1 after uptake by cells was studied by postembedding labeling techniques. On the plasma membrane the biotin-GM1 was detectable in the form of patches (0.1 micrometer in diameter), in caveola-like structures and, to a much lesser extent, in coated pits or vesicles. During endocytic uptake, the biotin-GM1 became detectable in organelles identified as late endosomes and lysosomes. The intracellular distribution of the biotin-GM1 was compared to the localization of the EGF receptor in EGF-stimulated fibroblasts. Both the biotin-GM1 and the EGF receptor were transported to intraendosomal and intralysosomal membranes, indicating that both membrane constituents follow the same pathway of endocytosis. Our observations show that biotin-GM1 can be successfully incorporated into the plasma membrane and be used as a tool for morphological detection of its pathway to lysosomes.

Gangliosides are transported from the plasma membrane to intralysosomal membranes as revealed by immuno-electron microscopy

A biotin-labeled derivative of the ganglioside GM1 (biotin-GM1) was used to study its transport along the endocytic pathway of cultured fibroblasts by immuno-electron microscopy. Using electron dense endocytic tracers we could demonstrate that late endosomes and lysosomes of these cells are long living organelles with a high content of internal membranes. Our studies show that during endocytosis the biotin-GM1 was transported to these intraendosomal and intralysosomal membranes. These observations support the hypothesis that glycosphingolipids (GSL) are preferentially degraded in intralysosomal vesicles.

Binding and selective detection of the secretory N-terminal domain of the alzheimer amyloid precursor protein on cell surfaces

The secretory N-terminal domain of the Alzheimer amyloid precursor protein (sAPP) evokes specific responses in cells on binding to their surfaces. Because APP is expressed in a large variety of cell types, the localization of sAPP binding requires detection techniques that selectively recognize sAPP as a ligand. For this purpose, we prepared antibodies against recombinant sAPP695 (sAPPrec) previously expressed in E. coli. Such antibodies were found to distinguish between sAPPrec and cellular APP or sAPP, as shown by immunocytochemistry and by immunoblot. In addition, they allowed the selective localization of bound sAPPrec on cell surfaces without any signal from cellular APP or sAPP. Saturation of sAPPrec binding to cell surfaces, as determined radiometrically, was reached at 10 nM [125I]-sAPPrec. Binding was specific because it was almost completely inhibited by a 100-fold excess of unlabeled sAPPrec. This specificity of binding was confirmed by surface plasmon resonance spectroscopy. Binding of sAPPrec to cell surfaces occurred in patches and was dependent on the state of cell differentiation. The sAPPrec used in this study contains heparin binding sites, but enzymatic removal of cell surface associated heparin did not affect sAPPrec binding. Aldehyde fixation of cells strongly inhibited their ability to bind sAPPrec. The data point to a fixation-sensitive sAPPrec binding protein which is detectable in the form of patches and therefore is part of assembled cell surface microdomains.

Modulation of cytokine release from mononuclear cells by prostacyclin, IL-4 and IL-13

In a previous study, we reported that cicaprost, a stable prostacyclin analogue can inhibit the release of granulocyte-macrophage colony-stimulating factor (GM-CSF) from activated human peripheral mononuclear blood cells (PBMCs). Since interleukin (IL-4) and IL-13 have been shown to inhibit the release of cytokines from PBMCs we tested the hypothesis that prostacyclin in combination with IL-4 or IL-13 can act synergistically to modulate the release of IL-10, generally associated with anti-inflammatory properties, and the pro-inflammatory cytokine tumour necrosis factor alpha (TNF-alpha). For this purpose, PBMCs were isolated over Ficoll, stimulated with lipopolysaccharide (LPS) and incubated in the presence of cicaprost, IL-4 or IL-13. There was a significant reduction in TNF-alpha as well as IL-10 secretion from LPS-stimulated PBMCs following incubation with IL-4 or IL-13. In contrast, cicaprost reduced the secretion of TNF-alpha but led to a slight enhancement of IL-10 release from PBMCs. When LPS-activated PBMCs were incubated in the presence of cicaprost and IL-4 or IL-13 there was a selective, synergistic inhibition of the TNF-alpha release which was not observed for IL-10. Thus, our data suggest that prostacyclin can synergize with cytokines to selectively inhibit the release of pro-inflammatory cytokines from PBMCs.

Atypical antineutrophil cytoplasmic antibodies with perinuclear fluorescence in chronic inflammatory bowel diseases and hepatobiliary disorders colocalize with nuclear lamina proteins

Antineutrophil cytoplasmic antibodies (ANCA) are frequently associated with chronic inflammatory bowel diseases (IBD) and hepatobiliary disorders. However, their target antigens have not been identified yet. Recently, we observed an atypical perinuclear ANCA fluorescence (p-ANCA) together with an intranuclear staining using ANCA-positive sera from patients with IBD and hepatobiliary disorders. This observation suggests that the target antigens are localized within the nucleus of neutrophilic granulocytes. To further investigate this hypothesis, we examined sera from patients with ulcerative colitis, primary sclerosing cholangitis, autoimmune hepatitis or systemic vasculitis on ethanol or formaldehyde-fixed neutrophils using confocal laser scanning microscopy and immunoelectron microscopy. Counterstaining with propidium iodide, a DNA-specific dye, showed that ANCA-positive sera in IBD and heptobiliary disorders react with intranuclear antigens at the nuclear periphery of the neutrophils. Double immunolabeling techniques revealed that nuclear lamina proteins, lamins A, C and B1, and lamin B receptor were colocalized with the antigen(s) recognized by atypical p-ANCA. No colocalization was observed with classical p-ANCA and antibodies against histones (H1-H4). Our study showed that atypical p-ANCA are antinuclear antibodies reactive with granulocyte-specific antigens present in the nuclear lamina.

Extracellularly occurring histone H1 mediates the binding of thyroglobulin to the cell surface of mouse macrophages

Thyroglobulin is the major secretory protein of thyroid epithelial cells. Part of thyroglobulin reaches the circulation of vertebrates by transcytosis across the epithelial wall of thyroid follicles. Clearance of thyroglobulin from the circulation occurs within the liver via internalization of thyroglobulin by macrophages. Here we have analyzed the interaction of thyroglobulin with the cell surface of J774 macrophages with the aim to identify the possible thyroglobulin-binding sites on macrophages. Binding of thyroglobulin to J774 cells was saturated at approximately 100 nM thyroglobulin with a Kd of 50 nM, and it was competed by the ligand itself. Preincubation of J774 cells with thyroglobulin resulted in downregulation of thyroglobulin-binding sites, indicating internalization of thyroglobulin and its binding proteins. By affinity chromatography, two proteins from J774 cells were identified as thyroglobulin-binding proteins with an apparent molecular mass of approximately 33 kD. Unexpectedly, both proteins were identified as histone H1 by protein sequencing. The occurrence of histone H1 at the plasma membrane was further proven by biotinylation or immunolabeling of J774 cells. The in vitro interaction between histone H1 and thyroglobulin was analyzed by surface plasmon resonance that revealed a Kd at 46 nM. In situ, histone H1 was colocalized to FITC-Tg-containing endocytic compartments of Kupffer cells, i.e., liver macrophages. We conclude that histone H1 is detectable at the cell surface of macrophages where it serves as a thyroglobulin-binding protein and mediates thyroglobulin endocytosis.

Iodination of mature cathepsin D in thyrocytes as an indicator for its transport to the cell surface

Thyrocytes are known for their ability to iodinate thyroglobulin from which the thyroid hormones are generated. In the intact thyroid gland the iodination process is almost exclusively executed at the apical plasma membrane of thyroid epithelial cells. Here, we show that freshly isolated thyrocytes iodinated polypeptides other than thyroglobulin and that one of the major iodinated polypeptides was the mature form of the lysosomal protease cathepsin D (CD). The detection of mature CD as an iodinated polypeptide suggested that a fraction of the lysosomally maturated enzyme was delivered to the apical plasma membrane where it became available for iodination. After labeling of thyrocytes with [35S]methionine/cysteine overnight part of the mature CD was released into the culture medium. This was abolished by inhibiting maturation of CD with NH4Cl, indicating that mature CD appeared in the medium after its proteolytic maturation in an acidic compartment. Besides CD other soluble lysosomal polypeptides like the beta-N-acetylhexosaminidase and the sphingolipid-activating protein D (Sap D) were iodinated and partially secreted as mature polypeptides. In contrast, the membrane-associated lysosomal ceramidase was iodinated and partially secreted as immature single-chain enzyme and not as fully maturated two-chain enzyme. These data indicate that a portion of mature CD and other soluble lysosomal enzymes is delivered from lysosomes to the cell surface whereas some membrane-associated enzymes from the terminal lysosomal compartment are efficiently excluded from this process.

From differentiation to proliferation: the secretory amyloid precursor protein as a local mediator of growth in thyroid epithelial cells

In various species, thyrotropin (TSH) is known to stimulate both differentiation and proliferation of thyroid follicle cells. This cell type has also been shown to express members of the Alzheimer amyloid precursor (APP) protein family and to release the secretory N-terminal domain of APP (sAPP) in a TSH-dependent fashion. In this study on binding to the cell surfaces, exogenously added recombinant sAPP stimulated phosphorylation mediated by mitogen-activated protein kinase and effectively evoked proliferation in the rat thyroid epithelial cell line FRTL-5. To see whether this proliverative effect of sAPP is of physiological relevance, we used antisense techniques to selectively inhibit the expression of APP and the proteolytic release of sAPP by cells grown in the presence of TSH. The antisense-induced inhibition was detected by immunoblot, immunoprecipitation, and immunocytochemical analyses. After the reduced APP expression and sAPP secretion, we observed a strong suppression of the TSH-induced cell proliferation down to 35%. Recombinant sAPP but not TSH was able to overcome this antisense effect and to completely restore cell proliferation, indicating that sAPP acts downstream of TSH, in that it is released from thyroid epithelial cells during TSH-induced differentiation. We propose that sAPP operates as an autocrine growth factor mediating the proliferative effect of TSH on neighboring thyroid epithelial cells.

Paracrine interaction between hepatocytes and macrophages after extrathyroidal proteolysis of thyroglobulin

Thyroglobulin (Tg), the precursor of the thyroid hormones triiodothyronine (T3) and thyroxine (T4), is known to derive from thyroid epithelial cells. Part of Tg reaches the circulation as an intact molecule by transcytosis across the epithelial wall of thyroid follicles. Circulating Tg is a potential ligand for the asialoglycoprotein receptor of hepatocytes. In this report we show, however, that clearance of circulating Tg occurred exclusively by endocytosis in liver macrophages, whereas hepatocytes did not participate in this process. The biological significance of this Tg uptake by the macrophages might consist in an increase of thyroid hormones in close proximity to the macrophages, thereby affecting the hepatocyte metabolism. To test this hypothesis, co-cultures of hepatocytes and macrophages were incubated with Tg, which resulted in the release of thyroid hormones and in a significant increase in the activity of lipogenesis and of hepatocellular key enzymes of the hexose monophosphate shunt. This effect of Tg could be mimicked by equivalent amounts of T3 or T4 exclusively in the co-cultures. When hepatocytes were incubated with thyroid hormones in the absence of macrophages, no or only little effect was observed, indicating that the interaction of macrophages and hepatocytes was a prerequisite for the stimulation of the hepatocellular metabolism. We conclude that the paracrine effect on HepG2 cells results from the degradation of Tg in J774 cells. Apparently, this process is not confined to the release of thyroid hormones, but it requires the interaction of both cell types, possibly mediated by an additional, as yet unknown stimulus.

Epithelial folding in vitro: studies on the cellular mechanism underlying evagination of thyrocyte monolayers

Epithelial monolayers in suspension culture fold in a way which closely resembles epithelial evagination. We have used freshly isolated segments of porcine thyroid follicles to study the mechanism underlying this evagination process. Epithelial folding was accompanied by dramatic changes in cell shape: the cells elongated and apical cell surfaces widened, whereas the basal cell portions were narrowed to about 20% of their original width. Apparently, enzymatic separation of thyroid epithelial cells from their underlying extracellular matrix resulted in an extension of the lateral cell-cell interactions on the expense of the basal cell surface area. Epithelial folding in vitro was Ca2+ dependent and reversibly blocked by cytochalasin D, by which the reorganization of the F-actin network was disturbed. This inhibitory effect was also observed by the action of cAMP analogues known to cause rounding of cells by their effect on cortical F-actin. Moreover, evagination in vitro was reversibly blocked at intracellular pH values of 5.8 and below. Under these conditions, protein phosphorylation was entirely inhibited. Inhibitors of protein kinases, specifically of myosin light chain kinase, were able to disrupt the evagination process, suggesting that protein phosphorylation, presumably of the myosin light chain, was essential for folding. We conclude that enzymatic separation of epithelial monolayers from their extracellular matrix initiated a cascade consisting of extended cell-cell interactions of the lateral plasma membranes and of reorganization of the apical actin-myosin network, finally resulting in profound changes in cell shape characteristic of epithelial evagination.

Multimerization of thyroglobulin (TG) during extracellular storage: isolation of highly cross-linked TG from human thyroids

Thyroglobulin (TG) is the major soluble protein of the thyroid and is known to be extracellularly stored for future liberation of thyroid hormones. We have developed techniques for the isolation of an insoluble storage form of human TG present in the follicle lumen. The application of these techniques yielded insoluble and translucent colloid globules varying in size (50-500 microns) and shape and consisting primarily of densely packed TG. Intact colloid globules exhibited the imprints of the apical cell surfaces of thyrocytes that had surrounded the colloid globules in situ. Hence, in size and surface morphology, isolated colloid globules represent authentic lumenal content. Based on the total protein of single colloid globules and their volume, an average protein concentration of 590 mg/mL was calculated. The presence of protein disulfide isomerase in colloid globules and in the secretory product of cultured thyrocytes suggests its involvement in the extracellular multimerization of human TG. Native colloid globules increased their volume considerably upon reduction of disulfide bonds; they were completely dissolved by treatment with dithiothreitol and SDS. The results show that part of extracellular human TG undergoes multimerization, primarily by the formation of intermolecular disulfide bonds, thus allowing the storage of TG at excessively high, previously unknown, concentrations.

Growth regulation of rat thyrocytes (FRTL-5 cells) by the secreted ectodomain of beta-amyloid precursor-like proteins

The TSH-dependent expression of amyloid precursor-like proteins and the secretion of their ectodomain (sAPP) in rat thyroids coincide with increased rates of thyrocyte proliferation. To analyze whether the secretion of sAPP and the proliferation of thyrocytes are regulatorily linked, we employed [3H]thymidine or 5-bromo-2'-deoxyuridine assays and found that conditioned culture medium stimulated the proliferation of FRTL-5 cells depending on the content of sAPP. These observations prompted experiments with sAPP-derived peptides known to stimulate the growth of APP-deficient fibroblasts. Using autoradiography and radiochemical assays, we observed that an iodinated 19-mer sAPP peptide was bound specifically to the surface of FRTL-5 cells. Binding of this peptide was followed by a 2- to 8-fold increase in cell proliferation, which reached a plateau at 1 nM. This effect was significant only when cells were cultured in nonconfluent monolayers, and contact inhibition did not interfere. Our observations indicate that sAPP and sAPP-derived peptides increased the proportion of proliferation-competent FRTL-5 cells and suggest that sAPP may be a new member in the family of peptides involved in the growth regulation of thyrocytes.

Evidence for extracellularly acting cathepsins mediating thyroid hormone liberation in thyroid epithelial cells

Thyroglobulin (Tg) is the major secretory product of thyroid epithelial cells and is stored in the lumen of thyroid follicles at high concentrations. Thyroid hormone liberation is assumed to occur separately from this storage compartment within lysosomes. However, for the transfer of Tg to lysosomes, mechanisms to solubilize the luminal content must precede its endocytosis, because part of the luminal Tg occurs in a covalently cross-linked form. Here, by immunoprecipitation and immunoblotting we show that the majority of procathepsin B or L and a fraction of mature cathepsin B are released from porcine thyrocytes in vitro. Released cathepsins were detectable on the cell surface of the thyrocytes by immunocytochemistry and amounted to 27% of the total cathepsin B. Cytochemical studies revealed the proteolytic activity of cathepsin B at neutral pH on the cell surface of thyrocytes. Therefore, the possibility of extracellular proteolysis by cathepsins was investigated by incubating plasma membrane preparations, conditioned media, or lysosomes with Tg. The liberation of thyroid hormones was quantitated by RIA, and the degradation of Tg was determined by SDS-PAGE. Extracellular and plasma membrane-associated proteases rapidly mediated up to 54% of the total T4 liberation by limited proteolysis of Tg at neutral pH under conditions where cysteine proteases were reactivated. We propose that released and proteolytically active cysteine protease i.e. cathepsins B and L, provide thyrocytes with a pathway of limited extracellular proteolysis of Tg before endocytosis.

Prostacyclin modulates granulocyte/macrophage colony-stimulating factor release by human blood mononuclear cells

Although production and immunological activity of granulocyte-macrophage colony stimulating factor (GM-CSF) have been implicated in the pathogenesis of various disorders, little has been reported concerning the factors involved in the regulation of GM-CSF release. Therefore, we examined the effect of the stable prostacyclin agonist, cicaprost, on the in vitro production of GM-CSF by peripheral blood mononuclear cells (PBMC) obtained from normal subjects by enzyme-linked immunosorbent assay (ELISA) and reverse transcriptase polymerase chain reaction (RT-PCR). Incubation of PBMC (10(6) cells/ml1) with the bacterial lipopolysaccharide (LPS; 0.1 microgram/ml) for 24 h caused a more than 10-fold concentration-dependent increase of GM-CSF release (401 +/- 58 pg/ml x 10(6) cells-1). Addition of cicaprost (0.01 ng/ml to 1 microgram/ml) resulted in a concentration- and time-dependent reduction of LPS-induced GM-CSF secretion by PBMC with a mean IC50 of 6.7 ng/ml (n = 9). Furthermore, cicaprost also inhibited the LPS-elicited expression of GM-CSF mRNA, as determined by RT-PCR. These results demonstrate that prostacyclin inhibits LPS-induced GM-CSF release and that its effects are related to the level of transcription. Hence, our data suggest that cicaprost or related PGI2 agonists may represent immunomodulators of mononuclear cell function and may offer a therapeutic approach to GM-CSF-mediated inflammatory disorders.

Localization and regulated release of Alzheimer amyloid precursor-like protein in thyrocytes

BACKGROUND

Dysregulation in the processing of the Alzheimer precursor protein (APP) is thought to be central to the deposition of the beta-A4 peptide and to the pathogenesis of Alzheimer's disease. Expression and release of APP has also been known to mediate cell-matrix interactions and to participate in the regulation of cell proliferation. It has also been shown that APP is a member of a family of closely related proteins. This family comprises different splice-forms of APP and APP-like proteins (APP/APLP). Because of the specific processing of exportable proteins, thyrocytes represent a particularly useful cell type for the study of the processing of APP/APLP (especially proteolysis and iodination) as an indication of cell surface expression and for the study of the regulation of these functions.

EXPERIMENTAL DESIGN

Rats were treated in vivo with propylthiouracil, which is known to cause a rise of serum thyroid-stimulating hormone (TSH) levels and maximum stimulation of thyroid function and growth. The expression of APP/APLP was analyzed in rat thyroid tissue and in a continuous cell line (FRTL-5) by immunofluorescence staining and by sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunoblotting. Using FRTL-5 cells, secretion and turnover were analyzed by biosynthetic radiolabeling and immunoprecipitation of APP/APLP.

RESULTS

APP/APLP was detected in follicle cells and in the follicle lumen of resting thyroid glands. In propylthiouracil-treated rats, the complete endocytic removal of the luminal content coincided with the pronounced visualization of APP/APLP in the extrafollicular space, where it was associated with proliferating endothelial cells and fibroblasts. In FRTL-5 cells, APP/APLP was localized mainly in the Golgi complex and in compartments along the endocytic pathway, including lysosomes, where degradation of APP/APLP occurred. Mature and immature forms of APP/APLP became iodinated upon reaching the plasma membrane. Part of the extracellular portion of APP/APLP was released by these cells into the culture medium by TSH-dependent cleavage and secretion mechanisms.

CONCLUSIONS

The observations show the expression, maturation, and secretion of APP/APLP in thyrocytes and the up-regulation of these processes by TSH. Part of the immature APP/APLP appeared on the cell surface as indicated by its iodination. Apparently, this portion of immature APP/APLP escaped maturation during its transport to the cell surface.

In vivo iodination of a misfolded proinsulin reveals co-localized signals for Bip binding and for degradation in the ER

The signal for degradation of proteins in the endoplasmic reticulum (ER) is thought to be the exposure of internal domains which are buried when the protein has adopted its correct conformation and which are also exposed in assembly intermediates. This raises the question of why the intermediates are not degraded. We developed a system based on the peroxidase-catalyzed iodination of tyrosine residues which continuously monitors the exposure of internal domains of proinsulin. In CHO cells this system discriminated between assembly intermediates of wild type (wt) proinsulin and misfolded proinsulin, as shown by the exclusive iodination of a misfolded mutant which was finally degraded in the ER. Iodination in vitro showed that the assembly intermediates of wt proinsulin also exposed internal domains. This iodination was inhibited by the addition of the molecular chaperone Bip which was co-immunoprecipitated with proinsulin in CHO cells. The results obtained with the mutant proinsulin support the assumption that exposed internal domains represent the signal for degradation in the ER. Observations of wt proinsulin show that Bip masks internal domains of normal assembly intermediates during the entire assembly process, thereby suppressing their degradation. We propose that internal domains contain co-localized signals for Bip binding and for degradation.

Regulated O-glycosylation of the Alzheimer beta-A4 amyloid precursor protein in thyrocytes

In thyrocytes, the beta-amyloid precursor protein (beta-APP) is expressed, proteolytically cleaved and released into the extracellular space in a TSH-dependent fashion. Immunocytochemically, beta-APP was detectable mainly in the stacked Golgi cisternae indicating the accumulation in this organelle. Because this unusual immunoreactivity might be related to the Golgi-specific posttranslational processing we studied the glycosylation of beta-APP and the possible regulation of this process. For this purpose we used FRTL-5 cells which showed that the degree of glycosylation was also TSH dependent. Glycosidase digestion experiments revealed that only the O-glycans, not the N-glycans, of beta-APP were regulated by TSH. Using enzyme digestion and lectin precipitation analyses we showed that O-glycosylation involved mainly alpha 2,6-sialylated Gal 1-3 GalNAc-alpha-core glycans (approximately 85%) whereas the 2,3 linked sialic acids amounted to only approximately 15% of total sialic acid residues. Upon stimulation with TSH, O-glycosylation as measured by the degree of sialylation increased by a factor of approximately 1.7 thereby raising the molecular mass of mature beta-APP by 4 to 5 kDa above that from control cells. This process coincided with the accumulation of a proteolytically derived 8.5 kDa C-terminal beta-APP fragment indicating that the proteolytic processing of mature beta-APP was not inhibited by its O-glycosylation. When cells were stimulated with TSH in the presence of cycloheximide, the Golgi cisternae lost their predominant immunoreactivity for beta-APP and were rapidly emptied (within 30 min). Hence, under the conditions of normal protein synthesis, the Golgi cisternae may operate as a storage compartment for beta-APP.

Identification of iodinated proteins in cultured thyrocytes and their possible significance for thyroid hormone formation

The thyroid gland is known to generate the iodinated hormones T3 and T4 from the prohormone thyroglobulin. In this report we examined whether polypeptides other than thyroglobulin are iodinated and hormonogenic in thyrocytes and the prerequisites for their iodination. In primary cultures of porcine thyrocytes, a substantial portion of organified radioiodine was incorporated into cellular proteins other than thyroglobulin. Some of these were identified by immunoprecipitation. They included proteins of the extracellular matrix, plasma membrane proteins, and lysosomal enzymes, which follow in part a secretion and recapture pathway. All of these proteins come into contact with the iodinating system of thyrocytes located on the apical plasma membrane and possess iodination consensus sequences. Immunoprecipitation with T3- or T4-specific antibodies showed that thyroid hormones were detectable only within thyroglobulin. This was confirmed by an analysis of the iodoamino acids of thyroglobulin, cathepsin-D (representing a secretory protein), and aminopeptidase-N (a membrane-integrated protein) by two-dimensional TLC, which revealed the presence of T3 and T4 only within the polypeptide chain of thyroglobulin. These results indicate that iodoproteins other than thyroglobulin do not participate in the generation of thyroid hormones in situ.

Extrathyroidal release of thyroid hormones from thyroglobulin by J774 mouse macrophages

Thyroglobulin appears in the circulation of vertebrates at species-specific concentrations. We have observed that the clearance of thyroglobulin from the circulation occurs in the liver by macrophages. Here we show that the thyroid hormones T3 and T4 were released by incubation of mouse macrophages (J774) with thyroglobulin. Thyroid hormone release was a fast process, with an initial rate of approximately 20 pmol T4/mg per min and approximately 0.6 pmol T3/mg per min, indicating that macrophages preferentially release T4. The bulk of released thyroid hormones appeared after 5 min of incubation of macrophages with thyroglobulin, whereas degradation of the protein was detectable only after several hours. During internalization of thyroglobulin, endocytic vesicles and endosomes were reached at 5 min and lysosomes at 60 min. T4 release started extracellularly by secreted proteases and continued along the endocytic pathway of thyroglobulin, whereas T3 release occurred mainly intracellularly when thyroglobulin had reached the lysosomes. This shows that the release of both hormones occurred at distinct cellular sites. Our in vitro observations suggest that macrophages in situ represent an extrathyroidal source for thyroid hormones from circulating thyroglobulin.

Endocytosis of thyroglobulin is not mediated by mannose-6-phosphate receptors in thyrocytes. Evidence for low-affinity-binding sites operating in the uptake of thyroglobulin

Thyroglobulin, the major secretory product of thyrocytes, is the macromolecular precursor of thyroid hormones. After its synthesis, thyroglobulin follows a complex secretion, storage and recapture pathway to lysosomes. Porcine thyroglobulin was shown to carry the mannose 6-phosphate-(Man6P)-recognition marker on its N-linked glycans. Since the cation-independent Man6P receptor could also be found on the apical plasma membrane of porcine thyrocytes, we examined the significance of the Man6P signal for the transport of thyroglobulin. Here, we present data implying that Man6P receptors are not relevant for endocytosis of thyroglobulin in thyrocytes. Instead, we provide evidence for the existence of specific, low-affinity-binding sites for thyroglobulin on the apical plasma membrane of thyrocytes responsible for endocytosis of thyroglobulin. Binding studies with intact, polar-organized porcine thyrocytes grown on collagen-coated filters revealed cooperative and saturable binding of thyroglobulin to the apical-plasma-membrane domain at relatively high concentrations of thyroglobulin (20 microM). These observations show that low-affinity interactions between thyroglobulin and the apical plasma membrane play a key role in endocytosis of thyroglobulin and hormone formation in the thyroid. The data in this publication have been published as an abstract [Lemansky, P. and Herzog, V. (1991) J. Cell Biol. 115, 261a].

Isolation of insoluble secretory product from bovine thyroid: extracellular storage of thyroglobulin in covalently cross-linked form

Extracellular storage of thyroglobulin (TG) is an important prerequisite for maintaining constant levels of thyroid hormones in vertebrates. Storage of large amounts is made possible by compactation of TG in the follicle lumen with concentrations of at least 100-400 mg/ml. We recently observed that the luminal content from bovine thyroids can be isolated in an intact state and be separated from soluble TG. For this purpose, bovine thyroid tissue was homogenized and subjected to various steps of purification. This procedure resulted in a pellet of single globules measuring 20-120 microns in diameter. Scanning electron microscopy revealed a unique cobblestone-like surface pattern of isolated globules, showing in detail the impressions of the apical plasma membranes of thyrocytes which had formerly surrounded the luminal content before tissue homogenization. Isolated thyroid globules were rapidly digested by trypsin but extremely resistant to various protein solubilization procedures. Homogenization of isolated globules resulted in the release of approximately 3% of total protein, showing that only a minor proportion of TG was loosely incorporated in thyroid globules whereas approximately 22% appeared to be interconnected with the globule matrix by disulfide bridges. Analysis by SDS-gel electrophoresis and immunoblotting confirmed that the protein released by this procedure consisted of TG. The vast majority (approximately 75%) of the globule matrix protein was found to be covalently cross-linked by non-disulfide bonds. TG in isolated globules was highly iodinated (approximately 55 iodine atoms per 12-S TG subunit) suggesting that the covalent nondisulfide cross-linking occurs in part during the iodination of TG and that this process involves the formation of intermolecular dityrosine bridges. Mechanisms must exist which solubilize or disperse the insoluble luminal content prior to endocytosis of TG.

Transcytosis in thyroid follicle cells: regulation and implications for thyroglobulin transport

In order to analyze quantitatively the translocation of plasma membrane during endocytosis and transcytosis and the regulation of these processes in thyroid follicle cells, the apical cell surfaces of resting and TSH-stimulated inside-out follicles were labeled with cationized ferritin. Morphometric analyses showed that the rates of endocytosis and transcytosis are TSH-dependent. More interestingly, whereas the effect of TSH on endocytosis was transient (with a maximum at 16 min), the effect on transcytosis continued to increase until the end of the experiment (i.e, 70 min). During 1 h of endocytosis, the fraction of membrane involved in transcytosis increased by a factor 4 upon TSH stimulation, corresponding to about 12% of the internalized apical plasma membrane area. Cooling to 15 degrees C slowed down, but did not block endocytosis entirely, whereas transcytosis and transfer to lysosomes were totally inhibited In order to quantitate transcytosis of thyroglobulin (TG) and to ascertain whether this molecule undergoes cleavage during transcytosis, inside-out follicles were incubated in a medium containing 3H-labeled TG in the presence of TSH; upon washing and reopening of follicles, the luminal fluid containing TG after transcytosis was found to contain about 10% of the total radioactivity taken up by follicle cells. Transcytosed TG proved to be unmodified with respect to its electrophoretic mobility. We conclude that (i) the fraction of transcytosed TG corresponds approximately to the fraction of membrane involved in this process, (ii) TG does not undergo cleavage during transcytosis, (iii) endocytosis and transcytosis are regulated by TSH but differ in their kinetics after stimulation, and (iv) transcytosis is affected by temperature in a similar way as transfer to lysosomes, suggesting the existence of a common gating step for both pathways.

Classical pathway of complement activation in normal and diseased human glomeruli

Monoclonal antibodies (mAb) reactive against complement components involved in the classical activation pathway were applied in an indirect immunoperoxidase technique for the histological study of normal and diseased human renal tissues. Prominent staining with antibodies against the C4d fragment was seen in all glomeruli and some renal arteriolar walls. The C4d staining was mesangial with light microscopy, whereas the subendothelial site of the glomerular basement membrane (GBM) also appeared to be positive in immunoelectron microscopy. In similar localization, albeit with distinctly weaker intensity, IgM and C4 binding protein (C4bp) were detected. In kidney biopsies from patients with various types of glomerulonephritis, C4d reactive antibodies stained the glomerular structures in a strong, diffuse or granular pattern in contrast to the more segmental distribution and weaker staining intensity in normal kidney specimens. Increased amounts of C4d, occasionally also of C4b, were paralleled in diseased kidney tissues by distinct deposits of IgM and/or IgG in the presence of C4bp. This study suggests that the C4d fragment in normal human glomeruli is indicative of a continuous, local complement activation via the classical pathway induced by the physiological deposition of IgM-containing immune complexes.

Translocation arrest by reversible folding of a precursor protein imported into mitochondria. A means to quantitate translocation contact sites

Passage of precursor proteins through translocation contact sites of mitochondria was investigated by studying the import of a fusion protein consisting of the NH2-terminal 167 amino acids of yeast cytochrome b2 precursor and the complete mouse dihydrofolate reductase. Isolated mitochondria of Neurospora crassa readily imported the fusion protein. In the presence of methotrexate import was halted and a stable intermediate spanning both mitochondrial membranes at translocation contact sites accumulated. The complete dihydrofolate reductase moiety in this intermediate was external to the outer membrane, and the 136 amino acid residues of the cytochrome b2 moiety remaining after cleavage by the matrix processing peptidase spanned both outer and inner membranes. Removal of methotrexate led to import of the intermediate retained at the contact site into the matrix. Thus unfolding at the surface of the outer mitochondrial membrane is a prerequisite for passage through translocation contact sites. The membrane-spanning intermediate was used to estimate the number of translocation sites. Saturation was reached at 70 pmol intermediate per milligram of mitochondrial protein. This amount of translocation intermediates was calculated to occupy approximately 1% of the total surface of the outer membrane. The morphometrically determined area of close contact between outer and inner membranes corresponded to approximately 7% of the total outer membrane surface. Accumulation of the intermediate inhibited the import of other precursor proteins suggesting that different precursor proteins are using common translocation contact sites. We conclude that the machinery for protein translocation into mitochondria is present at contact sites in limited number.