Role of tight junctions in signal transduction: an update

Tight junctions (TJs), which are the most apically located of the intercellular junctional complexes, have a barrier function and a fence function. Recent studies show that they also participate in signal transduction mechanisms. TJs are modulated by intracellular signaling pathways including protein kinase C, mitogen-activated protein kinase, and NF-κB, to affect the epithelial barrier function in response to diverse stimuli. TJs are also regulated by various cytokines, growth factors, and hormones via signaling pathways. To investigate the regulation of TJ molecules via signaling pathways in human epithelial cells under normal and pathological conditions, we established a novel model of human telomerase reverse transcriptase-transfected human epithelial cells. In this review, we describe the recent progress in our understanding of the role of TJs for signal transduction under normal conditions in upper airway epithelium, pancreatic duct epithelial cells, hepatocytes, and endometrial epithelial cells, and in pathological conditions including cancer and infection

Transmembrane proteins of tight junctions

AbstractTight junctions contribute to the paracellular barrier, the fence dividing plasma membranes, and signal transduction, acting as a multifunctional complex in vertebrate epithelial and endothelial cells. The identification and characterization of the transmembrane proteins of tight junctions, claudins, junctional adhesion molecules (JAMs), occludin and tricellulin, have led to insights into the molecular nature of tight junctions. We provide an overview of recent progress in studies on these proteins and highlight their roles and regulation, as well as their functional significance in human diseases

Experimental effect of retinoic acids on apoptosis during the development of diabetic retinopathy

Nami Nishikiori1,2, Makoto Osanai2, Hideki Chiba2, Takashi Kojima2, Shuichiro Inatomi1,2, Hiroshi Ohguro1, Norimasa Sawada2Departments of 1Ophthalmology and 2Pathology, Sapporo Medical University School of MedicinePurpose: This study was conducted to investigate whether retinoic acids (RAs) had any effect on apoptosis during the development of diabetic retinopathy.Methods: To investigate whether RAs had any effect on apoptosis during the development of diabetic retinopathy, we housed 32 C57BL/6 male mice and induced diabetes in 24 by intra peritoneal injections of streptozotocin (STZ; Sigma, St Louis, MO) and treated 16 of the diabetic mice with the RAs, all-trans-retinoic acid (ATRA) (seven mice) and 4-[(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)carboxamido] benzoic acid (Am580) (nine mice). The other eight mice were used as diabetic controls. We then measured apoptosis in the retina by TdT-dUTP terminal nick-end labeling assay.Results: RAs inhibited the apoptosis of retinal cells in diabetic retinopathy. Many apoptotic cells were observed in retinas of the eight diabetic control mice (mean value and SD: 37.8 ± 6.9), whereas when diabetic mice were treated with RAs, the number of apoptotic cells significantly decreased (mean value and SD: 9.9 ± 6.4 for the seven ATRA-treated diabetic mice and 9.8 ± 5.9 for the nine Am580-treated diabetic mice) (p < 0.05).Conclusion: Treatment with RAs decreases apoptosis during the development of diabetic retinopathy.Keywords: retinoic acids, apoptosis, diabetic retinopathy, glial cell line-derived neurotrophic facto

The nuclear receptor hepatocyte nuclear factor 4α acts as a morphogen to induce the formation of microvilli

Microvilli are actin-based organelles found on apical plasma membranes that are involved in nutrient uptake and signal transduction. Numerous components, including ezrin/radixin/moesin (ERM) proteins, have been identified that link filamentous actins to transmembrane proteins, but the signals driving microvillus biogenesis are not known. In this study, we show that the conditional and/or ectopic expression of a nuclear receptor, hepatocyte nuclear factor 4α (HNF4α), triggers microvillus morphogenesis. We also demonstrate that HNF4α expression induces ERM-binding phosphoprotein 50 (EBP50) expression and that attenuation of EBP50 using RNA interference inhibits microvillus development. We conclude that HNF4α acts as a morphogen to trigger microvillus formation

A Newly Established Cell Line from Normal Human Bone Responds to 1alpha, 25-Dihydroxyvitamin D3, Retinoic Acid and Transforming Growth Factor-beta1

We have recently established a new osteoblastic cell line, designated SV-HFO, from normal human bone by immortalization with simian virus 40. In the present study, we examined the effects of diffusive factors on the expression of osteoblastic phenotype in SV-HFO cell line. lα, 25-dihydroxyvitamin D? in-duced the expression of alkaline phosphatase (ALP) and osteocalcin. Retinoic acid down-regulated the expression of ALP, whereas it up-regulated the expres-sion of osteocalcin. Transforming growth factor-β?, reduced the expression of both osteoblastic properties. These effects were time- and dose-dependent. These results show that the SV-HFO cell line maintains responsiveness to these diffusive factors. This cell line is suitable model for studying both metabolism and multistep carcinogenesis of human bone

Effects of the Protein Phosphatase Inhibitors, Okadaic Acid and Vanadate, on Localization of Occludin in Primary Cultures of Rat Hepatocytes

To elucidate whether protein phosphorylation is associated with the loca-lization of the tight junction protein occludin, we determined the changes of occludin protein expression in primary cultures of rat hepatocytes after treat-ment with the protein phosphatase inhibitors okadaic acid and vanadate. After 2 h of treatment with 1myu M okadaic acid or 5 mM vanadate, occludin immunoreactivity showing continuous lines in non-treated cells changed to a few spots on the plasma membrane. In western blots, broad bands above the occludin protein (65 kD) became conspicuous after treatment with okadaic acid and vanadate. We treated the same samples with alkaline phosphatase to examine whether the broad bands depended on the changes in the phos-phorylation states of occludin protein. The broad bands disappeared and the occludin was observed as a narrow band corresponding to 65 kD. Neither a significant change in the mRNA of occludin nor a change in the immunoreac-tivity of the tight junction associated protein, ZO-1, was observed after treatment with okadaic acid or vanadate. These results suggested that the phosphorylation of occludin is closely associated with localization of the protein in cultured hepatocytes and that protein phosphatase inhibitors affect the loca-lization of occludin but not ZO-1 on the plasma membrane

Isolation of cDNA Encoding 7H6-Reactive Polypeptide Defines a New Class of Protein with alpha-Helical Coiled-Coil Structure and DA-Box Similar to Yeast Chromosomal Segregation Proteins

7H6 monoclonal antibody was recently developed in our laboratory by im-munizing mice with a bile canaliculus-rich fraction of the rat liver. The anti-body reacted with a novel 155 Kd polypeptide designated 7H6 antigen that specifically localizes at tight junctions of various epithelia. Correlations of the paracellular barrier function of the tight junction with expression of the 7H6 antigen at the cell border have suggested important roles of this polypeptide for the maintenance of tight junctional functions. As the first step for the analysis of the antigen at the molecular level, we isolated a series of cDNA clones encod-ing 7H6-reactive polypeptides. Five clones were isolated by immunoscreening. Among them a clone designated RL5.3 which carries the largest 5.3Kb insert was characterized in this study. Both plaque screening and immunoblotting of the fusion protein produced by the RL5.3 clone with lysogen confirmed that the pro-tein specifically reacts with the 7H6 monoclonal antibody. Using DNA fragmentsof the RL5.3 clone, 21 clones were further identified. Studies with restriction enzymes and probe hybridization revealed that all the cDNA clones were derived from a single class of transcripts. A partial sequence identified one open reading frame with an α-helical coiled-coil structure and highly conserved aspartate (D)- alanine (A) residues with a helix-loop-helix structure corresponding to DA- box. Since this domain has been specifically found in yeast chromosomal segregation proteins (SMC1, CUT3 and CUT14), the polypeptide encoded by the RL5.3 clone provides the first rodent counterpart of these protein family. Yeast is known to be lethal when SMC and CUT proteins are deleted, suggesting essential roles of these proteins for cell cycle progression as a regulator for chromosomal segregation. Identification of a mammalian counterpart of this pro-tein family may give us some clues for a better understanding of fundamental regulatory mechanisms in the function of tigh junctions