Stephen S M Chung

The University of Hong Kong, Hong Kong, Hong Kong

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Publications (39)162.66 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Previously, exchange protein directly activated by cAMP 2 (Epac2) and PKA were known to play a role in glucose-stimulated insulin secretion (GSIS) by pancreatic β cells. The present study shows that Epac1 mRNA is also expressed by β cells. Therefore, we generated mice and embryonic stem (ES) cells with deletion of the Epac1 gene to define its role in β-cell biology and metabolism. The homozygous Epac1-knockout (Epac1(-/-)) mice developed impaired glucose tolerance and GSIS with deranged islet cytoarchitecture, which was confirmed by isolated islets from adult Epac1(-/-) mice. Moreover, Epac1(-/-) mice developed more severe hyperglycemia with increased β-cell apoptosis and insulitis after multiple low-dose streptozotocin (MLDS; 40 mg/kg) treatment than Epac1(+/+) mice. Interestingly, Epac1(-/-) mice also showed metabolic defects, including increased respiratory exchange ratio (RER) and plasma triglyceride (TG), and more severe diet-induced obesity with insulin resistance, which may contributed to β-cell dysfunction. However, islets differentiated from Epac1(-/-) ES cells showed insulin secretion defect, reduced Glut2 and PDX-1 expression, and abolished GLP-1-stimulated PCNA induction, suggesting a role of Epac1 in β-cell function. The current study provides in vitro and in vivo evidence that Epac1 has an important role in GSIS of β cells and phenotype resembling metabolic syndrome. Kai, A. K. L., Lam, A. K. M., Chen, Y., Tai, A. C. P., Zhang, X., Lai, A. K. W., Yeung, P. K. K., Tam, S., Wang, J., Lam, K. S., Vanhoutte, P. M., Bos, J. L., Chung, S. S. M., Xu, A., Chung, S. K. Exchange protein activated by cAMP 1 (Epac1)-deficient mice develop β-cell dysfunction and metabolic syndrome.
    The FASEB Journal 06/2013; · 5.70 Impact Factor
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    ABSTRACT: Increased level of endothelin-1 (ET-1), a potent vasoconstrictor, has been found in the cerebral spinal fluid (CSF) of patients with multi-infarction dementia, suggesting a possible role of ET-1 in cognitive deficit associated with stroke. Previously, we have reported that synthesis of ET-1 is induced in endothelial cells in hypoxic/ischemic conditions. Transgenic mice over-expressing endothelin-1 in endothelial cells (TET-1) developed systemic hypertension and showed more severe brain damage after transient ischemia. To further understand the significance of endothelial ET-1 in cognitive deficit, we subjected adult TET-1 mice to 30 min middle cerebral artery occlusion (MCAO) with 7 days reperfusion. At baseline, TET-1 mice showed similar locomotor activity, emotion and cognitive function compared to non-transgenic (NTg) mice. However, after 30 min MCAO and 7 days reperfusion, although the sensorimotor function measured by neurological scores was recovered in both genotypes, TET-1 mice showed increased anxiety-like behavior in the open field test and impaired spatial learning and reference memory in the Morris water maze. Parallel with these behavioral changes, TET-1 mice showed more severe brain damage with blood-brain-barrier breakdown (BBB), reactive astrogliosis, increased caspase-3, and increased peroxiredoxin 6 (Prx6) expressions around blood vessels in the ipsilateral hippocampus, compared to that of NTg mice, suggesting that ET-1 over-expression in the endothelial cells leads to more severe BBB breakdown and increased oxidative stress which may resulted in neuronal apoptosis and glial reactivity, which might contribute to the emotional changes and cognitive deficits after short-term ischemia with long-term reperfusion.
    Neurobiology of Learning and Memory 01/2013; · 3.33 Impact Factor
  • Fred K C Chan, Stephen S M Chung, Irene O Ng, Sookja K Chung
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    ABSTRACT: Deleted in liver cancer 2 (DLC2) is a novel Rho GTPase-activating protein that regulates RhoA activity. DLC2 is ubiquitously expressed in most tissues, including the brain, spinal cord and peripheral nerves, and is thought to be involved in actin cytoskeletal reorganization. Unlike DLC1-deficient mice, DLC2-deficient mice (DLC2(-/-)) are viable and without gross anatomical abnormalities. Interestingly, DLC2(-/-) mice exhibit hyperalgesia to noxious thermal stimuli and inflammation-inducing chemicals, such as formalin and acetic acid. There was no difference in the structure or morphology of cutaneous or sural nerves between DLC2(+/+) and DLC2(-/-) mice. However, sensory nerve conduction velocity in DLC2(-/-) mice was significantly higher than that in DLC2(+/+) mice, whereas motor nerve conduction velocity was not affected. After formalin injection, DLC2(-/-) mice showed increased RhoA activity in the spinal cord and an increased number of phosphorylated ERK1/2-positive cells. The inflammatory hyperalgesia in DLC2(-/-) mice appeared to be mediated through the activation of RhoA and ERK1/2. Taken together, DLC2 plays a key role in pain modulation during inflammation by suppressing the activation of RhoA and ERK to prevent an exaggerated pain response, and DLC2(-/-) mice provide a valuable tool for further understanding the regulation of inflammatory pain.
    Neurosignals 12/2011; 20(2):112-26. · 2.56 Impact Factor
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    ABSTRACT: The level of endothelin-1 (ET-1), a potent vasoconstrictor, was associated with retinopathy under ischemia. The effects of endothelial endothelin-1 (ET-1) over-expression in a transgenic mouse model using Tie-1 promoter (TET-1 mice) on pathophysiological changes of retinal ischemia were investigated by intraluminal insertion of a microfilament up to middle cerebral artery (MCA) to transiently block the ophthalmic artery. Two-hour occlusion and twenty-two-hour reperfusion were performed in homozygous (Hm) TET-1 mice and their non-transgenic (NTg) littermates. Presence of pyknotic nuclei in ganglion cell layer (GCL) was investigated in paraffin sections of ipsilateral (ischemic) and contralateral (non-ischemic) retinae, followed by measurement of the thickness of inner retinal layer. Moreover, immunocytochemistry of glial fibrillary acidic protein (GFAP), glutamine synthetase (GS) and aquaporin-4 (AQP4) peptides on retinal sections were performed to study glial cell reactivity, glutamate metabolism and water accumulation, respectively after retinal ischemia. Similar morphology was observed in the contralateral retinae of NTg and Hm TET-1 mice, whereas ipsilateral retina of NTg mice showed slight structural and cellular changes compared with the corresponding contralateral retina. Ipsilateral retinae of Hm TET-1 mice showed more significant changes when compared with ipsilateral retina of NTg mice, including more prominent cell death in GCL characterized by the presence of pyknotic nuclei, elevated GS immunoreactivity in Müller cell bodies and processes, increased AQP-4 immunoreactivity in Müller cell processes, and increased inner retinal thickness. Thus, over-expression of endothelial ET-1 in TET-1 mice may contribute to increased glutamate-induced neurotoxicity on neuronal cells and water accumulation in inner retina leading to edema.
    PLoS ONE 01/2011; 6(10):e26184. · 3.53 Impact Factor
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    ABSTRACT: Osmotic response element binding protein (OREBP) is a Rel-like transcription factor critical for cellular osmoresponses. Previous studies suggest that hypertonicity-induced accumulation of OREBP protein might be mediated by transcription activation as well as posttranscriptional mRNA stabilization or increased translation. However, the underlying mechanisms remain incompletely elucidated. Here, we report that microRNAs (miRNAs) play critical regulatory roles in hypertonicity-induced induction of OREBP. In renal medullary epithelial mIMCD3 cells, hypertonicity greatly stimulates the activity of the 3'-untranslated region of OREBP (OREBP-3'UTR). Furthermore, overexpression of OREBP-3'UTR or depletion of miRNAs by knocking-down Dicer greatly increases OREBP protein expression. On the other hand, significant alterations in miRNA expression occur rapidly in response to high NaCl exposure, with miR-200b and miR-717 being most significantly down-regulated. Moreover, increased miR-200b or miR-717 causes significant down-regulation of mRNA, protein and transcription activity of OREBP, whereas inhibition of miRNAs or disruption of the miRNA-3'UTR interactions abrogates the silencing effects. In vivo in mouse renal medulla, miR-200b and miR-717 are found to function to tune OREBP in response to renal tonicity alterations. Together, our results support the notion that miRNAs contribute to the maximal induction of OREBP to participate in cellular responses to osmotic stress in mammalian renal cells.
    Nucleic Acids Research 01/2011; 39(2):475-85. · 8.81 Impact Factor
  • Irina G Obrosova, Stephen S M Chung, Peter F Kador
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    ABSTRACT: Diabetes mellitus is associated with a 5-fold higher prevalence of cataracts, which remains a major cause of blindness in the world. Typical diabetic cataracts contain cortical and/or posterior subcapsular opacities. Adult onset diabetic cataracts also often contain nuclear opacities. Mechanisms of diabetic cataractogenesis have been studied in less detail than those of other diabetic complications. Both animal and human studies support important contribution of increased aldose reductase activity. Surgical extraction is the only cure of diabetic cataract today. An improved understanding of pathogenetic mechanisms, together with finding effective therapeutic agents, remain highest priority for diabetic cataract-related research and pharmaceutical development.
    Diabetes/Metabolism Research and Reviews 03/2010; 26(3):172-80. · 2.97 Impact Factor
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    ABSTRACT: Aldose reductase (AR), the first enzyme in the polyol pathway, has been implicated in a wide variety of physiological and pathological functions, such as diabetic vascular and neural complications. It is known that diabetes mellitus can exacerbate brain and retina damage after ischemic injuries. However, the underlying mechanisms are not clear. In the present study, we made use of db/db mice with an AR null mutation (AR(-/-)db/db) to understand better the role of AR in the pathogenesis of brain and retina ischemic injuries under diabetic conditions. Cerebral and retinal ischemia was induced by transient middle cerebral artery occlusion in control and diabetic mice either with or without an AR null mutation. Mice were evaluated for neurological deficits after 30 min of ischemia and 23.5 hr of reperfusion. Our results showed that the diabetic db/db mice had significantly more severe neurological deficit and larger brain infarct size than the nondiabetic mice. Compared with wild-type db/db mice, the AR(-/-)db/db mice had significantly lower neurological scores, smaller brain infarct areas, and less hemispheric brain swelling. Retinal swelling was also significantly decreased in the AR(-/-)db/db mice. Less swelling in the brain and retina of the AR(-/-)db/db mice correlated with less expression of the water channel aquaporin 4. Taken together, these data clearly show that deletion of AR leads to less severe brain and retinal ischemic injuries in the diabetic db/db mouse. The present study indicates that inhibition of AR in diabetics may protect against damage in the brain and retina following ischemic reperfusion injury.
    Journal of Neuroscience Research 02/2010; 88(9):2026-34. · 2.97 Impact Factor
  • Justin W C Leung, Stephen S M Chung, Sookja K Chung
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    ABSTRACT: Endothelin-1 (ET-1) is up-regulated in the endothelial cells and astrocytes under ischemia. Transgenic mice with astrocytic ET-1 over-expression (GET-1) showed more severe neurological deficit and larger infarct after transient middle cerebral artery occlusion (MCAO). Here, the significance of endothelial ET-1 in ischemic brain injury was investigated using transgenic mice with the endothelial ET-1 over-expression (TET-1). Increased ET-1 level was observed in the TET-1 brain infarct core after transient MCAO. ET(A) receptor expression was induced in the penumbra and ET(A) antagonist (A-147627) partially normalized the infarct volume and neurological deficit. In the infarct core of TET-1 brain, superoxide, nitrotyrosine, and gp91(phox) levels were increased. TET-1 brain displayed increased matrix metalloproteinase-2 expression, water content, immunoglobulin leakage and decreased occludin level in the ipsilateral hemisphere indicative of BBB breakdown and hemispheric edema. Interestingly, AQP-4 expression was increased in the penumbra of TET-1 brain following transient MCAO leading to the water accumulation. Taken together, endothelial ET-1 over-expression and ETA receptor activation contributes to the increased oxidative stress, water accumulation and BBB breakdown after transient MCAO leading to more severe neurological deficit and increased infarct.
    Brain research 03/2009; 1266:121-9. · 2.46 Impact Factor
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    ABSTRACT: Osmotic Response Element-Binding Protein (OREBP), also known as TonEBP or NFAT5, is a unique transcription factor. It is hitherto the only known mammalian transcription factor that regulates hypertonic stress-induced gene transcription. In addition, unlike other monomeric members of the NFAT family, OREBP exists as a homodimer and it is the only transcription factor known to bind naked DNA targets by complete encirclement in vitro. Nevertheless, how OREBP interacts with target DNA, also known as ORE/TonE, and how it elicits gene transcription in vivo, remains unknown. Using hypertonic induction of the aldose reductase (AR) gene activation as a model, we showed that OREs contained dynamic nucleosomes. Hypertonic stress induced a rapid and reversible loss of nucleosome(s) around the OREs. The loss of nucleosome(s) was found to be initiated by an OREBP-independent mechanism, but was significantly potentiated in the presence of OREBP. Furthermore, hypertonic induction of AR gene was associated with an OREBP-dependent hyperacetylation of histones that spanned the 5' upstream sequences and at least some exons of the gene. Nevertheless, nucleosome loss was not regulated by the acetylation status of histone. Our findings offer novel insights into the mechanism of OREBP-dependent transcriptional regulation and provide a basis for understanding how histone eviction and transcription factor recruitment are coupled.
    PLoS ONE 01/2009; 4(12):e8435. · 3.53 Impact Factor
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    ABSTRACT: Diabetes is a major contributing factor in cataract development. In animal models where cataracts develop within days or weeks of diabetes it is well established that osmotic stress from the accumulation of sorbitol leads to cataract development. This mechanism might explain the rare cases of acute cataract sometimes found in patients with uncontrolled sustained hyperglycemia but cannot account for the vast majority of cataracts that developed after years of diabetes. Thus, a model that can simulate diabetic slow-developing cataract is needed. The contribution of osmotic and oxidative stress in cataract development in sorbitol dehydrogenase (SDH) deficient mice, a model for slow-developing cataract in diabetic patients was determined. Contribution of osmotic stress was assessed by HPLC measurement of sorbitol and by observing the effect of blocking sorbitol accumulation by aldose reductase (AR) null mutation in the SDH deficient mice. Contribution of oxidative stress was assessed by observing the effect of vitamin E treatment and the effect of null mutation of glutathione peroxidase-1 (Gpx-1) on cataract development in these mice. Lenticular sorbitol level was significantly increased in the SDH deficient mice, and blocking sorbitol accumulation by the AR null mutation prevented cataract development, demonstrating the contribution of osmotic stress in cataract development. SDH deficiency did not affect lens oxidative stress status. However, treatment with vitamin E significantly reduced the incidence of cataract, and Gpx-1 deficiency exacerbated cataract development in these mice. Our findings suggest that chronic oxidative stress impaired the osmoregulatory mechanism of the lens. This was not evident until modest increases in lens sorbitol increased the demand of its osmoregulatory function. This osmoregulatory dysfunction model is supported by the fact that the activity of Na+/K+-ATPase, the key regulator of cellular ions and water balance, was dramatically reduced in the precataractous lenses of the SDH deficient mice, and that treatment with vitamin E prevented the loss of Na+/K+-ATPase activity. This osmoregulatory dysfunction model might explain why diabetic patients who control their blood glucose moderately well are still susceptible to develop cataract.
    Experimental Eye Research 09/2008; 87(5):454-61. · 3.03 Impact Factor
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    ABSTRACT: The osmotic response element-binding protein (OREBP), also known as tonicity enhancer-binding protein (TonEBP) or NFAT5, is the only known osmo-sensitive transcription factor that mediates cellular adaptations to extracellular hypertonic stress. Although it is well documented that the subcellular localization and transactivation activity of OREBP/TonEBP are tightly regulated by extracellular tonicity, the molecular mechanisms involved remain elusive. Here we show that nucleocytoplasmic trafficking of OREBP/TonEBP is regulated by the dual phosphorylation of Ser-155 and Ser-158. Alanine scanning mutagenesis revealed that Ser-155 is an essential residue that regulates OREBP/TonEBP nucleocytoplasmic trafficking. Tandem mass spectrometry revealed that Ser-155 and Ser-158 of OREBP/TonEBP are both phosphorylated in living cells under hypotonic conditions. In vitro phosphorylation assays further suggest that phosphorylation of the two serine residues proceeds in a hierarchical manner with phosphorylation of Ser-155 priming the phosphorylation of Ser-158 and that these phosphorylations are essential for nucleocytoplasmic trafficking of the transcription factor. Finally, we have shown that the pharmacological inhibition of casein kinase 1 (CK1) abolishes the phosphorylation of Ser-158 and impedes OREBP/TonEBP nuclear export and that recombinant CK1 phosphorylates Ser-158. Knockdown of CK1alpha1L, a novel isoform of CK1, inhibits hypotonicity-induced OREBP/TonEBP nuclear export. Together these data highlight the importance of Ser-155 and Ser-158 in the nucleocytoplasmic trafficking of OREBP/TonEBP and indicate that CK1 plays a major role in regulating this process.
    Journal of Biological Chemistry 07/2008; 283(25):17624-34. · 4.65 Impact Factor
  • Sookja K. Chung, Stephen S. M. Chung
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    ABSTRACT: Neuropathy is one of the major complications of long-term diabetes. Despite many years of intense research by a number of laboratories, the pathogenetic mechanisms of this disease are still not completely understood. Likely contributing factors of this disease include the polyol pathway, nonenzymatic glycation, protein kinase-C activation, hexosamine pathway, and overproduction of superoxide by the mitochondrial respiratory chain. Their roles in the pathogenesis of diabetic neuropathy are mainly supported by pharmacological studies, which often have inherent problem of uncertain drug specificity and availability. This article reviews the recent studies using transgenic and gene knockout mice to examine the role of polyol pathway, nonenzymatic glycation, poly(adenosine 5′-diphosphate [ADP]-ribose) polymerase, and neurofilaments in diabetic neuropathy. The results of these studies confirm some of the findings from drug experiments and also settle some controversies. These genetic studies avoid some of the problems of using chemical inhibitors, but they also have inherent problems of their own. The prospect of using more sophisticated inducible transgene expression, and conditional gene ablation technologies to circumvent these problems are discussed. It is expected that the number of genetically engineered mutant mice will increase exponentially in the near future and some of them will undoubtedly contribute to our understanding of diabetic neuropathy.
    01/2008: pages 51-67;
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    ABSTRACT: Retinal ischemic injury is common in patients with diabetes, atherosclerosis, hypertension, transient ischemia attack and amaurosis fugax. Previously, signs of ischemic stress, such as pericyte loss, blood-retinal barrier breakdown and neovascularization, which can lead to occlusion of retinal vessels, have been prevented in diabetic db/db mice with aldose reductase (AR) null mutation. To determine the role in retinal ischemic injury of AR and sorbitol dehydrogenase (SDH), the first and second enzymes in the polyol pathway, mice with deletion of AR (AR(-/-)) or SDH-mutation (SDH(-/-)), or C57BL/6N mice treated with AR or SDH inhibitors were subjected to transient retinal artery occlusion (2h of occlusion and 22h of reperfusion) by the intraluminal suture method. Neuronal loss and edema observed in wildtype (AR(+/+)) retinas after transient ischemia were prevented in the retinas of AR(-/-) mice or C57BL/6N mice treated with an AR inhibitor, Fidarestat. Fewer TUNEL-positive cells and smaller accumulations of nitrotyrosine and poly(ADP-ribose) were also observed in the retinas of AR(-/-) mice. However, SDH(-/-) mice and C57BL/6N mice treated with SDH inhibitor, CP-470,711, were not protected against ischemia-induced retinal damage. Taken together, AR contributes to retinal ischemic injury through increased edema and free radical accumulation. Therefore, AR inhibition should be considered for the treatment of retinal ischemic injury often observed in diabetic patients.
    Experimental Eye Research 12/2007; 85(5):608-16. · 3.03 Impact Factor
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    ABSTRACT: Previously, we reported that transgenic mice overexpressing endothelin-1 in astrocytes showed more severe neurological deficits and increased infarct after transient focal ischemia. In those studies, we also observed increased level of aldose reductase (AR), the first and rate-limiting enzyme of the polyol pathway, which has been implicated in osmotic and oxidative stress. To further understand the involvement of the polyol pathway, the mice with deletion of enzymes in the polyol pathway, AR, and sorbitol dehydrogenase (SD), which is the second enzyme in this pathway, were challenged with similar cerebral ischemic injury. Deletion of AR-protected animals from severe neurological deficits and large infarct, whereas similar protection was not observed in mice with SD deficiency. Most interestingly, AR(-/-) brains showed lowered expression of transferrin and transferrin receptor with less iron deposition and nitrotyrosine accumulation. The protection against oxidative stress in AR(-/-) brain was also associated with less poly(adenosine diphosphate-ribose) polymerase (PARP) and caspase-3 activation. Pharmacological inhibition of AR by Fidarestat also protected animals against cerebral ischemic injury. These findings are the first to show that AR contributes to iron- and transferrin-related oxidative stress associated with cerebral ischemic injury, suggesting that inhibition of AR but not SD may have therapeutic potential against cerebral ischemic injury.
    Journal of Cerebral Blood Flow & Metabolism 09/2007; 27(8):1496-509. · 5.40 Impact Factor
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    ABSTRACT: The deleted in liver cancer 2 (DLC2) is a tumor suppressor gene, frequently found to be underexpressed in hepatocellular carcinoma. DLC2 is a multidomain protein containing a sterile alpha-motif (SAM) domain, a GTPase-activating protein (GAP) domain, and a lipid-binding StAR-related lipid-transfer (START) domain. The SAM domain of DLC2, DLC2-SAM, exhibits a low level of sequence homology (15-30%) with other SAM domains, and appears to be the prototype of a new subfamily of SAM domains found in DLC2-related proteins. In the present study, we have determined the three-dimensional solution structure of DLC2-SAM using NMR methods together with molecular dynamics simulated annealing. In addition, we performed a backbone dynamics study. The DLC2-SAM packed as a unique four alpha-helical bundle stabilized by interhelix hydrophobic interactions. The arrangement of the four helices is distinct from all other known SAM domains. In contrast to some members of the SAM domain family which form either dimers or oligomers, both biochemical analyses and rotational correlation time (tau(c)) measured by backbone 15N relaxation experiments indicated that DLC2-SAM exists as a monomer in solution. The interaction of DLC2-SAM domain with sodium dodecyl sulfate (SDS) micelles and 1,2-dimyristoyl-sn-glycerol-3-phosphatidylglycerol (DMPG) phospholipids was examined by CD and NMR spectroscopic techniques. The DLC2-SAM exhibits membrane binding properties accompanied by minor loss of the secondary structure of the protein. Deletion studies showed that the self-association of DLC2 in vivo does not require SAM domain, instead, a protein domain consisting of residues 120-672 mediates the self-association of DLC2.
    Proteins Structure Function and Bioinformatics 07/2007; 67(4):1154-66. · 3.34 Impact Factor
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    ABSTRACT: Phospholipase D (PLD), a highly regulated enzyme that generates the second messenger phosphatidic acid, functions in signal transduction, membrane trafficking and cytoskeletal reorganization. PLD is thought to be involved in the pathogenesis of diabetic complications by activating PKC. Since PKC and PLD are present in the lens we sought to determine if PLD plays a role in diabetic cataract development. We developed transgenic mice that overexpress PLD2, one of the two mammalian isoforms of PLD. These mice developed congenital nuclear cataracts, but not diabetic cataracts. Histological analysis revealed vacuole formation in the fiber cells, mediated potentially by the substantially increased Na,K-ATPase activity. In the presence of the aldose reductase overexpressing transgene that increases lens osmotic pressure, these double transgenic mice developed more severe congenital cataract and became susceptible to develop diabetic cataract. Together, these data suggest that increased PLD2 activity in the lens under hyperglycemic condition might impair its osmoregulatory mechanism and reduce its ability to cope with the osmotic stress triggered by sorbitol accumulation.
    Experimental Eye Research 11/2006; 83(4):939-48. · 3.03 Impact Factor
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    ABSTRACT: The osmotic response element-binding protein (OREBP), also known as tonicity enhancer-binding protein (TonEBP) or NFAT5, regulates the hypertonicity-induced expression of a battery of genes crucial for the adaptation of mammalian cells to extracellular hypertonic stress. The activity of OREBP/TonEBP is regulated at multiple levels, including nucleocytoplasmic trafficking. OREBP/TonEBP protein can be detected in both the cytoplasm and nucleus under isotonic conditions, although it accumulates exclusively in the nucleus or cytoplasm when subjected to hypertonic or hypotonic challenges, respectively. Using immunocytochemistry and green fluorescent protein fusions, the protein domains that determine its subcellular localization were identified and characterized. We found that OREBP/TonEBP nuclear import is regulated by a nuclear localization signal. However, under isotonic conditions, nuclear export of OREBP/TonEBP is mediated by a CRM1-dependent, leucine-rich canonical nuclear export sequence (NES) located in the N terminus. Disruption of NES by site-directed mutagenesis yielded a mutant OREBP/TonEBP protein that accumulated in the nucleus under isotonic conditions but remained a target for hypotonicity-induced nuclear export. More importantly, a putative auxiliary export domain distal to the NES was identified. Disruption of the auxiliary export domain alone is sufficient to abolish the nuclear export of OREBP/TonEBP induced by hypotonicity. By using bimolecular fluorescence complementation assay, we showed that CRM1 interacts with OREBP/TonEBP, but not with a mutant protein deficient in NES. Our findings provide insight into how nucleocytoplasmic trafficking of OREBP/TonEBP is regulated by changes in extracellular tonicity.
    Journal of Biological Chemistry 09/2006; 281(33):23870-9. · 4.65 Impact Factor
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    ABSTRACT: The exaggerated flux through polyol pathway during diabetes is thought to be a major cause of lesions in the peripheral nerves. Here, we used aldose reductase (AR)-deficient (AR(-/-)) and AR inhibitor (ARI)-treated mice to further understand the in vivo role of polyol pathway in the pathogenesis of diabetic neuropathy. Under normal conditions, there were no obvious differences in the innervation patterns between wild-type AR (AR(+/+)) and AR(-/-) mice. Under short-term diabetic conditions, AR(-/-) mice were protected from the reduction of motor and sensory nerve conduction velocities observed in diabetic AR(+/+) mice. Sorbitol levels in the sciatic nerves of diabetic AR(+/+) mice were increased significantly, whereas sorbitol levels in the diabetic AR(-/-) mice were significantly lower than those in diabetic AR(+/+) mice. In addition, signs of oxidative stress, such as increased activation of c-Jun NH(2)-terminal kinase (JNK), depletion of reduced glutathione, increase of superoxide formation, and DNA damage, observed in the sciatic nerves of diabetic AR(+/+) mice were not observed in the diabetic AR(-/-) mice, indicating that the diabetic AR(-/-) mice were protected from oxidative stress in the sciatic nerve. The diabetic AR(-/-) mice also excreted less 8-hydroxy-2'-deoxyguanosine in urine than diabetic AR(+/+) mice. The structural abnormalities observed in the sural nerve of diabetic AR(+/+) mice were less severe in the diabetic AR(-/-) mice, although it was only mildly protected by AR deficiency under short-term diabetic conditions. Signs of oxidative stress and functional and structural abnormalities were also inhibited by the ARI fidarestat in diabetic AR(+/+) nerves, similar to those in diabetic AR(-/-) mice. Taken together, increased polyol pathway flux through AR is a major contributing factor in the early signs of diabetic neuropathy, possibly through depletion of glutathione, increased superoxide accumulation, increased JNK activation, and DNA damage.
    Diabetes 08/2006; 55(7):1946-53. · 7.90 Impact Factor
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    ABSTRACT: To investigate the underlying causes for aldose reductase deficiency-induced diabetes insipidus, we carried out studies with three genotypic groups of mice. These included wild-type mice, knockout mice, and a newly created bitransgenic line that was homozygous for both the aldose reductase null mutation and an aldose reductase knockin transgene driven by the kidney-specific cadherin promoter to direct transgene expression in the collecting tubule epithelial cells. We found that from early renal developmental stages onward, urine osmolality did not exceed 1,000 mosmol/kgH2O in aldose reductase-deficient mice. The functional defects were correlated with significant renal cellular and structural abnormalities that included cell shrinkage, apoptosis, disorganized tubular and vascular structures, and segmental atrophy. In contrast, the transgenic aldose reductase expression in the bitransgenic mice largely but incompletely rescued urine concentrating capacity and significantly improved renal cell survival, cellular morphology, and renal structures. Together, these results suggest that aldose reductase not only plays important roles in osmoregulation and medullary cell survival but may also be essential for the full maturation of the urine concentrating mechanism.
    American journal of physiology. Renal physiology 08/2006; 291(1):F186-95. · 3.61 Impact Factor
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    ABSTRACT: Mesenchymal stem cells (MSCs) from bone marrow are believed to be an ideal cell source for cardiomyoplasty; however, cellular electrophysiology is not understood. The present study was designed to investigate ion channels in undifferentiated rat MSCs. It was found that three types of outward currents were present in rat MSCs, including a small portion of Ca(2+)-activated K(+) channel (I(KCa)) sensitive to inhibition by iberiotoxin and/or clotromazole, a delayed rectifier K(+) current (IK(DR)), and a transient outward K(+) current (I(to)). In addition, tetrodotoxin (TTX)-sensitive sodium current (I(Na.TTX)) and nifedipine-sensitive L-type Ca(2+) current (I(Ca.L)) were found in a small population of rat MSCs. Moreover, reverse transcription-polymerase chain reaction revealed the molecular evidence of mRNA for the functional ionic currents, including Slo and KCNN4 for I(KCa); Kv1.4 for I(to); Kv1.2 and Kv2.1 for IK(DR); SCN2a1 for I(Na.TTX); and CCHL2a for I(Ca.L). These results demonstrate for the first time that multiple functional ion channel currents (i.e., I(KCa), I(to), IK(DR), I(Na.TTX), and I(Ca.L)) are present in rat MSCs from bone marrow; however, physiological roles of these ion channels remain to be studied.
    Stem Cells 07/2006; 24(6):1519-28. · 7.70 Impact Factor

Publication Stats

959 Citations
162.66 Total Impact Points

Institutions

  • 2002–2013
    • The University of Hong Kong
      • • Department of Anatomy
      • • Department of Physiology
      • • Department of Chemistry
      Hong Kong, Hong Kong
  • 2008–2009
    • The Chinese University of Hong Kong
      • • Department of Anatomical and Cellular Pathology
      • • State Key Laboratory of Oncology in South China
      Hong Kong, Hong Kong
  • 2006
    • Queen Mary Hospital
      Hong Kong, Hong Kong
    • Lands Department of The Government of the Hong Kong Special Administrative Region
      Hong Kong, Hong Kong