Enhanced expression of uridine diphosphate-N-acetylglucosaminyl transferase (OGT) in a stable, tetracycline-inducible HeLa cell line using histone deacetylase inhibitors: kinetics of cytosolic OGT accumulation and nuclear translocation

Microbial-Derived Butyrate Promotes Epithelial Barrier Function through IL-10 Receptor-Dependent Repression of Claudin-2

Commensal interactions between the enteric microbiota and distal intestine play important roles in regulating human health. Short-chain fatty acids (SCFAs), such as butyrate, produced through anaerobic microbial metabolism represent a major energy source for the host colonic epithelium and enhance epithelial barrier function through unclear mechanisms. Separate studies revealed that the epithelial anti-inflammatory IL-10 receptor α subunit (IL-10RA) is also important for barrier formation. Based on these findings, we examined if SCFAs promote epithelial barrier through IL-10RA-dependent mechanisms. Using human intestinal epithelial cells (IECs), we discovered that SCFAs, particularly butyrate, enhanced IEC barrier formation, induced IL-10RA mRNA, IL-10RA protein, and transactivation through activated Stat3 and HDAC inhibition. Loss and gain of IL-10RA expression directly correlates with IEC barrier formation and butyrate represses permeability-promoting claudin-2 tight-junction protein expression through an IL-10RA-dependent mechanism. Our findings provide a novel mechanism by which microbial-derived butyrate promotes barrier through IL-10RA-dependent repression of claudin-2.

O-GlcNAcylation of histone deacetylases 1 in hepatocellular carcinoma promotes cancer progression

Hepatocellular carcinoma (HCC) is a malignant tumor originating in the liver. Previous studies have indicated that O-GlcNAc transferase (OGT) and histone deacetylase-1 (HDAC1) play important roles in the pathogenesis of HCC. In the present study, we investigated the physical link between OGT and HDAC1. The O-GlcNAcylation of HDAC1 is overexpressed in HCC. We found that HDAC1 has two major sites of O-GlcNAcylation in its histone deacetylase domain. HDAC1 O-GlcNAcylation increases the activated phosphorylation of HDAC1, which enhances its enzyme activity. HDAC1 O-GlcNAc mutants promote the p21 transcription regulation through affecting the acetylation levels of histones from chromosome, and then influence the proliferation of HCC cells. We also found that mutants of O-GlcNAcylation site of HDAC1 affect invasion and migration of HepG2 cells. E-cadherin level is highly up-regulated in HDAC1 O-GlcNAc mutant-treated liver cancer cells, which inhibit the occurrence and development of HCC. Our findings suggest that OGT promotes the O-GlcNAc modification of HDAC1in the development of HCC. Therefore, inhibiting O-GlcNAcylation of HDAC1 may repress the progression of HCC.

Butyrate and other short-chain fatty acids increase the rate of lipolysis in 3T3-L1 adipocytes

We determined the effect of butyrate and other short-chain fatty acids (SCFA) on rates of lipolysis in 3T3-L1 adipocytes. Prolonged treatment with butyrate (5 mM) increased the rate of lipolysis approximately 2–3-fold. Aminobutyric acid and acetate had little or no effect on lipolysis, however propionate stimulated lipolysis, suggesting that butyrate and propionate act through their shared activity as histone deacetylase (HDAC) inhibitors. Consistent with this, the HDAC inhibitor trichostatin A (1 µM) also stimulated lipolysis to a similar extent as did butyrate. Western blot data suggested that neither mitogen-activated protein kinase (MAPK) activation nor perilipin down-regulation are necessary for SCFA-induced lipolysis. Stimulation of lipolysis with butyrate and trichostatin A was glucose-dependent. Changes in AMP-activated protein kinase (AMPK) phosphorylation mediated by glucose were independent of changes in rates of lipolysis. The glycolytic inhibitor iodoacetate prevented both butyrate- and tumor necrosis factor-alpha-(TNF-α) mediated increases in rates of lipolysis indicating glucose metabolism is required. However, unlike TNF-α– , butyrate-stimulated lipolysis was not associated with increased lactate release or inhibited by activation of pyruvate dehydrogenase (PDH) with dichloroacetate. These data demonstrate an important relationship between lipolytic activity and reported HDAC inhibitory activity of butyrate, other short-chain fatty acids and trichostatin A. Given that HDAC inhibitors are presently being evaluated for the treatment of diabetes and other disorders, more work will be essential to determine if these effects on lipolysis are due to inhibition of HDAC.

Dynamic O-GlcNAcylation and its roles in the cellular stress response and homeostasis

O-linked N-acetyl-β-D-glucosamine (O-GlcNAc) is a ubiquitous and dynamic post-translational modification known to modify over 3,000 nuclear, cytoplasmic, and mitochondrial eukaryotic proteins. Addition of O-GlcNAc to proteins is catalyzed by the O-GlcNAc transferase and is removed by a neutral-N-acetyl-β-glucosaminidase (O-GlcNAcase). O-GlcNAc is thought to regulate proteins in a manner analogous to protein phosphorylation, and the cycling of this carbohydrate modification regulates many cellular functions such as the cellular stress response. Diverse forms of cellular stress and tissue injury result in enhanced O-GlcNAc modification, or O-GlcNAcylation, of numerous intracellular proteins. Stress-induced O-GlcNAcylation appears to promote cell/tissue survival by regulating a multitude of biological processes including: the phosphoinositide 3-kinase/Akt pathway, heat shock protein expression, calcium homeostasis, levels of reactive oxygen species, ER stress, protein stability, mitochondrial dynamics, and inflammation. Here, we will discuss the regulation of these processes by O-GlcNAc and the impact of such regulation on survival in models of ischemia reperfusion injury and trauma hemorrhage. We will also discuss the misregulation of O-GlcNAc in diseases commonly associated with the stress response, namely Alzheimer's and Parkinson's diseases. Finally, we will highlight recent advancements in the tools and technologies used to study the O-GlcNAc modification.

Roles of GPR41 and GPR43 in leptin secretory responses of murine adipocytes to short chain fatty acids

GPR41 is reportedly expressed in murine adipose tissue and mediates short chain fatty acid (SCFA)-stimulated leptin secretion by activating Galpha(i). Here, we agree with a contradictory report in finding no expression of GPR41 in murine adipose tissue. Nevertheless, in the presence of adenosine deaminase to minimise Galpha(i) signalling via the adenosine A1 receptor, SCFA stimulated leptin secretion by adipocytes from wild-type but not GPR41 knockout mice. Expression of GPR43 was reduced in GPR41 knockout mice. Acetate but not butyrate stimulated leptin secretion in wild-type mesenteric adipocytes, consistent with mediation of the response by GPR43 rather than GPR41. Pertussis toxin prevented stimulation of leptin secretion by propionate in epididymal adipocytes, implicating Galpha(i) signalling mediated by GPR43 in SCFA-stimulated leptin secretion.

Role of insulin, adipocyte hormones, and nutrient-sensing pathways in regulating fuel metabolism and energy homeostasis: a nutritional perspective of diabetes, obesity, and cancer

Traditionally, nutrients such as glucose and amino acids have been viewed as substrates for the generation of high-energy molecules and as precursors for the biosynthesis of macromolecules. However, it is now apparent that nutrients also function as signaling molecules in functionally diverse signal transduction pathways. Glucose and amino acids trigger signaling cascades that regulate various aspects of fuel and energy metabolism and control the growth, proliferation, and survival of cells. Here, we provide a functional and regulatory overview of three well-established nutrient signaling pathways-the hexosamine signaling pathway, the mTOR (mammalian target of rapamycin) signaling pathway, and the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway. Nutrient signaling pathways are interconnected, coupled to insulin signaling, and linked to the release of metabolic hormones from adipose tissue. Thus, nutrient signaling pathways do not function in isolation. Rather, they appear to serve as components of a larger "metabolic regulatory network" that controls fuel and energy metabolism (at the cell, tissue, and whole-body levels) and links nutrient availability with cell growth and proliferation. Understanding the diverse roles of nutrients and delineating nutrient signaling pathways should facilitate drug discovery research and the search for novel therapeutic compounds to prevent and treat various human diseases such as diabetes, obesity, and cancer.

Ataxin-10 interacts with O-linked beta-N-acetylglucosamine transferase in the brain

Modification by O-GlcNAc involves a growing number of eucaryotic nuclear and cytosolic proteins. Glycosylation of intracellular proteins is a dynamic process that in several cases competes with and acts as a reciprocal modification system to phosphorylation. O-Linked beta-N-acetylglucosamine transferase (OGT) levels are highest in the brain, and neurodegenerative disorders such as Alzheimer disease have been shown to involve abnormally phosphorylated key proteins, probably as a result of hypoglycosylation. Here, we show that the neurodegenerative disease protein ataxin-10 (Atx-10) is associated with cytoplasmic OGT p110 in the brain. In PC12 cells and pancreas, this association is competed by the shorter OGT p78 splice form, which is down-regulated in brain. Overexpression of Atx-10 in PC12 cells resulted in the reconstitution of the Atx-10-OGT p110 complex and enhanced intracellular glycosylation activity. Moreover, in an in vitro enzyme assay using PC12 cell extracts, Atx-10 increased OGT activity 2-fold. These data indicate that Atx-10 might be essential for the maintenance of a critical intracellular glycosylation level and homeostasis in the brain.

Discovery of O-GlcNAc transferase inhibitors

O-GlcNAcylation of serine and threonine residues is a dynamic and essential post-translational modification involved in signaling pathways in eukaryotes. Studies of O-GlcNAcylation would be aided by small-molecule inhibitors of O-GlcNAc transferase (OGT), the sole enzyme know to mediate this modification, but discovery of such molecules has been hampered by poor expression of cloned OGT and lack of suitable high-throughput screens. This Communication describes the development an expression system to access large amounts of the catalytic domain of OGT and the implementation of a fluorescence-based substrate analogue displacement assay that has led to the discovery of a set of OGT inhibitors. This work lays the foundation for both structural and functional analysis of the catalytic domain of OGT.

Turnover and characterization of UDP-N-acetylglucosaminyl transferase in a stably transfected HeLa cell line

To estimate the turnover of UDP-N-acetylglucosaminyl transferase (OGT), we exposed stably transfected HeLa cells to tetracycline for 16h to induce OGT gene expression and increase cytosolic enzyme levels. Removal of tetracycline led to a progressive decrease in OGT activity (after a 6h lag period), yielding an estimated OGT half-life of 13h. A similar half-life (12h) was obtained by measuring the loss of biosynthetically labeled OGT ([35S]methionine pulse-chase experiments). Since OGT turnover was relatively slow, it is unlikely that changes in OGT gene expression or protein expression play a role in the short-term regulatory actions mediated by the hexosamine signaling pathway. We also found that the overexpressed 110kDa murine OGT subunit (recombinant enzyme) was enzymatically similar to the endogenous holoenzyme derived from rat brain tissue. Thus, stably transfected HeLa cells provide an abundant source of enzyme that can be used to study the structure, function, and regulation of OGT.

Potential regulation of nuclear UDP-N-acetylglucosaminyl transferase (OGT) by substrate availability: ability of chromatin protein to bind UDP-N-acetylglucosamine and reduce OGT-mediated O-Linked glycosylation

UDP-N-acetylglucosaminyl transferase (OGT) resides in both cytosolic and nuclear compartments and catalyzes O-linked glycosylation of various proteins. In the current study, we have extracted protein from nuclear DNA (chromatin protein) using 0.2% NP-40 detergent. Addition of chromatin protein to either cytosolic or nuclear preparations (containing abundant OGT) resulted in a dose-dependent loss of OGT activity. Since chromatin-mediated loss of OGT activity could be restored by immunopurification of OGT, we conclude that loss of enzyme activity is not due to direct inactivation of OGT. Addition of UDP-galactose (to saturate potential UDP binding proteins) effectively restored OGT activity in cytosol containing chromatin protein. This indicates that chromatin protein inhibits OGT activity by binding UDP-GlcNAc. These studies suggest that nuclear substrate availability may comprise one of the in vivo mechanisms regulating OGT activity and O-linked glycosylation of nuclear proteins. This is potentially significant, since most transcription factors are O-linked glycosylated and such post-translational modifications can alter gene expression.

Differential effects of vanadate on UDP-N-acetylglucosaminyl transferase activity derived from cytosol and nucleosol

UDP-N-acetylglucosaminyl transferase (OGT) is a key enzyme of a novel signal transduction pathway that regulates protein function through O-linked glycosylation. In the current study, we found that sodium vanadate potently inhibits OGT activity in brain cytosol (IC50 = 55 microM) and nucleosol (IC50 = 150 microM), but fails to alter activity of a related enzyme (UDP-galactosyltransferase). Vanadate also inhibits OGT activity in cytosol (IC50 of 2.3 microM) and nucleosol (IC50 of 130) derived from a stable HeLa cell line that overexpresses OGT. When HeLa cytosol was immunopurified to separate OGT from other cellular proteins, vanadate still inhibited OGT activity (IC50 = 2 microM). We conclude that OGT derived from cytosol exhibits greater vanadate sensitivity than nucleosol OGT and that a large difference exists (25-fold) in vanadate sensitivity when comparing OGT activity in different cell types (IC50 of 55 microM for brain cytosol vs. 2.3 microM for HeLa cytosol). Understanding the mechanism(s) by which a tyrosine phosphatase inhibitor differentially reduces OGT activity should lead to new insights into OGT function and regulation.