Protein O-N-acetylglucosaminylation modulates promoter activities of cyclic AMP response element and activator protein 1 and enhances E-selectin expression on HuH-7 human hepatoma cells

What turns CREB on? And off? And why does it matter?

Altered expression and function of the transcription factor cyclic AMP response-binding protein (CREB) has been identified to play an important role in cancer and is associated with the overall survival and therapy response of tumor patients. This review focuses on the expression and activation of CREB under physiologic conditions and in tumors of distinct origin as well as the underlying mechanisms of CREB regulation by diverse stimuli and inhibitors. In addition, the clinical relevance of CREB is summarized, including its use as a prognostic and/or predictive marker as well as a therapeutic target.

Nutrient regulation of signaling and transcription

In the early 1980s, while using purified glycosyltransferases to probe glycan structures on surfaces of living cells in the murine immune system, we discovered a novel form of serine/threonine protein glycosylation (O-linked β-GlcNAc; O-GlcNAc) that occurs on thousands of proteins within the nucleus, cytoplasm, and mitochondria. Prior to this discovery, it was dogma that protein glycosylation was restricted to the luminal compartments of the secretory pathway and on extracellular domains of membrane and secretory proteins. Work in the last 3 decades from several laboratories has shown that O-GlcNAc cycling serves as a nutrient sensor to regulate signaling, transcription, mitochondrial activity, and cytoskeletal functions. O-GlcNAc also has extensive cross-talk with phosphorylation, not only at the same or proximal sites on polypeptides, but also by regulating each other's enzymes that catalyze cycling of the modifications. O-GlcNAc is generally not elongated or modified. It cycles on and off polypeptides in a time scale similar to phosphorylation, and both the enzyme that adds O-GlcNAc, the O-GlcNAc transferase (OGT), and the enzyme that removes O-GlcNAc, O-GlcNAcase (OGA), are highly conserved from C. elegans to humans. Both O-GlcNAc cycling enzymes are essential in mammals and plants. Due to O-GlcNAc's fundamental roles as a nutrient and stress sensor, it plays an important role in the etiologies of chronic diseases of aging, including diabetes, cancer, and neurodegenerative disease. This review will present an overview of our current understanding of O-GlcNAc's regulation, functions, and roles in chronic diseases of aging.

Iron down-regulates leptin by suppressing protein O-GlcNAc modification in adipocytes, resulting in decreased levels of O-glycosylated CREB

We previously reported that iron down-regulates transcription of the leptin gene by increasing occupancy of phosphorylated cAMP response element-binding protein (pCREB) at two sites in the leptin gene promoter. Several nutrient-sensing pathways including O-GlcNAcylation also regulate leptin. We therefore investigated whether O-glycosylation plays a role in iron- and CREB-mediated regulation of leptin. We found that high iron decreases protein O-GlcNAcylation both in cultured 3T3-L1 adipocytes and in mice fed high-iron diets and down-regulates leptin mRNA and protein levels. Glucosamine treatment, which bypasses the rate-limiting step in the synthesis of substrate for glycosylation, increased both O-GlcNAc and leptin, whereas inhibition of O-glycosyltransferase (OGT) decreased O-GlcNAc and leptin. The increased leptin levels induced by glucosamine were susceptible to the inhibition by iron, but in the case of OGT inhibition, iron did not further decrease leptin. Mice with deletion of the O-GlcNAcase gene, either via whole-body heterozygous deletion or through adipocyte-targeted homozygous deletion, exhibited increased O-GlcNAc levels in adipose tissue and increased leptin levels that were inhibited by iron. Of note, iron increased the occupancy of pCREB and decreased the occupancy of O-GlcNAcylated CREB on the leptin promoter. These patterns observed in our experimental models suggest that iron exerts its effects on leptin by decreasing O-glycosylation and not by increasing protein deglycosylation and that neither O-GlcNAcase nor OGT mRNA and protein levels are affected by iron. We conclude that iron down-regulates leptin by decreasing CREB glycosylation, resulting in increased CREB phosphorylation and leptin promoter occupancy by pCREB.

Glucosamine abrogates the stem cell factor + endothelin-1-induced stimulation of melanogenesis via a deficiency in MITF expression due to the proteolytic degradation of CREB in human melanocytes

We have already reported that glucosamine (GlcN) distinctly abrogates the pigmentation of human epidermal equivalents stimulated by stem cell factor + endothelin-1 (SE). In this study, we characterized the molecular mechanism involved in the anti-melanogenic effects of GlcN using normal human melanocytes (NHMs) in culture. The SE-stimulated gene (12 h) and protein (24 h) expression levels of melanocyte-specific proteins (at the indicated times post-stimulation) were significantly abrogated by pretreatment with GlcN for 72 h. Western blotting analysis of the phosphorylation of intracellular signaling molecules in the MAPK pathway revealed that despite the significantly decreased level of total CREB protein at all times post-stimulation, the SE-stimulated phosphorylation of ERK, CREB and MITF is not attenuated at 15 min post-stimulation in GlcN-treated NHMs. However, the SE-stimulated protein expression level of total MITF at 2 and 6 h post-stimulation was significantly abrogated by 72 h pretreatment with GlcN. Consistently, pretreatment with GlcN for 72 h abrogated the stimulated gene and protein expression levels of MITF at 1 h and 2 h post-stimulation, respectively. Analysis of gene and protein expression levels also demonstrated that pretreatment with GlcN for 72 h significantly reduced the protein levels of CREB and MITF without affecting their gene expression levels prior to the SE stimulation. Silencing with a CREB siRNA distinctly abrogated the SE-stimulated expression of MITF (at 2 h post-stimulation) and melanocyte-specific proteins (at 24 h post-stimulation). Similarly, transfection of MITF siRNA markedly abrogated the SE-stimulated expression of MITF protein and melanocyte-specific proteins at 2 and 24 h post-stimulation, respectively. Finally, the decreased levels of CREB and MITF proteins induced by 72 h pretreatment with GlcN were abrogated by the co-addition of the proteosomal degradation inhibitor MG132. These findings suggest that the anti-melanogenic effect elicited by GlcN is mediated via the decreased expression of MITF which results from the attenuated transcriptional activity of CREB due to proteolytic degradation.

Apricot attenuates oxidative stress and modulates of Bax, Bcl-2, caspases, NFκ-B, AP-1, CREB expression of rats bearing DMBA-induced liver damage and treated with a combination of radiotherapy

We evaluated the ability of apricot to attenuate apoptosis and oxidative stress developed during the process of 7,12-dimethylbenz[a]anthracene (DMBA) and radiotherapy in the liver of rats bearing liver damage. Fifty female Wistar rats were divided into 7 groups; (i) normal control rats; (ii) rats fed with standard diet with apricot (20%), (ii) rats fed with standard diet and administrated 6 gray radiotherapy with Co 60 device applied to a single fraction, (iv) rats fed with standard diet and administered intraperitoneally DMBA (20mg/kg), (v) rats fed with standard diet and administered DMBA and 6 gray radiotherapy, (vi) rats fed with standard rat diet and administered DMBA and supplemented apricot, (vii) rats fed with standard diet supplemented apricot administered DMBA and radiotherapy (RT) for 6weeks. Expression of Bax, caspase 3, and glutathione activity decreased in the liver but liver expression of NF-κB, AP-1, CREB, Bcl-2 and ALT, AST, 5'NT, MDA, NO levels increased in DMBA-induced liver damage rats. In conclusion, the results suggest that apricot supplementation and irradiation given in combination, offer maximum protection against DMBA-induced hepatic carcinogenesis.

Prolonged high glucose suppresses phorbol 12-myristate 13-acetate and ionomycin-induced interleukin-2 mRNA expression in Jurkat cells

BACKGROUND

The disturbance of immunological responses is a complication of diabetes mellitus.

METHODS AND RESULTS

We cultured Jurkat cells in 11.1 (normal) and 22.2 mmol/l (high) glucose for 12 weeks and stimulated them with 10 nmol/l phorbol 12-myristate 13-acetate (PMA) and 500 nmol/l ionomycin. RT-PCR revealed that induced interleukin (IL)-2 mRNA expression levels were suppressed in high glucose cultures compared to those in normal glucose. Promoter activities of IL-2, nuclear factor of activated T cells (NFAT), and activator protein-1 (AP-1), after 6 h stimulation with PMA and ionomycin, gradually decreased in high glucose cultures to approximately 20% of those in normal glucose at 12 weeks. The prolonged culture in high glucose increased inducible cAMP early repressor (ICER) II mRNA and protein levels, and overexpression of ICER II dose-dependently suppressed promoter activities of IL-2, NFAT, and AP-1. Moreover, ICER II mRNA expression was transiently induced by stimulation with PMA and ionomycin in normal glucose cultures; however, with high glucose, the induction disappeared.

CONCLUSION

These results indicate that ICER II protein accumulates during prolonged culture in high glucose and suppresses IL-2 mRNA expression in Jurkat cells.