Palmitate-induced activation of the hexosamine pathway in human myotubes: increased expression of glutamine:fructose-6-phosphate aminotransferase

Selective decrease of membrane-associated PKC-alpha and PKC-epsilon in response to elevated intracellular O-GlcNAc levels in transformed human glial cells

Increased flux through the hexosamine biosynthetic pathway (HBP) has been shown to affect the activity and translocation of certain protein kinase C (PKC) isoforms. It has been suggested that this effect is due to increases in the beta-O-linked N-acetylglucosamine (O-GlcNAc) modification. Herein, we demonstrate the effect of increasing the O-GlcNAc modification on the translocation of select PKC isozymes in a human astroglial cell line. Treating cells with either 8 mM d-glucosamine (GlcN), 5 mM streptozotocin (STZ), or 80 muM O-(2-acetamido-2-deoxy-d-glucopyranosylidene)amino-N-phenylcarbamate (PUGNAc) produced a significant increase in the O-GlcNAc modification on both cytosolic and membrane proteins; however, both the level and rate of O-GlcNAc increase varied with the compound. GlcN treatment resulted in a rapid, transient translocation of PKC-betaII that was maximal after 3 h (73+/-8%) and also produced a 48+/-15% decrease in membrane-associated PKC-epsilon after 9 h of treatment. Similar to GlcN treatment, STZ and PUGNAc treatment also resulted in decreased levels of PKC-epsilon in the membrane fraction. Significant decreases were seen as early as 5 h and, by 9 h of treatment, had decreased by 87+/-6% with STZ and 73+/-7% with PUGNAc. Unlike GlcN, both STZ and PUGNAc produced a decrease in PKC-alpha membrane levels by 9 h posttreatment (78+/-10% with STZ and 66+/-8% with PUGNAc) while neither compound produced any changes in PKC-betaII translocation. In addition, none of the three compounds affected membrane levels of PKC-iota. Altogether, these results demonstrate a novel link between increased levels of the O-GlcNAc modification and the regulation of specific PKC isoforms.

The tumor suppressor HIC1 (hypermethylated in cancer 1) is O-GlcNAc glycosylated

HIC1 (hypermethylated in cancer 1) is a transcriptional repressor containing five Krüppel-like C(2)H(2) zinc fingers and an N-terminal dimerization and autonomous repression domain called BTB/POZ. Here, we demonstrate that full-length HIC1 proteins are modified both in vivo and in vitro with O-linked N-acetylglucosamine (O-GlcNAc). This is a highly dynamic glycosylation found within the cytosolic and the nuclear compartments of eukaryotes. Analysis of [(3)H]Gal-labeled tryptic peptides indicates that HIC1 has three major sites for O-GlcNAc glycosylation. Using C-terminal deletion mutants, we have shown that O-GlcNAc modification of HIC1 proteins occurred preferentially in the DNA-binding domain. Nonglycosylated and glycosylated forms of full-length HIC1 proteins separated by wheat germ agglutinin affinity purification, displayed the same specific DNA-binding activity in electrophoretic mobility shift assays proving that the O-GlcNAc modification is not directly implicated in the specific DNA recognition of HIC1. Intriguingly, N-terminal truncated forms corresponding to BTB-POZ-deleted proteins exhibited a strikingly differential activity, as the glycosylated truncated forms are unable to bind DNA whereas the unglycosylated ones do. Electrophoretic mobility shift assays performed with separated pools of glycosylated and unglycosylated forms of a construct exhibiting only the DNA-binding domain and the C-terminal tail of HIC1 (residues 399-714) and supershift experiments with wheat germ agglutinin or RL-2, an antibody raised against O-GlcNAc residues, fully corroborated these results. Interestingly, these truncated proteins are O-GlcNAc modified in their C-terminal tail (residues 670-711) and not in the DNA-binding domain, as for the full-length proteins. Thus, the O-GlcNAc modification of HIC1 does not affect its specific DNA-binding activity and is highly sensitive to conformational effects, notably its dimerization through the BTB/POZ domain.

Palmitate, but not unsaturated fatty acids, induces the expression of interleukin-6 in human myotubes through proteasome-dependent activation of nuclear factor-kappaB

Circulating interleukin-6 (IL-6), insulin, and free fatty acid (FFA) concentrations are associated with impaired insulin action in obese and type 2 diabetic individuals. However, a causal relationship between elevated plasma FFAs and IL-6 has not been shown. Because skeletal muscle represents a major target of impaired insulin action, we studied whether FFAs may affect IL-6 expression in human myotubes. We demonstrate that specifically saturated FFAs, e.g. palmitate (0.25 mm), induce IL-6 mRNA expression and protein secretion by a proteasome-dependent mechanism that leads to a rapid and chronic activation of nuclear factor-kappaB. Insulin, high glucose concentrations, or unsaturated FFAs did not activate IL-6 expression. In fact, the unsaturated FFA linoleate inhibited palmitate-induced IL-6 production. Because inhibition of palmitate metabolism by the acyl-CoA synthetase inhibitor triacsin C did not abolish IL-6 expression, it appears that the palmitate molecule per se exerts the observed effects. Furthermore, we show that in human myotubes, IL-6 activates the phosphorylation of signal transducer and activator of transcription 3 in concentrations similar to hepatocytes. However, no inhibitory effect of IL-6 on insulin action, determined as phosphatidylinositol 3-kinase association with insulin receptor substrate-1, Akt phosphorylation, and glycogen synthesis, was detected. We conclude that IL-6 expression may be modulated by the composition of circulating FFA, e.g. by diet, and that skeletal muscle cells could be target cells for IL-6.