okadaic-acid and acadesine

Browning, the conversion of white adipose tissue (WAT) to a beige phenotype, has gained interest as a strategy to induce weight loss and improve insulin resistance in metabolic disorders. However, for hypermetabolic conditions stemming from burn trauma or cancer cachexia, browning is thought to contribute to energy wasting and supraphysiological nutritional requirements. Metformin's impact on this phenomenon and underlying mechanisms have not been explored.. We used both a murine burn model and human ex vivo adipose explants to assess metformin and 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR)'s effects on the development of subcutaneous beige adipose. Enzymes involved in fat homeostasis and browning, as well as mitochondrial dynamics, were assessed to determine metformin's effects.. Treatment with the biguanide metformin lowers lipolysis in beige fat by inducing protein phosphatase 2A (PP2A) independently of adenosine monophosphate kinase (AMPK) activation. Increased PP2A activity catalyzes the dephosphorylation of acetyl-CoA carboxylase (Ser 79) and hormone sensitive lipase (Ser 660), thus promoting fat storage and the "whitening" of otherwise lipolytic beige adipocytes. Moreover, co-incubation of metformin with the PP2A inhibitor okadaic acid countered the anti-lipolytic effects of this biguanide in human adipose. Additionally, we show that metformin does not activate this pathway in the WAT of control mice and that AICAR sustains the browning of white adipose, offering further evidence that metformin acts independently of this cellular energy sensor.. This work provides novel insights into the mechanistic underpinnings of metformin's therapeutic benefits and potential as an agent to reduce the lipotoxicity associated with hypermetabolism and adipose browning.

Topics: Acetyl-CoA Carboxylase; Adipocytes, Beige; Adipose Tissue, White; Adult; Aminoimidazole Carboxamide; AMP-Activated Protein Kinases; Animals; Burns; Disease Models, Animal; Humans; Lipolysis; Metformin; Mice; Mice, Inbred C57BL; Mitochondria; Okadaic Acid; Oxidative Phosphorylation; Protein Phosphatase 2; Ribonucleosides; Sterol Esterase; Subcutaneous Fat

Glycine N-methyltransferase (GNMT) is an abundant cytosolic enzyme that catalyses the methylation of glycine into sarcosine, coupled with conversion of the methyl donor, S -adenosylmethionine (AdoMet), into S -adenosylhomocysteine (AdoHcy). GNMT is believed to play a role in monitoring the AdoMet/AdoHcy ratio, and hence the cellular methylation capacity, but regulation of the enzyme itself is not well understood. In the present study, treatment of isolated rat hepatocytes with the protein phosphatase inhibitor okadaic acid, was found to induce an overphosphorylation of GNMT, as shown by proteomic analysis. The analysis comprised two-dimensional gel electrophoretic separation of (32)P-labelled phosphoproteins and identification of individual protein spots by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry. The identity of GNMT was verified by N-terminal Edman sequencing of tryptic peptides. Chromatographic separation of proteolytic peptides and (32)P-labelled amino acids suggested that GNMT was phosphorylated within a limited region, and only at serine residues. GNMT phosphorylation could be suppressed by naringin, an okadaic acid-antagonistic flavonoid. To assess the possible functional role of GNMT phosphorylation, the effect of okadaic acid on hepatocytic AdoMet and AdoHcy levels was examined, using HPLC separation for metabolite analysis. Surprisingly, okadaic acid was found to have no effect on the basal levels of AdoMet or AdoHcy. An accelerated AdoMet-AdoHcy flux, induced by the addition of methionine (1 mM), was likewise unaffected by okadaic acid. 5-Aminoimidazole-4-carboxamide riboside, an activator of the hepatocytic AMP-activated protein kinase, similarly induced GNMT phosphorylation without affecting AdoMet and AdoHcy levels. Activation of cAMP-dependent protein kinase by dibutyryl-cAMP, reported to cause GNMT phosphorylation under cell-free conditions, also had little effect on hepatocytic AdoMet and AdoHcy levels. Phosphorylation of GNMT would thus seem to play no role in regulation of the intracellular AdoMet/AdoHcy ratio, but could be involved in other GNMT functions, such as the binding of folates or aromatic hydrocarbons.

Topics: Aminoimidazole Carboxamide; Animals; Bucladesine; Cell-Free System; Enzyme Inhibitors; Flavanones; Flavonoids; Glycine; Glycine N-Methyltransferase; Hepatocytes; Methyltransferases; Okadaic Acid; Peptide Fragments; Phosphorylation; Rats; Ribonucleosides; S-Adenosylhomocysteine; S-Adenosylmethionine

Although it is now clearly established that a number of genes involved in glucose and lipid metabolism are up-regulated by high glucose concentrations in both liver and adipose tissue, the signaling pathway arising from glucose to the transcriptional machinery is still poorly understood. We have analyzed the regulation of fatty acid synthase gene expression by glucose in cultured rat hepatocytes. Glucose (25 mM) induces an activation of the transcription of the fatty acid synthase gene, and this effect is markedly reduced by incubation of the cells with okadaic acid, an inhibitor of protein phosphatases 1 and 2A. A similar reduction in glucose-activated fatty acid synthase gene expression is obtained by incubation with 5-amino-imidazolecarboxamide riboside, a cell-permeable activator of the AMP-activated protein kinase. Taken together, these results indicate that the glucose-induced expression of the fatty acid synthase gene involves a phosphorylation/dephosphorylation mechanism and suggest that the AMP-activated protein kinase plays an important role in this process. This is the first evidence that implicates the AMP-activated protein kinase in the regulation of gene expression. AMP-activated protein kinase is the mammalian analog of SNF1, a kinase involved in yeast in the transcriptional regulation of genes by glucose.

Topics: Adenosine Monophosphate; Aminoimidazole Carboxamide; Animals; Cells, Cultured; Enzyme Activation; Enzyme Inhibitors; Fatty Acid Synthases; Female; Gene Expression Regulation; Glucokinase; Glucose; Liver; Nucleotides; Okadaic Acid; Phosphoprotein Phosphatases; Phosphorylation; Protein Kinases; Rats; Rats, Wistar; Ribonucleosides; Transcriptional Activation