Activation of acetyl-CoA carboxylase by a glutamate- and magnesium-sensitive protein phosphatase in the islet beta-cell.
Acetyl-CoA carboxylase (ACC) catalyzes the formation of malonyl-CoA, a precursor in the biosynthesis of long-chain fatty acids, which have been implicated in physiological insulin secretion. The catalytic function of ACC is regulated by phosphorylation (inactive)-dephosphorylation (active). In this study we investigated whether similar regulatory mechanisms exist for ACC in the pancreatic islet beta-cell. ACC was quantitated in normal rat islets, human islets, and clonal beta-cells (HIT-15 or INS-1) using a [(14)C]bicarbonate fixation assay. In the beta-cell lysates, ACC was stimulated by magnesium in a concentration-dependent manner. Of all the dicarboxylic acids tested, only glutamate, albeit ineffective by itself, significantly potentiated magnesium-activated ACC in a concentration-dependent manner. ACC stimulation by glutamate and magnesium was maximally demonstrable in the cytosolic fraction; it was markedly reduced by okadaic acid (OKA) in concentrations (<50 nmol/l) that inhibited protein phosphatase 2A (PP2A). Furthermore, pretreatment of the cytosolic fraction with anti-PP2A serum attenuated the glutamate- and magnesium-mediated activation of ACC, thereby suggesting that ACC may be regulated by an OKA-sensitive PP2A-like enzyme. Streptavidin-agarose chromatography studies have indicated that glutamate- and magnesium-mediated effects on ACC are attributable to activation of ACC's dephosphorylation; this suggests that the stimulatory effects of glutamate and magnesium on ACC might involve activation of an OKA-sensitive PP2A-like enzyme that dephosphorylates and activates ACC. In our study, 5-amino-imidazolecarboxamide (AICA) riboside, a stimulator of AMP kinase, significantly inhibited glucose-mediated activation of ACC and insulin secretion from isolated beta-cells. Together, our data provide evidence for a unique regulatory mechanism for the activation of ACC in the pancreatic beta-cell, leading to the generation of physiological signals that may be relevant for physiological insulin secretion.
Kowluru
A
A
Department of Pharmaceutical Sciences, 610 Shapero Hall, Wayne State University, Detroit, MI 48202, USA. akowluru@wizard.pharm.wayne.edu
Chen
H Q
HQ
Modrick
L M
LM
Stefanelli
C
C
D000103:Acetyl-CoA Carboxylase / Q000378:metabolism*; D000263:Adenylate Kinase / Q000378:metabolism; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002478:Cells, Cultured; D005947:Glucose / Q000494:pharmacology; D018698:Glutamic Acid / Q000378:metabolism*; D006801:Humans; D007515:Islets of Langerhans / Q000201:enzymology*; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D054648:Protein Phosphatase 2; D051381:Rats; D012263:Ribonucleosides / Q000494:pharmacology
D012263:Ribonucleosides; D000620:Aminoimidazole Carboxamide; D018698:Glutamic Acid; C011651:acadesine; D000263:Adenylate Kinase; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2; D000103:Acetyl-CoA Carboxylase; D008274:Magnesium; D005947:Glucose