Further Evidence for the Regulation of Acetyl-CoA Carboxylase Activity by a Glutamate- and Magnesium-Activated Protein Phosphatase in the Pancreatic β Cell: Defective Regulation in the Diabetic GK Rat Islet

Abstract

We previously identified and characterized a glutamate- and magnesium-sensitive PP2A-like phosphatase (GAPP), which dephosphorylated and activated acetyl-CoA carboxylase (ACC) in the islet beta cell. Herein, we studied potential regulatory mechanisms by which GAPP is activated by glutamate and magnesium, and also quantitated the degree of activation, by glutamate- and magnesium, of ACC in normal rat islets and islets derived from the diabetic Goto-Kakizaki (GK) rat, a model for type 2 diabetes in humans. Our findings indicate that magnesium, but not glutamate, specifically activates the post-translational carboxylmethylation (CML) of the 36 kDa catalytic subunit of GAPP. Okadaic acid (OKA), which inhibits GAPP-mediated activation of ACC, also reduced the magnesium-stimulated CML of the catalytic subunit of GAPP in all the beta cell preparations studied. These data suggest that the CML step may be necessary for magnesium- and glutamate-mediated activation of ACC. We also observed a marked attenuation in magnesium- and glutamate-facilitated activation of ACC activity in islets derived from the GK rat. Together, our findings raise an interesting possibility that inhibition of GAPP-catalyzed inactivation of ACC (and subsequent reduction in the generation of long-chain fatty acids) could contribute toward the abnormalities in insulin secretion demonstrable in this animal model for type 2 diabetes.

Agonist-modulated Regulation of AMP-activated Protein Kinase (AMPK) in Endothelial Cells

“…Interestingly, the increase in AMPK phosphorylation seen with caveolin-1 knockdown appeared to effectively "uncouple" VEGF-and S1P-stimulated AMPK phosphorylation, suggesting that abrogation of the inhibitory effect of caveolin-1 on AMPK phosphorylation with caveolin-1 siRNA might eclipse the otherwise stimulatory effect of these receptor-dependent AMPK agonists. We note that other kinases, such as protein kinase A (57), as well as phosphoprotein phosphatases (58) have been implicated in ACC regulation. It is plausible that these other mechanisms may play a role in enhancing ACC activity when AMPK phosphorylation is stimulated following caveolin-1 knockdown.…”

Localization of a nuclear serine/threonine protein phosphatase in insulin-secreting INS-1 cells: potential regulation by IL-1β

“…They include: (i) a cytosolic glutamate and magnesium-sensitive PP2A-like activity, which we have shown to mediate the dephosphorylation and activation of acetyl CoA carboxylase in the β cell [45,46]; (ii) a mitochondrial ceramide-activated protein phosphatase, which appears to dephosphorylate and inactivate pro-apoptotic proteins, such as Bcl-2 [47,48]; (iii) a PP2A-like activity that we characterized in islet lysates which is inhibited by glycolytic and tricarboxylic acid cycle intermediates; we speculate that this form of PP2A is required for glucose-stimulated insulin secretion [16]; and (iv) the currently identified nuclear PP4, which could play regulatory roles in IL-1β-mediated metabolic dysfunction and demise. Studies are in progress to determine the precise identity and the subunit composition of these phosphatases to further decipher their novel roles in the β cell function.…”

Depletion of the catalytic subunit of protein phosphatase-2A (PP2Ac) markedly attenuates glucose-stimulated insulin secretion in pancreatic β-cells

“…Future studies will need to conclusively identify the A and B subunits of the PP2A, as well as the locus (or loci) of action PP2A which regulate GSIS. Lastly, we believe that our current demonstration of importance of PP2Ac should provide basis for future work to understand the nature of and the mechanisms underlying the defects in OKA-sensitive phosphatase signaling steps that we recently reported in islets from the Goto-Kakizaki rat, a model for spontaneous type 2 diabetes [18].…”

“…Pancreatic islets from male Sprague-Dawley rats (200-250 g body wt; Harlan Laboratories) were isolated by collagenase digestion method as we described in [18,19].…”