[Show abstract][Hide abstract] ABSTRACT: The liver responds to an increase in blood glucose levels in the postprandial state by uptake of glucose and conversion to glycogen. Liver glycogen synthase (GYS2), a key enzyme in glycogen synthesis, is controlled by a complex interplay between the allosteric activator glucose-6-phosphate (G6P) and reversible phosphorylation through GS kinase-3 and glycogen-associated form of protein phosphatase 1. Here we initially performed mutagenesis analysis and identified a key residue (Arg582) required for activation of GYS2 by G6P. We then employed GYS2 Arg582Ala knockin (+/R582A) mice in which G6P-mediated GYS2 activation has been profoundly impaired (60-70%), while sparing regulation through reversible phosphorylation. R582A-mutant-expressing hepatocytes showed significantly reduced glycogen synthesis with glucose and insulin or glucokinase activator, which resulted in channeling glucose/G6P towards glycolysis and lipid synthesis. GYS2(+/R582A) mice were modestly glucose intolerant and displayed significantly reduced glycogen accumulation with feeding or glucose load in vivo. These data show that G6P-mediated activation of GYS2 plays a key role in controlling glycogen synthesis and hepatic glucose-G6P flux control and thus whole-body glucose homeostasis.
[Show abstract][Hide abstract] ABSTRACT: Many pharmaceuticals used to treat cancer target the cell cycle or mitotic spindle dynamics, such as the anti-tumor drug, paclitaxel, which stabilizes microtubules. Here we show that, in cells arrested in mitosis with the spindle toxins, nocodazole, or paclitaxel, the endogenous protein phosphatase 4 (Ppp4) complex Ppp4c-R2-R3A is phosphorylated on its regulatory (R) subunits, and its activity is inhibited. The phosphorylations are blocked by roscovitine, indicating that they may be mediated by Cdk1-cyclin B. Endogenous Ppp4c is enriched at the centrosomes in the absence and presence of paclitaxel, nocodazole, or roscovitine, and the activity of endogenous Ppp4c-R2-R3A is inhibited from G 1/S to the G 2/M phase of the cell cycle. Endogenous γ-tubulin and its associated protein, γ-tubulin complex protein 2, both of which are essential for nucleation of microtubules at centrosomes, interact with the Ppp4 complex. Recombinant γ-tubulin can be phosphorylated by Cdk1-cyclin B or Brsk1 and dephosphorylated by Ppp4c-R2-R3A in vitro. The data indicate that Ppp4c-R2-R3A regulates microtubule organization at centrosomes during cell division in response to stress signals such as spindle toxins, paclitaxel, and nocodazole, and that inhibition of the Ppp4 complex may be advantageous for treatment of some cancers.
[Show abstract][Hide abstract] ABSTRACT: Abnormal regulation of brain glycogen metabolism is believed to underlie insulin-induced hypoglycaemia, which may be serious or fatal in diabetic patients on insulin therapy. A key regulator of glycogen levels is glycogen targeted protein phosphatase 1 (PP1), which dephosphorylates and activates glycogen synthase (GS) leading to an increase in glycogen synthesis. In this study, we show that the gene PPP1R3F expresses a glycogen-binding protein (R3F) of 82.8 kDa, present at the high levels in rodent brain. R3F binds to PP1 through a classical 'RVxF' binding motif and substitution of Phe39 for Ala in this motif abrogates PP1 binding. A hydrophobic domain at the carboxy-terminus of R3F has similarities to the putative membrane binding domain near the carboxy-terminus of striated muscle glycogen targeting subunit G(M)/R(GL), and R3F is shown to bind not only to glycogen but also to membranes. GS interacts with PP1-R3F and is hyperphosphorylated at glycogen synthase kinase-3 sites (Ser640 and Ser644) when bound to R3F(Phe39Ala). Deprivation of glucose or stimulation with adenosine or noradrenaline leads to an increased phosphorylation of PP1-R3F bound GS at Ser640 and Ser644 curtailing glycogen synthesis and facilitating glycogen degradation to provide glucose in astrocytoma cells. Adenosine stimulation also modulates phosphorylation of R3F at Ser14/Ser18.
Journal of Neurochemistry 06/2011; 118(4):596-610. · 4.24 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Activation of 5'-AMP-activated protein kinase (AMPK) is believed to be the mechanism by which the pharmaceuticals, metformin and phenformin, exert their beneficial effects for treatment of type 2 diabetes. These biguanide drugs elevate 5'-AMP, which allosterically activates AMPK and promotes phosphorylation on Thr172 of AMPK catalytic α subunits. Although kinases phosphorylating this site have been identified, phosphatases that dephosphorylate it are unknown. The aim of this study is to identify protein phosphatase(s) that dephosphorylate AMPKα-Thr172 within cells. Our initial data indicated that members of the protein phosphatase Mg/Mn(2+)-dependent [corrected] (PPM) family and not those of the PPP family of protein serine/threonine phosphatases may be directly or indirectly inhibited by phenformin. Using antibodies raised to individual Ppm phosphatases that facilitated the assessment of their activities, phenformin stimulation of cells was found to decrease the Mg(2+)/Mn(2+)-dependent [corrected] protein serine/threonine phosphatase activity of Ppm1E and Ppm1F, but not that attributable to other PPM family members, including Ppm1A/PP2Cα. Depletion of Ppm1E, but not Ppm1A, using lentiviral-mediated stable gene silencing, increased AMPKα-Thr172 phosphorylation approximately three fold in HEK293 cells. In addition, incubation of cells with low concentrations of phenformin and depletion of Ppm1E increased AMPK phosphorylation synergistically. Ppm1E and the closely related Ppm1F interact weakly with AMPK and assays with lysates of cells stably depleted of Ppm1F suggest [corrected] that this phosphatase contributes to dephosphorylation of AMPK. The data indicate that Ppm1E and probably PpM1F are in cellulo AMPK phosphatases and that Ppm1E is a potential anti-diabetic drug target.