Association of PP1 with Its Regulatory Subunit AKAP149 Is Regulated by Serine Phosphorylation Flanking the RVXF Motif of AKAP149†

PEPC [PEP (phosphoenolpyruvate) carboxylase] is a tightly controlled enzyme located at the core of plant C-metabolism that catalyses the irreversible β-carboxylation of PEP to form oxaloacetate and Pi. The critical role of PEPC in assimilating atmospheric CO2 during C4 and Crassulacean acid metabolism photosynthesis has been studied extensively. PEPC also fulfils a broad spectrum of non-photosynthetic functions, particularly the anaplerotic replenishment of tricarboxylic acid cycle intermediates consumed during biosynthesis and nitrogen assimilation. An impressive array of strategies has evolved to co-ordinate in vivo PEPC activity with cellular demands for C4–C6 carboxylic acids. To achieve its diverse roles and complex regulation, PEPC belongs to a small multigene family encoding several closely related PTPCs (plant-type PEPCs), along with a distantly related BTPC (bacterial-type PEPC). PTPC genes encode ~110-kDa polypeptides containing conserved serine-phosphorylation and lysine-mono-ubiquitination sites, and typically exist as homotetrameric Class-1 PEPCs. In contrast, BTPC genes encode larger ~117-kDa polypeptides owing to a unique intrinsically disordered domain that mediates BTPC's tight interaction with co-expressed PTPC subunits. This association results in the formation of unusual ~900-kDa Class-2 PEPC hetero-octameric complexes that are desensitized to allosteric effectors. BTPC is a catalytic and regulatory subunit of Class-2 PEPC that is subject to multi-site regulatory phosphorylation in vivo. The interaction between divergent PEPC polypeptides within Class-2 PEPCs adds another layer of complexity to the evolution, physiological functions and metabolic control of this essential CO2-fixing plant enzyme. The present review summarizes exciting developments concerning the functions, post-translational controls and subcellular location of plant PTPC and BTPC isoenzymes.

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Muscarinic-receptor-mediated inhibition of insulin-like growth factor-1 receptor-stimulated phosphoinositide 3-kinase signalling in 1321N1 astrocytoma cells

Biochemical Journal ◽

10.1042/bj20031700 ◽

2004 ◽

Vol 379 (3) ◽

pp. 641-651 ◽

Cited By ~ 15

Author(s):

Ian H. BATTY ◽

Ian N. FLEMING ◽

C. Peter DOWNES

Keyword(s):

Growth Factor ◽

Protein Kinase ◽

Muscarinic Receptors ◽

Kinase Activity ◽

Serine Phosphorylation ◽

Regulatory Subunit ◽

Insulin Like Growth Factor ◽

Phosphatase Inhibitor ◽

Astrocytoma Cells ◽

1321N1 Astrocytoma

In 1321N1 astrocytoma cells, stimulation of the IGF-1 (insulin-like growth factor-1) receptor increased the association of PI3K [phosphoinositide (PI) 3-kinase] activity with IRS-1 (insulin re-ceptor substrate 1), and increased the cellular concentration of PtdIns(3,4,5)P3. Carbachol, acting on M3 muscarinic receptors, inhibited insulin-, but not PDGF (platelet-derived growth factor)-, stimulated responses by ≈50%. The inhibition of IRS-1-associated PI3K activity by carbachol (i) was rapid (<1 min), persistent (≥60 min) and potent (half-maximal concentration ≈1 µM); (ii) was reproduced by stimuli for several phospholipase-C-coupled receptors; (iii) was prevented by the inhibition of protein kinase C, but not by chelation of intracellular Ca2+; and (iv) was not blocked or reproduced by inhibitors or stimuli respectively of mitogen-activated protein kinase, PI3K, protein kinase B or the mammalian target of rapamycin. However, the effects of carbachol were prevented by sodium vanadate, a protein tyrosine phosphatase inhibitor, and were accompanied by reduced insulin-stimulated IRS-1 tyrosine phosphorylation and recruitment of the 85 kDa regulatory subunit of PI3K to IRS-1, but not by reduced IGF-1 receptor kinase activity. The inhibitory effect of carbachol was reproduced by okadaic acid, a protein serine/threonine phosphatase inhibitor, but not by PDGF, yet all three agents stimulated the serine phosphorylation of IRS-1 at residues Ser312, Ser616 and Ser636/639, albeit to different extents. Thus muscarinic receptors may inhibit insulin signalling by promoting IRS-1 tyrosine dephosphorylation and/or by uncoupling IRS-1 from the stimulated IGF-1 receptor by stimulating IRS-1 serine phosphorylation. However, the proportion of IRS-1 molecules phosphorylated at a particular site or the phosphorylation of additional IRS-1 serine residues other than those noted above must be important.

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IκB kinase β (IKKβ) does not mediate feedback inhibition of the insulin signalling cascade

Biochemical Journal ◽

10.1042/bj20112037 ◽

2012 ◽

Vol 442 (3) ◽

pp. 723-732 ◽

Cited By ~ 5

Author(s):

Graeme I. Lancaster ◽

Beata Skiba ◽

Christine Yang ◽

Hayley T. Nicholls ◽

Katherine G. Langley ◽

...

Keyword(s):

Tyrosine Phosphorylation ◽

Feedback Inhibition ◽

Insulin Signalling ◽

Serine Phosphorylation ◽

Regulatory Subunit ◽

Iκb Kinase ◽

Pkb Phosphorylation ◽

Signalling Cascade

In the present study, we have examined whether IKKβ [IκB (inhibitor of nuclear factor κB) kinase β] plays a role in feedback inhibition of the insulin signalling cascade. Insulin induces the phosphorylation of IKKβ, in vitro and in vivo, and this effect is dependent on intact signalling via PI3K (phosphoinositide 3-kinase), but not PKB (protein kinase B). To test the hypothesis that insulin activates IKKβ as a means of negative feedback, we employed a variety of experimental approaches. First, pharmacological inhibition of IKKβ via BMS-345541 did not potentiate insulin-induced IRS1 (insulin receptor substrate 1) tyrosine phosphorylation, PKB phosphorylation or 2-deoxyglucose uptake in differentiated 3T3-L1 adipocytes. BMS-345541 did not prevent insulin-induced IRS1 serine phosphorylation on known IKKβ target sites. Secondly, adenovirus-mediated overexpression of wild-type IKKβ in differentiated 3T3-L1 adipocytes did not suppress insulin-stimulated 2-deoxyglucose uptake, IRS1 tyrosine phosphorylation, IRS1 association with the p85 regulatory subunit of PI3K or PKB phosphorylation. Thirdly, insulin signalling was not potentiated in mouse embryonic fibroblasts lacking IKKβ. Finally, insulin treatment of 3T3-L1 adipocytes did not promote the recruitment of IKKβ to IRS1, supporting our findings that IKKβ, although activated by insulin, does not promote direct serine phosphorylation of IRS1 and does not contribute to the feedback inhibition of the insulin signalling cascade.

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Insulin Resistance in Human Preeclamptic Placenta Is Mediated by Serine Phosphorylation of Insulin Receptor Substrate-1 and -2

The Journal of Clinical Endocrinology & Metabolism ◽

10.1210/jc.2005-1965 ◽

2006 ◽

Vol 91 (2) ◽

pp. 709-717 ◽

Cited By ~ 53

Author(s):

Marco Scioscia ◽

Khalid Gumaa ◽

Sirilaksana Kunjara ◽

Malcolm A. Paine ◽

Luigi E. Selvaggi ◽

...

Keyword(s):

Insulin Resistance ◽

Insulin Receptor ◽

Insulin Signaling ◽

Serine Phosphorylation ◽

Regulatory Subunit ◽

Insulin Receptor Substrate ◽

Insulin Receptor Substrate 1 ◽

Cross Sectional ◽

Maternal Risk ◽

Therapeutic Implications

Context: Preeclampsia is a severe complication of human pregnancy often associated with maternal risk factors. Insulin resistance represents a major risk for developing preeclampsia during pregnancy. Objective: A putative second messenger of insulin, inositol phosphoglycan P type (P-IPG), was previously shown to be highly increased during active preeclampsia. Its association with insulin resistance was investigated. Design and Setting: A cross-sectional study was carried out in a referral center. Patients: Nine preeclamptic (PE) and 18 healthy women were recruited and matched for maternal age, body mass index, parity, and ethnicity in a 1:2 ratio. Placental specimens were collected immediately after delivery. Intervention: Placental tissue was incubated with insulin and P-IPG production assessed. Insulin signaling proteins were subsequently studied by immunoblotting. Results: P-IPG extracted from human term placentas upon incubation with insulin was found to be far lower in those with preeclampsia than controls (P < 0.001). Immunoblotting studies revealed serine phosphorylation of insulin receptor substrate-1 and -2 in PE placentas (P < 0.001) with downstream impairment of insulin signaling. The activation of the p85 regulatory subunit of phosphatidylinositol 3- kinase was markedly decreased in PE samples (P < 0.001). Conclusions: These findings highlight the importance of P-IPG in active preeclampsia and demonstrate a substantially different response to the insulin stimulus of human PE placentas. Acquired alterations in activation of proteins involved in insulin signaling may play a role in the complex pathogenesis of preeclampsia, probably as a consequence of the immunological dysfunction that occurs in this syndrome. These results seem to confirm an insulin-resistant state in PE placenta and shed a different light on its role in the pathogenesis of this disease with potential therapeutic implications.

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Prolonged exposure to GH impairs insulin signaling in the heart

Journal of Molecular Endocrinology ◽

10.1530/jme-11-0066 ◽

2011 ◽

Vol 47 (2) ◽

pp. 167-177 ◽

Cited By ~ 13

Author(s):

J G Miquet ◽

J F Giani ◽

C S Martinez ◽

M C Muñoz ◽

L González ◽

...

Keyword(s):

Transgenic Mice ◽

Insulin Signaling ◽

Glucose Transporter ◽

Prolonged Exposure ◽

Interstitial Fibrosis ◽

Total Content ◽

Direct Consequence ◽

Insulin Injection ◽

Serine Phosphorylation ◽

Regulatory Subunit

Acromegaly is associated with cardiac hypertrophy, which is believed to be a direct consequence of chronically elevated GH and IGF1. Given that insulin is important for cardiac growth and function, and considering that GH excess induces hyperinsulinemia, insulin resistance, and cardiac alterations, it is of interest to study insulin sensitivity in this tissue under chronic conditions of elevated GH. Transgenic mice overexpressing GH present cardiomegaly and perivascular and interstitial fibrosis in the heart. Mice received an insulin injection, the heart was removed after 2 min, and immunoblotting assays of tissue extracts were performed to evaluate the activation and abundance of insulin-signaling mediators. Insulin-induced tyrosine phosphorylation of the insulin receptor (IR) was conserved in transgenic mice, but the phosphorylation of IR substrate 1 (IRS1), its association with the regulatory subunit of the phosphatidylinositol 3-kinase (PI3K), and the phosphorylation of AKT were decreased. In addition, total content of the glucose transporter GLUT4 was reduced in transgenic mice. Insulin failed to induce the phosphorylation of the mammalian target of rapamycin (mTOR). However, transgenic mice displayed increased basal activation of the IR/IRS1/PI3K/AKT/mTOR and p38 signaling pathways along with higher serine phosphorylation of IRS1, which is recognized as an inhibitory modification. We conclude that GH-overexpressing mice exhibit basal activation of insulin signaling but decreased sensitivity to acute insulin stimulation at several signaling steps downstream of the IR in the heart. These alterations may be associated with the cardiac pathology observed in these animals.

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Sequential phosphorylation of insulin receptor substrate-2 by glycogen synthase kinase-3 and c-Jun NH2-terminal kinase plays a role in hepatic insulin signaling

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00534.2007 ◽

2008 ◽

Vol 294 (2) ◽

pp. E307-E315 ◽

Cited By ~ 37

Author(s):

Hadar Sharfi ◽

Hagit Eldar-Finkelman

Keyword(s):

Insulin Receptor ◽

Tyrosine Phosphorylation ◽

Insulin Signaling ◽

Glycogen Synthase ◽

Serine Phosphorylation ◽

Glycogen Synthase Kinase ◽

Regulatory Subunit ◽

Insulin Receptor Substrate ◽

Phosphorylation Sites ◽

Hepatic Insulin Signaling

Serine phosphorylation of insulin receptor substrate (IRS) proteins is a potential inhibitory mechanism in insulin signaling. Here we show that IRS-2 is phosphorylated by glycogen synthase kinase (GSK)-3. Phosphorylation by GSK-3 requires prior phosphorylation of its substrates, prompting us to identify the “priming kinase.” It was found that the stress activator anisomycin enhanced the ability of GSK-3 to phosphorylate IRS-2. Use of a selective c-Jun NH2-terminal kinase (JNK) inhibitor and cells overexpressing JNK implicated JNK as the priming kinase. This allowed us to narrow down the number of potential GSK-3 phosphorylation sites within IRS-2 to four regions that follow the motif SXXXSP. IRS-2 deletion mutants enabled us to localize the GSK-3 and JNK phosphorylation sites to serines 484 and 488, respectively. Mutation at serine 488 reduced JNK phosphorylation of IRS-2, and mutation of each site separately abolished GSK-3 phosphorylation of IRS-2. Treatment of H4IIE liver cells with anisomycin inhibited insulin-induced tyrosine phosphorylation of IRS-2; inhibition was reversed by pretreatment with the JNK and GSK-3 inhibitors. Moreover, overexpression of JNK and GSK-3 in H4IIE cells reduced insulin-induced tyrosine phosphorylation of IRS-2 and its association with the p85 regulatory subunit of phosphatidylinositol 3-kinase. Finally, both GSK-3 and JNK are abnormally upregulated in the diabetic livers of ob/obmice. Together, our data indicate that IRS-2 is sequentially phosphorylated by JNK and GSK-3 at serines 484/488 and provide evidence for their inhibitory role in hepatic insulin signaling.

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