Arsenic disrupts neuronal insulin signaling through increasing free PI3K-p85 and decreasing PI3K activity

Abstract TNFα, which activates three different MAPKs [ERK, p38, and jun amino terminal kinase (JNK)], also induces insulin resistance. To better understand the respective roles of these three MAPK pathways in insulin signaling and their contribution to insulin resistance, constitutively active MAPK/ERK kinase (MEK)1, MAPK kinase (MKK6), and MKK7 mutants were overexpressed in 3T3-L1 adipocytes using an adenovirus-mediated transfection procedure. The MEK1 mutant, which activates ERK, markedly down-regulated expression of the insulin receptor (IR) and its major substrates, IRS-1 and IRS-2, mRNA and protein, and in turn reduced tyrosine phosphorylation of IR as well as IRS-1 and IRS-2 and their associated phosphatidyl inositol 3-kinase (PI3K) activity. The MKK6 mutant, which activates p38, moderately inhibited IRS-1 and IRS-2 expressions and IRS-1-associated PI3K activity without exerting a significant effect on the IR. Finally, the MKK7 mutant, which activates JNK, reduced tyrosine phosphorylation of IRS-1 and IRS-2 and IRS-associated PI3K activity without affecting expression of the IR, IRS-1, or IRS-2. In the context of our earlier report showing down-regulation of glucose transporter 4 by MEK1-ERK and MKK6/3-p38, the present findings suggest that chronic activation of ERK, p38, or JNK can induce insulin resistance by affecting glucose transporter expression and insulin signaling, though via distinctly different mechanisms. The contribution of ERK is, however, the strongest.

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Protein Kinase C-ζ Phosphorylates Insulin Receptor Substrate-1, -3, and -4 But Not -2: Isoform Specific Determinants of Specificity in Insulin Signaling

Endocrinology ◽

10.1210/en.2007-1595 ◽

2008 ◽

Vol 149 (5) ◽

pp. 2451-2458 ◽

Cited By ~ 23

Author(s):

Sihoon Lee ◽

Edward G. Lynn ◽

Jeong-a Kim ◽

Michael J. Quon

Keyword(s):

Protein Kinase C ◽

Protein Kinase ◽

Insulin Receptor ◽

Negative Feedback ◽

Insulin Signaling ◽

Kinase Assay ◽

Insulin Receptor Substrate ◽

Kinase C ◽

Pi3k Activity ◽

Pkc Ζ

Protein kinase C-ζ, a downstream effector of phosphatidylinositol 3-kinase (PI3K), phosphorylates insulin receptor substrate (IRS)-1 on serine residues impairing activation of PI3K in response to insulin. Because IRS-1 is upstream from PI3K, this represents a negative feedback mechanism that may contribute to signal specificity in insulin action. To determine whether similar feedback pathways exist for other IRS isoforms, we evaluated IRS-2, -3, and -4 as substrates for PKC-ζ. In an in vitro kinase assay, purified recombinant PKC-ζ phosphorylated IRS-1, -3 and -4 but not IRS-2. Similar results were obtained with an immune-complex kinase assay demonstrating that wild-type, but not kinase-deficient mutant PKC-ζ, phosphorylated IRS-1, -3, and -4 but not IRS-2. We evaluated functional consequences of serine phosphorylation of IRS isoforms by PKC-ζ in NIH-3T3IR cells cotransfected with epitope-tagged IRS proteins and either PKC-ζ or empty vector control. Insulin-stimulated IRS tyrosine phosphorylation was impaired by overepxression of PKC-ζ for IRS-1, -3, and -4 but not IRS-2. Significant insulin-stimulated increases in PI3K activity was coimmunoprecipitated with all IRS isoforms. In cells overexpressing PKC-ζ there was marked inhibition of insulin-stimulated PI3K activity associated with IRS-1, -3 and -4 but not IRS-2. That is, PI3K activity associated with IRS-2 in response to insulin was similar in control cells and cells overexpressing PKC-ζ. We conclude that IRS-3 and -4 are novel substrates for PKC-ζ that may participate in a negative feedback pathway for insulin signaling similar to IRS-1. The inability of PKC-ζ to phosphorylate IRS-2 may help determine specific functional roles for IRS-2.

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Acidosis impairs insulin receptor substrate-1-associated phosphoinositide 3-kinase signaling in muscle cells: consequences on proteolysis

AJP Renal Physiology ◽

10.1152/ajprenal.00440.2003 ◽

2004 ◽

Vol 287 (4) ◽

pp. F700-F706 ◽

Cited By ~ 51

Author(s):

Harold A. Franch ◽

Sina Raissi ◽

Xiaonan Wang ◽

Bin Zheng ◽

James L. Bailey ◽

...

Keyword(s):

Insulin Receptor ◽

Protein Degradation ◽

Insulin Signaling ◽

Muscle Cells ◽

Insulin Receptor Substrate ◽

Insulin Receptor Substrate 1 ◽

Pi3k Activity ◽

Tnf Α ◽

Phosphoinositide 3 Kinase ◽

In Cells

Chronic acidosis is a stimulus for proteolysis in muscle in vivo, but the mechanism of this response is unknown. We tested the hypothesis that acidosis or TNF-α, a cytokine whose production increases in acidosis, regulates proteolysis by inhibiting insulin signaling through phosphoinositide 3-kinase (PI3K). In cultured L6 myotubes, acidified (pH 7.1) media did not accelerate the basal protein degradation rate, but it inhibited insulin’s ability to suppress proteolysis. Insulin receptor substrate-1 (IRS-1)-associated PI3K activity was not altered in cells acidified for 10 min but was strongly inhibited in cells incubated at pH 7.1 for 24 h. Phosphorylation of Akt was also suppressed by acidification for 24 h. Acidification did not induce changes in IRS-1 abundance, insulin-stimulated IRS-1 tyrosine phosphorylation, or the amount of PI3K p85 regulatory subunit. In contrast to acidification, TNF-α suppressed proteolysis in the presence or absence of insulin but had no effect on IRS-1-associated PI3K activity. To establish that the PI3K pathway can regulate protein degradation in muscle, we measured proteolysis in cells after inhibition of PI3K activity with LY-294002 or infection with an adenovirus encoding a dominant negative PI3K p85α-subunit. Both approaches inhibited insulin-induced suppression of proteolysis to a degree similar to that seen with acidification. We conclude that acidosis accelerates protein degradation by impairing insulin signaling through PI3K in muscle cells.

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Selective PPARγ modulator INT131 normalizes insulin signaling defects and improves bone mass in diet-induced obese mice

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00569.2011 ◽

2012 ◽

Vol 302 (5) ◽

pp. E552-E560 ◽

Cited By ~ 29

Author(s):

Dae Ho Lee ◽

Hu Huang ◽

Kangduk Choi ◽

Christos Mantzoros ◽

Young-Bum Kim

Keyword(s):

Type 2 Diabetes ◽

Bone Mass ◽

Insulin Signaling ◽

Tolerance Test ◽

Peroxisome Proliferator ◽

Mineral Density ◽

Peroxisome Proliferator Activated Receptor ◽

Insulin Resistant ◽

Pi3k Activity

INT131 is a potent non-thiazolidinedione (TZD)-selective peroxisome proliferator-activated receptor-γ modulator being developed for the treatment of type 2 diabetes. In preclinical studies and a phase II clinical trial, INT131 has been shown to lower glucose levels and ameliorate insulin resistance without typical TZD side effects. To determine whether the insulin-sensitizing action of INT131 is mediated by effects on insulin-mediated glucose homeostasis and insulin signaling, high-fat diet-induced obese (DIO) insulin-resistant mice treated with INT131 were studied. INT131's effects on bone density were also investigated. Treatment with INT131 enhanced systemic insulin sensitivity, as revealed by lower insulin levels in the fasted state and an increase in the area above the curve during an insulin tolerance test. These effects were independent of changes in adiposity. Insulin-stimulated PI3K activity in skeletal muscle and adipose tissue of DIO mice was significantly reduced ∼50–65%, but this was restored completely by INT131 therapy. The INT131 effects on PI3K activity are most likely due to increased IRS-1 tyrosine phosphorylation. Concurrently, insulin-mediated Akt phosphorylation also increased after INT131 treatment in DIO mice. Importantly, INT131 therapy caused a significant increase in bone mineral density without alteration in circulating osteocalcin in these mice. These data suggest that a newly developed insulin-sensitizing agent, INT131, normalizes obesity-related defects in insulin action on PI3K signaling in insulin target tissues by a mechanism involved in glycemic control. If these data are confirmed in humans, INT131 could be used for treating type 2 diabetes without loss in bone mass.

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Folate and vitamin B-12 deficiencies additively impaired memory function and disturbed the gut microbiota in amyloid-β infused rats

International Journal for Vitamin and Nutrition Research ◽

10.1024/0300-9831/a000624 ◽

2019 ◽

pp. 1-13 ◽

Cited By ~ 2

Author(s):

Sunmin Park ◽

Sunna Kang ◽

Da Sol Kim

Keyword(s):

Insulin Resistance ◽

Gut Microbiota ◽

Insulin Signaling ◽

Gut Microbiome ◽

Metabolic Diseases ◽

Memory Function ◽

Amyloid Β ◽

Impaired Memory ◽

Ca1 Region ◽

Folate And Vitamin B12

Abstract. Folate and vitamin B12(V-B12) deficiencies are associated with metabolic diseases that may impair memory function. We hypothesized that folate and V-B12 may differently alter mild cognitive impairment, glucose metabolism, and inflammation by modulating the gut microbiome in rats with Alzheimer’s disease (AD)-like dementia. The hypothesis was examined in hippocampal amyloid-β infused rats, and its mechanism was explored. Rats that received an amyloid-β(25–35) infusion into the CA1 region of the hippocampus were fed either control(2.5 mg folate plus 25 μg V-B12/kg diet; AD-CON, n = 10), no folate(0 folate plus 25 μg V-B12/kg diet; AD-FA, n = 10), no V-B12(2.5 mg folate plus 0 μg V-B12/kg diet; AD-V-B12, n = 10), or no folate plus no V-B12(0 mg folate plus 0 μg V-B12/kg diet; AD-FAB12, n = 10) in high-fat diets for 8 weeks. AD-FA and AD-VB12 exacerbated bone mineral loss in the lumbar spine and femur whereas AD-FA lowered lean body mass in the hip compared to AD-CON(P < 0.05). Only AD-FAB12 exacerbated memory impairment by 1.3 and 1.4 folds, respectively, as measured by passive avoidance and water maze tests, compared to AD-CON(P < 0.01). Hippocampal insulin signaling and neuroinflammation were attenuated in AD-CON compared to Non-AD-CON. AD-FAB12 impaired the signaling (pAkt→pGSK-3β) and serum TNF-α and IL-1β levels the most among all groups. AD-CON decreased glucose tolerance by increasing insulin resistance compared to Non-AD-CON. AD-VB12 and AD-FAB12 increased insulin resistance by 1.2 and 1.3 folds, respectively, compared to the AD-CON. AD-CON and Non-AD-CON had a separate communities of gut microbiota. The relative counts of Bacteroidia were lower and those of Clostridia were higher in AD-CON than Non-AD-CON. AD-FA, but not V-B12, separated the gut microbiome community compared to AD-CON and AD-VB12(P = 0.009). In conclusion, folate and B-12 deficiencies impaired memory function by impairing hippocampal insulin signaling and gut microbiota in AD rats.

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