Attenuated Akt Phosphorylation Research Articles

Cellular Insulin Resistance Disrupts Leptin-Mediated Control of Neuronal Signaling and Transcription

Central resistance to the actions of insulin and leptin is associated with the onset of obesity and type 2 diabetes mellitus, whereas leptin and insulin signaling is essential for both glucose and energy homeostasis. Although it is known that leptin resistance can lead to attenuated insulin signaling, whether insulin resistance can lead to or exacerbate leptin resistance is unknown. To investigate the molecular events underlying crosstalk between these signaling pathways, immortalized hypothalamic neuronal models, rHypoE-19 and mHypoA-2/10, were used. Prolonged insulin exposure was used to induce cellular insulin resistance, and thereafter leptin-mediated regulation of signal transduction and gene expression was assessed. Leptin directly repressed agouti-related peptide mRNA levels but induced urocortin-2, insulin receptor substrate (IRS)-1, IRS2, and IR transcription, through leptin-mediated phosphatidylinositol 3-kinase/Akt activation. Neuronal insulin resistance, as assessed by attenuated Akt phosphorylation, blocked leptin-mediated signal transduction and agouti-related peptide, urocortin-2, IRS1, IRS2, and insulin receptor synthesis. Insulin resistance caused a substantial decrease in insulin receptor protein levels, forkhead box protein 1 phosphorylation, and an increase in suppressor of cytokine signaling 3 protein levels. Cellular insulin resistance may cause or exacerbate neuronal leptin resistance and, by extension, obesity. It is essential to unravel the effects of neuronal insulin resistance given that both peripheral, as well as the less widely studied central insulin resistance, may contribute to the development of metabolic, reproductive, and cardiovascular disorders. This study provides improved understanding of the complex cellular crosstalk between insulin-leptin signal transduction that is disrupted during neuronal insulin resistance.

Disruption of the Selenocysteine Lyase-Mediated Selenium Recycling Pathway Leads to Metabolic Syndrome in Mice

Selenium (Se) is an essential trace element used for biosynthesis of selenoproteins and is acquired either through diet or cellular recycling mechanisms. Selenocysteine lyase (Scly) is the enzyme that supplies Se for selenoprotein biosynthesis via decomposition of the amino acid selenocysteine (Sec). Knockout (KO) of Scly in a mouse affected hepatic glucose and lipid homeostasis. Mice lacking Scly and raised on an Se-adequate diet exhibit hyperinsulinemia, hyperleptinemia, glucose intolerance, and hepatic steatosis, with increased hepatic oxidative stress, but maintain selenoprotein levels and circulating Se status. Insulin challenge of Scly KO mice results in attenuated Akt phosphorylation but does not decrease phosphorylation levels of AMP kinase alpha (AMPKα). Upon dietary Se restriction, Scly KO animals develop several characteristics of metabolic syndrome, such as obesity, fatty liver, and hypercholesterolemia, with aggravated hyperleptinemia, hyperinsulinemia, and glucose intolerance. Hepatic glutathione peroxidase 1 (GPx1) and selenoprotein S (SelS) production and circulating selenoprotein P (Sepp1) levels are significantly diminished. Scly disruption increases the levels of insulin-signaling inhibitor PTP1B. Our results suggest a dependence of glucose and lipid homeostasis on Scly activity. These findings connect Se and energy metabolism and demonstrate for the first time a unique physiological role of Scly in an animal model.

Mammalian Target of Rapamycin Complex 2 (mTORC2) Negatively Regulates Toll-like Receptor 4-mediated Inflammatory Response via FoxO1

Activation of the PI3K pathway plays a pivotal role in regulating the inflammatory response. The loss of mTORC2 has been shown to abrogate the activation of Akt, a critical downstream component of PI3K signaling. However, the biological importance of mTORC2 in innate immunity is currently unknown. Here we demonstrate that rictor, a key component of mTORC2, plays a critical role in controlling the innate inflammatory response via its ability to regulate FoxO1. Upon LPS stimulation, both rictor-deficient mouse embryonic fibroblasts (MEFs) and rictor knockdown dendritic cells exhibited a hyperinflammatory phenotype. The hyperinflammatory phenotype was due to a defective Akt signaling axis, because both rictor-deficient MEFs and rictor knockdown dendritic cells exhibited attenuated Akt phosphorylation and kinase activity. Analysis of downstream Akt targets revealed that phosphorylation of FoxO1 was impaired in rictor-deficient cells, resulting in elevated nuclear FoxO1 levels and diminished nuclear export of FoxO1 upon LPS stimulation. Knockdown of FoxO1 attenuated the hyperinflammatory phenotype exhibited by rictor-deficient MEFs. Moreover, FoxO1 deletion in dendritic cells attenuated the capacity of LPS to induce inflammatory cytokine expression. These findings identify a novel signaling pathway by which mTORC2 regulates the TLR-mediated inflammatory response through its ability to regulate FoxO1.

Guggulsterone induces heme oxygenase-1 expression through activation of Nrf2 in human mammary epithelial cells: PTEN as a putative target

Guggulsterone (GS) [4,17(20)-pregnadiene-3,16-dione] is a phytosterol found in the gum resin of the Commiphora mukul. GS exists naturally in two stereoisomers: E-GS (cis-GS) and Z-GS (trans-GS). In this study, the effects of both isomers on expression of the cytoprotective enzyme heme oxygenase-1 (HO-1) were evaluated in human mammary epithelial (MCF10A) cells. NF-E2-related factor 2 (Nrf2) is considered a master regulator in activating antioxidant response element (ARE)-driven expression of HO-1 and many other antioxidant/cytoprotective proteins. cis-GS upregulated the transcription and protein expression of HO-1 to a greater extent than did trans-GS. cis-GS treatment enhanced nuclear translocation and ARE-binding activity of Nrf2. MCF10A cells transfected with an ARE luciferase construct exhibited significantly elevated Nrf2 transcriptional activity upon cis-GS treatment compared with cells transfected with the control vector. In addition, silencing of the Nrf2 gene abrogated cis-GS-induced expression of HO-1. Incubation of MCF10A cells with cis-GS increased phosphorylation of Akt. The pharmacological inhibition of phosphoinositide 3-kinase (PI3K), an upstream kinase responsible for Akt phosphorylation, abrogated cis-GS-induced Nrf2 nuclear translocation. Pretreatment with the thiol-reducing agents attenuated Akt phosphorylation, Nrf2 activation and HO-1 expression, suggesting that cis-GS may cause thiol modification of an upstream signaling modulator. Phosphatase and Tensin Homologue Deleted on Chromosome 10 (PTEN) is a negative regulator of the PI3K-Akt axis. The mutation in cysteine 124 present in the catalytic domain of PTEN abolished cis-GS-induced HO-1 expression as well as Akt phosphorylation. Whether this cysteine is a 'bona fide' target of cis-GS in its activation of Nrf2 needs additional investigation.

NOD1 activation induces proinflammatory gene expression and insulin resistance in 3T3‐L1 adipocytes

Chronic inflammation is associated with obesity and insulin resistance. However, the underlying mechanisms are not fully understood. Pattern recognition receptors (PRRs) play critical roles in innate immune response. Here we report a role of Nucleotide‐binding Oligomerization Domain protein 1 (NOD1), a member of PRR, in adipocyte inflammation and insulin resistance. mRNA level of NOD1 is increased by 4 fold in differentiated 3T3‐L1 adipocytes compared with preadipocytes. Stimulation of NOD1 with a synthetic ligand Tri‐DAP induces proinflammatory chemokine MCP‐1, RANTES and cytokine TNF‐α and MIP‐2 mRNA expression and protein secretion. Moreover, stimulation of NOD1 with Tri‐DAP suppresses insulin signaling as revealed by attenuated Akt phosphorylation on Ser 473 and Thr 308 and subsequent insulin‐induced glucose uptake in 3T3‐L1 adipocytes. In addition, NOD1 mRNA is also increased upon differentiation of primary culture derived from human fat. Stimulation of NOD1 induces similar proinflammatory chemokine/cytokine mRNA expression and protein secretion. Together, our results suggest that NOD1 may be involved in adipose inflammation and insulin resistance. The work is supported by The University of Tennessee faculty start‐up fund and WHNRC USDA/ARS program funds.

Regulation of FoxO1 by mTORC2 negatively controls Toll-like receptor responses (111.24)

Abstract Activation of the Akt pathway plays a pivotal role in regulating the inflammatory response. Rictor is a component of the mTORC2 complex, and its loss has been shown to abrogate the phosphorylation of Akt at serine 473. However, the biological importance of mTORC2 in innate immunity is currently unknown. Here we demonstrate that rictor plays a critical role in TLR-signaling via its ability to regulate FoxO1. Genetic deletion or siRNA-mediated knockdown of rictor enhanced the production of pro-inflammatory cytokines upon LPS-stimulation, whereas the levels of the anti-inflammatory cytokine IL-10 were suppressed. The hyper-inflammatory phenotype was due to a defective Akt signaling axis, since rictor-deficient cells or rictor knockdown DC exhibited attenuated Akt phosphorylation and kinase activity, whereas complementation with a constitutively active Akt ablated the enhanced inflammatory response. Analysis of downstream targets showed phosphorylation of FoxO1 was impaired in rictor-deficient or rictor knockdown cells. The loss of rictor resulted in elevated levels of nuclear FoxO1 and an inability to export FoxO1 into the cytoplasm upon TLR-stimulation, an event that was restored upon complementation with a constitutively active Akt. FoxO1 deletion in DC resulted in an attenuated TLR mediated inflammatory response in which rictor knockdown was unable to rescue. These findings identify a novel signaling pathway by which mTORC2 regulates the TLR-mediated inflammatory response

Knockdown of STEAP4 inhibits insulin-stimulated glucose transport and GLUT4 translocation via attenuated phosphorylation of Akt, independent of the effects of EEA1

The aim of this study was to investigate whether the early endosome antigen1 (EEA1) and/or PI3K pathway is involved in the molecular mechanisms underlying the effects of the six-transmembrane protein of prostate 4 (STEAP4; also called STAMP2 and TIARP) on the insulin sensitivity of human adipocytes. Our data demonstrated that siRNA-mediated STEAP4 deficiency significantly decreased insulin-stimulated glucose transport in mature human adipocytes by decreasing GLUT4 translocation to the plasma membrane through attenuated Akt phosphorylation. We further found that EEA1 may not be involved in the mechanisms underlying the effects of STEAP4 on insulin-stimulated glucose uptake and GLUT4 translocation, as indicated by the results that i) STEAP4 does not alter the effects of EEA1 on insulin-stimulated glucose uptake and GLUT4 translocation; ii) STEAP4 does not modify the expression of EEA1 protein; and iii) STEAP4 does not interact with EEA1 according to FRET analysis. In conclusion, this study revealed that the knockdown of STEAP4 inhibits insulin-stimulated glucose transport and GLUT4 translocation via the attenuated phosphorylation of Akt, independent of the effects of EEA1.

Paeonol Oxime Inhibits bFGF-Induced Angiogenesis and Reduces VEGF Levels in Fibrosarcoma Cells

BackgroundWe previously reported the anti-angiogenic activity of paeonol isolated from Moutan Cortex. In the present study, we investigated the negative effect of paeonol oxime (PO, a paeonol derivative) on basic fibroblast growth factor (bFGF)-mediated angiogenesis in human umbilical vein endothelial cells (HUVECs) (including tumor angiogenesis) and pro-survival activity in HT-1080 fibrosarcoma cell line.Methodology/Principal FindingsWe showed that PO (IC50 = 17.3 µg/ml) significantly inhibited bFGF-induced cell proliferation, which was achieved with higher concentrations of paeonol (IC50 over 200 µg). The treatment with PO blocked bFGF-stimulated migration and in vitro capillary differentiation (tube formation) in a dose-dependent manner. Furthermore, PO was able to disrupt neovascularization in vivo. Interestingly, PO (25 µg/ml) decreased the cell viability of HT-1080 fibrosarcoma cells but not that of HUVECs. The treatment with PO at 12.5 µg/ml reduced the levels of phosphorylated AKT and VEGF expression (intracellular and extracelluar) in HT-1080 cells. Consistently, immunefluorescence imaging analysis revealed that PO treatment attenuated AKT phosphorylation in HT-1080 cells.Conclusions/SignificanceTaken together, these results suggest that PO inhibits bFGF-induced angiogenesis in HUVECs and decreased the levels of PI3K, phospho-AKT and VEGF in HT-1080 cells.

Troglitazone inhibits cell proliferation by attenuation of epidermal growth factor receptor signaling independent of peroxisome proliferator-activated receptor γ

Peroxisome proliferator-activated receptors (PPAR) belong to the nuclear hormone receptor superfamily of ligand-dependent transcription factors. Recent results have shown that agonists of PPARgamma, such as troglitazone (TGZ), can inhibit cell proliferation and promote cell differentiation independent of PPARgamma. In the present study, we provide evidence that TGZ may bind directly to EGFR and trigger its signaling and internalization independent of PPARgamma. Detailed studies revealed that prolonged incubation with TGZ effectively attenuated EGFR signaling by targeting the receptor to the endo-lysosomal degradation machinery. Although the extracellular signal-regulated kinase-signaling pathway was transiently activated by TGZ in EGFR overexpressing cancer cells, inhibition of EGF-induced Akt phosphorylation most likely accounted for the growth arrest of tumor cells caused by TGZ at pharmacologically achievable concentrations. Therefore, we have provided a new line of evidence indicating that TGZ inhibits cell proliferation by promoting EGFR degradation and attenuating Akt phosphorylation.

Role of Integrin-linked Kinase in Regulating Phosphorylation of Akt and Fibroblast Survival in Type I Collagen Matrices through a β1 Integrin Viability Signaling Pathway

A beta1 integrin phosphatidylinositol 3-kinase/Akt pathway regulates fibroblast survival in collagen matrices. When fibroblasts attach to collagen, Akt becomes phosphorylated, providing a survival signal. In contrast, in response to mechanical forces generated during collagen contraction, Akt is dephosphorylated and fibroblasts undergo apoptosis. The kinase(s) responsible for regulating Akt phosphorylation in response to matrix-derived mechanical signals are unclear. Integrin-linked kinase (ILK) is associated with the beta1 integrin in the focal adhesion complex and as such is a candidate kinase that may regulate Akt phosphorylation and fibroblast viability. Nevertheless, there is no direct evidence that matrix-derived mechanical forces regulate cell viability by modulating ILK activity. Here, we show that ILK activity decreased in response to collagen matrix contraction, which correlated with Akt dephosphorylation and induction of fibroblast apoptosis. In contrast, enforced activation of beta1 integrin by activating antibody preserved ILK and Akt activity during collagen matrix contraction, and this is associated with protection from collagen contraction-induced apoptosis. Knock-down of ILK by small, interfering RNA (siRNA) attenuated Akt phosphorylation in response to ligation of beta1 integrin by collagen or activating antibody and enhanced fibroblast apoptosis in response to collagen contraction. Kinase dead ILK attenuated Akt phosphorylation and enhanced fibroblast apoptosis, whereas hyperactive and wild type ILK augmented Akt phosphorylation and protected fibroblasts from apoptosis. Constitutively active Akt preserved Akt activity and rescued ILK siRNA-treated fibroblasts from collagen contraction-induced apoptosis. These data establish that matrix-derived mechanical forces sensed by beta1 integrin are capable of modulating ILK activity which regulates fibroblast viability via an Akt-dependent mechanism.

PKB/AKT and ERK regulation of caspase-mediated apoptosis by methylseleninic acid in LNCaP prostate cancer cells

Methylselenol has been implicated as an active metabolite for the anticancer effect of selenium in part through the induction of cancer cell apoptosis. Since inactivation of the AKT/protein kinase B negative regulator gene PTEN (phosphatase and tensin homologue deleted on chromosome 10) is common in prostate cancer (PCa), we compared PTEN wild-type DU145 PCa cells (low basal AKT activity) with PTEN-mutant LNCaP PCa cells (high basal AKT activity) for their apoptosis responses to the methylselenol precursor methylseleninic acid (MSeA) and sodium selenite, an inorganic salt. Our results show that LNCaP cells withstood approximately 4 times higher doses of MSeA than DU145 cells, although they were slightly more sensitive than the latter to selenite-induced apoptosis. Treatment by MSeA modestly attenuated AKT phosphorylation and increased phospho-ERK1/2 in LNCaP cells. Selenite treatment increased the phosphorylation of p53 Ser15 and both kinases, but the selenite-induced apoptosis was not influenced by chemical inhibitors of either kinase. In contrast, PI3K/AKT inhibitors greatly sensitized LNCaP cells to apoptosis induced by MSeA, accompanied by increased mitochondrial release of cytochrome c and multiple caspase activation without changing p53 Ser15 phosphorylation. The apoptosis was further accentuated by extracellular signal regulated kinases 1 and 2 (ERK1/2) inhibition without further increase in cytochrome c release. The general caspase inhibitor z-VAD-fmk completely blocked MSeA-induced apoptosis when both kinases were inhibited, whereas a caspase-8 inhibitor exerted a greater protection than did a caspase-9 inhibitor. Transfection of DU145 cells with a constitutively active AKT increased their resistance to MSeA-induced apoptosis. In summary, AKT played an important role in regulating apoptosis sensitivity of LNCaP and DU145 cells to MSeA. An MSeA-induced activation of ERK1/2 in LNCaP cells also contributed to resistance to apoptosis. However, these kinases did not significantly regulate caspase-mediated apoptosis induced by selenite in LNCaP cells. These findings support the differential involvement of these protein kinase pathways in regulating apoptosis induction by different forms of selenium.

Elevated Akt Phosphorylation as an Indicator of Renal Tubular Epithelial Cell Stress

Characterization of the phosphoinositide 3-kinase-signaling pathway in a human renal tubular epithelial cell (TEC) line HKC-8 revealed high levels of Akt phosphorylation in serum-starved cultures. In contrast to Erk1/2, little additional phosphorylation of Akt was observed after cytokine or serum stimulation. Replacement of the conditioned medium attenuated Akt phosphorylation such that 90 min after the addition of warmed serum-free media, Akt phosphorylation had fallen sufficiently to allow an epidermal growth factor-stimulated increase to be detected readily. Although the mechanism by which the phosphoinositide 3-kinase/Akt pathway is activated in serum-starved TEC is unknown, the mediator responsible is secreted from these cells. Thus, conditioned media removed from a dish of quiescent TECs stimulated Akt phosphorylation in washed TEC cultures within 10 min. Biochemical characterization of the bioactive agent identified a heat labile factor of small apparent molecular mass. The basal level of Akt phosphorylation observed in serum-starved cultures was inhibited by wortmannin at concentrations that demonstrated its dependence on 3-phosphoinositide synthesis (IC(50) = 8 nm). Regular removal of conditioned media from TEC cultures and its replacement with serum free media resulted in a sustained attenuation of Akt phosphorylation. Interestingly, after 5 days of this treatment, washed TEC cultures contained a greater number of viable cells than cultures maintained in conditioned media throughout. This observation was not explained by a difference in the rate of DNA synthesis. Instead, the number of cells undergoing apoptosis increased markedly in the unwashed cultures. Consequently, we propose that in HKC-8 cells Akt phosphorylation is up-regulated in an effort to minimize cell death. This stress-activated response is initiated by a factor secreted into the conditioned medium that stimulates the phosphoinositide 3-kinase signaling pathway.