Reconstitution of phosphoinositide 3-kinase-dependent insulin signaling in a cell-free system

PHLDA2 is a key oncogene-induced negative feedback inhibitor of EGFR/ErbB2 signaling via interference with AKT signaling

Pleckstrin homology-like domain family A member 2 (PHLDA2) is located within the tumor suppressor region of 11p15, and its expression is suppressed in several malignant tumor types. We recently identified PHLDA2 as a robustly induced, novel downstream target of oncogenic EGFR/ErbB2 signaling. In an immunohistochemical study, we find that PHLDA2 protein expression correlates positively with AKT activation in human lung cancers corroborating our data that PHLDA2 is induced upon oncogenic activation and might serve as a biomarker for AKT pathway activation. We show that PHLDA2 overexpression inhibits AKT phosphorylation while decreased PHLDA2 expression increases AKT activity. We further find that PHLDA2 competes with the PH domain of AKT for binding of membrane lipids, thereby directly inhibiting AKT translocation to the cellular membrane and subsequent activation. Indeed, PHLDA2 overexpression suppresses anchorage-independent cell growth and decreased PHLDA2 expression results in increased cell proliferation and reduced sensitivity to targeted agents of EGFR/ErbB2-driven cancers demonstrating functional relevance for this interaction. In summary, our studies demonstrate that PHLDA2 is strongly regulated by EGFR/ErbB2 signaling and inhibits cell proliferation via repressing AKT activation in lung cancers in a negative feedback loop. We highlight a novel action for PHLDA2 as a potential biomarker for AKT pathway activation.

Antisense oligonucleotide suppression of human IGF-1R inhibits the growth and survival of in vitro cultured epithelial ovarian cancer cells

Background

Preclinical evaluation of the anti-neoplastic activity of antisense oligonucleotide (AS) suppression of human insulin-like growth factor I receptor (IGF-IR) in human epithelial ovarian cancer (EOC).

Methods

Ovarian cancer cells from 36 patients with EOC were investigated under serum-free tissue culture conditions. IGF-I production was evaluated by standard ELISA. IGF-IR and phosphorylated IRS-1, AKT, and MAP kinase expression and protein levels were evaluated by immunohistochemistry and Western blotting. Cancer cell growth and proliferation assays were performed in triplicates using MTT assay. Apoptosis was evaluated by TUNNEL assay.

Results

All ovarian cancer tissue samples tested produced IGF-I and expressed IGF-IR, supporting the existence of an autocrine loop. Treatment of primary ovarian cancer cell lines with an IGF-1R AS inhibited growth and proliferation and decreased clonogenicity in soft agar assay. AS treatment was demonstrated to inhibit the expression of IGF-1R and decrease the concentration of phosphorylated IRS-1, AKT, and MAP kinase signaling protein downstream of the IGF-IR. We also observed that the IGF-1R AS sensitized cancer cell lines to cisplatin in vitro through the PI3K pathway.

Conclusions

IGF-IR enhances the proliferation and tumorigenicity of human ovarian cancer cells and inhibition of IGF-IR by AS oligonucleotide treatment potentiates the activity of cisplatin in vitro. Therefore, IGF-1R is a potential molecular target in ovarian cancer.

Endoglin regulates PI3-kinase/Akt trafficking and signaling to alter endothelial capillary stability during angiogenesis

Endoglin (CD105) is an endothelial-specific transforming growth factor β (TGF-β) coreceptor essential for angiogenesis and vascular homeostasis. Although endoglin dysfunction contributes to numerous vascular conditions, the mechanism of endoglin action remains poorly understood. Here we report a novel mechanism in which endoglin and Gα-interacting protein C-terminus-interacting protein (GIPC)-mediated trafficking of phosphatidylinositol 3-kinase (PI3K) regulates endothelial signaling and function. We demonstrate that endoglin interacts with the PI3K subunits p110α and p85 via GIPC to recruit and activate PI3K and Akt at the cell membrane. Opposing ligand-induced effects are observed in which TGF-β1 attenuates, whereas bone morphogenetic protein-9 enhances, endoglin/GIPC-mediated membrane scaffolding of PI3K and Akt to alter endothelial capillary tube stability in vitro. Moreover, we employ the first transgenic zebrafish model for endoglin to demonstrate that GIPC is a critical component of endoglin function during developmental angiogenesis in vivo. These studies define a novel non-Smad function for endoglin and GIPC in regulating endothelial cell function during angiogenesis.

Hepatic regulation of fatty acid synthase by insulin and T3: evidence for T3 genomic and nongenomic actions

Fatty acid synthase (FAS) is a key enzyme of hepatic lipogenesis responsible for the synthesis of long-chain saturated fatty acids. This enzyme is mainly regulated at the transcriptional level by nutrients and hormones. In particular, glucose, insulin, and T(3) increase FAS activity, whereas glucagon and saturated and polyunsaturated fatty acids decrease it. In the present study we show that, in liver, T(3) and insulin were able to activate FAS enzymatic activity, mRNA expression, and gene transcription. We localized the T(3) response element (TRE) that mediates the T(3) genomic effect, on the FAS promoter between -741 and -696 bp that mediates the T(3) genomic effect. We show that both T(3) and insulin regulate FAS transcription via this sequence. The TRE binds a TR/RXR heterodimer even in the absence of hormone, and this binding is increased in response to T(3) and/or insulin treatment. The use of H7, a serine/threonine kinase inhibitor, reveals that a phosphorylation mechanism is implicated in the transcriptional regulation of FAS in response to both hormones. Specifically, we show that T(3) is able to modulate FAS transcription via a nongenomic action targeting the TRE through the activation of a PI 3-kinase-ERK1/2-MAPK-dependent pathway. Insulin also targets the TRE sequence, probably via the activation of two parallel pathways: Ras/ERK1/2 MAPK and PI 3-kinase/Akt. Finally, our data suggest that the nongenomic actions of T(3) and insulin are probably common to several TREs, as we observed similar effects on a classical DR4 consensus sequence.

Insulin-stimulated Interaction between Insulin Receptor Substrate 1 and p85α and Activation of Protein Kinase B/Akt Require Rab5

Binding of insulin to the insulin receptor initiates a cascade of protein phosphorylation and effector recruitment events leading to the activation of multiple distinct signaling pathways. Previous studies suggested that the diversity and specificity of insulin signal transduction are accomplished by both subcellular localization of receptor and the selective activation of downstream signaling molecules. The small GTPase Rab5 is a key regulator of endocytosis. Three Rab5 isoforms (Rab5a, -5b, and -5c) have been identified. Here we exploited the RNA interference technique to specifically knock down individual Rab5 isoforms to determine the cellular function of Rab5 in distinct insulin signaling pathways. Small interference RNA against a single Rab5 isoform had no effect on protein kinase B (PKB)/Akt or MAPK activation by insulin in NIH3T3 cells overexpressing human insulin receptor. However, simultaneous knockdown of all three Rab5 isoforms dramatically attenuated PKB/Akt activation by insulin without affecting MAPK activation. This inhibition of PKB/Akt activation was because of the impaired interaction between insulin receptor substrate 1 and the p85alpha subunit of phosphatidylinositol 3-kinase. These results indicate a requirement of Rab5 in presenting p85 to insulin receptor substrate 1. Additional evidence supporting a role for Rab5 was suggested by studies with GAPex-5, a vps9 domain containing exchange factor. Down-regulation of GAPex-5 impaired insulin-stimulated PKB/Akt activation. Collectively, this study indicates the involvement of Rab5 in insulin signaling.

A miniaturized cell-based fluorescence resonance energy transfer assay for insulin-receptor activation

This report describes the development, optimization, and implementation of a miniaturized cell-based assay for the identification of small-molecule insulin mimetics and potentiators. Cell-based assays are attractive formats for compound screening because they present the molecular targets in their cellular environment. A fluorescence resonance energy transfer (FRET) cell-based assay that measures the insulin-dependent colocalization of Akt2 fused with either cyan fluorescent protein or yellow fluorescent protein to the cellular membrane was developed. This ratiometric FRET assay was miniaturized into a robust, yet sensitive 3456-well nanoplate assay with Z' factors of approximately 0.6 despite a very small assay window (less than twofold full activation with insulin). The FRET assay was used for primary screening of a large compound collection for insulin-receptor agonists and potentiators. To prioritize compounds for further development, primary hits were tested in two additional assays, a biochemical time-resolved fluorescence resonance energy transfer assay to measure insulin-receptor phosphorylation and a translocation-based imaging assay. Results from the three assays were combined to yield 11 compounds as potential leads for the development of insulin mimetics or potentiators.

mTOR.RICTOR is the Ser473 kinase for Akt/protein kinase B in 3T3-L1 adipocytes

The insulin-signaling pathway leading to the activation of Akt/protein kinase B has been well characterized except for a single step, the phosphorylation of Akt at Ser-473. Double-stranded DNA-dependent protein kinase (DNA-PK), ataxia telangiectasia mutated (ATM) gene product, integrin-linked kinase (ILK), protein kinase Calpha (PKCalpha), and mammalian target of rapamycin (mTOR), when complexed to rapamycin-insensitive companion of mTOR (RICTOR), have all been identified as playing a critical role in Akt Ser-473 phosphorylation. However, the apparently disparate results reported in these studies are difficult to evaluate, given that different stimuli and cell types were examined and that all of the candidate proteins have never been systematically studied in a single system. Additionally, none of these studies were performed in a classical insulin-responsive cell type or tissue such as muscle or fat. We therefore examined each of these candidates in 3T3-L1 adipocytes. In vitro kinase assays, using different subcellular fractions of 3T3-L1 adipocytes, revealed that phosphatidylinositol 3,4,5-trisphosphate-stimulated Ser-473 phosphorylation correlated well with the amount of DNA-PK, mTOR, and RICTOR but did not correlate with levels of ATM, ILK, and PKCalpha. PKCalpha was completely absent from compartments with Ser-473 phosphorylation activity. Although purified DNA-PK could phosphorylate a peptide derived from Akt that contains amino acid Ser-473, it could not phosphorylate full-length Akt2. Vesicles immunoprecipitated from low density microsomes using antibodies directed against mTOR or RICTOR had phosphatidylinositol 3,4,5-trisphosphate-stimulated Ser-473 activity that was sensitive to wortmannin but not staurosporine. In contrast, immunopurified low density microsome vesicles containing ILK could not phosphorylate Akt on Ser-473 in vitro. Small interference RNA knockdown of RICTOR, but not DNA-PK, ATM, or ILK, suppressed insulin-activated Ser-473 phosphorylation and, to a lesser extent, Thr-308 phosphorylation in 3T3-L1 adipocytes. Based on our cell-free kinase and small interference RNA results, we conclude that mTOR complexed to RICTOR is the Ser-473 kinase in 3T3-L1 adipocytes.

GLUT4 trafficking in a test tube

Insulin regulates glucose transport in muscle and fat cells by stimulating the translocation of GLUT4 from intracellular vesicles to the plasma membrane. In this issue of Cell Metabolism, Holman and colleagues reconstitute this process in vitro, providing a system that promises new breakthroughs in our understanding of this important metabolic process.

Insulin signaling meets vesicle traffic of GLUT4 at a plasma-membrane-activated fusion step

A hypothesis that accounts for most of the available literature on insulin-stimulated GLUT4 translocation is that insulin action controls the access of GLUT4 vesicles to a constitutively active plasma-membrane fusion process. However, using an in vitro fusion assay, we show here that fusion is not constitutively active. Instead, the rate of fusion activity is stimulated 8-fold by insulin. Both the magnitude and time course of stimulated in vitro fusion recapitulate the cellular insulin response. Fusion is cell cytoplasm and SNARE dependent but does not require cell cytoskeleton. Furthermore, insulin activation of the plasma-membrane fraction of the fusion reaction is the essential step in regulation. Akt from the cytoplasm fraction is required for fusion. However, the participation of Akt in the stimulation of in vitro fusion is dependent on its in vitro recruitment onto the insulin-activated plasma membrane.

The major target of the endogenously generated reactive oxygen species in response to insulin stimulation is phosphatase and tensin homolog and not phosphoinositide-3 kinase (PI-3 kinase) in the PI-3 kinase/Akt pathway

Phosphoinositide-3 kinase (PI-3 kinase) and its downstream signaling molecules PDK-1 and Akt were analyzed in SK-N-SH and SK-N-BE(2) human neuroblastoma cell lines. When cells were stimulated with insulin, PI-3 kinase was activated in both cell lines, whereas the translocation of PDK-1 to the membrane fraction and phosphorylated Akt were observed only in SK-N-SH cells. Analyses of the insulin-mediated reactive oxygen species (ROS) generation and Phosphatase and Tensin homolog (PTEN) oxidation indicate that PTEN oxidation occurred in SK-N-SH cells, which can produce ROS, but not in SK-N-BE(2) cells, which cannot increase ROS in response to insulin stimulation. When SK-N-SH cells were pretreated with the NADPH oxidase inhibitor diphenyleneiodonium chloride before insulin stimulation, insulin-mediated translocation of PDK-1 to the membrane fraction and phosphorylation of Akt were remarkably reduced, whereas PI-3 kinase activity was not changed significantly. These results indicate that not only PI-3 kinase activation but also inhibition of PTEN by ROS is needed to increase cellular level of phosphatidylinositol 3,4,5-trisphosphate for recruiting downstream signaling molecules such as PDK-1 and Akt in insulin-mediated signaling. Moreover, the ROS generated by insulin stimulation mainly contributes to the inactivation of PTEN and not to the activation of PI-3 kinase in the PI-3 kinase/Akt pathway.

Regulation of insulin action by ceramide: dual mechanisms linking ceramide accumulation to the inhibition of Akt/protein kinase B

The sphingolipid ceramide negatively regulates insulin action by inhibiting Akt/protein kinase B (PKB), a serine/threonine kinase that is a central regulator of glucose uptake and anabolic metabolism. Despite considerable attention, the molecular mechanism accounting for this action of ceramide has remained both elusive and controversial. Herein we utilized deletion constructs encoding two different functional domains of Akt/PKB to identify which region of the enzyme conferred responsiveness to ceramide. Surprisingly the findings obtained with these separate domains reveal that ceramide blocks insulin stimulation of Akt/PKB by two independent mechanisms. First, using the isolated pleckstrin homology domain, we found that ceramide specifically blocks the translocation of Akt/PKB, but not its upstream activator phosphoinositide-dependent kinase-1, to the plasma membrane. Second, using a construct lacking this pleckstrin homology domain, which does not require translocation for activation, we found that ceramide stimulates the dephosphorylation of Akt/PKB by protein phosphatase 2A. Collectively these findings identify at least two independent mechanisms by which excessive ceramide accumulation in peripheral tissues could contribute to the development of insulin resistance. Moreover the results obtained provide a unifying theory to account for the numerous dissenting reports investigating the actions of ceramide toward Akt/PKB.

Genetic Manipulation of Mammary Gland Development and Lactation

The mammalian genome is believed to contain some 30,000 to 40,000 different genes. Of these an estimated 42% have no known function. Genetically engineered mouse models (GEMM) have been a powerful tool available for determining gene function in vivo. In the mammary gland, a variety of genetic engineering approaches have been applied successfully to understanding the importance of specific gene products to mammary gland development and lactation. Our own laboratory has applied genetically engineered mice to facilitate understanding of the regulation of mammary gland development and lactation by insulin-like growth factors (IGF) and by the transcription factor, upstream stimulatory factor (USF-2). Our studies on transgenic mice that overexpress IGF-I have demonstrated the importance of IGF-dependent signaling pathways to maintenance of mammary epithelial cells during the declining phase of lactation. Our analysis of early developmental processes in mammary tissue from mice that carry a targeted mutation in the IGF-I receptor gene suggests that IGF-dependent stimulation of cell cycle progression is more important to early mammary gland development than potential antiapoptotic effects. Lastly, our studies on mice that carry a targeted mutation of the Usf2 gene have demonstrated that this gene is necessary for normal lactation and have highlighted the importance of this gene to the maintenance of protein synthesis. These studies, as well as studies of others, have highlighted both the strengths and limitations inherent in the use of GEMM. Limitations serve as the driving force behind development of new experimental strategies and genetic engineering schemes that will allow for a full understanding of gene function within the mammary gland.

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3beta, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85alpha. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.

Phosphoinositide-dependent kinase-2 is a distinct protein kinase enriched in a novel cytoskeletal fraction associated with adipocyte plasma membranes

By recombining subcellular components of 3T3-L1 adipocytes in a test tube, early insulin signaling events dependent on phosphatidylinositol 3-kinase (PI 3-kinase) were successfully reconstituted, up to and including the phosphorylation of glycogen synthase kinase-3 by the serine/threonine kinase, Akt (Murata, H., Hresko, R.C., and Mueckler, M. (2003) J. Biol. Chem. 278, 21607-21614). Utilizing the advantages provided by a cell-free methodology, we characterized phosphoinositide-dependent kinase 2 (PDK2), the putative kinase responsible for phosphorylating Akt on Ser-473. Immunodepleting cytosolic PDK1 from an in vitro reaction containing plasma membrane and cytosol markedly inhibited insulin-stimulated phosphorylation of Akt at the PDK1 site (Thr-308) but had no effect on phosphorylation at the PDK2 site (Ser-473). In contrast, PDK2 activity was found to be highly enriched in a novel cytoskeletal subcellular fraction associated with plasma membranes. Akt isoforms 1-3 and a kinase-dead Akt1 (K179A) mutant were phosphorylated in a phosphatidylinositol 3,4,5-trisphosphate-dependent manner at Ser-473 in an in vitro reaction containing this novel adipocyte subcellular fraction. Our data indicate that this PDK2 activity is the result of a kinase distinct from PDK1 and is not due to autophosphorylation or transphosphorylation of Akt.