05-217 Sigma-AldrichAnti-PI3 Kinase Antibody, p85, N-SH2,

AMP-activated protein kinases (AMPK) are heterotrimeric, αβγ, serine/threonine kinases. The γ3-AMPK subunit is particularly interesting in muscle physiology because 1) it is specifically expressed in skeletal muscle, 2) α2β2γ3 is the AMPK heterotrimer activated during exercise in humans, and 3) it is down-regulated in humans after a training period. However, mechanisms underlying this decrease of γ3-AMPK expression remained unknown. We investigated whether the expression of AMPK subunits and particularly that of γ3-AMPK are regulated by the PPARβ pathway. We report that PPARβ activation with GW0742 induces a rapid (2 h) and sustained down-regulation of γ3-AMPK expression both in mouse skeletal muscles and in culture myotubes. Concomitantly, phosphorylation levels of both AMPK and acetyl-coenzyme A carboxylase are rapidly modified. The γ3-AMPK down-regulation is also observed in muscles from young and adult transgenic mice with muscle-specific overexpression of peroxisome proliferator-activated receptor β (PPARβ). We showed that γ3-AMPK down-regulation is a rapid physiological muscle response observed in mouse after running exercise or fasting, two situations leading to PPARβ activation. Finally, using C2C12, we demonstrated that dose and time-dependent down-regulation of γ3-AMPK expression upon GW0742 treatment, is due to decrease γ3-AMPK promoter activity.

Insulin inhibits forkhead O class (FoxO) transcription factors, which down-regulate the expression of genes involved in metabolism, cell cycle arrest, and apoptosis. After being phosphorylated by protein kinase B (PKB) on S253 in its DNA-binding domain, Foxo1 is phosphorylated on T24 and additional sites, which overall triggers its nuclear exclusion. During this process, Foxo1 is thought to retain some transcriptional activity and signaling potential. To evaluate this Foxo1 action, we used a Foxo1-ADA mutant that is constitutively nuclear due to mutation of T24 and S316 to A and harbors a mutation of S253 to D. Adenoviral-mediated expression of Foxo1-ADA in hepatocytes activates PKB and MAPK pathways more than expression of wild-type or of a transactivation domain-deleted mutant (Delta256). PKB activation cannot be accounted for by a Foxo1-mediated increase in upstream signaling components such as insulin receptor substrate 1 or 2 or by Foxo1-mediated down-regulation of Tribbles homolog 3. In contrast, Foxo1-ADA increases p38 activity, and p38 is required for effects of Foxo1 on PKB, at least in part. We propose that Foxo1 turns on a feed-forward loop, relayed by p38 and acting to amplify both PKB activation and Foxo1 inhibition. To conclude, key signaling pathways are activated in hepatocytes through nuclear Foxo1.

The Src homology 2 (SH2) domains of the p85 subunit of phosphatidylinositol 3'-kinase have been shown to bind to the tyrosine-phosphorylated platelet-derived growth factor receptor (PDGFR). Previously, we have demonstrated that p85 SH2 domains can also bind to the serine/threonine kinase A-Raf via a unique phosphorylation-independent interaction. In this report, we describe a new phosphotyrosine-independent p85 SH2-binding protein, ankyrin 3 (Ank3). In general, ankyrins serve a structural role by binding to both integral membrane proteins at the plasma membrane and spectrin/fodrin proteins of the cytoskeleton. However, smaller isoforms of Ank3 lack the membrane domain and are localized to late endosomes and lysosomes. We found that p85 binds directly to these smaller 120- and 105-kDa Ank3 isoforms. Both the spectrin domain and the regulatory domain of Ank3 are involved in binding to p85. At least two domains of p85 can bind to Ank3, and the interaction involving the p85 C-SH2 domain was found to be phosphotyrosine-independent. Overexpression of the 120- or 105-kDa Ank3 proteins resulted in significantly enhanced PDGFR degradation and a reduced ability to proliferate in response to PDGF. Ank3 overexpression also differentially regulated signaling pathways downstream from the PDGFR. Chloroquine, an inhibitor of lysosomal-mediated degradation pathways, blocked the ability of Ank3 to enhance PDGFR degradation. Immunofluorescence experiments demonstrated that both small Ank3 isoforms colocalized with the lysosomal-associated membrane protein and with p85 and the PDGFR. These results suggest that Ank3 plays an important role in lysosomal-mediated receptor down-regulation, likely through a p85-Ank3 interaction.

Oncogenic mutations of the receptor tyrosine kinase KIT are encountered in myeloid leukemia and various solid tumors, including gastrointestinal stromal tumors. We previously identified the human oncogenic germ line mutant KIT(K642E), a substitution in the tyrosine kinase 1 domain (TK1D) in a familial form of gastrointestinal stromal tumors. The effects of oncogenic KIT mutants on cell signaling and regulation are complex. Cellular models are valuable basic tools to tailor novel strategies on specific cellular and molecular bases for tumors expressing KIT oncogenic mutants. Murine KIT(WT) and the murine homologues of human KIT oncogenic mutants, further referred to as KIT(K641E) and KIT(del559), a point deletion in the juxtamembrane domain (JMD), were stably expressed in IL-3-dependent Ba/F3 cells. Major differences in the constitutively activation of Akt/PKB, MAP kinases and STATs pathways were observed between KIT(K641E) and KIT(del559), whereas KIT ligand elicited responses in both mutants. Noteworthy, the protein level of the phosphoinositide phosphatase SHIP1, but not SHIP2 and PTEN, was reduced in KIT(K641E) only while inhibition of KIT phosphorylation reversibly raised SHIP1 level in both JMD and TK1D oncogenic mutants, unraveling the control of SHIP protein level by KIT phosphorylation.

Rab5 and Rab4 are small monomeric GTPases localized on early endosomes and function in vesicle fusion events. These Rab proteins regulate the endocytosis and recycling or degradation of activated receptor tyrosine kinases such as the platelet-derived growth factor receptor (PDGFR). The p85alpha subunit of phosphatidylinositol 3'-kinase contains a BH domain with sequence homology to GTPase activating proteins (GAPs), but has not previously been shown to possess GAP activity. In this report, we demonstrate that p85alpha has GAP activity toward Rab5, Rab4, Cdc42, Rac1 and to a lesser extent Rab6, with little GAP activity toward Rab11. Purified recombinant Rab5 and p85alpha can bind directly to each other and not surprisingly, the p85alpha-encoded GAP activity is present in the BH domain. Because p85alpha stays bound to the PDGFR during receptor endocytosis, p85alpha will also be localized to the same early endosomal compartment as Rab5 and Rab4. Taken together, the physical co-localization and the ability of p85alpha to preferentially stimulate the down-regulation of Rab5 and Rab4 GTPases suggests that p85alpha regulates how long Rab5 and Rab4 remain in their GTP-bound active state. Cells expressing BH domain mutants of p85 show a reduced rate of PDGFR degradation as compared with wild type p85 expressing cells. These cells also show sustained activation of the mitogen-activated protein kinase and Akt pathways. Thus, the p85alpha protein may play a role in the down-regulation of activated receptors through its temporal control of the GTPase cycles of Rab5 and Rab4.

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.

Insulin receptor substrate-1 (IRS-1) plays an essential role in mediating the insulin signals that trigger mitogenesis, lipid synthesis, and uncoupling protein-1 gene expression in mouse brown adipocytes. Expression of IRS-3 is restricted mainly to white adipose tissue; expression of this IRS protein is virtually absent in brown adipocytes. We have tested the capacity of IRS-3 to mediate insulin actions in IRS-1-deficient brown adipocytes. Thus, we expressed exogenous IRS-3 in immortalized IRS-1-/- brown adipocytes at a level comparable with that of endogenous IRS-3 in white adipose tissue. Under these conditions, IRS-3 signaling in response to insulin was observed, as revealed by tyrosine phosphorylation of IRS-3, and the activation of phosphatidylinositol (PI) 3-kinase associated with this recombinant protein. However, although insulin promoted the association of Grb-2 with recombinant IRS-3 in IRS-1-/- cells, the exogenous expression of this IRS family member failed to activate p42/44 MAPK and mitogenesis in brown adipocytes lacking IRS-1. Downstream of PI 3-kinase, IRS-3 expression restored insulin-induced Akt phosphorylation, which is impaired by the lack of IRS-1 signaling. Whereas the generation of IRS-3 signals enhanced adipocyte determination and differentiation-dependent factor 1/sterol regulatory element-binding protein (ADD-1/SREBP-1c) and fatty acid synthase mRNA and protein expression, activation of this pathway was unable to reconstitute CCAAT/enhancer-binding protein alpha and uncoupling protein-1 transactivation and gene expression in response to insulin. Similar results were obtained following insulin-like growth factor-I stimulation. In brown adipocytes expressing the IRS-3F4 mutant, the association of the p85alpha regulatory subunit via Src homology 2 binding was lost, but insulin nevertheless induced PI 3-kinase activity and Akt phosphorylation in a wortmannin-dependent manner. In contrast, activation of IRS-3F4 signaling failed to restore the induction of ADD-1/SREBP-1c and fatty acid synthase gene expression in IRS-1-deficient brown adipocytes. These studies demonstrate that recombinant IRS-3 may reconstitute some, but not all, of the signals required for insulin action in brown adipocytes. Thus, our data further implicate a unique role for IRS-1 in triggering insulin action in brown adipocytes.

Binding of interferon alpha (IFN alpha) to its receptor induces activation of the Tyk-2 and Jak-1 tyrosine kinases and tyrosine phosphorylation of multiple downstream signaling elements, including the Stat components of the interferon-stimulated gene factor 3 (ISGF-3). IFN alpha also induces tyrosine phosphorylation of IRS-1, the principle substrate of the insulin receptor. In this study we demonstrate that various Type I IFNs rapidly stimulate tyrosine phosphorylation of IRS-2. This is significant since IRS-2 is the major IRS protein found in hematopoietic cells. The IFN alpha-induced phosphorylated form of IRS-2 associates with the p85 regulatory subunit of the phosphatidylinositol 3'-kinase, suggesting that this kinase participates in an IFN alpha-signaling cascade downstream of IRS-2. We also provide evidence for an interaction of IRS-2 with Tyk-2, suggesting that Tyk-2 is the kinase that phosphorylates this protein during IFN alpha stimulation. A conserved region in the pleckstrin homology domain of IRS-2 may be required for the interaction of IRS-2 with Tyk-2, as shown by the selective binding of glutathione S-transferase (GST) fusion proteins containing the IRS-2-IH1PH or IRS-1-IH1PH domains to Tyk-2 but not other Janus kinases in vitro.