Microinjection of the SH2 domain of the 85-kilodalton subunit of phosphatidylinositol 3-kinase inhibits insulin-induced DNA synthesis and c-fos expression

Carboxy-terminal domain of Cas differentially modulates c-Jun expression, DNA synthesis, and membrane ruffling induced by insulin, EGF, and IGF-1

p130 Crk-associated substrate (Cas) is an adaptor protein associating with many other signaling proteins and regulates a various biological processes including cell adhesion, migration, and growth factor stimulation. However, the exact functional role of Cas in growth factor signaling pathway was poorly understood. Here we investigated the role of Cas and its domains in the effects of insulin, EGF, and IGF-1 on c-Jun gene expression, DNA synthesis, cytoskeletal reorganization. We found that microinjection of anti-Cas antibody and C-terminal domain of Cas (Cas-CT) specifically inhibited EGF-induced, but not insulin- or IGF-1-induced, c-Jun expression. Cell cycle progression and cytoskeleton reorganization induced by insulin and EGF, but not by IGF-1, were inhibited by microinjected anti-Cas and Cas-CT. In contrast, microinjection of the substate domain (Cas-SD) of Cas did not have any inhibitory effects. These results revealed that the Cas-CT is differentially implicated in insulin and EGF-mediated, but not IGF-1-mediated, c-Jun expression, DNA synthesis and membrane ruffling.

Oncogenic N-Ras Stimulates SRF-Mediated Transactivation via H3 Acetylation at Lysine 9

Signal transduction pathways regulate the gene expression by altering chromatin dynamics in response to mitogens. Ras proteins are key regulators linking extracellular stimuli to a diverse range of biological responses associated with gene regulation. In mammals, the three ras genes encode four Ras protein isoforms: H-Ras, K-Ras4A, K-Ras4B, and N-Ras. Although emerging evidence suggests that Ras isoforms differentially regulate gene expressions and are functionally nonredundant, the mechanisms underlying Ras specificity and Ras signaling effects on gene expression remain unclear. Here, we show that oncogenic N-Ras acts as the most potent regulator of SRF-, NF-κB-, and AP-1-dependent transcription. N-Ras-RGL2 axis is a distinct signaling pathway for SRF target gene expression such as Egr1 and JunB, as RGL2 Ras binding domain (RBD) significantly impaired oncogenic N-Ras-induced SRE activation. By monitoring the effect of Ras isoforms upon the change of global histone modifications in oncogenic Ras-overexpressed cells, we discovered that oncogenic N-Ras elevates H3K9ac/H3K23ac levels globally in the chromatin context. Importantly, chromatin immunoprecipitation (ChIP) assays revealed that H3K9ac is significantly enriched at the promoter and coding regions of Egr1 and JunB. Collectively, our findings define an undocumented role of N-Ras in modulating of H3 acetylation and in gene regulation.

Insulin Signaling in the Control of Glucose and Lipid Homeostasis

A continuous supply of glucose is necessary to ensure proper function and survival of all organs. Plasma glucose levels are thus maintained in a narrow range around 5 mM, which is considered the physiological set point. Glucose homeostasis is controlled primarily by the liver, fat, and skeletal muscle. Following a meal, most glucose disposals occur in the skeletal muscle, whereas fasting plasma glucose levels are determined primarily by glucose output from the liver. The balance between the utilization and production of glucose is primarily maintained at equilibrium by two opposing hormones, insulin and glucagon. In response to an elevation in plasma glucose and amino acids (after consumption of a meal), insulin is released from the beta cells of the islets of Langerhans in the pancreas. When plasma glucose falls (during fasting or exercise), glucagon is secreted by α cells, which surround the beta cells in the pancreas. Both cell types are extremely sensitive to glucose concentrations, can regulate hormone synthesis, and are released in response to small changes in plasma glucose levels. At the same time, insulin serves as the major physiological anabolic agent, promoting the synthesis and storage of glucose, lipids, and proteins and inhibiting their degradation and release back into the circulation. This chapter will focus mainly on signal transduction mechanisms by which insulin exerts its plethora of effects in liver, muscle, and fat cells, focusing on those pathways that are crucial in the control of glucose and lipid homeostasis.

Functional roles of BCAR3 in the signaling pathways of insulin leading to DNA synthesis, membrane ruffling and GLUT4 translocation

Breast cancer anti-estrogen resistance 3 (BCAR3) is an SH2-containing signal transducer and is implicated in tumorigenesis of breast cancer cells. In this study, we found that BCAR3 mediates the induction of ERK activation and DNA synthesis by insulin, but not by IGF-1. Specifically, the SH2 domain of BCAR3 is involved in insulin-stimulated DNA synthesis. Differential tyrosine-phosphorylated patterns of the BCAR3 immune complex were detected in insulin and IGF-1 signaling, suggesting that BCAR3 is a distinct target molecule of insulin and IGF-1 signaling. Moreover, microinjection of BCAR3 inhibitory materials inhibited membrane ruffling induced by insulin, while this did not affect insulin-mediated GLUT4 translocation. Taken together, these results demonstrated that BCAR3 plays an important role in the signaling pathways of insulin leading to cell cycle progression and cytoskeleton reorganization, but not GLUT4 translocation.

PKC-beta1 mediates glucose-induced Akt activation and TGF-beta1 upregulation in mesangial cells

Accumulation of glomerular matrix is a hallmark of diabetic nephropathy. The serine/threonine kinase Akt mediates glucose-induced upregulation of collagen I in mesangial cells through transactivation of the EGF receptor (EGFR). In addition, in renal tubular cells, glucose-induced secretion of TGF-beta requires phosphoinositide-3-OH kinase, suggesting a possible role for Akt in the modulation of TGF-beta expression, but the mechanisms of Akt activation and its involvement in TGF-beta regulation are unknown. Here, in primary mesangial cells, high glucose induced AktS473 phosphorylation, which correlates with its activation, in a protein kinase C beta (PKC-beta)-dependent manner. Glucose led to PKC-beta1 membrane translocation and association with Akt, and PKC-beta1 immunoprecipitated from glucose-treated cells phosphorylated recombinant Akt on S473. PKC is known to mediate glucose-induced TGF-beta1 upregulation through the transcription factor AP-1; here, inhibitors of phosphoinositide-3-OH kinase, PKC-beta and Akt, and dominant-negative Akt all prevented glucose-induced activation of AP-1 and upregulation of TGF-beta1. Finally, pharmacologic and dominant negative inhibition of EGFR blocked glucose-induced activation of PKC-beta1, phosphorylation of AktS473, activation of AP-1, and upregulation of TGF-beta1. In vivo, the PKC-beta inhibitor ruboxistaurin prevented Akt activation in the renal cortex of diabetic rats. In conclusion, PKC-beta1 is an Akt S473 kinase in glucose-treated mesangial cells, and TGF-beta1 transcriptional upregulation requires EGFR/PKC-beta1/Akt signaling. New therapeutic approaches for diabetic nephropathy may result from targeting components of this pathway, particularly the initial EGFR transactivation.

BCAR3 regulates EGF-induced DNA synthesis in normal human breast MCF-12A cells

BCAR3 (breast cancer anti-estrogen resistance 3) is a signal transducer containing an SH2 domain, a proline/serine-rich domain and a GDP-exchange factor homologous domain, whose role in signaling pathways is currently unclear. Furthermore, BCAR3 is implicated in anti-estrogen resistance of breast cancer cells. In the present study, we investigated the functional role of BCAR3 in a mitogenic signaling pathway of EGF in non-tumorigenic human breast epithelial MCF-12A cells. Microinjection of an anti-BCAR3 antibody, siRNAs targeting BCAR3 and an SH2 domain of BCAR3 inhibited EGF-induced DNA synthesis. Direct association of BCAR3 with activated EGF receptor and Cas was observed. Lastly, microinjection of a BCAR3 expression plasmid induced DNA synthesis. These findings suggest that the BCAR3 protein, through its SH2 domain, is involved in the signaling pathways of EGF leading to cell cycle progression, and that BCAR3 itself is part of a mitogenic signaling pathway.

Collagen I induction by high glucose levels is mediated by epidermal growth factor receptor and phosphoinositide 3-kinase/Akt signalling in mesangial cells

AIMS/HYPOTHESIS

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. Recent data have linked the serine/threonine kinase protein kinase B (Akt) to matrix modulation. Here, we studied its role in high glucose-induced collagen elaboration by mesangial cells.

METHODS

Primary rat mesangial cells were treated with high glucose levels (30 mmol/l) or mannitol as osmotic control. Western blots, northern blots, ELISA and immunohistochemistry were used for assessment. Diabetes was induced in rats by streptozotocin.

RESULTS

Phosphorylated Akt at S473 (pAktS473), corresponding to Akt activation, was seen in diabetic glomeruli. In mesangial cells, high glucose levels induced pAktS473 by 20 min. This was sustained to 72 h, while mannitol had no effect. Akt activation by kinase assay and phosphorylation on threonine 308 was also observed. Phosphoinositide 3-kinase (PI3K) inhibitors LY294002 (20 micromol/l) and wortmannin (100 nmol/l) prevented pAktS473. Collagen IA1 transcript and collagen I protein upregulation by high glucose levels were inhibited by PI3K blockade, as was collagen I secretion into the medium (ELISA). Dominant-negative Akt overexpression also inhibited high glucose-induced collagen IA1 transcript and collagen I protein production. Since signalling through the epidermal growth factor receptor (EGFR) can activate PI3K-Akt, we studied its activation by high glucose levels. EGFR was correspondingly activated by 10 min; mannitol had no effect. EGFR activation was also seen in glomeruli from diabetic rats and co-localised with collagen IA1 in diabetic glomeruli. Specific EGFR inhibition (AG1478, 5 micromol/l or dominant-negative EGFR) blocked high glucose-induced pAktS473, phosphorylation on threonine 308 and activation of the EGFR downstream target p44 extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase. Finally, EGFR inhibition also blocked high glucose-induced collagen I upregulation at transcriptional and protein levels.

CONCLUSIONS/INTERPRETATION

We conclude that EGFR-PI3K-Akt signalling mediates high glucose-induced collagen I upregulation in mesangial cells and that this pathway is activated in diabetic glomeruli. Targeting its components may provide a new therapeutic approach to diabetic kidney disease.

Neuregulin 1-Beta cytoprotective role in AML 12 mouse hepatocytes exposed to pentachlorophenol

Neuregulins are a family of growth factor domain proteins that are structurally related to the epidermal growth factor. Accumulating evidence has shown that neuregulins have cyto- and neuroprotective properties in various cell types. In particular, the neuregulin-1 Beta (NRG1-Beta) isoform is well documented for its antiinflammatory properties in rat brain after acute stroke episodes. Pentachlorophenol (PCP) is an organochlorine compound that has been widely used as a biocide in several industrial, agricultural, and domestic applications. Previous investigations from our laboratory have demonstrated that PCP exerts both cytotoxic and mitogenic effects in human liver carcinoma (HepG2) cells, primary catfish hepatocytes and AML 12 mouse hepatocytes. We have also shown that in HepG2 cells, PCP has the ability to induce stress genes that may play a role in the molecular events leading to toxicity and tumorigenesis. In the present study, we hypothesize that NRG1-Beta will exert its cytoprotective effects in PCP-treated AML 12 mouse hepatocytes by its ability to suppress the toxic effects of PCP. To test this hypothesis, we performed the MTT-cell respiration assay to assess cell viability, and Western-blot analysis to assess stress-related proteins as a consequence of PCP exposure. Data obtained from 48 h-viability studies demonstrated a biphasic response; showing a dose-dependent increase in cell viability within the range of 0 to 3.87 microg/mL, and a gradual decrease within the concentration range of 7.75 to 31.0 microg/mL in concomitant treatments of NRG1-Beta+PCP and PCP. Cell viability percentages indicated that NRG1-Beta+PCPtreated cells were not significantly impaired, while PCP-treated cells were appreciably affected; suggesting that NRG1-Beta has the ability to suppress the toxic effects of PCP. Western Blot analysis demonstrated the potential of PCP to induce oxidative stress and inflammatory response (c-fos), growth arrest and DNA damage (GADD153), proteotoxic effects (HSP70), cell cycle arrest as consequence of DNA damage (p53), mitogenic response (cyclin- D1), and apoptosis (caspase-3). NRG1-Beta exposure attenuated stress-related protein expression in PCP-treated AML 12 mouse hepatocytes. Here we provide clear evidence that NRG1-Beta exerts cytoprotective effects in AML 12 mouse hepatocytes exposed to PCP.

Quantitative immunodetection of key elements of polyphosphoinositide signal transduction in osteoblasts from arthritic patients shows a direct correlation with cell proliferation

Phosphoinositides play an essential role in diverse cellular functions such as cell proliferation, cytoskeletal regulation, intracellular vesicle trafficking, motility, cell metabolism and death. Alteration of these pathways is common to many diseases. In this study, we show that osteoblasts from patients affected by osteoarthritis (OA) and by rheumatoid arthritis (RA) present a decreased cell proliferation and a reduced expression of the key elements of polyphosphoinositide signal transduction such as phosphatidylinositol-3-kinase (PI 3K), phospholipase C gamma1 (PLCgamma1), and protein kinase C zeta (PKCzeta) compared to the post-traumatic (PT) patients. Our results suggest that a correlation may exist between the reduced osteoblast proliferation observed in OA and RA patients and the lowered expression of PI 3K, PLCgamma1, and PKCzeta enzymes. The reduced proliferation rate of osteoblasts in response to these signal transduction effectors could counteract the evolution of arthritic disease.

Increased p85/55/50 expression and decreased phosphotidylinositol 3-kinase activity in insulin-resistant human skeletal muscle

Insulin resistance is predominantly characterized by decreased insulin-stimulated glucose uptake into skeletal muscle. In the current study, we have assessed various aspects of the phosphatidylinositol (PI) 3-kinase pathway in skeletal muscle biopsies obtained from normal, obese nondiabetic, and type 2 diabetic subjects, before and after a 5-h insulin infusion. We found a highly significant inverse correlation between in vivo insulin sensitivity (as measured by the glucose infusion rate) and increased protein expression of p85/55/50, protein kinase C (PKC)-theta activity, levels of pSer307 insulin receptor substrate (IRS)-1 and p-Jun NH2-terminal kinase (JNK)-1, and myosin heavy chain IIx fibers. Increased basal phosphorylation of Ser307 IRS-1 in the obese and type 2 diabetic subjects corresponds with decrease in insulin-stimulated IRS-1 tyrosine phosphorylation, PI 3-kinase activity, and insulin-induced activation of Akt and, more prominently, PKC-zeta/lambda. In summary, increased expression of the PI 3-kinase adaptor subunits p85/55/50, as well as increased activity of the proinflammatory kinases JNK-1, PKC-theta, and, to a lesser extent, inhibitor of kappaB kinase-beta, are associated with increased basal Ser307 IRS-1 phosphorylation and decreased PI 3-kinase activity and may follow a common pathway to attenuate in vivo insulin sensitivity in insulin-resistant subjects. These findings demonstrate interacting mechanisms that can lead to impaired insulin-stimulated PI 3-kinase activity in skeletal muscle from obese and type 2 diabetic subjects.

Akt mediates mechanical strain-induced collagen production by mesangial cells

Increased glomerular hydrostatic pressure is an important determinant of glomerulosclerosis and can be modeled by in vitro exposure of mesangial cells to cyclic mechanical strain. Stretched mesangial cells increase extracellular matrix protein production, the hallmark of glomerulosclerosis. Recent data indicate that the serine/threonine kinase Akt may be involved in matrix modulation. Thus, Akt activation and matrix synthesis in stretched mesangial cells were studied. Exposure of mesangial cells to 1 Hz cyclic strain led to prompt Akt activation, which was biphasic to 24 h. Activation was dependent on signaling through phosphatidylinositol-3-kinase and required EGF receptor transactivation. Inhibition of signaling through the PDGF receptor, Src kinase, or cytoskeletal disruption failed to prevent strain-induced Akt activation. Collagen type 1A1 transcript expression, promoter activation, and protein secretion were increased by stretch at 24 h and were dependent on phosphatidylinositol-3 kinase. Overexpression of dominant-negative Akt inhibited strain-induced collagen 1A1 production. Conversely, overexpression of constitutively active Akt led to increased collagen 1A1 upregulation and secretion. Finally, Akt activation was observed in the glomeruli of remnant rat kidneys, a model marked by increased intraglomerular pressure. The authors conclude that mechanical strain induces Akt activation in mesangial cells through a mechanism requiring phosphatidylinositol-3-kinase and EGF receptor transactivation. Type 1 collagen production is dependent on Akt and can be induced by Akt overexpression. Akt activation is observed in remnant kidneys in vivo. Thus, the role of Akt in progression of chronic hemodynamic glomerular disease is worthy of further exploration.

Interactions between Activating Signal Cointegrator-2 and the Tumor Suppressor Retinoblastoma in Androgen Receptor Transactivation

Activating signal cointegrator-2 (ASC-2), a cancer-amplified transcription coactivator of nuclear receptors and numerous other transcription factors, was previously shown to contain two LXXLL motifs, each of which interacts with a distinct set of nuclear receptors. In this work, we showed that ASC-2 has an indirect, separate binding site for androgen receptor (AR). Interestingly, this region overlapped with the direct interaction interfaces with the tumor suppressor retinoblastoma (Rb). Although ASC-2 alone stimulated AR transactivation in cotransfections of HeLa cells, ectopic expression of Rb effected ASC-2 to act as a transcription coactivator of AR in Rb-null Saos2 cells. These results, along with the previous report in which AR was shown to directly interact with Rb (Yeh, S., Miyamoto, H., Nishimura, K., Kang, H., Ludlow, J., Hsiao, P., Wang, C., Su, C., and Chang C. (1998) Biochem. Biophys. Res. Commun. 248, 361-367), suggest that the AR-ASC-2 interactions in vivo may involve Rb. Thus, ASC-2 appears to contain at least three distinct nuclear receptor interaction domains.

Possible dominant-negative mutation of the SHIP gene in acute myeloid leukemia

The SH2 domain-containing inositol 5'-phosphatase (SHIP) is crucial in hematopoietic development. To evaluate the possible tumor suppressor role of the SHIP gene in myeloid leukemogenesis, we examined primary leukemia cells from 30 acute myeloid leukemia (AML) patients, together with eight myeloid leukemia cell lines. A somatic mutation at codon 684, replacing Val with Glu, was detected in one patient, lying within the signature motif 2, which is the phosphatase active site. The results of an in vitro inositol 5'-phosphatase assay revealed that the mutation reduced catalytic activity of SHIP. Leukemia cells with the mutation showed enhanced Akt phosphorylation following IL-3 stimulation. K562 cells transfected with the mutated SHIP-V684E cDNA showed a growth advantage even at lower serum concentrations and resistance to apoptosis induced by serum deprivation and exposure to etoposide. These results suggest a possible role of the mutated SHIP gene in the development of acute leukemia and chemotherapy resistance through the deregulation of the phosphatidylinositol-3,4,5-triphosphate (PI(3,4,5)P3)/Akt signaling pathway. This is the first report of a mutation in the SHIP gene in any given human cancer, and indicates the need for more attention to be paid to this gene with respect to cancer pathogenesis.

The role of PI 3-kinase in the UVB-induced expression of c-fos

The role of the PI 3-kinase signaling pathway in UVB-induced c-fos gene expression was investigated in a human keratinocyte cell line, HaCaT. The enzymatic activity of PI 3-kinase was increased threefold by 250 J/m(2) UVB. Inhibition of PI 3-kinase activity, via expression of a mutant p85 subunit or treatment with wortmannin, resulted in decreased levels of c-fos promoter activity and c-fos protein. Two members of the PI 3-kinase signaling pathway, Akt and GSK-3beta, were also found to affect c-fos transactivation. Expression of dominant negative Akt or wild-type GSK-3beta significantly inhibited UVB-induced c-fos promoter activity. In addition, when GSK-3beta activity was inhibited by lithium chloride, both c-fos promoter activity and protein levels increased. These results demonstrate that both Akt activation and GSK-3beta inactivation are required in the UVB-induction of c-fos. Our results demonstrate for the first time that UVB induction of c-fos is in part mediated by the PI 3-kinase signaling pathway in the HaCaT cell line. By identifying the multiple signaling pathways that are induced by UVB and contribute to the induction of c-fos expression, more drug targets may be identified to aid attempts to prevent and treat skin cancer.

Retinoid-dependent antagonism of serum response factor transactivation mediated by transcriptional coactivator proteins

Transcriptional coactivators SRC-1 and p300 specifically interact with liganded-nuclear receptors and also modulate other transcription factors, including serum response factor (SRF). Here, we report that retinoids repress transactivation by SRF and specific interactions exist between the DNA binding domains of SRF and retinoic acid and retinoid X receptors. We further demonstrate that the repression may involve retinoid-dependent competition for a limiting amount of SRC-1 and p300 between SRF and retinoid receptors. We propose that the well-defined anti-proliferative action of retinoids could, at least in part, result from this novel transrepressive action on the mitogenic transcription factor SRF.

Vps34p differentially regulates endocytosis from the apical and basolateral domains in polarized hepatic cells

Using a microinjection approach to study apical plasma membrane protein trafficking in hepatic cells, we found that specific inhibition of Vps34p, a class III phosphoinositide 3 (PI-3) kinase, nearly perfectly recapitulated the defects we reported for wortmannin-treated cells (Tuma, P.L., C.M. Finnegan, J.-H Yi, and A.L. Hubbard. 1999. J. Cell Biol. 145:1089-1102). Both wortmannin and injection of inhibitory Vps34p antibodies led to the accumulation of resident apical proteins in enlarged prelysosomes, whereas transcytosing apical proteins and recycling basolateral receptors transiently accumulated in basolateral early endosomes. To understand how the Vps34p catalytic product, PI3P, was differentially regulating endocytosis from the two domains, we examined the PI3P binding protein early endosomal antigen 1 (EEA1). We determined that EEA1 distributed to two biochemically distinct endosomal populations: basolateral early endosomes and subapical endosomes. Both contained rab5, although the latter also contained late endosomal markers but was distinct from the transcytotic intermediate, the subapical compartment. When PI3P was depleted, EEA1 dissociated from basolateral endosomes, whereas it remained on subapical endosomes. From these results, we conclude that PI3P, via EEA1, regulates early steps in endocytosis from the basolateral surface in polarized WIF-B cells. However, PI3P must use different machinery in its regulation of the apical endocytic pathway, since later steps are affected by Vps34p inhibition.

Phosphatidylinositol 3-kinase is necessary but not sufficient for thrombopoietin-induced proliferation in engineered Mpl-bearing cell lines as well as in primary megakaryocytic progenitors

Thrombopoietin and its receptor (Mpl) support survival and proliferation in megakaryocyte progenitors and in BaF3 cells engineered to stably express Mpl (BaF3/Mpl). The binding of thrombopoietin to Mpl activates multiple kinase pathways, including the Jak/STAT, Ras/Raf/MAPK, and phosphatidylinositol 3-kinase pathways, but it is not clear how these kinases promote cell cycling. Here, we show that thrombopoietin induces phosphatidylinositol 3-kinase and that phosphatidylinositol 3-kinase is required for thrombopoietin-induced cell cycling in BaF3/Mpl cells and in primary megakaryocyte progenitors. Treatment of BaF3/Mpl cells and megakaryocytes with the phosphatidylinositol 3-kinase inhibitor LY294002 inhibited mitotic and endomitotic cell cycl-ing. BaF3/Mpl cells treated with thrombopoietin and LY294002 were blocked in G(1), whereas megakaryocyte progenitors treated with thrombopoietin and LY294002 showed both a G(1) and a G(2) cell cycle block. Expression of constitutively active Akt in BaF3/Mpl cells restored the ability of thrombopoietin to promote cell cycling in the presence of LY294002. Constitutively active Akt was not sufficient to drive proliferation of BaF3/Mpl cells in the absence of thrombopoietin. We conclude that in BaF3/Mpl cells and megakaryocyte progenitors, thrombopoietin-induced phosphatidylinositol 3-kinase activity is necessary but not sufficient for thrombopoietin-induced cell cycle progression. Phosphatidylinositol 3-kinase activity is likely to be involved in regulating the G(1)/S transition.

Regulation of phosphoinositide metabolism, Akt phosphorylation, and glucose transport by PTEN (phosphatase and tensin homolog deleted on chromosome 10) in 3T3-L1 adipocytes

To investigate the roles of PTEN (phosphatase and tensin homolog deleted on chromosome 10) in the regulation of 3-position phosphorylated phosphoinositide metabolism as well as insulin-induced Akt phosphorylation and glucose metabolism, wild-type PTEN and its phosphatase-dead mutant (C124S) with or without an N-terminal myristoylation tag were overexpressed in Sf-9 cells and 3T3-L1 adipocytes using baculovirus and adenovirus systems, respectively. When expressed in Sf-9 cells together with the p110alpha catalytic subunit of phosphoinositide 3-kinase, myristoylated PTEN markedly reduced the accumulations of both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate induced by p110alpha. In contrast, overexpression of the C124S mutants apparently increased these accumulations. In 3T3-L1 adipocytes, insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate were markedly suppressed by overexpression of wild-type PTEN with the N-terminal myristoylation tag, but not by that without the tag. On the contrary, the C124S mutants of PTEN enhanced insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. Interestingly, the phosphorylation level of Akt at Thr308 (Akt2 at Thr309), but not at Ser473 (Akt2 at Ser474), was revealed to correlate well with the accumulation of phosphatidylinositol 3,4,5-trisphosphate modified by overexpression of these PTEN proteins. Finally, insulin-induced increases in glucose transport activity were significantly inhibited by the overexpression of myristoylated wild-type PTEN, but were not enhanced by expression of the C124S mutant of PTEN. Therefore, in conclusion, 1) PTEN dephosphorylates both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in vivo, and the C124S mutants interrupt endogenous PTEN activity in a dominant-negative manner. 2) The membrane targeting process of PTEN may be important for exerting its function. 3) Phosphorylations of Thr309 and Ser474 of Akt2 are regulated differently, and the former is regulated very sensitively by the function of PTEN. 4) The phosphorylation level of Ser474, but not that of Thr309, in Akt2 correlates well with insulin-stimulated glucose transport activity in 3T3-L1 adipocytes. 5) The activity of endogenous PTEN may not play a major role in the regulation of glucose transport activity in 3T3-L1 adipocytes.

Role of the cytosolic phospholipase A2-linked cascade in signaling by an oncogenic, constitutively active Ha-Ras isoform

Activation of Ras signaling by growth factors has been associated with gene regulation and cell proliferation. Here we characterize the contributory role of cytosolic phospholipase A(2) in the oncogenic Ha-Ras(V12) signaling pathway leading to activation of c-fos serum response element (SRE) and transformation in Rat-2 fibroblasts. Using a c-fos SRE-luciferase reporter gene, we showed that the transactivation of SRE by Ha-Ras(V12) is mainly via a Rac-linked cascade, although the Raf-mitogen-activated protein kinase cascade is required for full activation. In addition, Ha-Ras(V12)-induced DNA synthesis was significantly attenuated by microinjection of recombinant Rac(N17), a dominant negative mutant of Rac1. To identify the mediators downstream of Rac in the Ha-Ras(V12) signaling, we investigated the involvement of cytosolic phospholipase A(2). Oncogenic Ha-Ras(V12)-induced SRE activation was significantly inhibited by either pretreatment with mepacrine, a phospholipase A(2) inhibitor, or cotransfection with the antisense oligonucleotide of cytosolic phospholipase A(2). We also found cytosolic phospholipase A(2) to be situated downstream of Ha-Ras(V12) in a signal pathway leading to transformation. Together, these results are indicative of mediatory roles of Rac and cytosolic phospholipase A(2) in the signaling pathway by which Ha-Ras(V12) transactivates c-fos SRE and transformation. Our findings point to cytosolic phospholipase A(2) as a novel potential target for suppressing oncogenic Ha-Ras(V12) signaling in the cell.

Differential usage of signal transduction pathways defines two types of serum response factor target gene

Activation of the transcription factor serum response factor (SRF) is dependent on Rho-controlled changes in actin dynamics. We used pathway-specific inhibitors to compare the roles of actin dynamics, extracellular signal-regulated kinase (ERK) signaling, and phosphatidylinositol 3-kinase in signaling either to SRF itself or to four cellular SRF target genes. Serum, lysophosphatidic acid, platelet-derived growth factor, and phorbol 12-myristate 13-acetate (PMA) each activated transcription of a stably integrated SRF reporter gene dependent on functional RhoA GTPase. Inhibition of mitogen-activated protein kinase-ERK kinase (MEK) signalling reduced activation of the SRF reporter by all stimuli by about 50%, except for PMA, which was effectively blocked. Inhibition of phosphatidylinositol 3-kinase slightly reduced reporter activation by serum and lysophosphatidic acid but substantially inhibited activation by platelet-derived growth factor and PMA. Reporter induction by all stimuli was absolutely dependent on actin dynamics. Regulation of the SRF (srf) and vinculin (vcl) genes was similar to that of the SRF reporter gene; activation by all stimuli was Rho-dependent and required actin dynamics but was largely independent of MEK activity. In contrast, activation of fos and egr1 occurred independently of RhoA and actin polymerization but was almost completely dependent on MEK activation. These results show that at least two classes of SRF target genes can be distinguished on the basis of their relative sensitivity to RhoA-actin and MEK-ERK signaling pathways.

Insulin signals to prenyltransferases via the Shc branch of intracellular signaling

We assessed the roles of insulin receptor substrate-1 (IRS-1) and Shc in insulin action on farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) using Chinese hamster ovary (CHO) cells that overexpress wild-type human insulin receptors (CHO-hIR-WT) or mutant insulin receptors lacking the NPEY domain (CHO-DeltaNPEY) or 3T3-L1 fibroblasts transfected with adenoviruses that express the PTB or SAIN domain of IRS-1 and Shc, the pleckstrin homology (PH) domain of IRS-1, or the Src homology 2 (SH2) domain of Shc. Insulin promoted phosphorylation of the alpha-subunit of FTase and GGTase I in CHO-hIR-WT cells, but was without effect in CHO-DeltaNPEY cells. Insulin increased FTase and GGTase I activities and the amounts of prenylated Ras and RhoA proteins in CHO-hIR-WT (but not CHO-DeltaNPEY) cells. Overexpression of the PTB or SAIN domain of IRS-1 (which blocked both IRS-1 and Shc signaling) prevented insulin-stimulated phosphorylation of the FTase and GGTase I alpha-subunit activation of FTase and GGTase I and subsequent increases in prenylated Ras and RhoA proteins. In contrast, overexpression of the IRS-1 PH domain, which impairs IRS-1 (but not Shc) signaling, did not alter insulin action on the prenyltransferases, but completely inhibited the insulin effect on the phosphorylation of IRS-1 and on the activation of phosphatidylinositol 3-kinase and Akt. Finally, overexpression of the Shc SH2 domain completely blocked the insulin effect on FTase and GGTase I activities without interfering with insulin signaling to MAPK. These data suggest that insulin signaling from its receptor to the prenyltransferases FTase and GGTase I is mediated by the Shc pathway, but not the IRS-1/phosphatidylinositol 3-kinase pathway. Shc-mediated insulin signaling to MAPK may be necessary (but not sufficient) for activation of prenyltransferase activity. An additional pathway involving the Shc SH2 domain may be necessary to mediate the insulin effect on FTase and GGTase I.

Additivity of the proliferative effects of HGF/SF and EGF on hepatocytes

The additivity of DNA synthesis induced by hepatocyte growth factor/scatter factor (HGF/SF) and epidermal growth factor (EGF) was revealed in periportal hepatocytes (PPH), perivenous hepatocytes (PVH), and primary hepatocytes. Furthermore, additivity of the signal transduction pathway of HGF/SF and EGF was investigated (i.e., the activity of mitogen-activated protein kinase (MAPK) induced by HGF/SF and EGF), but it was not seen in PPH, PVH, or primary hepatocytes, although wortomannin, a PI 3-kinase inhibitor, abolished the additivity. The additivity of DNA synthesis induced by HGF/SF and EGF was not related to hepatocyte heterogeneity, but to a difference in the signal transduction pathway, probably another pathway that is different from the classical MAPK (MAPK/ERK1,2) path.

Epidermal growth factor-induced phosphatidylinositol 3-kinase activation and DNA synthesis. Identification of Grb2-associated binder 2 as the major mediator in rat hepatocytes

In previous work we showed that the phosphatidylinositol 3-kinase (PI3-kinase), not the mitogen-activated protein kinase, pathway is necessary and sufficient to account for insulin- and epidermal growth factor (EGF)-induced DNA synthesis in rat hepatocytes. Here, using a dominant-negative p85, we confirmed the key role of EGF-induced PI3-kinase activation and sought to identify the mechanism by which this is effected. Our results show that EGF activates PI3-kinase with a time course similar to that of the association of p85 with three principal phosphotyrosine proteins (i. e. PY180, PY105, and PY52). We demonstrated that each formed a distinct p85-associated complex. PY180 and PY52 each constituted about 10% of EGF-activated PI3-kinase, whereas PY105 was responsible for 80%. PY105 associated with Grb2 and SHP-2, and although it behaved like Gab1, none of the latter was detected in rat liver. We therefore cloned a cDNA from rat liver, which was found to be 95% homologous to the mouse Grb2-associated binder 2 (Gab2) cDNA sequence. Using a specific Gab2 antibody, we demonstrated its expression in and association with p85, SHP-2, and Grb2 upon EGF treatment of rat hepatocytes. Gab2 accounted for most if not all of the PY105 species, since immunoprecipitation of Gab2 with specific antibodies demonstrated parallel immunodepletion of Gab2 and PY105 from the residual supernatants. We also found that the PI3-kinase activity associated with Gab2 was totally abolished by dominant negative p85. Thus, Gab2 appears to be the principal EGF-induced PY protein recruiting and activating PI3-kinase and mitogenesis.

Regulation of cyclin D(1) expression and DNA synthesis by phosphatidylinositol 3-kinase in airway smooth muscle cells

We have shown in bovine tracheal myocytes that extracellular signal-regulated kinase (ERK) and Rac1 function as upstream activators of transcription from the cyclin D(1) promoter. We now examine the role of phosphatidylinositol (PI) 3-kinase in this process. PI 3-kinase activity was increased by platelet-derived growth factor (PDGF) and attenuated by the PI 3-kinase inhibitors wortmannin and LY294002. These inhibitors also decreased cyclin D(1) promoter activity, protein abundance, and DNA synthesis. Overexpression of the active catalytic subunit of PI 3-kinase (p110(PI) (3-K)CAAX) was sufficient to activate the cyclin D(1) promoter. Wortmannin and LY294002 failed to attenuate PDGF-induced ERK activation, and overexpression of p110(PI) (3-K)CAAX was insufficient to activate ERK. p110(PI) (3-K)CAAX-induced cyclin D(1) promoter activity was not blocked by PD98059, an inhibitor of mitogen-activated protein kinase/ERK kinase. We next examined whether PI 3-kinase and the 21-kD guanidine triphosphatase Rac1 regulate cyclin D(1) promoter activity by similar mechanisms. p110(PI) (3-K)CAAX-induced cyclin D(1) promoter activity was decreased by two inhibitors of Rac1-mediated signaling, catalase and diphenylene iodonium. Further, PDGF, PI 3-kinase, and Rac1 each activated the cyclin D(1) promoter at the cyclic adenosine monophosphate response element binding protein (CREB)/activating transcription factor (ATF)-2 binding site, as evidenced by expression of a CREB/ATF-2 reporter plasmid. Finally, PI 3-kinase and Rac1-induced CREB/ATF-2 transactivation were each inhibited by catalase. Together, these data suggest that in airway smooth muscle (ASM) cells, PI 3-kinase regulates transcription from the cyclin D(1) promoter and DNA synthesis in an ERK-independent manner. Further, PI 3-kinase and Rac1 regulate ASM cell cycle traversal via a common cis-regulatory element in the cyclin D(1) promoter.

Dexamethasone inhibits insulin-like growth factor signaling and potentiates myoblast apoptosis

In the critically ill, glucocorticoids induce myopathy, combining profound protein catabolism and mild myotubular death. Insulin-like growth factors (IGFs) inhibit muscle catabolism through activation of phosphatidylinositol 3-kinase (PI3K). Using rat L6 myoblasts, we show that IGF-I also acts through PI3K to inhibit apoptosis induced by hyperosmolar metabolic stress with 300 mM mannitol. We find that the glucocorticoid dexamethasone inhibits this antiapoptotic effect of IGF-I by impairing PI3K signaling. Dexamethasone induces overexpression of the PI3K subunit p85alpha, which, in turn, competes with the complete PI3K heterodimer for binding at insulin receptor substrate-1, inhibiting PI3K activation. Dexamethasone blocks IGF-I-induced phosphorylation of Akt, a PI3K-dependent process. Increased cellular p85alpha abundance, induced by either 10 microM dexamethasone or transient transfection with a plasmid coding for p85alpha, significantly inhibits IGF-I rescue from apoptosis induced by mannitol, as indicated by both loss of cell viability and increased activity of caspase-3 by fluorogenic assay. Conversely, constitutively active PI3K inhibits death induced by mannitol, even in the presence of dexamethasone. These findings may have particular relevance in the pathogenesis of acute steroid myopathy in critical illness, in which catabolic glucocorticoid effects combine with acute metabolic stressors, including sepsis, fasting, and chemical denervation.

Persistent activation of phosphatidylinositol 3-kinase causes insulin resistance due to accelerated insulin-induced insulin receptor substrate-1 degradation in 3T3-L1 adipocytes

Recently, we have reported that the overexpression of a membrane-targeted phosphatidylinositol (PI) 3-kinase (p110CAAX) stimulated p70S6 kinase, Akt, glucose transport, and Ras activation in the absence of insulin but inhibited insulin-stimulated glycogen synthase activation and MAP kinase phosphorylation in 3T3-L1 adipocytes. To investigate the mechanism of p110CAAX-induced cellular insulin resistance, we have now studied the effect of p110CAAX on insulin receptor substrate (IRS)-1 protein. Overexpression of p110CAAX alone decreased IRS-1 protein levels to 63+/-10% of control values. Insulin treatment led to an IRS-1 gel mobility shift (most likely caused by serine/threonine phosphorylation), with subsequent IRS-1 degradation. Moreover, insulin-induced IRS-1 degradation was enhanced by expression of p110CAAX (61+/-16% vs. 13+/-15% at 20 min, and 80+/-8% vs. 41+/-12% at 60 min, after insulin stimulation with or without p110CAAX expression, respectively). In accordance with the decreased IRS-1 protein, the insulin-stimulated association between IRS-1 and the p85 subunit of PI 3-kinase was also decreased in the p110CAAX-expressing cells, and IRS-1-associated PI 3-kinase activity was decreased despite the fact that total PI 3-kinase activity was increased. Five hours of wortmannin pretreatment inhibited both serine/threonine phosphorylation and degradation of IRS-1 protein. These results indicate that insulin treatment leads to serine/threonine phosphorylation of IRS-1, with subsequent IRS-1 degradation, through a PI 3-kinase-sensitive mechanism. Consistent with this, activated PI 3-kinase phosphorylates IRS-1 on serine/threonine residues, leading to IRS- 1 degradation. The similar finding was observed in IRS-2 as well as IRS-1. These results may also explain the cellular insulin-resistant state induced by chronic p110CAAX expression.

The tumor suppressor PTEN negatively regulates insulin signaling in 3T3-L1 adipocytes

PTEN is a tumor suppressor with sequence homology to protein-tyrosine phosphatases and the cytoskeleton protein tensin. PTEN is capable of dephosphorylating phosphatidylinositol 3,4, 5-trisphosphate in vitro and down-regulating its levels in insulin-stimulated 293 cells. To study the role of PTEN in insulin signaling, we overexpressed PTEN in 3T3-L1 adipocytes approximately 30-fold above uninfected or control virus (green fluorescent protein)-infected cells, using an adenovirus gene transfer system. PTEN overexpression inhibited insulin-induced 2-deoxy-glucose uptake by 36%, GLUT4 translocation by 35%, and membrane ruffling by 50%, all of which are phosphatidylinositol 3-kinase-dependent processes, compared with uninfected cells or cells infected with control virus. Microinjection of an anti-PTEN antibody increased basal and insulin stimulated GLUT4 translocation, suggesting that inhibition of endogenous PTEN function led to an increase in intracellular phosphatidylinositol 3,4,5-trisphosphate levels, which stimulates GLUT4 translocation. Further, insulin-induced phosphorylation of downstream targets Akt and p70S6 kinase were also inhibited significantly by overexpression of PTEN, whereas tyrosine phosphorylation of the insulin receptor and IRS-1 or the phosphorylation of mitogen-activated protein kinase were not affected, suggesting that the Ras/mitogen-activated protein kinase pathway remains fully functional. Thus, we conclude that PTEN may regulate phosphatidylinositol 3-kinase-dependent insulin signaling pathways in 3T3-L1 adipocytes.

Insulin-mediated cell proliferation and survival involve inhibition of c-Jun N-terminal kinases through a phosphatidylinositol 3-kinase- and mitogen-activated protein kinase phosphatase-1-dependent pathway

We previously reported that long term treatment with insulin led to sustained inhibition of c-Jun N-terminal kinases (JNKs) in CHO cells overexpressing insulin receptors. Here we investigated the signaling molecules involved in insulin inhibition of JNKs, focusing on phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase phosphatase-1 (MKP-1). In addition, we examined the relevance of JNK inhibition for insulin-mediated proliferation and survival. Insulin inhibition of JNKs was mediated by PI 3-K, as it was blocked by wortmannin and LY294002 and required the de novo synthesis of a phosphatase(s), as it was abolished by orthovanadate and actinomycin D. MKP-1 was a good candidate because 1) insulin stimulation of MKP-1 expression correlated with insulin inhibition of JNKs; 2) insulin stimulation of MKP-1 expression, like insulin inhibition of JNKs, was mediated by PI 3-K; and 3) the transient expression of an antisense MKP-1 RNA reduced the insulin inhibitory effect on JNKs. The overexpression of a dominant negative JNK1 mutant increased insulin stimulation of DNA synthesis and mimicked the protective effect of insulin against serum withdrawal-induced apoptosis. The overexpression of wild-type JNK1 or antisense MKP-1 RNA reduced the proliferative and/or antiapoptotic responses to insulin. Altogether, these results demonstrate that insulin inhibits JNKs through a PI 3-K- and MKP-1-dependent pathway and provide evidence for a key role for JNK inhibition in insulin regulation of proliferation and survival.

Ras signaling through PI3K confers hormone-independent proliferation that is compatible with differentiation

Hormones are specialized mitogens that stimulate proliferation in their differentiated target cells. Thyrotropin (TSH), the physiologic regulator of thyroid cells, stimulates cAMP-mediated proliferation and thyroid-specific gene expression. The mitogenic effects of TSH require Ras, therefore Ras activation should be compatible with the maintenance of thyroid differentiation. However, expression of activated Ras extinguishes the differentiated phenotype of thyroid cells. One explanation for this apparent paradox is the selective utilization of Ras effector pathways. We tested the hypothesis that Ras signaling through PI3K mediates the mitogenic effects of TSH in cells which retain their differentiated character. Expression of a Ras effector mutant (RasV12S35) that signals preferentially through Raf-1, although sufficient to confer TSH-independent proliferation, abolished hormone-regulated expression of thyroglobulin and the sodium/iodide symporter. In contrast, expression of a Ras mutant (RasV12C40) that binds selectively to PI3K conferred TSH-independent proliferation without marked effects on thyroid-specific gene expression. Unlike the inhibitory effects of TSH on the proliferation of RasV12S35-expressing cells, TSH enhanced RasV12C40-stimulated proliferation by further increasing the activity of p70s6k, an important mediator of the mitogenic effects of TSH and RasV12C40. These results demonstrate that channeling Ras-dependent signals to PI3K confers TSH with the ability to stimulate proliferation in differentiated cells. Oncogene (2000) 19, 924 - 932.

The PI 3-kinase isoforms p110(alpha) and p110(beta) have differential roles in PDGF- and insulin-mediated signaling

Phosphoinositide 3′-kinases constitute a family of lipid kinases implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. Phosphoinositide 3′-kinases that bind to the platelet-derived growth factor receptor are composed of two subunits: the p85 subunit acts as an adapter and couples the catalytic p110 subunit to the activated receptor. There are different isoforms of p85 as well as of p110, the individual roles of which have been elusive. Using microinjection of inhibitory antibodies specific for either p110(alpha) or p110(beta) we have investigated the involvement of the two p110 isoforms in platelet-derived growth factor- and insulin-induced actin reorganization in porcine aortic endothelial cells. We have found that antibodies against p110(alpha), but not antibodies against p110(beta), inhibit platelet-derived growth factor-stimulated actin reorganization, whereas the reverse is true for inhibition of insulin-induced actin reorganization. These data indicate that the two phosphoinositide 3′-kinase isoforms have distinct roles in signal transduction pathways induced by platelet-derived growth factor and insulin.

Epidermal growth factor and insulin-induced deoxyribonucleic acid synthesis in primary rat hepatocytes is phosphatidylinositol 3-kinase dependent and dissociated from protooncogene induction

The mitogenic response to insulin and epidermal growth factor (EGF) was studied in subconfluent and confluent cultures of primary rat hepatocytes. In subconfluent cultures, wortmannin, LY294002, and rapamycin reversed insulin- and EGF-induced [3H]thymidine incorporation into DNA. The mitogen-activated protein kinase (MAPK) kinase 1 (MEK1) inhibitor PD98059 was without significant effect on either insulin- or EGF-induced [3H]thymidine incorporation. Insulin treatment did not alter levels of messenger RNAs (mRNAs) for c-fos, c-jun, and c-myc. EGF induced an increase in c-myc, but not c-fos or c-jun, mRNA levels in subconfluent hepatocyte cultures. This increase in c-myc mRNA was abolished by PD98059. In confluent cells that could not be induced to synthesize DNA, EGF treatment also promoted an increase in c-myc mRNA to levels seen in subconfluent cultures. This increase was also abrogated by PD98059. These data indicate that in primary rat hepatocyte cultures, 1) the phosphoinositol 3-kinase pathway, perhaps through p70s6k activation, regulates DNA synthesis in response to insulin and EGF; 2) the MAPKpathway is not involved in insulin- and EGF-induced DNA synthesis; and 3) p44/42 MAPKs are involved the induction of c-myc mRNA levels, although this induction is not required for DNA synthesis. These studies define two distinct signal transduction pathways that independently mediate growth-related responses in a physiologically relevant, normal cell system.

A nuclear factor, ASC-2, as a cancer-amplified transcriptional coactivator essential for ligand-dependent transactivation by nuclear receptors in vivo

Many transcription coactivators interact with nuclear receptors in a ligand- and C-terminal transactivation function (AF2)-dependent manner. We isolated a nuclear factor (designated ASC-2) with such properties by using the ligand-binding domain of retinoid X receptor as a bait in a yeast two-hybrid screening. ASC-2 also interacted with other nuclear receptors, including retinoic acid receptor, thyroid hormone receptor, estrogen receptor alpha, and glucocorticoid receptor, basal factors TFIIA and TBP, and transcription integrators CBP/p300 and SRC-1. In transient cotransfections, ASC-2, either alone or in conjunction with CBP/p300 and SRC-1, stimulated ligand-dependent transactivation by wild type nuclear receptors but not mutant receptors lacking the AF2 domain. Consistent with an idea that ASC-2 is essential for the nuclear receptor function in vivo, microinjection of anti-ASC-2 antibody abrogated the ligand-dependent transactivation of retinoic acid receptor, and this repression was fully relieved by coinjection of ASC-2-expression vector. Surprisingly, ASC-2 was identical to a gene previously identified during a search for genes amplified and overexpressed in breast and other human cancers. From these results, we concluded that ASC-2 is a bona fide transcription coactivator molecule of nuclear receptors, and its altered expression may contribute to the development of cancers.

Role of the actin cytoskeleton in insulin action

Insulin has diverse effects on cells, including stimulation of glucose transport, gene expression, and alterations of cell morphology. The hormone mediates these effects by activation of signaling pathways which utilize, 1) adaptor molecules such as the insulin receptor substrates (IRS), the Src and collagen homologs (Shc), and the growth factor receptor binding protein 2 (Grb2); 2) lipid kinases such as phosphatidylinositol 3-kinase (PI 3-Kinase); 3) small G proteins; and 4) serine, threonine, and tyrosine kinases. The activation of such signaling molecules by insulin is now well established, but we do not yet fully understand the mechanisms integrating these seemingly diverse pathways. Here, we discuss the involvement of the actin cytoskeleton in the propagation and regulation of insulin signals. In muscle cells in culture, insulin induces a rapid actin filament reorganization that coincides with plasma membrane ruffling and intense accumulation of pinocytotic vesicles. Initiation of these effects of insulin requires an intact actin cytoskeleton and activation of PI 3-kinase. We observed recruitment PI 3-kinase subunits and glucose transporter proteins to regions of reorganized actin. In both muscle and adipose cells, actin disassembly inhibited early insulin-induced events such as recruitment of glucose transporters to the cell surface and enhanced glucose transport. Additionally, actin disassembly inhibited more prolonged effects of insulin, including DNA synthesis and expression of immediate early genes such as c-fos. Intact actin filaments appear to be essential for mediation of early events such as association of Shc with Grb2 in response to insulin, which leads to stimulation of gene expression. Preliminary observations support a role for focal adhesion signaling complexes in insulin action. These observations suggest that the actin cytoskeleton facilitates propagation of the morphological, metabolic, and nuclear effects of insulin by regulating proper subcellular distribution of signaling molecules that participate in the insulin signaling pathway.

Role of the Src homology 2 (SH2) domain and C-terminus tyrosine phosphorylation sites of SH2-containing inositol phosphatase (SHIP) in the regulation of insulin-induced mitogenesis

To examine the role of SHIP in insulin-induced mitogenic signaling, we used a truncated SHIP lacking the SH2 domain (deltaSH2-SHIP) and a Y917/1020F-SHIP (2F-SHIP) in which two tyrosines contributing to Shc binding were mutated to phenylalanine. Wild-type (WT)-, deltaSH2-, and 2F-SHIP were transiently transfected into Rat1 fibroblasts overexpressing insulin receptors (HIRc). Insulin-stimulated tyrosine phosphorylation of WT-SHIP and deltaSH2-SHIP, whereas tyrosine phosphorylation of 2F-SHIP was not detectable, indicating that 917/1020-Tyr are key phosphorylation sites on SHIP. Although SHIP can bind via its 917/1020-Tyr residues and SH2 domain to Shc PTB domain and 317-Tyr residue, respectively, insulin-induced SHIP association with Shc was more greatly decreased in 2F-SHIP cells than that in deltaSH2-SHIP cells. Insulin stimulation of Shc association with Grb2, which is important for p21ras-MAP kinase activation, was decreased by overexpression of WT- and 2F-SHIP. Importantly, insulin-induced Shc x Grb2 association was not detectably reduced in deltaSH2-SHIP cells. In accordance with the extent of Shc association with Grb2, insulin-induced MAP kinase activation was relatively decreased in both WT-SHIP and 2F-SHIP cells, but not in deltaSH2-SHIP cells. To examine the functional role of SHIP in insulin's biological action, insulin-induced mitogenesis was compared among these transfected cells. Insulin stimulation of thymidine incorporation and bromodeoxyuridine incorporation was decreased in WT-SHIP cells compared with that of control HIRc cells. Expression of 2F-SHIP also significantly reduced insulin-induced mitogenesis, whereas it was only slightly affected by overexpression of deltaSH2-SHIP. Furthermore, the reduction of insulin-induced mitogenesis in WT-SHIP cells was partly compensated by coexpression of Shc. These results indicate that SHIP plays a negative regulatory role in insulin-induced mitogenesis and that the SH2 domain of SHIP is important for its negative regulatory function.

Bcl3, an IκB Protein, Stimulates Activating Protein-1 Transactivation and Cellular Proliferation

Bcl3, an IkappaB protein, was originally isolated as a putative proto-oncogene in a subset of B cell chronic lymphocytic leukemias. Bcl3 was subsequently shown to associate tightly with and transactivate the NFkappaB p50 or p52 homodimer. Herein, we show that Bcl3 stimulates the activating protein-1 (AP-1) transactivation, either alone or in conjunction with transcription integrators steroid receptor coactivator-1 and CREB-binding protein/p300. The C-terminal 158 residues of Bcl3 exhibited an autonomous transactivation function and interacted with specific subregions of the AP-1 components c-Jun and c-Fos, CREB-binding protein/p300, and steroid receptor coactivator-1, as demonstrated by the yeast and mammalian two-hybrid tests as well as glutathione S-transferase pull-down assays. In addition, anti-HA antibody co-precipitated c-Jun from HeLa cells co-expressing c-Jun and HA-tagged Bcl3, consistent with the idea that Bcl3 directly associates with AP-1 in vivo. Furthermore, microinjection of Bcl3 expression vector into Rat-1 fibroblast cells significantly enhanced DNA synthesis and expression of c-jun, one of the cellular target genes of AP-1. These results suggest that Bcl3 may directly participate in the tumorigenesis processes as a novel transcription coactivator of the mitogenic transcription factor AP-1 in vivo.

Intracellular movement of green fluorescent protein-tagged phosphatidylinositol 3-kinase in response to growth factor receptor signaling

Phosphatidylinositol 3-kinase (PI 3-kinase) is a lipid kinase which has been implicated in mitogenesis, protein trafficking, inhibition of apoptosis, and integrin and actin functions. Here we show using a green fluorescent protein-tagged p85 subunit that phosphatidylinositol 3-kinase is distributed throughout the cytoplasm and is localized to focal adhesion complexes in resting NIH-3T3, A431, and MCF-7 cells. Ligand stimulation of an epidermal growth factor receptor/c-erbB-3 chimera expressed in these cells results in a redistribution of p85 to the cell membrane which is independent of the catalytic activity of the enzyme and the integrity of the actin cytoskeleton. The movement is, however, dependent on the phosphorylation status of the erbB-3 chimera. Using rhodamine-labeled epidermal growth factor we show that the phosphatidylinositol 3-kinase and the receptors colocalize in discrete patches on the cell surface. Low concentrations of ligand cause patching only at the periphery of the cells, whereas at high concentrations patches were seen over the whole cell surface. Using green fluorescent protein-tagged fragments of p85 we show that binding to the receptor requires the NH(2)-terminal part of the protein as well as its SH2 domains.

Synergistic role of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase cascade in the regulation of insulin receptor trafficking

To examine the molecular mechanism of insulin receptor trafficking, we investigated the intracellular signaling molecules that regulate this process in Rat1 fibroblasts overexpressing insulin receptors. Cellular localization of insulin receptors was assessed by confocal laser microscopy with indirect immunofluorescence staining. Insulin receptors were visualized diffusely in the basal state. Insulin treatment induced the change of insulin receptor localization to perinuclear compartment. This insulin-induced insulin receptor trafficking was not affected by treatment of the cells with PI3-kinase inhibitor (wortmannin), whereas treatment with MEK [mitogen-activated protein (MAP) kinase-Erk kinase] inhibitor (PD98059) partly inhibited the process in a dose-dependent manner. Interestingly, treatment with both wortmannin and PD98059 almost completely inhibited insulin receptor trafficking. The functional importance of PI3-kinase and MAP kinase in the trafficking process was directly assessed by using single cell microinjection analysis. Microinjection of p85-SH2 and/or catalytically inactive MAP kinase ([K71A]Erk1) GST fusion protein gave the same results as treatment with wortmannin and PD98059. Furthermore, to determine the crucial step for the requirement of PI3-kinase and MAP kinase pathways, the effect of wortmannin and PD98059 on insulin receptor endocytosis was studied. Insulin internalization from the plasma membrane and subsequent insulin degradation were not affected by treatment with wortmannin and PD98059. In contrast, insulin receptor down-regulation from the cell surface and insulin receptor degradation, after prolonged incubation with insulin, were markedly impaired by the treatment. These results suggest that PI3-kinase and MAP kinase pathways synergistically regulate insulin receptor trafficking at a step subsequent to the receptor internalization.

Activation of mitogenic pathways by albumin in kidney proximal tubule epithelial cells: implications for the pathophysiology of proteinuric states

Albumin is filtered into the proximal tubule in large quantities in nephrotic states. It has been proposed that this protein may have a toxic effect on tubular epithelial cells and may be responsible for the initiation of interstitial inflammation and scarring. The mitogenic effect of recombinant human albumin in wild-type opossum kidney cells and in similar cells transfected with a dominant negative p85 subunit (deltap85) of phopshatidylinositide 3-kinase (PI 3-kinase) has been studied. This study demonstrates that recombinant human albumin stimulates proliferation of opossum kidney cells in culture. This effect is mediated via PI 3-kinase, and is inhibited by wortmannin and deltap85 expression. Albumin stimulates PI 3-kinase activity in opossum kidney cells as determined by three different experimental procedures. Recombinant albumin also stimulates pp70(s6) kinase activity in a kinase cascade downstream of PI 3-kinase. Activity of pp70(s6) kinase is essential for albumin-induced proliferation of opossum kidney cells. It is proposed that this mitogenic pathway may have a critical role in proximal tubular homeostasis and pathophysiology of proteinuric states.

Molecular cloning of rat SH2-containing inositol phosphatase 2 (SHIP2) and its role in the regulation of insulin signaling

SH2-containing inositol 5'-phosphatase (SHIP) plays a negative regulatory role in hematopoietic cells. We have now cloned the rat SHIP isozyme (SHIP2) cDNA from skeletal muscle, which is one of the most important target tissue of insulin action. Rat SHIP2 cDNA encodes a 1183-amino-acid protein that is 45% identical with rat SHIP. Rat SHIP2 contains an amino-terminal SH2 domain, a central 5'-phosphoinositol phosphatase activity domain, and a phosphotyrosine binding (PTB) consensus sequence and a proline-rich region at the carboxyl tail. Specific antibodies to SHIP2 were raised and the function of SHIP2 was studied by stably overexpressing rat SHIP2 in Rat1 fibroblasts expressing human insulin receptors (HIRc). Endogenous SHIP2 underwent insulin-mediated tyrosine phosphorylation and phosphorylation was markedly increased when SHIP2 was overexpressed. Although overexpression of SHIP2 did not affect insulin-induced tyrosine phosphorylation of the insulin receptor beta-subunit and Shc, subsequent association of Shc with Grb2 was inhibited, possibly by competition between the SH2 domains of SHIP2 and Grb2 for the Shc phosphotyrosine. As a result, insulin-stimulated MAP kinase activation was reduced in SHIP2-overexpressing cells. Insulin-induced tyrosine phosphorylation of IRS-1, IRS-1 association with the p85 subunit of PI3-kinase, and PI3-kinase activation were not affected by overexpression of SHIP2. Interestingly, although both PtdIns-(3,4,5)P3 and PtdIns(3,4)P2 have been implicated in the regulation of Akt activity in vitro, overexpression of SHIP2 inhibited insulin-induced Akt activation, presumably by its 5'-inositol phosphatase activity. Furthermore, insulin-induced thymidine incorporation was decreased by overexpression of SHIP2. These results indicate that SHIP2 plays a negative regulatory role in insulin-induced mitogenesis, and regulation of the Shc. Grb2 complex and of the downstream products of PI3-kinase provides possible mechanisms of SHIP2 action in insulin signaling.

Membrane-targeted phosphatidylinositol 3-kinase mimics insulin actions and induces a state of cellular insulin resistance

Phosphatidylinositol (PI) 3-kinase plays an important role in various insulin-stimulated biological responses including glucose transport, glycogen synthesis, and protein synthesis. However, the molecular link between PI 3-kinase and these biological responses is still unclear. We have investigated whether targeting of the catalytic p110 subunit of PI 3-kinase to cellular membranes is sufficient and necessary to induce PI 3-kinase dependent signaling responses, characteristic of insulin action. We overexpressed Myc-tagged, membrane-targeted p110 (p110(CAAX)), and wild-type p110 (p110(WT)) in 3T3-L1 adipocytes by adenovirus-mediated gene transfer. Overexpressed p110(CAAX) exhibited approximately 2-fold increase in basal kinase activity in p110 immunoprecipitates, that further increased to approximately 4-fold with insulin. Even at this submaximal PI 3-kinase activity, p110(CAAX) fully stimulated p70 S6 kinase, Akt, 2-deoxyglucose uptake, and Ras, whereas, p110(WT) had little or no effect on these downstream effects. Interestingly p110(CAAX) did not activate MAP kinase, despite its stimulation of p21(ras). Surprisingly, p110(CAAX) did not increase basal glycogen synthase activity, and inhibited insulin stimulated activity, indicative of cellular resistance to this action of insulin. p110(CAAX) also inhibited insulin stimulated, but not platelet-derived growth factor-stimulated mitogen-activated protein kinase phosphorylation, demonstrating that the p110(CAAX) induced inhibition of mitogen-activated protein kinase and insulin signaling is specific, and not due to some toxic or nonspecific effect on the cells. Moreover, p110(CAAX) stimulated IRS-1 Ser/Thr phosphorylation, and inhibited IRS-1 associated PI 3-kinase activity, without affecting insulin receptor tyrosine phosphorylation, suggesting that it may play an important role as a negative regulator for insulin signaling. In conclusion, our studies show that membrane-targeted PI 3-kinase can mimic a number of biologic effects normally induced by insulin. In addition, the persistent activation of PI 3-kinase induced by p110(CAAX) expression leads to desensitization of specific signaling pathways. Interestingly, the state of cellular insulin resistance is not global, in that some of insulin's actions are inhibited, whereas others are intact.

Arrest of endosome acidification by bafilomycin A1 mimics insulin action on GLUT4 translocation in 3T3-L1 adipocytes

In insulin-sensitive fat and muscle cells, the major glucose transporter GLUT4 is constitutively sequestered in endosomal tubulovesicular membranes, and moves to the cell surface in response to insulin. While sequence information within GLUT4 appears to be responsible for its constitutive intracellular sequestration, the regulatory elements and mechanisms that enable this protein to achieve its unique sorting pattern under basal and insulin-stimulated conditions are poorly understood. We show here that arrest of endosome acidification in insulin-sensitive 3T3-L1 adipocytes by bafilomycin A1, a specific inhibitor of the vacuolar proton pump, results in the rapid and dose-dependent translocation of GLUT4 from the cell interior to the membrane surface; the effects of maximally stimulatory concentrations of bafilomycin A1 (400-800 nM) were equivalent to 50-65% of the effects of acute insulin treatment. Like insulin, bafilomycin A1 induced the redistribution of GLUT1 and Rab4, but not that of other proteins whose membrane localization has been shown to be insulin-insensitive. Studies to address the mechanism of this effect demonstrated that neither autophosphorylation nor internalization of the insulin receptor was altered by bafilomycin A1 treatment. Bafilomycin-induced GLUT4 translocation was not blocked by cell pretreatment with wortmannin. Taken together, these data indicate that arrest of endosome acidification mimics insulin action on GLUT4 and GLUT1 translocation by a mechanism distal to insulin receptor and phosphatidylinositol 3-kinase activation, and suggest an important role for endosomal pH in the membrane dynamics of the glucose transporters.

Phosphatidylinositol 3-kinase translocation to the nucleus is induced by interleukin 1 and prevented by mutation of interleukin 1 receptor in human osteosarcoma Saos-2 cells

Although interleukin 1 (IL-1) functions have been extensively characterized, the mechanisms by which IL-1 signals are transduced from the plasma membrane to the nucleus are less known. Recent evidence indicates that phosphatidylinositol 3-kinase (PI3-kinase) could be activated by a direct association with the activated IL-1 receptor. In this study we analyzed the effects of IL-1 on the intracellular distribution of PI3-kinase in wild-type Saos-2 human osteosarcoma cells, and in cell clones overexpressing type I IL-1 receptor (IL-1RI). PI3-kinase intracellular distribution displays two distinct patterns. In quiescent cells, PI3-kinase is distributed through the cytoplasm, although a portion is present in the nucleus; following stimulation with IL-1, PI3-kinase is redistributed, increasing in the nuclear compartment. Both immunoblotting and immunofluorescence data indicate that IL-1 causes a rapid and transient translocation of PI3-kinase from the cytoplasm to the nucleus. This phenomenon is prevented by PI3-kinase inhibitors, suggesting that the maintenance of PI3-kinase activity is essential for IL-1-induced translocation. Indeed, in cell clones stably transfected with Y479F receptor mutant, in which the binding of the enzyme to the activated receptor is blocked, IL-1-induced PI3-kinase translocation to the nucleus is completely prevented. These data suggest that PI3-kinase translocation to the nucleus upon IL-1R activation is an early event in IL-1 signaling mechanism, and may be involved in transcriptional activation.

Distinct roles for the p110alpha and hVPS34 phosphatidylinositol 3'-kinases in vesicular trafficking, regulation of the actin cytoskeleton, and mitogenesis

We have examined the roles of the p85/ p110alpha and hVPS34 phosphatidylinositol (PI) 3'-kinases in cellular signaling using inhibitory isoform-specific antibodies. We raised anti-hVPS34 and anti-p110alpha antibodies that specifically inhibit recombinant hVPS34 and p110alpha, respectively, in vitro. We used the antibodies to study cellular processes that are sensitive to low-dose wortmannin. The antibodies had distinct effects on the actin cytoskeleton; microinjection of anti-p110alpha antibodies blocked insulin-stimulated ruffling, whereas anti-hVPS34 antibodies had no effect. The antibodies also had different effects on vesicular trafficking. Microinjection of inhibitory anti-hVPS34 antibodies, but not anti-p110alpha antibodies, blocked the transit of internalized PDGF receptors to a perinuclear compartment, and disrupted the localization of the early endosomal protein EEA1. Microinjection of anti-p110alpha antibodies, and to a lesser extent anti-hVPS34 antibodies, reduced the rate of transferrin recycling in CHO cells. Surprisingly, both antibodies inhibited insulin-stimulated DNA synthesis by 80%. Injection of cells with antisense oligonucleotides derived from the hVPS34 sequence also blocked insulin-stimulated DNA synthesis, whereas scrambled oligonucleotides had no effect. Interestingly, the requirement for p110alpha and hVPS34 occurred at different times during the G1-S transition. Our data suggest that different PI 3'-kinases play distinct regulatory roles in the cell, and document an unexpected role for hVPS34 during insulin-stimulated mitogenesis.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.