Bifurcation of lipid and protein kinase signals of PI3Kgamma to the protein kinases PKB and MAPK.

Phosphoinositide 3-kinases (PI3Ks) activate protein kinase PKB (also termed Akt), and PI3Kgamma activated by heterotrimeric guanosine triphosphate-binding protein can stimulate mitogen-activated protein kinase (MAPK). Exchange of a putative lipid substrate-binding site generated PI3Kgamma proteins with altered or aborted lipid but retained protein kinase activity. Transiently expressed, PI3Kgamma hybrids exhibited wortmannin-sensitive activation of MAPK, whereas a catalytically inactive PI3Kgamma did not. Membrane-targeted PI3Kgamma constitutively produced phosphatidylinositol 3,4, 3,4,5-trisphosphate and activated PKB but not MAPK. Moreover, stimulation of MAPK in response to lysophosphatidic acid was blocked by catalytically inactive PI3Kgamma but not by hybrid PI3Kgammas. Thus, two major signals emerge from PI3Kgamma: phosphoinositides that target PKB and protein phosphorylation that activates MAPK.

Decreased RAGE expression in peripheral blood mononuclear cells of patients with rheumatoid arthritis.

OBJECTIVE: Interactions between the multiligand receptor for advanced glycation end products (RAGE) and its proinflammatory ligands (AGEs, S100/calgranulins, HMBG1, Mac-1) may contribute to inflammatory responses playing a key role in the pathogenesis of chronic inflammatory diseases such as in rheumatoid arthritis (RA). Peripheral blood mononuclear cells (PBMCs) participate in the development of chronic inflammatory diseases. This study investigated expression of the RAGE variants endogenous secretory RAGE (esRAGE), N-truncated RAGE (NtRAGE) and complete RAGE (cRAGE: encoding full-length RAGE, esRAGE and NtRAGE) in PBMCs of patients with RA in comparison to healthy control subjects (controls) and to patients with Crohn's disease (CD) as another chronic inflammatory disease. METHODS: The cRAGE, esRAGE and NtRAGE mRNA expression levels of PBMCs from controls, RA and CD patients were measured by real-time PCR. The RAGE protein expression was determined by Western blot analysis and the esRAGE plasma levels by ELISA. RESULTS: PBMCs of RA patients showed significantly decreased mRNA expression for cRAGE (46%), esRAGE (54.0%) and NtRAGE (52%) in comparison to healthy controls (100%). For CD patients, also a down-regulation but to a lower extent was found (cRAGE: 79%; esRAGE: 76%; NtRAGE: 69%). Related to controls, RA PBMCs showed a significantly reduced protein expression of full-length RAGE (53%) as well as significantly decreased esRAGE plasma concentrations (70%). CONCLUSION: The down-regulation of RAGE isoforms in RA PBMCs may contribute to reduced intracellular responses mediated by the cell-standing receptor as well as to a lowered capability of trapping inflammatory ligands by circulating esRAGE.

Morg1(+/-) heterozygous mice are protected from experimentally induced focal cerebral ischemia.

Focal cerebral ischemia (stroke) and reperfusion injury leads to acute and chronic brain damage. The increase of the hypoxia-inducible transcription factor alpha (HIF-α), an important transcription factor for several genes, may attenuate ischemic brain injury. We recently identified a new WD-repeat protein designated Morg1 (MAPK organizer 1) that interacts with prolyl hydroxylase 3 (PHD3), an important enzyme involved in the regulation of HIF-1α and HIF-2α expression. While homozygous Morg1(-/-) mice are embryonically lethal, heterozygous Morg1(+/-) mice have a normal phenotype. Brain vasculature as well as systolic blood pressure in Morg1(+/-) mice were indistinguishable from wild-type (WT) animals. We show here that Morg1(+/-) mice were partially protected from cerebral ischemia/reperfusion injury in comparison to WT (Morg1(+/+)) animals using the middle cerebral artery occlusion model (MCAO). Morg1(+/-) mice compared with WT animals revealed a significantly reduced infarct volume as detected by Nissl and Map 2 staining despite a similar restriction of blood flow in both mice genotypes as measured by laser Doppler flowmetry. Immunohistochemistry revealed specific Morg1 expression in reactive astrocytes in the ipsilateral (ischemic) hemisphere in Morg1(+/-) and WT mice, especially in the penumbral regions. In the contralateral hemisphere, Morg1 was not detectable. Furthermore, Morg1 mRNA expression was significantly enhanced in the ischemic brain of WT, but not in ischemic brain tissue obtained from Morg1(+/-) animals. However, HIF-1α was expressed with the same intensity in Morg1(+/-) and WT mice with no difference between the ipsilateral and contralateral hemispheres. No positive staining for HIF-2α was found in ischemic (ipsilateral) and non-ischemic (contralateral) brain regions in Morg1(+/+) and Morg1(+/-) mice. Almost no PHD3 staining was found in the contralateral hemispheres of either WT or heterozygous Morg1(+/-) mice. Transcript expression for the HIF1α-dependent genes erythropoietin (Epo) and vascular endothelial growth factor 164 (VEGF 164) were significantly reduced in the ischemic brain from Morg1(+/-) mice. Positive staining for PHD3 in the ipsilateral hemisphere of WT mice was suggested to occur in astrocytes. A compensatory increase in Morg1 expression in astrocytes in the penumbra may negatively influence infarct volume. It appears that these effects are independent of the PHD3-HIF1α axis.

Characterization of recombinant phosphatidylinositol 4-kinase beta reveals auto- and heterophosphorylation of the enzyme.

Phosphatidylinositol (PI) 4-kinases catalyze the synthesis of PI 4-phosphate, an important intermediate for the synthesis of membrane polyphosphoinositides, regulators of multiple cellular functions. Two mammalian PI 4-kinases have been cloned, a 230-kDa enzyme (alpha-form) and a 110-kDa (beta-form), both of which are inhibited by >0.1 microm concentrations of the PI 3-kinase inhibitor, wortmannin (WT). In the present study, we created a glutathione S-transferase-PI4Kbeta fusion protein for expression in Escherichia coli. The purified protein was biologically active and phosphorylated PI in its 4-position with WT sensitivity and kinetic parameters that were identical to those of purified bovine brain PI4Kbeta. In addition to its lipid kinase activity, the enzyme exhibited autophosphorylation that was enhanced by Mn(2+) ions and inhibited by WT and another PI 3-kinase inhibitor, LY 294002. The recombinant protein was unable to transphosphorylate, but its isolated C-terminal catalytic domain still displayed autophosphorylation, suggesting that the autophosphorylation site resides within the C-terminal catalytic domain of the protein and is held in position by intramolecular interactions. Autophosphorylation inhibited subsequent lipid kinase activity, which was reversed upon dephosphorylation, by protein phosphatases, PP1 and PP2A(1), suggesting that it may represent a regulatory mechanism for the enzyme. Phosphorylation of endogenous or overexpressed PI4Kbeta was also observed in COS-7 cells; however, the in vivo phosphorylation of the expressed protein was only partially inhibited by WT and also occurred in a catalytically inactive form of the enzyme, indicating the presence of additional phosphorylation site(s). Successful bacterial expression of PI4Kbeta should aid research on the structure-function relationships of this protein as well as of other, structurally related enzymes.

Retinoic acid induces selective expression of phosphoinositide 3-kinase gamma in myelomonocytic U937 cells.

Retinoic acid provokes growth inhibition and differentiation of the human leukemic cell line U937 to macrophage-like cells. We report that treatment of U937 cells with all-trans retinoic acid (ATRA), but not the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, results in an increased gene expression of the phosphoinositide 3-kinase (PI3K) species PI3Kgamma. PI3Kgamma expression was transcriptionally elevated, indicating that the PI3Kgamma gene may be a direct target for ATRA. In contrast to its effect on PI3Kgamma expression, ATRA did neither affect the levels of the PI3K species beta and delta nor the adapter proteins p85 and p101. Enhanced expression by ATRA of PI3Kgamma correlated with an increase of PI3K lipid kinase activity. Additionally, ATRA induced significant and lasting stimulation of mitogen-activated protein kinase/Erk2 activity. This effect was sensitive to the PI3K inhibitors wortmannin or LY294002 and, therefore, attributed to the upregulation of PI3Kgamma expression. Our findings suggest that sustained MAPK activation via PI3Kgamma precedes ATRA -dependent differentiation or growth inhibition.

Inhibition of Na,K-ATPase activates PI3 kinase and inhibits apoptosis in LLC-PK1 cells.

In the present study we used LLC-PK1 cells, a porcine renal proximal tubular cell line, to investigate whether PI3 kinase activation was involved in the anti-apoptotic effect of ouabain, a specific inhibitor of Na,K-ATPase. Apoptosis was induced by actinomycin D (Act D, 5 microM) and assessed by appearance of hypodiploid nuclei and DNA fragmentation. Ouabain attenuated Act D-induced apoptotic response in a dose-dependent manner. Incubation in a low K(+) medium (0.1 mM) which is another way to decrease Na,K-ATPase activity also had anti-apoptotic effect. Both ouabain and low K(+) medium increased the PI3 kinase activity in p85 immunoprecipitates. Ouabain, as well as incubation in the low K(+) medium, also increased the phosphorylation of Akt. Inhibition of PI3 kinase by either wortmannin or LY294002 reversed the cytoprotective effect of ouabain. These data together indicate that inhibition of Na,K-ATPase activates PI3 kinase in LLC-PK1 cells which could then exert the cytoprotective effect.