Phosphatidylinositol 3,5-bisphosphate plays a role in the activation and subcellular localization of mechanistic target of rapamycin 1

The kinase complex mechanistic target of rapamycin 1 (mTORC1) plays an important role in controlling growth and metabolism. We report here that the stepwise formation of phosphatidylinositol 3-phosphate (PI(3)P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)P(2)) regulates the cell type-specific activation and localization of mTORC1. PI(3)P formation depends on the class II phosphatidylinositol 3-kinase (PI3K) PI3K-C2α, as well as the class III PI3K Vps34, while PI(3,5)P(2) requires the phosphatidylinositol-3-phosphate-5-kinase PIKFYVE. In this paper, we show that PIKFYVE and PI3K-C2α are necessary for activation of mTORC1 and its translocation to the plasma membrane in 3T3-L1 adipocytes. Furthermore, the mTORC1 component Raptor directly interacts with PI(3,5)P(2). Together these results suggest that PI(3,5)P(2) is an essential mTORC1 regulator that defines the localization of the complex.     

Autophagy works out

Autophagy is generally considered to be a cytoprotective response to stress, whether in the form of nutrient deprivation or the presence of dysfunctional organelles. He et?al. now show in Nature that exercise-induced autophagy is needed for some of the beneficial effects of exercise on metabolism (He et?al., 2012).     

Synthesis of phosphatidylinositol in rat liver microsomes is accompanied by the rapid formation of lysophosphatidylinositol

In mammalian cells, newly synthesized phosphatidylinositol (PI) has a fatty acid composition similar to its precursors, phosphatidic acid and CDP-diacylglycerol (DAG). It is then remodelled by deacylation/reacylation cycles to the predominant form, 1-stearoyl, 2-arachidonoyl PI. Incubation of dipalmitoyl CDP-DAG, [3H]inositol and Mg2+ with rat liver microsomes results in the rapid synthesis of PI, along with the simultaneous formation of multiple species of lysoPI. Analysis of the kinetics of formation of PI and lysoPI reveals no lag in the formation of lysoPI from PI. Moreover, evaluation of the concentration dependencies indicate nearly identical apparent Km values for PI synthesis compared with lysoPI synthesis for the substrates inositol (180 microM) and CDP-DAG (100 microM). The dependence on pH and the requirement for Mg2+ or Mn2+ are nearly identical for PI and lysoPI formation and the labelling of both lipids is similarly inhibited by submicromolar concentrations of calcium and by NEM. These results suggest that the formation of lysoPI is dependent on the initial, rate-limiting synthesis of PI. Pulse-chase analysis of the labelling of these lipids indicates that PI and lysoPI rapidly equilibrate after the initial slow synthesis of PI. In addition, it appears that only newly synthesized PI is involved in lysoPI formation. The extent of lysoPI formation depends upon the fatty acid composition of the added CDP-DAG. A number of experimental approaches demonstrate that lysoPI is not formed when pre-existing microsomal PI is labelled by head group exchange, perhaps because this PI has already undergone remodelling to polyenoic forms. These data suggest that the rapid deacylation of newly synthesized PI may represent the first step in PI remodeling.     

The specific protein phosphatase inhibitor okadaic acid differentially modulates insulin action.

The pleiotropic nature of insulin action suggests diverse mechanisms of signal transduction for the hormone. The specific protein phosphatase inhibitor, okadaic acid, is utilized to differentiate metabolic pathways that may be regulated by phosphorylation or dephosphorylation of key enzymes. In H-35 hepatoma cells, okadaic acid inhibits insulin-stimulated glycogen synthesis with an IC50 of 400 nM. In contrast, activation of lipogenesis by insulin is inhibited with an IC50 of 50 nM okadaic acid. The toxin also inhibits stimulation of lipogenesis in these cells by the insulin-sensitive inositol glycan enzyme modulator. In isolated rat adipocytes, insulin-stimulated lipogenesis is also inhibited by okadaic acid with an IC50 of approximately 1,700 nM. The antilipolytic effect of insulin in these cells is more sensitive to okadaic acid, exhibiting an IC50 of 150 nM. Maximal activation of lipogenesis by insulin is dramatically reduced by okadaic acid with no effect on the concentration required for half-maximal activation, whereas the sensitivity of insulin-induced antilipolysis is attenuated by okadaic acid, with no apparent reduction in the maximal effect of the hormone. Taken together, these data suggest that specific phosphatases may be differentially involved in some of the metabolic pathways regulated by insulin.     

Doxorubicin (Adriamycin) Cardiomyopathy—A Critical Review

Despite its vast utility in clinical oncology, the use of doxorubicin hydrochloride (Adriamycin) is limited by a potentially fatal cardiomyopathy. The following critical review, which examines the natural course, histopathologic effects, risk factors and monitoring indicators of this toxicity, also analyzes recent research of proposed mechanisms, including free radical formation with depletion of detoxifying enzymes, inhibition of vital enzyme systems and alterations in relative calcium concentrations. Prevention of the adverse reaction has been attempted by using such agents as α-tocopherol, selenium sulfide, coenzyme Q₁₀, sulfhydryl donors, nucleosides and razoxane, and via liposomal carriage and alternative methods of administration.     

The function of glycosyl phosphoinositides in hormone action

The molecular events involved in the cellular actions of insulin remain unexplained. Some of the acute actions of the hormone may be due to the intracellular generation of a chemical substance which modulates certain enzyme activities. Such an enzyme-modulating substance has been identified as an inositol phosphate-glycan, produced by the insulin-sensitive hydrolysis of a glycosyl-phosphatidylinositol (glycosyl-PtdIns) precursor. This precursor glycolipid is structurally similar to the glycosyl-phosphoinositide membrane protein anchor. The exposure of fat, liver or muscle cells to insulin results in the hydrolysis of glycosyl-PtdIns, giving rise to the inositol phosphate glycan and diacylglycerol. This hydrolysis reaction is catalysed by a glycosyl-PtdIns-specific phospholipase C. This enzyme has been characterized and purified from a plasma membrane fraction of liver. This reaction also results in the acute release of certain glycosyl-PtdIns-anchored proteins from the cell surface. Elucidation of the functional role of glycosyl-phosphoinositides in the generation of second messengers or the release of proteins may provide further insights into the pleiotropic nature of insulin action.     

Spatial determinants of specificity in insulin action

Insulin is a potent stimulator of intermediary metabolism, however the basis for the remarkable specificity of insulin's stimulation of these pathways remains largely unknown. This review focuses on the role compartmentalization plays in insulin action, both in signal initiation and in signal reception. Two examples are discussed: (1) a novel signalling pathway leading to the phosphorylation of the caveolar coat protein caveolin, and (2) a recently identified scaffolding protein, PTG, involved directly in the regulation of enzymes controlling glycogen metabolism.