A fluorescent inositol phosphate glycan stimulates lipogenesis in rat adipocytes by extracellular activation alone

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

The Biomedical Uses of Inositols: A Nutraceutical Approach to Metabolic Dysfunction in Aging and Neurodegenerative Diseases

Inositols are sugar-like compounds that are widely distributed in nature and are a part of membrane molecules, participating as second messengers in several cell-signaling processes. Isolation and characterization of inositol phosphoglycans containing myo- or d-chiro-inositol have been milestones for understanding the physiological regulation of insulin signaling. Other functions of inositols have been derived from the existence of multiple stereoisomers, which may confer antioxidant properties. In the brain, fluctuation of inositols in extracellular and intracellular compartments regulates neuronal and glial activity. Myo-inositol imbalance is observed in psychiatric diseases and its use shows efficacy for treatment of depression, anxiety, and compulsive disorders. Epi- and scyllo-inositol isomers are capable of stabilizing non-toxic forms of β-amyloid proteins, which are characteristic of Alzheimer’s disease and cognitive dementia in Down’s syndrome, both associated with brain insulin resistance. However, uncertainties of the intrinsic mechanisms of inositols regarding their biology are still unsolved. This work presents a critical review of inositol actions on insulin signaling, oxidative stress, and endothelial dysfunction, and its potential for either preventing or delaying cognitive impairment in aging and neurodegenerative diseases. The biomedical uses of inositols may represent a paradigm in the industrial approach perspective, which has generated growing interest for two decades, accompanied by clinical trials for Alzheimer’s disease.

Insulin-mimicking bioactivities of acylated inositol glycans in several mouse models of diabetes with or without obesity

Insulin-mimetic species of low molecular weight are speculated to mediate some intracellular insulin actions. These inositol glycans, which are generated upon insulin stimulation from glycosylphosphatidylinositols, might control the activity of a multitude of insulin effector enzymes. Acylated inositol glycans (AIGs) are generated by cleavage of protein-free GPI precursors through the action of GPI-specific phospholipase C (GPI-PLC) and D (GPI-PLD). We synthesized AIGs (IG-1, IG-2, IG-13, IG-14, and IG-15) and then evaluated their insulin-mimicking bioactivities. IG-1 significantly stimulated glycogen synthesis and lipogenesis in 3T3-L1 adipocytes and rat isolated adipocytes dose-dependently. IG-2 significantly stimulated lipogenesis in rat isolated adipocytes dose-dependently. IG-15 also enhanced glycogen synthesis and lipogenesis in 3T3-L1 adipocytes. The administration of IG-1 decreased plasma glucose, increased glycogen content in liver and skeletal muscles and improved glucose tolerance in C57B6N mice with normal diets. The administration of IG-1 decreased plasma glucose in STZ-diabetic C57B6N mice. The treatment of IG-1 decreased plasma glucose, increased glycogen content in liver and skeletal muscles and improved glucose tolerance in C57B6N mice with high fat-diets and db/db mice. The long-term treatment of IG-1 decreased plasma glucose and reduced food intake and body weight in C57B6N mice with high fat-diets and ob/ob mice. Thus, IG-1 has insulin-mimicking bioactivities and improves glucose tolerance in mice models of diabetes with or without obesity.

Chemical biology of glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositols (GPIs) are complex glycolipids that are covalently linked to the C-terminus of proteins as a posttranslational modification. They anchor the attached protein to the cell membrane and are essential for normal functioning of eukaryotic cells. GPI-anchored proteins are structurally and functionally diverse. Many GPIs have been structurally characterized but comprehension of their biological functions, beyond the simple physical anchoring, remains largely speculative. Work on functional elucidation at a molecular level is still limited. This Review focuses on the roles of GPI unraveled by using synthetic molecules and summarizes the structural diversity of GPIs, as well as their biological and chemical syntheses.

Synthetic inositol phosphoglycans related to GPI lack insulin-mimetic activity

Insulin signaling has been suggested, at least in part, to be affected by an insulin-mimetic species of low molecular weight. These inositol phosphoglycans (IPGs) are generated upon growth hormone/cytokine stimulation and control the activity of a multitude of insulin effector enzymes. The minimal structural requirements of IPGs for insulin-mimetic action have been debated. Two types of IPGs were suggested, and the IPG-A type resembles the core glycan of glycosylphosphatidylinositol (GPI)-anchors. In fact, purified GPI-anchors of lower eukaryotic origin have been shown to influence glucose homeostasis. To elucidate active IPGs, a collection of synthetic IPGs designed on the basis of previous reports of activity were tested for their insulin-mimetic activity. In vitro and ex vivo assays in rodent adipose tissue as well as in vivo analyses in mice were employed to test the synthetic IPGs. None of the IPGs we tested mimic insulin actions as determined by PKB/Akt phosphorylation and quantification of glucose transport and lipogenesis. Furthermore, none of the IPGs had any effect in in vivo insulin tolerance assays. In stark contrast to previous claims, we conclude that neither of the compounds tested is insulin-mimetic.

Synthetic phosphoinositolglycans regulate lipid metabolism between rat adipocytes via release of GPI-protein-harbouring adiposomes

A novel molecular mechanism for the regulation of lipid metabolism by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in rat adipocytes was recently elucidated. It encompasses the translocation of the glycosylphosphatidylinositol-anchored (GPI-) and (c)AMP degrading enzymes Gce1 and CD73 from detergent-insoluble glycolipid-enriched microdomains of the plasma membrane (DIGs) to intracellular lipid droplets (LD), the incorporation of Gce1 and CD73 into vesicles (adiposomes) which are then released from donor adipocytes and finally the transfer of Gce1 and CD73 from the adiposomes to acceptor adipocytes, where they degrade (c)AMP at the LD surface. Here the stimulation of esterification and inhibition of lipolysis by synthetic phosphoinositolglycans (PIGs), such as PIG37, which represents the glycan component of the GPI anchor, are shown to be correlated to translocation from DIGs to LD and release into adiposomes of Gce1 and CD73. PIG37 actions were blocked upon disruption of DIGs, inactivation of PIG receptor and removal of adiposomes from the incubation medium as was true for those induced by palmitate, H2O2 or glimepiride. In contrast, only the latter actions were dependent on the GPI-specific phospholipase C (GPI-PLC), which may generate PIGs, or on exogenous PIG37 in case of inhibited GPI-PLC. At submaximal concentrations PIG37 and palmitate, H2O2 or glimepiride acted in synergistic fashion. These data suggest that PIGs provoke the transfer of GPI-proteins from DIGs via LD and adiposomes of donor adipocytes to acceptor adipocytes and thereby mediate the regulation of lipid metabolism by palmitate, H2O2 and glimepiride between adipocytes.

The biological activity of structurally defined inositol glycans

BACKGROUND: The inositol glycans (IGs) are glycolipid-derived carbohydrates produced by insulin-sensitive cells in response to insulin treatment. IGs exhibit an array of insulin-like activities including stimulation of lipogenesis, glucose transport and glycogen synthesis, suggesting that they may be involved in insulin signal transduction. However, because the natural IGs are structurally heterogeneous and difficult to purify to homogeneity, an understanding of the relationship between structure and biological activity has relied principally on synthetic IGs of defined structure. DISCUSSION: This article briefly describes what is known about the role of IGs in signal transduction and reviews the specific biological activities of the structurally defined IGs synthesized and tested to date. CONCLUSION: A pharmacophore for IG activity begins to emerge from the reviewed data and the structural elements necessary for activity are summarized.