Emerging role of inositol monophosphatase in cancer

Inositol monophosphatase (IMPase) is an enzyme with two homologs-IMPA1 and IMPA2-that is responsible for dephosphorylating myo-inositol monophosphate to generate myo-inositol. IMPase has been extensively studied in neuropsychiatric diseases and is regarded as a susceptibility gene. Recently, emerging evidence has implied that IMPase is linked to cancer development and progression and correlates with patient survival outcomes. Interestingly, whether it acts as a tumor-promoter or tumor-suppressor is inconsistent among different research studies. In this review, we summarize the latest findings on IMPase in cancer, focusing on exploring the underlying mechanisms for its pro- and anticancer roles. In addition, we discuss the potential methods of IMPase regulation in cancer cells and the possible approaches for IMPase intervention in clinical practice.

Strain-specific clearance of seed-dependent tau aggregation by lithium-induced autophagy

Different conformational strains of tau have been implicated in the clinicopathological heterogeneity of tauopathies. In this study, we hypothesized that distinct strains are degraded in a different manner. Lithium, a drug for bipolar disorder, had previously been reported to reduce aggregation-prone protein content by promoting autophagy. Here, we assessed the effects of lithium on tau aggregates using different tauopathy brain seeds. SH-SY5Y cells were transfected with C-terminal tau fragment Tau-CTF24 (residues 243-441), and Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and corticobasal degeneration (CBD) brain seeds were introduced. After 48-h lithium treatment, sarkosyl-insoluble fractions were prepared. Lithium treatment was found to reduce the amount of insoluble tau and p62, and increase LC3-II levels along with the number of autophagic vacuoles in AD-seeded cells. The effects were lower in case of CBD seeds, and comparable between PSP and AD seeds. An inhibitor of myo-inositol monophosphatase (IMPase) also demonstrated similar effects. Overall, the study suggested that aggregated tau protein is degraded by lithium-induced autophagy, influencing IMPase in a strain-specific manner.

Crystallization and structure of ebselen bound to Cys141 of human inositol monophosphatase

Inositol monophosphatase (IMPase) is inhibited by lithium, which is the most efficacious treatment for bipolar disorder. Several therapies have been approved, or are going through clinical trials, aimed at the replacement of lithium in the treatment of bipolar disorder. One candidate small molecule is ebselen, a selenium-containing antioxidant, which has been demonstrated to produce lithium-like effects both in a murine model and in clinical trials. Here, the crystallization and the first structure of human IMPase covalently complexed with ebselen, a 1.47 Å resolution crystal structure (PDB entry 6zk0), are presented. In the complex with human IMPase, ebselen in a ring-opened conformation is covalently attached to Cys141, a residue located away from the active site. IMPase is a dimeric enzyme and in the crystal structure two adjacent dimers share four ebselen molecules, creating a tetramer with approximate 222 symmetry. In the crystal structure presented in this publication, the active site in the tetramer is still accessible, suggesting that ebselen may function as an allosteric inhibitor or may block the binding of partner proteins.

Co-crystallization of human inositol monophosphatase with the lithium mimetic L-690,330

Lithium, which is still the gold standard in the treatment of bipolar disorder, has been proposed to inhibit inositol monophosphatase (IMPase) and is hypothesized to exert its therapeutic effects by attenuating phosphatidylinositol (PI) cell signalling. Drug-discovery efforts have focused on small-molecule lithium mimetics that would specifically inhibit IMPase without exhibiting the undesired side effects of lithium. L-690,330 is a potent bisphosphonate substrate-based inhibitor developed by Merck Sharp & Dohme. To aid future structure-based inhibitor design, determination of the exact binding mechanism of L-690,330 to IMPase was of interest. Here, the high-resolution X-ray structure of human IMPase in complex with L690,330 and manganese ions determined at 1.39 Å resolution is reported.

Crystal structure of cbbF from Zymomonas mobilis and its functional implication

A phosphate group at the C1-atom of inositol-monophosphate (IMP) and fructose-1,6-bisphosphate (FBP) is hydrolyzed by a phosphatase IMPase and FBPase in a metal-dependent way, respectively. The two enzymes are almost indiscernible from each other because of their highly similar sequences and structures. Metal ions are bound to residues on the β1- and β2-strands and one mobile loop. However, FBP has another phosphate and FBPases exist as a higher oligomeric state, which may discriminate FBPases from IMPases. There are three genes annotated as FBPases in Zymomonas mobilis, termed also cbbF (ZmcbbF). The revealed crystal structure of one ZmcbbF shows a globular structure formed by five stacked layers. Twenty-five residues in the middle of the sequence form an α-helix and a β-strand, which occupy one side of the catalytic site. A non-polar Leu residue among them is protruded to the active site, pointing out unfavorable access of a bulky charged group to this side. In vitro assays have shown its dimeric form in solution. Interestingly, two β-strands of β1 and β2 are disordered in the ZmcbbF structure. These data indicate that ZmcbbF might structurally belong to IMPase, and imply that its active site would be reorganized in a yet unreported way.

Potential role and therapeutic interests of myo-inositol in metabolic diseases

Several inositol isomers and in particular myo-inositol (MI) and D-chiro-inositol (DCI), were shown to possess insulin-mimetic properties and to be efficient in lowering post-prandial blood glucose. In addition, abnormalities in inositol metabolism are associated with insulin resistance and with long term microvascular complications of diabetes, supporting a role of inositol or its derivatives in glucose metabolism. The aim of this review is to focus on the potential benefits of a dietary supplement of myo-inositol, by far the most common inositol isomer in foodstuffs, in human disorders associated with insulin resistance (polycystic ovary syndrome, gestational diabetes mellitus or metabolic syndrome) or in prevention or treatment of some diabetic complications (neuropathy, nephropathy, cataract). The relevance of such a nutritional strategy will be discussed for each context on the basis of the clinical and/or animal studies. The dietary sources of myo-inositol and its metabolism from its dietary uptake to its renal excretion will be also covered in this review. Finally, the actual insights into inositol insulin-sensitizing effects will be addressed and in particular the possible role of inositol glycans as insulin second messengers.