Inositol phosphates and inositol phospholipids: how big is the iceberg?

Inositol phosphates and phosphoinositides in health and disease

In the past two decades, considerable progress has been made toward understanding inositol phosphates and PI metabolism. However, there is still much to learn. The present challenge is to understand how inositol phosphates and PIs are compartmentalized, identify new targets of inositol phosphates and PIs, and elucidate the mechanisms underlying spatial and temporal regulation of the enzymes that metabolize inositol phosphates and PIs. Answers to these questions will help clarify the mechanisms of the diseases associated with these molecules and identify new possibilities for drug design.

Ca2+-dependent elevation of inositol 1,4,5-trisphosphate level induced by freezing or homogenization of tissues and cells

Various cells and tissues contain high basal levels of inositol 1,4,5-trisphosphate, raising questions about the functional significance of inositol 1,4,5-trisphosphate in some tissues such as the heart. We used intact tissue and isolated cells from heart and liver of adult rats to examine if different fixation procedures might artificially elevate the level of inositol 1,4,5-trisphosphate. The basal level of inositol 1,4,5-trisphosphate in intact, freeze-clamped cardiac tissue from adult rats was 10 times higher than in isolated, non-frozen cardiomyocytes, while freeze-clamped liver contained approximately 4 times higher inositol 1,4,5-trisphosphate levels than isolated, non-frozen hepatocytes. Stimulation with norepinephrine induced a significant increase in the inositol 1,4,5-trisphosphate level in isolated cardiomyocytes, whereas no significant increase was observed in freeze-clamped cardiac tissue. Freezing of isolated cardiomyocytes or hepatocytes before extraction increased basal inositol 1,4,5-trisphosphate levels 3 times. In cellular homogenates prepared in the presence of EGTA and stored at 4 degrees , readdition of calcium resulted in a time-dependent increase in inositol 1,4,5-trisphosphate mass and a decrease in the mass of phosphatidylinositol 4,5-bisphosphate (PIP(2)). The reaction was essentially complete within 30 sec. in homogenates from cardiomyocytes, while PIP(2) hydrolysis was slower in hepatocyte homogenates. Perfusion of intact rat hearts with EGTA present during the last 2 min. of perfusion, followed by freeze-clamping, resulted in basal inositol 1,4,5-trisphosphate levels comparable to those in isolated cardiomyocytes, and norepinephrine stimulation increased inositol 1,4,5-trisphosphate mass by approximately 80%. The presence of EGTA did not significantly affect PIP(2) levels in perfused hearts. The results suggest that freezing or homogenization of intact tissue and isolated cells may result in Ca(2+)-dependent activation of phospholipase C, leading to high basal inositol 1,4,5-trisphosphate levels that may mask agonist-induced changes.

Enzyme-assisted total synthesis of the optical antipodes D-myo-inositol 3,4,5-trisphosphate and D-myo-inositol 1,5, 6-trisphosphate: aspects of their structure-activity relationship to biologically active inositol phosphates

Unambiguous total syntheses of both optical antipodes of the enantiomeric pair D-myo-inositol 3,4,5-trisphosphate (Ins(3,4,5)P3) and D-myo-inositol 1,5,6-trisphosphate (Ins(1,5,6)P3) are described. The ring system characteristic of myo-inositol was constructed de novo from p-benzoquinone. X-ray data for the enzymatically resolved (1S,2R,3R,4S)-1,4-diacetoxy-2,3-dibromocyclohex-5-ene enabled the unequivocal assignment of the absolute configuration. Subsequent transformations under stereocontrolled conditions led to enantiopure C2-symmetrical 1,4-(di-O-benzyldiphospho)conduritol B derivatives. Their synthetic potential was exploited to prepare Ins(3,4,5,6)P4 and Ins(1,4,5,6)P4 in three steps. With a recently identified and partially purified InsP5/InsP4 phosphohydrolase from Dictyostelium discoideum, these enantiomers could be converted to the target compounds, Ins(3,4,5)P3 and Ins(1,5,6)P3, on a preparative scale. An HPLC system employed for both purification of the inositol phosphates and analytical runs ensured that the products were isomerically homogeneous. The sensitivity of detection achieved by a complexometric postcolumn derivatization method indicates that the complexation properties of Ins(3,4,5)P3/Ins(1,5,6)P3 resemble those of Ins(1,2,3)P3, a compound with antioxidant potential. The set of inositol phosphates synthesized was used to clarify structural motifs important for molecular recognition by p42(IP4), a high-affinity Ins(1,3,4,5)P4/PtdIns(3,4,5)P3-specific binding protein from pig cerebellum.

Multiple inositol polyphosphate phosphatase: evolution as a distinct group within the histidine phosphatase family and chromosomal localization of the human and mouse genes to chromosomes 10q23 and 19

Multiple inositol polyphosphate phosphatase is the only enzyme known to hydrolyze the abundant metabolites inositol pentakisphosphate and inositol hexakisphosphate. We have previously demonstrated that the chick homolog of multiple inositol polyphosphate phosphatase, designated HiPER1, has a role in growth plate chondrocyte differentiation. The relationship of these enzymes to intracellular signaling is obscure, and as part of our investigation we have examined the murine ((MMU)Minpp1) and human ((HSA)MINPP1) homologs. Northern blot analysis demonstrated expression of ((MMU)Minpp1 in a variety of mouse tissues, comparable to the expression of other mammalian homologs, but less restricted than the expression of HiPER1 in chick. A purified (MMU)Minpp1 fusion protein cleaved phosphate from inositol (1,3,4,5)-tetrakisphosphate and para-nitrophenyl phosphate. When the presumptive active site histidine was altered to alanine by site-directed mutagenesis, enzyme activity was abolished, confirming the classification of (MMU)Minpp1 as a histidine phosphatase. The amino acid sequences of the murine and human MINPP proteins share >80% identity with the rat enzyme and >56% identity with HiPER1, with conservation of the C-terminal consensus sequence that retains proteins in the endoplasmic reticulum. The intron/exon structure of the mammalian (MMU)Minpp1 and (HSA)MINPP1 genes is also conserved compared to the chick HiPER1 gene. Sequence analysis of plant and fruit fly MINPP homologs supports the hypothesis that the MINPP enzymes constitute a distinct evolutionary group within the histidine phosphatase family. We have mapped (HSA)MINPP1 to human chromosome 10q23 by fluorescence in situ hybridization, YAC screening, and radiation hybrid mapping. This assignment places (HSA)MINPP1 in a region of chromosome 10 that is frequently mutated in human cancers and places (HSA)MINPP1 proximal to the tumor suppressor PTEN, which maps to 10q23.3. Using a radiation hybrid panel, we localized (MMU)Minpp1 to a region of mouse chromosome 19 that includes the murine homolog of Pten. The evolutionary conservation of this novel enzyme within the inositol polyphosphate pathway suggests a significant role for multiple inositol polyphosphate phosphatase throughout higher eukaryotes.

2-Deoxy derivative is a partial agonist of the intracellular messenger inositol 3,4,5,6-tetrakisphosphate in the epithelial cell line T84

We have synthesized the first deoxy analogues of myo-inositol 3,4,5, 6-tetrakisphosphate (1) [Ins(3,4,5,6)P4], rac-2-deoxy-myo-inositol 3, 4,5,6-tetrakisphosphate (rac-2), 2-deoxy-myo-inositol 1,4,5, 6-tetrakisphosphate (ent-2), and rac-1-deoxy-myo-inositol 3,4,5, 6-tetrakisphosphate (rac-3). In order to evaluate the binding properties of the three derivatives to the yet unidentified intracellular binding sites for Ins(3,4,5,6)P4, the analogues were converted to membrane-permeant derivatives. Starting with common inositol precursors, various forms of Barton-McCombie deoxygenation and classical protection/deprotection procedures yielded the desired precursors rac-1-O-butyryl-2-deoxy-myo-inositol (rac-12), ent-3-O-butyryl-2-deoxy-myo-inositol (ent-12), and rac-2-O-butyryl-1-deoxy-myo-inositol (rac-19), respectively. Phosphorylation and subsequent deprotection yielded rac-2, ent-2, and rac-3. Alternatively, phosphorylation followed by alkylation with acetoxymethyl bromide gave the membrane-permeant derivatives 1-O-butyryl-2-deoxy-myo-inositol 3,4,5,6-tetrakisphosphate octakis(acetoxymethyl) ester (rac-5), 3-O-butyryl-2-deoxy-myo-inositol 1,4,5,6-tetrakisphosphate octakis(acetoxymethyl) ester (ent-5), and 2-O-butyryl-1-deoxy-myo-inositol 3,4,5,6-tetrakisphosphate octakis(acetoxymethyl) ester (rac-6), respectively. We examined the potency of the membrane-permeant deoxy derivatives in inhibition of calcium-mediated chloride secretion (CaMCS) in intact T84 cells. Compared to the 1,2-di-O-butyryl-myo-inositol 3,4,5, 6-tetrakisphosphate octakis(acetoxymethyl) ester (4), the membrane-permeant derivative of Ins(3,4,5,6)P4 (1), the 2-deoxy derivative (rac-5) exhibited a slightly weaker inhibitory effect, while the enantiomerically pure 2-deoxy-Ins(1,4,5,6)P4 (ent-5) and the 1-deoxy derivative (rac-6) were inactive. As expected, the effect was stereoselective. Thus, the 1-hydroxyl group is apparently essential for binding and the inhibitory effect of Ins(3,4,5,6)P4 on chloride secretion, whereas the 2-hydroxyl group plays a less important role.