Two different mechanisms for activation of cyclic PIP synthase: by a G protein or by protein tyrosine phosphorylation

Activated Inositol Phosphate, Substrate for Synthesis of Prostaglandylinositol Cyclic Phosphate (Cyclic PIP)-The Key for the Effectiveness of Inositol-Feeding

The natural cyclic AMP antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), is biosynthesized from prostaglandin E (PGE) and activated inositol phosphate (n-Ins-P), which is synthesized by a particulate rat-liver-enzyme from GTP and a precursor named inositol phosphate (pr-Ins-P), whose 5-ring phosphodiester structure is essential for n-Ins-P synthesis. Aortic myocytes, preincubated with [3H] myo-inositol, synthesize after angiotensin II stimulation (30 s) [3H] pr-Ins-P (65% yield), which is converted to [3H] n-Ins-P and [3H] cyclic PIP. Acid-treated (1 min) [3H] pr-Ins-P co-elutes with inositol (1,4)-bisphosphate in high performance ion chromatography, indicating that pr-Ins-P is inositol (1:2-cyclic,4)-bisphosphate. Incubation of [3H]-GTP with unlabeled pr-Ins-P gave [3H]-guanosine-labeled n-Ins-P. Cyclic PIP synthase binds the inositol (1:2-cyclic)-phosphate part of n-Ins-P to PGE and releases the [3H]-labeled guanosine as [3H]-GDP. Thus, n-Ins-P is most likely guanosine diphospho-4-inositol (1:2-cyclic)-phosphate. Inositol feeding helps patients with metabolic conditions related to insulin resistance, but explanations for this finding are missing. Cyclic PIP appears to be the key for explaining the curative effect of inositol supplementation: (1) inositol is a molecular constituent of cyclic PIP; (2) cyclic PIP triggers many of insulin's actions intracellularly; and (3) the synthesis of cyclic PIP is decreased in diabetes as shown in rodents.

Metformin's Mechanism of Action Is Stimulation of the Biosynthesis of the Natural Cyclic AMP Antagonist Prostaglandylinositol Cyclic Phosphate (Cyclic PIP)

Metformin is the leading drug for treating type 2 diabetics, but the mechanism of action of metformin, despite some suggested mechanisms such as the activation of the AMP-kinase, is largely unknown. Among its many positive effects are the reduction of blood glucose levels, the inhibition of cyclic AMP synthesis, gluconeogenesis and an increase in sensitivity to insulin. Recent studies have described the natural antagonist of cyclic AMP, prostaglandylinositol cyclic phosphate. Synthesis of cyclic PIP is stimulated in all organs by hormones such as insulin and also by drugs such as metformin. Its primary action is to trigger the dephosphorylation of proteins/enzymes, phosphorylated on serine/threonine residues. Cyclic PIP triggers many of the regulations requested by insulin. The parallels between the beneficial effects of metformin and the regulations triggered by cyclic PIP suggest that the mechanism of action of this key drug may well be explained by its stimulation of the synthesis of cyclic PIP.

Prostaglandylinositol cyclic phosphate, the natural antagonist of cyclic AMP

While searching for a counterpart to cyclic AMP, a new compound was found to inhibit adenylate cyclase. It was identified as prostaglandyl-(15-4')-myo-inositol (1':2'-cyclic)-phosphate (cyclic PIP). The substrates for its biosynthesis are prostaglandin E (PGE) and the novel inositol phosphate, guanosine diphospho-4-myo-inositol 1:2-cyclic phosphate (n-IP). The basic regulatory properties of cyclic PIP are to inhibit dose-dependently protein kinase A (PKA) and to seven-fold activate protein ser/thr phosphatase holoenzyme. These regulations occur as rapidly as the activation of PKA by cyclic AMP. Such regulatory properties are essential for the meticulous regulation of the equilibrium between the phospho- and de-phospho-form of interconvertible enzymes. The synthesis of cyclic PIP is stimulated by insulin and noradrenaline (α-receptor action). The insulin-stimulated cyclic PIP synthase is active in a tyrosine-phosphorylated state. A comparable characterization of the adrenaline-stimulated cyclic PIP synthase is still incomplete. In streptozotocin-diabetic rats, the hormonal stimulation of cyclic PIP synthesis decreases with time. Cyclic PIP synthesis is activated by biguanides as metformin two to four-fold and by antihypertensive drugs two-fold. Inhibition of cyclic PIP synthesis leads to a metabolic state as observed in early-stage type-2 diabetes. In summary, all living cells synthesize cyclic PIP, which switches on anabolism, whereas cyclic AMP triggers catabolism.

Prostaglandylinositol cyclic phosphate synthase activity in the liver of insulin-resistant rhesus monkeys before and after a euglycemic hyperinsulinemic clamp

Prostaglandylinositol cyclic phosphate (cPIP), functionally a cAMP antagonist, is a novel, low-molecular weight mediator of insulin action. Both essential hypertension and type 2 diabetes may be associated with a reduction of cPIP synthesis. In intact cells and in plasma membranes, cPIP synthesis is stimulated by insulin, which activates cPIP synthase by tyrosine phosphorylation. We measured the activities of cPIP synthase in the homogenates of freeze-clamped and then lyophilized liver samples from five insulin-resistant, adult rhesus monkeys, obtained under basal fasting conditions and again under maximal insulin stimulation during a euglycemic hyperinsulinemic clamp. The mean cPIP synthase activity in basal samples (0.33 +/- 0.09 pmol/min/mg protein) was not significantly different at the end of the clamp (0.24 +/- 0.11 pmol/min/mg protein). Basal cPIP synthase activityVoL 12, No. 1, 2001 was directly related to both basal cAMP content and basal fractional activity of cAMP-dependent protein kinase (PKA): r=0.85, p<0.05 and r=0.86, p<0.05, respectively. In turn, insulin-stimulated cPIP synthase activity was inversely related to both the insulin-stimulated fractional activity of PKA (r=0.89, p<0.02) and the insulin-stimulated total PKA activity: r=0.94, p<0.005. The findings suggest that in the liver of insulin-resistant rhesus monkeys, cPIP synthase activity, which leads to the synthesis of the low-molecular weight mediator cPIP, may oppose cAMP synthesis and PKA activity.

Prostaglandylinositol cyclic phosphate (cPIP): a novel second messenger of insulin action. Comparative analysis of two kinds of "insulin mediators"

Insulin induces a broad spectrum of effects over a wide time interval. It also stimulates the phosphorylation of some cellular proteins, while decreasing the state of phosphorylation of others. These observations indicate the presence of different, but not necessarily mutually exclusive, pathways of insulin action. One well-known pathway represents a phosphorylation cascade initiated by the tyrosine kinase activity of the insulin receptor followed by involvement of different MAP-kinases. Another pathway suggests the existence of low molecular weight insulin mediators whose synthesis and/or release is initiated by insulin. Comparable analysis of two kinds of insulin mediators, namely inositolphosphoglycans and prostaglandylinositol cyclic phosphate (cPIP), has been carried out. It has been shown that the expression of a number of enzymes, such as phospholipase A(2), phospholipase C, cyclo-oxygenase and IRS-1-like enzyme, could regulate the biosynthesis of cPIP in both normal and diabetes-related conditions. Data on the activity of a key enzyme of cPIP biosynthesis termed cPIP synthase (IRS-1-like enzyme) in various monkey tissues before and twice during an euglycemic hyperinsulinemic clamp have been presented. It has been concluded that in vivo insulin increases cPIP synthase activity in both liver and subcutaneous adipose tissue of lean normal monkeys. It has been also suggested that abnormal production of cPIP could be related to several pathologies including glucocorticoid-induced insulin resistance and diabetic embryopathy. Further studies on cPIP and other types of insulin mediators are necessary to aid our understanding of insulin action.

Phosphatidylinositol 3-kinase and prostaglandylinositol cyclic phosphate (cyclic PIP), a mediator of insulin action, in the signal transduction of insulin

It has been suggested that downstream signaling from the insulin receptor to the level of the protein kinases and protein phosphatases is accomplished by prosta-glandylinositol cyclic phosphate (cyclic PIP), a proposed second messenger of insulin. However, evidence points also to both phosphatidylinositol 3-kinase, which binds to the tyrosine phosphorylated insulin receptor substrate-1, and the Ras complex in insulin's downstream signaling. We have examined whether a correlation exists between these various observations. It was found that wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase, prevented insulin-induced, as well as cyclic PIP-induced activation of glucose transport, indicating that PI 3-kinase action on glucose transport involves downstream signaling of both insulin and cyclic PIP. Wortmannin has no effect on cyclic PIP synthase activity nor on the substrate production for cyclic PIP synthesis either, indicating that the functional role of PI 3-kinase is exclusively downstream of cyclic PIP.

Prostaglandin deficiency promotes sensitization of adenylyl cyclase

Inhibition of prostaglandin synthesis by the drug indomethacin suppresses the synthesis of the cyclic AMP antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), and leads to a metabolic state comparable to type II diabetes. It was of interest whether prostaglandin-deficiency likewise causes sensitization of adenylyl cyclase, as this has been reported for the diabetic state. In liver plasma membranes of indomethacin-treated male rats, basal and forskolin-stimulated cyclic AMP synthesis remained unchanged when compared to untreated control rats. In control rats, stimulation of cyclic AMP synthesis by fluoride (2.2-fold) or glucagon (3.5-fold) was much lower than stimulation by forskolin (6.6-fold). In contrast, in indomethacin-treated rats, stimulation of cAMP synthesis by fluoride (4.6-fold) or glucagon (5.2-fold) nearly matched the stimulation by forskolin (6.4-fold). The level of alpha1-adrenergic receptors was slightly reduced, from 450 to 320 fmol/mg protein, by the indomethacin treatment. Independent of the treatment by indomethacin, stimulation of cyclic AMP synthesis by adrenaline failed, in agreement with the low density of adrenergic beta-receptors. In conclusion, PGE deficiency sensitizes adenylyl cyclase in rat liver for G protein-coupled receptors (glucagon) and also for fluoride.