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

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.