Phospholipids, insulin

Cui, F., Shi, K., Zhang, L., Tao, A., Kawashima, Y. (2006). Biodegradable nanoparticles loaded with insulin-phospholipid complex for oral delivery preparation, in vitro characterization and in vivo evaluation. Journal of Controlled Release, 114, 242-250. 

Akt is activated by binding of plasma membrane phospholipids downstream of insulin receptors, growth and survival factor receptors in a phosphoinositide 3-kinases dependent manner. In humans, there are three genes in the Akt family Aktl, Akt2 and Akt3. Their respective fimctions are still under investigation. 

Phospholipid Insulin Receptor Tyrosin Kinases Growth Factors... 

Family of enzymes phosphorylating phosphatidylinositol (Ptdlns), PtdIns(4)phosphate, and PtdIns(4,5)phosphate in the 3-position. The Ptdlns(3 phospholipids are second messengers in processes like cell growth, cytoskeletal rearrangement, and vesicular transport. PI 3-kinases are heterodimers composed of a catalytic and a regulatory subunit. The enzymes are activated by insulin, many growth factors, and by a variety of cytokines. Their activity can be inhibited by wortmannin and LY294002. 

An important question arises about the effects of phospholipid composition and the function of membrane-bound enzymes. The phospholipid composition and cholesterol content in cell membranes of cultured cells can be modified, either by supplementing the medium with specific lipids or by incubation with different types of liposomes. Direct effects of phospholipid structure have been observed on the activity of the Ca2+-ATPase (due to changes in the phosphorylation and nucleotide binding domains) [37]. Evidence of a relationship between lipid structure and membrane functions also comes from studies with the insulin receptor [38]. Lipid alteration had no influence on insulin binding, but modified the kinetics of receptor autophosphorylation. 

Liposomes were formed from 1,2-dipalmitoylphosphatidylcholine (DPPC) and cholesterol (Choi) and the effect of liposomal entrapment on pulmonary absorption of insulin was related to oligomerization of insulin (Liu et al. 1993). Instillation of both dimeric and hexameric insulin produced equivalent duration of hypoglycemic response. However, the initial response from the hexameric form was slightly slower than that from dimeric insulin, probably due to lower permeability across alveolar epithelium of the hexameric form caused by larger molecular size. The intratracheal administration of liposomal insulin enhanced pulmonary absorption and resulted in an absolute bioavailability of 30.3%. Nevertheless, a similar extent of absorption and hypoglycemic effects was obtained from a physical mixture of insulin and blank liposomes and from liposomal insulin. This suggests a specific interaction of the phospholipid with the surfactant layer or even with the alveolar membrane. 

Phospholipids, such as DPPC, act as absorption enhancers in the lung. A significantly higher reduction in blood glucose levels was observed with a DPPC-insulin physical mixture compared to liposome-insulin following intratracheal instillation into rats (Figure 10.5) (Mitra et al. 2001). In this study, insulin alone, 1 U/kg, resulted... 

Li, Y.P. and Mitra, A.K., (1996). Effects of phospholipid chain length, concentration, charge, and vesicle size on pulmonary insulin absorption. Pharmaceut. Res., 13, 76-79. 

Yang, T.Z., et al. 2002. Phospholipid deformable vesicles for buccal delivery of insulin. Chem Pharm Bull Tokyo) 50 749. 

Lecithin (phosphatidylcholine) is a phospholipid, which may be isolated from either egg yolk or soybeans. It is commercially available in high purity for medical uses and has been used to enhance the absorption of insulin in vivo [26]. The antibiotic sodium fusidate, a steroid similar in molecular structure to bile salts has also been shown to have permeation enhancing properties for insulin in vitro [41]. 

Soybean-derived sterol mixture (SS), soybean-derived steryl glucosides (SG), and their individual components have been extensively studied for their ability to promote the nasal absorption of drugs, particularly insulin [79,80], Maitani et al. [79] demonstrated that the nasal administration of SG plus insulin to rabbits resulted in significant reductions in blood glucose. The effect of SG was dose dependent to 1%, with a plateau being reached thereafter. Muramatsu et al. [81] have demonstrated that SG perturbs the phospholipids in artificial membranes (i.e., liposomes). Furthermore, circular dichroism studies with insulin in the presence or absence of SG have indicated that the enhancer had little effect on the dissociation of insulin hexamers to monomers. These results suggest that the action of SS and SG involves interaction with the nasal membrane rather than interaction with insulin molecules. 

Since its discovery, isolation, and purification in the early twentieth century, insulin has been administered to diabetic patients exclusively by injection until the recent introduction of inhaled insulin. Insulin possesses certain physiochemical properties that contribute to its limited absorption from the gastrointestinal tract, and requires subcutaneous injection to achieve clinically relevant bioavailability. With a molecular size of 5.7 kDa, insulin is a moderately sized polypeptide composed of two distinct peptide chains designated the A chain (21 amino acid residues) and the B chain (30 amino acid residues) and joined by two disulfide bonds. Like all polypeptides, insulin is a charged molecule that cannot easily penetrate the phospholipid membrane of the epithelial cells that line the nasal cavity. Furthermore, insulin monomers self-associate into hexameric units with a molecular mass greater than 30 kDa, which can further limit its passive absorption. Despite these constraints, successful delivery of insulin via the nasal route has been reported in humans and animals when an absorption enhancer was added to the formulation. 

It was shown, however, that the ability of insulin to release GIPs could be mimicked by an inositol phospholipid-specific phospholipase C. This enzyme is distinct from the phospholipidase C which acts to break down polyphosphatidylinositols as it acts specifically on phosphatidylinositol and has been used to release proteins which are anchored to membranes by the covalent attachment to phosphatidylinositol... 

Fig. 4. Proposed production of a glycan-inositol phosphate mediator by insulin. This summarizes Sal-tiel and Cuatrecasas concept for the production of novel mediators through insulin s action. It may, by analogy with the receptor-mediated stimulation of inositol phospholipid metabolism, involve a G-pro-tein. Here it is postulated that the putative Glns may play such a role.

There is now considerable evidence to suggest that hormones which stimulate inositol phospholipid metabolism do so through a distinct G-protein [90-92]. It is thus possible that insulin might activate a G-protein in order to stimulate the proposed phosphatidylinositol-specific phospholipase C claimed to produce the GIPs insulin mediator (Fig. 4). 

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