Effect of phospholipase treatment on insulin receptor signal transduction

Chronic caloric restriction alters muscle membrane fatty acid content

Chronic caloric restriction (CR) has been demonstrated to increase longevity in lower species and studies are ongoing to evaluate its effect in higher species. A consistent metabolic feature of CR is improved insulin sensitivity and lowered lifetime glycemia, yet the mechanism responsible is currently unknown. However, the membrane's physiochemical properties, as determined by phospholipid composition, have been related to insulin action in animal and human studies and CR has been reported to alter membrane lipid content. We evaluated muscle membrane fatty acid content in rodents randomized to CR versus control diets for up to 29 months. CR was observed to increase the membrane content of C22:6 (docosahexaenoate) and to decrease C18:2 content. The membrane lipid content was related to insulin levels but not to parameters assessing glycemic control. This study suggests that membrane lipids, in particular 22:6, may contribute to the variation in insulin sensitivity seen with age.

Incorporation of exogenous lipids modulates insulin signaling in the hepatoma cell line, HepG2

The lipid content of cultured cells can be experimentally modified by supplementing the culture medium with specific lipids or by the use of phospholipases. In the case of the insulin receptor, these methods have contributed to a better understanding of lipid disorder-related diseases. Previously, our laboratory demonstrated that experimental modification of the cellular lipid composition of an insulin-sensitive rat hepatoma cell line (ZHC) resulted in an alteration in insulin receptor binding and biological action (Bruneau et al., Biochim. Biophys. Acta 928 (1987) 287-296/297-304). In this paper, we have examined the effects of lipid modification in another hepatoma cell line, HepG2. Exogenous linoleic acid (LA, n-6), eicosapentaenoic acid (EPA, n-3) or hemisuccinate of cholesterol (CHS) was added to HepG2 cells, to create a cellular model in which membrane composition was modified. In this model, we have shown that: (1) lipids were incorporated in treated HepG2 cells, but redistributed differently when compared to treated ZHC cells; (2) that insulin signaling events, such as insulin receptor autophosphorylation and the phosphorylation of the major insulin receptor substrate (IRS-1) were altered in response to the addition of membrane lipids or cholesterol derived components; and (3) different lipids affected insulin receptor signaling differently. We have also shown that the loss of insulin receptor autophosphorylation in CHS-treated cells can be correlated with a decreased sensitivity to insulin. Overall, the results suggest that the lipid environment of the insulin receptor may play an important role in insulin signal transduction.

Modulatory effects of peroxovanadates on insulin receptor binding

The insulin-mimetic effects exhibited by vanadate, hydrogen peroxide, and some peroxovanadates have recently been shown to occur, at least in part, through an activation of the insulin receptor tyrosine kinase activity. In this study, we examine the effects of these compounds on insulin receptor binding using receptor preparations from human placental membranes. Among the 16 vanadium(V)-peroxo complexes studied, the [VO(O2)2(bipy)]- ion, where bipy = 2,2'-bipyridine, was found to increase insulin receptor binding by 24%, whereas the [VO(O2)2(en)]- ion, where en = ethylenediamine, was found to reduce insulin receptor binding by about the same amount under steady-state conditions. Scatchard analysis of the binding data indicates that the observed effect of the [VO(O2)2(bipy)]- ion on insulin receptor binding is exerted mainly at the high-capacity low-affinity sites. Furthermore, this modulatory effect is reversible and requires a continuous presence of the compound. By perturbing the membrane environment of the insulin receptor, we have shown that an intact membrane structure is essential for an observable effect. The observed modulation of insulin receptor binding by peroxovanadates is interpreted in terms of a ternary complex model in which the peroxovanadate acts as an allosteric effector modulating the binding equilibrium between insulin and its receptor.

Inhibition of the insulin receptor tyrosine kinase by phosphatidic acid

The lipid second messenger, phosphatidic acid, inhibits the intrinsic tyrosine kinase activity of the insulin receptor in detergent-lipid mixed micelles or in reconstituted membranes. Enzymatic studies revealed that this lipid second messenger inhibits the catalytic activity of partially purified insulin receptor without affecting the affinity of the receptor for insulin. Selectivity in the protein-lipid interaction is suggested by the inability of several other acidic lipids to affect the kinase activity of the receptor and by the relative insensitivity of the inhibition to increasing ionic strength and, in some cases, micelle surface charge. Lysophosphatidic acid and phosphatidic acids with short acyl chains do not affect significantly the receptor's kinase activity, suggesting that hydrophobic interactions are involved in the inhibition. Thus, both a high affinity interaction of the insulin receptor with the phosphate headgroup and a stabilizing hydrophobic interaction with the acyl chains contribute to the inhibitory protein-lipid interaction. The selective sensitivity of the insulin receptor to phosphatidic acid suggests that the receptor-mediated generation of this lipid in the plasma membrane could negatively modulate insulin receptor function.

Role of rat liver plasma membrane phospholipids in regulation of protein kinase activities

The role of rat liver plasma membrane phospholipids in the regulation of protein kinase A, protein kinase C and tyrosine kinase activities was investigated. Plasma membrane composition was modified by phospholipase A2, phospholipase C and phospholipase D treatment and subsequent incorporation of various phospholipids. Phospholipase A2 deactivated the three types of protein kinases, while phospholipase C and D affected the enzymes in a different manner. Phosphatidylcholine and sphingomyelin were found to be the most effective activators of protein kinase A and tyrosine kinase. Incorporation of sphingomyelin and phosphatidylserine into partially delipidated plasma membranes resulted in a significant stimulation of protein kinase C activity. Since sphingomyelin appeared to be a specific effector of the three types of protein kinases under investigation, one might suggest that its role in cellular signaling could be manifested via regulation of protein kinase C as well as via modulation of protein kinase A and tyrosine kinase activities.

Heterogeneity of glycosylphosphatidylinositol-anchored alkaline phosphatase of calf intestine

A method is described for large-scale purification of glycosylphosphatidylinositol-anchored alkaline phosphatase from intestinal mucosa and chyme to homogeneity. Both enzyme preparations contain approximately 2 mol fatty acid/mol subunit and exhibit a very similar fatty acid composition with octadecanoate and hexadecanoate as prevalent components. No significant differences between native glycosylPtdIns-anchored and hydrophilic alkaline phosphatases from both sources were found regarding Km, Vmax, the type of inhibition and inhibition constants of the amino acids L-leucine, L-phenylalanine, and L-tryptophan. The purified enzymes of both sources yield diacylglycerol and phosphatidic acid, after treatment with phosphatidylinositol-specific phospholipase C (PtdIns-PLC) and glycosylphosphatidylinositol phospholipase D (PLD), respectively. Enzyme preparations of both sources appear as heterogeneous mixtures of five fractions separable by octyl-Sepharose chromatography. Fraction I corresponds to the anchorless enzyme, fractions II-V differ in their susceptibility to phospholipases. Fractions II and IV are completely split by PtdIns-PLC or PLD action, almost 50% of fraction III is split by PtdIns-PLC, while fraction V is resistant. The susceptibility of these two fractions toward the action of PLD is considerably higher. Fatty acid analysis yields molar ratios of fatty acids/alkaline phosphatase subunit of 1.78, 2.58, 2.24, and 3.37 for fractions II, III, IV, and V, respectively. Aggregates of glycosylPtdIns-anchored alkaline phosphatase of all fractions are seen in native PAGE in the presence of Triton X-100. By gel chromatography in the presence of Brij 35, fractions II-V form stable multiple aggregates of dimers and may bind different amounts of the detergent. These data, together with fatty acid analysis, can be interpreted by the following model. Fractions II and IV are tetramers and octamers with two molecules fatty acid/subunit. Fraction III is a tetramer, bearing one additional fatty acid molecule, localized on the dimer. Fraction V is an octamer, containing glycosylPtdIns-anchor molecules with three molecules fatty acids/anchor molecule. The additional fatty acid residue is possibly located on inositol and responsible for the reduced susceptibility to PtdIns-PLC. The similarity of all measured parameters of both enzymes suggests that the glycosylPtdIns-anchored alkaline phosphatase of the mucosa is released into the chyme without changing the anchor molecule constituents.

Effect of membrane phospholipid composition and fluidity on rat liver plasma membrane tyrosine kinase activity

1. The effect of membrane phospholipid composition and fluidity on tyrosine kinase activity was investigated in rat liver plasma membranes. 2. The phospholipid composition has been modified by in vitro enrichment of plasma membranes with different phospholipids in the presence of lipid transfer proteins and by partial delipidation with exogenous phospholipases A2, C and D and subsequent enrichment with phosphatidylglycerol. 3. Phosphatidylglycerol and dioleoylglycerophosphocholine caused dramatic elevation of this activity, while phosphatidylserine and phosphatidylethanolamine were less effective. Enrichment with dipalmitoylglycerophosphocholine and sphingomyeline reduced tyrosine kinase activity.

Reconstitution studies of lipid effects on insulin-receptor kinase activation

Insulin receptors extracted from human placenta were reconstituted by dialysis into well-characterized lipid vesicles. For all types of lipids studied, vesicles were shown to be unilamellar, about 120 nm in diameter. The incorporation of lectin-purified insulin receptors was assessed by cosedimentation of 125I-insulin binding and [32P]phospholipids in a sucrose gradient. The insulin-binding activity was not modified by the composition of the lipid vesicles. However, tyrosine kinase activation appeared to be more sensitive to its lipid environment. Mixtures of phosphatidylcholine/phosphatidylserine or phospholipids/phosphatidylserine, in ratios of 1-4, increased the insulin-induced tyrosine kinase activation in a dose-dependent manner. In contrast, experiments performed in the presence of phosphatidylinositol showed a decrease in the enzyme stimulation. These results indicate an opposing involvement of these two anionic phospholipids in the kinase activation. Inclusion of cholesterol (10-30%) into phosphatidylcholine vesicles reduced kinase activation, which was drastically inhibited by 30% cholesterol. The effect of a total extract of brain gangliosides was biphasic, stimulatory at low concentration (5-10%), but with a reverse effect at higher concentrations. These results stress the importance of the lipid environment for insulin-receptor signaling, particularly for the insulin-induced activation of its beta-subunit kinase.