Changes in the insulin-sensitive glycosyl-phosphatidyl-inositol signalling system with aging in rat hepatocytes

Age-related changes in the response of rat adipocytes to insulin: evidence for a critical role for inositol phosphoglycans and cAMP

Adipose tissue plays a pivotal role in ageing and longevity; many studies, both human and animal, have focussed on the effects of food limitation. Here we present a new model based on striking differences between two 'normal' inbred strains of albino Wistar rats the Charles River (CR) and Harlan Olac (HO) that have marked differences in age-related accumulation of fat and insulin-stimulated rates of glucose incorporation into lipid in the epididymal fat pads (EFP). The incorporation [U-(14)C]glucose into lipid by adipocytes showed that the CR group had a twofold higher basal rate of lipogenesis and a greater response to insulin in vitro, exceptionally, adipocytes from CR group maintained the high response to insulin to late adulthood while retaining the lower EFP weight/100 g body weight. Inositol phosphoglycan A-type (IPG-A), a putative insulin second messenger, was 3.5-fold higher and cAMP significantly lower per EFP in the CR versus HO groups. Plasma insulin levels were similar and plasma leptin higher in CR versus HO groups. The anomaly of a higher rate of lipogenesis and response to insulin and lower EFP weight in the CR group is interpreted as the resultant effect of a faster turnover of lipid and stimulating effect of leptin in raising fatty acid oxidation by muscle, potentially key to the lower accumulation of visceral fat. The metabolic profile of the CR strain provides a template that could be central to therapies that may lead to the lowering of both adipose and non-adipocyte lipid accumulation in humans in ageing.

Age-related changes of insulin receptors, plasma insulin and glucose level

The effects of aging on hepatic and erythrocyte insulin receptors have been investigated in 6, 12, 18 and 21-months-old compare to 3-months-old rats. Plasma insulin was elevated in 6, 12 and 18-months-old rats. Specific binding of insulin in liver was increased at the age of 8 months and accompanied with increase in concentration of low affinity binding sites, while specific binding to erythrocytes as well as concentration of both classes of binding sites was increased in 6-months-old rats. The protein and mRNA content of hepatic receptor were decreased only in the oldest animals. Plasma glucose was elevated starting from 12-months-old rats, while, after decrease in 6-months-old animals, citrulline was raised in the oldest group. The results demonstrating that specific binding of insulin in liver and erythrocytes and the concentration of binding sites in both tissues were not decreased during aging, as well as the absence of changes in affinity of insulin binding sites do not point out to occurrence of insulin resistance. However, the increase in insulinemia in the middle of lifespan, elevated plasma glucose and citrulline as well as decrease of hepatic receptor protein and mRNA content in the oldest animals indicate some age-related changes in insulin signaling.

The role of glycosyl-phosphatidylinositol in signal transduction

Glycosyl-phosphatidylinositol (GPI) lipids have a structural role as protein anchors to the cell surface. In addition, they are implicated in hormone, growth factor and cytokine signal transduction. Their phosphodiesteric hydrolysis mediated by an activated phospholipase results in the generation of water soluble oligosaccharide species termed the inositol phosphoglycan (IPG). This product has been demonstrated to possess biological properties when added exogenously to cells mimicking the biological effects of a variety of extracellular ligands. This may be accomplished since IPG is generic for a family of closely related species which are released in a tissue-specific manner and additionally have cell-specific targets. Micro-organic synthesis has recently been able to shed new light on this topic by the introduction of defined oligosaccharide analogues of IPG for the assessment of their biological activity. These have complemented the findings observed with purified IPG from biological sources thus strengthening the belief that the GPI/IPG signalling system represents a truly novel aspect of transmembrane signalling.

Distribution and levels of [125I]IGF-I, [125I]IGF-II and [125I]insulin receptor binding sites in the hippocampus of aged memory-unimpaired and -impaired rats

The insulin-like growth factors (IGF-I and IGF-II) and insulin are localized within distinct brain regions and their respective functions are mediated by specific membrane receptors. High densities of binding sites for these growth factors are discretely and differentially distributed throughout the brain, with prominent levels localized to the hippocampal formation. IGFs and insulin, in addition to their growth promoting actions, are considered to play important roles in the development and maintenance of normal cell functions throughout life. We compared the anatomical distribution and levels of IGF and insulin receptors in young (five month) and aged (25 month) memory-impaired and memory-unimpaired male Long Evans rats as determined in the Morris water maze task in order to determine if alterations in IGF and insulin activity may be related to the emergence of cognitive deficits in the aged memory-impaired rat. In the hippocampus, [125I]IGF-I receptors are concentrated primarily in the dentate gyrus (DG) and the CA3 sub-field while high amounts of [125I]IGF-II binding sites are localized to the pyramidal cell layer, and the granular cell layer of the DG. [125I]insulin binding sites are mostly found in the molecular layer of the DG and the CA1 sub-field. No significant differences were found in [125I]IGF-I. [125I]IGF-II or [125I]insulin binding levels in any regions or laminae of the hippocampus of young vs aged rats. and deficits in cognitive performance did not relate to altered levels of these receptors in aged memory-impaired vs aged memory-unimpaired rats. Other regions. including various cortical areas, were also examined and failed to reveal any significant differences between the three groups studied. It thus appears that IGF-I, IGF-II and insulin receptor sites are not markedly altered during the normal ageing process in the Long Evans rat, in spite of significant learning deficits in a sub-group (memory-impaired) of aged animals. Hence. recently reported changes in IGF-I receptor messenger RNA levels in aged memory-impaired rats are apparently not reflected at the level of the translated protein.

The role of Src kinase in the potentiation by ethanol of cytokine- and endotoxin-mediated nitric oxide synthase expression in rat hepatocytes

This study demonstrates that exposure of primary rat hepatocytes or mouse BNL Cl.2 liver cell line to ethanol causes potentiation of tumor necrosis factor-alpha (TNF-alpha)- and lipopolysaccharide (LPS)-stimulated nitrite accumulation. The potentiating effect of ethanol (0.02-2 mM) appears to be time and concentration dependent. Consistent with nitrite production, the amount of inducible nitric oxide synthase (iNOS) mRNA and protein is initially detected at 4 hr after treatment with TNF-alpha/LPS/ethanol. Furthermore, the capability of these agents to induce iNOS expression is primarily determined by the age of the animals. Interestingly, antioxidants such as N-acetylcysteine (NAC), ascorbic acid, or alpha-tocopherol fail to inhibit TNF-alpha/LPS/ethanol-induced increase in iNOS protein. In addition, several kinase inhibitors, including staurosporine, genistein, curcumin, and herbimycin A, were used to examine their effects on this induction. Among them, only herbimycin A potently inhibits the accumulation of nitrite and iNOS expression. In vitro kinase assay verifies that Src tyrosine kinase is rapidly activated with a peak at 1 hr after treatment with TNF-alpha/LPS/ethanol but is not activated by these agents singly or doubly. As expected, herbimycin A can block Src kinase activity under circumstances in which iNOS expression is also inhibited. However, our results do not indicate that the mitogen-activated protein kinase is activated after treatment with these agents. The study results suggest that Src tyrosine kinase plays a prominent role in transducing the signal to induce iNOS expression in hepatocytes treated with TNF-alpha/LPS/ethanol.

Cell signalling by inositol phosphoglycans from different species

The discovery of glycosyl-phosphatidylinositol (GPI) molecules and their products has given new insight into the field of signal transduction. In the last decade a novel mechanism of protein attachment to membranes has emerged, which involves a covalent linkage of the protein to the glycan moiety of a GPI. The discovery that GPI-anchored proteins are ubiquitous throughout the eukaryotes was followed by the observation that uncomplexed GPI molecules are implicated in signal transduction for a diversity of hormones and growth factors. The hydrolysis of free-GPI generates a novel second messenger: the inositol phosphoglycan (IPG). The aim of this article is to review the role of IPG and IPG-like molecules in signal transduction and to discuss future research directions.