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

Abstract

An inositol-phosphate glycan (InsP glycan), which is the polar head group of an insulin-sensitive glycosyl-phosphatidylinositol (glycosyl-PtdIns), has been reported to mimic some insulin actions when added to different types of cells. In connection with this, a specific, time-dependent and energy-dependent transport system for this InsP glycan has been identified in isolated rat hepatocytes [Alvarez, J. F., Shchez-Arias, J. A., Guadaiio, A., Estevez, F., Varela, I., Feliu, J. E. & Mato, J. M. (1991) Biochem. J. 274,369-3741. Here we have investigated the glycosyl-PtdIns-dependent insulin-signalling system in hepatocytes isolated from either 3-month-old or 24-month-old rats. Aging reduced the stimulatory effect of insulin on [U-'4C]glucose incorporation into glycogen, caused a significant decrease in basal glycosyl-PtdIns levels and blocked the insulin-mediated hydrolysis of this lipid. In 24-month-old rats, we also observed a diminution in the rate of hepatocyte InsP-glycan uptake and a marked reduction of the stimulatory effect of this compound on glycogen synthesis. These results support the hypothesis that insulin resistance associated with aging is accompanied by an impairment of the glycosyl-PtdIns-dependent cellular signalling system.In recent years, reports from different laboratories have supported the involvement of a phospholipase-C-catalyzed hydrolysis of a glycosyl-phosphatidylinositol (glycosylPtdIns) in the mechanism of insulin action (for reviews see [l-31). The polar head group generated by the hydrolysis of glycosyl-PtdIns is an inositol-phosphate glycan (InsP glycan), which seems to contain a hexosamine-inositol-phosphate moiety and several additional sugars [4-71. This InsP glycan has been reported to simulate the effects of insulin on the activity of certain insulin-modulated enzymes in cell-free systems [8-101. In intact cells, InsP glycan also mimics some of the metabolic actions of insulin on lipid and glucose metabolism, cyclic-AMP levels, amino-acid uptake, pattern of phosphorylatioddephosphorylation of proteins and mRNA expression [ 11 -181.It is well established that insulin promotes the release of InsP glycan to the extracellular medium [19, 201. In fact, the majority (approximately 80%) of insulin-sensitive glycosylPtdIns in rat hepatocytes has been localized at the outer cell surface [21]. Moreover, addition of anti-(InsP glycan) immunoglobulins to cultured BC3H1 myocytes selectively blocked insulin-induced activation of pyruvate dehydrogenase and provoked the accumulation of insulin-mediator activity in the extracellular medium [22].As a possible connection between the extracellular release of InsP glycan and its intracellular effects, we have recently identified a specific, time-dependent and energy-dependent InsP-glycan-transport system for this compound in isolated rat hepatocytes, which is antagonized by micromolar Although experimental evidence supporting the implication of glycosylated forms of PtdIns in the signal-transduction mechanism of insulin is increasingly strong, li...

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

“…The present results on the decreases in lipogenesis and response to insulin and IPG-A with ageing in adipocytes from HO rats is in accord with the studies of Sanchez-Arias et al (1993) on the insulin-sensitive glycosyl-phosphatidyl-inositol signalling system with ageing in rat hepatocytes. These authors demonstrated that not only did ageing reduce the stimulating effect of insulin on [U-14 C]glucose incorporation into glycogen, but also caused a significant decrease in basal glycosyl-phosphatidyl-inositol levels and diminution in the rate of hepatocyte InsP-glycan uptake.…”

“…Rodent models have given considerable insight into the changes occurring in adipose tissue with ageing (Narimiya et al 1984;Carrascosa et al 1989;Sanchez-Arias et al 1993;Molero et al 2002;Villar et al 2006;Escriva et al 2007). In these studies a wide range of parameters have been measured including, among others, insulin receptor kinase activities, MAP kinase, Akt and GLUT 4 translocation, measurement of adipocytederived cytokines including leptin, resistin and adiponectin, in addition the effects of age and moderate food restriction on insulin sensitivity changes have been examined (Carrascosa et al 1989;Molero et al 2002;Villar et al 2006;Escriva et al 2007).…”

The Role of Src Kinase in the Potentiation by Ethanol of Cytokine- and Endotoxin-Mediated Nitric Oxide Synthase Expression in Rat Hepatocytes

“…It is well documented that many physiological functions or signaling pathways in rat hepatocytes could be affected during the aging process. For example, the expression of heat shock protein 70, epidermal growth factor-stimulated DNA synthesis, or insulin-induced glucosyl-phosphatidylinositol signaling could be reduced or attenuated in rat hepatocytes from old rats (27)(28)(29). The biological role of NO 2…”