Alteration of phosphoinositide metabolism, protein phosphorylation, and carbohydrate levels in sciatic nerve from Wistar fatty diabetic rats

Biology of obesity: lessons from animal models of obesity

Obesity is an epidemic problem in the world and is associated with several health problems, including diabetes, cardiovascular disease, respiratory failure, muscle weakness, and cancer. The precise molecular mechanisms by which obesity induces these health problems are not yet clear. To better understand the pathomechanisms of human disease, good animal models are essential. In this paper, we will analyze animal models of obesity and their use in the research of obesity-associated human health conditions and diseases such as diabetes, cancer, and obstructive sleep apnea syndrome.

Growth factors promote inositol uptake in BC3H1 cells

BC3H1 cells induced to differentiate by serum withdrawal were found to incorporate substantially less [3H]inositol into their phosphoinositides than cells induced to differentiate by growth in the presence of high serum. This decrease was found to be due to a decline in the rate of [3H]inositol uptake by the serum-starved cells. Addition of purified growth factors such as TGF-beta, EGF and FGF to these cells promoted inositol uptake and lead to an increase in the incorporation of [3H]inositol into phosphoinositides. Stimulation of inositol uptake by TGF-beta required at least a 24 hr exposure to the growth factor. These data indicate that growth factors regulate phosphoinositide metabolism at many different levels including at the level of inositol uptake.

Insulin-sensitive myelin basic protein phosphorylation on tyrosine residues

Rat brain plasma membranes were solubilized in detergent and a glycoprotein-enriched fraction was obtained by lectin affinity chromatography. This glycoprotein fraction contained insulin receptors, as well as protein kinases capable of phosphorylating some exogenously added substrates such as MAP2 (microtubule associated protein 2) and MBP (myelin basic protein), but not ribosomal protein S6. Phosphoamino acid analysis of MAP2 and MBP showed that phosphotyrosine residues, as well as phosphoserine/phosphotheronine residues, were present in both proteins under basal conditions. Whereas the addition of insulin to the rat brain membrane glycoprotein fraction in vitro had no effect on MAP2 phosphorylation, MBP phosphorylation was stimulated 2.7-fold in response to insulin. This phenomenon was dose-dependent, with half-maximal stimulation of MBP phosphorylation observed with 2 nM insulin. Phosphoamino acid analysis of MBP indicated that insulin stimulated the phosphorylation of tyrosine residues nearly three-fold, whereas the phosphorylation of serine or threonine residues was not increased. These results identify MBP as a substrate for the rat brain insulin receptor tyrosine-specific protein kinase in vitro.

A myo-inositol pool utilized for phosphatidylinositol synthesis is depleted in sciatic nerve from rats with streptozotocin-induced diabetes

Peripheral nerve from experimentally diabetic rats exhibits lowered levels of myo-inositol (MI) and decreased incorporation of [3H]MI into phosphatidylinositol (PI). There are indications that diminished PI turnover may be causally related to reduced Na+,K(+)-ATPase activity in diabetic nerve. We have investigated whether a metabolic compartment of MI that is essential for PI synthesis is decreased in this tissue. Sciatic nerve segments from streptozotocin-induced diabetic and age-matched normal rats were incubated in vitro with either 32Pi or [3H]cytidine in the presence of propranolol. This cationic amphiphilic agent redirected nerve phospholipid metabolism to produce enhanced 32P incorporation into PI and decreased labeling of phosphatidylcholine and phosphatidyl-ethanolamine. The accumulation of phosphatidyl CMP (CMP-PA) was also demonstrated by chromatographic and enzymatic means. The incorporation of [3H]cytidine into CMP-PA in normal nerve increased up to 15-fold when 0.6 mM propranolol was present. In diabetic nerve, the liponucleotide incorporated 2- to 3-fold more isotope and was more readily labeled at lower drug concentrations as compared to normal nerve. The buildup of [3H]CMP-PA was reduced in a dose-dependent manner in the presence of MI in the incubation medium at concentrations up to 3 mM. However, if MI was added after liponucleotide accumulation, preformed CMP-PA could not be utilized for PI synthesis. The difference in liponucleotide labeling between normal and diabetic nerve was nearly abolished at 0.3 mM medium MI, a concentration much less than the level of cyclitol in the tissue. These results strongly suggest the presence in nerve of a pool of MI that is not in equilibrium with the bulk of nerve MI and that is preferentially used for PI synthesis. This metabolic compartment is depleted in diabetic nerve but can be readily replenished by exogenous MI and may correspond to the MI pool that has been proposed to be required for the turnover of a portion of tissue PI involved in maintenance of normal Na+,K(+)-ATPase activity.

Effect of fructose supplementation on sorbitol accumulation and myo-inositol metabolism in cultured neuroblastoma cells exposed to increased glucose concentrations

Aldose reductase activity is increased in neuroblastoma cells grown in media containing 30 mM fructose and/or 30 mM glucose. Neuroblastoma cells cultured in media supplemented with increased concentrations of glucose and fructose amass greater amounts of sorbitol than do cells exposed to media containing only high glucose concentrations. The increase in sorbitol content is dependent on the fructose and glucose concentration in the media. The increase in sorbitol content caused by exposing neuroblastoma cells to media containing 30 mM glucose/30 mM fructose is due to a protein synthesis sensitive mechanism and not to an alteration in the redox state. The addition of sorbinil to media containing 30 mM glucose blocks the increase in sorbitol content. In contrast, sorbinil treatment of media containing 30 mM glucose/30 mM fructose does not totally block the increase in sorbitol levels. myo-Inositol accumulation and incorporation into inositol phospholipids and intracellular myo-inositol content are decreased in cells chronically exposed to media containing 30 mM glucose or 30 mM glucose/30 mM fructose compared to cells cultured in unsupplemented media or media containing 30 mM fructose. However, maximal depletion of myo-inositol accumulation and intracellular content occurs earlier in cells exposed to media containing 30 mM glucose/30 mM fructose than in cells exposed to media supplemented with 30 mM glucose. Sorbinil treatment of media containing 30 mM glucose/30 mM fructose maintains cellular myo-inositol accumulation and incorporation into phospholipids at near normal levels. myo-Inositol content in neuroblastoma cells chronically exposed to media containing 30 mM glucose or 30 mM glucose/30 mM fructose recovers within 72 h when the cells are transferred to unsupplemented media or media containing 30 mM fructose. In contrast, the sorbitol content of cells previously exposed to media containing 30 mM glucose or 30 mM glucose/30 mM fructose then transferred into media containing 30 mM fructose remains elevated compared to the sorbitol content of cells transferred into unsupplemented media. These data suggest that fructose may be activating or increasing sorbinil-resistant aldose reductase activity as well as partially blocking sorbitol dehydrogenase activity. The presence of increased concentrations of fructose in combination with increased glucose levels may enhance alterations in cell metabolism and properties due to increased sorbitol levels.

Diabetic neuropathy in db/db mice develops independently of changes in ATPase and aldose reductase. A biochemical and immunohistochemical study

ATPase activity was investigated in sciatic and optic nerves of female mutant diabetic C57Bl/Ks (db/db) mice and age-matched control mice (db/m and m/m). Nerves from animals aged 50, 70, 125, 180 and 280 days were assayed in vitro for ATPase activity in the presence or absence of ouabain: the ouabain-sensitive fraction contained Na+,K(+)-ATPase. Enzymatic activity was compared within and between age-matched groups. No significant difference in Na+,K(+)-ATPase activity was detected between the diabetic and control mice, whether expressed as mumol Pi/h-1 formed per gramme wet weight or per nerve (protein content). The activity decreased by about 25% in both the sciatic and optic nerves of the oldest animals. These results were strikingly similar in all groups, regardless of the type of nerve examined, confirming that the development of neuropathy in this animal model is unrelated to the postulated derangement of Na+,K(+)-ATPase activity. Among possible explanations, a lack of polyol pathway activation was investigated by staining the sciatic nerves of animals from all groups with the peroxidase-antiperoxidase procedure using a polyclonal antiserum raised against the enzyme aldose reductase. Histological sections of all nerves were consistently negative, suggesting that these animals actually lack the enzyme involved in activating the self-perpetuating metabolic cycle leading to deranged nerve function. The db/db mouse appears to present particular biochemical changes which merit attention with a view to clarifying the pathogenesis of diabetic neuropathy.