Metabolic Functions of Myo-inositol

Inositol based non-viral vectors for transgene expression in human cervical carcinoma and hepatoma cell lines

Myo-Inositol (INO) is a biomolecule with crucial functions in many aspects. In this study, hyperbranched copolymers for gene delivery were synthesized based on inositol and low molecular weight polyethylenimine. The capacity of INO-PEIs to load plasmid DNA and their biocompatibility was demonstrated. A tumor target ligand, folic acid (FA), which was widely used for drug delivery systems, was subsequently conjugated to INO-PEIs and resulted in INO-PEI-FA copolymers. The polymers were then evaluated on their activity to mediate transgene expression in mammalian cell lines. As indicated, INO-PEIs were able to mediate efficient transgene expression, which was particularly noticeable in carcinoma cell line HeLa. INO-PEI-FA further improved the efficiency in HepG2. Distribution of INO-PEI-FA polymers in non-carcinoma NIH 3T3 and carcinoma HeLa cell lines was discussed.

Kinetics of myo-inositol transport in rat ocular lens

Myo-inositol (MI) influx as a function of concentration in rat lens consisted of a saturable component, fit by a rectangular hyperbola, and a linear component which was more distinct at high myo-inositol concentrations suggesting passive diffusion. The hyperbolic component was half-maximally saturated (Kt) at 61.3 microM and had a maximal transport rate (Jmax) of 44.6 mumol/kg wet wt/h. The linear component had an apparent permeability coefficient of 1.44 x 10(-6) s-1. Sorbitol, which distributed rapidly in the extracellular space (6.83 ml/100 g wet wt), also appeared to enter the intracellular space with a permeability coefficient of 1.37 x 10(-6) s-1, similar to that of myo-inositol. The influx of myo-inositol was critically dependent on the concentration of extracellular sodium consistent with a sodium-myo-inositol cotransport. The kinetics of influx activation by sodium suggested an apparent 2:1 coupling ratio for sodium and myo-inositol. When potassium was used as sodium substitute, a significantly stronger influx inhibition was observed than with nondepolarizing sodium substitutes, indicating that myo-inositol was driven by the electrochemical gradient of sodium rather than the chemical gradient only. Reducing the extracellular Na concentration increased the MI concentration at which transport was half-maximally activated, suggesting an ordered binding sequence of Na followed by MI. Myo-inositol influx was competitively inhibited by phlorizin with an inhibitory coefficient (Ki) of 35 microM. Phloretin also was capable of inhibition but with a much lesser efficacy. Myo-inositol desaturates from the lens at a rate of 0.00862 h-1. Approximately 19% of the efflux can be inhibited with phlorizin, suggesting that it represents carrier-mediated flux. The phlorizin insensitive flux has a rate of 0.00695 h-1 or 1.93 x 10(-6) s-1, similar to the Na-independent passive influx. MI influx is due to a Na-dependent, phlorizin-sensitive active transport while the efflux consists largely of a phlorizin-independent passive leakage.

Reduced sodium pump activity in inositol-deficient HL-60 cells: no evidence of control by protein kinase C

HL-60 cells were cultured in normal and inositol-deficient media. The inositol-deficient cells showed reduced sodium pump activity, as measured by ouabain-sensitive 86Rb+ uptake. The protein kinase C inhibitors staurosporine and H7 did not affect uptake in either normal or inositol-deficient cells. However, U73122, a steroidal inhibitor of phosphoinositidase C, inhibited uptake in both types of cells. Activators of protein kinase C had no effect on Rb+ entry. The inositol deficiency is not considered to affect the sodium pump by a mechanism involving diacylglycerol and protein kinase C.

Myo-inositol transport in the lens of galactose-maintained rats

Lens myo-inositol (MI) content is regulated by a pump-leak system consisting of an active Na-dependent MI transport and its passive permeability through the membrane. We measured the active MI uptake and membrane permeability in lenses of rats maintained on a 50% galactose diet for 1, 3 and 7 days. After only 1 day of galactose feeding, active MI uptake in the lens was reduced dramatically by 74% compared to age-matched control lenses; by day 3, active MI transport was decreased by 89% and it was undetectable by day 7. The passive membrane permeability was determined by measuring (a) the passive MI influx and (b) the 3H-sorbitol flux. After 1 day of galactose feeding, the membrane permeability increased such that within 3 days it increased to 5-6 fold. Galactose feeding also led to a rapid increase in lens polyol content. After 1 day, lens polyol increased to 53 mumol/g wet wt compared to a control value of 0.35 mumol/g wet wt and increased further to 65 and 72 mumol/g wet wt after 3 and 7 days of galactose feeding respectively. Lens galactose accumulation was low (3 mumol/g wet wt) up to 7 days; however, it was rapidly increased after 7 days. Our results indicate that galactose feeding rapidly interfered with MI homeostasis by a severe depression of active MI transport and a rapid increase in membrane permeability. These interferences of MI homeostasis correlate with the appearance of high polyol levels.

Polypeptide and glycoprotein abnormalities in dorsal root ganglia of streptozotocin-diabetic rats

Observations were made on the polypeptide and glycoprotein composition of dorsal root ganglia from streptozotocin-induced diabetic rats by sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis (PAGE). Silver staining of one-dimensional gels failed to demonstrate any differences between diabetic and control animals. In two-dimensional studies, good resolution of polypeptides with a mass greater than 70 kDa was not obtained, but a number of important abnormalities in the polypeptide composition of diabetic ganglia were detected. Some polypeptides recognized in the gels from control ganglia were present in high concentrations in diabetic ganglia; other polypeptides, particularly a number of basic polypeptides of low molecular mass, were only identified in the diabetic rats. Three major polypeptides showed a small shift in their isoelectric point in the diabetic animals. The glycoprotein content of the ganglia was examined by lectin binding to both one- and two-dimensional gel separations. An increase in total glycoprotein content was evident in the diabetic ganglia. A number of polypeptides with a molecular mass between 70 and 110 kDa showed heavy glycosylation. Altered glycosylation of some specific polypeptides of lower molecular mass was also seen, 7 of these showing increased and 3 reduced glycosylation. The significance of these findings is discussed.

In vitro correction of impaired Na+-K+-ATPase in diabetic nerve by protein kinase C agonists

Diminished Na+-K+-ATPase activity in diabetic peripheral nerve plays a central role in the early electrophysiological, metabolic, and morphological abnormalities of experimental diabetic neuropathy. The defect in Na+-K+-adenosinetriphosphatase (ATPase) regulation in diabetic nerve is linked experimentally to glucose- and sorbitol-induced depletion of nerve myo-inositol but is not fully understood at a molecular level. Therefore, regulation of nerve Na+-K+-ATPase activity by phosphoinositide-derived diacylglycerol was explored as the putative link between myo-inositol depletion and the Na+-K+-ATPase impairment responsible for slowed saltatory conduction in diabetic animal models. In vitro exposure of endoneurial preparations from alloxan-diabetic rabbits to two protein kinase C agonists, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate and 1,2-(but not 1,3-) dioctanoyl-sn-glycerol, for as little as 1 min completely and specifically corrected the 40% decreased enzymatically measured ouabain-sensitive ATPase activity. Neither of these agonists affected ouabain-sensitive ATPase activity in endoneurial preparations derived from nondiabetic controls. These observations are compatible with the hypothesis that metabolites of electrically stimulated phosphoinositide turnover such as diacylglycerol acutely regulate nerve Na+-K+-ATPase activity, probably via protein kinase C, thereby tightly coupling energy-dependent Na+-K+-antiport with impulse conduction in peripheral nerve. Glucose-induced depletion of myo-inositol presumably limits phosphoinositide turnover and diacylglycerol production, thereby disrupting this putative regulatory mechanism for Na+-K+-ATPase in diabetic peripheral nerve.

Reduced Na+ + K+-ATPase activity in peripheral nerve of streptozotocin-diabetic rats: a role for protein kinase C?

Six weeks after induction of diabetes, the rate of ouabain-sensitive 86Rb+ accumulation, a parameter which reflects Na+ + K+-ATPase pumping activity, was significantly reduced in endoneurial preparations of sciatic nerve from untreated diabetic rats compared with that in control rats (Trial, 1, 0.19 +/- 0.09 versus 0.48 +/- 0.13 pmol/min per mg wet weight of tissue, p less than 0.001; Trial 2, 0.27 +/- 0.16 versus 0.47 +/- 0.18, p less than 0.01). This decrease in ouabain-sensitive 86Rb+ uptake was not observed in nerves from diabetic rats maintained on sorbinil (an aldose reductase inhibitor) or myo-inositol diets. Protein kinase C activity was demonstrated in the soluble fraction of a sciatic nerve homogenate by assaying for lipid-activated, Ca+-dependent phosphorylation of calf thymus histone. No significant difference in the time course of kinase C activity was observed between cytosol fractions of nerve homogenates from control and diabetic rats (control, 6.22 +/- 0.97 pmol 32P incorporated/mg cytosol protein in 50 min; diabetic, 5.32 +/- 0.71). Three low molecular weight neural proteins (each with Mr less than 29,000) were identified as substrates for protein kinase C.

Pathogenesis and prevention of diabetic neuropathy

Diabetic neuropathy, long-recognized as an important but complex and poorly understood clinical complication of diabetes, is finally yielding to more than a decade of intense clinical and laboratory investigation. At least one basic biochemical mechanism involving sorbitol and MI metabolism, phosphoinositides, protein kinase C, and the (Na,K)-ATPase has been identified that can rationally account for the neurotoxicity of glucose. This biochemical sequence has been examined in some detail in vitro, but some of its elements, such as the link between abnormal sorbitol and MI metabolism, and between protein kinase C and the (Na,K)-ATPase, remain the subject of ongoing investigation. Through its effect on the (Na,K)-ATPase, this metabolic sequence can explain both the rapidly-reversible functional impairment and the early structural lesions of nerve fibers, such as paranodal swelling in acute diabetes. Extrapolation of early paranodal swelling to the more advanced stages of nerve fiber damage remains somewhat speculative, although axo-glial dysjunction is a likely intermediate step. Impaired axonal transport or microvascular dysfunction may be additional contributing factors, possibly also related to abnormal sorbitol and MI metabolism. Blunted phosphoinositide-mediated signal transduction could potentially explain a putative insensitivity to neurotrophic factors and a diminished regenerative response in diabetic neuropathy. Human morphometric studies and ARI trials support the relevance of these pathogenetic processes to human diabetic neuropathy, and suggest that specific metabolic therapy with agents such as ARIs hold promise as important new elements in the treatment and possibly prevention of diabetic neuropathy.

Sorbitol, phosphoinositides, and sodium-potassium-ATPase in the pathogenesis of diabetic complications

During the past decade, our appreciation of the original experiments with myo-inositol supplementation in diabetic rats has greatly expanded. The effects of myo-inositol on nerve conduction are now explained by concepts that were largely unappreciated in 1976, including the fundamental role of phosphoinositide metabolism in cell regulation and the importance of the activity of sodium-potassium-ATPase in nerve conduction. Aldose reductase inhibitors firmly link defects in myo-inositol metabolism to activation of the polyol pathway in diabetes; the resulting "sorbitol-myo-inositol hypothesis" has been extended from its application to the lenses and peripheral nerves to most of the tissues involved with diabetic complications. These biochemical mechanisms provide a new framework within which to explore the complex interactions between hyperglycemia and the vascular, genetic, and environmental variables in the pathogenesis of diabetic complications. It is anticipated that these endeavors will result in the appearance of new classes of therapeutic agents, the first of which--the aldose reductase inhibitors--has emerged from the laboratory and is now undergoing extensive clinical testing. These efforts are very likely to result in the appearance of new treatment methods that may dramatically lighten the burden of chronic complications in patients with diabetes.

Phosphoinositides provide a regulatory mechanism of surface charge and active transport

Yeast cells, when grown in the presence of arsenate, are capable of accumulating phosphoinositides (PI) at the expense of inhibiting their degradation more than their synthesis. PI levels return to normal when the cells are cultured or exposed to media without arsenate. These reversible changes are employed as a tool to test the effect of inositide accumulation and dynamics on several membrane properties. In the PI-rich cells, phosphate and arsenate transport from low external concentrations (high affinity systems), as well as the transport of glycine, which enter the cells accompanied by protons, were increased. The proton ejection energized by glucose is also enhanced in the PI-rich cells that show a more efficient potassium inflow at pH 4.0-4.5. The membrane surface potential of the PI-rich cells was found to be 2-times higher than that of the normal cells, in agreement with the 2-fold increment in the PI. All the above mentioned alterations in membrane properties are reverted when the PI content of the PI-rich cells is reduced to the level of normal cells. The results show the participation of the phosphoinositides in the formation, maintenance and regulation of the membrane surface potential and their possible influence upon transport mechanisms.

Experimental diabetes mellitus impairs the function of the retinal pigmented epithelium

The retinal pigmented epithelium (RPE), which influences the composition of the retinal extracellular fluid, is significantly affected in diabetes. Changes in RPE morphology, permeability, and electrophysiology in experimentally diabetic animals have been described. To facilitate the study of diabetes-related changes in RPE metabolism, we applied the techniques of quantitative histochemistry to pure samples of RPE and individual retinal layers from eyes of normal and alloxan-diabetic rabbits. Glucose within the RPE approximated serum levels in both normal and diabetic animals. Other changes in diabetics included increased sorbitol, decreased myo-inositol, elevated total Na, and loss of measurable Na+-K+-ATPase activity within the RPE. The altered ion metabolism was associated with a progressive decrease in the amplitude of the RPE-generated c-wave of the electroretinogram. The deterioration of the c-wave was arrested by treatment of the diabetic animals with either myo-inositol supplementation or with sorbinil, an inhibitor of aldose reduction. Diabetic alterations in the RPE might impair the ability of the tissue to maintain normal transport functions. The subsequently altered composition of the extracellular environment of the retina may play an important role in the pathogenesis of diabetic retinopathy.

The nutritional significance, metabolism, and function of myo-inositol and phosphatidylinositol in health and disease

Recent advances in nutritional and biochemical research have substantiated the importance of inositol as a dietary and cellular constituent. The processes involved in the metabolism of inositol and its derivatives in mammalian tissues have been characterized both in vivo and at the enzyme level. Biochemical functions elucidated for phosphatidylinositol in biological membranes include the mediation of cellular responses to external stimuli, nerve transmission, and the regulation of enzyme activity through specific interactions with various proteins. Inositol deficiency in animals has been shown to produce an accumulation of triglyceride in liver, intestinal lipodystrophy, and other abnormalities. The metabolic mechanisms giving rise to these latter phenomena have been extensively studied as a function of dietary inositol. Altered metabolism of inositol has been documented in patients with diabetes mellitus, chronic renal failure, galactosemia, and multiple sclerosis. A moderate increase in plasma and nerve inositol levels by dietary supplementation has been suggested as a means of treating diabetic neuropathy, although excessively high levels, such as are found in uremic patients, may be neurotoxic. A thorough consideration of the biochemical functions of inositol and a further characterization of various diseases with the aid of appropriate animal models may suggest a possible role for inositol and other dietary components in their prevention and treatment

Inactivation of (Na-++K-+)-stimulated ATPase by a cytotoxic protein from cobra venom in relation to its lytic effects on cells

The mechanism of action of the cytotoxic protein P6 isolated from cobra venom (Naja naja) which shows preferential cytotoxicity particularly to Yoshida sarcoma cells has been studied by its effects on the membrane-bound enzyme (Na-++K-+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) of a variety of cell systems. Evidence obtained with Yoshida sarcoma cells, dog and human erythrocytes and three tissue culture cell lines KB (human oral carcinoma), Hela (human cervix carcinoma) and L-132 (human lung embryonic) shows that inhibition of (Na-++K-+)-ATPase by the P6 protein can be correlated with its lytic activity. (Na-++k-+)-ATPase of Yoshida sarcoma membrane fragments inactivated by P6 protein could be reconstituted by the addition of phosphatidylserine and phosphatidic acid. It is conceivable that lysis of cells by the P6 protein may be due to an imbalance of K-+ and Na-+ in the cell which leads to swelling and disintegration of the membrane structure. Observations indicate that the P6 protein combines with membrane constituents of susceptible cells. The overall evidence suggests that both the specificity of its protein structure and the highly basic nature of the P6 protein are factors which enable it to compete with the lipid moiety maintaining the (Na-++k-+)-ATPase of the susceptible cells in proper conformation for activity.