Ambient glucose and aldose reductase-induced myo-inositol depletion modulate basal and carbachol-stimulated inositol phospholipid metabolism and diacylglycerol accumulation in human retinal pigment epithelial cells in culture.

Abstract

Physiological hyperglycemia has been speculated to alter phosphoinositide (PPI; inositol phospholipid) signal transduction in cells prone to diabetic complications by two separate mass-action mechanisms with antiparallel putative effects on diacylglycerol (DAG): (i) sorbitol-induced depletion of myo-inositol leads to diminished PPI synthesis and turnover and DAG release, and (it) elevated glucose-derived DAG precursors enhance de novo DAG synthesis. Because the first mechanism is mediated by aldose reductase (AR2), which converts glucose to sorbitol, the effects of glucose on basal and stimulated PPI signaling were explored in lines of cultured human retinal pigment epithelial cells differing widely in their basal AR2 gene expression and enzymatic activity. The results suggest that the effects of glucose on PPI signaling vary inversely with the level of AR2 activity and parallel the extent of AR2-induced myo-inositol depletion.The hyperglycemia of diabetes has been speculated to alter phosphoinositide (PPI; inositol phospholipid) signal transduction, especially basal and agonist-stimulated intracellular diacylglycerol (DAG) mass, in cells prone to diabetic complications by one oftwo mechanisms. In the first, exemplified by peripheral nerve in experimental diabetes, glucoseinduced aldose reductase (AR2)-mediated sorbitol accumulation induces reciprocal depletion of myo-inositol (MI), such that it becomes rate-limiting for critical components of PPI synthesis and arachidonoyl-DAG production that regulate Na+, K+-ATPase activity, leading to slow nerve conduction (1-3). This biochemical response pattern to glucose has been modeled in cultured human retinal pigment epithelial (RPE) cells (2, 4) expressing aberrantly high levels of the AR2 gene (5) and enzyme (6), which is responsible for conversion of glucose to sorbitol. However, the detailed interaction between glucose-induced MI depletion and inhibition of the phosphatidylinositol (PI) synthase reaction, for which MI and CDPdiglyceride (CDP-DG) function as cosubstrates, remains controversial (1, 7). Several independent lines of investigation suggest that this interaction may be quite complex, involving functionally discrete metabolic pools of MI, PPI, and/or PI synthase (1-3). In the second metabolic response pattern to glucose, exhibited in vitro by retinal endothelial and glomerular cells with relatively low AR2 activity (1,8,9), DAG content, glucose incorporation into DAG, and protein kinase C translocation from the cytosolic to the membrane fraction are all increased after exposure to hyperglycemic concentrations of glucose (10-12). This pattern has beenThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. ascribed to enhanced de novo DAG synthesis from glucosederived precursors such as a-glycerophosphate and phosphatidic acid consequent to (13) or independent of (10-12) sorbitol pathway activation ...

Inositol and antioxidant supplementation: Safety and efficacy in pregnancy

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Nutritional and Acquired Deficiencies in Inositol Bioavailability. Correlations with Metabolic Disorders

“…By considering the structural similarities between myo-Ins and glucose, this result is far from surprising. Inositol uptake in cell cultures may be significantly inhibited by 20 mM glucose concentration [ 108 ]—a value currently used in laboratory investigations—and this finding is of utmost importance for properly modeling of in vitro experiments. In vitro, high ambient glucose attenuates myo-inositol concentrating capability via competitive inhibition.…”

Oxidative stress and dysregulation of the taurine transporter in high-glucose-exposed human Schwann cells: implications for pathogenesis of diabetic neuropathy

“…This pathway increases intracellular fructose, sorbitol, and protein glycation while reducing the concentration of NADPH, an essential cofactor for regenerating reduced glutathione (GSH) (9, 46). Increased sorbitol production can contribute to osmotic imbalance, resulting in a compensatory reduction of other osmolytes, such as myoinositol and taurine (42,65,74). Deficiency of these "compatible osmolytes" may result in their becoming rate limiting for normal metabolic functions (64,66).…”