The Effect of Glucose and Galactose Toxicity on Myo-inositol Transport and Metabolism in Human Skin Fibroblasts in Culture

Abstract

ABSTRACT. Myo-inositol transport and metabolism were studied in cultured human skin fibroblasts exposed to potentially toxic levels of glucose or galactose. Although variable among I I different cell lines, the myo-inositol level in confluent cells, ranging from 10-50 nmol/mg protein, was constant with passage. A high-affinity transport system for myo-inositol had an apparent Kt of 55 pM and V, of 16 pmol/min/mg protein. No obvious relationship existed between cellular levels and transport capacity. Dependency on sodium was complex. When medium sodium was lowered to 23 mM, myo-inositol uptake ceased after about 1 h. However, the initial rate of myo-inositol uptake only showed a sodium dependence at low myo-inositol concentrations. Both phloretin and phloridzin inhibited myo-inositol uptake. Phloridzin had a Ki of 60 pM. and phloretin was either a noncompetitive or uncompetitive inhibitor. Glucose and galactose were only weak competitive inhibitors, with a K1 of 30 mM and 65 mM, respectively. After 24 h of incubation with myo-[2-3Hjinositol, only 10% of the total cell label was incorporated into phospholipid. Compared with control media with 5 mM glucose, the incubation of confluent cells in media with 20 mM glucose had little effect on intracellular glucose and sorbitol, whereas cells incubated in control media supplemented with 5 mM galactose showed a large increase in galactose and polyol levels. In media with more than 200 WM of myo-inositol, neither treatment had an effect on myo-inositol levels after 24 h. The uptake and incorporation of 11 pM myo-[2-3Hjinositol and incorporation into phospholipid were studied after cells had been previously exposed for 24 to 48 h to media supplemented with 15 mM glucose or galactose. Compared with controls, fibroblasts with a 24-h exposure to 20 m M glucose showed a 10% decrease in myo-inositol uptake. When the exposure was extended to 48 h, preconditioning with galactose as well as glucose elicited the same 10% reduction in uptake. Phosphoinositide labeling in fibroblasts exposed to 20 mM glucose was reduced in parallel. These cells offer a unique opportunity for the study of sugar toxicity in human tissue: they can be exposed to high levels of glucose without significant glucose or polyol accumulation~or can be made to accumulate polyol by exposure to moderate levels of galactose. The expression of a hexose-induced reduction in myo-inositol transport required 24 to 48 h of exposure of the fibroblasts to elevated concentrations of glucose or galactose and may not be related to a competitive inhibitory

The Chemopreventive Agent Myoinositol Inhibits Akt and Extracellular Signal-Regulated Kinase in Bronchial Lesions from Heavy Smokers

“…In women, blood plasma levels of 100 μmol/L were achieved after a single oral dose of 100 mg/kg myoinositol (for a 150-lb subject, this equates to an oral dose of 6.8 g). Furthermore, it has been reported that in most mammalian cells or tissues, the content of myoinositol is 5- to 500-fold higher than the level in plasma due to an effective, active transport system for myoinositol (31). Because the phase I trial with heavy smokers used myoinositol at doses 2- to 3-fold higher than that used by Groenen et al for 2 to 3 months (during which time accumulation may have occurred), it is plausible that the concentrations of myoinositol used in vitro are relevant to that achieved in humans.…”

Expression of the sodium–myo‐inositol cotransporter SMIT2 at the apical membrane of Madin‐Darby canine kidney cells

“…MI also acts as a compatible osmolyte when cells are challenged by variations in extracellular tonicity (Burg et al 1997). Although this activity is of the greatest importance in the renal medulla, where the tonicity of the interstitial milieu fluctuates widely and constantly, other tissues such as brain, liver, fibroblasts and retinal endothelium also show a similar use of MI as an osmolyte when cell volume homeostasis is challenged (Berry et al 1994; Weise et al 1996; Karihaloo et al 1997). It is believed that intracellular MI levels are controlled by regulation of MI transport across the plasma membrane (Strange et al 1994; Weise et al 1996; Burg et al 1997).…”

Ultraviolet A induces transport of compatible organic osmolytes in human dermal fibroblasts

“…These osmolytes are part of stress response of NHKs to UVA radiation. Taurine and myo-inositol are also taken up by human skin fibroblasts and fibroblast lines (14)(15)(16). Taurine and myo-inositol are also taken up by human skin fibroblasts and fibroblast lines (14)(15)(16).…”

“…Accordingly, UVA induces the expression of the osmolyte transporter BGT-1, SMIT and TAUT in NHKs which leads to an increased uptake of osmolytes, especially taurine (13). Taurine and myo-inositol are also taken up by human skin fibroblasts and fibroblast lines (14)(15)(16). Interestingly, taurine release is induced by oxidative stress and inhibited by antioxidants in NIH3T3 mouse fibroblasts (17).…”