L-fucose is a potent inhibitor of myo-inositol transport and metabolism in cultured neuroblastoma cells

Hydroxysteroid 17-β dehydrogenase 14 (HSD17B14) is an L-fucose dehydrogenase, the initial enzyme of the L-fucose degradation pathway

L-Fucose (6-deoxy-L-galactose), a monosaccharide abundant in glycolipids and glycoproteins produced by mammalian cells, has been extensively studied for its role in intracellular biosynthesis and recycling of GDP-L-fucose for fucosylation. However, in certain mammalian species, L-fucose is efficiently broken down to pyruvate and lactate in a poorly understood metabolic pathway. In the 1970s, L-fucose dehydrogenase, an enzyme responsible for the initial step of this pathway, was partially purified from pig and rabbit livers and characterized biochemically. However, its molecular identity remained elusive until recently. This study reports the purification, identification, and biochemical characterization of the mammalian L-fucose dehydrogenase. The enzyme was purified from rabbit liver approximately 340-fold. Mass spectrometry analysis of the purified protein preparation identified mammalian hydroxysteroid 17-β dehydrogenase 14 (HSD17B14) as the sole candidate enzyme. Rabbit and human HSD17B14 were expressed in HEK293T and Escherichia coli, respectively, purified, and demonstrated to catalyze the oxidation of L-fucose to L-fucono-1,5-lactone, as confirmed by mass spectrometry and NMR analysis. Substrate specificity studies revealed that L-fucose is the preferred substrate for both enzymes. The human enzyme exhibited a catalytic efficiency for L-fucose that was 359-fold higher than its efficiency for estradiol. Additionally, recombinant rat HSD17B14 exhibited negligible activity towards L-fucose, consistent with the absence of L-fucose metabolism in this species. The identification of the gene-encoding mammalian L-fucose dehydrogenase provides novel insights into the substrate specificity of enzymes belonging to the 17-β-hydroxysteroid dehydrogenase family. This discovery also paves the way for unraveling the physiological functions of the L-fucose degradation pathway, which remains enigmatic.

The insulin-sensitizing mechanism of myo-inositol is associated with AMPK activation and GLUT-4 expression in human endometrial cells exposed to a PCOS environment

Polycystic ovary syndrome (PCOS) is an endocrine-metabolic disorder characterized by hyperandrogenism and ovulatory dysfunction but also obesity and hyperinsulinemia. These characteristics induce an insulin-resistant state in tissues such as the endometrium, affecting its reproductive functions. Myo-inositol (MYO) is an insulin-sensitizing compound used in PCOS patients; however, its insulin-sensitizing mechanism is unclear. To understand the relationship of MYO with insulin action in endometrial cells, sodium/myo-inositol transporter 1 (SMIT-1) (MYO-transporter), and MYO effects on protein levels related to the insulin pathway were evaluated. SMIT-1 was assessed in endometrial tissue from women with normal weight, obesity, insulin resistance, and PCOS; additionally, using an in vitro model of human endometrial cells exposed to an environment resembling hyperinsulinemic-obese-PCOS, MYO effect was evaluated on p-AMPK and GLUT-4 levels and glucose uptake by Western blot, immunocytochemistry, and confocal microscopy, respectively. SMIT-1 was detected in endometrial tissue from all groups and decreased in PCOS and obesity (P < 0.05 vs. normal weight). In the in vitro model, PCOS conditions decreased p-AMPK levels, while they were restored with MYO (P < 0.05). The diminished GLUT-4 protein levels promoted by PCOS environment were restored by MYO through SMIT-1 and p-AMPK-dependent mechanism (P < 0.05). Also, MYO restored glucose uptake in cells under PCOS condition through a p-AMPK-dependent mechanism. Finally, these results were similar to those obtained with metformin treatment in the same in vitro conditions. Consequently, MYO could be a potential insulin sensitizer through its positive effects on insulin-resistant tissues as PCOS-endometrium, acting through SMIT-1, provoking AMPK activation and elevated GLUT-4 levels and, consequently, increase glucose uptake by human endometrial cells. Therefore, MYO may be used as an effective treatment option in insulin-resistant PCOS women.

Insulin Treatment Attenuates Small Nerve Fiber Damage in Rat Model of Type 2 Diabetes

INTRODUCTION

Current clinical guidelines for management of diabetic peripheral neuropathy (DPN) emphasize good glycemic control. However, this has limited effect on prevention of DPN in type 2 diabetic (T2D) patients. This study investigates the effect of insulin treatment on development of DPN in a rat model of T2D to assess the underlying causes leading to DPN.

METHODS

Twelve-week-old male Sprague-Dawley rats were allocated to a normal chow diet or a 45% kcal high-fat diet. After eight weeks, the high-fat fed animals received a mild dose of streptozotocin to induce hyperglycemia. Four weeks after diabetes induction, the diabetic animals were allocated into three treatment groups receiving either no insulin or insulin-releasing implants in a high or low dose. During the 12-week treatment period, blood glucose and body weight were monitored weekly, whereas Hargreaves' test was performed four, eight, and 12 weeks after treatment initiation. At study termination, several blood parameters, body composition, and neuropathy endpoints were assessed.

RESULTS

Insulin treatment lowered blood glucose in a dose-dependent manner. In addition, both doses of insulin lowered lipids and increased body fat percentage. High-dose insulin treatment attenuated small nerve fiber damage assessed by Hargreaves' test and intraepidermal nerve fiber density compared to untreated diabetes and low-dose insulin; however, neuropathy was not completely prevented by tight glycemic control. Linear regression analysis revealed that glycemic status, circulating lipids, and sciatic nerve sorbitol level were all negatively associated with the small nerve fiber damage observed.

CONCLUSION

In summary, our data suggest that high-dose insulin treatment attenuates small nerve fiber damage. Furthermore, data also indicate that both poor glycemic control and dyslipidemia are associated with disease progression. Consequently, this rat model of T2D seems to fit well with progression of DPN in humans and could be a relevant preclinical model to use in relation to research investigating treatment opportunities for DPN.

Purification and characterization of the L-fucose transporter

L-Fucose is a monosaccharide present in low levels in the serum. It is, however, a common structural component of glycoproteins. L-Fucose is accumulated in eukaryotic cells by a specific, facilitative diffusion transport system which has been designated the fucose transporter. In this study, purification of the transporter from mouse brain was performed by detergent extraction followed by ion-exchange and reactive dye ligand column chromatography. Purification was followed using a transport assay into reconstituted liposomes. A 111-fold purification with 5% yield was achieved from the crude homogenate. The apparent molecular weight of the protein was 57 kDa. Transport was found to be saturable. The K(m) and V(max) values are estimated at 3 microM and 275 pmol/min/mg, respectively. The tissue distribution of fucose transport was examined in liver, kidney, heart, lung, spleen, brain, muscle, adipose, ovary, pancreas, and thymus. Some fucose transport was found in all tissues examined. Very low levels were observed in the liver relative to all other tissues examined. The only monosaccharide which could inhibit the uptake of L-[5,6-(3)H]fucose was fucose itself.

A model of inositol compartmentation in astrocytes based upon efflux kinetics and slow inositol depletion after uptake inhibition

Intracellular compartmentation of inositol was demonstrated in primary cultures of mouse astrocytes, incubated in isotonic medium, by determination of efflux kinetics after "loading" with [3H]inositol. Three kinetically different compartments were delineated. The largest and most slowly exchanging compartment had a halflife of approximately 9 hr. This slow release leads to retention of a sizeable amount of pre-accumulated inositol in the tissue 24 hr after the onset of uptake inhibition, as confirmed by the observation that the inositol uptake inhibitor fucose caused a larger inhibition of unidirectional inositol uptake than of inositol pool size, measured as accumulated [3H]inositol after 24 hr of combined exposure to the inhibitor and the labeled isotope. Based upon the present observations and literature data, it is suggested that the large, slowly exchanging compartment is largely membrane-associated and participating in signaling via the phosphatidylinositide second messenger system, whereas inositol functioning as an osmolyte is distributed in the cytosol and located in one or both of the compartments showing a faster release.

Correction of Leukocyte Adhesion Deficiency Type II With Oral Fucose

We describe a simple, noninvasive, and effective therapy for leukocyte adhesion deficiency type II (LAD II), a rare inherited disorder of fucose metabolism. This disorder leads to an immunodeficiency caused by the absence of carbohydrate-based selectin ligands on the surface of neutrophils as well as to severe psychomotor and mental retardation. The fucosylation defect in LAD II fibroblasts can be corrected by addition of L-fucose to the culture medium. This prompted us to initiate dietary fucose therapy on a patient with LAD II. Oral supplementation of fucose in this patient induced the expression of fucosylated selectin ligands on neutrophils and core fucosylation of serum glycoproteins. During 9 months of treatment, infections and fever disappeared, elevated neutrophil counts returned to normal, and psychomotor capabilities improved.

Correction of Leukocyte Adhesion Deficiency Type II With Oral Fucose

We describe a simple, noninvasive, and effective therapy for leukocyte adhesion deficiency type II (LAD II), a rare inherited disorder of fucose metabolism. This disorder leads to an immunodeficiency caused by the absence of carbohydrate-based selectin ligands on the surface of neutrophils as well as to severe psychomotor and mental retardation. The fucosylation defect in LAD II fibroblasts can be corrected by addition of L-fucose to the culture medium. This prompted us to initiate dietary fucose therapy on a patient with LAD II. Oral supplementation of fucose in this patient induced the expression of fucosylated selectin ligands on neutrophils and core fucosylation of serum glycoproteins. During 9 months of treatment, infections and fever disappeared, elevated neutrophil counts returned to normal, and psychomotor capabilities improved.

Effect of L-fucose and D-glucose concentration on L-fucoprotein metabolism in human Hep G2 cells and changes in fucosyltransferase and alpha-L-fucosidase activity in liver of diabetic rats

L-Fucose is a monosaccharide that is present at low concentrations in serum and is a normal constituent of glycoproteins. In some pathological conditions, such as cancer, rheumatoid arthritis, and diabetes, there is an abnormal fucosylation of acute phase serum proteins. Because most serum proteins are produced in the liver, we have examined L-fucose accumulation, metabolism, and secretion of L-fucose-containing proteins in human Hep G2 liver cells. Accumulation of L-fucose by Hep G2 cells approached 3.5 nmol/mg protein after a 48 h incubation. This accumulation appears similar to accumulation in other cells, which we have shown occurs via a specific transport protein. Exogenous L-fucose was incorporated into protein in both O- and N-linked glycosidic linkages. After a 48 h incubation, 61% of the accumulated L-fucose was incorporated into protein and secreted into the medium, whereas 39% of the L-fucose remaining in the cells was incorporated into integral membrane proteins. Utilizing reverse-phase high-performance liquid chromatographic separation of L-[5,6-(3)H]fucose-containing proteins and detection by scintillation counting, we determined that two major fucoproteins and numerous minor fucoproteins were produced and secreted by normal Hep G2 cells. This elution profile was unchanged when glucose-conditioned cells were examined. By size-separating secreted proteins by nondenaturing HPLC we determined that the size of the two major fucoproteins were approximately 60 and approximately 100 kDa. In these studies we also examined the effect of diabetes on hepatic fucosyltransferase and serum alpha-L-fucosidase activity and found that the activity of these enzymes is increased by 40 and 100%, respectively in diabetic rats.

Osmotic regulation of Na-myo-inositol cotransporter mRNA level and activity in endothelial and neural cells

Myo-inositol (MI) is an important factor in the synthesis of phosphoinositides, and as an osmolyte, MI contributes to the regulation of cell volume. In cells of renal origin, hypertonicity causes an increase in sodium-dependent MI transporter (SMIT) mRNA levels and MI transport. However, it is unknown whether changes in osmolarity regulate transport of MI in neural or endothelial cells. IN these studies, neural and endothelial cells were exposed to hyperosmotic medium for up to 48 h, and the effect on MI transport was determined. Transport of MI was maximally increased by exposing the cells to hyperosmotic medium for 24 h. Kinetic analysis of high-affinity MI transport demonstrated an increase in the apparent maximal velocity with no significant change in the apparent Km. The hyperosmotic induction of MI transport was blocked by the addition of cycloheximide, indicating a requirement for protein synthesis, and was associated with increased levels of SMIT mRNA. In contrast to the effect of hypertonicity, exposure of neural and endothelial cells to hypotonic conditions caused a decrease in SMIT mRNA levels and MI transport in endothelial cells. These studies demonstrate that, in extrarenal cell types, changes in osmolarity also regulate SMIT activity and mRNA levels.

Reduced Na+/K+ adenosine triphosphatase activity and motor nerve conduction velocity in L-fucose-fed rats is reversible after dietary normalization

Development of early defects in diabetic neuropathy has been linked to metabolic abnormalities and is considered reversible. To further address some of the questions concerning the contribution by metabolic derangements to the development of neural defects and reversibility, we have developed an animal model, by feeding rats a diet containing 20% L-fucose, that develops neural defects similar to those that occur in streptozotocin-induced diabetic rats. After 6 weeks on a 20% L-fucose diet, myo-inositol content and Na+/K+ adenosine triphosphatase (ATPase) activity of the sciatic nerve were significantly reduced, as was the motor nerve conduction velocity (MNCV). L-Fucose is a monosaccharide that occurs in low concentrations in normal serum but is increased in diabetic patients. In cultured cells, L-fucose, at concentrations that occur in diabetic circulation, is a competitive inhibitor of myo-inositol uptake. The purpose of the present study was to compare the sequential pattern of the reversibility of the slowing of MNCV with ouabain-inhibited sciatic nerve Na+/K+ ATPase activity and myo-inositol content in rats fed a diet containing 20% L-fucose for a period of 6 weeks followed by a normal diet lasting up to 2 weeks. Unbound L-fucose levels in the serum returned to normal in less than 24 hours of the rats being placed on the normal diet. Normalization of slowed MNCV after removing L-fucose-fed rats from the L-fucose diet followed a pattern of recovery similar to the recovery of sciatic nerve ouabain-inhibited Na+/K+ ATPase activity, with complete recovery occurring within 7 days of the rats being placed on the normal diet. In contrast, myo-inositol content of the sciatic nerve remained decreased following 3 days on the normal diet, and required 14 days for complete normalization. Results from these studies suggest that a causal relationship may exist for reduced Na+/K+ ATPase activity and MNCV in L-fucose-fed rats, and that a measurable decrease in myo-inositol content may not be necessary for the development of these defects in the sciatic nerve.

L-fucose reduces collagen and noncollagen protein production in cultured cerebral microvessel endothelial cells

L-fucose is a monosaccharide which is present in low concentrations in normal serum but is increased in diabetes, cancer, and inflammatory diseases. The contribution that abnormal L-fucose levels make to the progression of these disorders is unknown. In a previous study we showed that increased L-fucose concentration reduced proliferation and proteoglycan production by cultured cerebral microvessel endothelial cells. In the present study we show that exposing cerebral microvessel endothelial cells for 2 weeks to medium containing an increased concentration of L-fucose causes a significant decrease in collagen and to a lesser extent noncollagen protein production. The effect of L-fucose on collagen and noncollagen protein production is concentration-dependent: 1 mM L-fucose causes a significant decrease in collagen production but has no effect on noncollagen protein production; a 5 mM L-fucose concentration causes a maximum decrease in both collagen and noncollagen protein production. This defect is unrelated to the reduction in myo-inositol uptake caused by L-fucose and is not prevented by aminoguanidine. Collagen production can be improved by restoring L-fucose-conditioned cells to normal medium. Culturing cells for 2 weeks in medium containing 10 mM L-fucose resulted in a 50% decrease in collagen production, which was restored to 75% of control after cells were transferred to normal medium for 7 days. In contrast, noncollagen protein production was totally restored after 3 days in normal medium. Increasing levels of L-fucose in serum of rats also resulted in a decrease in collagen production. Collagenase digestible incorporation of L-[2,3,4,5-3H]proline into protein of the articular cartilage from rats fed a diet containing 20% L-fucose for 3 weeks was reduced by about 40% compared to rats fed a normal diet. The decrease in collagen production in L-fucose fed rats was less than the reduction that occurred in streptozotocin-induced diabetic rats. These data suggest that changes in L-fucose concentration itself may be a factor in the regulation of collagen production.

Decreased myo-inositol uptake is associated with reduced bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol content in cultured neuroblastoma cells exposed to L-fucose

L-Fucose is a potent, competitive inhibitor of myo-inositol transport by cultured mammalian cells. Chronic exposure of neuroblastoma cells to L-fucose causes a concentration-dependent decrease in myo-inositol content, accumulation, and incorporation into phosphoinositides. In these studies, L-fucose supplementation of culture medium was used to assess the effect of decreased myo-inositol metabolism and content on bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol production. Chronic exposure of cells to 30 mM L-fucose caused a sustained decrease in bradykinin-stimulated, but not basal, 3H-inositol phosphate release and 32P incorporation into phosphatidylinositol in cells incubated in serum-free, unsupplemented medium. In addition, 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was not altered in L-fucose-conditioned cells. Acute exposure of cells to serum-free medium containing 30 mM L-fucose did not affect either basal or bradykinin-stimulated 32P incorporation into phosphatidylinositol. Basal diacylglycerol content was decreased by 20% in cells chronically exposed to 30 mM L-fucose, although analysis of the molecular species profile revealed no compositional change. Bradykinin stimulated diacylglycerol production in neuroblastoma cells by increasing the hydrolysis of both phosphoinositides and phosphatidylcholine. Bradykinin-stimulated production of total diacylglycerol was similar for control and L-fucose-conditioned cells. However, there was a decrease in the bradykinin-induced generation of the 1-stearoyl-2-arachidonoyl diacylglycerol molecular species in the cells chronically exposed to 30 mM L-fucose. This molecular species accounts for about 70% of the composition of phosphoinositides, but only 10% of phosphatidylcholine. The results suggest that a decrease in myo-inositol uptake results in diminished agonist-induced phosphatidylinositol synthesis and phosphoinositide hydrolysis in cultured neuroblastoma cells grown in L-fucose-containing medium.

Elevated levels of glucose and L-fucose reduce 22Na+ uptake and whole cell Na+ current in cultured neuroblastoma cells

Na+ flux was studied in cultured neuroblastoma cells grown in medium containing increased glucose or L-fucose concentrations. Chronic exposure of neuroblastoma cells to 30 mM glucose or 30 mM L-fucose caused a decrease in ouabain-sensitive and veratridine-stimulated 22Na+ uptake compared with cells cultured in unsupplemented medium. The Na+ current, determined by using whole-cell configuration of the patch clamp, was also decreased in these cells. Tetrodotoxin (3 microM), which blocked whole cell Na+ currents, also blocked veratridine-stimulated 22Na+ accumulation. Culturing cells in medium containing 30 mM fructose as an osmotic control had no effect on Na+ flux. Specific [3H]saxitoxin binding was not affected by 30 mM glucose or 30 mM L-fucose compared with cells grown in unsupplemented medium, suggesting that the number of Na+ channels was not decreased. These studies suggest that exposing cultured neuronal cells to conditions that occur in the diabetic milieu alters Na+ transport and Na(+)-channel activity.

Reversal of hyperglycemic-induced defects in myo-inositol metabolism and Na+/K+ pump activity in cultured neuroblastoma cells by normalizing glucose levels

myo-Inositol accumulation and incorporation into phosphoinositides was decreased in neuroblastoma cells chronically exposed to medium containing 30 mmol/L glucose or 30 mmol/L galactose. In addition, the intracellular content of myo-inositol and phosphatidylinositol was decreased and the sorbitol or galactitol content increased in cells cultured for 2 weeks in medium containing 30 mmol/L glucose or 30 mmol/L galactose, respectively. Na+/K+ adenosine triphosphatase (ATPase) transport activity was also significantly decreased by long-term exposure of neuroblastoma cells to medium containing 30 mmol/L glucose or 30 mmol/L galactose. When glucose-conditioned cells were placed in medium containing a normal glucose concentration for 24 hours, myo-inositol metabolism and content, phosphatidylinositol levels, and Na+/K+ pump activity were restored or completely returned to normal values. These functions were also significantly improved, except for the phosphatidylinositol content, which was increased by 55%, when galactose-conditioned cells were incubated for 24 hours in unsupplemented medium. The polyol content of the glucose- or galactose-conditioned cells was also significantly reduced. Returning the cells to normal glucose levels for 1 to 3 hours did not completely restore myo-inositol metabolism. Improved myo-inositol metabolism and content, sorbitol levels, and Na+/K+ ATPase transport activity were also obtained within 24 hours when cells chronically exposed to medium supplemented with 30 mmol/L glucose were placed in medium containing 30 mmol/L glucose and 0.4 mmol/L sorbinil. The phosphatidylinositol content of these cells was improved by approximately 30%. Cells prelabeled for 24 hours with [U-14C]sorbitol metabolize more than 50% of the [U-14C]sorbitol during a 24-hour incubation in unsupplemented medium. These studies conducted at the cellular level suggest that restoration of normal myo-inositol metabolism, polyol content, and Na+/K+ pump activity altered by hyperglycemic conditions occurs rapidly following normalization of glucose concentration.

Effect of bradykinin on cytosolic calcium in neuroblastoma cells using the fluorescent indicator fluo-3

Neuroblastoma cells were used to examine the effect of chronic exposure to increased concentrations of glucose, galactose, or L-fucose on bradykinin-stimulated intracellular calcium release using the calcium indicator fluo-3. Bradykinin caused a concentration dependent increase in the intracellular calcium concentration and phosphoinositide hydrolysis in neuroblastoma cells. Norepinephrine, carbachol, serotonin, and thapsigargin also increased the calcium concentration. Treatment of the cells with 10(-6) M bradykinin exhausts calcium release such that the successive treatment of the cells with norepinephrine, carbachol, or serotonin results in no secondary response. In contrast, bradykinin treatment of the cells following exposure to norepinephrine, carbachol, or serotonin caused a secondary increase in calcium release. These results suggest that several hormone responsive calcium pools may exist in neuroblastoma cells or that norepinephrine, carbachol, or serotonin may not fully stimulate calcium release. Bradykinin-stimulated calcium release is not effected by chronic exposure of the cells to increased concentrations of glucose, galactose, or L-fucose. Suggesting that hormone-stimulated calcium release is not an abnormality that develops in neural cells exposed to conditions that mimic the diabetic milieu. In addition, these studies provide evidence that fluo-3 is a good fluorescent indicator for the study of calcium mobilization in cultured neuroblastoma cells.

Effect of L-fucose on proliferation and myo-inositol metabolism in cultured cerebral microvessel and aortic endothelial cells

Decreased myo-inositol metabolism possibly contributes to the development of diabetic complications including micro and macrovascular disease. Previous studies have shown that hyperglycemia may be partially responsible for this defect. We have found that L-fucose, a monosaccharide present in low concentrations in normal circulation and found to be elevated in diabetes, causes defects in cultured endothelial cells, including alterations in myo-inositol metabolism and proliferation. Murine cerebral microvessel and bovine aortic endothelial cells take up L-fucose from the medium in a time and concentration-dependent manner. Both acute and chronic exposure of these cultured endothelial cells to media containing L-fucose at concentrations that may exist in diabetic sera cause a significant decrease in the accumulation of myo-inositol and its incorporation into inositol phospholipids. There is a concomitant decrease in the intracellular levels of myo-inositol. Kinetic analysis of the effect of L-fucose on myo-inositol uptake suggests that L-fucose competitively inhibits the transport of myo-inositol, exhibiting a Ki in the range of 1.6-4.1 mM for both cell types. Endothelial cells exposed to L-fucose concentrations of 0.5-20 mM exhibit depressed rates of proliferation in a concentration-dependent fashion. Furthermore, L-fucose causes a concentration-dependent decrease in synthesis of proteoglycan by cultured cerebral microvessel endothelial cells as measured by incorporation of 35S; however, this effect is not observed in the aortic endothelia. These data suggest that L-fucose at concentrations that may exist in diabetic sera may impair myo-inositol metabolism and proliferation of the vascular endothelium.