Differential expression of the GLUT1 and GLUT4 glucose transporters during differentiation of L6 muscle cells

Elevated GLUT 1 level in crude muscle membranes from diabetic Zucker rats despite a normal GLUT 1 level in perineurial sheaths

Recently, we demonstrated that approximately 60% of GLUT 1 in a crude membrane fraction of rat skeletal muscle originates from perineurial sheaths. To study the in vivo regulation of GLUT 1 expression in different tissues in muscles, we measured the level of GLUT 1 in crude muscle membranes and in perineurial sheaths in diabetic (fa/fa) Zucker rats and lean controls, with and without metformin treatment. The GLUT 1 concentration in perineurial sheaths was identical in all four groups of rats, both when measured by quantitative immunofluorescence and by immunoblotting and densitometry. In a fraction of crude membranes of soleus muscles GLUT 1 expression was more than two-fold higher in (fa/fa) rats than in lean controls (p < 0.005). Metformin treatment significantly elevated GLUT 1 in control rats (p < 0.05) and tended to decrease GLUT 1 in diabetic rats (p < 0.075). The expressions of GLUT 1 and GLUT 4 in crude muscle membranes were inversely correlated (p < 0.01), and GLUT 1 expression correlated positively with fasting glucose (p < 0.05). In conclusion, GLUT 1 expression in perineurial sheaths is unaffected by alterations in glucose homeostasis and by the genes responsible for obesity and diabetes in the Zucker rat. GLUT 1 expression in a crude membrane fraction of soleus muscle is increased in the diabetic animals, likely due to an increased expression in muscle cells proper.

Structural disruption of the trans-Golgi network does not interfere with the acute stimulation of glucose and amino acid uptake by insulin-like growth factor I in muscle cells

The effects of insulin-like growth factor I (IGF-I) on glucose and amino acid uptake were investigated in fully differentiated L6 muscle cells, in order to determine whether the two processes are functionally related. Transport of both glucose and amino acid (methylaminoisobutyric acid, MeAIB) was activated rapidly in response to IGF-I. Stimulation reached a peak within 30 min and was sustained for up to 90 min. Maximal activation of either glucose or MeAIB transport was achieved at 3 nM IGF-I; the half-maximal activation (ED50) of glucose transport was at 107 pM and that of MeAIB transport was at 36 pM. Stimulation of amino acid uptake occurred in the absence or presence of glucose, suggesting that this response is not secondary to increased glucose intake. Incubation of cells for 1 h with Brefeldin A (5 micrograms/ml), which disassembles the Golgi apparatus and inhibits the secretory pathway in eukaryotic cells, had no effect on the acute IGF-I activation of glucose and MeAIB transport. Moreover, Brefeldin A caused wide redistribution of the trans-Golgi antigen TGN38, as assessed by subcellular fractionation, without affecting the distribution of glucose transporters. The finding that the degree of activation, time response and sensitivity to IGF-I and Brefeldin A were similar for both glucose and MeAIB transport suggests commonalities in the IGF-I mechanism of recruitment of glucose transporters and stimulation of amino acid transport through System A. An integral trans-Golgi network does not appear to be required for the acute IGF-I stimulation of glucose or amino acid transport, even though stimulation of glucose transport occurs through recruitment of glucose transporters from intracellular stores in these cells. We propose that the donor site of glucose transporters (and perhaps of amino acid transporters) involved in the acute response to IGF-I lies beyond the trans-Golgi network, perhaps in an endosomal compartment in close proximity to the plasma membrane.

Glucose transport in human skeletal muscle cells in culture. Stimulation by insulin and metformin

Primary human muscle cell cultures were established and the regulation of glucose transport was investigated. Primary cultures were allowed to proceed to the stage of myotubes through fusion of myoblasts or were used for clonal selection based on fusion potential. In clonally selected cultures, hexose (2-deoxy-glucose) uptake into myotubes was linear within the time of study and inhibitable by cytochalasin B (IC50 = 400 nM). Cytochalasin B photolabeled a protein(s) of 45,000-50,000 D in a D-glucose-protectable manner, suggesting identity with the glucose transporters. In the myotube stage, the cells expressed both the GLUT1 and GLUT4 glucose transporter protein isoforms at an average molar ratio of 7:1. Preincubation in media of increasing glucose concentrations (range 5-25 mM) progressively decreased the rate of 2-deoxyglucose uptake. Insulin elevated 2-deoxyglucose uptake in a dose-dependent manner, with half maximal stimulation achieved at 3.5 nM. Insulin also stimulated the transport of the nonmetabolizable hexose 3-O-methylglucose, as well as the activity of glycogen synthase, responsible for nonoxidative glucose metabolism. The oral antihyperglycemic drug metformin stimulated the cytochalasin B-sensitive component of both 2-deoxyglucose and 3-O-methylglucose uptake. Maximal stimulation was observed at 8 h of exposure to 50 microM metformin, and this effect was not prevented by incubation with the protein-synthesis inhibitor cycloheximide. The relative effect of metformin was higher in cells incubated in 25 mM glucose than in 5 mM glucose, consistent with its selective action in hyperglycemic conditions in vivo. Metformin (50 microM for 24 h) was more effective than insulin (1 microM for 1 h) in stimulating hexose uptake and the hormone was effective on top of the stimulation caused by the biguanide, suggesting independent mechanisms of action.

Detection of the GLUT3 facilitative glucose transporter in rat L6 muscle cells: regulation by cellular differentiation, insulin and insulin-like growth factor-I

The GLUT3 facilitative glucose transporter protein was found to be expressed in rat L6 muscle cells. It was detected at both the myoblast and myotube stage. GLUT3 protein content per mg of total membrane protein increased significantly during L6 cell differentiation. Subcellular fractionation demonstrated that the GLUT3 protein was predominantly localized in plasma membrane-enriched fractions of either myoblasts or myotubes. Short-term exposure of L6 myotubes to IGF-I or insulin caused a redistribution of GLUT3 protein from an intracellular membrane fraction to the plasma membrane, without affecting total membrane GLUT3 protein content. Long-term exposure of L6 myotubes to IGF-I produced an increase of GLUT3 protein in total membranes and all subcellular membrane fractions, especially the plasma membrane. We propose that the GLUT3 glucose transporter may play an important role in glucose metabolism in developing muscle.

Stimulation of glucose transport in L6 muscle cells by long-term intermittent stretch-relaxation

Skeletal muscle stretch increases resting metabolism and causes hypertrophy. We have examined the effect of mechanical stretch in vitro on glucose transport activity and transporter contents in L6 muscle cells. Long-term (24-48 h) stretch-relaxation (25% maximal elongation at 30 cycles per min) of cell monolayers significantly increased glucose uptake by 1.6- to 2-fold in myotubes but not in myoblasts. The presence of serum was required for the stretch-relaxation induced increase in glucose uptake. Cycloheximide inhibited the mechanical stimulation of glucose uptake, and the latter response was not additive to the stimulatory effect of long-term exposure to insulin. GLUT1 and GLUT4 glucose transporter contents were not changed in total cell membranes from mechanically stimulated cells relative to controls. These results indicate that mechanical stimulation through passive stretch may be an important regulation of nutrient uptake in fetal myotubes independent of innervation.

Regulation of glucose transport and GLUT1 glucose transporter expression by O2 in muscle cells in culture

The effect of varying cellular oxygenation on L6 muscle cell 2-deoxy-D-glucose transport, glucose utilization, lactate production, and expression of GLUT1 and GLUT4 transport proteins was investigated. Incubation of L6 myotubes in 3% O2 (mimicking a state of hypoxia) elevated glucose uptake by 6.5-fold over 48 h relative to cells incubated in 21% O2 (normoxia). Incubation of L6 cells in hyperoxic conditions (50% O2) significantly depressed glucose uptake by 0.4-fold. These effects were fully reversible. Incubation in 3% O2 also caused lactate accumulation and enhanced glucose consumption from the medium. Hypoxia elevated 2-deoxy-D-glucose transport even when the concentration of glucose in the medium was kept constant, suggesting that glucose deprivation alone was not responsible for increased cellular glucose uptake. Incubation in 3% O2 also elevated 3-O-methylglucose uptake but not amino acid uptake. Cycloheximide prevented the hypoxia-induced increase in glucose uptake, indicating that de novo synthesis of glucose transport-related proteins was the major means by which cells increased glucose uptake. The content of GLUT1 glucose transporter was significantly elevated in total membranes of cells incubated in 3% O2 and depressed in membranes from cells incubated in hyperoxic conditions, whereas GLUT4 expression was not affected. These results indicate that hypoxia induces an adaptive response of increasing cellular glucose uptake through elevated expression of GLUT1 in an attempt to maintain supply of glucose for utilization by nonoxidative pathways.

Acute and long-term effects of insulin-like growth factor I on glucose transporters in muscle cells. Translocation and biosynthesis

Insulin-like growth factor I (IGF-I) rapidly (less than 10 min) stimulated glucose uptake into myotubes of the L6 muscle cell line, at concentrations that act specifically on IGF-I receptors. Uptake remained stimulated at a steady level for 1-2 h, after which a second stimulation occurred. The first phase was insensitive to inhibition of protein synthesis. Subcellular fractionation demonstrated that it was accompanied by translocation of glucose transporters (both GLUT1 and GLUT4) to the plasma membrane from intracellular membranes. Translocation sufficed to explain the first phase increase in glucose transport, and there was no change in the total cellular content of GLUT1 or GLUT4 glucose transporters. The second phase of stimulation was inhibitable by cycloheximide, and involved a net increase in either GLUT1 or GLUT4 transporter content, which was reflected in an increase in transporter number in plasma membranes. These results define a cellular mechanism of metabolic action of IGF-I in muscle cells; furthermore, they suggest that IGF-I has acute metabolic effects that mimic those of insulin, bypassing action on the insulin receptor.

Acute and chronic signals controlling glucose transport in skeletal muscle

Glucose transport into muscle cells occurs through facilitated diffusion mediated primarily by the GLUT1 and GLUT4 glucose transporters. These transporter proteins are controlled by acute and chronic exposure to insulin, glucose, muscle contraction, and hypoxia. We propose that acute responses occur through recruitment of pre-formed glucose transporters from an intracellular storage site to the plasma membrane. In contrast, chronic control is achieved by changes in transporter biosynthesis and protein stability. Using subcellular fractionation of rat skeletal muscle, recruitment of GLUT4 glucose transporters to the plasma membrane is demonstrated by acute exposure to insulin in vivo. The intracellular pool appears to arise from a unique organelle depleted of transverse tubule, plasma membrane, or sarcoplasmic reticulum markers. In diabetic rats, GLUT4 content in the plasma membranes and in the intracellular pool is reduced, and incomplete insulin-dependent GLUT4 recruitment is observed, possibly through a defective incorporation of transporters to the plasma membrane. The lower content of GLUT4 transporters in the muscle plasma membranes is reversed by restoration of normoglycemia with phlorizin treatment. In some muscle cells in culture, GLUT1 is the only transporter expressed yet they respond to insulin, suggesting that this transporter can also be regulated by acute mechanisms. In the L6 muscle cell line, GLUT1 transporter content diminishes during myogenesis and GLUT4 appears after cell fusion, reaching a molar ratio of about 1:1 in the plasma membrane. Prolonged exposure to high glucose diminishes the amount of GLUT1 protein in the plasma membrane by both endocytosis and reduced biosynthesis, and lowers GLUT4 protein content in the absence of changes in GLUT4 mRNA possibly through increased protein degradation. These studies suggest that the relative contribution of each transporter to transport activity, and the mechanisms by which glucose exerts control of the glucose transporters, will be key subjects of future investigations.