Stimulation of glucose metabolism by lectins in isolated white fat cells

Addition of 5 μg/ml concanavalin A to isolated white fat cells in the presence of 1 % albumin maximally stimulated the conversion of d-[1-¹⁴C]glucose to CO₂, glyceride-glycerol and fatty acids over a 1 h incubation period; as little as 1 μg/ml agglutinin increased fat cell glucose oxidation more than 2-fold. Labelled CO₂ production in the presence of concanavalin A was linear for at least 90 min and was inhibited by 40 mM α-methyl-d-glucoside which had little effect on basal or insulin-stimulated glucose oxidation. The effect of a submaximal concentration of the agglutinin was additive to that of submaximal but not maximal concentrations of insulin.Concanavalin A caused agglutination of fat cells which could be readily detected by light microscopy. Digestion of fat cells with 0.5 mg/ml trypsin for 15 min did not affect subsequent agglutination and inhibited the increased glucose oxidation due to concanavalin A by less than 30%. Thus the action of concanavalin A was much less sensitive to trypsinization of fat cells than insulin since trypsin under the above conditions completely abolished the effect of insulin. An anti-blood group A agglutinin from Phaseolus lunatus and Lens culanaris agglutinin also markedly stimulatedfat cell glucose conversion to CO₂. Agglutinin-stimulated glucose metabolism was inhibited by phloretin. This binding of several types of specific plant lectins to fat cell membrane glycoprotein(s) and/or glycolipid(s) apparently initiates events which results in increased glucose transport.     

Barbiturates suppress glucose utilization by inhibition of hexokinase in neuroblastoma cells.

Exposure to the pentobarbital potently inhibited the 2-deoxy glucose uptake in cultured neuroblastoma cells. The inhibition was assumed to be due to saturation of the uptake in the early stage where the incorporation was linear in the nontreated cells. On the contrary, the incorporation of 3-O-methyl glucose, another glucose analog which is not phosphorylated by hexokinase, was not altered by the treatment with pentobarbital. These results suggest that the suppression of hexokinase is involved in the above-mentioned effect of pentobarbital.     

Absence of alpha-adrenergic inhibition of lipolysis in swine adipose tissue

Adipose tissue slices from young and older pigs and genetically obese pigs were incubated to demonstrate alpha-adrenergic inhibition of lipolysis as found by other investigators in dog, guinea-pig, hamster, human and rabbit adipose tissue. Purported alpha-adrenergic agonists (amidephrine, clonidine, methoxamine, phenylephrine) did not inhibit basal or catecholamine-stimulated lipolysis. Purported alpha-adrenergic antagonists (dihydroergotamine, phenoxybenzamine, phentolamine, prazosin, yohimbine) did not enhance basal or stimulated lipolysis. Adipose tissue from pigs is different from that of most species but similar to that of rats with no alpha-adrenergic inhibition of lipolysis.     

Growth hormone inhibition of glucagon- and cAMP-induced lipolysis by chicken adipose tissue in vitro

The ability of growth hormone (GH) to inhibit the early (first hour) lipolytic response to glucagon and cAMP was investigated using chicken adipose tissue explants in vitro. In the first hour of incubation, GH inhibited glucagon, 8-bromo-3',5'-cyclic adenosine monophosphate (8-bromo-cAMP), and 1-isobutyl-3-methyl-xanthine (IBMX) induced glycerol release. The antilipolytic effect of GH was dose dependent, with inhibition of glucagon and 8-bromo-cAMP observed in the presence of as little as 100 ng/ml GH. In the fourth hour of incubation (late lipolytic response), GH (10, 100, or 1000 ng/ml) enhanced the lipolytic action of glucagon.     

Stimulation of glucose phosphorylation by fructose in isolated rat hepatocytes

The phosphorylation of glucose was measured by the formation of [3H]H2O from [2-3H]glucose in suspensions of freshly isolated rat hepatocytes. Fructose (0.2 mM) stimulated 2-4-fold the rate of phosphorylation of 5 mM glucose although not of 40 mM glucose, thus increasing the apparent affinity of the glucose phosphorylating system. A half-maximal stimulatory effect was observed at about 50 microM fructose. Stimulation was maximal 5 min after addition of the ketose and was stable for at least 40 min, during which period 60% of the fructose was consumed. The effect of fructose was reversible upon removal of the ketose. Sorbitol and tagatose were as potent as fructose in stimulating the phosphorylation of 5 mM glucose. D-Glyceraldehyde also had a stimulatory effect but at tenfold higher concentrations. In contrast, dihydroxyacetone had no significant effect and glycerol inhibited the detritiation of glucose. Oleate did not affect the phosphorylation of glucose, even in the presence of fructose, although it stimulated the formation of ketone bodies severalfold, indicating that it was converted to its acyl-CoA derivative. These results allow the conclusion that fructose stimulates glucokinase in the intact hepatocyte. They also suggest that this effect is mediated through the formation of fructose 1-phosphate, which presumably interacts with a competitive inhibitor of glucokinase other than long-chain acyl-CoAs.     

Effect of polyene antibiotics on the isolated kidney mitochondria of dogs

The effect of amphotericin B and nystatin on mitochondria isolated from the dog kidneys was studied. It was shown that incubation of the isolated kidney mitocondria with amphotericin B or nystatin at a wide range of their concentrations, i.e. from 0.1 to 100 gamma per 0.5 mg of the mitochondrial protein did not affect the activity of succinate dehydrogenase of these cell microstructures. The absence of changes in the succinate dehydrogenase activity of the mitochondria under the effect of the polyenic antibiotics is significant from the viewpoint of elucidating their molecular mechanisms of the nephrotoxic effect.     

Stimulation of PAI-1 and adipokines by glucose in human adipose tissue in vitro

Adipokines such as Plasminogen activator inhibitor-1 (PAI-1), interleukin (IL)-8, and tumor necrosis factor (TNF)-alpha are elevated in patients with obesity, insulin resistance, and type 2 diabetes. In the present study, we investigated whether glucose affected the production of these adipokines in human adipose tissue in vitro. Glucose (up to 35mM) increased secretion of PAI-1 (p<0.01) and IL-8 (p<0.01), but not TNF-alpha, in a dose- and time-dependent manner. Half-maximal stimulatory concentration of glucose was about 1mM. Glucosamine (5mM) decreased production of PAI-1 (p<0.05) and IL-8 (p<0.05), indicating that the hexosamine biosynthesis pathway is not involved in the glucose-induced increment in adipokine secretion. The present data demonstrate that glucose increases PAI-1 and IL-8 secretion. However, glucose concentrations above 5mM had no additional effects on adipokine secretion, suggesting that mechanisms other than diabetes/insulin resistance-related hyperglycemia may be involved in the observed elevation of these adipokines.     

Impaired insulin-mediated inhibition of lipolysis and glucose transport with aging

Regulation of hormone action with aging has been extensively studied; adipocytes provide an interesting model for some of these questions. We have compared the ability of insulin to stimulate glucose uptake and suppress lipolysis in adipocytes isolated from two month and twelve month-old rats. The ability of insulin to stimulate maximal glucose transport was decreased in adipocytes from the older rats (P less than 0.001); as well, insulin's EC50 was also higher (P less than 0.01) in these cells. Furthermore, these defects were present when insulin-stimulated glucose transport was measured in the presence or absence of adenosine deaminase which metabolizes endogenously released adenosine. Endogenously released adenosine is a stimulator of glucose transport and an inhibitor of lipolysis. Maximal suppression of isoproterenol-induced lipolysis by insulin was similar when adipocytes isolated from the two age groups were incubated in the absence of adenosine deaminase. However, maximal insulin-mediated suppression of lipolysis was found to be significantly decreased (P less than 0.001) in adipocytes isolated from older rats when the experiments were done in the presence of adenosine deaminase; also, insulin's EC50 was increased in these cells under these conditions (P less than 0.001). These results emphasize the importance of the adenosine receptor in modulating the response of isolated adipocytes to insulin, particularly for lipolysis, and document the presence of age-associated defects in insulin regulation of both glucose transport and lipolysis.