Regulation of glycolysis in rat aorta

Cytokine stimulation of aerobic glycolysis in hematopoietic cells exceeds proliferative demand

The relationship between growth factor-dependent cell growth and proliferation and the up-regulation of cellular metabolism required to support these processes remains poorly defined. Here, we demonstrate that cell growth, proliferation, and glucose metabolism are coordinately regulated by interleukin-3 (IL-3) in cytokine-dependent cells. Surprisingly, glycolytic activity is stimulated to a greater extent than would be expected based on the rate of cell growth or proliferation. IL-3 signaling exerts a direct effect on glycolytic commitment independent of cell growth control. These results are not restricted to IL-3 as the cytokines IL-7 and IL-2 have similar effects on glucose metabolism when assayed in factor-dependent cell lines or primary lymphocytes, respectively. Growth factor stimulation leads cells to consume less oxygen and produce more lactate per glucose, indicative of conversion from oxidative to glycolytic metabolism. The enforced rate of glucose metabolism is in excess of that required to support cell growth; accordingly, if extracellular glucose is reduced, cells retain the ability to grow and proliferate by derepressing oxidative metabolism. These data suggest that the high rate of glycolysis observed in response to growth factor stimulation is a primary effect rather than a homeostatic response to increased cell growth.

Effect of palmitate on carbohydrate utilization and Na/K-ATPase activity in aortic vascular smooth muscle from diabetic rats

Several investigators have reported that carbohydrate metabolism is suppressed in blood vessels from diabetic (Db) rats. However, it is not known if metabolites from the reciprocal increase in oxidation of long-chain fatty acids that accompanies insulin-deficiency exacerbates the suppression of this pathway in the Db blood vessels. Such inhibition may have particularly deleterious consequences in vascular smooth muscle since aerobic glycolysis is believed to preferentially fuel the sarcolemmal Na/K ATPase in this tissue. Therefore, this study evaluated the effect of physiological (0.4 mM) and elevated (1.2 mM) concentrations of the long-chain fatty acid palmitate on both carbohydrate utilization and Na/K-ATPase activity in aorta from insulin-deficient Db rat. Thoracic aorta were removed from 10 week Db (streptozotocin 60 mg/Kg , i.v.) or control (C) rats and intima-media aortic preparations were incubated in the absence or presence of palmitate. Glycolysis (microM/g dry wt/h) and glucose oxidation (microM/g dry wt/h) were quantified using 3H-glucose and 14C-glucose, respectively. Na/K-ATPase activity was estimated by the measurement of 86rubidium uptake in the absence and presence of 2 mM ouabain. In the absence of exogenous palmitate, glycolysis (p < 0.05), glucose oxidation (p < 0.01) and the estimated ATP production from exogenous glucose were decreased in aorta from Db rat. However, despite this diminished rate of glycolysis, Na/K ATPase activity was similar in Db and C aorta. Palmitate (0.4 mM) inhibited Na/K ATPase activity and glucose oxidation to a similar extent in both Db and C but had no effect on glycolysis in either group. Elevation of palmitate to 1.2 mM had no additional inhibitory effect on glucose oxidation, Na/K ATPase activity or glycolysis in either the Db or C aorta. The metabolism of exogenous palmitate restored the ATP production in Db to control values. These data demonstrate that, despite the diminished glycolysis and glucose oxidation demonstrated in the Db tissue, Na/K ATPase activity was comparable in the C and Db aorta, in the absence or presence of exogenous long-chain fatty acid. Therefore, the accelerated oxidation of palmitate in diabetic vascular smooth muscle had no additional inhibitory effect on glycolysis or Na/K ATPase activity. These data suggest that Na/K ATPase activity in vascular smooth muscle is not impaired by the altered pattern of substrate utilization that occurs in insulin-deficient Db rats.

Interferon-gamma and tumor necrosis factor synergize to induce nitric oxide production and inhibit mitochondrial respiration in vascular smooth muscle cells

Nitric oxide (NO) is an important signal substance in cell-cell communication and can induce relaxation of blood vessels by activating guanylate cyclase in smooth muscle cells (SMCs). NO is synthesized from L-arginine by the enzyme NO synthase, which is present in endothelial cells. It was recently shown that SMCs may themselves produce NO or an NO-related compound. We have studied NO production and its effects on energy metabolism in cultured rat aortic smooth muscle cells. It was observed that the cytokines, interferon-gamma and tumor necrosis factor-alpha, synergistically induced an arginine-dependent production of NO in these cells. This was associated with an inhibition of complex I (NADH: ubiquinone oxidoreductase) and complex II (succinate: ubiquinone oxidoreductase) activities of the mitochondrial respiratory chain, suggesting that NO blocks mitochondrial respiration in these cells. Lactate accumulated in the media of the cells, implying an increased anaerobic glycolysis, but there was no reduction of viability. An NO-dependent inhibition of mitochondrial respiration and a switch to anaerobic glycolysis would reduce energy production of the SMCs. This would in turn reduce the contractile capacity of the cell and might represent another NO-dependent vasodilatory mechanism. It could be of particular importance in inflammation, since cytokines released by inflammatory cells may induce autocrine NO production in SMCs.