Recent reports emphasize the importance of mitochondria in white adipose tissue biology. In addition to their crucial role in energy homeostasis, mitochondria are the main site of reactive oxygen species generation. When moderately produced, they function as physiological signaling molecules. Thus, mitochondrial reactive oxygen species trigger hypoxia-dependent gene expression. Therefore the present study tested the implication of mitochondrial reactive oxygen species in adipocyte differentiation and their putative role in the hypoxiadependent effect on this differentiation. Pharmacological manipulations of mitochondrial reactive oxygen species generation demonstrate a very strong and negative correlation between changes in mitochondrial reactive oxygen species and adipocyte differentiation of 3T3-F442A preadipocytes. Moreover, mitochondrial reactive oxygen species positively and specifically control expression of the adipogenic repressor CHOP-10/ GADD153. Hypoxia (1% O 2 ) strongly increased reactive oxygen species generation, hypoxia-inducible factor-1 and CHOP-10/GADD153 expression, and inhibited adipocyte differentiation. All of these hypoxia-dependent effects were partly prevented by antioxidants. By using hypoxia-inducible factor-1␣ (HIF-1␣)-deficient mouse embryonic fibroblasts, HIF-1␣ was shown not to be required for hypoxia-mediated CHOP-10/GADD153 induction. Moreover, the comparison of hypoxia and CoCl 2 effects on adipocyte differentiation of wild type or HIF-1␣ deficient mouse embryonic fibroblasts suggests the existence of at least two pathways dependent or not on the presence of HIF-1␣. Together, these data demonstrate that mitochondrial reactive oxygen species control CHOP-10/GADD153 expression, are antiadipogenic signaling molecules, and trigger hypoxia-dependent inhibition of adipocyte differentiation.White adipose tissue is the main energy store in adult mammals and displays great plasticity according to the energy needs of the organism. Adipocyte differentiation results from a subtle balance of sequential and interdependent transcription factors expression that activate or inhibit promoters of adipogenic genes.

The physiological signaling mechanisms that link glucose sensing to the electrical activity in metabolism-regulating hypothalamus are still controversial. Although ATP production was considered the main metabolic signal, recent studies show that the glucose-stimulated signaling in neurons is not totally dependent on this production. Here, we examined whether mitochondrial reactive oxygen species (mROS), which are physiologically generated depending on glucose metabolism, may act as physiological sensors to monitor the glucose-sensing response. Transient increase from 5 to 20 mmol/l glucose stimulates reactive oxygen species (ROS) generation on hypothalamic slices ex vivo, which is reversed by adding antioxidants, suggesting that hypothalamic cells generate ROS to rapidly increase glucose level. Furthermore, in vivo, data demonstrate that both the glucose-induced increased neuronal activity in arcuate nucleus and the subsequent nervous-mediated insulin release might be mimicked by the mitochondrial complex blockers antimycin and rotenone, which generate mROS. Adding antioxidants such as trolox and catalase or the uncoupler carbonyl cyanide m-chlorophenylhydrazone in order to lower mROS during glucose stimulation completely reverses both parameters. In conclusion, the results presented here clearly show that the brain glucosesensing mechanism involved mROS signaling. We propose that this mROS production plays a key role in brain metabolic signaling. Diabetes 55: 2084 -2090, 2006 E lucidating the signaling mechanisms by which cells sense nutrient or metabolic status, a vital process in energy homeostasis, is of prime importance. Glucose-sensing mechanisms have been mainly characterized in two tissues, both in the pancreas (at the ␤-cell level) and in the brain (the so-called "glucose-stimulated" or "glucose-inhibited" neurons) (1,2). The cellular and molecular mechanisms underlying such glucose responsiveness appear to share similarities in the two glucose responsive cells (i.e., transport and phosphorylation by GLUT2 and glucokinase, respectively) and the consequent closure of ATP-sensitive K ϩ channels (K ATP channels) and calcium influx (3-5). Although ATP production used to be considered the main metabolic signal, recent studies show that the glucose-excited signaling in pancreatic ␤-cells and neurons is not totally dependent on this production. Within the hypothalamus, a previous work showed that glucose challenge monitors K ATP closure independently of ATP level (6), and more recent data demonstrated that glucose-induced depolarization might occur through a new K ATP channel-independent mechanism, at least in some hypothalamic arcuate neurons (7). These studies suggest that ATP-independent intracellular signaling mechanisms leading to the stimulation of hypothalamic neurons by glucose might be present.Transient increase in glucose metabolism generates the key substrates NADH and FADH 2 for the mitochondria, and their use increases electron formation without modifying other complex constraints along the res...

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