Effect of glucose on lipolysis and energy metabolism in fat cells

Possible involvement of AMP-activated protein kinase in obesity resistance induced by respiratory uncoupling in white fat

The AMP-activated protein kinase (AMPK) cascade is a sensor of cellular energy charge that promotes catabolic and inhibits anabolic pathways. However, the role of AMPK in adipocytes is poorly understood. We show that transgenic expression of mitochondrial uncoupling protein 1 in white fat, which induces obesity resistance in mice, is associated with depression of cellular energy charge, activation of AMPK, downregulation of adipogenic genes, and increase in lipid oxidation. Activation of AMPK may explain the complex metabolic changes in adipose tissue of these animals and our results support a role for adipocyte AMPK in the regulation of storage of body fat.

Impaired noradrenaline-induced lipolysis in white fat of aP2-Ucp1 transgenic mice is associated with changes in G-protein levels

In vitro experiments suggest that stimulation of lipolysis by catecholamines in adipocytes depends on the energy status of these cells. We tested whether mitochondrial uncoupling proteins (UCPs) that control the efficiency of ATP production could affect lipolysis and noradrenaline signalling in white fat in vivo. The lipolytic effect of noradrenaline was lowered by ectopic UCP1 in white adipocytes of aP2-Ucp1 transgenic mice, overexpressing the UCP1 gene from the aP2 gene promoter, reflecting the magnitude of UCP1 expression, the impaired stimulation of cAMP levels by noradrenaline and the reduction of the ATP/ADP ratio in different fat depots. Thus only subcutaneous but not epididymal fat was affected. UCP1 also down-regulated the expression of hormone-sensitive lipase and lowered its activity, and altered the expression of trimeric G-proteins in adipocytes. The adipose tissue content of the stimulatory G-protein alpha subunit was increased while that of the inhibitory G-protein alpha subunits decreased in response to UCP1 expression. Our results support the idea that the energy status of cells, and the ATP/ADP ratio in particular, modulates the lipolytic effects of noradrenaline in adipose tissue in vivo. They also demonstrate changes at the G-protein level that tend to overcome the reduction of lipolysis when ATP level in adipocytes is low. Therefore, respiratory uncoupling may exert a broad effect on hormonal signalling in adipocytes.