Despite its importance in terms of energy homeostasis, the role of AMP-activated protein kinase in adipose tissue remains controversial. Initial studies have described an anti-lipolytic role for AMP-activated protein kinase, whereas more recent studies have suggested the converse. Thus we have addressed the role of AMP-activated protein kinase in adipose tissue by modulating AMP-activated protein kinase activity in primary rodent adipocytes using pharmacological activators or by adenoviral expression of dominant negative or constitutively active forms of the kinase. We then studied the effects of AMPactivated protein kinase activity modulation on lipolytic mechanisms. Finally, we analyzed the consequences of a genetic deletion of AMP-activated protein kinase in mouse adipocytes. AMP-activated protein kinase activity in adipocytes is represented mainly by the ␣ 1 isoform and is induced by all of the stimuli that increase cAMP in adipocytes, including fasting. When AMP-activated protein kinase activity is increased by 5-aminoimidazole-4-carboxamide-riboside, phenformin, or by the expression of a constitutively active form, isoproterenol-induced lipolysis is strongly reduced. Conversely, when AMP-activated protein kinase activity is decreased either by a dominant negative form or in AMP-activated protein kinase ␣ 1 knock-out mice, lipolysis is increased. We present data suggesting that AMP-activated protein kinase acts on hormone-sensitive lipase by blocking its translocation to the lipid droplet. We conclude that, in mature adipocytes, AMP-activated protein kinase activation has a clear anti-lipolytic effect.

OBJECTIVES:Adipocyte is the only cell whose size may vary dramatically in physiological conditions. We hypothesized that increase in fat cell size per se could modulate several signalling pathways by changing the relationships between the cell and the extracellular matrix. The aim of the current study was (i) to examine whether within the same fat depot, metabolic functions of adipocyte were modified by cell size and (ii) if such an adaptation exists, to look for an integrin/extracellular-signal-regulated kinases (ERKs) signalling pathway. RESULTS: We isolated two populations of adipocytes with different volumes (67 and 22 Â 10 3 mm 3 ) within the same adipose location. In large compared to small fat cells, fatty acid synthase and lipoprotein lipase activities were increased two-and sevenfold, respectively; GLUT4 protein concentration and leptin expression were increased three-fold; lipolytic capacity was increased four-fold. The integrin/ERK signalling pathway could be the one responsible for the adaptation of adipose functions to cell size. In large compared with small adipocytes, we showed that b 1 -integrins are present in adipose membranes and at a higher concentration in large than in small cells. In isolated adipocytes, stimulation of b 1 -integrins with a specific monoclonal antibody results in ERK 1 and ERK 2 activation. In large compared to small cells, cytoplasmic concentrations of these two mitogen-activated protein kinases were increased two-fold, whereas their activities were increased 10-fold. CONCLUSION: A b 1 -integrin/ERKs signalling pathway is present in mature adipocyte. Increase in cell size, by modifying the relationships between cell and extracellular matrix, could turn on this pathway. Since ERKs can modulate transcription factors and subsequently modulate gene expression important for adipose function, this pathway could play an important role in the adaptation of adipose functions to cell size.