Fat burning: Foxy business

Lipidomics Gateway (27 May 2009) [doi:10.1038/lipidmaps.2009.8]

Insulin regulates adipocyte fat metabolism by limiting FoxO1 nuclear access, reducing transcription of a lipolytic enzyme.

FoxO1 adapts metabolism to starvation. FFA, free fatty acids; TG, triglycerides; VLDL, very low-density lipoprotein.

The complete hydrolysis of stored triglycerides is achieved by different lipolytic enzymes at the tri-, di- and mono- stages. The first step is rate-limiting and is performed by adipose triglyceride lipase (ATGL). One of the many ways that insulin promotes fat storage is by decreasing the expression of ATGL through an unknown mechanism. Now Kandror and colleagues have shown that the transcription factor FoxO1 promotes ATGL expression and that insulin prevents this by causing retention of FoxO1 in the cytoplasm.

The FoxO family of forkhead transcription factors participates in many cellular processes and is subject to complex regulation, including control of nuclear—cytoplasmic shuttling. In response to insulin signaling, phosphorylation by Akt, which is itself regulated by products of phosphatidylinositol-3-kinase activity, promotes binding of a chaperone to FoxO1, exposing a nuclear export signal and disrupting a nuclear localization signal. This leads to cytoplasmic retention of FoxO1 and a reduction in transcription of target genes. Despite its recognized role in metabolism and expression in adipocytes, the identity of FoxO1 target genes in fat tissue is largely unknown.

Using 3T3-L1 adipocytes, Kandror and colleagues found that insulin increased the expression of ATGL message and protein. This was prevented by inhibition of the PI3K—Akt pathway. Bioinformatic analysis of the ATGL promoter identified at least two FoxO1-binding sites, and expression of FoxO1 in differentiating adipocytes peaked to coincide with the induction of ATGL expression. To confirm the ability of FoxO1 to control ATGL expression the authors expressed a luciferase reporter construct containing the ATGL promoter, and found that its expression was increased approximately six-fold by FoxO1 and ten-fold by a constitutively active FoxO1 mutant. In mature adipocytes, short interfering RNA (siRNA)-mediated knockdown of FoxO1 decreased the expression of ATGL and attenuated the rate of lipolysis.

The authors then used immunofluorescence and subcellular fractionation to demonstrate that cytoplasmic retention of FoxO1 in response to insulin occurs in adipocytes. Furthermore, insulin decreased the association of FoxO1 with the ATGL promoter, as shown by chromatin immunoprecipitation. Knockdown of FoxO1 appeared to affect ATGL expression and ATGL-mediated lipolysis directly and exclusively: expression of other lipolytic components and factors implicated in ATGL regulation, but not in the FoxO1 cascade, was unchanged. In addition, FoxO1 expression appears to be sufficient for the effect of insulin on expression of ATGL: in mouse embryonic fibroblasts that did not express detectable FoxO1, insulin did not regulate ATGL expression. Stable overexpression of FoxO1 in these cells restored the effect.

FoxO1 is thought to participate in adaptation to fasting, switching muscle metabolism from use of carbohydrates to oxidation of lipids. Activating lipolysis in adipocytes is clearly one way that this could be achieved. This study has implications for the understanding of diabetes: insulin receptor signaling appears to affect lipolysis in adipocytes through the same PI3K/Akt/FoxO1 pathway that controls gluconeogenesis in the liver, potentially uniting a 'glucocentric' and a 'lipocentric' theory of the development of diabetes.

Emma Leah