Ji et al. BMC Genomics 2012, 13:441 http://www.biomedcentral.com/1471-2164/13/441 between the two array platforms must be kept in mind, these data suggest that adipose tissue metabolism in chicken is at least as sensitive to energy status as hepatic metabolism. Our results indicate that both fatty acid syn- thesis and storage are dynamically regulated by energy sta- tus in chicken adipose tissue, despite its modest (~ 15%) contribution to the amount of stored fatty acids. Both fasted and insulin-neutralized birds exhibited sig- nificant increases in plasma glucagon. Parallel elevations in plasma NEFA suggested that this resulted in significant lip- olysis of stored triacylglycerol in both treatment groups. During fasting, a considerable percentage of the liberated fatty acids are re-esterified in adipocytes, and only a small fraction traditionally have been thought to be oxidized in the mitochondria of adipocytes through beta oxidation [34]. However, recent studies in mice and in human adi- pose tissue demonstrate that in some conditions fatty acid oxidation in white adipose tissue is considerable and may be an important determinant of obesity [35-37]. Consistent with this concept, we found significant increases in a num- ber of key enzymes that mediate mobilization of fatty acids and their oxidation, including the rate-limiting enzymes in both mitochondrial and peroxisomal fatty acid oxidation (CPT1A and ACOX1, respectively). We measured tissue levels of beta-hydroxybutyrate, a ketone product of beta oxidation, to confirm that changes in gene expression had functional consequences and found them to be signifi- cantly elevated in adipose tissue of fasted vs. fed chickens. Levels were numerically but not statistically higher in insulin-neutralized adipose tissue (data not shown). Qualitatively, fasting-induced changes in gene expression resemble those induced by the fibrate class of drugs, which activate PPARα and promote fatty acid oxidation in white adipose tissue and are used clinically to treat hyper- lipidemia [35,37-40]. These data suggest that dietary acti- vation of PPARα, for example through supplementation with fatty acids that preferentially bind and activate this member of the PPAR family [41], may be a means to at- tenuate fat deposition in commercial broilers. Such action may underlie the reduced abdominal fat mass reported in broilers that were fed diets rich in n-3 PUFA [42]. Both fasting and insulin neutralization elicited marked upregulation of PDK4. PDK4 is a nutrient sensing fuel switch that phosphorylates and inactivates pyruvate de- hydrogenase, which shifts fuel use from glucose to fatty acids and spares glucose for the brain during periods of fasting [23,43-45]. PDK4 also enhances glycerol synthesis in white adipose tissue by shunting pyruvate into glycero- neogenesis, at least in the fed state [46]. Hepatic and skel- etal muscle expression of PDK4 is increased by fatty acids, acetyl CoA, NADH and the diabetic state and decreased by insulin and pyruvate (rev. in [23,47]). Little is known about PDK4 in chicken, but a recent study suggests it acts as a glycogen sensor in muscle and thus plays comparable Page 11 of 16 roles to those in mammals [48]. In mouse white adipose tissue, PDK4 expression was shown to be induced by acti- vation of p38MAPK [49], which we found to be signifi- cantly up-regulated with fasting and, to a lesser extent, with insulin neutralization [19]. Although PDK4 was up- regulated in both treatment groups, and both groups showed evidence of increased lipolysis [18], only fasted chickens presented a gene expression signature and tissue beta-hydroxybutyrate levels that were clearly indicative of fatty acid oxidation. Although we did not measure malonyl-CoA levels, we predict that they were reduced with fasting, but not insulin neutralization, based on reduced expression of ACACA. Malonyl-CoA allosteri- cally binds and inhibits CPT1A, minimizing fatty acid transport and subsequent oxidation in mitochondria [50]. With insulin neutralization, increased PDK4 may thus be more aligned with the demand for glycerol needed to re- esterify fatty acids liberated by lipolysis [46]. Additional experiments are needed to confirm that manipulation of PDK4 alters fatty acid oxidation in chicken adipose tissue and to delineate its relative contributions to fatty acid oxi- dation and glyceroneogenesis under varying metabolic states. If manipulation of PDK4 does alter fatty acid oxida- tion, our results highlight this pathway as a potential tar- get for reducing fatness, which has relevance for both poultry and humans. Microarray data indicate that the effects of fasting in chicken adipose tissue extend beyond metabolism. GO analysis highlighted pathways such as cell cycle and cytokine-cytokine receptor interaction that are most likely related to changes in the stromal vascular fraction, which contains proliferating preadipocytes and cells of the immune system. In particular, a number of genes that regulate multiple steps in adipogenesis were signifi- cantly altered by fasting. Chickens rapidly accumulate abdominal fat after hatch, and until approximately 7 weeks of age this is due more to formation of new adi- pocytes than to adipocyte hypertrophy [11]. Adipocytes arise from mesenchymal stem cells in a two stage process of lineage commitment to an adipocyte fate, fol- lowed by differentiation of fibroblast-like preadipocytes into mature fat-storing cells [51]. Members of both the Wnt (MSC lineage commitment) and TGFβ/BMP (MSC lineage commitment and preadipocyte competence) sig- naling pathways were significantly regulated by fasting. Fasting down-regulated expression of CEBPα and PPARγ, two transcription factors that orchestrate the cascade of gene expression changes that lead to terminal adipocyte differentiation [51]. Expression of other adipo- genic mediators including fibroblast growth factor 2 (FGF2), fibroblast growth factor receptor 1 (FGFR1) [52], and nuclear receptor corepressor 1 (NCOR1) [53] were also significantly regulated by fasting. Collectively, these changes suggest that adipocyte number in