Fat Body dSir2 Regulates Muscle Mitochondrial Physiology and Energy Homeostasis Nonautonomously and Mimics the Autonomous Functions of dSir2 in Muscles

Abstract

Sir2 is an evolutionarily conserved NAD؉ -dependent deacetylase which has been shown to play a critical role in glucose and fat metabolism. In this study, we have perturbed Drosophila Sir2 (dSir2) expression, bidirectionally, in muscles and the fat body. We report that dSir2 plays a critical role in insulin signaling, glucose homeostasis, and mitochondrial functions. Importantly, we establish the nonautonomous functions of fat body dSir2 in regulating mitochondrial physiology and insulin signaling in muscles. We have identified a novel interplay between dSir2 and dFOXO at an organismal level, which involves Drosophila insulinlike peptide (dILP)-dependent insulin signaling. By genetic perturbations and metabolic rescue, we provide evidence to illustrate that fat body dSir2 mediates its effects on the muscles via free fatty acids (FFA) and dILPs (from the insulin-producing cells [IPCs]). In summary, we show that fat body dSir2 is a master regulator of organismal energy homeostasis and is required for maintaining the metabolic regulatory network across tissues. Metabolic and energy homeostasis are tightly linked to insulin signaling and fat metabolism, which together become crucial determinants of organismal physiology (1, 2). From a clinical perspective, dysfunctions in insulin signaling and metabolic defects affect each other, as seen in obesity and type 2 diabetes (2, 3). Comorbidity of these conditions arises because abrogated fat metabolism has been implicated in insulin resistance and because alterations in insulin signaling are known to regulate the expression of fat metabolism genes. However, the interplay between insulin signaling, fat metabolism, and mitochondrial functions in the etiology of metabolic diseases is still unclear (4).Recent reports in mammals and flies clearly show that SIRT1/ Sir2 plays an important role in fat metabolism (5-12) and affects starvation survival (5). Additionally, ablation of SIRT1 in liver and muscles has been shown to result in an insulin resistance-like phenotype (12, 13). However, it is not clear if fat metabolism and systemic insulin signaling are regulated independently by Sir2/ SIRT1. An important link between insulin signaling and fat metabolism is the FOXO family of transcription factors. FOXOs are typically associated with the transcription of genes downstream to insulin/insulin-like growth factor (IGF) signaling, which include lipogenic and lipolytic genes (14, 15). Although SIRT1 regulates FOXO-dependent transcription (16), the importance of the SIRT1-FOXO cross talk in mediating the effects of insulin signaling at the organismal level is poorly addressed (17)(18)(19). In this respect, it is important to delineate the Sir2/SIRT1-dependent fat phenotype from insulin resistance in the liver and in peripheral tissues (muscles), in addition to investigating the role of FOXO in regulating these effects.Impaired insulin signaling has been associated with mitochondrial dysfunctions and defects in energy homeostasis. Several reports highlight the role of SIRT1 in affecti...

Functional implications of Drosophila insulin-like peptides in metabolism, aging, and dietary restriction

“…Finally, dSir2-mediated regulation of these two dilps is shown to act independently of dFOXO, a forkhead box-O transcription factor. Transcript levels of dilp2 and dilp5 were up-regulated in flies expressing both dSir2 RNAi and dFOXO-TM (constitutively active dFOXO) constructs in their fat body similar to the levels observed in dSir2 RNAi flies (Banerjee et al, 2013). …”

“…A recent study demonstrated a role of dSir2, the Drosophila homolog of mammalian histone deacetylase SIRT1 in regulating the expression of dilp2 and dilp5 where systemic knockdown of dSir2 up-regulates those two dilps (Banerjee et al, 2012). In addition, fat body-specific knockdown of dSir2 is sufficient to up-regulate dilp2 and dilp5 expression with changes in dilp3 transcript levels in those flies not reported (Banerjee et al, 2012, 2013). Finally, dSir2-mediated regulation of these two dilps is shown to act independently of dFOXO, a forkhead box-O transcription factor.…”

Identification of a Tissue-Restricted Isoform of SIRT1 Defines a Regulatory Domain that Encodes Specificity

“…SIRT1 is a key factor that regulates insulin sensitivity across species, including via AKT deacetylation (Banerjee et al, 2013; Liang et al, 2009; Sundaresan et al, 2011). Assaying for SIRT1-AKT interactions revealed that AKT was efficiently pulled down with SIRT1-FL and to a lesser extent with SIRT1-ΔE2 (Figure 4D).…”

Eat to reproduce: a key role for the insulin signaling pathway in adult insects

“…Although the fruit fly brain does not seem to readily release ILPs in the presence of glucose (Geminard et al, 2009), some electrical properties of ILP-producing cells change in the presence of glucose, further suggesting a link between ILPs and carbohydrate availability (Fridell et al, 2009; Kreneisz et al, 2010). Other studies demonstrate a link with fat availability and mobilization (Banerjee et al, 2012, 2013). Although much remains to be learned about the precise regulation of insect metabolism by the ISP, it seems that this pathway is not only linked with amino acid availability, but also with fat and sugar availability, which would correspond with its presumed role of nutrient sensor.…”