HFD-induced obesity is associated with an increased risk for diseases, including cancer, diabetes, and heart disease. The polygenic and/or multi-organ nature of HFD-induced obesity makes it difficult to determine the relative contribution to each of these diseases. In order to examine these complicated interactions in a simple system, we established a HFD-induced obesity model in Drosophila to elucidate the underlying mechanisms. We used the genetic versatility of the Drosophila model along with sophisticated cardiac function assays to investigate the effects of insulin-TOR mediated metabolic regulation and the crosstalk between organs exposed to excess dietary fat. We find that HFD-fed flies become obese, develop metabolic syndrome and exhibit severe symptoms of cardiac lipotoxicity. The deleterious HFD-induced effects are alleviated by genetic manipulations of metabolic regulators or by directly altering lipid metabolism either systemically, in adipose tissue, or specifically (autonomously) in the heart.
We provide evidence that Drosophila
fed a HFD exhibit central features of mammalian metabolic syndrome, including elevated lipid levels and changes in insulin and glucose homeostasis. Furthermore, the effects of the HFD-induced obesity on the heart are profound, including diminished contractility, conduction blocks and structural defects. The HFD-induced elevation in TG levels is an important marker for this collection of phenotypes as high TG levels are associated with disruptions of lipid and glucose homeostasis, mitochondrial function and other processes (Schaffer 2003
, Unger 2003
, Ouwens et al., 2005
; Van Gaal et al., 2006
, van Herpen and Schrauwen-Hinderling 2008
), all of which may contribute to high lipid accumulation and heart phenotypes.
Because the insulin-TOR pathway is a key integrator of metabolism, we initiated a comprehensive investigation of both the systemic and tissue-specific effects of altering insulin-TOR signaling under HFD conditions. We found that reduction of pathway activity blocks HFD-induced increased lipid levels in Drosophila
. Recent studies have begun to connect insulin-TOR signaling to the regulation of lipid metabolism in flies and mammals (Luong et al., 2006
; Li et al. 2010
; Lee et al. 2010
; this study). For example, the increased lipid synthesis caused by insulin treatment is blocked by reduction of TOR function, and the activation of the TOR pathway leads to fat accumulation (Luong et al., 2006
; Porstmann et al., 2008
; Li et al., 2010
). In addition, TOR function in lipogenesis (and heart function) may be mediated by activation of Sterol Regulatory Element Binding Protein (SREBP) and its lipogenic target genes, including FAS
(HY Lim, R.T.B., S.O. and R.B., unpubl.). Therefore, modulation of lipid metabolism is a likely mechanism that mediates the effect of TOR signaling on the HFD phenotype.
To determine the contributions of lipid metabolism to the HFD-induced obesity phenotypes, we examined a key regulator of lipid utilization: the Bmm (ATGL) gene is required for the breakdown of lipid droplets, and encodes a triacylglycerol lipase that is conserved from nematodes to mammals (Gronke et al., 2005
; this study). Drosophila Bmm
mutants show significant increases in TGs, while ectopic expression results in the reverse effect of lowering TG levels. Previous studies in mice have shown that systemic mutants of ATGL
caused increases in lipid accumulation in non-adipose such as the heart (Haemmerle et al., 2006
; Hirano et al., 2008
). Here, we find that reduction of TOR function leads to increased Bmm
RNA levels and Bmm
overexpression prevents the HFD-induced elevation of TG levels. In the heart, Bmm
overexpression protects against cardiac dysfunction and fat accumulation inflicted by a HFD. These protective effects by Bmm
may stem from modified lipid utilization by the mitochondria. Additionally, lipid droplet lipolysis may also liberate fatty acid ligands for nuclear receptors, which may contribute to changes in mitochondrial biogenesis (Palanker et al., 2009
; Finck et al., 2003
). Altered insulin-TOR signaling is likely to also likely to increase activity of translation factors involved in mitochondrial function (Zid et al. 2009
). Lastly, increases in autophagy have been implicated in the regulation of lipid metabolism via the breakdown of lipid droplets (Kovsan et al., 2009
). Thus, reduction of insulin-TOR signaling can coordinately lead to changes in lipid metabolism, which in turn affects organismal physiology under excess dietary fat conditions.
A critical step in the regulation of lipid synthesis involves SREBP, which in flies responds to decreased levels of the major membrane lipid phosphatidylethanolamine (PE) by increasing FAS
expression (Rawson, 2003
). In examining the role of lipid synthesis in relation to the HFD-induced obesity phenotypes, we observed that the transcript levels of FAS
decrease in TOR mutants, and that FAS
knockdown reduces the deleterious effects of HFD, possibly because of reduced lipid synthesis. Consistent with this idea, loss of SREBP
function in mammalian models leads to low TG levels (Valet et al., 2002
; Bentzinger et al., 2008
), and loss of Drosophila easily shocked (eas),
which encodes an ethanolamine kinase critical for PE synthesis, leads to increased levels of the active form of SREBP, increased FAS expression and thus elevated TG levels (HY Lim and R. B., unpubl.). In addition, this effect is also observed in mammalian hepatocytes where TOR function is required for SREBP activation (Li et al., 2010
), and it has been proposed that TOR serves as an important fork in the road of diabetic insulin resistance separating gluconeogenesis from lipogenesis (Li et al. 2010
; Laplante and Sabatini 2009; 2010
). Collectively, these data support the idea that TOR is required to mediate HFD-induced obesity and ensuing (cardiac) organ defects via multiple mechanisms. Interestingly, reducing or blocking fat accumulation mimics the protective effects of lowered insulin-TOR signaling under a HFD. However, it remains to be determined what it is about the accumulating fat that is detrimental to an organism: Is the fat itself or a side effect of its accumulation that is causing metabolic and physiological dysregulation?
Our data indicate that heart dysfunction due to the HFD treatment are the result of autonomous changes within the heart, as evidenced by the increased cardiac TG levels; and cardiac-only reduction of insulin-TOR signaling protects the heart from dysfunction and fat accumulation. Importantly, our findings show that inhibition of insulin-TOR signaling or ensuing fat accumulation in the heart itself can significantly prevent cardiac dysfunction despite the presence of elevated systemic TG levels. In addition, HFD-induced obesity also influences heart function via the adipose tissue since blocking insulin-TOR function in the adipose can also prevent the HFD obesity heart phenotypes. Thus, non-autonomous cross-talk factors, which may include hormones or metabolites, also contribute to HFD-induced heart dysfunction. The Drosophila model will help to understand the basis of such cross-talk.
The fact that flies become obese on a HFD has important implications on its own. Theories for the increased frequency and appearance of obesity include the thrifty gene hypothesis, which tries to understand why the incidence of obesity may be increasing despite its deleterious effects. These ideas state that recent evolutionary selection pressure on people experiencing frequent famines concentrated genetic variants that increased the ability to provide sufficient nutrients to an organism in times of lean, but also predisposed to obesity in times of plenty. Alternatively, the potential for HFD-induced obesity may have arisen early in evolution, perhaps independently of external selection pressure, via deregulation of metabolic responses in a multi-cellular organism. In this report, we provide evidence to suggest that the capacity for HFD-induced obesity and its associated complications are evolutionary ancient and is latent in core nutrient sensing pathways that becomes manifest upon exposure to dietary extremes that bypass the homeostatic threshold and dynamic range.
The discovery of these HFD-induced obesity phenotypes in the Drosophila genetic model will permit a detailed dissection of obesity phenotypes, especially with regard to the cardiac lipotoxicity effects (and possibly mimicking aspects of diabetic cardiomyopathy). In particular, we can now attempt to understand the various contributions of insulin resistance, fat accumulation, and fatty acid oxidation to the HFD-induced obesity phenotypes, including timing requirements. In summary, the advent of the Drosophila HFD-induced obesity model opens up many new horizons to study deregulated processes and diseases of chronic lipid excess.