Accelerated Fatty Acid Oxidation in Muscle Averts Fasting-induced Hepatic Steatosis in SJL/J Mice
Department of Molecular Genetics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390-9046, USA. Journal of Biological Chemistry
(Impact Factor: 4.57).
08/2009; 284(36):24644-52. DOI: 10.1074/jbc.M109.034397
The accumulation of triglycerides (TG) in the liver, designated hepatic steatosis, is characteristically associated with obesity and insulin resistance, but it can also develop after fasting. Here, we show that fasting-induced hepatic steatosis is under genetic control in inbred mice. After a 24-h fast, C57BL/6J mice and SJL/J mice both lost more than 20% of body weight and approximately 60% of total body TG. In C57BL/6J mice, TG accumulated in liver, producing frank steatosis. In striking contrast, SJL/J mice failed to accumulate any hepatic TG even though they lost nearly as much adipose tissue mass as the C57BL/6J mice. Mice from five other inbred strains developed fasting-induced steatosis like the C57BL/6J mice. Measurements of the uptake of free fatty acids (FA) in vivo and in vitro demonstrated that SJL/J mice were protected from steatosis because their heart and skeletal muscle took up and oxidized twice as much FA as compared with C57BL/6J mice. As a result of this muscle diversion, serum-free FA and ketone bodies rose much less after fasting in SJL/J mice as compared with C57BL/6J mice. When livers of SJL/J and C57BL/6J mice were perfused with similar concentrations of FA, the livers took up and esterified similar amounts. We conclude that SJL/J mice express one or more variant genes that lead to enhanced FA uptake and oxidation in muscle, thereby sparing the liver from FA overload in the fasting state.
Available from: Jamie E Moscovitz
- "Prolonged food deprivation induces a dramatic change of energy utilization and metabolism in mammalian model, via increasing the release of fatty acids from WAT, and enhancing fatty acid oxidation in liver ; this results in rapid body fat mobilization, and dramatically increases hepatic lipid accumulation . Guan et al. reported that mice undergoing 24 hours of fasting exhibited excessive hepatic triglyceride (TG) accumulation and obvious hepatic steatosis, further suggesting that prolonged food deprivation induces dramatic energy mobilization and promotes fatty liver development process . "
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ABSTRACT: The purpose of this study was to determine whether Nrf2 activation, via Keap1-knockdown (Keap1-KD), regulates lipid metabolism and mobilization induced by food deprivation (e.g. fasting).
Male C57BL/6 (WT) and Keap1-KD mice were either fed ad libitum or food deprived for 24 hours. After fasting, WT mice exhibited a marked increase in hepatic lipid accumulation, but Keap1-KD mice had an attenuated increase of lipid accumulation, along with reduced expression of lipogenic genes (acetyl-coA carboxylase, stearoyl-CoA desaturase-1, and fatty acid synthase) and reduced expression of genes related to fatty acid transport, such as fatty acid translocase/CD36 (CD36) and Fatty acid transport protein (FATP) 2, which may attribute to the reduced induction of Peroxisome proliferator-activated receptor (Ppar) α signaling in the liver. Additionally, enhanced Nrf2 activity by Keap1-KD increased AMP-activated protein kinase (AMPK) phosphorylation in liver. In white adipose tissue, enhanced Nrf2 activity did not change the lipolysis rate by fasting, but reduced expression of fatty acid transporters - CD36 and FATP1, via a PPARα-dependent mechanism, which impaired fatty acid transport from white adipose tissue to periphery circulation system, and resulted in increased white adipose tissue fatty acid content. Moreover, enhanced Nrf2 activity increased glucose tolerance and Akt phosphorylation levels upon insulin administration, suggesting Nrf2 signaling pathway plays a key role in regulating insulin signaling and enhanced insulin sensitivity in skeletal muscle.
Enhanced Nrf2 activity via Keap1-KD decreased fasting-induced steatosis, pointing to an important function of Nrf2 on lipid metabolism under the condition of nutrient deprivation.
Available from: PubMed Central
- "Nonetheless, the link between insulin and the liver’s handling of lipids is not completely understood and likely is more complex than a simple linear relationship. For instance, NAFLD is also increased in states of low insulin such as poorly controlled type-1 diabetes (T1DM) ,  and prolonged fasting , . "
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ABSTRACT: Fatty liver disease (FLD) is commonly associated with insulin resistance and obesity, but interestingly it is also observed at low insulin states, such as prolonged fasting. Thus, we asked whether insulin is an independent modulator of hepatic lipid accumulation.
In mice we induced, hypo- and hyperinsulinemia associated FLD by diet induced obesity and streptozotocin treatment, respectively. The mechanism of free fatty acid induced steatosis was studied in cell culture with mouse liver cells under different insulin concentrations, pharmacological phosphoinositol-3-kinase (PI3K) inhibition and siRNA targeted gene knock-down. We found with in vivo and in vitro models that lipid storage is increased, as expected, in both hypo- and hyperinsulinemic states, and that it is mediated by signaling through either insulin receptor substrate (IRS) 1 or 2. As previously reported, IRS-1 was up-regulated at high insulin concentrations, while IRS-2 was increased at low levels of insulin concentration. Relative increase in either of these insulin substrates, was associated with an increase in liver-specific fatty acid transport proteins (FATP) 2&5, and increased lipid storage. Furthermore, utilizing pharmacological PI3K inhibition we found that the IRS-PI3K pathway was necessary for lipogenesis, while FATP responses were mediated via IRS signaling. Data from additional siRNA experiments showed that knock-down of IRSs impacted FATP levels.
States of perturbed insulin signaling (low-insulin or high-insulin) both lead to increased hepatic lipid storage via FATP and IRS signaling. These novel findings offer a common mechanism of FLD pathogenesis in states of both inadequate (prolonged fasting) and ineffective (obesity) insulin signaling.
Available from: Lourdes Cruz-Garcia
- "GENES & DEVELOPMENT 289 adipose lipolysis (Lin et al. 2005; Guan et al. 2009). Furthermore , there is an inverse association between hepatic lipid levels and serum ketone body concentrations in fasted mice (Lin et al. 2005). "
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ABSTRACT: To find new genes that influence liver lipid mass, we performed a genetic screen for zebrafish mutants with hepatic steatosis, a pathological accumulation of fat. The red moon (rmn) mutant develops hepatic steatosis as maternally deposited yolk is depleted. Conversely, hepatic steatosis is suppressed in rmn mutants by adequate nutrition. Adult rmn mutants show increased liver neutral lipids and induction of hepatic lipid biosynthetic genes when fasted. Positional cloning of the rmn locus reveals a loss-of-function mutation in slc16a6a (solute carrier family 16a, member 6a), a gene that we show encodes a transporter of the major ketone body β-hydroxybutyrate. Restoring wild-type zebrafish slc16a6a expression or introducing human SLC16A6 in rmn mutant livers rescues the mutant phenotype. Radiotracer analysis confirms that loss of Slc16a6a function causes diversion of liver-trapped ketogenic precursors into triacylglycerol. Underscoring the importance of Slc16a6a to normal fasting physiology, previously fed rmn mutants are more sensitive to death by starvation than are wild-type larvae. Our unbiased, forward genetic approach has found a heretofore unrecognized critical step in fasting energy metabolism: hepatic ketone body transport. Since β-hydroxybutyrate is both a major fuel and a signaling molecule in fasting, the discovery of this transporter provides a new direction for modulating circulating levels of ketone bodies in metabolic diseases.
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