Accelerated Fatty Acid Oxidation in Muscle Averts Fasting-induced Hepatic Steatosis in SJL/J Mice

Michael S. Brown,Hong-Ping Guan,Joseph L. Goldstein,Guosheng Liang

doi:10.1074/jbc.m109.034397

Michael S. Brown, Hong-Ping Guan + Show 2 more

Open Access

https://doi.org/10.1074/jbc.m109.034397

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Abstract

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.

Highlights

Under fasting conditions, insulin falls and the inhibitory effect of insulin on adipose tissue lipolysis is diminished
When we corrected for isotopic dilution by the different serum levels of fatty acids (FA) in fasted C57BL/6J mice (1.21 mM) and SJL/J mice (0.57 mM), the corrected total FA uptake was 2.3-fold higher in C57BL/6J liver compared with SJL/J liver (Fig. 7B)
The current studies began with the observation that C57BL/6J mice and SJL/J mice differ dramatically in their susceptibility to fasting-induced hepatic steatosis even though synthesis in primary hepatocytes isolated from fasted C57BL/6J and SJL/J mice and incubated at 37 °C for the indicated time with 1.5 mM [3H]glycerol (14.6 ϫ 103 dpm/nmol)

Summary

EXPERIMENTAL PROCEDURES

Isotopes and Other Materials—We obtained etomoxir, [1,2,3-3H]glycerol (39.8 Ci/mmol), [carboxy-14C]palmitic acid (53 mCi/mmol), and [9,10-3H]palmitic acid (50 Ci/mmol) from Sigma and 3H-labeled water (5 Ci/ml) and (R)-2-[9,10-3H]bromopalmitic acid (40 Ci/mmol) from American Radiolabeled Chemicals. 30 min after injection, each mouse was anesthetized, and 500 ␮l of blood was removed from the retroorbital sinus for measurement of the serum content of 3H or 14C radioactivity in duplicate. For measurement of TG synthesis from [3H]water, [3H]glycerol, or [14C]palmitate, portions of liver (200 mg) were homogenized and subjected to Folch extraction as described above, followed by thin-layer chromatography (TLC) and scintillation counting as described [11]. Mice were fasted for 24 h and anesthetized by injecting 30 mg/kg sodium pentobarbital intraperitoneally, after which a catheter was inserted into the jugular vein. Fifty microliters of blood was taken from the jugular vein for serum FA determination, after which sodium [3H]bromopalmitate-albumin (39 ϫ 106 cpm; 1 nmol) was injected. Various tissues were removed and used for lipid extraction and for measurement of radioactivity by scintillation counting as described [11].

Total RNA was prepared from mouse livers using an RNA

Findings

DISCUSSION