Impaired Mitochondrial Fat Oxidation Induces FGF21 in Muscle

SummaryFatty acids are the primary fuel source for skeletal muscle during most of our daily activities, and impaired fatty acid oxidation (FAO) is associated with insulin resistance. We have developed a mouse model of impaired FAO by deleting carnitine palmitoyltransferase-1b specifically in skeletal muscle (Cpt1bm−/−). Cpt1bm−/− mice have increased glucose utilization and are resistant to diet-induced obesity. Here, we show that inhibition of mitochondrial FAO induces FGF21 expression specifically in skeletal muscle. The induction of FGF21 in Cpt1b-deficient muscle is dependent on AMPK and Akt1 signaling but independent of the stress signaling pathways. FGF21 appears to act in a paracrine manner to increase glucose uptake under low insulin conditions, but it does not contribute to the resistance to diet-induced obesity

Impaired mitochondrial fat oxidation induces adaptive remodeling of muscle metabolism

© 2015, National Academy of Sciences. All rights reserved. The correlations between intramyocellular lipid (IMCL), decreased fatty acid oxidation (FAO), and insulin resistance have led to the hypothesis that impaired FAO causes accumulation of lipotoxic intermediates that inhibit muscle insulin signaling. Using a skeletal muscle-specific carnitine palmitoyltransferase-1 KO model, we show that prolonged and severe mitochondrial FAO inhibition results in increased carbohydrate utilization, along with reduced physical activity; increased circulating nonesterified fatty acids; and increased IMCLs, diacylglycerols, and ceramides. Perhaps more importantly, inhibition of mitochondrial FAO also initiates a local, adaptive response in muscle that invokes mitochondrial biogenesis, compensatory peroxisomal fat oxidation, and amino acid catabolism. Loss of its major fuel source (lipid) induces an energy deprivation response in muscle coordinated by signaling through AMP-activated protein kinase (AMPK) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) to maintain energy supply for locomotion and survival. At the whole-body level, these adaptations result in resistance to obesity

A low fat diet ameliorates pathology but retains beneficial effects associated with CPT1b knockout in skeletal muscle

<div><p>Inhibiting fatty acid oxidation is one approach to lowering glucose levels in diabetes. Skeletal muscle specific Carnitine Palmitoyltransferase 1b knockout mice (Cpt1b^m-/-) comprise a model of impaired fat oxidation; and have decreased fat mass and enhanced glucose disposal and muscle oxidative capacity compared to controls. However, unfavorable effects occur relative to controls when Cpt1b^m-/- mice are fed a 25% fat diet, including decreased activity and fat free mass and increased intramuscular lipid and serum myoglobin. In this study we explore if a low fat, high carbohydrate diet can ablate the unfavorable effects while maintaining the favorable phenotype in Cpt1b^m-/- mice. Mice were fed either 10% fat (low fat) or 25% fat (chow) diet. Body composition was measured biweekly and indirect calorimetry was performed. Low fat diet abolishes the decreased activity, fat, and fat free mass seen in Cpt1b^m-/- mice fed chow diet. Low fat diet also reduces serum myoglobin levels in Cpt1b^m-/- mice and diminishes differences in IGF-1 seen between Cpt1b^m-/- mice and control mice fed chow diet. Glucose tolerance tests reveal that glucose clearance is improved in Cpt1b^m-/- mice relative to controls regardless of diet, and serum analysis shows increased levels of muscle derived FGF21. Electron microscopic analyses and measurements of mRNA transcripts show increased intramuscular lipids, FGF21, mitochondrial and oxidative capacity markers regardless of diet. The favorable metabolic phenotype of Cpt1b^m-/- mice therefore remains consistent regardless of diet; and a combination of a low fat diet and pharmacological inhibition of CPT1b may offer remedies to reduce blood glucose.</p></div

Diet effects on Cpt1b^m-/- adiposity.

<p>Control mice (red dashed lines, N = 10 animals per time point on chow and N = 8 animals per time point on low fat diet) and Cpt1b^m-/- mice (black lines, N = 9 animals per time point on chow and N = 12 animals per time point on low fat diet) were monitored for 24 weeks to assess gain of Weight, Fat, and FFM. Results are compared between groups fed low fat diet (A-C) and chow diet (D-F). Asterisks indicate significance with P ≤ 0.05.</p

Low fat diet restores levels of IGF-1, myoglobin, activity, and serum lipids in Cpt1b^m-/- mice.

<p>(A) Serum levels of IGF-1 are plotted for control (white circles) and Cpt1b^m-/- (black squares) mice for each of the diets tested at 4–6 months (N = 10 animals per genotype for low fat diet and N = 14 animals per genotype for chow diet). (B) Serum myoglobin levels in animals fed either low fat or chow diets (N = 9 animals per genotype for low fat diet; and N = 10 control and 8 Cpt1b^m-/- for chow diet). (C-D) Activity variation is plotted for control (white circles) and Cpt1b^m-/- (black squares) mice fed chow diet (C) (N = 9 animals per genotype) or low fat diet (D) (N = 12 animals per genotype). Grey sections indicate periods of darkness, while white sections indicate periods of light. (E-F) Serum levels of NEFA (E) and serum levels of 3-hydroxybutyrate (F) in control (white) and Cpt1b^m-/- (black) mice fed chow or low fat diet (N = 9 for control animals and N = 10 for Cpt1b^m-/- animals). Asterisks indicate significance with P ≤ 0.05.</p

Changes in Cpt1b^m-/- relative to controls fed chow and low fat diets.

<p>Changes in Cpt1b^m-/- relative to controls fed chow and low fat diets.</p

Diet has little effect on glucose clearance and muscle physiology of Cpt1b^m-/- mice.

<p>(A-B) GTT are shown for control (white circles) and Cpt1b^m-/- (black squares) mice that were fed (A) low fat diet (N = 8 animals per genotype) or (B) chow diet (N = 15 animals per genotype). (C) Levels in control (white) and Cpt1b^m-/- mice (black) of FGF21 mRNA in red quadriceps muscle (left panel), and FGF21 protein in serum; (right panel) (N = 8 animals per genotype for low fat diet and N = 10 animals per genotype for chow diet). (D) EM showing increased IMCL in soleus of Cpt1b^m-/- (right) mice relative to controls (left). (E-F) (N = 4 animals per genotype for low fat diet and N = 6 animals per genotype for chow diet) Relative levels of markers of mitochondrial biogenesis and lipid usage in control (white) and Cpt1b^m-/- (black) mouse red quadriceps muscle from mice fed (E) low fat diet or (F) chow diet. Asterisks indicate significance with P ≤ 0.05.</p

Increased caloric intake as a result of low fat feeding explains increased gain of FFM.

<p>(A) ANCOVA analysis conducted by varying FFM as a function of genotype and average daily Kcal intake. White circles represent control mice, while black circles represent Cpt1b^m-/- mice. <b>(</b>B) Average weekly Kcal intake is plotted for control (white) and Cpt1b^m-/- (black) mice during this food switching study. (C) Average daily food intake was monitored for control (white circles) and Cpt1b^m-/- (black squares) mice that were fed 25% fat diet for 125 days, followed by switching to low fat diet. (D) Average weekly weight gain during this food switching study is shown for control (white) and Cpt1b^m-/- (black) mice. N = 15 animals per genotype for chow diet and N = 10 animals per genotype for low fat diet. Asterisks indicate significance with P ≤ 0.05.</p