Muscle-specific deletion of comparative gene identification-58 (CGI-58) causes muscle steatosis but improves insulin sensitivity in male mice

Abstract

Intramyocellular accumulation of lipids is often associated with insulin resistance. Deficiency of comparative gene identification-58 (CGI-58) causes cytosolic deposition of triglyceride (TG)-rich lipid droplets in most cell types, including muscle due to defective TG hydrolysis. It was unclear, however, whether CGI-58 deficiency-induced lipid accumulation in muscle influences insulin sensitivity. Here we show that muscle-specific CGI-58 knockout mice relative to their controls have increased glucose tolerance and insulin sensitivity on a Western-type high-fat diet, despite TG accumulation in both heart and oxidative skeletal muscle and cholesterol deposition in heart. Although the intracardiomyocellular lipid deposition results in cardiac ventricular fibrosis and systolic dysfunction, muscle-specific CGI-58 knockout mice show increased glucose uptake in heart and soleus muscle, improved insulin signaling in insulin-sensitive tissues, and reduced plasma concentrations of glucose, insulin, and cholesterol. Hepatic contents of TG and cholesterol are also decreased in these animals. Cardiac steatosis is attributable, at least in part, to decreases in cardiac TG hydrolase activity and peroxisome proliferator-activated receptor-α/peroxisome proliferator-activated receptor-γ coactivator-1-dependent mitochondrial fatty acid oxidation. In conclusion, muscle CGI-58 deficiency causes cardiac dysfunction and fat deposition in oxidative muscles but induces a series of favorable metabolic changes in mice fed a high-fat diet.

Figure 1.

Muscle-specific inactivation of CGI-58 causes cardiac and soleus steatosis. A, Muscle-specific deletion of CGI-58. Western blotting was performed using homogenates of tissues from adult mCgi58-KO (KO) mice and their littermates (Con). B, The gross appearance of hearts at necropsy showing the normal heart from control mice and the fatty and enlarged heart from mCgi58-KO mice. C, Increases in percentages of heart-to-body weight ratios in mCgi58-KO mice (n = 4–6) (**, P < .0001). D and E, H&E and Oil red-O staining showing LD and neutral lipid deposition in the LV (D) and soleus (E) muscles of mCgi58-KO mice. F, Representative electron micrographs of heart taken at ×2900. G, Representative electron micrographs of heart taken at ×9300, showing cardiac mitochondria and LDs.

Figure 2.

Tissue lipids at necropsy. mCgi58-KO and control mice (n = 4–6) on the HFD for 12 weeks were fasted for 4 hours and then killed for tissue collection. A, Tissue TG content. B, Tissue contents of total cholesterol (TC) and free cholesterol (FC). *, P < .05 and **, P < .01.

Figure 3.

Reduced TG hydrolase activity and fatty acid oxidation in the heart of mCgi58-KO mice. A, Cardiac TG hydrolase activity (n = 5). B, Western blottings of ATGL. GAPDH was used as a loading control. C, Relative mRNA levels in the ventricular tissue of control and mCgi58-KO mice (n = 4). D, Cardiac fatty acid oxidation activity (n = 5). E, Cardiac contents of total acyl-carnitine (AC) and AC species (n = 6). All tissue samples were collected from 4-hour-fasted mice fed the HFD for 12 weeks. *, P < .05; **, P < .01.

Figure 4.

mCgi58-KO mice have increased glucose tolerance and insulin sensitivity. A, GTT in mCgi58-KO mice (n = 7) and control littermates (n = 6) on HFD for 8 weeks. B, ITTs in mCgi58-KO mice (n = 12) and control littermates (n = 12) on HFD for 10 weeks. C, Basal glucose uptake in heart (n = 5) and soleus (n = 3). D, Cardiac and soleus mRNA levels of GLUTs in 4-hour-fasted mice fed the HFD for 12 weeks (n = 4). E, Western blot analysis (left panel) of Akt phosphorylation at serine 473, total Akt, and GAPDH (as loading controls). Right panel, Densitometry analysis of the blots. *, P < .05 and **, P < .01.

Figure 5.

mCgi58-KO mice have cardiac dysfunction and ventricular remodeling. A, Echocardiograph (M-Mode) showing LV systolic function in mice on HFD for 10 weeks. B, Ventricular mRNA levels of genes related to cardiac function and fibrosis in mice on HFD for 12 weeks (n = 4). C, Trichrome staining of ventricular heart tissues from mice on HFD for 12 weeks. *, P < .05 and **, P < .01.