Cessation of daily exercise dramatically alters precursors of hepatic steatosis in Otsuka Long-Evans Tokushima Fatty (OLETF) rats

Abstract

The purpose of this study was to delineate potential mechanisms initiating the onset of hepatic steatosis following the cessation of daily physical activity. Four-week-old, hyperphagic/obese Otsuka Long-Evans Tokushima Fatty rats were given access to voluntary running wheels for 16 weeks to prevent the development of hepatic steatosis. The animals were then suddenly transitioned to a sedentary condition as wheels were locked (wheel lock; WL) for 5 h (WL5), 53 h (WL53) or 173 h (WL173). Importantly after the cessation of daily exercise (5-173 h), no changes occurred in body weight, fat pad mass (omental and retroperitoneal), food intake, serum insulin, hepatic triglycerides or in the exercise-suppressed hepatic stearoyl-CoA desaturase-1 and peroxisome proliferator-activated receptor-gamma protein content. However, complete hepatic fatty acid oxidation and mitochondrial enzyme activities were highest at WL5 and WL53 and dropped significantly to SED levels by WL173. In addition, cessation of daily exercise quickly increased the hepatic protein contents of fatty acid synthase and acetyl-coenzyme A carboxylase (ACC), reduced ACC phosphorylation status, and dramatically increased hepatic malonyl-CoA concentration. This study is the first to show that the sudden cessation of daily exercise in a hyperphagic/obese model activates a subgroup of precursors and processes known to initiate hepatic steatosis, including decreased hepatic mitochondrial oxidative capacity, increased hepatic expression of de novo lipogenesis proteins, and increased hepatic malonyl CoA levels; each probably increasing the susceptibility to non-alcoholic fatty liver disease.

Figures

Figure 1. Representative images of haematoxylin and eosin (H&E) and Oil-Red O staining

Oil-Red O staining in WL5 (A), WL53 (C), WL173 (E) and SED (G). Red droplets indicate neutral lipid staining. Quantification of Oil-Red O lipid staining is shown in Table 2. H&E staining from WL5 (B), WL53 (D), WL173 (F) and SED (H).

Figure 2. Effects of 5 h, 53 h and 173 h of physical inactivity on liver citrate synthase (A), β-HAD (B) and cytochrome c oxidase (C) activities, cytochrome c protein content (D), and the complete oxidation of palmitate (E)

Values (means ±s.e.m.; n= 6–8) with different letters are significantly different (P < 0.05).

Figure 3. Effects of 5 h, 53 h and 173 h of physical inactivity on liver ACC (A), pACC (B), malonyl-CoA (C), FAS (D), SCD-1 (E) and PPARγ (F) protein content

Representative Western blots for each and AMPKα and pAMPK are shown in G. Values (means ±s.e.m.; n= 6–8) with different letters are significantly different (P < 0.05).