The propensity to store triglyceride within hepatocytes is related to low mitochondrial content and associated low rates of fatty acid β-oxidation, which is exceeded by hepatic FFA uptake (Fig. 1B). Similarly, lower fasting and glucose-stimulated insulin concentrations after exercise training44 may reduce insulin-mediated hepatic conversion of FFAs to triglycerides (Fig. 1B). Unfortunately, human studies examining direct hepatic effects of exercise therapy on hepatocellular biochemistry are restricted by the limitation of obtaining liver tissue, and no human data are available. Sedentary rats genetically bred for low
aerobic capacity have higher sterol regulatory element binding protein 1c (SREBP-1c), a transcription factor that regulates genes which promote triglyceride synthesis,
with associated reductions in hepatic mitochondrial volume density and capacity for fatty acid oxidation.50 However, it is difficult to dissociate Selleckchem KU-60019 these adaptations from factors external to the liver. For instance, when compared selleck kinase inhibitor with that of high-fitness rats, those with low aerobic capacity had increased adiposity, including visceral adiposity and insulin resistance, which is known to increase hepatic fatty acid synthesis via SREBP-1c.51 More recently, Rector et al. have shown that hepatic fatty acid oxidation increases and de novo lipogenesis declines with exercise training in rodent models of obesity and type 2 diabetes, but initiation of sedentary behavior elevates hepatic triglyceride (Fig. 1B). The latter was accompanied by enzyme alterations which initiate hepatic fat accumulation.52, 53 In human NAFLD, variability in the expression this website of peroxisome proliferator-activated receptor-delta, which is involved in the regulation of hepatic mitochondrial biogenesis, has been shown to affect liver fatness. Namely, homozygous and heterozygous carriers of the rs1053049, rs6902123, and rs2267668 single-nucleotide polymorphisms experienced less pronounced reductions in visceral and hepatic fat in response to lifestyle intervention.54
The signal for these adaptations may be adenosine monophosphate-activated protein kinase (AMPK), whose activity is increased during and after exercise in rodents.55 Although direct studies of exercise training are absent, AMPK activation is known to attenuate malonyl-coenzyme A and subsequently to increase fatty acid entry and oxidation within mitochondria (perhaps due to hepatic acetyl-coenzyme A carboxylase inhibition),55 and reduce lipid synthesis and insulin resistance.55 These effects are modulated by adipokines, particularly adiponectin, which up-regulates AMPK in both skeletal muscle and liver and also reduces hepatic glucose production. Although adiponectin concentration has been shown to increase following significant weight loss (∼10% body weight), an independent effect of exercise is yet to be established.