Obesity and its associated disorders such as type 2 diabetes, coronary heart diseases and non-alcoholic fatty liver disease (NAFLD) are currently global health problems. There is accumulating evidence that obesity is closely associated with impaired free fatty acid (FFA) metabolism as well as insulin resistance and inflammation . Excessive release of FFA from visceral fat adipocytes leads to the production of inflammatory and proatherogenic proteins through activation of the NFκB and c-Jun NH2-terminal kinase (JNK) pathways in skeletal muscle, liver and endothelial cells, and promotes atherosclerotic vascular disease (ASVD) and NAFLD.
Fatty acid translocase CD36 mediates uptake of FFA from circulation and intracellular transport of long-chain fatty acids in diverse cell types such as monocytes, platelets, macrophages, microvascular endothelial cells, adipocytes, muscle cells, enterocytes, and hepatocytes . Mice deficient of CD36 exhibit defective uptake and utilization of fatty acids. Excessive fatty acid uptake mediated by CD36 plays an important role in hepatic steatosis . The expression level of CD36 is very low in normal liver tissues , but is drastically increased in the liver tissues of high-fat diet (HFD)-induced fatty liver mice and those of human NAFLD. Conversely, forced expression of CD36 in liver causes hepatic steatosis in the absence of HFD . There is extensive evidence showing that CD36 plays significant roles in hepatic steatosis, suggesting that CD36 can be a potential drug target against NAFLD.
Since the first report in 2007, which demonstrated the effect of molecular hydrogen on brain infarction , hydrogen has been shown to protect against a variety of diseases including oxidative stress-related diseases, inflammation and allergy in in vivo and in vitro models as well as in humans . In the metabolic diseases, hydrogen attenuates oxidative stress and improves lipid, glucose and energy metabolism in patients and animal models of hepatic steatosis and atherosclerosis, but the underlying molecular mechanisms remain largely unknown [8–11]. Although the hydrogen effects have been ascribed to a selective scavenging of hydroxyl radicals, we previously reported that hydrogen attenuates type I allergy via inhibiting intracellular signaling pathways, providing the first evidence that hydrogen modulates signaling pathways . We also demonstrated that hydrogen suppresses LPS/IFNγ-induced phosphorylation of apoptosis signal-regulating kinase 1 (ASK1) and its downstream signaling molecules, p38, JNK and NFκB, resulting in inhibition of iNOS expression and NO production in macrophages . Based on these findings, we proposed a hypothesis that hydrogen may act as a modulator of signaling pathways, thereby exhibiting protective effects against various diseases. Consistent with our hypothesis, it has been recently reported that hydrogen inhibits signaling pathways in animal models of acute liver injury  and amyloid-beta-induced Alzheimer’s disease .
In the present study, in order to understand the underlying mechanisms of hydrogen effects on lipid metabolism disorders and atherosclerosis, we examined if hydrogen could attenuate fatty acid intake and lipid accumulation caused by palmitate overload in human hepatoma HepG2 cells. We then investigated whether hydrogen could modulate signaling pathways after palmitate overload as well as CD36 expression after hydrogen treatment in this cell culture model of hepatic steatosis.