34 Several strains check details that developed the most pronounced liver injury, C57BL/10J and NZW/LacJ, also exhibited increased levels of Grp78 and Chop and an increase in Chop transcript. Notably, these ER stress markers were not induced consistently in other strains with high liver injury, which suggests that ER stress may not be a requisite event in alcoholic liver disease. Alternatively, it is also likely that selective persistence of ER chaperone and CHOP expression is evidence of failure to adapt to chronic unfolded protein response,42 thus serving as a prodeath factor that exacerbates liver injury caused by alcohol.
ER stress has also been implicated as one of the regulatory mechanisms in hepatocyte lipid metabolism.28 A key interconnectedness between hepatic steatosis and ER stress, including the physiological role of the ER stress protein find more sensors in lipid homeostasis, has been demonstrated in several recent publications.43 In this study we observed an unexpected down-regulation of Srebf1 and lack of induction of Cebpa in strains with high liver injury and liver steatosis. In prior work, up-regulation of SREBP1 and lipogenesis
has been observed, albeit in a mouse strain not studied here. The difference may be related to the severity of ER stress or other unknown factors. A down-regulation of transcription factors involved in lipid synthesis has also been suggested as a sign of failure to adapt to chronic ER stress. For example, steatosis develops in the liver of tunicamycin-treated mice and is associated with unresolved ER stress, prolonged up-regulation of Chop, and inhibition of metabolic master regulators.28 In addition, silencing of SREBP1 in vitro has led to dramatic loss of cell viability by way of induction of apoptosis.44 Most recent studies demonstrated that the decreased SAM/SAH ratio as a consequence of hyperhomocysteinemia appears to have a key role, as it can affect the ratio of phosphatidylcholine to phosphatidylethanolamine in ER membrane that could either lead to increased processing of SREBP145 or ER stress response.46 In Caenorhabditis elegans, decreased
SAM/SAH leads to decreased phosphatidylcholine/phosphatidylethanolamine ratio in ER, resulting in transcription-independent activation of SREBP1 and induction MCE of lipogenesis and one-carbon metabolism.45 However, the latter compensatory attempt to correct SAM/SAH may be impaired by the effects of alcohol. Although the precise mechanism of alcohol-induced effects on one-carbon metabolism remain to be determined and additional studies are needed to further investigate the differences in the role of ER stress in apoptosis and steatohepatitis among susceptible and resistant strains, our data clearly point to the genetic factors that may control adaptation to ER stress as one of the key events in the predisposition to alcoholic liver disease.