Fatty acid-induced lipotoxicity inhibits choline metabolism independent of ER stress in mouse primary hepatocytes

ABSTRACT

Choline is an essential nutrient that is critical component of the membrane phospholipid phosphatidylcholine (PC), the neurotransmitter acetylcholine and the methylation pathway. In the liver specifically, PC is the major membrane constituent and can be synthesized by the CDP-choline or the phosphatidylethanolamine (PE) N-methyltransferase (PEMT) pathway. With the continuing global rise in the rates of obesity and non-alcoholic fatty liver disease, we sought to explore how excess fatty acids (FA), typical of an obesity and hepatic steatosis, affect choline uptake and metabolism in primary hepatocytes. Our results demonstrate that hepatocytes chronically treated with palmitate, but not oleate or a mixture, had decreased choline uptake, which was associated with lower choline incorporation into PC and lower expression of choline transport proteins. Interestingly, a reduction in the rate of degradation spared PC levels in response to palmitate when compared to control. PE synthesis was slightly diminished; however, no compensatory changes in the PEMT pathway were observed. We next hypothesized that ER stress may be a potential mechanism by which palmitate treatment diminished choline. However, when we exposed primary hepatocytes to the common ER stress inducing compound tunicamycin, choline uptake, contrary to our expectation was augmented, concomitant with the transcript expression of choline transporters. Moreover, tunicamycin-induced ER stress divorced the observed increase in choline uptake from CDP-choline pathway flux since ER stress significantly diminished the incorporation and total PC content, similar to PE. Conclusion: Therefore, our results suggest that the altered FA milieu seen in obesity and fatty liver disease progression may adversely affect choline metabolism, but that compensatory mechanisms work to maintain phospholipid homeostasis.

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