Hepatic Transthyretin knockdown alleviates NAFLD by enhancing SERCA2 function and inhibiting endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is considered a key trigger in the nonalcoholic fatty liver disease (NAFLD). Dysfunction of sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2) leads to insufficient ER Ca2+ storage, thus activating ER stress. Transthyretin (TTR), expressed and secreted by the liver, may function as a new "hepatokine" related with insulin resistance (IR). However, the role of TTR in NAFLD remains unclear. We firstly found the hepatic Ttr mRNA expression was elevated in NAFLD patients. TTR over-expression exacerbated steatosis in fatty livers, while TTR knockdown alleviated IR, steatosis, inflammation, and upregulated SERCA2 expression, leading to reduced ER stress in NAFLD mice. Consistently, TTR knockdown alleviated lipid deposition in steatotic hepatocytes. TTR stimulation of hepatocytes increased cytosolic Ca{superscript 2} and decreased ER Ca{superscript 2}. Conversely, in steatotic AML12 cells, TTR knockdown increased ER calcium storage and mitigated the cytosolic calcium pulse induced by thapsigargin. Then using immunofluorescence and co-immunoprecipitation, we further confirmed the interaction between TTR and SERCA2. In conclusion, TTR knockdown ameliorated liver steatosis and inflammation in NAFLD, which might be related to the improvement of cellular calcium homeostasis and ER stress by reversing SERCA2 function.