Signaling dynamics of palmitate-induced ER stress responses mediated by ATF4 in HepG2 cells

The three key things from the in silico simulation and experimental results are: 1) palmitate induces different signaling pathways (PKR (double-stranded RNA-activated protein kinase), PERK (PKR-like ER kinase), PKA (cyclic AMP (cAMP)-dependent protein kinase A) in a time dependent-manner, 2) both ATF4 and CREB1 (cAMP-responsive element-binding protein 1) interact with the Atf4 promoter to contribute to a prolonged accumulation of ATF4, and 3) CREB1 is involved in ER-stress induced apoptosis upon palmitate treatment, by regulating ATF4 expression and possibly Ca2+ dependent-CaM (calmodulin) signaling pathway.The in silico model helped to delineate the essential signaling pathways in palmitate-mediated apoptosis.Elevated serum free fatty acids (FFA) and hepatic lipid accumulation in non-adipose tissues can lead to cellular dysfunction or cell death, due in part to the diversion of unoxidized FFAs to nonoxidative pathways, resulting in lipoapoptosis [1]. Excess amounts of nonesterified FFAs that fail to convert to triglyceride in liver cells, enhance the risk for hepatocellular lipoapoptosis, a pathogenic feature observed in non-alcoholic steatohepatitis (NASH) [2]. The mechanisms involved in FFA-induced toxicity have remained unresolved, however recent studies suggest that hepatic lipoapoptosis arises predominantly from FFA-induced lipotoxic stress of intracellular organelles, in particular the endoplasmic reticulum (ER) and mitochondria [3].The ER is one of the largest organelles in the cells, and perturbing ER homeostasis or inducing ER stress has profound effects on cell survival. Cellular perturbations, such as alterations in calcium storage in the ER lumen or an imbalance in the luminal-oxidizing environment will cause ER stress. This stress is sensed by the cells through three ER transmembrane proteins, inositol requiring enzyme (IRE) 1α, PKR-like ER kinase (PERK), and activating transcription factor (ATF) 6α [4]. They activate signaling processes to restore ER