Interference with the HNF4-dependent gene regulatory network diminishes ER stress in hepatocytes
In all eukaryotic cell types, the unfolded protein response (UPR) upregulates factors that promote protein folding and misfolded protein clearance to help alleviate endoplasmic reticulum (ER) stress. Yet ER stress in the liver is uniquely accompanied by the suppression of metabolic genes, the coordination and purpose of which is largely unknown. Here, we combined in silico machine learning, in vivo liver-specific deletion of the master regulator of hepatocyte differentiation HNF4a, and in vitro manipulation of hepatocyte differentiation state to determine how the UPR regulates hepatocyte identity, and toward what end. Machine learning identified a cluster of correlated genes that were profoundly suppressed by persistent ER stress in the liver. These genes, which encode diverse functions including metabolism, coagulation, drug detoxification, and bile synthesis, are likely targets of the master regulator of hepatocyte differentiation HNF4a. The response of these genes to ER stress was phenocopied by liver-specific deletion of HNF4a. Strikingly, while deletion of HNF4a exacerbated liver injury in response to an ER stress challenge, it also diminished UPR activation and partially preserved ER ultrastructure, suggesting attenuated ER stress. Conversely, pharmacological maintenance of hepatocyte identity in vitro enhanced sensitivity to stress. Together, our findings suggest that the UPR regulates hepatocyte identity through HNF4a to protect ER homeostasis even at the expense of liver function. Overall design: Hnf4a floxed (fl/fl) male mice of variable age (8 - 12 weeks old) were injected with AAV-TBG-iCre or AAV-TBG-eGFP (2.5 x 10^11 GC/mice). A week later, Hnf4a -/- or fl/fl mice were injected intraperitoneally with 0.5 mg/kg tunicamycin (TM) or vehicle control (DMSO). Each experimental group has 3-4 mice. 14h post tunicamycin injection, mice were sacrificed, and total RNA was prepared from resected livers.