Endoplasmic reticulum (ER) stress activates the unfolded protein response (UPR), a highly conserved signaling cascade that
functions to alleviate stress and promote cell survival. If, however, the cell is unable to adapt and restore homeostasis,
the UPR activates pathways that promote apoptotic cell death. The molecular mechanisms governing the critical transition from
adaptation and survival to initiation of apoptosis remain poorly understood. We aim to determine the role of hepatic Xbp1,
a key mediator of the UPR, in controlling the adaptive response to ER stress in the liver. Liver-specific Xbp1 knockout mice
(Xbp1LKO) and Xbp1fl/fl control mice were subjected to varying levels and duration of pharmacologic ER stress. Xbp1LKO and
Xbp1fl/fl mice showed robust and equal activation of the UPR acutely after induction of ER stress. By 24 hours, Xbp1fl/fl
controls showed complete resolution of UPR activation and no liver injury indicating successful adaptation to the stress.
Conversely Xbp1LKO mice showed ongoing UPR activation associated with progressive liver injury, apoptosis, and ultimately
fibrosis by day 7 after induction of ER stress. These data indicate that hepatic XBP1 controls the adaptive response of the
UPR and is critical to restoring homeostasis in the liver in response to ER stress.