Gli1 Activation and Protection Against Hepatic Encephalopathy is Suppressed by Circulating Transforming Growth Factor β1 in Mice

Background & Aims: Hepatic encephalopathy (HE) is a neurologic disorder that develops during liver failure. Few studies exist investigating systemic-central signaling during HE outside of inflammatory signaling. The transcription factor Gli1, which can be modulated by hedgehog signaling or transforming growth factor β1 (TGFβ1) signaling, has been shown to be protective in various neuropathies. We measured Gli1 expression in brain tissues from mice and evaluated how circulating TGFβ1 and canonical hedgehog signaling regulate its activation. Methods: Mice were injected with azoxymethane (AOM) to induce liver failure and HE in the presence of Gli1 Vivo-morpholinos (to mediate knockdown), the hedgehog inhibitor cyclopamine, smoothened Vivo-morpholinos, a smoothened agonist, or TGFβ-neutralizing antibodies. Molecular analyses were used to assess Gli1, hedgehog signaling, and TGFβ1 signaling in the liver and brain of AOM mice and HE patients. Results: Gli1 expression was increased in brains of AOM mice and in HE patients. Intra-cortical infusion of Gli1 Vivo-morpholinos exacerbated the neurologic deficits of AOM mice. Measures to modulate hedgehog signaling had no effect on HE neurological decline. Levels of TGFβ1 increased in the liver and serum of mice following AOM administration. TGFβ neutralizing antibodies slowed neurologic decline following AOM administration without significantly affecting liver damage. TGFβ1 inhibited Gli1 expression via a SMAD3-dependent mechanism. Conversely, inhibiting TGFβ1 increased Gli1 expression. Conclusions: Cortical activation of Gli1 protects mice from induction of HE. TGFβ1 suppresses Gli1 in neurons via SMAD3 and promotes neurologic decline. Strategies to activate Gli1 or inhibit TGFβ1 signaling might be developed to treat patients with HE.