Upstream Regulators and Downstream Signals of YAP/TAZ in NAFLD
Abstract
Non-alcoholic fatty liver disease (NAFLD) affects 25% of the world population underpinning the urgent need to understand molecular pathways and develop effective therapeutic interventions. Recently, yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) have emerged as two important transcriptional regulators implicated in liver fibrosis and injury. Despite the growing recognition of YAP/TAZ in several liver diseases, YAP/TAZ expression has not been extensively characterized in hepatic cells, and the upstream regulators of this pathway in the setting of liver injury remain poorly understood. Here, we examined the expression pattern of YAP/TAZ in NAFLD patient samples and demonstrated that PAR receptors could be regulators of the YAP/TAZ pathway.
Archival human sections of normal liver and NAFLD including cases of simple steatosis, steatohepatitis, steatohepatitis with various stages of fibrosis and cirrhosis were analyzed using immunostaining and the activation status of YAP/TAZ and PAR receptors was monitored through assessing subcellular localization. The expression pattern of YAP in various hepatic cell types was characterized by colocalization with clinically validated cellular markers. An in vitro reporter assay was used to directly assess YAP/TAZ activation by PAR1 and PAR2 agonists.
Confocal imaging combined with standardized image analysis revealed increased nuclear YAP expression in Kupffer cells, myofibroblasts and cholangiocytes as NAFLD progressed from steatosis to fibrosis and cirrhosis. YAP/TAZ co-activators could drive inflammation in myofibroblasts and Kupffer cells and promote bile duct regeneration in cholangiocytes. PAR receptors showed different expression patterns in diseased livers indicative of receptor upregulation and activation. In complimentary in vitro studies, specific PAR1 and PAR2 agonists increased YAP/TAZ nuclear activity providing further evidence that PAR receptors can directly regulate this pathway. Immunostaining analysis and in vitro assays indicated that PARs are disease relevant regulators of YAP/TAZ in the liver and may be targeted to modulate YAP/TAZ activation. Together, our findings can help guide the development of novel therapeutic agents for treating NAFLD.