The Functions And Mechanisms Of YAP In Hepatocellular Carcinoma Progression In Hypoxic Microenvironment

Hepatocellular carcinoma is a common malignant tumor of the digestive system.Although the treatment methods are diverse,the overall prognosis is still poor.Studies have confirmed that hypoxic microenvironment is closely related to the progression and poor prognosis of liver cancer,however,the regulatory mechanisms have not been fully studied.Hypoxia can promote tumor cell apoptosis resistance,invasion,metastasis and so on.At the same time,HIF-1α is a key regulator in hypoxic condition.YAP is a critical effector downstream of the Hippo pathway.Studies have shown that YAP is activated in the hypoxic microenviront of breast cancer,but how that YAP regulates the function of liver cancer cells under hypoxic condition is not well understood.First,we found that human liver cancer specimens not only highly expressed YAP but their paralogous TAZ,and there was expression imbalance between them by immunoblotting assays.After knockdown,overexpression or administration of Verteporfin,we found that both YAP and TAZ knockdown or degradation which could accelerate apoptosis of liver cancer cells under hypoxic condition through flow cytometry and immunoblotting.Besides,we carried out tumor xenografts imaging in vivo and following IHC,by which we confirmed the above hypothesis.Second,studies have shown that liver cancer with high YAP expression and activation is more malignant.We have found that hypoxia can promote YAP nuclear translocation,activation and colocalization with HIF-1α by immunofluorescence experiments.Through IHC of liver cancer tissues,we further confirmed that YAP and HIF-1α colocalize in the nucleus.Using endogenous and exogenous coimmunoprecipitation,we found that activated YAP can interact with HIF-1α.Again,utilizing YAP overexpression and knockdown stable celllines,we found that under hypoxic condition YAP can promote liver cancer cells migration and invasion by Transwell assays,orthotopic tumor xenografts and tail vein injections of cancer cells.Finally,we found that YAP can stabilize HIF-1α and slow down the rate of proteasomal degradation by immunoblotting,CHX assays and so on.Making use of stable celllines with HIF-1α knockdown or overexpression,we discovered that HIF-1α can promote EMT of liver cancer cells under hypoxic condition through immunoblotting experiments.In the present study,we confirmed that YAP and its paralogous TAZ regulate apoptosis of liver cancer cells in hypoxic microenvironment.On the other hand,we proved that hypoxia promotes nucleus translocation of YAP that stabilizes HIF-1α by slowing proteasomal degradation.And then HIF-1α mediates EMT which promotes migration and invasion of liver cancer cells.