More important, our study also shows that HuR regulates HSC activation, which likely results in the reduced fibrosis observed in vivo after HuR silencing. HSC activation is highly regulated, with hundreds of genes up- and down-regulated.5 Modulation of mRNA stability and translation rates plays an important role in the regulation of gene expression during liver fibrosis development and hepatic stellate activation.1 Here, we show that HSC activation in vitro and in vivo after BDL is accompanied by an increase in HuR. HuR silencing significantly reduces the expression of HSC activation markers. Importantly, we observed that HuR mediates the response of two of the principal mediators of HSC activation (PDGF and TGF-β).30,

31 These data, together with the finding that HSC from human samples of hepatic cirrhosis expressed HuR, suggest Pexidartinib cost that HuR has a significant role in fibrosis development after liver injury by controlling HSC activation itself, in addition to see more liver damage and inflammation. HuR regulates PDGF-induced proliferation and migration, controlling the expression of several genes involved in these processes. PDGF binding to its receptor leads to the sequential activation of RAF photo-oncogene serine/threonine-protein kinase, MEK, and ERK1/2. ERK

signaling is involved in PDGF-stimulated mitogenesis, migration, and chemotaxis. PI3K also mediates PDGF-induced proliferation, migration, and chemotaxis, at least in part, through ERK-independent pathways.30 Here, we demonstrated that ERK1/2, but not PI3K, regulates the cytoplasmic translocation of HuR. PDGF also induces LKB1 (Ser428) phosphorylation through ERK activation.22 LKB1 has been classically described as a tumor suppressor,32 but seems to have the Idoxuridine opposite role in the liver, controlling HuR nucleocytoplasmic shuttling and proliferation in HGF-stimulated hepatocytes and during apoptosis in hepatoma cell lines.8, 9 Here, we also identified LKB1 as a downstream target of ERK1/2 in PDGF-stimulated HSCs, and silencing LKB1 significantly reduced PDGF-induced migration and proliferation. These functions of LKB1 are possibly mediated by HuR activity,

because LKB1 regulates the nucleocytoplasmic shuttling of HuR and both regulate the expression of a common set of mRNAs. It is known that LKB1 phosphorylates and regulates AMPK; however, we observed that PDGF-induced HuR cytosolic localization was independent of AMPK activity. This observation is in agreement with previous work describing that AMPK exerts antiproliferative properties in HSCs,23, 24 as well as with studies in melanoma cells, which show that LKB1 can be active without affecting AMPK activity.22 Previous studies have shown that PI3K and ERK are activated in HSCs in vivo after liver injury.33, 34 Here, we found that, similarly, LKB1 (Ser428) phosphorylation is also expressed in vivo in activated HSCs in two animal models of hepatic fibrosis (i.e.