Roles of Estrogen Signaling in Vertebrate Liver

Abstract

Estrogen (E2) signaling has been implicated in many biological processes, particularly in reproductive biology, organ development and carcinogenesis. E2 plays crucial roles in adult reproductive organ growth and size determination; however, the impact of E2 on non-reproductive organs such as liver is not well understood. In this dissertation work, I have investigated the roles of E2 signaling throughout embryonic liver development and adult liver homeostasis. I demonstrate in Chapter 2 that early activation of E2 signaling during hepatic formation in the zebrafish embryo from 24 hour post fertilization (hpf) progressively decreased liver size, with the most severe impact on hepatic differentiation. These E2 effects are mediated by estrogen receptor 2b (esr2b) as specifically generated esr2b knockout embryos blocked estrogenic effects and had increased liver size. Importantly, while pharmacological blockade of estrogen receptors increased liver size, it negatively impacted cholangiocyte formation, suggesting the potential role of E2 signaling in determining hepatic lineages: promoting biliary tree while suppressing hepatocyte formation. I describe further in Chapter 3 that activation of E2 signaling after hepatic differentiation increased liver size. This increase in liver size induced by E2 exposure was blocked in our generated G protein-coupled estrogen receptor 1 (gper1) knockout embryos, demonstrating that E2 signals via gper1 to drive liver growth. Intriguingly, impacts of E2 on liver size were observed after a brief 5 hour E2 exposure, suggesting that E2 activates downstream rapid signaling. Epistasis analysis revealed that E2 signals via gper1 to activate Akt/mTOR signaling to promote liver growth. Indeed, hypomorphic mtor mutant embryos blocked estrogenic effects on liver size. E2/GPER1 signaling further promoted liver regeneration in a hepatocyte-targeted ablation model. Importantly, E2 accelerated, whereas loss of GPER1 in gper1 knockout or in GPER1 inhibitor-treated adults significantly reduced, liver cancer incidence and progression, specifically in males. Finally, E2 increased proliferation of primary human hepatocytes and GPER1 expression increased in human liver tissues with cirrhosis and hepatocellular carcinomas, suggesting its positive role during liver carcinogenesis. Taken together, I have provided substantially novel mechanistic insight into the roles of E2 signaling during normal liver organogenesis, regeneration as well as liver cancer.