The ER-α36/EGFR signaling loop promotes growth of hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is the common primary liver cancer and the third leading cause of cancer related mortality worldwide. It is generally thought that the estrogen-signaling pathway is not related to the development and progression of human HCC. However, accumulating evidences indicate the existence of a rapid estrogen signaling in HCC cells that is able to promote cell growth. However, the receptor that mediates the rapid estrogen signaling in HCC cells has not been established. Previously, our laboratory identified a variant of ER-α, ER-α36, and found that ER-α36 mediates the rapid estrogen signaling such as the activation of the MAPK/ERK signaling in breast carcinoma cells. Our current experiments studied the role of the rapid estrogen signaling mediated by ER-α36 in growth of HCC HepG2 and PLC/PRF/5 cells that highly express ER-α36 and found these cells were strongly responsive to the rapid estrogen signaling. Knockdown of ER-α36 expression in these HCC cells using the shRNA method attenuated their responsiveness to estrogen and destabilized EGFR protein. ER-α36 mediated estrogen-induced phosphorylation of Src and the MAPK/ERK as well as cyclin D1 expression. In addition, there existed an ER-α36/EGFR positive regulatory loop in HCC cells that was important for the maintenance and positive regulation of HCC tumorsphere cells. Our results thus indicated that the rapid estrogen receptor is mediated by ER-α36 in HCC cells through the EGFR/Src/ERK signaling pathway and suggested that the ER-α36/EGFR signaling loop is a potential target to develop novel therapeutic approaches for HCC treatment.