A Dual Role of HGF-ERK Signaling in IL6/HGF-induced Hepatocyte Proliferation

Primary hepatocytes are difficult to expand in vitro. Hepatocyte growth factor (HGF) and epidermal growth factor (EGF) have long been used to maintain primary hepatocytes in vitro but with limited growth promotion effect. Previously, we reported that the combination of interleukin 6 (IL6) with HGF and EGF (or the combination of IL6 with either of these growth factors) could support almost infinite growth of hepatocytes. However, the downstream pathways of these growth factors in hepatocyte proliferation remain elusive.

Transcriptome analysis was carried out to analyze the activation of pathways under various culture condition. Pathway inhibitors were used in cell culture and a hepatectomy mouse model to study the function of different pathways in hepatocyte growth and liver regeneration.

We surprisingly discovered, that under growth condition, the transforming growth factor β (TGFβ) pathway, which is typically linked to growth inhibition, is activated. TGFβ activation is mainly induced by HGF via mitogen-activated protein kinase (MAPK)- extracellular regulated protein kinase (ERK) signaling; blocking the TGFβ pathway significantly enhances H6 (HGF and IL6)-induced hepatocyte expansion in vitro. However, blocking the MAPK-ERK signaling almost completely blocks H6-induced cell growth, indicating a dual effect of the ERK pathway. Further study demonstrates that the HGF-ERK pathway could also upregulate the protein level of YAP and the downstream genes associated with the Hippo-YAP signaling pathway, exerting a positive effect in hepatocyte proliferation. In a mouse partial hepatectomy model, blockade of the activated TGFβ signaling also significantly promotes liver regeneration in vivo.

We report a dual role of HGF-ERK signaling in hepatocyte culture. By blocking the growth-inhibiting TGFβ pathway, we could further promote H6-induced hepatocyte proliferation.