The Role of Embryonic Stem Cell-expressed RAS (ERAS) in the Maintenance of Quiescent Hepatic Stellate Cells

Abstract

Hepatic stellate cells (HSCs) were recently identified as liverresident mesenchymal stem cells. HSCs are activated after liver injury and involved in pivotal processes, such as liver development, immunoregulation, regeneration, and also fibrogenesis. To date, several studies have reported candidate pathways that regulate the plasticity of HSCs during physiological and pathophysiological processes. Here we analyzed the expression changes and activity of the RAS family GTPases and thereby investigated the signaling networks of quiescent HSCs versus activated HSCs. For the first time, we report that embryonic stem cell-expressed RAS (ERAS) is specifically expressed in quiescent HSCs and down-regulated during HSC activation via promoter DNA methylation. Notably, in quiescent HSCs, the high level of ERAS protein correlates with the activation of AKT, STAT3, mTORC2, and HIPPO signaling pathways and inactivation of FOXO1 and YAP. Our data strongly indicate that in quiescent HSCs, ERAS targets AKT via two distinct pathways driven by PI3K␣/␦ and mTORC2, whereas in activated HSCs, RAS signaling shifts to RAF-MEK-ERK. Thus, in contrast to the reported role of ERAS in tumor cells associated with cell proliferation, our findings indicate that ERAS is important to maintain quiescence in HSCs.

The RAS-Effector Interface: Isoform-Specific Differences in the Effector Binding Regions

“…It could be argued that the isolated RB domain of PI3Kα (consisting of the amino acids 169–301) may lack additional binding determinants, when compared to the 50-fold higher affinity obtained with the isolated RB domain of PI3Kγ (amino acids 144–1102) (Tables 1 and 2) [36]. A recent cell-based study has shown that RB domain of PI3Kα (aa 127–314) is sufficient to bind to ERAS, a newly discovered member of the RAS family, but obviously not to HRAS [5,61]. However, the immunoprecipitation studies have revealed the endogenous PI3K isoforms α and γ interact with almost same affinity with both ERAS and HRAS [5].…”

“…A recent cell-based study has shown that RB domain of PI3Kα (aa 127–314) is sufficient to bind to ERAS, a newly discovered member of the RAS family, but obviously not to HRAS [5,61]. However, the immunoprecipitation studies have revealed the endogenous PI3K isoforms α and γ interact with almost same affinity with both ERAS and HRAS [5]. These data suggest that RB domain of PI3K is sufficient for a tight interaction with ERAS but clearly requires additional capacity to properly associate with HRAS.…”

“…Constructs for the expression of human HRAS, KRAS, NRAS, RRAS1 and RRAS2 isoforms were described previously [5]. …”

“…Signal transduction implies physical association of these proteins with a spectrum of functionally diverse downstream effectors, e . g ., CRAF, PI3Kα, RALGDS, PLCε and RASSF5, and their activation [1,4,5,6,7,8,9,10]. CRAF, a serine/threonine kinase, activates the MEK-ERK axis and controls gene expression and cell proliferation [11].…”

Research progress on therapeutic targeting of quiescent cancer cells

“…Previous studies have identified embryonic stem cell-expressed RAS (ERAS) specifically expressed in quiescence hepatic stellate cells (HSCs) and significantly downregulated during HSC activation owing to an increase in promotor DNA methylation. ERAS targets AKT through two different pathways driven by PI3K and mTORC2 in G0 cells whereas in activated HSCs, RAS signaling is transferred to RAF-MEK-ERK [25]. ERAS is thus responsible for the maintenance of quiescence in HSCs.…”