Abstract
Endoplasmic reticulum (ER) stress activates three principal signaling pathways, collectively known as the unfolded protein response, leading to translational and transcriptional control mechanisms that dictate the cell's response as adaptive or apoptotic. The present study illustrates that for HepG2 human hepatocellular carcinoma cells the signaling pathways triggered by ER stress extend beyond the three principal pathways to include mitogen-activated protein kinase (MAPK) signaling, leading to activation of transcription from the early growth response 1 (EGR1) gene. Analysis provided evidence for a SRC-RAS-RAF-MEK-ERK cascade mechanism that leads to enhanced phosphorylation of the transcription factor ELK1. ELK1 and serum response factor (SRF) are constitutively bound to the EGR1 promoter and are phosphorylated by nuclear localized ERK. The promoter abundance of both phospho-SRF and phopsho-ELK1 was increased by ER stress, but the SRF phosphorylation was transient. Knockdown of ELK1 had little effect on the basal EGR1 mRNA content, but completely blocked the increase in response to ER stress. Conversely, knockdown of SRF suppressed basal EGR1 mRNA content, but had only a small effect on the induction by ER stress. This research highlights the importance of MAPK signaling in response to ER stress and identifies ELK1 as a transcriptional mediator and the EGR1 gene as a target.