RESUMEN
The metastasis suppressor, N-myc downstream regulated gene-1 (NDRG1), is a stress response protein that is involved in the inhibition of multiple oncogenic signaling pathways. Initial studies have linked NDRG1 and the endoplasmic reticulum (ER) stress response. Considering this, we extensively examined the mechanism by which NDRG1 regulates the ER stress response in pancreatic and colon cancer cells. We also examined the anti-cancer agent, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT), which induces NDRG1 expression and causes ER stress. The expression of NDRG1 was demonstrated to regulate the three main arms of the ER stress response by: (1) increasing the expression of three major ER chaperones, binding immunoglobulin protein (BiP), calreticulin, and calnexin; (2) suppressing the protein kinase, RNA-activated (PKR)-like ER kinase (PERK); (3) inhibiting the inositol-requiring kinase 1α (IRE1α) arm; and (4) increasing the cleavage of activating transcription factor 6 (ATF6). An important finding was that NDRG1 enhances the anti-proliferative and anti-migratory activity of Dp44mT. This increased efficacy could be related to the following effects in the presence of Dp44mT and NDRG1, namely: markedly increased activation of the PERK target, eukaryotic translation initiation factor 2α (eIF2α); the maintenance of activating transcription factor 4 (ATF4) expression; high cytosolic Ca+2 that increases the sensitivity of cells to apoptosis via activation of the calmodulin-dependent kinase II (CaMKII) signaling cascade; and increased pro-apoptotic C/EBP-homologous protein (CHOP) expression. Collectively, this investigation dissects the molecular mechanisms through which NDRG1 manipulates the ER stress response and its ability to potentiate the activity of the potent anti-cancer agent, Dp44mT.