Abstract
Activating transcription factor 6 (ATF6) is one of three principle endoplasmic reticulum (ER) stress response proteins and becomes activated when ER homeostasis is perturbed. ATF6 functions to increase ER capacity by stimulating transcription of ER-resident chaperone genes such as GRP78. Using an antibody that recognizes active ATF6α-p50, we found that active ATF6α was detected in insulinoma cells and rodent islets even under basal conditions and the levels were further increased by ER stress. To examine the function of ATF6α-p50, we depleted endogenous ATF6α-p50 levels using small interfering RNA in insulinoma cells. Knockdown of endogenous ATF6α-p50 levels by ∼60% resulted in a reduction in the steady-state levels of GRP78 mRNA and protein levels in nonstressed cells. Furthermore, ATF6α knockdown resulted in an apoptotic phenotype. We hypothesized that removal of the ATF6α branch of the unfolded protein response (UPR) would result in ER stress. However, neither the PKR-like endoplasmic reticulum kinase (PERK), nor the inositol requiring enzyme 1 (IRE1) pathways of the UPR were significantly activated in ATF6α knockdown cells, although these cells were more sensitive to ER stress-inducing compounds. Interestingly, phosphorylation of JNK, p38, and c-Jun were elevated in ATF6α knockdown cells and inhibition of JNK or p38 kinases prevented apoptosis. These results suggest that ATF6α may have a role in maintaining β-cell survival even in the absence of ER stress.
Publication types
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Research Support, Non-U.S. Gov't
MeSH terms
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Activating Transcription Factor 6 / genetics*
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Activating Transcription Factor 6 / metabolism*
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Animals
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Apoptosis / physiology
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Cell Line, Tumor
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Cell Survival / physiology
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Cells, Cultured
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Endoplasmic Reticulum / genetics
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Endoplasmic Reticulum / metabolism
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Endoplasmic Reticulum Chaperone BiP
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Gene Knockdown Techniques
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HeLa Cells
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Heat-Shock Proteins / genetics
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Heat-Shock Proteins / metabolism
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Humans
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Insulin-Secreting Cells / cytology*
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Insulin-Secreting Cells / metabolism*
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Insulinoma / genetics
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Insulinoma / metabolism
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Islets of Langerhans / metabolism
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MAP Kinase Kinase 4 / genetics
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MAP Kinase Kinase 4 / metabolism
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Membrane Proteins / genetics
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Membrane Proteins / metabolism
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Mice
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Phosphorylation
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Protein Serine-Threonine Kinases / genetics
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Protein Serine-Threonine Kinases / metabolism
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Proto-Oncogene Proteins c-jun / genetics
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Proto-Oncogene Proteins c-jun / metabolism
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RNA, Messenger / genetics
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Rats
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Signal Transduction
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Unfolded Protein Response
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eIF-2 Kinase / genetics
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eIF-2 Kinase / metabolism
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p38 Mitogen-Activated Protein Kinases / genetics
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p38 Mitogen-Activated Protein Kinases / metabolism
Substances
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Activating Transcription Factor 6
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Atf6 protein, mouse
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Endoplasmic Reticulum Chaperone BiP
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HSPA5 protein, human
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Heat-Shock Proteins
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Hspa5 protein, mouse
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Membrane Proteins
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Proto-Oncogene Proteins c-jun
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RNA, Messenger
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Ern2 protein, mouse
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Ern2 protein, rat
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PERK kinase
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Protein Serine-Threonine Kinases
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eIF-2 Kinase
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p38 Mitogen-Activated Protein Kinases
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MAP Kinase Kinase 4