Unfolded protein response suppresses CEBPA by induction of calreticulin in acute myeloid leukaemia.
The unfolded protein response (UPR) is triggered by the accumulation of misfolded proteins within the endoplasmic reticulum (ER). The role of the UPR during leukemogenesis is unknown so far. Here, we studied the induction of mediators of the UPR in leukaemic cells of AML patients. Increased expression of the spliced variant of the X-box binding protein 1 (XBP1s) was detected in 17.4% (16 of 92) of AML patients. Consistent with activated UPR, this group also had increased expression of ER-resident chaperones such as the 78 kD glucose-regulated protein (GRP78) and of calreticulin. Conditional expression of calreticulin in leukaemic U937 cells was found to increase calreticulin binding to the CEBPA mRNA thereby efficiently blocking translation of the myeloid key transcription factor CEBPA and ultimately affecting myeloid differentiation. Consequently, leukaemic cells from AML patients with activated UPR and thus increased calreticulin levels showed in fact suppressed CEBPA protein expression. We identified two functional ER stress response elements (ERSE) in the calreticulin promoter. The presence of NFY and ATF6, as well as an intact binding site for YY1 within these ERSE motifs were essential for mediating sensitivity to ER stress and activation of calreticulin. Thus, we propose a model of the UPR being activated in a considerable subset of AML patients through induction of calreticulin along the ATF6 pathway, thereby ultimately suppressing CEBPA translation and contributing to the block in myeloid differentiation.
Schardt
Julian A
JA
Department of Medical Oncology, University Hospital Bern and University of Bern, Switzerland.
Eyholzer
Marianne
M
Timchenko
Nikolai A
NA
Mueller
Beatrice U
BU
Pabst
Thomas
T
D051702:Activating Transcription Factor 6 / Q000378:metabolism; D017398:Alternative Splicing / Q000235:genetics; D023081:CCAAT-Binding Factor / Q000378:metabolism; D022762:CCAAT-Enhancer-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D037282:Calreticulin / Q000235:genetics / Q000378:metabolism*; D045744:Cell Line, Tumor; D004268:DNA-Binding Proteins / Q000235:genetics; D004721:Endoplasmic Reticulum / Q000473:pathology; D000091342:Endoplasmic Reticulum Chaperone BiP; D015973:Gene Expression Regulation, Leukemic; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism; D006801:Humans; D015470:Leukemia, Myeloid, Acute / Q000235:genetics / Q000378:metabolism*; D022423:Myeloid Cells / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D014176:Protein Biosynthesis; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors / Q000235:genetics; D014158:Transcription, Genetic; D015533:Transcriptional Activation / Q000235:genetics; D056811:Unfolded Protein Response; D000071717:X-Box Binding Protein 1; D050986:YY1 Transcription Factor / Q000378:metabolism
D051702:Activating Transcription Factor 6; D023081:CCAAT-Binding Factor; D022762:CCAAT-Enhancer-Binding Proteins; C515848:CEBPA protein, human; D037282:Calreticulin; D004268:DNA-Binding Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; C000717247:HSPA5 protein, human; D006360:Heat-Shock Proteins; C492061:NFYA protein, human; D000071701:Regulatory Factor X Transcription Factors; D014157:Transcription Factors; D000071717:X-Box Binding Protein 1; C000605543:XBP1 protein, human; D050986:YY1 Transcription Factor