Leukemic HRX fusion proteins inhibit GADD34-induced apoptosis and associate with the GADD34 and hSNF5/INI1 proteins.
One of the most common chromosomal abnormalities in acute leukemia is a reciprocal translocation involving the HRX gene (also called MLL, ALL-1, or HTRX) at chromosomal locus 11q23, resulting in the formation of HRX fusion proteins. Using the yeast two-hybrid system and human cell culture coimmunoprecipitation experiments, we show here that HRX proteins interact directly with the GADD34 protein. We have found that transfected cells overexpressing GADD34 display a significant increase in apoptosis after treatment with ionizing radiation, indicating that GADD34 expression not only correlates with apoptosis but also can enhance apoptosis. The amino-terminal third of the GADD34 protein was necessary for this observed increase in apoptosis. Furthermore, coexpression of three different HRX fusion proteins (HRX-ENL, HRX-AF9, and HRX-ELL) had an anti-apoptotic effect, abrogating GADD34-induced apoptosis. In contrast, expression of wild-type HRX gave rise to an increase in apoptosis. The difference observed here between wild-type HRX and the leukemic HRX fusion proteins suggests that inhibition of GADD34-mediated apoptosis may be important to leukemogenesis. We also show here that GADD34 binds the human SNF5/INI1 protein, a member of the SNF/SWI complex that can remodel chromatin and activate transcription. These studies demonstrate, for the first time, a gain of function for leukemic HRX fusion proteins compared to wild-type protein. We propose that the role of HRX fusion proteins as negative regulators of post-DNA-damage-induced apoptosis is important to leukemia progression.
Adler
H T
HT
VA Puget Sound Health Care System, Seattle, Washington 98108, USA.
Chinery
R
R
Wu
D Y
DY
Kussick
S J
SJ
Payne
J M
JM
Fornace
A J
AJ
Tkachuk
D C
DC
D000943:Antigens, Differentiation; D017209:Apoptosis; D001665:Binding Sites; D018797:Cell Cycle Proteins; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins / Q000378:metabolism*; D005720:Gamma Rays; D015972:Gene Expression Regulation, Neoplastic / Q000528:radiation effects; D011495:Histone-Lysine N-Methyltransferase; D006801:Humans; D007938:Leukemia / Q000378:metabolism*; D009154:Mutation; D051788:Myeloid-Lymphoid Leukemia Protein; D015514:Oncogene Proteins, Fusion / Q000378:metabolism*; D011485:Protein Binding; D054645:Protein Phosphatase 1; D011506:Proteins / Q000378:metabolism*; D011519:Proto-Oncogenes; D000071796:SMARCB1 Protein; D017384:Sequence Deletion; D014157:Transcription Factors; D014407:Tumor Cells, Cultured; D020798:Two-Hybrid System Techniques
D000943:Antigens, Differentiation; D018797:Cell Cycle Proteins; D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; C497422:KMT2A protein, human; D015514:Oncogene Proteins, Fusion; D011506:Proteins; D000071796:SMARCB1 Protein; C513266:SMARCB1 protein, human; D014157:Transcription Factors; D051788:Myeloid-Lymphoid Leukemia Protein; D011495:Histone-Lysine N-Methyltransferase; C087519:PPP1R15A protein, human; D054645:Protein Phosphatase 1