Dichotomy of AML1-ETO Functions: Growth Arrest versus Block of Differentiation

Abstract

The fusion gene AML1-ETO is the product of t(8;21)(q22;q22), one of the most common chromosomal translocations associated with acute myeloid leukemia. To investigate the impact of AML1-ETO on hematopoiesis, tetracycline-inducible AML1-ETO-expressing cell lines were generated using myeloid cells. AML1-ETO is tightly and strongly induced upon tetracycline withdrawal. The proliferation of AML1-ETO⁺ cells was markedly reduced, and most of the cells eventually underwent apoptosis. RNase protection assays revealed that the amount of Bcl-2 mRNA was decreased after AML1-ETO induction. Enforced expression of Bcl-2 was able to significantly delay, but not completely overcome, AML1-ETO-induced apoptosis. Prior to the onset of apoptosis, we also studied the ability of AML1-ETO to modulate differentiation. AML1-ETO expression altered granulocytic differentiation of U937T-A/E cells. More significantly, this change of differentiation was associated with the down-regulation of CCAAT/enhancer binding protein α (C/EBPα), a key regulator of granulocytic differentiation. These observations suggest a dichotomy in the functions of AML1-ETO: (i) reduction of granulocytic differentiation correlated with decreased expression ofC/EBPα and (ii) growth arrest leading to apoptosis with decreased expression of CDK4, c-myc, andBcl-2. We predict that the preleukemic AML1-ETO⁺ cells must overcome AML1-ETO-induced growth arrest and apoptosis prior to fulfilling their leukemogenic potential.

ACKNOWLEDGMENTS

We thank John Reed, Stephen Nimer, Nancy Speck, Beatrice Muller, and Alan Friedman for valuable discussion and Gerald Grosveld, Scott Hiebert, Warren Pear, Gary Nolan, Jonathan Licht, Stanley Korsmeyer, Philip Koeffler, Kimiko Shimizu, Scott Kogan, and John Schuetz for DNA constructs, cell lines, and antibodies. We thank Ernest Beutler and Kenneth Ritchie for editing the manuscript.

This work was supported by National Institutes of Health grant CA72009 and American Cancer Society grant LBC-99438. D.E.Z. is a Leukemia and Lymphoma Society Scholar. S.A.B. was supported by a fellowship from the Lady Tata Memorial Trust. The Stein Endowment Fund partially supported the Departmental Molecular Biology Service Laboratory for DNA Sequencing and Oligonucleotide Synthesis.