ABSTRACT
We utilized a mouse model of acute promyelocytic leukemia (APL) to investigate how aberrant activation of cytokine signaling pathways interacts with chimeric transcription factors to generate acute myeloid leukemia. Expression in mice of the APL-associated fusion, PML-RARA, initially has only modest effects on myelopoiesis. Whereas treatment of control animals with interleukin-3 (IL-3) resulted in expanded myelopoiesis without a block in differentiation, PML-RARA abrogated differentiation that normally characterizes the response to IL-3. Retroviral transduction of bone marrow with an IL-3-expressing retrovirus revealed that IL-3 and promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) combined to generate a lethal leukemia-like syndrome in <21 days. We also observed that a constitutively activated mutant IL-3 receptor, beta(c)V449E, cooperated with PML-RARalpha in leukemogenesis, whereas a different activated mutant, beta(c)I374N, did not. Analysis of additional mutations introduced into beta(c)V449E showed that, although tyrosine phosphorylation of beta(c) is necessary for cooperation, the Src homology 2 domain-containing transforming protein binding site is dispensable. Our results indicate that chimeric transcription factors can block the differentiative effects of growth factors. This combination can be potently leukemogenic, but the particular manner in which these types of mutations interact determines the ability of such combinations to generate acute myeloid leukemia.
Subject(s)
Cytokines/metabolism , Leukemia, Myeloid, Acute/metabolism , Neoplasm Proteins/metabolism , Oncogene Proteins, Fusion/metabolism , Signal Transduction , Animals , Bone Marrow Cells/metabolism , Cell Differentiation , Flow Cytometry , Immunophenotyping , Interleukin-3/metabolism , Karyotyping , Leukemia, Myeloid, Acute/etiology , Mice , Mutation , Neoplasm Proteins/genetics , Oncogene Proteins, Fusion/genetics , Phosphorylation , Receptors, Interleukin-3/metabolism , Retroviridae/genetics , Spleen/cytology , Time Factors , Tyrosine/metabolism , src Homology DomainsABSTRACT
CCAAT/enhancer binding proteins (C/EBPs) are a family of factors that regulate cell growth and differentiation. These factors, particularly C/EBPalpha and C/EBPepsilon, have important roles in normal myelopoiesis. In addition, loss of C/EBP activity appears to have a role in the pathogenesis of myeloid disorders including acute myeloid leukemia (AML). Acute promyelocytic leukemia (APL) is a subtype of AML in which a role for C/EBPs has been postulated. In almost all cases of APL, a promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) fusion protein is expressed as a result of a t(15;17)(q22;q12) chromosomal translocation. PML-RARalpha inhibits expression of C/EBPepsilon, whereas all-trans retinoic acid (tRA), a differentiating agent to which APL is particularly susceptible, induces C/EBPepsilon expression. PML-RARalpha may also inhibit C/EBPalpha activity. Thus, the effects of PML-RARalpha on C/EBPs may contribute to both the development of leukemia and the unique sensitivity of APL to tRA. We tested the hypothesis that increasing the activity of C/EBPs would revert the leukemic phenotype. C/EBPalpha and C/EBPepsilon were introduced into the FDC-P1 myeloid cell line and into leukemic cells from PML-RARA transgenic mice. C/EBP factors suppressed growth and induced partial differentiation in vitro. In vivo, enhanced expression of C/EBPs prolonged survival. By using a tamoxifen-responsive version of C/EBPepsilon, we observed that C/EBPepsilon could mimic the effect of tRA, driving neutrophilic differentiation in leukemic animals. Our results support the hypothesis that induction of C/EBP activity is a critical effect of tRA in APL. Furthermore, our findings suggest that targeted modulation of C/EBP activities could provide a new approach to therapy of AML.
