CCAAT/Enhancer binding proteins repress the leukemic phenotype of acute myeloid leukemia

Abstract

AbstractCCAAT/enhancer binding proteins (C/EBPs) are a family of factors that regulate cell growth and differentiation. These factors, particularly C/EBPα and C/EBPε, 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 α (PML-RARα) fusion protein is expressed as a result of a t(15;17)(q22;q12) chromosomal translocation. PML-RARα inhibits expression of C/EBPε, whereas all-trans retinoic acid (tRA), a differentiating agent to which APL is particularly susceptible, induces C/EBPε expression. PML-RARα may also inhibit C/EBPα activity. Thus, the effects of PML-RARα 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/EBPα and C/EBPε 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/EBPε, we observed that C/EBPε 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.Supplemental figure. C/EBPs suppress growth of murine myeloid leukemia in vivo. Bone marrow and spleen cells of leukemic mice were harvested and transduced with retroviruses expressing MIG (control), hC/EBPα, or hC/EBPε. At 24 ours after the second round of infection, GFP+ cells were sorted and intravenously injected into sublethally irradiated (4.5 Gy) healthy FVB/N females. (A) PML-RARα leukemia #1111, 50,000 cells per animal. Control mice n = 5, C/EBPα mice n = 5, C/EBPε mice n = 4. (B) PML-RARα leukemia #1111, 25,000 cells per animal. Control mice n = 4, C/EBPα mice n = 5, C/EBPε mice n = 5. (C) PML-RARα leukemia #935, 50,000 cells per animal. Control mice n = 6, C/EBPa mice n = 6, C/EBPε mice n = 6. (A, B) For PML-RARα leukemia #1111 recipients of C/EBP transduced cells survived longer. This prolongation of survival approached statistical significance for C/EBPε when 50,000 cells were used (A, p=.06) and was marked in recipients of 25,000 C/EBPε transduced cells (B, mean increase in survival 26 days, median 23 days, p=.02). (C) Both C/EBPα and C/EBPε modestly prolonged the survival of mice that received PML-RARα leukemia #935 cells (C/EBPα: mean increase in survival 3 days, median 3 days, P =.003; C/EBPε: mean increase in survival 6 days, median 6.5 days, P =.00001).