Myeloid leukemia factor is a conserved regulator of RUNX transcription factor activity involved in hematopoiesis

Abstract

Defining the function of the genes that, like RUNX1, are deregulated in blood cell malignancies represents an important challenge. Myeloid leukemia factors (MLFs) constitute a poorly characterized family of conserved proteins whose founding member, MLF1, has been associated with acute myeloid leukemia in humans. To gain insight into the functions of this family, we investigated the role of the Drosophila MLF homolog during blood cell development. Here we report that mlf controls the homeostasis of the Drosophila hematopoietic system. Notably, mlf participates in a positive feedback loop to fine tune the activity of the RUNX transcription factor Lozenge (LZ) during development of the crystal cells, one of the two main blood cell lineages in Drosophila. At the molecular level, our data in cell cultures and in vivo strongly suggest that MLF controls the number of crystal cells by protecting LZ from degradation. Remarkably, it appears that the human MLF1 protein can substitute for MLF in the crystal cell lineage. In addition, MLF stabilizes the human oncogenic fusion protein RUNX1-ETO and is required for RUNX1-ETO-induced blood cell disorders in a Drosophila model of leukemia. Finally, using the human leukemic blood cell line Kasumi-1, we show that MLF1 depletion impairs RUNX1-ETO accumulation and reduces RUNX1-ETO-dependent proliferation. Thus, we propose that the regulation of RUNX protein levels is a conserved feature of MLF family members that could be critical for normal and pathological blood cell development.fruit fly | cancer | proteasome

Myeloid leukemia factor

“…24 Several non-exclusive hypotheses could explain this protective effect. First, MLF protein could favor the interaction between RUNX and their cofactor CBFß, which is required to prevent RUNX degradation by the proteasome.…”

“…Remarkably, dMLF also stabilizes RUNX1-ETO and is required for its activity in vivo in a Drosophila model of leukemia. 24 Moreover, knocking-down hMLF1 impairs RUNX1-ETO accumulation and reduces RUNX1-ETOdependent proliferation in a human leukemic cell line. LZ and RUNX1-ETO only share homology through their DNAbinding domain, which characterizes all RUNX family members.…”

“…16 Interestingly, loss of dMLF in the eye only resulted in a slight downregulation of LZ levels, suggesting that the regulation of LZ stability is tissue specific. 24 Post-transcriptional modifications of RUNX proteins have been implicated in the regulation of their turnover 28 and MLF could impinge on these modifications. Alternatively, MLF may stabilize RUNX proteins by promoting their association with chromatin or their nuclear import.…”

Subcellular localization of full-length human myeloid leukemia factor 1 (MLF1) is independent of 14-3-3 proteins

“…Further indications of the potential oncogenic activity of MLF1 are its inhibition of erythropoietin-induced differentiation and p27 Kip1 accumulation [4] and the observation that a disturbance of MLF1 shuttling affects p53 stability and susceptibility to transformation [5]. In addition, recent results suggest that MLF1 is critical for normal and pathological blood cell development [6,7]. MLF1 was originally described in a translocation between MLF1 on chromosome 3 and nucleophosmin (NPM) on chromosome 5, which yields the oncogene NPM-MLF1 [8].…”

“…Interestingly, although MLF1 localizes to the cytoplasm in non-hemopoietic cells [8], it preferentially resides in the nucleus of hemopoietic cells [8][9][10][11]. Beyond these reports, information on the physiological function of MLF1 is limited and is derived mostly from studies identifying MLF1 interaction partners, such as CSN3 [12], MLF1IP [13,14], MADM [15], Manp, and the 14-3-3 proteins [11,[5][6][7][8][9][10][11][12][13][14][15][16][17]]. 14-3-3 proteins are eukaryotic proteins of 25 to 30 kDa that influence a variety of physiological processes [18].…”