The Role of Hox Proteins and Interferon Regulatory Factors in Myeloid Differentiation.

During myelopoiesis, differentiating phagocytes develop functional competence and undergo proliferation arrest and eventual programmed cell death. This process involves transcriptional regulation of genes which mediate the innate immune response, mitotic arrest, and apoptosis. A number of transcription factor families play important roles in regulating such genes, including Hox proteins and interferon regulatory factors (IRFs). Disordered expression of Hox proteins is associated with myeloid leukemogenesis. Hox proteins are homeodomain transcription factors that are organized in four paralog groups (A–D). Expression of HoxA7-11 (the ABD-HOXA genes) is characteristic of myeloid progenitors. ABD-HOXA transcription decreases with CD34+ to CD34− progression, and persistent expression of these genes is found in poor prognosis leukemia. Abd Hox proteins regulate genes that are involved in multiple aspects of myelopoiesis. For example, HoxA10 represses transcription of the genes encoding gp91^phox and p67^phox in myeloid progenitors (the CYBB and NCF2 genes, respectively). In contrast, HoxA9 activates transcription of these genes as differentiation proceeds. Since these are the rate-limiting NADPH oxidase components, HoxA proteins influence the innate immune response. HoxA10 activates transcription of the gene encoding Beta3 integrin, thereby further facilitating NADPH oxidase activation and influencing adhesion. HoxA10 also activates transcription of DUSP4, the gene encoding MAP kinase phosphatase 2 (Mkp2). Mkp2 antagonizes the activity of c-Jun N-terminal kinases (Jnk). Since HoxA10-activation of DUSP4 decreases during myelopoiesis, decreased Mkp2 expression in mature phagocytes facilitates apoptosis via Jnk. Interferon regulatory factors (IRF) also regulate multiple aspects of myelopoiesis. IRF1 and ICSBP/IRF8 activate transcription of the CYBB and NCF2 genes in cooperation with the ets protein PU.1. ICSBP/IRF8 also contributes to phagocyte function by activating genes encoding TLR4, IL12, and Nramp. Additionally, IRF proteins regulate cell cycle progression and proliferation. IRF2, ICSBP/IRF8, and PU.1 activate transcription of gene encoding Neurofibromin 1, thereby downregulating the proliferative response to cytokines such as GM-CSF, M-CSF, and G-CSF. ICSBP/IRF8 and PU.1 also activate the gene encoding Ink4b, thereby also influencing proliferation. In myeloid progenitors, ICSBP/IRF8 influences cell survival by repressing transcription of PTPN13, the gene encoding Fas-associated phosphatase 1 (Fap1), a Fas-antagonist. Decreased ICSBP/IRF8-induced PTPN13 repression during myelopoiesis increases susceptibility of mature phagocytes to Fas-induced apoptosis. Such studies reveal that multiple aspects of myelopoiesis are regulated by common sets of transcription factors. This may suggest therapeutic targets for myeloid leukemias or other disorders of myeloid development.