c-REL is a Transcriptional Target of Mesoderm Inducer in Xenopus Like 1 (MIXL1)

MIXL1, an evolutionarily conserved, paired-type homeobox transcription factor induced by BMP4/TGFb signaling, is a critical regulator of embryonic and adult hematopoiesis. Several lines of evidence implicate MIXL1 in hematopoietic transformation: (i) Aberrant MIXL1 expression is seen in human CML ( Chronic Myelogenous Leukemia) in blast crisis, AML (Acute myelogenous leukemia), B cell lymphomas and pediatric ALL (Acute lymphocytic leukemia). (ii) Retroviral transduction of Mixl1 induces AML in murine models. Nonetheless, mechanisms underlying MIXL1 mediated proliferative, survival advantages are unknown. The goal of my studies is to understand if and how aberrant MIXL1 expression contributes to leukemogenesis. As a first step, I sought to determine transcriptional targets of MIXL1. Using MIXL1 overexpression lines established in the human myelomonocytic leukemia cell line, U937, I performed global chromatin immunoprecipitation coupled sequencing (ChIP-Seq), expression profiling studies I identified several putative targets. Of these, the proto-oncogene c-REL was an important target. Both ectopically expressed and endogenous MIXL1 bound REL promoters in myelogenous leukemia cells. Furthermore, cREL was induced under MIXL1 over-expression conditions in the leukemic cell line U937. Targeted shRNA mediated knockdown of endogenous MIXL1 in AML cell lines down regulated REL expression. c-REL the cellular homolog of the viral oncogene v-rel encoded by the oncogenic reticuloendotheliosis retrovirus, is a member of the Nf-κB/Rel gene family. c-REL transcriptionally activates proliferation and immune response and represses apoptosis through upregulation of anti-apoptotic genes. c-REL plays a critical role in hematopoietic differentiation. It is required for B-cell differentiation, and dendritic cells to activate T cells. In the myeloid lineage, c-REL is critical for macrophage mediated innate immunity. My findings suggest that transcriptional up regulation of REL by MIXL1, therefore would promote survival or proliferation through activation of the Nf-κB pathway in AML cells. Consistent with these established activities of c-REL, expression of two anti-apoptotic genes BCL2A1 and BCL2L1 increased under MIXL1 over-expression conditions and decreased under conditions of MIXL1 knockdown by targeted shRNA in U937 cells. In summary, my studies identify c-REL to be a novel mediator of MIXL1-induced survival signals in leukemia