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Hypoxia-induced mixed-lineage leukemia 1 regulates glioma stem cell tumorigenic potential.

ABSTRACT: Normal stem cells reside in functional niches critical for self-renewal and maintenance. Neural and hematopoietic stem cell niches, in particular, are characterized by restricted availability of oxygen and the resulting regulation by hypoxia-inducible factors (HIFs). Glioblastoma multiforme (GBM) is the most common malignant brain tumor and also contains high degrees of hypoxia. Heterogeneity within the neoplastic compartment has been well characterized in GBM and may be derived from genetic and epigenetic sources that co-evolve during malignant progression. Recent experimental evidence has supported the importance of hypoxia in glioma stem cell (GSC) niches. We hypothesized that HIFs require epigenetic-modifying proteins to promote tumor malignancy in GBM. Here we demonstrate that in GBM the histone methyltransferase mixed-lineage leukemia 1 (MLL1) is induced by hypoxia and enhances hypoxic responses. Loss of MLL1 reduces the expression of HIF transcripts and HIF2α protein. Targeting MLL1 by RNA interference inhibited the expression of HIF2α and target genes, including vascular endothelial growth factor (VEGF). GSCs expressed higher levels of MLL1 than matched non-stem tumor cells and depletion of MLL1 reduced GSC self-renewal, growth, and tumorigenicity. These studies have uncovered a novel mechanism mediating tumor hypoxic responses linking microenvironmental regulation of epigenetic-modifying proteins to cellular heterogeneity and provide rationale for the design of more sophisticated clinical approaches targeting epigenetic regulation.

SUBMITTER: Heddleston JM

PROVIDER: S-EPMC3229666 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:The stem cell factor spalt-like transcription factor 4 (SALL4) plays important roles in normal hematopoiesis and also in leukemogenesis. We previously reported that SALL4 exerts its effect by recruiting important epigenetic factors such as DNA methyltransferases DNMT1 and lysine-specific demethylase 1 (LSD1/KDM1A). Both of these proteins are critically involved in mixed lineage leukemia (MLL)-rearranged (MLL-r) leukemia, which has a very poor clinical prognosis. Recently, SALL4 has been further linked to the functions of MLL and its target gene homeobox A9 (HOXA9). However, it remains unclear whether SALL4 is indeed a key player in MLL-r leukemia pathogenesis.Using a mouse bone marrow retroviral transduction/ transplantation approach combined with tamoxifen-inducible, CreERT2-mediated Sall4 gene deletion, we studied SALL4 functions in leukemic transformation that was induced by MLL-AF9-one of the most common MLL-r oncoproteins found in patients. In addition, the underlying transcriptional and epigenetic mechanisms were explored using chromatin immunoprecipitation (ChIP) sequencing (ChIP-Seq), mRNA microarray, qRT-PCR, histone modification, co-immunoprecipitation (co-IP), cell cycle, and apoptosis assays. The effects of SALL4 loss on normal hematopoiesis in mice were also investigated.In vitro and in vivo studies revealed that SALL4 expression is critically required for MLL-AF9-induced leukemic transformation and disease progression in mice. Loss of SALL4 in MLL-AF9-transformed cells induced apoptosis and cell cycle arrest at G1. ChIP-Seq assay identified that Sall4 binds to key MLL-AF9 target genes and important MLL-r or non-MLL-r leukemia-related genes. ChIP-PCR assays indicated that SALL4 affects the levels of the histone modification markers H3K79me2/3 and H3K4me3 at MLL-AF9 target gene promoters by physically interacting with DOT1-like histone H3K79 methyltransferase (DOT1l) and LSD1/KDM1A, and thereby regulates transcript expression. Surprisingly, normal Sall4 f/f /CreERT2 mice treated with tamoxifen or vav-Cre-mediated (hematopoietic-specific) Sall4 -/- mice were healthy and displayed no significant hematopoietic defects.Our findings indicate that SALL4 critically contributes to MLL-AF9-induced leukemia, unraveling the underlying transcriptional and epigenetic mechanisms in this disease and suggesting that selectively targeting the SALL4 pathway may be a promising approach for managing human MLL-r leukemia.

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Hypoxia-induced mixed-lineage leukemia 1 regulates glioma stem cell tumorigenic potential.

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