HoxA10 Influences Protein Ubiquitination by Activating Transcription of ARIH2, the Gene Encoding Triad1

Abstract

HoxA10 is a homeodomain transcription factor that is maximally expressed in myeloid progenitor cells. An increase in HoxA10 expression correlates with poor prognosis in human acute myeloid leukemia (AML). Consistent with this scenario, HoxA10 overexpression in murine bone marrow induces a myeloproliferative neoplasm that advances AML over time. Despite the importance of HoxA10 for leukemogenesis, few genuine HoxA10 target genes have been identified. The current study identified ARIH2, the gene encoding Triad1, as a HoxA10 target gene. We identified two distinct HoxA10-binding cis elements in the ARIH2 promoter and determined that HoxA10 activates these cis elements in myeloid cells. Triad1 has E3 ubiquitin ligase activity, and we found that HoxA10-overexpressing myeloid cells exhibited a Triad1-dependent increase in protein ubiquitination. Therefore, these studies have identified the regulation of protein ubiquitination as a novel function of Hox transcription factors. Forced overexpression of Triad1 has been show previously to inhibit colony formation by myeloid progenitor cells. In contrast, HoxA10-overexpressing myeloid progenitor cells exhibited increased proliferation in response to low doses of various cytokines. We found that Triad1 knockdown further increased cytokine-induced proliferation in HoxA10-overexpressing cells. Therefore, these studies have identified a HoxA10 target gene that antagonizes the overall influence of overexpressed HoxA10 on myeloproliferation. This result suggests that the consequences of HoxA10 overexpression reflect a balance between the target genes that facilitate and antagonize proliferation. These results have implications for understanding the mechanisms of leukemogenesis in AML with Hox overexpression.

HoxA10 Protein Regulates Transcription of Gene Encoding Fibroblast Growth Factor 2 (FGF2) in Myeloid Cells

“…This result is consistent with the multiple HoxA10-binding sites found in other target genes, including CYBB, NCF2, DUSP4, and TGF␤-2 (19,21,22,(35)(36)(37). We found that HoxA10-binding affinity for the two FGF2 cis elements is approximately equivalent.…”

The Leukemia-associated Mll-Ell Oncoprotein Induces Fibroblast Growth Factor 2 (Fgf2)-dependent Cytokine Hypersensitivity in Myeloid Progenitor Cells

“…Oligonucleotides probes were prepared, and DNA binding was analyzed by electrophoretic mobility shift assay (EMSA) and DNA affinity purification techniques as described (18,(21)(22)(23)(24)(25)(26). For EMSA, antibody to HoxA9 or HoxA10 (or irrelevant control antibody) was included in binding assays.…”

“…We used chromatin immunoprecipitation-based screening techniques to identify HoxA10 target genes that might be relevant to leukemogenesis (21)(22)(23)(24)(25)(26). In these studies we identified the gene encoding fibroblast growth factor 2 (Fgf2, or basic Fgf) as a HoxA10 target gene (26).…”

HoxA10 Terminates Emergency Granulopoiesis by Increasing Expression of Triad1

“…Expression of Triad1 is known to increase during granulopoiesis and impair proliferation of myeloid progenitor cells, but substrates for Triad1 in hematopoietic cells are not defined (17-21). In epithelial cells, Triad1-dependent ubiquitination results in lysosomal degradation versus re-cycling (and sustained signaling) of the receptors for epidermal growth factor and growth hormone (22).…”

“…We previously identified tandem cis elements in the ARIH2 promoter that are activated by the homeodomain transcription factor, HoxA10 (21). Unlike ARIH2 −/− mice, HOXA10 −/− mice are viable; unlike mice transplanted with ARIH2 −/− bone marrow, HOXA10 −/− mice do not exhibit steady state immune activation (23).…”