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

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.