A role of TGFbeta/BMP in leukemogenesis through interaction between Smad4 and Hoxa9

Hematopoiesis is tightly regulated by the continuous proliferation and differentiation of hematopoietic cells. Understanding the molecular basis for the regulation of hematopoiesis may help to elucidate the pathogenesis of hematological disorders such as leukemia and to identify new therapeutic targets.; Transforming growth factor beta (TGFbeta) is a crucial regulator of hematopoiesis. However, the molecular mechanism of how TGFbeta modulates hematopoiesis is not well characterized. In this dissertation, the studies demonstrate that TGFbeta/bone morphogenetic protein (BMP) regulates the bone marrow transformation capability of Hoxa9 and Nup98-Hoxa9 through Smad4 and show how Smad4 mutations identified in human acute myeloid leukemia patients may affect its normal function.; Smad4 directly interacts with the homeodomain of Hoxa9, which is conserved in all 39 homeobox (Hox) proteins and functions as their DNA-binding domain. This interaction blocks the DNA-binding ability of Nup98-Hoxa9 and, thus, suppresses its down stream gene transcription. Mapping data revealed that the amino-terminus of Smad4 is responsible for this interaction. Overexpression of this Hoxa9 interaction domain of Smad4 was sufficient to inhibit the leukemic transformation capability of Nup98-Hoxa9. Thus, our studies establish a novel mechanism by which TGFbeta/BMP regulates hematopoiesis and raise the possibility that Hox DNA-binding activity may serve as a novel therapeutic intervention for those leukemias that involve Hox deregulations.; In addition, Smad4 mutants exhibited significantly decreased protein stability. Importantly, we found that F-box protein beta-transducin-repeat-containing protein 1(beta-TrCP1) in Skp1-cullin-F box protein (SCF) E3 ligase interacts with Smad4, and exhibits stronger interaction affinity with the acute myelogenous leukemia-derived Smad4 mutants. Consequently, E3 ligase complex SCF beta-TrCP1 mediates ubiquitination of Smad4 mutants. When small interference RNA (siRNA)-induced F-box protein beta-TrCP1 gene silencing was used, the protein steady-state level of Smad4 was elevated in acute myelogenous leukemia cells.