RESUMO
Tropomyosin-related kinase A (TRKA) translocations have oncogenic potential and have been found in rare cases of solid tumors. Accumulating evidence indicates that TRKA and its ligand, nerve growth factor (NGF), may play a role in normal hematopoiesis and may be deregulated in leukemogenesis. Here, we report a comprehensive evaluation of TRKA signaling in normal and leukemic cells. TRKA expression is highest in common myeloid progenitors and is overexpressed in core binding factor and megakaryocytic leukemias, especially Down syndrome-related AML. Importantly, NGF can rescue GM-CSF dependent TF-1 AML cells, but does not drive proliferation in other TRKA-expressing lines. Although TRKA expression is heterogeneous between and within AML samples, NGF stimulation broadly induces ERK signaling, demonstrating the functional ability of AML cells to respond to NGF/TRKA signaling. However, neither shRNA knockdown nor pharmacologic inhibition have significant anti-proliferative effects on human AML cells in vitro and in vivo. Thus, despite functional NGF/TRKA signaling, the importance of TRKA in AML remains unclear.
RESUMO
Although aberrant Notch activation contributes to leukemogenesis in T cells, its role in acute myelogenous leukemia (AML) remains unclear. Here, we report that human AML samples have robust expression of Notch receptors; however, Notch receptor activation and expression of downstream Notch targets are remarkably low, suggesting that Notch is present but not constitutively activated in human AML. The functional role of these Notch receptors in AML is not known. Induced activation through any of the Notch receptors (Notch1-4), or through the Notch target Hairy/Enhancer of Split 1 (HES1), consistently leads to AML growth arrest and caspase-dependent apoptosis, which are associated with B cell lymphoma 2 (BCL2) loss and enhanced p53/p21 expression. These effects were dependent on the HES1 repressor domain and were rescued through reexpression of BCL2. Importantly, activated Notch1, Notch2, and HES1 all led to inhibited AML growth in vivo, and Notch inhibition via dnMAML enhanced proliferation in vivo, thus revealing the physiological inhibition of AML growth in vivo in response to Notch signaling. As a novel therapeutic approach, we used a Notch agonist peptide that led to significant apoptosis in AML patient samples. In conclusion, we report consistent Notch-mediated growth arrest and apoptosis in human AML, and propose the development of Notch agonists as a potential therapeutic approach in AML.