Oncogenic signaling in acute myeloid leukemia

High CD123 levels enhance proliferation in response to IL-3, but reduce chemotaxis by downregulating CXCR4 expression

High expression of the α chain of the interleukin-3 receptor (IL-3Rα; CD123) is a hallmark of acute myeloid leukemia (AML) leukemic stem cells (LSCs). Elevated CD123 expression is part of the diagnostic immunophenotyping of myeloid leukemia, and higher expression is associated with poor prognosis. However, the biological basis of the poorer prognosis is unclear, and may include heightened IL-3 signaling and non-cell autonomous interactions with the bone marrow (BM) microenvironment. We used TF-1 cells expressing different levels of CD123 and found elevated CD123 levels amplified the proliferative response to exogenous IL-3 and maintained viability in reducing IL-3 concentrations. This was associated with stronger activation of STAT5, Akt, and extracellular signal-regulated kinase 1/2 in vitro. Surprisingly, in vivo e14.5 fetal liver cells transduced with retroviral constructs to express high CD123 failed to engraft in syngeneic recipients. In exploring the underlying mechanism for this, we found that CXCR4, a key molecule involved in LSC/BM interactions, was specifically downregulated in CD123 overexpressing cells in a manner dependent on IL-3 signaling. CXCR4 downregulation was sufficient to alter the chemotactic response of hematopoietic cells to stromal derived factor-1 (SDF-1). Thus, we propose that the overexpression of CD123 in AML LSC dictates their location by altering CXCR4/SDF-1 interaction in the BM, raising the possibility that this mechanism underpins the egress of BM AML LSC and more mature cells into the circulation.

SOCS1 cooperates with FLT3-ITD in the development of myeloproliferative disease by promoting the escape from external cytokine control

Activating mutations in the receptor tyrosine kinase FLT3 are frequently found in acute myelogenous leukemia patients and confer poor clinical prognosis. It is unclear how leukemic blasts escape cytokine control that regulates normal hematopoiesis. We have recently demonstrated that FLT3-internal tandem duplication (ITD), when localized to the biosynthetic compartment, aberrantly activates STAT5. Here, we show that one of the target genes induced by STAT5 is suppressor of cytokine signaling (SOCS)1—a surprising finding for a known tumor suppressor. Although SOCS1 expression in murine bone marrow severely impaired cytokine-induced colony growth, it failed to inhibit FLT3-ITD–supported colony growth, indicating resistance of FLT3-ITD to SOCS1. In addition, SOCS1 coexpression did not affect FLT3-ITD–mediated signaling or proliferation. Importantly, SOCS1 coexpression inhibited interferon-α and interferon-γ signaling and protected FLT3-ITD hematopoietic cells from interferon-mediated growth inhibitory effects. In a murine bone marrow transplantation model, the coexpression of SOCS1 and FLT3-ITD significantly shortened the latency of a myeloproliferative disease compared with FLT3-ITD alone (P < .01). Mechanistically, SOCS proteins shield FLT3-ITD from external cytokine control, thereby promoting leukemogenesis. The data demonstrate that SOCS1 acts as a conditional oncogene, providing novel molecular insights into cytokine resistance in oncogenic transformation. Restoring cytokine control may provide a new way of therapeutic intervention.

Profiling the cancer genome

Cancer profiling studies have had a profound impact on our understanding of the biology of cancers in a number of ways, including providing insights into the biological heterogeneity of specific cancer types, identification of novel oncogenes and tumor suppressors, and defining pathways that interact to drive the growth of individual cancers. Several large-scale genomic studies are underway that aim to catalog all biologically significant mutational events in each cancer type, and these findings will allow researchers to understand how mutational networks function within individual tumors. The identification of molecular predictive and prognostic tools to facilitate treatment decisions is an important step for individualized patient therapy and, ultimately, in improving patient outcomes. Whereas there are still significant challenges to implementing genomic testing and targeted therapy into routine clinical practice, rapid technological advancements provide hope for overcoming these obstacles.

An emerging role for bromodomain-containing proteins in chromatin regulation and transcriptional control of adipogenesis

Transcriptional co-activators, co-repressors and chromatin remodeling machines are essential elements in the transcriptional programs directed by the master adipogenic transcription factor PPARgamma. Many of these components have orthologs in other organisms, where they play roles in development and pattern formation, suggesting new links between cell fate decision-making and adipogenesis. This review focuses on bromodomain-containing protein complexes recently shown to play a critical role in adipogenesis. Deeper understanding of these pathways is likely to have major impact on treatment of obesity-associated diseases, including metabolic syndrome, cardiovascular disease and Type 2 diabetes. The research effort is urgent because the obesity epidemic is serious; the medical community is ill prepared to cope with the anticipated excess morbidity and mortality associated with diet-induced obesity.

E3 ligase-defective Cbl mutants lead to a generalized mastocytosis and myeloproliferative disease

Somatic mutations of Kit have been found in leukemias and gastrointestinal stromal tumors. The proto-oncogene c-Cbl negatively regulates Kit and Flt3 by its E3 ligase activity and acts as a scaffold. We recently identified the first c-Cbl mutation in human disease in an acute myeloid leukemia patient, called Cbl-R420Q. Here we analyzed the role of Cbl mutants on Kit-mediated transformation. Coexpression of Cbl-R420Q or Cbl-70Z with Kit induced cytokine-independent proliferation, survival, and clonogenic growth. Primary murine bone marrow retrovirally transduced with c-Cbl mutants and transplanted into mice led to a generalized mastocytosis, a myeloproliferative disease, and myeloid leukemia. Overexpression of these Cbl mutants inhibited stem cell factor (SCF)-induced ubiquitination and internalization of Kit. Both Cbl mutants enhanced the basal activation of Akt and prolonged the ligand-dependent activation. Importantly, transformation was observed also with kinase-dead forms of Kit and Flt3 in the presence of Cbl-70Z, but not in the absence of Kit or Flt3, suggesting a mechanism dependent on receptor tyrosine kinases, but independent of their kinase activity. Instead, transformation depends on the Src family kinase Fyn, as c-Cbl coimmunoprecipitated with Fyn and inhibition abolished transformation. These findings may explain primary resistance to tyrosine kinase inhibitors targeted at receptor tyrosine kinases.