Cyclopamine and Quercetin Suppress the Growth of Leukemia and Lymphoma Cells

Background: Hedgehog (Hh) and Wnt signaling pathways are involved in the stimulation of growth of leukemia and lymphoma cells. In the present study, whether or not the Hh inhibitor, cyclopamine, and the Wnt inhibitor, quercetin, suppress cell growth was investigated. Materials and Methods: The effects of cyclopamine and quercetin on the in vitro growth and protein expression of ten acute leukemia and B-cell lymphoma cell lines were examined. Results: Cyclopamine and quercetin suppressed cell growth and induced apoptosis in seven and eight cell lines respectively. Cyclopamine decreased the level of Gli1 protein, a target gene product of Hh signaling. Quercetin decreased the level of Notch1 protein and its active fragment in the DND-41 T-lymphoblastic leukemia cell line with constitutive Notch activation. Conclusion: Cyclopamine and quercetin suppress the growth of a number of leukemia and lymphoma cells. This finding suggests the potential use of these compounds in molecularly-targeted therapy for leukemia and lymphoma. Notch, Hedgehog (Hh), and Wnt signaling are involved in the self-renewal of hematopoietic stem cells and the growth of hematological malignancies (1-4). We have previously reported that Notch and Wnt signaling were involved in the self-renewal capacity of acute myeloblastic leukemia (AML) cells (4, 5). Sonic Hh has also been found to be expressed in cells derived from hematological malignancies (6). These pathways are considered as potential candidate targets for molecular therapy. We have previously demonstrated that Notch inhibitors suppressed the growth of leukemia cells (7). It is therefore possible that Hh and Wnt inhibitors might also suppress their growth. The Hh signaling cascade is as follows: in the absence of Hh ligands, the activity of Smoothened (Smo) is suppressed by Patched (Ptch), which is a receptor of Hh ligands; ligand binding to Ptch releases Smo, which then activates the Gli transcription factor. Gli induces the transcription of target genes such as Ptch1, cyclin D and Gli1 itself. The steroidal alkaloid cyclopamine blocks Hh signaling by direct binding to Smo (8). The Wnt signaling pathway is as follows: in the absence of Wnt ligands, glycogen synthase kinase 3β (GSK3β) phosphorylates β-catenin, which is then degraded by proteasomes; Wnt ligand binding to receptors inhibits the activity of GSK3β, resulting in the accumulation of β-catenin; the accumulated β-catenin translocates to the nucleus and induces the expression of growth-related genes (5). The flavonoid quercetin blocks Wnt signaling by inhibiting the transcriptional activity of β-catenin (9). In this study, the effects of cyclopamine and quercetin on the growth of leukemia and lymphoma cell lines in culture were investigated. Furthermore, the mechanisms responsible for their effects were examined, with the aim of establishing a potential therapeutic use for these drugs for the treatment of leukemia and lymphoma. Materials and Methods