We previously demonstrated that RARα2 expression is increased in CD138 selected plasma cells of relapsed multiple myelomas (MMs), and increased expression was linked to poor prognosis in newly diagnosed MM patients. In the present study, we demonstrate that increased RARα2 confers myeloma stem cell features. Higher expression of RARα2 was identified in the multiple myeloma stem cell (MMSC) fraction. Overexpression of RARα2 in bulk MM cell lines resulted in: 1) increased drug resistance; 2) increased clonogenic potential; 3) activation of both Wnt and Hedgehog (Hh) pathways; 4) increased side population and aldehyde dehydrogenase levels; and 5) increased expression of embryonic stem cell genes. The opposite effects were seen with RARα2 knockdown. We demonstrate that RARα2 induces drug resistance by activating the drug efflux pump gene ABCC3 and anti-apoptotic Bcl-2 family members. Inhibition of Wnt signaling or ABCC3 function could overcome drug resistance in RARα2 overexpressing MM cells. We also showed that in the 5TGM1 mouse model, targeting of the Wnt and Hh pathways using CAY10404, cyclopamine, or itraconazole significantly reduced the myeloma tumor burden and increased survival. Targeting RARα2 or its downstream signaling pathways provides a potential strategy to eliminate MMSC.
Bibliographical noteFunding Information:
The authors are grateful to Dr Justin Fishbaugh, Heath Vignes, and George Rasmussen for assistance with the use of flow cytometer and cell sorting and to Dr Thai Cao (University of Utah) and Garry Hauser (DNA Facility) for RT-PCR support. This work was supported by National Cancer Institute grants R01CA115399 (to G.T.), R01CA152105 (to F.Z.), and R21CA143887 (to F.Z.); the MMRF Senior (to F.Z., 2008 and 2010); the Leukemia Lymphoma Society TRP (to F.Z., 2010 and 2011); institutional start-up funds from the Department of Internal Medicine, Carver School of Medicine, University of Iowa (to F.Z. and G.T.); and the National Natural Science Foundation of China, China (no. 81228016 to F.Z. and J.S.).
This work was supported by National Cancer Institute grants R01CA115399 (to G.T.), R01CA152105 (to F.Z.), and R21CA143887 (to F.Z.); the MMRF Senior (to F.Z., 2008 and 2010); the Leukemia Lymphoma Society TRP (to F.Z., 2010 and 2011); institutional
© 2013 by The American Society of Hematology.