Transcriptional regulation of Rab21 GTPase in leukemia.

Rab GTPase family proteins have critical roles in membrane trafficking. Rab21 is a member of the Ras oncogene family, whose dysregulation has been associated with drug resistance in cancer. Regulation of Rab21 expression is still unknown. Here we present data that RAB21 expression is regulated by Casein Kinase II (CK2) and by the Ikaros protein in B-cell acute lymphoblastic leukemia (B-ALL). Ikaros is a DNA-binding protein, encoded by the Ikzf1 gene, which acts as a tumor suppressor in leukemia. Analysis of global DNA binding showed increased occupancy of Ikaros at promoter of the Rab21 gene in several B-ALL cell lines and primary B-ALL cells. The role of Ikaros in transcriptional regulation of RAB21 expression in B-ALL was determined by gain-of-function and loss-of-function experiments. Overexpression of Ikaros in B-ALL via retroviral transduction resulted in increased Ikaros binding to the RAB21 promoter and decreased RAB21 transcription. In contrast, shRNA-mediated knockdown of Ikaros inhibited Ikaros binding to the RAB21 promoter and resulted in an increased RAB21 expression. These data suggest that Ikaros directly repress RAB21 transcription. Our previous studies showed that CK2 directly phosphorylates Ikaros and reduces its DNA-binding affinity. We tested whether CK2 affects Ikaros' ability to regulate RAB21 expression in B-ALL. Treatment of B-ALL cells with a specific CK2 inhibitors, CX-4945, resulted in increased Ikaros binding at the RAB21 promoter and reduced RAB21 expression. The use of luciferase reporter assay showed that both wildtype Ikaros and Ikaros phosphoresistant mutants strongly repress Rab21 transcription, while phopshomimetic Ikaros mutants at CK2 phosphorylation sites lost the ability to repress RAB21 transcription. Together, these data demonstrate that Ikaros and CK2 regulate transcription and overall expression of RAB21 in B-cell ALL. Results uncovered an interplay between CK2 and Rab GTPase signaling pathways and suggest a mechanism by which CK2 regulates Rab21 activity in leukemia.