Identification of Ikaros As Novel Interaction Partner of Cyclin D1/CDK4 in MCL

Mantle cell lymphoma (MCL) is a B-cell non-Hodgkin lymphoma characterized by cyclin D1 (CycD1) overexpression. CycD1 physiologically works as a cell cycle regulator in conjunction with CDK4/6, inducing cell cycle progression in G1/S phase. The role of CycD1 in oncogenesis is manyfold, including promoting proliferation, regulating DNA damage response and controlling migration and invasiveness, but many of the interacting proteins and the functional mechanisms underlying their interaction remain unexplored. We therefore aimed to systematically screen for novel CycD1-interacting proteins in MCL using proximity-dependent biotin identification (BioID).

Using an MCL mouse model based on the cooperation of CycD1 and c-myc, we established lymphoma lines from mice expressing CycD1 linked to a bacterial biotin ligase BioID2. The approach enables covalent biotinylation of proteins closely interacting with CycD1 after the addition of biotin. After optimization of the labeling procedure, CycD1-interacting proteins in the MCL cell line were tagged and enriched by streptavidin pull-down, followed by mass spectrometry analysis. The association of CycD1 with candidate proteins was verified by co-immunoprecipitation (Co-IP) and immunofluorescence co-localization and investigated functionally by shRNA knockdown and inhibitor treatment.

Among other candidates, we found the transcription factor Ikaros (Ikzf1) to be specifically interacting with CycD1. The interaction in a complex consisting of CycD1, CDK4, and Ikaros was verified by Co-IP in human MCL (hMCL) cells, and CycD1 and Ikaros were found to co-localize in the cell nuclei of hMCL cells. Knockdown of Ikaros and treatment with the Ikaros degrader lenalidomide and also the novel degrader CFT7455 induced apoptosis, G1-phase arrest and decreased CycD1 expression, indicating a direct effect of Ikaros on CycD1 levels. In ongoing experiments, we observed that CDK4 inhibition may affect the phosphorylation of specific amino acids in Ikaros, altering the nuclear localization and transcriptional activity of Ikaros.

In summary, we identified Ikaros as a novel interaction partner of CycD1/CDK4 in MCL. Our results indicate that Ikaros can transcriptionally regulate and stabilize CycD1, and in turn, CycD1-CDK4 may phosphorylate Ikaros and affect Ikaros transcriptional activity. This feedback loop may play an important role in the pathogenesis of MCL and could serve as a novel potential therapeutic target in MCL.