Functional Analysis of Cyclin D1 in Mantle Cell Lymphoma (MCL) by Specific Lentiviral shRNA Mediated Knockdown.

Introduction: Cyclin D1 overexpression is the hallmark of MCL. However, the importance of cyclin D1 for the maintenance of MCL still remains to be defined. Therefore, the aim of this study is to elucidate the role of cyclin D1 overexpression using the siRNA technology in well-characterized MCL cell lines, as a model system. Material and Methods: A highly efficient cyclin D1-shRNA (96% knockdown) was identified using a lacZ-cyclin D1 fusion gene reporter system in HEK-293T cells. This shRNA was cloned into a lentiviral transfer vector carrying GFP as a reporter gene, which enables the detection of infected cells by FACS analysis. Seven MCL cell lines were analyzed (Granta 519, Jeko-1, Rec-1, Z-138, UPN-1, Hbl-2 and JVM-2), using appropriate controls. Western Blot analysis and qRT-PCR were performed to quantitate the knockdown effect. The effect of cyclin D1 knockdown on proliferation, cell cycle, and viability was analyzed by MTT assay and FACS analysis. Results: The infection rates varied among the different MCL cell lines. Rec-1 and Hbl-2 showed low infection rates (50%) even at high MOI’s (multiplicity of infection), whereas UPN-1 and JVM-2 had moderate infection rates (80%). Jeko-1, Granta 519 and Z-138 showed high infection rates (almost 100% of the cells). Despite the good tranfection rate, the downregulation of cyclin D1, as measured by Western Blot and qRT-PCR, was about 80% in Granta 519, and 65% in Jeko-1 and Z-138. No IFN response, as secondary effect was identified. Interestingly, no apoptosis was observed, and there was only a moderate retardation of growth (60% of control cells) with 10% shift from the S phase to G1 phase of the cell cycle when compared to the controls, suggesting that other cell cycle proteins might compensate, at least partially, for the loss of cyclin D1. Accordingly, cyclin D2 showed upregulation in Western blot analysis and qRT-PCR, whereas the phosphorylation status of retinoblastoma protein on Ser780 was reduced and the expression of the CDK inhibitor p27Kip1 increased. No changes were observed in the expression of cyclin D3, Cyclin E, CDK4 and CDK2. Conclusions: In this study, a system that enables the specific downregulation of cyclin D1 in MCL cell lines was established. Surprisingly, the downregulation of cyclin D1 in MCL cell lines resulted in only a moderate inhibition on cell growth with no apoptosis. The reasons for this might be 1) that the upregulation of cyclin D2 compensates for cyclin D1 downregulation, and/or 2) that the chromosomal translocation leading to cyclin D1 overexpression is an initiating event in MCL lymphomagenesis followed by secondary genetic events at later stages of the disease, which make cyclin D1 dispensable. This finding has important implications for MCL therapy, as strategies targeting only cyclin D1 might be hampered by the redundancy of the system, resulting in a low probability of treatment response.