Cyclin D1 is an essential regulator of cell cycle progression. Over-expression of cyclin D1 is associated with tumorigenesis both in solid cancers and in hematological malignancies such as mantle cell lymphoma (MCL), hairy cell leukemia and multiple myeloma (MM). The CCND1 gene coding for cyclin D1 exhibit an A/G polymorphism at the nucleotide 870 which is responsible for the generation of an alternative splice variant called form b. The cyclin D1b is identical to the typical form a, except in the C-terminal part. The cyclin D1b has lost the PEST domain responsible for protein degradation and the threonine 286, the site of phosphorylation by the glycogen synthase kinase-3 essential for the nuclear export. Cyclin D1b has been recently described as a nuclear oncogene (
Lu et al., Cancer Res 63, 7056, 2003
; Solomon et al., J Biol Chem 278, 30339, 2003
). Using real time quantitative RT-PCR, we have detected cyclin D1b mRNA in all cyclin D1a-expressing MM and MCL cell lines and in MCL patients. The cyclin D1b protein, detected with an antibody specific of the form b (a generous gift of Dr Alan Diehl) is present in a limited number of samples (1/6 MM and 1/1 MCL cell lines) and localized both in the nucleus and in the cytoplasm. We have generated B-cells conditionally (ecdysone system) expressing cyclin D1a or cyclin D1b proteins. B-cells were stimulated with ponasterone A to induce cyclin D1 synthesis and then with cycloheximide. The two alternative proteins forms displayed similar stability: calculated half-lives are 35 min and 50 min for a and b respectively. By co-immunoprecipitation and western blots experiments, we determined that, although cyclin D1b was able to bind efficiently CDK4 (to the same extent than cyclin D1a), cyclin D1b/CDK4 complexes weakly catalyzed the phosphorylation of pRB. We further showed by cytofluorometric analysis of propidium iodide-stained cells, that cyclin D1b did not allow the entry of quiescent cells within the cell cycle and did not allow the progression through the G1 phase. This implies that cyclin D1b plays a minor role in cell cycle regulation. However, cyclin D1b, such as cyclin D1a (Duquesne et al., Cell Death Different 8, 51, 2001
) potentiated the apoptosis induced by growth factor deprivation. It us tempting to speculate that the contribution of cyclin D1b for B-cell hemopathies does not occur through deregulation of the pRb pathway but more likely through deregulation of apoptotic pathways. We will study these aspects in the future.
