Alternative cyclin D1 forms a and b have different biological functions in the cell cycle of B lymphocytes

Cyclin D1 is an important regulator of the early phase of the cell cycle and the transcriptional machinery. It is often deregulated in human tumors of various origins and is considered to be an oncogene. The CCND1 gene encoding cyclin D1 generates two mRNAs by alternative splicing, leading to the production of two alternative proteins: a long form a (36 kDa) and a short form b (30–31 kDa) from which the C-terminal moiety required for protein stability and sub-cellular localization has been deleted. Both forms of RNA and protein have been detected in B-cell hemopathies, but their respective roles are unclear. We investigated the function of cyclin D1b in cell cycle regulation, by generating B-cell lines displaying conditional expression of isoform b. Comparisons of these cell lines (BD1b series) with previously obtained cell lines expressing cyclin D1a demonstrated that cyclin D1b had no cell cycle regulatory properties.