BCR signals target p27(Kip1) and cyclin D2 via the PI3-K signalling pathway to mediate cell cycle arrest and apoptosis of WEHI 231 B cells

Cross-linking of the B cell antigen receptor (BCR) on immature WEHI 231 B c ells results in GI cell cycle arrest and apoptosis. Here we investigated th e, molecular mechanisms that are necessary and sufficient for these changes to occur., We show that BCR stimulation of WEHI 231 cells results in down- regulation of cyclin D2 and up-regulation of p27(Kip1), which are associate d with pocket protein hypophosphorylation and E2F inactivation. Ectopic exp ression of p27(Kip1) by TAT-fusion protein or retroviral transduction is su fficient to cause G1 cell cycle arrest, followed by apoptosis. In contrast, overt expression of cyclin D2 overcomes, the cell cycle arrest and apoptos is induced by anti-IgM, indicating that downregulation of cyclin D2 is nece ssary for the cell cycle arrest and apoptosis activated by BCR stimulation. Thus, cyclin D2 and p27(Kip1) have opposing roles in these pathways and ou r data also suggest that cyclin D2 functions upstream of p27(Kip1) and the pRB pathway and therefore plays an essential part in integrating the signal s from BCR with the cell cycle machinery. We next investigated which signal transduction pathways triggered by the BCR regulate cell proliferation and apoptosis. via cyclin D2 and p27(Kip1). Inhibition of PI3-K signalling by LY294002 down-regulated cyclin D2 and up-regulated p27(Kip1) expression at both, protein and RNA levels, mimicking the effects of BCR-stimulation. Fur thermore, ectopic expression of a constitutively active form of AKT blocked the cell cycle arrest and apoptosis triggered by anti-IgM and also, abroga ted downregulation of cyclin D2 and up-regulation of p27(Kip1) expression i nduced by BCR-engagement. These results indicate that BCR activation target s: p27(Kip1) and cyclin D2 to mediate cell cycle arrest and apoptosis and t hat down-regulation of PI3-K/AKT activity post BCR stimulation is necessary for these to occur.