Regulation of p27(KIP1) in Epstein-Barr virus-immortalized lymphoblastoid cell lines involves non-apoptotic caspase cleavage

The cyclin-dependent kinase inhibitor p27(KIP1) plays a key role in control ling cell proliferation. Here we show that p27(KIP1) is commonly down-regul ated in B-cells immortalized by Epstein-Barr virus (EBV) (lymphoblastoid ce ll lines, LCLs). The significance of this event for the immortal phenotype of LCLs is implied by a requirement for active cdk2-containing complexes fo r continued proliferation, and by the ability of the residual p27 KIN to as sociate with cdk2. The mechanism of p27(KIP1) attenuation is post-translati onal, but inhibitor studies reveal that the mechanism does not rely heavily on the proteasome. Instead we find that LCLs contain an activity that clea ves a caspase recognition site present in p27(KIP1) (DPSD139). The activity is not associated with apoptosis and closely resembles a proliferation-ass ociated caspase activity we previously described in the p27(KIP1)-negative B-lymphoma-derived cell line BJAB. Importantly, proliferating LCLs contain a p27(KIP1) product that is consistent with cleavage at this site. Inhibiti on of caspase(s) in vivo modulates p27(KIP1) expression and strongly inhibi ts proliferation of IB4 cells. This inhibitor profile is identical to that displayed by the DPSD-directed caspase present in BJAB cells, suggesting th at the caspase may fulfil a general role in controlling p27(KIP1) expressio n in immortal lymphoid cell lines. Thus, apoptosis-independent cleavage app ears to contribute to the maintenance of the low basal levels of p27(KIP1) in B-cells immortalized by EBV.