Regulation of p27KIP1 in Epstein-Barr virus-immortalized lymphoblastoid cell lines involves non-apoptotic caspase cleavage.
The cyclin-dependent kinase inhibitor p27KIP1 plays a key role in controlling cell proliferation. Here we show that p27KIP1 is commonly down-regulated in B-cells immortalized by Epstein-Barr virus (EBV) (lymphoblastoid cell lines, LCLs). The significance of this event for the immortal phenotype of LCLs is implied by a requirement for active cdk2-containing complexes for continued proliferation, and by the ability of the residual p27KIP1 to associate with cdk2. The mechanism of p27KIP1 attenuation is post-translational, but inhibitor studies reveal that the mechanism does not rely heavily on the proteasome. Instead we find that LCLs contain an activity that cleaves a caspase recognition site present in p27KIP1 (DPSD139). The activity is not associated with apoptosis and closely resembles a proliferation-associated caspase activity we previously described in the EBV-negative B-lymphoma-derived cell line BJAB. Importantly, proliferating LCLs contain a p27KIP1 product that is consistent with cleavage at this site. Inhibition of caspase(s) in vivo modulates p27KIP1 expression and strongly inhibits proliferation of IB4 cells. This inhibitor profile is identical to that displayed by the DPSD-directed caspase present in BJAB cells, suggesting that the caspase may fulfil a general role in controlling p27KIP1 expression in immortal lymphoid cell lines. Thus, apoptosis-independent cleavage appears to contribute to the maintenance of the low basal levels of p27KIP1 in B-cells immortalized by EBV.
Frost
Victoria
V
School of Biological Sciences, University of Sussex, Brighton, East Sussex, BN1 9QG, UK1.
Delikat
Sylvie
S
School of Biological Sciences, University of Sussex, Brighton, East Sussex, BN1 9QG, UK1.
Al-Mehairi
Salama
S
School of Biological Sciences, University of Sussex, Brighton, East Sussex, BN1 9QG, UK1.
Sinclair
Alison J
AJ
School of Biological Sciences, University of Sussex, Brighton, East Sussex, BN1 9QG, UK1.
D001402:B-Lymphocytes / Q000378:metabolism* / Q000821:virology; D042846:CDC2-CDC28 Kinases; D020169:Caspases / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000378:metabolism*; D002455:Cell Division; D002461:Cell Line, Transformed; D051357:Cyclin-Dependent Kinase 2; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D015536:Down-Regulation; D004854:Herpesvirus 4, Human; D006801:Humans; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D025521:Tumor Suppressor Proteins / Q000378:metabolism*
D018797:Cell Cycle Proteins; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases; D020169:Caspases