Exploitation of a non-apoptotic caspase to regulate the abundance of the cdkI p27(KIP1) in transformed lymphoid cells

Expression of the cyclin dependent kinase inhibitor p27(KIP1) is intimately linked to the control of proliferation, and is itself regulated by transcription, translation, phosphorylation, protein stability or sequestration. p27(KIP1) is also regulated during apoptosis; cleavage occurs at DPSD(139)S and ESQD(108)V, by a sub-set of Z-VAD-fmk-sensitive caspases. We have identified a related but distinct mechanism that regulates p27(KIP1) in proliferating lymphoid cell lines. In a B-lymphoid cell line (BJAB), the abundance of p27(KIP1) oscillates inversely to proliferation; loss of full-length p27(KIP1) correlates with the appearance of a truncated version corresponding to cleavage at DPSD(139)S. A direct correlation exists between the appearance of truncated p27(KIP1) and the presence of an activity able to cleave Peptides representing DPSD(139)S and a Caspase-8 substrate (Ac-IETD-AMC) in vitro. This activity is inhibited by Ac-IETD-CHO but not Z-VAD-fmk in vitro. Furthermore a requirement for Caspase-8 has been excluded. The activity differs from the Apoptosis related p27(KIP1)-cleaving activity; indeed few cells undergoing Apoptosis are present in the population of proliferating cells. The activity is further distinguished by its inability to cleave a peptide based on ESQD(108)V in vitro, together with the lack of a corresponding cleavage product in vivo. Inhibition of the Caspase activity in vivo promotes an accumulation of full length p27(KIP1), as well as a decrease in cell proliferation. Together these studies highlight the importance of non-apoptotic caspases in regulating p27(KIP1) in transformed lymphoid cells.