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 transcr iption, translation, phosphorylation, protein stability or sequestration, p 27(KIP1) is also regulated during apoptosis; cleavage occurs at (DPSDS)-S-1 39 and ESQD(108)V, by a sub-set of z-VAD-fmk-sensitive caspases, We have id entified related but distinct mechanism that regulates p27(KIP1) in prolife rating lymphoid cell lines. In a B-lymphoid cell line (BJAB), the abundance of p27(KIP1) oscillates inversely to proliferation; loss of full-length p2 7(KIP1) correlates with the appearance of a truncated version corresponding to cleavage at (DPSDS)-S-139, A direct correlation exists between the appe arance of truncated p27(KIP1) and the presence of an activity able to cleav e peptides representing (DPSDS)-S-139 and a caspase-8 substrate (Ac-IETD-AM C) 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 acti vity differs from the apoptosis related p27(KIP1)-cleaving activity; indeed few cells undergoing apoptosis are present in the population of proliferat ing 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 corres ponding cleavage product in vivo. Inhibition of the caspase activity in viv o promotes an accumulation of full length p27(KIP1), as well as a decrease in cell proliferation. Together these studies highlight the importance of n on-apoptotic caspases in regulating p27(KIP1) in transformed lymphoid cells .