Exploitation of a non-apoptotic caspase to regulate the abundance of the cdkI p27KIP1 in transformed lymphoid cells

Abstract

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 DPSD¹³⁹S and ESQD¹⁰⁸V, 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 DPSD¹³⁹S. A direct correlation exists between the appearance of truncated p27^KIP1 and the presence of an activity able to cleave peptides representing DPSD¹³⁹S 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 ESQD¹⁰⁸V 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.