p27^KIP1 is down-regulated by two different mechanisms in human lymphoid cells undergoing apoptosis

Abstract

The cyclin-dependent kinase inhibitor p27^KIP1 is a crucial component of the mammalian restriction point, and as such is subject to multiple regulatory mechanisms. It has recently been shown that the abundance of p27^KIP1 is also regulated during apoptosis; p27^KIP1 is cleaved by a Z-VAD-fmk-sensitive caspase during apoptosis induced by growth factor deprivation in endothelial cells, and also following exposure of myeloid leukaemia cells to etoposide. Here, we investigate p27^KIP1 regulation in B- and T-lymphoid cells undergoing apoptosis. We observe that p27^KIP1 is down-regulated following exposure to a variety of apoptotic stimuli including an agonistic anti-Fas antibody, cycloheximide and etoposide. Further investigation revealed the existence of two different routes of p27^KIP1 regulation in lymphoid cells undergoing apoptosis. The first pathway is utilized by lymphoid cells stimulated through Fas, is abrogated in a caspase-8-deficient T-cell line, and is blocked by the caspase inhibitors Z-VAD-fmk and Boc-D-fmk. In contrast, the loss of p27^KIP1 in cells exposed to cycloheximide and etoposide occurs in the absence of caspase-8 or any Z-VAD-fmk- or Boc-D-fmk-sensitive caspase activities. Thus the down-regulation of p27^KIP1 is a common occurrence in lymphoid cells undergoing apoptosis but, depending on the apoptotic trigger, this can be affected by two different mechanisms.