1. ¹Department of Biology, Queens College and Graduate Center of the City University of New York, 65-30 Kissena Blvd., Flushing, NY 11367, USA
2. ²Department of Biological Sciences, St John's University, 8000 Utopia Parkway, Jamaica, NY 11439, USA

Correspondence: RA Lockshin, E-mail: lockshin@stjohns.edu

Abstract

Many cells die with apoptotic morphology and with documented activation of an effector caspase, but there are also many exceptions. Cells frequently display activation of other proteases, including granzymes, lysosomal cathepsins, matrix metalloproteinases, and proteasomal proteases, and others display morphologies that are not fully consistent with classical apoptosis. In some experimental situations, evidence of caspase-dependent death is indirect, demonstrating that the cell can activate caspases rather than that it does. In other situations, such as involution of mammary or prostate tissue, many cells display autophagic or other morphology different from apoptosis, and there is considerable evidence for the activation of a lysosomal system. Prior to total collapse and necrosis, cells that are in trouble can activate numerous physiological pathways toward self-destruction. Intrinsic or extrinsic routes to effector caspase activation are frequently the most rapid and efficient. If neither of these routes is immediately available, owing to mutation, genetic manipulation, inhibitor, or the biology of the cell, other routes may be followed, leading to variant forms of cell death that may display one or more characteristics of apoptosis. Experimental and therapeutic procedures must account for this possibility.