Rebuttal from Florian Lang and Else K. Hoffmann

Abstract

Orlov et al. (2013a,b) argue that cell volume changes during apoptosis are cell type specific and cell volume perturbations are not always sufficient for the triggering of cell death. They focus on three points: Early shrinkage is not always detected during apoptosis. We do agree (Lang & Hoffmann, 2012) that in some cells under specific challenges apoptosis occurs without preceding apoptotic volume decrease (AVD). In vascular muscle cells transfected with E1A-adenoviral protein AVD was observed following staurosporine treatment but not growth factor deprivation (Platonova et al. 2012). AVD is, however, observed and required for apoptosis in almost all cell types under almost all challenges. Decrease in intracellular ion concentration rather than cell shrinkage triggers apoptosis. In Ehrlich cells initiation of apoptosis did not result in any decrease in Cl− concentration and only minute decreases in K+ concentration during the essential initial AVD whereas secondary AVD was paralleled by substantial decrease in K+ concentration and increase of Na+ concentration (Poulsen et al. 2010). In resistant Ehrlich cells, lack of AVD rather than differences in ion concentrations explained protection against apoptosis (Poulsen et al. 2010). Clear evidence for the importance of the initial AVD has been provided utilizing other cell types (Maeno et al. 2006a,b). Hyperosmotic shrinkage does not yield consistent result in different cells. Again in a wide variety of cells apotosis is indeed triggered by hyperosmotic cell shrinkage (Lang & Hoffmann, 2012) and the ability to counteract cell shrinkage by regulatory volume increase (RVI) apparently determines the resistance to apoptosis (Bortner & Cidlowski, 2007; Numata et al. 2008). There are indeed well documented exceptions. Failure to undergo apoptosis upon cell shrinkage could in some cells be explained by RVI. It is mandatory that cells are kept shrunken during the entire experiment. In any case, excessive cell shrinkage activates proapoptotic signalling and inhibits survival signalling in the vast majority of cells (Hoffmann et al. 2009). Those signalling pathways are, however, not exclusively sensitive to cell volume but may be triggered by mechanisms other than shrinkage In conclusion, cell shrinkage resulting from opening of K+ and Cl− channels and inhibition of the regulatory volume increase mechanism seems to be, in the vast majority of cells and challenges, an essential step in the early apoptotic signalling. The involved signalling pathways can, however, under specific conditions in some cells be activated by other mechanisms thus rendering AVD dispensable.