Normotonic cell shrinkage because of disordered volume regulation is an early prerequisite to apoptosis

Abstract

A major hallmark of apoptosis is normotonic shrinkage of cells. Here, we studied the relation between apoptotic cell shrinkage and apoptotic cell death. Induction of the apoptotic volume decrease (AVD) under normotonic conditions was found to be coupled to facilitation of the regulatory volume decrease (RVD), which is known to be attained by parallel operation of Cl ؊ and K ؉ channels, under hypotonic conditions. Both the AVD induction and the RVD facilitation were found to precede cytochrome c release, caspase-3 activation, DNA laddering, and ultrastructural alterations in three cell types after apoptotic insults with two distinct apoptosis inducers. Also, the AVD was not prevented by a broad-spectrum caspase inhibitor. When the AVD induction and the RVD facilitation were prevented by blocking volume-regulatory Cl ؊ or K ؉ channels, these cells did not show succeeding apoptotic biochemical and morphological events and were rescued from death. Thus, it is concluded that the AVD, which is caused by disordered cell volume regulation, is an early prerequisite to apoptotic events leading to cell death.

Intracellular K+ inhibits apoptosis by suppressing the Apaf-1 apoptosome formation and subsequent downstream pathways but not cytochrome c release

“…Our observation of cytochrome c release even in high K þ medium could not agree with earlier reports that insisted elevated K þ inhibits apoptosis before mitochondrial perturbation. 16,17 This variation might have arisen due to the existence of multiple pathways of cytochrome c release governed by the nature of apoptotic stimuli and cell type. 23 A recent study has shown that 80 mM extracellular K þ markedly attenuated though never completely prevented the cytochrome c release from mitochondria in human neutrophils.…”

“…The results are inconsistent with others, which propose that blockage of K þ efflux also suppresses cytochrome c release. 16,17 The reasons for this discrepancy are presently unclear. A huge variation regarding the effect of K þ efflux on apoptosis depending upon cell type and nature of apoptotic insult 30 and similar diversities in the pathways of cytochrome c release might have contributed to these contrasting findings.…”

“…16 And other line of study demonstrated that Cl À and K þ channel blockers prevented early-phase apoptotic volume decrease (AVD) and staurosporine (STS)-induced cytochrome c release. 17 Both of these reports suggested the targets for anti-apoptotic effect of K þ to be upstream of the mitochondria. However, some studies showed that Z-VAD-FMK blocks both AVD and apoptosis in anti-Fas antibodytreated Jurkat cells, highlighting the role of caspases in the process.…”

Opening of plasma membrane voltage-dependent anion channels (VDAC) precedes caspase activation in neuronal apoptosis induced by toxic stimuli

“…1 An early morphological alteration occurring during apoptosis is cell shrinkage, which is associated with an increased cellular efflux of K þ and Cl À ions. [2][3][4][5][6][7][8] The reduced intracellular K þ concentration also facilitates the activation of apoptosisrelated proteases -the caspases. 9,10 Conversely, blocking K þ or Cl À channels prevents cell shrinkage and cell death.…”

“…9,10 Conversely, blocking K þ or Cl À channels prevents cell shrinkage and cell death. [5][6][7][8] While the K þ efflux depends on an increased number of active K þ channels in the plasma membrane, less is known about the Cl À efflux.…”