No abstract

Apoptosis is a cell-autonomous mode of death that is activated to eradicate superfluous, damaged, mutated, or aged cells. In addition to their role as the cell's powerhouse, mitochondria play a central role in the control of apoptosis. Thus, numerous pro-apoptotic molecules act on mitochondria and provoke the permeabilization of mitochondrial membranes. Soluble proteins contained in the mitochondrial intermembrane space are released through the outer membrane and participate in the organized destruction of the cell. Several among these lethal proteins can activate caspases, a class of cysteine proteases specifically activated in apoptosis, whereas others act in a caspase-independent fashion, by acting as nucleases (e.g., endonuclease G), nuclease activators (e.g., apoptosis-inducing factor), or serine proteases (e.g., Omi/HtrA2). In addition, mitochondria can generate reactive oxygen species, following uncoupling and/or inhibition of the respiratory chain. The diversity of mitochondrial factors participating in apoptosis emphasizes the central role of these organelles in apoptosis control and unravels novel mechanisms of cell death execution.

Hydroxychloroquine (HCQ) is a lysosomotropic amine with cytotoxic properties. Here, we show that HCQ induces signs of lysosomal membrane permeabilization (LMP), such as the decrease in the lysosomal pH gradient and the release of cathepsin B from the lysosomal lumen, followed by signs of apoptosis including caspase activation, phosphatidylserine exposure, and chromatin condensation with DNA loss. HCQ also induces mitochondrial membrane permeabilization (MMP), as indicated by the insertion of Bax into mitochondrial membranes, the conformational activation of Bax within mitochondria, the release of cytochrome c from mitochondria, and the loss of the mitochondrial transmembrane potential. To determine the molecular order among these events, we introduced inhibitors of LMP (bafilomycin A 1 ), MMP (Bcl-X L , wild-type Bcl-2, mitochondrion-targeted Bcl-2, or viral mitochondrial inhibitor of apoptosis from cytomegalovirus), and caspases (Z-VAD.fmk) into the system. Our data indicate that caspase-independent MMP is rate-limiting for LMP-mediated caspase activation. Mouse embryonic fibroblasts lacking the expression of both Bax and Bak are resistant against hydroxychloroquine-induced apoptosis. Such Bax À/À Bak À/À cells manifest normal LMP, yet fail to undergo MMP and subsequent cell death. The data reported herein indicate that LMP does not suffice to trigger caspase activation and that Bax/Bak-dependent MMP is a critical step of LMP-induced cell death.

The cyclin-dependent kinase 1 (Cdk1), formerly called Cdc2 (or p34 Cdc2 ), interacts with cyclin B1 to form an active heterodimer. The activity of Cdk1 is subjected to a complex spatiotemporary regulation, required to guarantee its scheduled contribution to the mitotic prophase and metaphase. Moreover, the activation of Cdk1 may be required for apoptosis induction in some particular pathways of cell killing. This applies to several clinically important settings, for instance to paclitaxel-induced killing of breast cancer cells, in which the ErbB2 receptor kinase can mediate apoptosis inhibition through inactivation of Cdk1. The activation of Cdk1 participates also in HIV-1-induced apoptosis, upstream of the p53-dependent mitochondrial permeabilization step. An unscheduled Cdk1 activation may contribute to neuronal apoptosis occurring in neurodegenerative diseases. Finally, the premature activation of Cdk1 can lead to mitotic catastrophe, for instance after irradiation-induced DNA damage. Thus, a cell type-specific modulation of Cdk1 might be taken advantage of for the therapeutic correction of pathogenic imbalances in apoptosis control.