Chemotherapy drug response in ovarian cancer cells strictly depends on a cathepsin D-Bax activation loop.

The ovarian cancer cell lines A2780 (wild-type p53) and NIHOVCAR3 (mutated p53) showed, respectively, sensitivity and resistance towards several chemotherapy drugs. We hypothesized that the two cell lines differ in their ability to activate the intrinsic death pathway and have, therefore, dissected the lysosome-mitochondrion signalling pathway by pharmacological inhibition or genetic manipulation of key regulators and executioners. Biochemical and morphological confocal fluorescence studies showed that: (1) In A2780 cells bcl-2 is expressed at an undetectable level, whereas Bax is expressed at a rather high level; by contrast, bcl-2 is highly expressed and Bax is expressed at extremely low levels in NIHOVCAR3 cells; (2) Chemotherapy treatment reduced the expression of bcl-2 in NIHOVCAR3 cells, yet these cells resisted to drug toxicity; (3) Cathepsin D (CD), not cathepsin B or L, mediates the activation of the mitochondrial intrinsic death pathway in A2780 cells; (4) Lysosome leakage and cytosolic relocation of CD occurs in the chemosensitive A2780 cells, not in the chemoresistant NIHOVCAR3 cells; (5) Bax is essential for the permeabilization of both lysosomes and mitochondria in A2780 cells exposed to chemotherapy drugs; (6) CD activity is mandatory for the oligomerization of Bax on both mitochondrial and lysosomal membranes; (7) Bax activation did not occur in the resistant NIHOVCAR3 cells despite their high content in CD. The present data are consistent with a model in which on treatment with a cytotoxic drug the activation of a CD-Bax loop leads to the generalized permeabilization of lysosomes and eventually of mitochondria, thus reaching the point of no return, and culminates with the activation of the caspase cascade. Our data also imply that dysfunctional permeabilization of lysosomes contributes to the development of chemoresistance.

Resveratrol-induced apoptosis depends on the lipid kinase activity of Vps34 and on the formation of autophagolysosomes.

In human colorectal DLD1 cancer cells, the dietary bioflavonoid resveratrol (RV) rapidly induced autophagy. This effect was reversible (on removal of the drug) and was associated with increased expression and cytosolic redistribution of the proteins Beclin1 and LC3 II. Supplementing the cells with asparagine (Asn) abrogated the Beclin-dependent autophagy. When applied acutely (2 h), RV was not toxic; however, reiterate chronic (48 h) exposure to RV eventually led to annexin V- and terminal deoxinucleotidyl transferase-mediated dUTP-biotin nick end labeling-positive cell death. This toxic effect was autophagy dependent, as it was prevented either by Asn, by expressing a dominant-negative lipid kinase-deficient class III phosphoinositide 3-phosphate kinase, or by RNA interference knockdown of Beclin1. Lamp2b silencing abolished the fusion of autophagosomes with lysosomes and preserved cell viability despite the ongoing formation of autophagosomes in cells chronically exposed to RV. The pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone inhibited RV-induced cell death, but not autophagy. These results uncover a novel pathway of RV cytotoxicity in which autophagy plays a dual role: (i) at first, it acts as a prosurvival stress response and (ii) at a later time, it switches to a caspase-dependent apoptosis pathway. The present data also indicate that genetic or epigenetic inactivation of autophagy proteins in cancer cells may confer resistance to RV-mediated killing.

Cathepsin D-Bax death pathway in oxidative stressed neuroblastoma cells.

Hydrogen peroxide, the major oxidoradical species in the central nervous system, has been involved in neuronal cell death and associated neurodegenerative diseases. In this study, we have investigated the involvement of the lysosomal pathway in the cytotoxic mechanism of hydrogen peroxide in human neuroblastoma cells. Alteration of lysosomal and mitochondrial membrane integrity was shown to be an early event in the lethal cascade triggered by oxidative stress. Desferrioxamine (DFO), an iron chelator that abolishes the formation of reactive oxygen species within lysosomes, prevented lysosome leakage, mitochondrial permeabilization and caspase-dependent apoptosis in hydrogen peroxide-treated cells. Inhibition of cathepsin D, not of cathepsin B, as well as small-interference RNA-mediated silencing of the cathepsin D gene prevented hydrogen peroxide-induced injury of mitochondria, caspase activation, and TUNEL-positive cell death. Cathepsin D activity was shown indispensable for translocation of Bax onto mitochondrial membrane associated with oxidative stress. DFO abolished both the cytosolic relocation of Cathepsin D and the mitochondrial relocation of Bax in hydrogen peroxide-treated cells. siRNA-mediated down-regulation of Bax expression protected the cells from oxidoradical injury. The present study identifies the lysosome as the primary target and the axis cathepsin D-Bax as the effective pathway of hydrogen peroxide lethal activity in neuroblastoma cells.

Folding, activity and targeting of mutated human cathepsin D that cannot be processed into the double-chain form.

The precursor of human cathepsin D (CD) is converted into the single-chain and the double-chain active polypeptides by subsequent proteolysis reactions taking place in the endosomal-lysosomal compartment and involving specific aminoacid sequences. We have mutagenized the region of aminoacids (comprising the beta-hairpin loop) involved in the latter proteolytic maturation step and generated a mutant CD that cannot be converted into the mature double-chain form. This mutant CD expressed in rodent cells reaches the lysosome and is stable as single-chain polypeptide, bears high-mannose type sugars, binds to pepstatin A and is enzymatically active, indicating that it is correctly folded. The present work provides new insights on the aminoacid region involved in the terminal processing of human CD and on the function of the processing beta-hairpin loop.

Resveratrol induces cell death in colorectal cancer cells by a novel pathway involving lysosomal cathepsin D.

In human colorectal cancer cells, the polyphenol resveratrol (RV) activated the caspase-dependent intrinsic pathway of apoptosis. This effect was not mediated via estrogen receptors. Pepstatin A, an inhibitor of lysosomal cathepsin D (CD), not (2S,3S)-trans-epoxysuccinyl-L-leucylamido-3-methylbutane ethyl ester, an inhibitor of cathepsins B and L, prevented RV cytotoxicity. Similar protection was attained by small interference RNA-mediated knockdown of CD protein expression. RV promoted the accumulation of mature CD, induced lysosome leakage and increased cytosolic immunoreactivity of CD. Inhibition of CD or its post-transcriptional down-regulation precluded Bax oligomerization, permeabilization of mitochondrial membrane, cytosolic translocation of cytochrome c, caspase 3 activation and terminal deoxinucleotidyl transferase-mediated dUTP-biotin nick end labeling positivity occurring in RV-treated cells. The present study identifies the lysosome as a novel target of RV activity and demonstrates a hierarchy of the proteolytic pathways involved in its cytotoxic mechanism in which the lysosomal CD acts upstream of the cytosolic caspase activation. Our data indicate that metabolic, pharmacologic or genetic conditions affecting CD expression and/or activity could reflect on the sensitivity of cancer cells to RV.