Enhancement of lysosomal osmotic sensitivity induced by the photooxidation of membrane thiol groups

Photochemical Internalization for Intracellular Drug Delivery. From Basic Mechanisms to Clinical Research

Photochemical internalisation (PCI) is a unique intervention which involves the release of endocytosed macromolecules into the cytoplasmic matrix. PCI is based on the use of photosensitizers placed in endocytic vesicles that, following light activation, lead to rupture of the endocytic vesicles and the release of the macromolecules into the cytoplasmic matrix. This technology has been shown to improve the biological activity of a number of macromolecules that do not readily penetrate the plasma membrane, including type I ribosome-inactivating proteins (RIPs), gene-encoding plasmids, adenovirus and oligonucleotides and certain chemotherapeutics, such as bleomycin. This new intervention has also been found appealing for intracellular delivery of drugs incorporated into nanocarriers and for cancer vaccination. PCI is currently being evaluated in clinical trials. Data from the first-in-human phase I clinical trial as well as an update on the development of the PCI technology towards clinical practice is presented here.

Photo-oxidation of proteins

Photo-induced damage to proteins occurs via multiple pathways. Direct damage induced by UVB (λ 280-320 nm) and UVA radiation (λ 320-400 nm) is limited to a small number of amino acid residues, principally tryptophan (Trp), tyrosine (Tyr), histidine (His) and disulfide (cystine) residues, with this occurring via both excited state species and radicals. Indirect protein damage can occur via singlet oxygen ((1)O(2)(1)Δ(g)), with this resulting in damage to Trp, Tyr, His, cystine, cysteine (Cys) and methionine (Met) residues. Although initial damage is limited to these residues multiple secondary processes, that occur both during and after radiation exposure, can result in damage to other intra- and inter-molecular sites. Secondary damage can arise via radicals (e.g. Trp, Tyr and Cys radicals), from reactive intermediates generated by (1)O(2) (e.g. Trp, Tyr and His peroxides) and via molecular reactions of photo-products (e.g. reactive carbonyls). These processes can result in protein fragmentation, aggregation, altered physical and chemical properties (e.g. hydrophobicity and charge) and modulated biological turnover. Accumulating evidence implicates these events in cellular and tissue dysfunction (e.g. apoptosis, necrosis and altered cell signaling), and multiple human pathologies.

Mechanism of Lysophosphatidylcholine-Induced Lysosome Destabilization

Lysosomal destabilization is critical for the organelle and living cells. Phospholipase A(2 )(PLA(2)) was shown to be able to destabilize lysosomes under some conditions. By what mechanism the enzyme affects lysosomal stability is not fully studied. In this study, we investigated the effects of lysophosphatidylcholine (lysoPC), a PLA(2)-produced lipid metabolite, on lysosomal ion permeability, osmotic sensitivity and stability. By measuring lysosomal beta-hexosaminidase free activity, membrane potential, proton leakage and their enzyme latency loss in hypotonic sucrose medium, we established that lysoPC could increase the lysosomal permeability to both potassium ions and protons and enhance lysosomal osmotic sensitivity. These changes in lysosomal membrane properties promoted entry of potassium ions into lysosomes via K(+)/H(+) exchange. The resultant osmotic imbalance across the membranes led to losses of lysosomal integrity. The enhancement of lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in osmotic shock. These results suggest that lysoPC may play a key role in PLA(2)-induced lysosomal destabilization.

Effects of phospholipase A2 on the lysosomal ion permeability and osmotic sensitivity

In this study, we investigated the mechanism of PLA(2)-induced lysosomal destabilization. Through the measurements of lysosomal beta-hexosaminidase free activity, their membrane potential, the intra-lysosomal pH and the lysosomal latency loss in hypotonic sucrose medium, we established that PLA(2) could increase the lysosomal membrane permeability to both potassium ions and protons. The enzyme could also enhance the organelle osmotic sensitivity. The increases in the lysosomal ion permeability promoted influx of potassium ions into the lysosomes via K(+)/H(+) exchange. The resulted osmotic imbalance across the lysosomal membranes osmotically destabilized the lysosomes. In addition, the enhancement of the lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in the osmotic stress. The results explain how PLA(2) destabilized the lysosomes.

Mechanism of cytosol phospholipase C and sphingomyelinase-induced lysosome destabilization

Lysosomal disintegration may cause apoptosis, necrosis and some diseases. However, mechanisms for these events are still unclear. In this study, we measured lysosomal beta-hexosaminidase free activity, membrane potential and intralysosomal pH. The results revealed that the cytosolic extracts of rat hepatocytes could increase the lysosomal permeability to both potassium ions and protons, and osmotically destabilize lysosomes via K(+)/H(+) exchange. The effects of cytosol on lysosomes could be completely abolished by D609, which inhibited both phospholipase C and sphingomyelinase, and partly prevented by sphingomyelinase inhibitor Ara-AMP, but not by the inhibitors of PLA(2). Moreover, purified phospholipase C could destabilize the lysosomes while phospholipase A(2) and phospholipase D did not produce such effects. The cytosolic phospholipases hydrolyzed lysosomal membrane phospholipids by 50%, which could be prevented by D609. Disintegration of the cytosol-treated lysosomes biphasically depended on the cytosolic [Ca(2+)]. The cytosol did not disintegrate lysosomes below 100 nM or above 10 muM cytosolic [Ca(2+)], but markedly destabilized lysosomes at about 340 nM [Ca(2+)]. The results suggest that cytosolic phospholipase C and sphingomyelinase may be responsible for the alterations in lysosomal stability by increasing the ion permeability.

Effects of Arachidonic Acid on the Lysosomal Ion Permeability and Osmotic Stability

In this study, we investigated the effects of arachidonic acid, a PLA2-produced lipid metabolite, on the lysosomal permeability, osmotic sensitivity and stability. Through the measurements of lysosomal beta-hexosaminidase free activity, membrane potential, intralysosomal pH, and lysosomal latency loss in hypotonic sucrose medium, we established that arachidonic acid could increase the lysosomal permeability to both potassium ions and protons, and enhance the lysosomal osmotic sensitivity. As a result, the fatty-acid-promoted entry of potassium ions into the lysosomes via K+/H+ exchange, which could produce osmotic imbalance across their membranes and osmotically destabilize the lysosomes. In addition, the enhancement of lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in osmotic shock. The results suggest that arachidonic acid may play a role in the lysosomal destabilization.

Lysosome destabilization by cytosolic extracts, putative involvement of Ca(2+)/phospholipase C

Lysosomal disintegration is a crucial event for living cells, but mechanisms for the event are still unclear. In this study, we established that the cytosolic extracts could enhance lysosomal osmotic sensitivity and osmotically destabilize the lysosomes. The cytosol also caused the lysosomes to become more swollen in the hypotonic sucrose medium. The results indicate that the cytosol induced an osmotic shock to the lysosomes and an influx of water into the organelle. Since the effects of cytosol on the lysosomes could be abolished by O-tricyclo[5.2.1.0(2,6)]dec-9-yl dithiocarbonate potassium salt (D609), a specific inhibitor of cytosolic phospholipase C (PLC), the PLC might play an important role in the lysosomal osmotic destabilization. The activity of cytosolic PLC and the extent of enzyme latency loss of the cytosol-treated lysosomes exhibited a similar biphasic dependence on the cytosolic Ca2+ concentration. In addition, the cytosol did not osmotically destabilize the lysosomes until the cytosolic calcium ions rose above 100 nM. It suggests that the destabilization effect of cytosol on the lysosomes is Ca(2+)-dependent.

Enhancement of lysosomal proton permeability induced by photooxidation of membrane thiol groups

Effects of photooxidation of membrane thiol groups on lysosomal proton permeability were studied by measuring intralysosomal pH with fluorescein isothiocyanate-dextran and monitoring proton leakage with p-nitrophenol. Methylene blue-mediated photooxidation of lysosomes decreased their membrane thiol groups and produced cross-linking of the membrane proteins, which was established by the measurement of residual membrane thiol groups with 5,5'-dithio-bis(2-nitrobenzoic acid) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, respectively. The cross-linking of proteins could be abolished by subsequent treatment of the photodamaged lysosomes with dithiothreitol, indicating that the proteins were linked via disulfide bonds. In addition, the photodamage of lysosomes raised the intralysosomal pH and caused leakage of the lysosomal protons, which could also be reversed by subsequent dithiothreitol treatment. This indicates that lysosomal proton permeability can be increased by photooxidation of the membrane thiol groups and recovered to the normal level by reduction of the groups.