ABSTRACT
BACKGROUND: Male transgenic mice expressing the human RAS gene on an FVB strain background develop adenocarcinoma of the breast between 7 and 8 weeks of age. We have utilized this mammary tumour model to investigate apoptotic responses following photodynamic therapy (PDT) with a chlorin-based, water-soluble photosensitizer. METHODS: Detection of apoptosis was accomplished by use of the antibody M30 against a neo-epitope of caspase-cleaved cytokeratin 18 that becomes available at an early stage of the apoptotic cascade. Mice bearing multiple tumours were injected with the photosensitizer intraperitoneally, and following a drug-light interval of 96h, 40J/cm(2) of 652nm laser light was applied to one tumour per animal, while the other tumours were protected from light to serve as host controls. The M30 antibody was used for standard immunohistochemistry of tumour sections and flow cytometric detection of epitope expression coupled to cell cycle analysis in tumour cell populations retrieved from paraffin blocks. RESULTS: M30 staining was significantly increased within 2h following light treatment and persisted until 96h after treatment. Flow cytometric analysis for the S-phase fraction (SPF) of tumour cells post-PDT showed a substantial decrease in SPF at 2h post PDT, and recovery of SPF within 96h. CONCLUSIONS: Cytokeratin 18 cleavage seems to be both an early and ongoing event during the cellular response to PDT. Calculating the M30/SPF ratio at both 2h and 96h suggested distinct cellular dynamics at early and late time points, and we propose the M30/SPF ratio as a tumour dynamic index (TDI) to monitor events post PDT.
ABSTRACT
Monoclonal antibodies that specifically recognize caspase cleaved K18 fragments or specific (phospho)epitopes on intact K8 and K18 were used for a detailed investigation of the temporal and causal relationship of proteolysis and phosphorylation in the collapse of the keratin cytoskeleton during apoptosis. Caspases involved in the specific proteolysis of keratins were analyzed biochemically using recombinant caspases and specific caspase inhibitors. Finally, the fate of the keratin aggregates was analyzed using the M30-ApoptoSense trade mark Elisa kit to measure shedding of caspase cleaved fragments into the supernatant of apoptotic cell cultures. From our studies, we conclude that C-terminal K18 cleavage at the (393)DALD/S site is an early event during apoptosis for which caspase 9 is responsible, both directly and indirectly by activating downstream caspases 3 and 7. Cleavage of the L1-2 linker region of the central alpha-helical rod domain is responsible for the final collapse of the keratin scaffold into large aggregates. Phosphorylation facilitates formation of these aggregates, but is not crucial. K8 and K18 remain associated in heteropolymeric aggregates during apoptosis. At later stages of the apoptotic process, that is, when the integrity of the cytoplasmic membrane becomes compromised, keratin aggregates are shed from the cells.
