The influence of photodynamic therapy with 5-aminolevulinic acid on senescent skin cancer cells.

Background: Senescent cells, which are resistant to apoptosis, accumulate with age and after ultraviolet (UV) radiation, chemotherapy and radiation therapy. Preventing or eliminating senescent cells may be crucial for protection against skin cancer development and improving tumour treatment. The aim of the present study was to investigate the potential of photodynamic therapy (PDT) with 5-aminolevulinic acid (ALA) to induce senescence in skin cancer cells and to eliminate senescent cells induced by chemotherapy (bleomycin) or UVA (315-400nm) exposure.

Methods: WM115 and A431 cells were incubated with 1mM ALA for 2 and 4h, respectively, before exposure to blue light (10mW/cm, 0-80s, 0-0.8J/cm). WM115 cells were treated once with 106J/cm (58.4mW/cm, 30.25min) UVA 6days before ALA-PDT or with 0.24IU/ml bleomycin for 7days to induce senescence before ALA-PDT. Cell viability was monitored by the MTT colorimetric assay. Senescent cells were detected using senescence-associated-beta-galactosidase (SA-β-Gal) staining and morphological changes (enlarged, flat cells).

Results: ALA-PDT caused a light dose dependent increase in senescence. ALA-PDT induced senescence very effectively only in WM115 cells but not in A431 cells, while similar cytotoxic effects were observed in both cell lines. After ALA-PDT with 0.4J/cm around 70% of survived WM115 cells were senescent, while only around 5% of A431 cells were senescent after ALA-PDT with 0.8J/cm.

Conclusion: ALA-PDT can induce premature senescence and kill senescent cells induced by ALA-PDT itself, UVA and chemotherapy (bleomycin). Light doses must be properly chosen to photoinactivate ALA-PDT-induced senescent cells.