Conclusions
Combination of PDT and low-toxicity drugs (such as aspirin) resulted in protracted inhibition of proteasome activity and induced apoptosis even in apoptosis-resistant cancer cells.
Methods
Cells were irradiated at doses ranging from 0.54 to 1.10 J cm(-2), and subsequently were incubated with aspirin at either high (10 and 5 mm) or low concentration (2.5 and 1.5 mm). Photofrin concentration and incubation time were constant (2.5 μg/ml and 16 h). Under these conditions, we analysed cell viability, colony-forming efficiency, cycle profile, expression patterns of specific proteins and ubiquitination state, after individual or combined administration.
Results
Treatment with either PDT or aspirin, rapidly induced proteasome malfunction and accumulation of cells in G(2)M, but did not induce apoptosis. However, when aspirin was added to cells (even at low concentrations) after PDT, the proteasome block was sustained. Moreover, significant cytotoxic effects, including apoptosis, were observed along with cytostatic effects (G(2)M accumulation/decreased colony formation). Conclusions: Combination of PDT and low-toxicity drugs (such as aspirin) resulted in protracted inhibition of proteasome activity and induced apoptosis even in apoptosis-resistant cancer cells.