|
Photodynamic therapy (PDT) leads to oxidative damage of cellular macromolecules, including numerous
proteins that undergo multiple modifications such as fragmentation, cross-linking and carbonylation that result
in protein unfolding and aggregation. Several mechanisms are involved in the protective responses to PDT
that include activation of transcription factors, heat shock proteins, antioxidant enzymes and antiapoptotic
pathways. Identification of these cytoprotective mechanisms might result in the design of more effective
combination strategies to improve the antitumor efficacy of PDT. By using various molecular biology
approaches, including microarray-based technologies we have identified genes that are up-regulated
following PDT. Subsequent experiments revealed that some of these gene products can become targets for
the combined therapeutic regimens encompassing PDT and selective small-molecule inhibitors. These
include superoxide dismutase (SOD-2), cyclooxygenase 2 (COX-2), heme oxygenase 1 (HO-1), and proteins
engaged in signaling endoplasmatic reticulum (ER) stress and unfolded protein response (UPR).
Since a major mechanism for elimination of carbonylated proteins is their degradation by
proteasomes, we hypothesized that a combination of PDT with proteasome inhibitors might lead to
accumulation of carbonylated proteins in ER, aggravated ER stress and potentiated cytotoxicity towards
tumor cells. Indeed, we observed that incubation of tumor cells with three different proteasome inhibitors,
including bortezomib, MG132 and PSI gave increased accumulation of carbonylated and ubiquitinated
proteins in PDT-treated cells. Proteasome inhibitors effectively sensitized tumor cells to PDT-mediated
cytotoxicity and augmented antitumor effects of PDT in vivo.
|
