Reduction of cancer cell viability by synergistic combination of photodynamic treatment with the inhibition of the Id protein family.

The inhibitor of DNA binding and cell differentiation (Id) proteins are dominant negative regulators of the helix-loop-helix transcription factor family and play a key role during development as well as in vascular disorders and cancer. In fact, impairing the Id-protein activity in cancer cells reduces cell growth and even chemoresistance. Recently, we have shown that a synthetic Id-protein ligand (1Y) consisting of a cyclic nonapeptide can reduce the viability of the two breast cancer cell lines MCF-7 and T47D and of the bladder cancer cells T24 to about 50% at concentrations ≥100μM. Moreover, the cyclopeptide displays both proapoptotic and antiproliferative effects on MCF-7 cells. Herein, we show that the cyclopeptide does not induce cell death at the dose of 5μΜ, but it still inhibits MCF-7 and T24 cell proliferation, which correlates with an increased protein level of the cell-cycle regulator p27Kip1 . Furthermore, 1Y-pretreated MCF-7, T47D, and T24 cells are more susceptible than untreated cells to the phototoxic effects of the three photosensitizers meta-tetra(hydroxyphenyl)chlorin, porfimer sodium, and hypericin, which are applied in photodynamic therapy (PDT). The combination of the Id-protein ligand with each of the light-activated photosensitizers shows synergistic effects on the reduction of cell viability. In conclusion, an Id-protein ligand with moderate cancer cell killing activity at concentrations ≥100μM can be applied at a 20-fold lower and barely toxic dose to raise the sensitivity of cancer cells towards phototoxicity associated with photodynamic treatment. This suggests the potential benefit of targeting the Id proteins in combined drug approaches for cancer therapy.