Increased cytotoxic effects of photodynamic therapy in IL-6 gene transfected cells via enhanced apoptosis

PDT has been reported to induce cancer cell expression of cytokines, such a s IL-6 and TNF-alpha, but it has been unclear whether cytokine expression b y cancer cells is directly related to the antitumor effect of PDT. We treat ed Lewis lung carcinoma (LLC) cells with a new photosensitizer, mono-L-aspa rtyl chlorin e6 (NPe6) and light from a diode laser and found that expressi on of the mRNA of IL-2, IL-6, and TNF-alpha was increased by NPe6-mediated- PDT 6 hr later. To elucidate the mechanism of the direct anti-tumor effect of cytokine expression, we examined the photosensitivity of cytokine-gene-t ransfected cells, namely LLC-IL-2, LLC-IL-6, and LLC-TNF-alpha cells, by MT T assay. The IL-6 gene transfected, LLC-IL-6 cells were significantly more sensitive to cytotoxic effects than the parent LLC cells and other cytokine gene-transfected cells. This Finding indicates that IL-6 expression modula tes cellular sensitivity to PDT and that IL-2 and TNF-alpha expressions doe s not, In addition, the apoptosis of LLC-IL-6 cells induced by NPe6-PDT was greater than in the other cells as determined by DNA fragmentation and sta ining of apoptotic nuclei. Because IL-6 has been reported to induce apoptos is by downregulating expression of Bcl-2, we analyzed the expression of apo ptosis-related Bcl-2, Bax, and cytochrome C by Western blot analysis. Decre ased expression of Bcl-2 and cytochrome C was observed in both LLC calls an d LLC-IL-6 cells. Bax protein increased in a time-dependent manner, and the ratio of Bax to Bcl-2 rose markedly after PDT in LLC-IL-6 cells, These res ults suggest that the increased sensitivity of LLC-IL-6 cells to PDT-induce d cytotoxicity results from the high ratio of Bax to Bcl-2 in the IL-6-depe ndent apoptotic pathway, In conclusion, IL-6 expression plays a role in cel lular sensitivity to PDT, and combination of IL-6 and PDT may provide a new strategy for cancer treatment. (C) 2001 Wiley-Liss, Inc.