Bystander effects of nitric oxide in anti-tumor photodynamic therapy

Abstract

Ionizing radiation of specifically targeted cells in a given population is known to elicit pro-death or pro-survival responses in non-targeted bystander cells, which often make no physical contact with the targeted ones. We have recently demonstrated a similar phenomenon for non-ionizing photodynamic therapy (PDT), showing that prostate cancer cells subjected to targeted photodynamic stress stimulated growth and migration of non-stressed, non-contacting bystander cells. Diffusible nitric oxide (NO) generated by stress-upregulated inducible nitric oxide synthase (iNOS) was shown to play a dominant role in these responses. Moreover, target-derived NO stimulated iNOS/NO induction in bystanders, suggesting a NO-mediated feed-forward field effect driven by targeted cells surviving the photodynamic challenge. In this research highlight, we will review these findings and discuss their potential negative implications on clinical PDT outcomes and how these might be mitigated through pharmacologic use of select iNOS inhibitors.

Keywords: Bystander effects; iNOS; iNOS inhibitors; nitric oxide; photodynamic therapy.

Figure 1. Effect of siRNA-induced iNOS knockdown in target cells on bystander cell proliferation

Prostate cancer PC3 cells were used, the targeted ones being dark-incubated with 1 mM ALA for 30 min in serum-free medium. After irradiation (light fluence ~1 J/cm²), cells were washed free of ALA and switched to serum-containing medium, after which the separating rings were removed. Bystander cell proliferation was then monitored over 48 h of dark incubation, cell counts being obtained by Image-J analysis of photomicrographs. Data from iNOS-kd cells and scrambled vector controls (Scr) are compared; values are means ± SEM (n=12). *P<0.005 vs. Scr; *P<0.002 vs. Scr. Adapted from Figure 9 in Ref. .

Figure 2. Schematic depicting bystander cell responses to NO generated by photodynamically stressed target cells

Both populations are irradiated, but only target cells sensitized with ALA-induced PpIX in mitochondria experience ¹O₂-mediated oxidative stress leading to iNOS/NO upregulation. The NO increases target cell resistance to apoptosis and surviving cells, as well as NO-stimulated bystanders, grow and migrate more aggressively.