Dual roles of nitric oxide in the regulation of tumor cell response and resistance to photodynamic therapy

Abstract

Photodynamic therapy (PDT) against cancer has gained attention due to the successful outcome in some cancers, particularly those on the skin. However, there have been limitations to PDT applications in deep cancers and, occasionally, PDT treatment resulted in tumor recurrence. A better understanding of the underlying molecular mechanisms of PDT-induced cytotoxicity and cytoprotection should facilitate the development of better approaches to inhibit the cytoprotective effects and also augment PDT-mediated cytotoxicity. PDT treatment results in the induction of iNOS/NO in both the tumor and the microenvironment. The role of NO in cytotoxicity and cytoprotection was examined. The findings revealed that NO mediates its effects by interfering with a dysregulated pro-survival/anti-apoptotic NF-κB/Snail/YY1/RKIP loop which is often expressed in cancer cells. The cytoprotective effect of PDT-induced NO was the result of low levels of NO that activates the pro-survival/anti-apoptotic NF-κB, Snail, and YY1 and inhibits the anti-survival/pro-apoptotic and metastasis suppressor RKIP. In contrast, PDT-induced high levels of NO result in the inhibition of NF-kB, Snail, and YY1 and the induction of RKIP, all of which result in significant anti-tumor cytotoxicity. The direct role of PDT-induced NO effects was corroborated by the use of the NO inhibitor, l-NAME, which reversed the PDT-mediated cytotoxic and cytoprotective effects. In addition, the combination of the NO donor, DETANONOate, and PDT potentiated the PDT-mediated cytotoxic effects. These findings revealed a new mechanism of PDT-induced NO effects and suggested the potential therapeutic application of the combination of NO donors/iNOS inducers and PDT in the treatment of various cancers. In addition, the study suggested that the combination of PDT with subtoxic cytotoxic drugs will result in significant synergy since NO has been shown to be a significant chemo-immunosensitizing agent to apoptosis.

The Antitumor Effect of Singlet Oxygen

“…Importantly, apoptosis induction by photodynamic therapy was inhibited in their experiments when the activity of NOS was blocked (54,55). Therefore, it is obvious that under the conditions of the test system described by Rapozzi et al, the concentration of singlet oxygen was not sufficiently high to cause direct apoptosis induction.…”

“…It is, therefore, attractive to speculate that induction of NOS by photodynamic therapy might have caused a sufficiently high concentration of NO for the transient inhibition of membrane-associated catalase, in analogy to the induction by the FAS system (6) or by interferons (52). As a result, the generation of secondary singlet oxygen, FAS receptor involvement, catalase inactivation and reactivation of intercellular ROS/RNS signalling might have been the dominant cause of apoptosis induction, in addition to the other effects of NO characterized by Rapozzi et al (54), Thus the selective uptake of the photosensitizer might have led to NO-mediated catalase inhibition, followed by selective inactivation of catalase by secondary singlet oxygen and subsequent ROS/RNS-dependent apoptosis-inducing signaling.…”

“…Rather, due to the established method and the chemical nature of photosensitizers presently used, classical photodynamic therapy is believed to be based on preferential uptake of photosensitizers by tumor tissue compared to nonmalignant tissue (53,54). At the time of photoactivation…”

Modulation of the Anti-Tumor Efficacy of Photodynamic Therapy by Nitric Oxide

“…For example, Rapozzi et al [70,71,72] reported that endogenous iNOS/NO can modulate pheophorbide a-sensitized photokilling of melanoma cells in distinct ways, depending on the intensity of photodynamic action. Thus, low level NO induced by modest photooxidative pressure signaled for cytoprotection via upregulation of anti-apoptotic NF-κB and Snail (zinc finger protein SNAI1), but down-regulation of RKIP (Raf kinase inhibitor protein).…”

“…Thus, low level NO induced by modest photooxidative pressure signaled for cytoprotection via upregulation of anti-apoptotic NF-κB and Snail (zinc finger protein SNAI1), but down-regulation of RKIP (Raf kinase inhibitor protein). In contrast, a higher level of NO induced by greater photooxidative pressure signaled for less protection (greater apoptosis) via down-regulation of NF-κB and Snail, but upregulation of RKIP [70,71,72]. Therefore, a pro- vs. anti-PDT signaling role for endogenous NO was demonstrated based on the level of photodynamic pressure applied.…”