In spite of initially promising responses, 5-year recurrence after photodynamic therapy (PDT) sustains high level and an increase in PDT effectiveness is needed. It has been demonstrated that gap junctional intercellular communication (GJIC) formed by Connexin (Cx)43 could improve the transfer of “death signal” between cells, thereby causing the enhancement of cytotoxicity of chemotherapeutics and suicide gene therapy. Nevertheless, whether Cx43-composed GJIC has an effect on PDT phototoxicity remains unknown. This study showed that Cx43-formed GJIC could improve PDT phototoxicity to tumor cells in vitro and in vivo. Specifically, Cx43-formed GJIC under the condition of high cellular density could improve PDT phototoxicity in Cx43-transfected HeLa cells and Cx43-expressing U87 glioma cells. This effect was remarkably inhibited when Cx43 was not expressed or Cx43-formed GJ channels were prohibited. Additionally, the presence of Cx43-mediated GJIC could decrease the mean RTV and tumor weights of xenografts after Photofrin-PDT. The improved PDT efficacy by Cx43-composed GJIC was correlated with stress signaling pathways mediated by ROS, calcium and lipid peroxide. The present study demonstrates the presence of Cx43-composed GJIC improves PDT phototoxicity and suggests that therapeutic strategies designed to upregulate the expression of Cx43 or enhance Cx43-mediated GJIC function may increase the sensitivity of malignant cell to PDT, leading to the increment of PDT efficacy. Oppositely, factors that retard Cx43 expression or prohibit the function of Cx43-mediated GJIC may cause insensitivity of malignant cells to PDT, leading to PDT resistance.

Background and Objective Photodynamic therapy (PDT) has been widely used to treat malignant tumors. Our previous studies indicated that connexin (Cx) 32‐ and Cx26‐composed gap junctional intercellular communication (GJIC) could improve the phototoxicity of PDT. However, the role of heterotypic Cx32/Cx26‐formed GJIC in PDT phototoxicity is still unknown. Thus, the present study was aimed to investigate the effect of Cx32/Cx26‐formed GJIC on PDT efficacy. Methods CCK8 assay was used to detect cell survival after PDT. Western blot assay was utilized to detect Cx32/Cx26 expression. “Parachute” dye‐coupling assay was performed to measure the function of GJ channels. The intracellular Ca2+ concentrations were determined using flow cytometer. ELISA assay was performed to detect the intracellular levels of PGE2 and cAMP. Results The present study demonstrates there is a Cx32/Cx26‐formed GJIC‐dependent reduction of phototoxicity when cells were exposure to low concentration of Photofrin. Such a protective action is missing at low cell density due to the lack of GJ coupling. Under high‐cell density condition, where there is opportunity for the cells to contact each other and form GJ, suppressing Cx32/Cx26‐formed GJIC by either inhibiting the expression of Cx32/Cx26 or pretreating with GJ channel inhibitor augments PDT phototoxicity after cells were treated with at 2.5 µg/ml Photofrin. The above results suggest that at low Photofrin concentration, the presence of Cx32/Cx26‐formed GJIC may decrease the phototoxicity of PDT, leading to the insensitivity of malignant cells to PDT treatment. The GJIC‐mediated PDT insensitivity was associated with Ca2+ and prostaglandin E2 (PGE2) signaling pathways. Conclusion The present study provides a cautionary note that for tumors expressing Cx32/Cx26, the presence of Cx32/Cx26‐composed GJIC may cause the resistance of tumor cells to PDT. Oppositely, treatment strategies designed to downregulate the expression of Cx32/Cx26 or restrain the function of Cx32/Cx26‐mediated GJIC may increase the sensitivity of malignant cell to PDT. Lasers Surg. Med. 51:301–308, 2019. © 2019 Wiley Periodicals, Inc.

Despite responses to initial treatment of photodynamic therapy (PDT) being promising, a recurrence rate exists. Thus, finding novel therapeutic targets to enhance PDT efficacy is an urgent need. Reports indicate that connexin (Cx) 40 plays an important role in tumor angiogenesis and growth. However, it is unknown whether Cx40-composed channels have effects on PDT efficacy. The study uniquely demonstrated that Cx40-formed channels could enhance the phototoxicity of PDT to malignant cells in vitro and in vivo . Specifically, Cx40-formed channels at high cell density could increase PDT photocytotoxicity. This action was substantially restricted when Cx40 expression was not induced or Cx40 channels were restrained. Additionally, the presence of Cx40-composed channels enhanced the phototoxicity of PDT in the tumor xenografts. The above results indicate that enhancing the function of Cx40-formed channels increases PDT efficacy. The enhancement of PDT efficacy mediated by Cx40 channels was related with intracellular pathways mediated by ROS and calcium pathways, but not the lipid peroxide-mediated pathway. This work demonstrates the capacity of Cx40-mediated channels to increase PDT efficacy and suggests that therapeutic strategies designed to maintain or enhance Cx40 expression and/or channels composed by Cx40 may increase the therapeutic efficacy of PDT.

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