Decreased phototoxicity of photodynamic therapy by Cx32/Cx26‐composed GJIC: A “Good Samaritan” effect

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 Ca 2+ concentrations were determined using flow cytometer. ELISA assay was performed to detect the intracellular levels of PGE 2 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 Ca 2+ and prostaglandin E 2 (PGE 2 ) 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.