Critical role of gap junction communication, calcium and nitric oxide signaling in bystander responses to focal photodynamic injury

// Bianca Calì 1,2 , Stefano Ceolin 3 , Federico Ceriani 1,3 , Mario Bortolozzi 1,3 , Andrielly H.R. Agnellini 1,2 , Veronica Zorzi 1,3 , Andrea Predonzani 4 , Vincenzo Bronte 1,5 , Barbara Molon 4,* and Fabio Mammano 1,3,6,* 1 Foundation for Advanced Biomedical Research, Venetian Institute of Molecular Medicine, Padua, Italy 2 University of Padua, Department of Surgery Oncology and Gastroenterology, Oncology and Immunology Section, Padua, Italy 3 University of Padua, Department of Physics and Astronomy, Padua, Italy 4 IRCCS, Istituto Oncologico Veneto, Padua, Italy 5 Verona University Hospital, Department of Pathology and Diagnostics, Immunology Section, Verona, Italy 6 Present address: CNR, Institute of Cell Biology and Neurobiology, Monterotondo (RM), Italy * These authors contributed equally to this work Correspondence to: Vincenzo Bronte, email: // Barbara Molon, email: // Fabio Mammano, email: // Keywords : cancer, photodynamic therapy, nitric oxide, calcium signaling, connexins Received : January 24, 2015 Accepted : February 14, 2015 Published : March 12, 2015 Abstract Ionizing and nonionizing radiation affect not only directly targeted cells but also surrounding “bystander” cells. The underlying mechanisms and therapeutic role of bystander responses remain incompletely defined. Here we show that photosentizer activation in a single cell triggers apoptosis in bystander cancer cells, which are electrically coupled by gap junction channels and support the propagation of a Ca 2+ wave initiated in the irradiated cell. The latter also acts as source of nitric oxide (NO) that diffuses to bystander cells, in which NO levels are further increased by a mechanism compatible with Ca 2+ -dependent enzymatic production. We detected similar signals in tumors grown in dorsal skinfold chambers applied to live mice. Pharmacological blockade of connexin channels significantly reduced the extent of apoptosis in bystander cells, consistent with a critical role played by intercellular communication, Ca 2+ and NO in the bystander effects triggered by photodynamic therapy.