Transcriptional activation of HIF-1 by a ROS-ERK axis underlies the resistance to photodynamic therapy.

Photodynamic therapy (PDT), a promising treatment option for cancer, involves the activation of a photosensitizer (PS) by local irradiation with visible light. Excitation of the PS leads to a series of photochemical reactions and consequently the local generation of harmful reactive oxygen species (ROS) causing limited or none systemic defects. However, the development of resistance to this promising therapy has slowed down its translation into the clinical practice. Thus, there is an increase need in understanding of the molecular mechanism underlying resistance to PDT. Here, we aimed to examine whether a relationship exists between PDT outcome and ROS-involvement in the resistance mechanism in photosensitized cancer cells. In order to recapitulate tumor architecture of the respective original tumor, we developed a multicellular three-dimensional spheroid system comprising a normoxic periphery, surrounding a hypoxic core. Using Me-ALA, a prodrug of the PS PpIX, in human colorectal spheroids we demonstrate that HIF-1 transcriptional activity was strongly up-regulated and mediates PDT resistant phenotype. RNAi knockdown of HIF-1 impairs resistance to PDT. Oxidative stress-mediated activation of ERK1/2 followed PDT was involved on positive modulation of HIF-1 transcriptional activity after photodynamic treatment. ROS scavenging and MEK/ERK pathway inhibition abrogated the PDT-mediated HIF-1 upregulation. Together our data demonstrate that resistance to PDT is in part mediated by the activation of a ROS-ERK1/2-HIF-1 axis, thus, identifying novel therapeutic targets that could be used in combination with PDT.