Molecular Responses to Photodynamic Therapy

Monitoring Molecular Responses to PDT

PDT stimulates the activation of several signaling pathways, amongst which the induction of heat shock proteins (HSPs) constitutes a strong response to the treatment generated reactive oxygen species (ROS)-induced oxidative stress. HSPs belong to a large family of protein chaperones involved in assisting protein folding and unfolding in cells. They are not only constitutively and ubiquitously expressed but are also inducible in response to variety of stressful conditions and are thus commonly referred in the literature as stress proteins. These inducible responses involve transcriptional activation mediated by a transcription factor known as the heat shock factor (HSF), which binds to a specific heat shock element (HSE) in the heat shock gene. Among the stress proteins, the HSP70 family is the most abundant and conserved and has been frequently proposed as a potential biomarker of cellular toxicity.

Morimoto, R.I., Genes & Development (1998)

In our lab, we have examined the PDT-stress-mediated HSP70 activation and its relation to cellular toxicity in a mouse tumor cell line which was transfected with a plasmid consisting of a fluorescent reporter gene (GFP) under the control of an hsp70 promoter. This allowed us to study the PDT-mediated inducible expression of GFP in living specimens in vitro and in vivo using fluorescence imaging. Our current studies will provide a significant opportunity to determine thresholds for gene induction which can be related to a threshold in PDT generated ROS.

Several recent studies have established that HSPs can play a pivotal role in antitumor immunity in vivo by activating dendritic cells and enhancing host cell infiltration. With this in mind, our present studies are also focussed on using sublethal doses of PDT to activate HSP induction, in a strategy where PDT is not intended to be curative but is used instead to prime a tumor-specific immune response.