CHOP activation by photodynamic therapy increases treatment induced photosensitization

Background and Objectives: C/EBP homologous protein (CHOP) is an endoplasmic reticulum (ER) stress inducible transcription factor involved in the development of apoptosis, growth arrest, and differentiation. CHOP deficient (chop -/-) mouse embryonic fibroblasts (MEFs) exposed to ER stresses such as tunicamycin exhibit decreased apoptosis and reduced toxicity when compared to chop +/+ control cells. Overexpression of the 70 kDa heat shock stress protein (HSP-70) can inhibit apoptotic pathways. The biological significance of photodynamic therapy (PDT) protocols that induce cellular damage resulting in differential CHOP and stress protein expression patterns was examined. Study Design/Materials and Methods: Wild type mouse radiation induced fibrosarcoma (RIF) cells as well as MEFs with chop +/+ and chop -/- genotypes were used with either a mitochondrial and ER localizing porphyrin (PH) photosensitizer or a lysosomal localizing chlorin (NPe6) photosensitizer. PDT induced cytotoxicity, apoptosis, and stress protein expression patterns were determined as a function of cell type and photosensitizer. Results: PH mediated PDT induced expression of CHOP and 78 kDa glucose regulated protein (GRP-78), but not HSP-70 while NPe6 mediated PDT induced protein expression of HSP-70 but did not activate CHOP or GRP78 expression. Enhanced apoptosis and toxicity were observed in chop +/+ cells following exposure to tunicamycin or PH mediated PDT when compared to identical treatments in chop -/- cells. NPe6 mediated PDT induced minimally detectable apoptosis in both chop +/+ and chop -/- cells and only a modest increase in survival for chop -/- cells. Conclusions: These results demonstrate that PDT activation of CHOP, GRP-78, and HSP-70 varied as a function of photosensitizer subcellular localization and that a single oxidative stress response was not observed following PDT. We also show that CHOP expression increased apoptosis following PH mediated PDT and that increased CHOP expression is associated with enhanced PDT photosensitization.