Tumor response to mTHPC-mediated photodynamic therapy exhibits strong correlation with extracellular release of HSP70

Background and objective: We investigated the relationship among heat shock protein 70 (hsp70) promoter activation, extracellular HSP70 protein levels, and tumor cure in an animal model of meso-tetrahydroxyphenyl chlorin (mTHPC; Foscan®)-mediated photodynamic therapy (PDT).

Materials and methods: Using Western blot analysis, we compared HSP70 protein levels in control and PDT-treated EMT6 cells with the amplitude of hsp70-promoter driven green fluorescent protein (GFP) expression in identically treated, stably transfected hsp70-GFP/EMT6 cells. A clonogenic survival assay was performed to assess the relationship among promoter activation, HSP70 levels, and cell survival in vitro. Tumor growth studies with this transfected cell line were performed to examine responses to fluences from 0.1 to 10 J cm(-2) , which ranged from sub-curative to curative. In vivo stereofluorescence and confocal fluorescence imaging were used to assess the temporal kinetics in hsp70 activation in tumors subjected to these fluences and the intratumor spatial correlation between hsp70 induction and extracellular levels of HSP70, respectively.

Results: Maximum GFP expression and HSP protein levels in cells were observed at PDT doses that corresponded to 30% cell survival. The relative changes in GFP and HSP70 protein accumulation as analyzed using Western immunoblots agreed very well, thereby confirming the validity of fluorescent reporter assessment of gene expression in our studies. In vivo imaging revealed that hsp70 promoter-driven GFP expression and accumulation of extracellular HSP70 in PDT-treated tumors subjected to non-curative doses exhibit minimal spatial correlation. There is a strong correlation between mTHPC-PDT doses that result in long-term tumor cure and those that cause high levels of surface exposed or extracellularly released HSP70s.

Conclusion: Treatment conditions that induce strong promoter activation do not correspond to tumor cure. PDT doses that result in long-term tumor growth control also produce significant accumulation of extracellular HSP70.