Mitochondrial localization and photodamage during photodynamic therapy with tetraphenylporphines

The subcellular localization sites of TPPS₄ and TPPS₁ and the subsequent cellular site damage during photodynamic therapy were investigated in CT-26 colon carcinoma cells using spectroscopic and electron microscopy techniques. The association of both porphyrins with the mitochondria was investigated and the implications of this association on cellular functions were determined. Spectrofluorescence measurements showed that TPPS₄ favors an aqueous environment, while TPPS₁ interacts with lipophilic complexes. The subcellular localization sites of each sensitizer were determined using spectral imaging. Mitochondrial-CFP transfected cells treated with porphyrins revealed localization of TPPS₁ in the peri-nuclear region, while TPPS₄ localized in the mitochondria, inducing structural damage and swelling upon irradiation, as shown by transmission electron microscopy. TPPS₄ fluorescence was detected in isolated mitochondria following irradiation. The photodamage induced a 38% reduction in mitochondrial activity, a 30% decrease in cellular ATP and a reduction in Na⁺ /K⁺-ATPase activity. As a result, cytosolic concentrations of Na⁺ and Ca²⁺ increased, and the level of K⁺ decreased. In contrast, the lipophilic TPPS₁ did not affect mitochondrial structure or function and ATP content remained unchanged. We conclude that TPPS₄ induces mitochondrial structural and functional photodamage resulting in an altered cytoplasmic ion concentration, while TPPS₁ has no effect on the mitochondria.