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The Peripheral Benzodiazepine Receptor in Photodynamic Therapy with the Phthalocyanine Photosensitizer Pc 4 ¶
Author(s) -
Morris Rachel L.,
Varnes Marie E.,
Kenney Malcolm E.,
Li YingSyi,
Azizuddin Kashif,
McEnery Maureen W.,
Oleinick Nancy L.
Publication year - 2002
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2002)0750652tpbrip2.0.co2
Subject(s) - photosensitizer , photodynamic therapy , phthalocyanine , peripheral , benzodiazepine , chemistry , pharmacology , photochemistry , biophysics , receptor , medicine , biology , biochemistry , organic chemistry
The peripheral benzodiazepine receptor (PBR) is an 18 kDa protein of the outer mitochondrial membrane that interacts with the voltage‐dependent anion channel and may participate in formation of the permeability transition pore. The physiological role of PBR is reflected in the high‐affinity binding of endogenous ligands that are metabolites of both cholesterol and heme. Certain porphyrin precursors of heme can be photosensitizers for photodynamic therapy (PDT), which depends on visible light activation of porphyrin‐related macrocycles. Because the apparent binding affinity of a series of porphyrin analogs for PBR paralleled their ability to photoinactivate cells, PBR has been proposed as the molecular target for porphyrin‐derived photocytotoxicity. The phthalocyanine (Pc) photosensitizer Pc 4 accumulates in mitochondria and structurally resembles porphyrins. Therefore, we tested the relevance of PBR binding on Pc 4–PDT. Binding affinity was measured by competition with 3 H‐PK11195, a high‐affinity ligand of PBR, for binding to rat kidney mitochondria (RKM) or intact Chinese hamster ovary (CHO) cells. To assess the binding of the Pc directly, we synthesized 14 C‐labeled Pc 4 and found that whereas Pc 4 was a competitive inhibitor of 3 H‐PK11195 binding to the PBR, PK11195 did not inhibit the binding of 14 C–Pc 4 to RKM. Further, 14 C–Pc 4 binding to RKM showed no evidence of saturation up to 10 μ M. Finally, when Pc 4–loaded CHO cells were exposed to activating red light, apoptosis was induced; Pc 4–PDT was less effective in causing apoptosis in a companion cell line overexpressing the antiapoptotic protein Bcl‐2. For both cell lines, PK11195 inhibited PDT‐induced apoptosis; however, the inhibition was transient and did not extend to overall cell death, as determined by clonogenic assay. The results demonstrate (1) the presence of low‐affinity binding sites for Pc 4 on PBR; (2) the presence of multiple binding sites for Pc 4 in RKM and CHO cells other than those that influence PK11195 binding; and (3) the ability of high supersaturating levels of PK11195 to transiently inhibit apoptosis initiated by Pc 4–PDT, with less influence on overall cell killing. We conclude that the binding of Pc 4 to PBR is less relevant to the photocytotoxicity of Pc 4–PDT than are other mitochondrial events, such as photodamage to Bcl‐2 and that the observed inhibition of Pc 4–PDT–induced apoptosis by PK11195 likely occurs through a mechanism independent of PBR.