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PK11195 Mediates
Enhanced Phototoxicity by Increasing Mitochondrial Uptake of Photosensitizer
via the Peripheral Benzodiazepine Receptor.
Morgan, Janet1,
Hanley, Jennifer1 and Oseroff, Allan1
Roswell Park Cancer Institute1
Abstract-
Photodynamic therapy (PDT) with hexyl pyropheophorbide-a (Photochlor),
and ALA-mediated protoporphyrin IX (PpIX) is enhanced by ligands of
the peripheral benzodiazepine receptor (PBR) such as PK11195. The mechanism
is poorly understood. The PBR is an outer mitochondrial membrane protein
responsible for the transport of certain molecules across the membrane.
Heme and its porphyrin precursors, such as PpIX, are ligands for the
PBR. Other photosensitizers such as Photochlor also bind to the PBR.
Another natural PBR ligand is diazepam binding inhibitor (DBI), a polypeptide
carrier of cholesterol. DBI transfers cholesterol via the PBR to the
inner mitochondrial membrane enzyme (P450scc) for side-chain cleavage-the
initial step in steroid synthesis, a process stimulated by PK11195.
We hypothesized that (1) PK11195 enhanced PDT phototoxicity was caused
by increased photosensitizer uptake by mitochondria, and (2) that it
was mediated by the cholesterol transport mechanism. Mitochondria harvested
from FaDu cells were used to test the effect of PK11195 on photosensitizer
transport by the PBR. A naturally occurring fragment of DBI (ODN), which
binds to PBR and contains the cholesterol binding site was used as a
carrier of photosensitizer. Free or bound (to ODN) Photochlor or PpIX
(at 0.1, 0.25 or 0.5  M)
were incubated with mitochondria (200 g
total protein per sample) in the presence or absence of PK11195 (0,
10, 50, or 100 M).
Mitochondria were washed and the photosensitizers measured by fluorescence
spectroscopy. PK11195 increased both free and bound photosensitizer
uptake in a concentration dependent manner. Higher levels of photosensitizer
were obtained with free photosensitizer, and may partly have been due
to non-specific uptake into membranes. We conclude that PK11195 enhanced
PDT may be due to increased mitochondrial levels of photosensitizer
mediated at the PBR through DBI. Supported by NIH CA55791 and the
Roswell Park Cancer Center Support Grant P30 CA 16056.
Keywords: mitochondria,
peripheral benzodiazepine receptor, diazepam binding inhibitor, Photochlor
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