ALA-Dependent Cancericides - Chlorophyll Biosynthesis and Technological Applications - page 439

19.4 Induction of Apoptosis in Leukemia Cells

by Modulators of Heme Biosynthesis

We have shown in preceding sections that the concurrent use of Oph, a tetrapyrrole
heme biosynthesis modulator, with ALA, significantly increases the efficacy
of ALA treatment of transformed cells by enhancing Proto accumulation.
Heme biosynthesis modulators such as Oph enhance Proto accumulation in
transformed cells in the presence of ALA, increase cell lysis in vitro and enhance
Proto-induced tumor necrosis in vivo with little damage to surrounding normal
tissue (see section below).
In insects, modulators have also been associated with the induction of a dark
death phenomenon, whereby treated tissues undergo cell death in the absence of
light (Chap. 18 ). The dark cell death, attributed in insects to the formation and
accumulation of Zn-Proto has also been observed as well, in cancer cells treated
with ALA and Oph. One important possibility in cancer cells is that in addition to
enhancing light-induced tumor necrosis by increasing endogenous Proto accumu-
lation, Oph might also cause cell death in darkness by inducing programmed cell
death, or apoptosis (Fisher 1994 ). Apoptosis is a type of cell death that exhibits
distinct morphological and biochemical characteristics. It plays a key role in the
mode of action of a diverse array of anti-tumor agents (Dive and Hickman 1991 ;
Eastman 1990 ; Lowe et al. 1993 ; Strasser et al. 1994 ) and in oncogenesis (Fannidi
et al. 1992 ; Hart et al. 1987 ; Symonds et al. 1994 ). In addition, many of the
intermediate signaling molecules involved in apoptosis, such as p53, are known
to cause growth arrest of neoplastic cells (Radvanyl et al. 1993 ; Rebeiz et al. 1994 ;
Yonish-Rouach et al. 1991 ; Zhu and Anasetti 1995 ), prior to apoptosis induction.
One important intracellular mechanism occurring during apoptosis induction is
the changes in mitochondrial transmembrane potential caused by the opening of
large pores, which are part of the permeability transition (PT) phenomenon
(Kroemer et al. 1995 ). The proteins that form these pores have not been identified,
but the mitochondrial peripheral-type benzodiazepine receptor (M-PBR) has been
implicated during PT (Kinally et al. 1993 ; Pastorino et al. 1994 ). The M-PBR may
also be involved in porphyrin/porphyrinogen transport since coproporphyrinogen
(Taketani et al. 1994 ), Proto (McEnery et al. 1992 ; Verma and Snyder 1988 ) and
heme (Taketani et al. 1995 ) bind to the M-PBR. We have recently shown that
coproporphyrinogen transport into mitochondria is enhanced by ATP (Rebeiz
et al. 1996a ), and the adenine nucleotide is a component of the M-PBR complex
(McEnery et al. 1992 ).
We have therefore investigated the role of apoptosis in the Oph-mediated dark
death phenomenon using MLA 144 leukemic T cells. While Oph and Proto, but not
ALA, induced growth arrest, only Oph induced apoptosis. The results also
suggested that induction of apoptosis by Oph may occur via the M-PBR and that
apoptosis contributes to the death of MLA 144 cells in darkness.