| Abstract |
Caki-1 cells were selected for 2'-deoxyribosyl-7-deazaadenine
(deoxytubercidin, dTub) resistance by exposure to incrementally increasing
dTub concentrations over a one-year period. The resulting cells, Caki-dTub,
were moderately resistant to dTub (5-fold). and were approximately
200-fold cross resistant to 2'-deoxyribosyl-2-chloroadenine (CldAdo). The
resistance to these two agents was stable during maintenance of the cells
in the absence of dTub. They were also cross resistant (20-fold) to the
deoxy-cytidine kinase substrates arabinosyl cytosine and gemcitahine.
Surprisingly, the Caki-dTub cells were collaterally sensitive
(approximately 20-fold) to the deoxycytidine kinase substrates, arabinosy1-
2-fluoroadenine (FaraA) and BCH-4556 [Beta-L-(-)-dioxola-necytidineJ,
suggesting that deoxycytidine kinase activity was retained in the
resistant cell line. The presence of kinase activity was confirmed by
measurement of the uptake of radiolabeled CldAdo into acid-insoluble
material of Caki-1 and Caki-dTub cells, and by measurement of kinase
activity in cell homogenates. The Caki-dTub cells were also cross
resistant to the organic cations tetraethylammonium (8-fold). cyanine 863
(11-fold), and quinidine (2.5-fold). Cytotoxic organic cations are thought
to express their carcinoma toxicity via upjake into mitochondria. A
mechanism for CldAdo action involves binding of its 5'-triphosphate
metabolite to Apaf-I that enhances cytochrome c activation of caspase-9 in
the induction of apoptosis (Proc. Natl. Acad. Sci. USA, 95: 9567-9571,
1998). Consequently, we tested whether cytochrome c release from
mitochondria might be reduced in the Caki-dTub cells. Release of
cytochrome c induced after 16-24h incubation in 1 uM CldAdo (toxic to Caki-1
but not Caki-dTub cells) was approximately 20% of the total cytochrome
c in both cell types. Thus, either cytochrome c release is not related to
the mechanism of cell killing in these cells or perhaps more likely, the
resistant cells are impaired in their ability to induce apoptosis at a
step beyond cytochrome c release. The cross resistance to some
deoxycytidine kinase sub-strates and collateral sensitivity to FaraA and
BCH-4556 in the Caki-dTub cells is a novel phenotype of potential clinical
importance.
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