Walker tumor cells express larger amounts of the antiapoptotic protein Bcl-2 and presents higher resistance to toxic concentrations of Ca2+ than the tumor cells K 562

Ca2+ homeostasis was studied in two tumor cell lines (Walker 256 and K 562) previously shown to exhibit different mitochondrial Ca2+ accumulation capa city. When intact, both cells present cytosolic Ca2+ concentrations within the range expected for mammalian cells, as determined through fura-2 fluore scence ratios. In order to study intracellular Ca2+ distribution, digitonin was used to permeabilize the plasma membrane without affecting intracellul ar organelle structure, as assessed using electron microscopy. Digitonin-pe rmeabilized Walker 256 cells incubated with Ca2+ presented uptake of the ca tion exclusively through mitochondrial activity. In addition, very large Ca 2+ loads were necessary to promote a disruption of Walker 256 mitochondrial membrane potential. K 562 cells presented active Ca2+ uptake through both nonmitochondrial and mitochondrial compartments and suffered disruption of mitochondrial membrane potential at lower Ca2+ loads than Walker 256 mitoch ondria. The higher Ca2+ resistance in Walker 256 cells could be attributed to Bcl-2 overexpression, as evidenced by immunocytochemical staining. Thus, we correlate natural Bcl-2 overexpression, observed in Walker 256 cells, w ith higher resistance to mitochondrial Ca2+ overload, as was shown previous ly in mitochondria from cells transfected with the bcl-2 gene. Drug Dev. Re s. 52:508-514, 2001. (C) 2001 Wiley-Liss, Inc.