| Research Abstract |
(1) Both irradiated ara-C-treated BV173 human leukemia cells, which harbor the wild-type p53 gene, resulted in apoptosis and induction of p53. The downstream genes of p53, Bax and Fas/APO-1 were activated in irradiation, but only Bax in ara-C treatment. Two-dimensional gel electrophoresis revealed that the p53 proteins in irradiated and ara-C-treated BV173 cells had different isoelectric points which converged to a single isoelectric point after phosphatase treatment. P53-serine 15, 33, and 392 in irradiated, ara-C-treated and doxorubicin-treated BV173 cells were phosphorylated with no difference.
(2) Transient Bax overexpression in K562 erythroleukemia cells caused significantly more apoptotic cells than the control. In wild type Bax-bearing K562 cells transfected with Tet-On Bax-inducible system, overexpression of Bax without any cytotoxic signal resulted in apoptosis caspase-3 activation, mitochondrial release of cytochrome c, and mitochondrial membrane potential change. A pancaspase inhibitor, zVAD-fmk, inhibited the apoptotic events in the presence of overexpression Bax on mitochondria. These findings suggest that Bax protein, when present above a threshold level, is sufficient to trigger apoptosis cascade, and its initiation requires simultaneous activation of caspases not mediated by mitochondrial cytochrome c release in K562 cells.
(3) In DLD-1 colon cancer cells, Tet-On-Bax-induced caspase activation and apoptosis were considered to be mediated by mitochondrial cytochrome c release. Bax overexpression also sensitized KATOIII gastric cancer cells to chemotherapeutic agent-induced apoptosis through enhancing the cytochrome c release.
Taken together, p53 proteins are differently phosphorylated by different cytotoxic stimuli, and Bax-induced caspase activation has two pathways ; one is mediated by mitochondrial cytochrome c release, and one not mediated by cytochrome c.
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