Postmitochondrial regulation of apoptosis by bicarbonate

Ion homeostasis may play a role in the regulation of apoptosis. The current study has shown such a role for bicarbonate (HCO(3)(-)). In apoptosis triggered by ATP depletion, the proapoptotic molecule Bax translocated from the cytosol to mitochondria, followed by cytochrome c release from the organelle, caspase activation, and development of apoptotic morphology. Apoptosis was significantly ameliorated, when HCO(3)(-) was omitted from the incubation medium. The HCO(3)(-) dependence was also demonstrated for apoptosis induced by staurosporine in HeLa cells. Of significance, when HCO(3)(-) was reintroduced, apoptosis was restored. The Cl(-)/HCO(3)(-) exchanger inhibitor DIDS suppressed apoptosis in HCO(3)(-)-containing medium, further supporting a role for intracellular HCO(3)(-) in apoptosis regulation. We subsequently examined HCO(3)(-)-dependent steps in the apoptotic cascade. Translocation of Bax and cytochrome c was not suppressed by the omission of HCO(3)(-), suggesting HCO(3)(-) regulation at postmitochondrial levels. In vitro reconstitution of caspase activation using exogenous cytochrome c and cytosolic extracts was not HCO(3)(-) dependent. HCO(3)(-) was not required for the enzymatic activity of recombinant caspases either. In conclusion, the results have provided compelling evidence for HCO(3)(-) regulation of apoptosis. Such regulation takes place at postmitochondrial levels, downstream of Bax/cytochrome c translocation.