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 (HCO₃^-). 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 HCO₃^- was omitted from the incubation medium. The HCO₃^- dependence was also demonstrated for apoptosis induced by staurosporine in HeLa cells. Of significance, when HCO₃^- was reintroduced, apoptosis was restored. The Cl^-/HCO₃^- exchanger inhibitor DIDS suppressed apoptosis in HCO₃^--containing medium, further supporting a role for intracellular HCO₃^- in apoptosis regulation. We subsequently examined HCO₃^--dependent steps in the apoptotic cascade. Translocation of Bax and cytochrome c was not suppressed by the omission of HCO₃^-, suggesting HCO₃^- regulation at postmitochondrial levels. In vitro reconstitution of caspase activation using exogenous cytochrome c and cytosolic extracts was not HCO₃^- dependent. HCO₃^- was not required for the enzymatic activity of recombinant caspases either. In conclusion, the results have provided compelling evidence for HCO₃^- regulation of apoptosis. Such regulation takes place at postmitochondrial levels, downstream of Bax/cytochrome c translocation.