Radiation-induced apoptosis in human myeloma cell line increases BCL-2/BAX dimer formation and does not result in BAX/BAX homodimerization

Abstract

A popular model of BCL-2 and BAX involvement in apoptosis suggests that upon apoptosis induction cytosolic BAX translocates to the mitochondria, where it displays the pro-apoptotic function, which involves its homodimerization. BCL-2 exerts anti-apoptotic function by forming heterodimers with BAX, thus neutralizing the pro-apoptotic activity of the latter. We have shown that irradiation of the human myeloma cell line RPMI-8226 induced apoptosis as determined by DNA degradation, cytochrome c release into cytoplasm and BCL-2 caspase-mediated cleavage. BCL-2 protein was present only in the membrane fraction, whereas BAX was found both in cytosol and membranes isolated from non-irradiated cells. Radiation induced moderate redistribution of BAX from cytosol to membranes with a concomitant increase in BCL-2/BAX heterodimer formation. Rapid and transient BCL-2 phosphorylation in membrane fractions of irradiated cells did not affect BCL-2/BAX heterodimerization. We failed to detect any BAX/BAX homodimers in apoptotic cells. Our findings show that in irradiated RPMI-8226 cells the formation of BCL-2/BAX heterodimers correlates with apoptosis. We conclude that BCL-2/BAX heterodimers are negative regulators of death protection, and our data agree with those who propose that BCL-2 does not require BAX to exert its survival function.