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

A popular model of BCL-2 and BAX involvement in apoptosis suggests that upo n apoptosis induction cytosolic BAX translocates to the mitochondria, where it displays the pro apoptotic function, which involves its homodimerizatio n, BCL-2 exerts anti-apoptotic function by forming heterodimers with BAX, t hus neutralizing the pro-apoptotic activity of the latter. We have shown th at irradiation of the human myeloma cell line RPMI-8226 induced apoptosis a s determined by DNA degradation, cytochrome c release into cytoplasm and BC L-2 caspase-mediated cleavage. BCL-2 protein was present only in the membra ne fraction, whereas BAX was found both in cytosol and membranes isolated f rom non-irradiated cells. Radiation induced moderate redistribution of BAX from cytosol to membranes with a concomitant increase in BCL-2/BAX heterodi mer formation. Rapid and transient BCL-2 phosphorylation in membrane fracti ons of irradiated cells did not affect BCL-2/BAX heterodimerization. We fai led 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 nega tive regulators of death protection, and our data agree with those who prop ose that BCL-2 does not require BAX to exert, its survival function. (C) 20 01 Wiley-Liss, Inc.