Alterations in the characteristic size distributions of subcellular scatterers at the onset of apoptosis: effect of Bcl-xL and Bax/Bak.

Optical scatter imaging is used to estimate organelle size distributions in immortalized baby mouse kidney cells treated with 0.4 microM staurosporine to induce apoptosis. The study comprises apoptosis competent iBMK cells (W2) expressing the proapoptotic proteins Bax/Bak, apoptosis resistant Bax/Bak null cells (D3), and W2 and D3 cells expressing yellow fluorescent protein (YFP) or YFP fused to the antiapoptotic protein Bcl-x(L) (YFP-Bcl-x(L)). YFP expression is diffuse within the transfected cells, while YFP-Bcl-x(L) is localized to the mitochondria. Our results show a significant increase in the mean subcellular particle size from approximately 1.1 to 1.4 microm in both Bax/Bak expressing and Bax/Bak null cells after 60 min of STS treatment compared to DMSO-treated control cells. This dynamic is blocked by overexpression of YFP-Bcl-x(L) in Bax/Bak expressing cells, but is less significantly inhibited by YFP-Bcl-x(L) in Bax/Bak null cells. Our data suggest that the increase in subcellular particle size at the onset of apoptosis is modulated by Bcl-x(L) in the presence of Bax/Bak, but it occurs upstream of the final commitment to programmed cell death. Mitochondrial localization of YFP-Bcl-x(L) and the finding that micron-sized particles give rise to the scattering signal further suggest that alterations in mitochondrial morphology may underlie the observed changes in light scattering.