BCL-xL-dependent light scattering by apoptotic cells

We measured the intensity ratio of wide-to-narrow angle scatter, optical scatter image ratio (OSIR), in single cells during apoptosis and after overexpression of the mitochondria-bound antiapoptotic protein BCL-xL. OSIR is sensitive to particle size/shape for objects with wavelength-scale dimensions, and was used as a morphometric measure of cellular response. Three cell variants were treated with staurosporine (STS): nontransfected parental CSM14.1, CSM14.1 stably expressing yellow fluorescent protein (YFP) with diffuse YFP fluorescence, and apoptosis-resistant CSM14.1 stably expressing the fusion protein construct YFP-BCL-xL with YFP fluorescence localized on the mitochondria. After treatment with 1 or 2 muM STS, the measured OSIR decreased monotonically by approximately 25% in the nontransfected and YFP variants, and reached a steady-state value 40-60 min after STS treatment. The decrease in OSIR at the onset of apoptosis preceded phosphatidyl serine exposure by 5 h. In the YFP-BCL-xL cell variant, the initial OSIR was already approximately 24% lower than the initial OSIR in YFP and nontransfected cells, and only decreased by <10% after STS treatment. Alterations in light scattering by cells overexpressing BCL-xL even before apoptosis induction raise interesting questions as to the role of BCL-xL in conferring apoptosis resistance by preconditioning the cells and possibly altering mitochondrial morphology.