BCL-X_L-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-X _L. 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-X_L with YFP fluorescence localized on the mitochondria. After treatment with 1 or 2 μM STS, the measured OSIR decreased monotonically by ∼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-X_L cell variant, the initial OSIR was already ∼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-X_L even before apoptosis induction raise interesting questions as to the role of BCL-X_L in conferring apoptosis resistance by preconditioning the cells and possibly altering mitochondrial morphology.