Limitations of the fluorescent probe viability assay

In vitro assessment of the efficacy/capacity of toxicants (e.g., cancer chemotherapeutic agents, environmental pollutants, etc.) to damage/kill cells and/or inhibit growth (cell duplication) requires accurate measurement of target cell viability as a function of exposure. Rapid measurement of viability, such as can be achieved employing fluorescent probes of metabolic function in combination with instrumental analysis, is highly desirable. However, we observe that exposure to chemicals (of unrelated type) complicates the interpretation of viability data and, in the case of perturbed cells, questions the validity of viability growth assays based on intrinsic enzyme activity. Viability commonly is determined flow cytometrically (FCM) by the carboxyfluorescein diacetate (CFDA)/propidium iodide (PI) assay. Nonfluorescent CFDA is taken up by diffusion and converted via cytoplasmic esterase-catalyzed hydrolysis to carboxyfluorescein (CF), a negatively charged fluorescent molecule that is retained (incompletely) by the cell. As such, if CF fluorescence intensity is a relative measure of enzyme activity, it also can be considered an index of cellular vigor (metabolic rate). It is generally accepted that the viable cell excludes both basic dyes, such as PI, and acidic dyes, such as trypan blue, and uptake is indicative of irreversible cellular injury presaging cell death. We observe that, following incubation for 4 h with 0.5-1.0 microM tributyltin (TBT), a potent environmental toxicant, murine erythroleukemic cells (MELC) exhibit enhanced (supranormal) CF fluorescence compared to control cells. Apparent cell volume (ACV) is unaltered, and because such cells exclude PI, they are considered viable in terms of the CFDA/PI assay. However, rate of growth (increase in cell number over 48 h) is depressed, suggesting that supranormal CF fluorescence, even in the absence of PI uptake, is indicative of cellular perturbation. In effect, although CF fluorescence is the product of an enzyme-catalyzed reaction and, therefore, an indicator of vital function (enzyme activity), it apparently is not a reliable index of cellular vigor. At higher TBT concentrations (greater than 1.0, but less than 50.0 microM), the cells exhibit both increased CF fluorescence and PI fluorescence and are growth inhibited. MELC exposed to the cancer chemotherapeutic agents adriamycin, m-AMSA, or crisnatol (Burroughs Wellcome 770U82) also exhibit increased cellular CF fluorescence. However, rate of growth is decreased and ACV increased. The latter, measured either as a function of electrical resistance (Coulter volume) or by the FCM parameter axial light loss could account for the increase in mean CF fluorescence.(ABSTRACT TRUNCATED AT 400 WORDS)