Use of intracellular pH and annexin-V flow cytometric assays to monitor apoptosis and its suppression by bcl-2 over-expression in hybridoma cell culture

Accurate identification and quantitation of apoptosis is essential for developing efficient strategies for optimisation of culture viability and productivity in cell lines of industrial significance. We have examined the possibility of using carboxy-seminaphthorhodafluor-1-acetoxymethylester (carboxy SNARF-1-AM), a pH sensitive fluoroprobe and FITC-labelled annexin V (AV), a probe specific to phosphatidylserine exposed on the surface of apoptotic cells, to monitor apoptosis and to determine the relationship between intracellular pH (pHi), apoptosis and cell cycle in hybridoma cells. Temporal changes in the distribution of proliferative capacity (S phase), metabolic activity (pHi), and cell death population dynamics were effectively and reliably determined using flow cytometry. Intracellular acidification was shown to precede the occurrence of apoptosis during batch culture and after treatment with campothecin, staurosporine and under adverse bioreactor conditions such as glutamine deprivation and oxygen deficiency. These results showed that the decrease in pHi can be used as an indicator of cellular deterioration and cell death. AV in combination with propidium iodide permitted the identification of viable, transient apoptotic and necrotic cells in heterogeneous cultures of control (PEF) cells. Hybridoma cells over-expressing bcl-2 were protected from intracellular acidification and phosphatidylserine exposure, which was associated with the suppression of apoptosis in these cells. A decrease in pHi was apparent even before the accumulation of the normally acidic G1 phase and the development of a sub-G1 region, characteristic of apoptotic cell behaviour. The pHi assay can therefore be used as a tool to predict future cell culture performance. (C) 1998 Elsevier Science B.V. All rights reserved.