Relationship between actin microfilaments and plasma membrane changes during apoptosis of neoplastic cell lines in different culture conditions

Ln this study we investigated the relationship between the reorganisation o f actin cytoskeleton and the changes at cell surface level (i.e. PS exposur e and blebbing) in two neoplastic cell lines during apoptosis: Chang liver cells (adherent culture) and promyelocytic HL-60 cells (suspension culture) , treated with the podophyllotoxin derivative VP16. The morphological analysis, performed by means of conventional fluorescence microscopy and confocal laser scanning microscopy, on Chang cells showed t hat onset and progress of the two processes are synchronised. The initial d isassembly of stress fibers was associated with the early PS exposure on th e cell surface. Moreover, the accumulation of actin at cortical level appea red strongly associated with an intense labelling for Annexin V and, in som e cases, especially in the areas of membrane blebbing. The double staining for actin and PS exposure, quantitatively analysed by flow cytometry in HL- 60 cells after different treatment times, demonstrated that the decrease of Annexin V binding in the late stages of apoptosis is associated with the s trong reduction of actin labelling probably also due to a proteolytic cleav age. These events were also partially related to variations of the function al state of mitochondria, by analysing cytofluorometrically the dissipation of the inner membrane potential (Delta Psi m).