Dual effect of pseudorabies virus growth factor (PRGF) displayed on actin cytoskeleton

Pseudorabies virus growth factor (PRGF) was shown to possess transforming a ctivity as well as transformation repressing activity in in vitro systems. In order to better understand these phenomena we studied actin cytoskeleton and its alterations induced by PRGF using normal human fibroblasts VH-10 a nd transformed cell line HeLa. For specific detection of filamentous actin cells were stained with phalloidin conjugated with fluorescein isothiocyana te (FITC)-phalloidin. PRGF was applied to VH-10 cells for various length of time from 10 min up to 48 h. The effect was very fast and changes in actin filament composition could be detected already after 10 min. Ln comparison to untreated cells the staining of treated cells was more diffuse and a nu mber of actin microfilaments in individual stress fibers became reduced. Af ter 30 min thick short actin bundles appeared in the perinuclear region. A 24-h exposure resulted in a large reduction of actin bundles. After additio nal 24 h a partial restoration of actin cytoskeleton in cells was observed. In transformed HeLa cells PRGF induced opposite process than in normal cel ls: the number of filamentous actin structures increased. We hypothesise th at PRGF may act as a transcription-like factor and may initiate changes in gene expression which consequently result in actin cytoskeleton alterations .