Ultrastructural cytoskeleton alterations and modification of actin expression in the NIH/3T3 cell line after transformation with Ha‐ras‐Activated oncogene

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Abstract
Cytoskeleton alterations of NIH/3T3 fibroblast monolayers transfected with Haras‐activated oncogene were studied by immunofluorescence, immunoelectron microscopy, and immunoelectrophoretic analysis of actin isoforms. Transformation foci were found to consist of cells with a round shape and rare stress fibers that spread sparsely, forming rare focal contacts and fibronexuses. The loss of stress fibers in transformed cells was confirmed by staining with rhodamine‐phalloidin and with a fluorescinated anti‐non‐muscle cell actin antibody. The transformed cells were anchored to the substrate prominently by filaments that contained fibronectin, as showed by immunoelectron microscopy. A down‐regulation of α‐actin isoform was observed by immunofluorescence and immunoblotting analysis using a specific monoclonal antibody. The diffuse distribution of α‐actin, lacking a specific association with stress fibers, challenges the hypothesis of a connection between α‐actin down‐regulation and stress fiber loss.
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