Protein kinase C is localized in focal contacts of normal but not transformed fibroblasts

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Abstract
Transformed cells differ from normal cells in that they fail to respond to normal signals for regulation of growth and differentiation. This disordered signal transduction probably contributes to maintenance of the transformed phenotype. Several lines of evidence suggest that changes in the Ca2+- and phospholipid-dependent protein kinase, protein kinase C (PKC), may be important for transformation. To determine the role of PKC in transformation, we compared the levels and subcellular distribution of total phorbol ester receptors and PKC in normal and SV40-transformed rat embryo fibroblasts (REF52 cells). We also used our α-PKC (Type 3)-specific monoclonal antibodies to compare α-PKC content and regulation. We found no differences in quantity or subcellular distribution of PKC in 100,000 × g soluble and pelletable fractions. Downmodulation, which represents a feedback loop for limiting PKC activity, occurs to the same extent in both cell types. A major difference between the normal and transformed cells was revealed by immunofluorescence of α-PKC. In normal cells, α-PKC is tightly associated with the cytoskeleton and appears to be organized into focal contacts because it colocalizes with talin. In contrast, in SV40-REF52 cells, α-PKC is not tightly associated with the cytoskeleton and does not colocalize with talin. The difference in subcellular localizations correlates with a loss of two α-PKC-binding proteins in the transformed cells. These results indicate that inappropriate subcellular location of α-PKC may contribute to maintenance of the transformed phenotype.