Polyoma Virus Middle Tumor Antigen Stimulates Membrane-associated Protein Kinase C at Lower Levels Than Required for Phosphatidylinositol Kinase Activation and Neoplastic Transformation

Twelve independent rat F111 cell lines expressing the polyoma virus middle tumor antigen (mT) under control of the dexamethasone-regulatable MMTV-LTR promoter were assayed for levels of membrane-associated protein kinase C (PKC) activity. Low background levels of mT antigen expression (approximately 2%), although insufficient for transformation, triggered a dramatic increase in PKC activity. Under the same conditions, levels of the mT-associated phosphatidylinositol kinase activity were low, indicating that this kinase might be a factor limiting transformation in this cell system.