S-phase induction and transformation of quiescent NIH 3T3 cells by microinjection of phospholipase C

Two inositol phospholipid-specific phospholipase C (PLC) isozymes (PLC-I and -II) have been purified from bovine brain. When PLC-I or PLC-II was microinjected into quiescent NIH 3T3 cells, a time- and dose-dependent induction of DNA synthesis occurred, as demonstrated by [3H]thymidine incorporation into nuclear DNA. In addition, ∼ 8 hr after PLC injection, NIH 3T3 fibroblasts appeared spindle-shaped, refractile, and highly vacuolated, displaying a morphology similar to transformed cells. The morphologic transformation was apparent for 26-30 hr after which the injected cells reverted back to a normal phenotype. Microinjected PLC at a high concentration was cytotoxic, dissolving the cytoplasmic membrane and leaving behind cellular ghosts. PLC is a key regulatory enzyme involved in cellular membrane signal transduction. Introduction of exogenous PLC into NIH 3T3 cells by microinjection induced a growth and oncogenic potential, as demonstrated by the ability of microinjected PLC to override the cellular G0 block, inducing DNA synthesis and morphologic transformation of growth-arrested fibroblast cells