Decline of signal transduction by phospholipase Cγ1 in IMR 90 human diploid fibroblasts at high population doubling levels

During cellular senescence in vitro, the cells do not respond mitogenically to serum growth factors at high population doubling levels. Phospholipase C activity in low PDL IMR 90 cells showed a 4.7-fold stimulation in response to 10% serum compared to 3.3-fold in high PDL cells when measured in whole cell extracts. Immunoaffinity purified tyrosine phosphorylated protein fraction showed a greater increase (5.2-fold) in phospholipase C activity in low PDL than high PDL cells (2.1-fold) in response to serum. Serum stimulated PLCγ1 activity was diminished in high PDL cells. Immunokinase assay of PLCγ1 immunoprecipitates from serum stimulated IMR 90 fibroblasts suggested that diminished enzymatic activity in high PDL cells is not due to less receptor coupled tyrosine phosphorylated PLCγ1 enzyme. Serum stimulated [³H]thymidine incorporation into DNA declined in parallel with the activity of PLCγ1, suggesting that its activation might play significant roles in this in vitro model for cellular senescence.