Decline of protein kinase C activation in response to growth stimulation during Senescence of IMR-90 human diploid fibroblasts

We have studied the activation of protein kinase C (PKC) during senescence of human IMR-90 fibroblasts by analyzing the phosphorylation of its in vivo substrate MARCKS (myristoylated alanine-rich C kinase substrate). It was found that the extent of TPA-induced phosphorylation of MARCKS was not significantly different between young and old IMR-90 fibroblasts. In contrast, the increase of MARCKS phosphorylation after serum stimulation was 4.5-fold in young fibroblasts as compared to 1.8-fold in old fibroblasts. Analysis of PKC by Western blotting showed that the levels of PKC were not changed during senescence of IMR-90 fibroblasts. However, the generation of diacylglycerol in response to serum stimulation declined in old fibroblasts. These results suggest that the efficiency of signal transduction mediated by diacylglycerol generation and PKC activation during the mitogenic response is age-dependent in human IMR-90 fibroblasts.