Agonist-specific differential changes of cellular signal transduction pathways in senescent human diploid fibroblasts

Changes in the signal transduction efficiency of senescent cells led us to compare the signaling events induced by two mitogenic agonists, platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA) in presenescent and senescent or near-senescent human diploid fibroblasts. When the changes in intracellular [Ca(2+)](i) were analyzed, both PDGF and LPA generated a rhythmic increase in [Ca(2+)](i) in presenescent cells. The frequency of calcium response was reduced and desensitized in PDGF-stimulated senescent cells, while response to a LPA-induced calcium signal was also reduced in frequency, though its magnitude was unaltered. PDGF treatment increased the fibrous actin (F-actin) level in presenescent cells but not in senescent cells in contrast to a reduced but visible increase in F-actin in LPA-treated senescent cells. The effect of PDGF on phospholipase D (PLD) activation was also reduced significantly, as a ca. 60-80% reduction of PLD activity was observed in PDGF-stimulated cells but only a little reduction in LPA-induced cells. Agonist-specific differential changes of cellular signaling events caused a differential effect on DNA synthesis after growth factor stimulation. We observed a dramatic (80-90%) reduction of [3H]thymidine incorporation into DNA in the PDGF-stimulated near-senescent cells. LPA resulted in a 2-3-fold increase in thymidine incorporation even in the near-senescent cells. These differences in the responses of senescent or near-senescent cells to PDGF- and LPA-stimulation raised questions about the differential changes of the respective signaling apparatuses induced by aging. Since PDGF signaling event was affected greatly by aging, we further examined the protein contents involved in PDGF signal transduction pathway. PDGF receptor (PDGFR), protein kinase C-alpha (PKC-alpha), phospholipase C-gamma1 (PLC-gamma1), and PLD1 were examined by Western blot analysis. The protein levels of PKC-alpha and PLC-gamma1 were unchanged, but those of PLD1 and PDGFR were reduced with age. The reduced content of PDGFR protein may be one of the important contributors to the failure of PDGF-stimulated signal transduction in human senescent fibroblasts. Our results strongly suggest that age-dependent agonist-specific changes in signaling events might be in charge of the functional deterioration of senescent cells through imbalance of signal responses.