Failure in S6 protein phosphorylation by serum stimulation of senescent human diploid fibroblasts, TIG-1.

When quiescent young or senescent human diploid cells, TIG-1, were metabolically labeled with 32Pi and stimulated with 10% fetal bovine serum, the phosphorylation of ribosomal S6 protein was enhanced in young cells but not in senescent cells while that of some other proteins were increased in both cells. Inability to stimulate the phosphorylation of S6 protein in senescent cells after serum addition may be the primary cause of the failure of enhancement in protein synthesis followed by the block of prereplicative events dependent on protein synthesis and thus of the failure of cells to enter S phase. However, when the cell-free preparation from serum-stimulated senescent cells was incubated with [gamma-32P]ATP, S6-kinase activity was stimulated and S6 in ribosomal fraction was susceptible to phosphorylation as observed in young cells. Differences in S6 phosphorylation of senescent cells between in vivo and in vitro was discussed.