Modifications of phosphoproteins and protein kinases occurring with in vitro aging of cultured human cells.

We have studied the age-related modifications of endogenous phosphorylations and protein kinases in seven different strains of cultured human cells (two fibroblastic and five human liver-derived strains). Endogenous phosphorylation of phosphopeptides by cells incubated in the presence of [32P]orthophosphate constantly changed with aging; these modifications could not be explained only by loss of replicative capacity of these cells. Total casein and phosvitin kinase activity did not change in the course of in vitro cell aging. Gel filtration chromatography and autophosphorylation of partially purified kinase preparations, however, seemed to indicate some age-related modifications of protein kinases. Electrophoresis of active enzymes demonstrated that some histone/protamine kinases decreased with aging while a new 3':5'-cyclic AMP-independent histone kinase band appeared progressively. It is proposed that new protein kinase isozymes could correspond to new genes progressively activated during cell aging and could be closely related to cell senescence and death.