Absence of three secreted proteins and presence of a 57-kDa protein related to irreversible arrest of cell growth.

Two subsets of proteins from the conditioned medium of normal human diploid fibroblasts were detected by NaDodSO4/polyacrylamide gel electrophoresis analysis; the presence of these subset is related to the nonproliferative state of in vitro aged cells. One subset consists of three proteins (molecular mass from 80 to 87 kDa) secreted by replicating cells in sparse cultures, as well as by quiescent cells in confluent or serum-starved cultures of young fibroblasts. These proteins disappear from the medium when cultures reach the state of senescence. The other subset consists of a single protein of 57 kDa, detected only in conditioned medium of senescent fibroblast cultures. The results suggest that when human fibroblasts reach senescence, they secrete the 57-kDa protein and concomitantly stop secreting the other three proteins normally found in the culture medium of young fibroblasts. Therefore, the alternative secretion of these protein subsets could specifically signify irreversible arrest of cell growth during in vitro cellular aging.