Cell cycle-dependent regulation of Ca2+ in young and senescent WI-38 cells

During the in vitro senescence of the normal human diploid cell line WI-38, there is a loss of ability to initiate DNA synthesis in response to mitogens. This loss of replicative capacity is reflected in an increasing average length of G1 and a decreasing fraction of cells in the rapidly proliferating pool. Calcium ion (Ca2+) has been shown to be important for progression through the cell cycle and we have measured two processes which contribute to the regulation of intracellular Ca2+ in the early, mid, and late G1 phase of the cell cycle. Basal intracellular Ca2+ concentrations in quiescent cells as well as initial transient mobilization of Ca2+ stores following mitogenic stimulation by growth factors were equivalent in young and senescent cells. Calmodulin levels in young, quiescent cells decreased 50% in the first 4-6 h after stimulation with fresh serum and then increased two- to fourfold immediately prior to entry into DNA synthesis. Senescent cells did not exhibit this cell cycle-dependent pattern and calmodulin levels remained generally constant throughout G1, sometimes increasing slightly, prior to S phase. These data suggest that perhaps the intracellular Ca2+ concentration is regulated differently in young and senescent cells. Indeed, the cell cycle regulation of calmodulin may be uncoupled from the cell cycle regulation of calmodulin mRNA. This difference could provide one mechanism for the failure of senescent cells to synthesize DNA following mitogenic stimulation.