Related Effects of Calcium and Serum on the G1 Phase of the Human W138 Fibroblast

Deprivation of extracellular Ca or serum inhibits the proliferation of WI38 human diploid fibroblasts. Under these conditions, the cells become quiescent at a point in the cell cycle typical of early G1 or G0 phase-arrested cells. Exit of the cells from this point in the cycle appears to require both the presence of serum and Ca simultaneously. If quiescent cells are serum-stimulated in low Ca medium (0.01 mM), they do not progress through G1 to the S phase, which normally requires 14-18 hr. However, they remain competent to do so. Addition of Ca for up to 48 hr after serum stimulation results in an equal fraction of the cells progressing G1 phase as compared to the presence of Ca at the time of serum addition. In contrast, if quiescent cells are serum-stimulated in the presence of Ca, which is then removed, the cells can remain competent to enter S phase for only 10-12 hr. Re-addition of Ca beyond this time does not allow G1 progression on a normal schedule. These data suggest that Ca and serum are both required to trigger, in whole or in part, the pleiotypic response. Ca appears also to render the cells competent to enter S phase, but this competence is labile; an observation consistent with the PDGF-induced competence observed previously in the 3T3 cell. These observations are in contrast to previous data from other cell types which suggest that Ca is required only in late G1 phase for successful entrance to S phase.