Effects of Calcium Depletion on Human Cells in Vitro and the Anomalous Behavior of the Human Melanoma Cell Line MM170

Abstract

Seven strains of normal human cells (fibroblastic, skin epithelioid, and amniotic) ceased to proliferate in medium depleted of free calcium ion by titration with ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA), whereas the growth of 9 of 10 human melanoma cell lines was not affected. Fibroblasts showed a rapid drop in thymidine pool size and decreased incorporation of thymidine and uridine when treated with EGTA, followed during the next 48 hr by a decrease in plasma membrane potential and by development of a proliferative block in the G₁ phase of the cell cycle. The calcium-independent melanoma line MM96 exhibited an early decrease in thymidine pool size and enhanced incorporation of nucleosides but continued to proliferate with little perturbation of the cell cycle or change in membrane potential. Tumor cell DNA may therefore be selectively labeled in the presence of normal cells. The anomalous, calcium-dependent melanoma line (MM170) showed an immediate increase in the thymidine pool size and in nucleoside incorporation and subsequently accumulated in G₁ and G₂ with diminution of membrane potential and of DNA and RNA synthesis. The proliferative block in MM170 cells could be reversed by addition of calcium ion or by replacement with control medium. Addition to the medium of all 8 nucleosides (50 µm), singly or together, did not prevent EGTA-induced cytostasis in fibroblasts or MM170; transport of thymidine across the cell membrane was enhanced by 24-hr EGTA treatment in fibroblasts, MM96, and MM170. Thus, although calcium affected thymidine utilization rapidly and differently in each of the three cell types, nucleoside starvation per se did not appear to be responsible for either type of proliferative block.