Ca2+ and Mg2+ requirements for growth are not concomitantly reduced during cell transformation

The effects of Mg²⁺ and Ca²⁺ deprivation on survival and growth of normal and transformed cultured cells were studied. Normal fibroblast from several origins (mouse, hamster and human) did not grow in 2 µM Mg²⁺ medium, whereas transformed fibroblasts (mouse and rat) and tumor cells of other types (mouse adrenocortical and rat glial cells) grew optimal. All transformed or tumorogenic cells that showed low growth requirement for Mg²⁺ were able to develop colonies in agarose suspension cultures, i.e., were anchorage-independent for growth. Mg²⁺ deprivation of normal cells did not lead to cell cycle arresting at G_o/ G_l and cell death accounts for the limited growth of these cells in medium containing low Mg²⁺ concentration. Contrary to Mg²⁺, starvation for serum or for Ca²⁺ caused normal cells to undergo cell cycle arrest at the G_o/G_l phase. During the progressive spontaneous transformation of Swiss mouse 3T3 fibroblasts, the decrease in growth requirements for Mg²⁺ and Ca²⁺ do not occur at the same time. The requirement for external Ca²⁺ lowers before the onset of anchorage-independent growth while the Mg²⁺ requirement only decreases after cells become anchorage-independent. Therefore the reductions in growth requirement for Mg²⁺ and Ca²⁺ that take place in cell transformation are not linked events.