Murine mammary tumour cells in vitro. I. The development of a quiescent state

Three mouse mammary tumour lines (66, 67, and 68H) derived from a single mouse mammary tumour were investigated for their growth kinetics and development of quiescent cells in unfed monolayer cultures. All three lines develop pure quiescent populations when grown in unfed plateau cultures. A dramatic cell-cycle redistribution accompanied the proliferating (P) to quiescent (Q) transition, with the percentage of cells having a G1 DNA content increasing from 50% in the P state to greater than 97% in the Q state. As the cultures progressed from exponential to plateau growth, a decrease of greater than or equal to 50% in cellular RNA was observed in all three lines. This property enables the clear identification of P v. Q cells by flow cytometry using the two-step acridine orange assay. Autoradiographic data verified that these plateau cells were quiescent since less than 2.5% of the cells incorporated [3H]TdR when labelled for approximately two doubling times. Further comparison of the P and Q cells showed that: (a) the Coulter volume of Q cells was approximately half that of P cells in all three lines; (b) viability, as measured by dye exclusion was greater than 95% in all cultures regardless of their proliferative state; and (c) colony-forming ability decreased as the cells entered the quiescent state. In each of these cell lines the development of Q-cell populations was marked by similar changes in all measured parameters. These quiescent tumour cells provide a relatively simple model to evaluate what, if any, important differences exist between the response of P v. Q cells to various therapeutic agents.