Cyclin D1 triggers autonomous growth of breast cancer cells by governing cell cycle exit.

Cyclin D1 controls G1-associated processes, including G0-to-G1 and G1-to-S transitions. This study demonstrates a novel aspect of cyclin D1 as a regulator of the transition between G1 and G0. Overexpression of cyclin D1 in MCF7 breast tumor cells resulted in a continued proliferation under low-serum conditions, whereas nonoverexpressing cells ceased to grow. This difference in growth was due to a reduced exit from G1 to G0 in cyclin D1-overexpressing cells. Our data therefore suggest a model in which cyclin D1 overexpression in tumor cells is responsible for hyperproliferation under growth factor-deprived conditions.