Induction of cytostasis in mammary carcinoma cells treated with the anticancer agent perillyl alcohol

The monoterpene perillyl alcohol (POH) has preventive and therapeutic effects in a wide variety of pre-clinical tumor models, including those for breast cancers, and is currently being tested in human phase I clinical trials. POH causes both cytostasis and apoptosis in rat mammary carcinomas. In vitro, POH inhibits cellular proliferation in a variety of mammalian cell lines. Here we investigated the mechanisms underlying cytostasis by studying the effects of POH on the cell cycle in vitro using the murine mammary transformed cell line TM6. In TM6 cells, POH causes an early G(1) cell-cycle block and slows the G(2)-M transition. An increase in pRB in its hypophosphorylated state is associated with the early G(1) block caused by POH. POH treatment inhibits two important targets in the cells during the G(1)-S transition: cyclin D1- and cyclin E-associated kinase. POH treatment leads to a reduction in cyclin D1 RNA and protein levels and prevents the formation of active cyclin D1-associated kinase complexes in synchronous cells during the exit of G(0) and entry into the cell cycle. In addition, POH treatment induces an increased association of p21(WAF1) with cyclin E-Cdk2 complexes, and inhibits the activating phosphorylation of Cdk2. All these effects of POH may contribute to the inhibition of the transition out of the G(1) phase of the cell cycle.