INDOLE-3-CARBINOL INHIBITS THE EXPRESSION OF CYCLIN-DEPENDENT KINASE-6 AND INDUCES A G(1) CELL-CYCLE ARREST OF HUMAN BREAST-CANCER CELLS INDEPENDENT OF ESTROGEN-RECEPTOR SIGNALING

Indole-3-carbinol (I3C), a naturally occurring component of Brassica v egetables such as cabbage, broccoli, and Brussels sprouts, has been sh own to reduce the incidence of spontaneous and carcinogen-induced mamm ary tumors, Treatment of cultured human MGF7 breast cancer cells with I36 reversibly suppresses the incorporation of [H-3]thymidine without affecting cell viability or estro,aem receptor (ER) responsiveness, Fl ow cytometry of propidium iodide stained cells revealed that I3C induc es a G(1) cell cycle arrest, Concurrent with the I3C-induced growth in hibition, Northern blot and Western blot analyses demonstrated that I3 C selectively abolished the expression of cyclin-dependent kinase 6 (C DK6) in a dose-and time-dependent manner. Furthermore, I3C inhibited t he endogenous retinoblastoma protein phosphorylation and CDK6 phosphor ylation of retinoblastoma in vi;ro to the same extent, After the MCF7 cells reached their maximal growth arrest, the levels of the p21 and p 27 CDK inhibitors increased by 50%, The antiestrogen tamoxifen also su ppressed MCF7 cell DNA synthesis but had no effect on CDK6 expression, while a combination of I3C and tamoxifen inhibited MCF7 cell growth m ore stringently than either agent alone. The I3C-mediated cell cycle a rrest and repression of CDK6 production were also observed in estrogen receptor-deficient MDA-MB-231 human breast cancer cells, which demons trates that this indole can suppress the growth of mammary tumor cells independent of estrogen receptor signaling Thus, our observations hav e uncovered a previously undefined antiproliferative pathway for I3C t hat implicates CDK6 as a target for cell cycle control in human breast cancer cells, Moreover, our results establish for the first time that CDK6 gene expression can be inhibited in response to an extracellular antiproliferative signal.