Blockade of Rac1 activity induces G1 cell cycle arrest or apoptosis in breast cancer cells through downregulation of cyclin D1, survivin, and X-linked inhibitor of apoptosis protein.

Rac1 GTPase regulates a variety of signaling pathways that are implicated in malignant phenotypes. Here, we show that selective inhibition of Rac1 activity by the pharmacologic inhibitor NSC23766 suppressed cell growth in a panel of human breast cancer cell lines, whereas it had little toxicity to normal mammary epithelial cells. NSC23766 elicits its cytotoxicity via two distinct mechanisms in a cell line-dependent manner: induction of G(1) cell cycle arrest in cell lines (MDA-MB-231, MCF7, and T47D) that express retinoblastoma (Rb) protein or apoptosis in Rb-deficient MDA-MB-468 cells. In MDA-MB-231 cells, Rac1 inhibition induced G(1) cell cycle arrest through downregulation of cyclin D1 and subsequent dephosphorylation/inactivation of Rb. By contrast, MDA-MB-468 cells underwent substantial apoptosis that was associated with loss of antiapoptotic proteins survivin and X-linked inhibitor of apoptosis protein (XIAP). Rac1 knockdown by RNAi interference confirmed the specificity of NSC23766 and requirement for Rac1 in the regulation of cyclin D1, survivin, and XIAP in breast cancer cells. Further, NF-kappaB, but not c-Jun NH(2)-terminal kinase or p38 pathways, mediates the survival signal from Rac1. Overall, our results indicate that Rac1 plays a central role in breast cancer cell survival through regulation of NF-kappaB-dependent gene products.