| Summary: | Despite improvements in diagnosis and treatment, breast cancer is still the most common cancer type among non-smoking females. TGF-β can inhibit breast cancer development by inducing cell cycle arrest in both, cancer cells and, as part of a senescence program in normal human mammary epithelial cells (HMEC). Moreover, TGF-β also drives cell migration and invasion, in part through the small GTPases Rac1 and Rac1b. Depletion of Rac1b or Rac1 and Rac1b in MDA-MB-231 or MDA-MB-435s breast cancer cells by RNA interference enhanced or suppressed, respectively, TGF-β1-induced migration/invasion. Rac1b depletion in MDA-MB-231 cells also increased TGF-β-induced p21WAF1 expression and ERK1/2 phosphorylation. Senescent HMEC (P15/P16), when compared to their non-senescent counterparts (P11/P12), presented with dramatically increased migratory activity. These effects were paralleled by elevated expression of genes associated with TGF-β signaling and metastasis, downregulated Rac1b, and upregulated Rac1. Our data suggest that acquisition of a motile phenotype in HMEC resulted from enhanced autocrine TGF-β signaling, invasion/metastasis-associated gene expression, and a shift in the ratio of antimigratory Rac1b to promigratory Rac1. We conclude that although enhanced TGF-β signaling is considered antioncogenic in HMEC by suppressing oncogene-induced transformation, this occurs at the expense of a higher migration and invasion potential.
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