TGF-β supervises centrosome amplification and tetraploidy in the mammary gland

At its earliest stages, prior to mutational loss of p53, breast cancer is characterized by centrosomal abnormalities (CA) and chromosomal instability (Lingle 2002) and shows signs of constitutive DNA damage response (Bartkova 2005). Genetic instability potentially occurs through impaired responses to damage from exogenous agents, and/or environmental stress within p53 competent cells. High doses of IR, an established carcinogen but also therapeutic of the breast induces CA within tumor cells (Sato 2000). It is unknown if IR or environmental stress is able to induce CA in normal p53 competent cells, and if, whether these can persist within a population and resolve with genetic instability. We show that the frequency of CA dramatically increases in the progeny of p53 competent human mammary epithelial cells surviving graded exposure to even low dose IR. The frequency of CA are altered by exogenous TGF-β, an extracellularly activated cytokine. Whereas the presence of exogenous TGF-β did not prevent the generation of CA its addition significantly decreased the levels of persistent CA. Also, the inhibition of endogenous TGF-β signaling in the epithelium increased the frequency of cells with CA even without IR. Within murine tissue, in-situ analysis confirmed this supervisory role of TGF-β on CA. Inhibition of TGF-β1 signaling within ex-vivo murine mammary epithelia augmented the persistence of CA, spindle abnormalities and consequent tetraploidy. Examination of CA within murine mammary glands of varying genetic backgrounds revealed a genetic interaction between p53 and TGF-β pathways in the supervision of CA and ploidy. Cumulatively these data demonstrate that in the absence of extracellular TGF-β1 p53 competent epithelial cells can accumulate CA and tetraploidy in response to DNA damaging agents or environmental stress. The role of TGF-β in suppressing CA directly links microenvironment and genomic integrity showing an additional level of defense against carcinogenesis.