The Potential Role of Integrin Regulation by Par6 in TGF-beta-induced Apoptosis

The Par6-polarity pathway regulates breast cancer metastasis, and more recently has been shown to regulate transforming growth factor β (TGFβ)-induced apoptosis. Integrins may mediate the regulation of TGFβ-induced apoptosis by Par6, as they are key regulators of cell polarity, survival and death. First, we confirmed that blocking Par6 activation significantly inhibits TGFβ-induced apoptosis in both monolayer and three-dimensional NMuMG (Normal Murine Mammary Gland) cell culture models. TGFβ altered the expression of β1 and β4 integrins in NMuMG monolayers. In addition, TGFβ significantly reduced the basal localization of α6 and β4 integrins in NMuMG three-dimensional acini-like structures (p < 0.001), which was dependent on both Par6 and TGFβ receptor I (TβRI)/SMAD activation. We went on to show that the activities of integrin pro-survival signaling mediators, NF-κB and FAK, were altered in response to TGFβ, and that blocking Par6 activation in the Par6/S345A mutant maintained polarity and basal α6 and β4 integrin expression in the presence of TGFβ in NMuMG three-dimensional structures, in addition to a significant increase in FAK activation. This suggests that TGFβ alters the expression, localization and downstream signaling of integrins, which may contribute to TGFβ-induced apoptosis