A bidirectional "α_vβ₃ integrin-ERK1/ERK2 MAPK" connection regulates the proliferation of breast cancer cells

In addition to their role in cell migration and adhesion, integrins elicit a series of transduction events that regulate cell-cycle progression and apoptosis in a process known as "outside-in" signaling. A second mode of integrin regulation known as "inside-out" signaling, in which the activation of major cell transduction cascades can influence the activation status of some integrins, has also been described. Here, we have assessed the role of the extracellular signal-regulated kinase (ERK1)/ERK2, mitogen-activated protein kinase (MAPK), and phospoinositide 3-kinase (PI-3′K) signaling pathways in the expression and function of α_vβ₃ integrin in breast cancer models. Pharmacological inhibition of MEK1 and MEK2 with U0126 drastically increased the levels of α_vβ ₃ in Heregulin (HRG)-overexpressing MDA-MB-231 cells (231/WT, 231/VEC) and derivatives transfected with the antisense orientation of the HRG-β2 full length cDNA (231/ASPOOL, 231/AS31). Interestingly, this was related to a significant decrease of viability and of the S- and G2/M subcompartment of the cell cycle in MDA MB 231 cells in response to U0126. Furthermore, specific inhibition of the PI-3′K pathway with LY294002 also induced an increase of α_vβ₃ levels but to a lesser extent. Moreover, pretreatment of MDA-MB-231 cells with U0126 antagonized the effects of small peptidomimetic α_vβ₃ antagonists. Remarkably, inhibition of the PI-3′K/AKT pathway did not exert the same effects, thus suggesting that the "outside-in" as well as the "inside-out" α_vβ₃-mediated signaling goes primarily through the ERK1/ERK2 MAPK pathway in MDA MB 231 breast cancer cells. Collectively, these results strongly suggest the existence of a bidirectional molecular connection α_vβ₃-ERK1/ ERK2 MAPK that would regulate breast cancer cells survival and proliferation.