This review examines the hypothesis that the function of the alpha 6beta 4 integrin is altered substantially as normal epithelia undergo malignant transformation and progress to invasive carcinoma and that the functions of this integrin contribute to the behavior of aggressive carcinoma cells. Specifically, alpha 6beta 4 functions primarily as an adhesion receptor in normal epithelia, often as a component of hemidesmosomes and associated with intermediate filaments. Factors in the host-tumor microenvironment have the potential to mobilize alpha 6beta 4 from hemidesmosomes and promote its association with F-actin in lamellae and filopodia, a process that is mediated by PKC-dependent phosphorylation of the beta 4 cytoplasmic domain. Importantly, this altered localization of alpha 6beta 4 appears to be coupled to an activation of its signaling potential, which may occur through its association with growth factor receptors or lipid rafts, possibilities that are not mutually exclusive. The primal signaling event triggered by alpha 6beta 4 appears to be activation of PI3-K and this activation has profound consequences on the migration, invasion and survival of carcinoma cells. Arguably, the ability of alpha 6beta 4 to stimulate the PI3-K-dependent translation of VEGF and possibly other growth factors may be the most significant contribution of this integrin to carcinoma because of the potential autocrine and paracrine effects of these factors.

We define a novel mechanism by which integrins regulate growth factor expression and the survival of carcinoma cells. Specifically, we demonstrate that the α6β4 integrin enhances vascular endothelial growth factor (VEGF) translation in breast carcinoma cells. The mechanism involves the ability of this integrin to stimulate the phosphorylation and inactivation of 4E-binding protein (4E-BP1), a translational repressor that inhibits the function of eukaryotic translation initiation factor 4E (eIF-4E). The regulation of 4E-BP1 phosphorylation by α6β4 derives from the ability of this integrin to activate the PI-3K–Akt pathway and, consequently, the rapamycin-sensitive kinase mTOR that can phosphorylate 4E-BP1. Importantly, we show that this α6β4-dependent regulation of VEGF translation plays an important role in the survival of metastatic breast carcinoma cells by sustaining a VEGF autocrine signaling pathway that involves activation of PI-3K and Akt. These findings reveal that integrin-mediated activation of PI-3K–Akt is amplified by integrin-stimulated VEGF expression and they provide a mechanism that substantiates the reported role of α6β4 in carcinoma progression.

The A6B4 integrin has been widely implicated in carcinoma function in vitro; however, in vivo data are scarce. To determine the importance of A6B4 in tumor progression, a SUM-159 breast carcinoma cell line that is essentially devoid of A6B4 expression was generated using an RNA interference strategy. Loss of A6B4 expression inhibits colony formation in soft agar assays, suggesting a vital role for A6B4 in survival signaling and anchorage-independent growth. Orthotopic injection of the B4-deficient cell line into the mammary fat pad of immunocompromised mice yielded significantly fewer and smaller tumors than the control cell line, revealing a role for the A6B4 integrin in tumor formation. Under conditions that mimicked the in vivo environment, decreased expression of the A6B4 integrin led to enhanced apoptosis as determined by the percentage of Annexin V-FITC+, PIÀ cells and the presence of caspase-3 cleavage products. Recombinant vascular endothelial growth factor (VEGF) significantly inhibited the cell death observed in the B4-deficient cell line, demonstrating the importance of VEGF expression in this survival pathway. Furthermore, loss of A6B4 expression leads to enhanced apoptosis and reduced expression of VEGF in breast carcinoma cells in vivo. Importantly, the specificity of A6B4 in both the in vitro and in vivo assays showed that reexpression of the B4 subunit into the B4-deficient cell line could rescue the functional phenotype. Taken together, these data implicate the A6B4 integrin in tumor formation by regulating tumor cell survival in a VEGF-dependent manner.