| Integrin β4 is a non-conventional β integrin subunit that pairs almost exclusively with integrin α6 to form the α6β4 heterodimer, a laminin receptor that organizes hemidesmosomes and also acts as a signaling hub linking the extracellular matrix to intracellular pathways controlling adhesion, survival, and migration in epithelial tissues and carcinomas. The α6β4 complex spans the plasma membrane with a classical integrin ectodomain that binds laminin-332 in the basement membrane, but β4 is distinguished by an unusually long cytoplasmic tail containing multiple fibronectin type III–like domains and tyrosine-rich regulatory motifs, which provide docking sites for cytolinker proteins such as plectin and BPAG1e in stable hemidesmosomes and for signaling molecules when the receptor is mobilized into motile structures. Hemidesmosome assembly is initiated by β4 through its cytoplasmic association with plectin, which connects the integrin to keratin intermediate filaments, while the extracellular α6β4 head engages laminin, thereby creating an anchoring unit that confers strong mechanical linkage between basal keratinocytes and the basement membrane; genetic ablation of β4 in mice eliminates hemidesmosomes, causes downregulation of α6, results in fragile attachment of stratified epithelia to the basal lamina, and leads to tissue disorganization reminiscent of junctional epidermolysis bullosa. Growth factor and chemokine signaling pathways can trigger phosphorylation-dependent remodeling of the β4 cytoplasmic tail, including protein kinase C–driven events that uncouple α6β4 from hemidesmosomal complexes and promote its redistribution to actin-rich lamellipodia and filopodia, where the integrin participates in dynamic adhesion turnover and drives chemotactic migration on laminin-containing matrices. Within these motile structures, β4 engages signaling networks that integrate with receptor tyrosine kinases; β4 forms complexes with c-Met and other receptors, and a key tyrosine residue in the β4 cytodomain (Y1494) controls recruitment and activation of Shp2 and Src, leading to sustained activation of downstream effectors such as Ras–ERK and PI3K–Akt, which support anchorage-independent growth and enhanced invasive behavior under hepatocyte growth factor stimulation. This dual capacity to nucleate hemidesmosomes under homeostatic conditions and to switch into a signaling-competent, migration-promoting state under growth factor or stress cues positions integrin β4 as a central regulator of epithelial plasticity, coordinating the balance between stable adhesion and motility. In clinical specimens, elevated expression or mislocalization of α6β4 from hemidesmosomes into nonjunctional membrane domains correlates with aggressive behavior and poor prognosis in several malignancies, including breast, colorectal, and lung cancers, and β4-dependent signaling is linked to enhanced survival under detachment stress and to increased invasion through basement membrane–like barriers. |
