ILK interactions and ILK-mediated signaling pathways. (A) This figure depicts the interaction of ILK with various proteins many of which, including ILK, are found at sites of integrin connections to the actin cytoskeleton. ILK can anchor to integrins by interacting with the cytoplasmic domain of certain β integrin subunits. The NH₂-terminal ankyrin repeats mediate interactions with ILKAP, a PP2C phosphatase, which negatively regulates ILK signaling, and PINCH, a LIM domain–only protein, which potentially couples ILK to growth factor receptors and PI 3-kinase by binding to another adaptor protein, Nck-2. PINCH is also required for the localization of ILK to cell adhesion sites and can be found in a stable complex with ILK and another ILK-binding protein, CH-ILKBP. The latter binds to ILK at its COOH terminus, which comprises the kinase domain. A closely related protein, affixin, binds to ILK in the same region, as does paxillin, via its LD1 domain. CH-ILKBP, paxillin, and probably other ILK-binding proteins can bind directly or indirectly to actin filaments, thus coupling integrins to the actin cytoskeleton via ILK. Signaling proteins, PKB, PDK-1, and GSK-3 can also interact with ILK, and ILK can phosphorylate PKB and GSK-3. Affixin is also a substrate of ILK, and the kinase activity of ILK appears to be involved in enhancing the ability of the affixin CH2 domain to inhibit cell spreading. (B) Signaling pathways activated by ILK. PTEN and ILKAP negatively regulate ILK activation. Activated ILK can directly phosphorylate PKB/Akt and GSK-3. Phosphorylation of PKB/Akt by ILK contributes to its activation, leading to suppression of apoptosis and anoikis. Phosphorylation of GSK-3 results in its inhibition, leading to stabilization of β-catenin and stimulation of AP-1 activity. β-catenin and CREB activation via ILK and GSK-3 contribute to the upregulation of cyclin D1, whereas AP-1 activation leads to stimulation of matrix metalloproteinase-9 expression. ILK is also involved in regulating Erk, since overexpression of ILK prevents Erk inactivation during myogenic differentiation. ILK can also directly phosphorylate MLC in a calcium-independent manner, thus potentially regulating smooth muscle contraction and possibly cell motility in nonmuscle cells.