The roles of integrin-linked kinase in the regulation of myogenic differentiation.

Myogenic differentiation is a highly orchestrated, multistep process that is coordinately regulated by growth factors and cell adhesion. We show here that integrin-linked kinase (ILK), an intracellular integrin- and PINCH-binding serine/threonine protein kinase, is an important regulator of myogenic differentiation. ILK is abundantly expressed in C2C12 myoblasts, both before and after induction of terminal myogenic differentiation. However, a noticeable amount of ILK in the Triton X-100-soluble cellular fractions is significantly reduced during terminal myogenic differentiation, suggesting that ILK is involved in cellular control of myogenic differentiation. To further investigate this, we have overexpressed the wild-type and mutant forms of ILK in C2C12 myoblasts. Overexpression of ILK in the myoblasts inhibited the expression of myogenic proteins (myogenin, MyoD, and myosin heavy chain) and the subsequent formation of multinucleated myotubes. Furthermore, mutations that eliminate either the PINCH-binding or the kinase activity of ILK abolished its ability to inhibit myogenic protein expression and allowed myotube formation. Although overexpression of the ILK mutants is permissive for the initiation of terminal myogenic differentiation, the myotubes derived from myoblasts overexpressing the ILK mutants frequently exhibited an abnormal morphology (giant myotubes containing clustered nuclei), suggesting that ILK functions not only in the initial decision making process, but also in later stages (fusion or maintaining myotube integrity) of myogenic differentiation. Additionally, we show that overexpression of ILK, but not that of the PINCH-binding defective or the kinase-deficient ILK mutants, prevents inactivation of MAP kinase, which is obligatory for the initiation of myogenic differentiation. Finally, inhibition of MAP kinase activation reversed the ILK-induced suppression of myogenic protein expression. Thus, ILK likely influences the initial decision making process of myogenic differentiation by regulation of MAP kinase activation.