Cell to matrix adhesion regulates cellular homeostasis in multiple ways. Integrin attachment to the extracellular matrix mediates this regulation through direct and indirect connections to the actin cytoskeleton, growth factor receptors, and intracellular signal transduction cascades. Disruption of this connection to the extracellular matrix has deleterious effects on cell survival. It leads to a specific type of apoptosis known as anoikis in most non-transformed cell types. Anchorage independent growth is a critical step in the tumorigenic transformation of cells. Thus, breaching the anoikis barrier disrupts the cell's defenses against transformation. This review examines recent investigations into the molecular mechanisms of anoikis to illustrate current understanding of this important process.

Integrins clearly play a key role in regulating both mitogenic signalling and cell migration. Thus integrins modulate the efficiency of the Erk (extracellular-signal-regulated kinase)/MAP kinase (mitogen-activated protein kinase) pathway, acting at several distinct levels. We have shown that both cAMP-dependent protein kinase and PAKs (p21-activated kinases) play a role in integrin regulation of the Erk pathway, acting primarily at the level of Raf-1. Integrins and PAKs also play a role in the control of cell migration. Thus we have discovered a novel protein that links the alpha5beta1 integrin to migration controlled by Rho-family GTPases. This protein, termed Nischarin, is a large cytosolic macromolecule that is not related to well-known protein families. The N-terminus of Nischarin interacts with a short segment of the cytoplasmic domain of the alpha5 integrin subunit. Overexpression of Nischarin alters actin organization and inhibits Rac-driven cell migration and tumour cell invasion. Use of effector domain mutants of Rac suggest that Nischarin acts downstream of Rac, probably at the level of PAK-family kinases. These studies emphasize the intricate connection between integrins and Rho-family GTPases and their effectors in controlling both mitogenesis and migration.

Nischarin, a novel intracellular protein, was originally identified as a binding partner for the a5b1 integrin. Here we show that Nischarin also interacts with members of the PAK family of kinases. The amino terminus of Nischarin preferentially binds to the carboxy-terminal domain of PAK1 when the kinase is in its activated conformation. Nischarin binding to PAK1 is enhanced by active Rac, with the three proteins forming a complex, while expression of the a5b1 integrin also increases the Nischarin/PAK1 association. Interaction with Nischarin strongly inhibits the ability of PAK1 to phosphorylate substrates. This effect on PAK kinase activity closely parallels Nischarin's ability to inhibit cell migration. Conversely, reduction of endogenous levels of Nischarin by RNA interference promotes cell migration. In addition, PAK1 and Nischarin colocalize in membrane ruffles, structures known to be involved in cell motility. Thus, Nischarin may regulate cell migration by forming inhibitory complexes with PAK family kinases.

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Nischarin regulates Rac1-dependent cell motility by interaction with and inhibition of the p21-activated kinase (PAK1). In addition to regulating the activation of PAK1, Rac1 controls multiple downstream pathways to regulate cell growth and differentiation, as well as cell motility. Signaling by a constitutively activated Rac1 mutant deficient in PAK binding (Rac1Q61L-40C) was examined to determine whether Nischarin impinges on these other Rac1 effector pathways. Nischarin formed immunoprecipitatable complexes with Rac1Q61L and Rac1Q61L-40C when the proteins were co-expressed.