From stop to go

Better known for its inhibitory cell-cycle function as a tumour suppressor, p27^Kip1 is extending its molecular repertoire. In Genes & Development, Arnaud Besson et al. tell us about its ability to regulate cell migration by modulating the small GTPase Rho.

Earlier work from other groups had shown that the motility of mouse embryonic fibroblasts (MEFs) from p27^−/− mice was impaired compared to MEFs from wild-type animals, and that increased expression of cytoplasmic p27^Kip1 stimulated cell migration. Besson et al. confirmed the effect of p27^Kip1 on cell migration, and went on to show that this could not be overcome by adding known motility factors, or Ras, a potent inducer of motility. Moreover, glioblastoma cells that were treated with antisense oligonucleotides targeted against p27^Kip1 showed a similar migration defect.

The activity of Rho must be carefully regulated, as it promotes the formation of stress fibres and focal adhesions, both of which inhibit cell migration by promoting too much cell adhesion. So the fact that p27^−/− cells had many stress fibres and focal adhesions, and integrin localization to focal adhesions was enhanced, indicated a potential activation of Rho in these cells (Rho also regulates integrin localization at focal adhesions). Indeed, the authors found higher amounts of Rho–GTP in p27^−/− cells than in wild-type cells, whereas the small GTPases Rac and Cdc42 were unaffected.

An effector of Rho, Rho kinase (Rock), often influences cell migration, so it was perhaps not surprising that treating p27^−/− cells with the Rock inhibitor Y27632 restored normal migration. p27^Kip1, therefore, seems to function upstream of Rock in controlling cell migration. However, unlike a close relative, p21^Waf1/Cip1, which directly inhibits Rock activity, p27^Kip1 had no effect on Rock. Instead, p27^Kip1 bound directly to Rho irrespective of whether it was GTP- or GDP-bound.

The interaction of p27^Kip1 with Rho occurred through the carboxyl terminus of p27^Kip1; its amino terminus binds to cyclins or cyclin-dependent kinases. These separate binding domains are consistent with the finding that a form of p27^Kip1 that cannot bind these cell-cycle regulators rescued the migratory defect of p27^−/− cells; wild-type p27^Kip1, as expected, restored cell motility in this situation. So the role of p27^Kip1 in migration is independent of its function in the cell cycle.

The consequence of the direct interaction between Rho and p27^Kip1 seems to be the inhibition of Rho activation by its guanine-nucleotide-exchange factors. So, depending on the levels of p27^Kip1, Rho activity will be inhibited and cell migration will be enhanced accordingly. Although the authors are careful to point out that this effect is probably cell-type-specific, p27^Kip1 is frequently inactivated in many cancers, often correlating with increased invasion. Inactivation can occur by exclusion from the nucleus, which might not only interfere with its inhibitory cell-cycle role, but also place it in a prime position for inhibiting Rho and increasing migration. And migration is probably just one of the ways in which the effect of p27^Kip1 on the cytoskeleton is manifest — how broadly p27^Kip1 might influence the many other processes that are controlled by the cytoskeleton remains to be seen.