Involvement of phosphatases in the anchorage-dependent regulation of ERK2 activation.

Activation of extracellular signal-regulated kinase (ERK) is known to be regulated by cell adhesion, namely "anchorage dependence". Most studies on the anchorage-dependent regulation have focused on the upstream activating components. We previously reported that the focal adhesion protein vinexin beta can induce the anchorage-independent activation of ERK2. We show here that vinexin beta-induced anchorage-independent activation of ERK2 involves prevention of the dephosphorylation of ERK2, but not the promotion of MEK1 or Raf1 activity. Furthermore, knockdown of vinexin beta resulted in a faster dephosphorylation of ERK2 in A549 cells. Moreover, the coexpression of MKP3/rVH6, an ERK2 specific phosphatase, suppressed the anchorage-independent activation of ERK2 induced by vinexin beta. These results suggest that vinexin beta can prevent the dephosphorylation of ERK2 stimulated by cell detachment, leading to the anchorage-independent activation of ERK2. Furthermore, we found that phosphatase activity directed against activated ERK2 was higher in suspended cells than in adherent cells. In addition, orthovanadate efficiently induces anchorage-independent activation of ERK2 without marked activation of MEK1 in NIH3T3 cells. These observations suggest that the anchorage dependence of ERK1/2 activation is regulated not only by upstream kinases, Raf1 and MEK, but also by phosphatases acting against ERK1/2 and that vinexin beta can induce anchorage-independent activation of ERK by preventing the inactivation of ERK1/2.