Disruption of Rho signal transduction upon cell detachment

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Abstract
Serum-soluble factors play a dominant role in the activation of the small GTPase RhoA. Cell adhesion also modulates RhoA activity but the effect is modest in the absence of serum. Here, we show that cell adhesion is required for serum-stimulated Rho signal transduction leading to myosin light chain (MLC) phosphorylation. Characterization of Rho-kinase substrates revealed that diphosphorylation of MLC at Thr-18 and Ser-19 (ppMLCT18/S19) and phosphorylation of the myosin-binding subunit (MBS) of myosin phosphatase at Thr-853 (pMBST853) were mostly Rho and Rho-kinase dependent in attached fibroblasts. MLC monophosphorylation at Ser-19 (pMLCS19) was partially dependent on Rho kinase, whereas phosphorylation of MBS at Thr-696 (pMBST696) and phosphorylation of CPI-17 at Thr-38 (pCPI-17T38) were mostly Rho-kinase independent. Cell detachment caused a significant reduction in pMLCS19 and a more dramatic decrease of ppMLCT18/S19 without inhibiting RhoA. pMBST853, pMBST696 and pCPI-17T38 were not significantly reduced, suggesting that myosin-phosphatase activity was little changed. Cells expressing active RhoA (RhoAV14) or Rho-kinase catalytic domain maintained elevated pMBST853 upon detachment but failed to support ppMLCT18/S19, indicating that the ability of Rho kinase to phosphorylate MLC is impaired. Reattachment to immobilized fibronectin resulted in a gradual recovery of Rho-kinase-induced ppMLCT18/S19 that is absent from the cells attached to poly-L-lysine. The convergence of signals from soluble factors and cell adhesion might therefore occur at the point of MLC phosphorylation, providing an effective mechanism for dynamic control of contractility during cell migration.