The RhoA-ROCK-PTEN pathway as a molecular switch for anchorage dependent cell behavior. 
The proliferation of anchorage-dependent cells of mesenchymal origin requires the attachment of the cells to substrates.  Thus, cells that are poorly attached to substrates exhibit retarded cell cycle progression or apoptotic death.  A major disadvantage of most polymers used in tissue engineering is their hydrophobicity; hydrophobic surfaces do not allow cells to attach firmly and, therefore, do not allow normal proliferation rates.  In this study, we investigated the molecular mechanism underlying the reduced proliferation rate of cells that are poorly attached to substrates.  There was an inverse relationship between the activity of the small GTPase RhoA (RhoA) and the cell proliferation rate.  RhoA activity correlated inversely with the strength of cell adhesion to the substrates.  The high RhoA activity in the cells poorly attached to substrates caused an increase in the activity of Rho-associated kinase (ROCK), a well-known effector of RhoA that upregulated the activity of phosphatase and tensin homolog (PTEN).  The resulting activated PTEN downregulated Akt activity, which is essential for cell proliferation.  Thus, the cells that were poorly attached to substrates showed low levels of cell proliferation because the RhoA-ROCK-PTEN pathway was hyperactive.  In addition, RhoA activity seemed to be related to focal adhesion kinase (FAK) activity.  Weak FAK activity in these poorly attached cells failed to downregulate the high RhoA activity that restrained cell proliferation.  Interestingly, reducing the expression of any component of the RhoA-ROCK-PTEN pathway rescued the proliferation rate without physico-chemical surface modifications.  Based on these results, we suggest that the RhoA-ROCK-PTEN pathway acts as a molecular switch to control cell proliferation and determine anchorage dependence.  In cells that are poorly attached to substrates, its inhibition is sufficient to restore cell proliferation without the need for physico-chemical modification of the material surface.