The small GTP-binding proteins Rho and Rac induce T cell adhesion to the mucosal addressin MAdCAM-1 in a hierarchical fashion

Here we report that an activator (AIF(4)(-)) of heterotrimeric GTP-binding proteins (G-proteins) and inhibitors (lovastatin and C3 exoenzyme) of small GTP-binding proteins regulate the induction of alpha 4 beta 7-mediated adh esion of TK-1 T lymphoma cells (alpha 4(+)beta 7(+)beta 1(-)) to the mucosa l addressin cell adhesion molecule MAdCAM-1. Activation of cell adhesion by AIF(4)(-) was abrogated by lovastatin, thereby establishing a link between heterotrimeric G-proteins and small GTP-binding proteins in the regulation of alpha 4 beta 7-mediated cell adhesion. Increased numbers of cells bound MAdCAM-1-coated microspheres following activation with AIF(4)(-), discount ing an obligatory role for cell spreading in alpha 4 beta 7-mediated cell a dhesion. MAdCAM-1-Fc dimers triggered ligand-induced clustering of alpha 4 beta 7 in response to AIF(4)(-) and Mn2+-induced activation of integrins. H ence alpha 4 beta 7 cluster formation may be responsible, at least in part, for inducing cell adhesion in response to both extracellular and intracell ular signals that impact on integrin function. Electroporation of constitut ively active V14RhoA and V12Rac1 recombinant proteins into TK-1 cells revea led that both RhoA and Rac1 induce alpha 4 beta 7 adhesion to MAdCAM-1. Act ivation is hierarchical since Rad is unable to directly activate alpha 4 be ta 7, but induces cell adhesion via RhoA, whereas the transient induction o f cell adhesion mediated by RhoA is dependent on the activities of protein 'cyrosine kinases and protein kinase(s) C.