GEF-H1 mediates tumor necrosis factor-α-induced Rho activation and myosin phosphorylation: Role in the regulation of tubular paracellular permeability

Tumor necrosis factor-α (TNF-α), an inflammatory cytokine, has been shown to activate the small GTPase Rho, but the underlying signaling mechanisms remained undefined. This general problem is particularly important in the kidney, because TNF-α, a major mediator of kidney injury, is known to increase paracellular permeability in tubular epithelia. Here we aimed to determine the effect of TNF-α on the Rho pathway in tubular cells (LLC-PK₁ and Madin-Darby canine kidney), define the upstream signaling, and investigate the role of the Rho pathway in the TNF-α-induced alterations of paracellular permeability. We show that TNF-α induced a rapid and sustained RhoA activation that led to stress fiber formation and Rho kinase-dependent myosin light chain (MLC) phosphorylation. To identify new regulators connecting the TNF receptor to Rho signaling, we applied an affinity precipitation assay with a Rho mutant (RhoG17A), which captures activated GDP-GTP exchange factors (GEFs). Mass spectrometry analysis of the RhoG17A-precipitated proteins identified GEF-H1 as a TNF-α-activated Rho GEF. Consistent with a central role of GEF-H1, its down-regulation by small interfering RNA prevented the activation of the Rho pathway. Moreover GEF-H1 and Rho activation are downstream of ERK signaling as the MEK1/2 inhibitor PD98059 mitigated TNF-α-induced activation of these proteins. Importantly TNF-αenhanced the ERK pathway-dependent phosphorylation of Thr-678 of GEF-H1 that was key for activation. Finally the TNF-α-induced paracellular permeability increase was absent in LLC-PK₁ cells stably expressing a non-phosphorylatable, dominant negative MLC. In summary, we have identified the ERK/GEF-H1/Rho/Rho kinase/phospho-MLC pathway as the mechanism mediating TNF-α-induced elevation of tubular epithelial permeability, which in turn might contribute to kidney injury.