Impaired cAMP and Rac 1 Signaling Contribute to TNF‐α‐induced Endothelial Barrier Breakdown in Microvascular Endothelium

Objective: In sepsis, tumor necrosis factor‐alpha (TNF‐α) contributes to endothelial barrier breakdown. The involvement of Rho A/rho kinase signaling has recently been challenged. Here, we tested the role of cAMP and Rac 1 signaling. Materials and Methods: For this study, we took in vivo measurements of hydraulic conductivity in postcapillary mesenteric venules of adult rats. Measurements of transendothelial electrical resistence (TER), fluorescein isothiocyanate–dextran flux, Western blotting, immunostaining, and enzyme‐linked immunosorbent assay–based measurements of cAMP levels and Rho‐GTPase activity in human microvascular endothelial cells. Results: TNF‐α disrupted endothelial barrier functions in vivo and in vitro. Under these conditions, Rho A activity was significantly increased, whereas Rac 1 activity was decreased and Cdc42 was unaltered. Moreover, cAMP levels were reduced. Rho kinase inhibition, using Y27632, did not prevent TNF‐α‐induced barrier breakdown. In contrast, preincubation with forskolin and rolipram (F/R) to increase cAMP and cytotoxic necrotizing factor 1 to activate Rac 1 and Rho A abolished TNF‐α‐induced barrier breakdown in vivo and in vitro. Moreover, inactivation of Rac 1 was blocked by F/R‐mediated increase of cAMP, whereas Rho A activation was only partially inhibited. Conclusion: Our data indicate that decrease of cAMP and Rac 1 inactivation, rather than Rho A activation, contribute to TNF‐α‐induced endothelial barrier breakdown in vivo and in vitro.