The major findings of the present study are that the MEK5-BMK1 pathway mediates flow-dependent inhibition of TNF signaling in EC as shown by a novel MEK5-selective inhibitor BMK1, the MAP kinase downstream of MEK5, was shown previously to play a critical role in EC function since both mek5
knockout mice rapidly die due to disruption of vascular integrity[10
]. Our previous studies have demonstrated that BMK1 plays an essential role in the ability of shear stress to prevent EC apoptosis[10
]. The present study further supports the key role of BMK1 in flow mediated anti-inflammatory effects. In particular, our data show a significant and specific role of BMK1 in inhibiting TNF-induced JNK activation.
Steady laminar flow inhibits inflammatory responses in EC by both short-term and long-term mechanisms. Flow regulates the activity of several transcription factors including NF-κB, Kruppel-like factor-2 (KLF2) and PPARγ. KLF2 and PPARγ are particularly relevant, since recent reports show these factors are regulated by a MEK5-ERK5 pathway. KLF2 is induced by laminar shear stress in cultured EC[25
], inhibits induction of VCAM-1, and a MEK5-BMK1-MEF2C pathway is required for up regulation of KLF2 [28
]. In EC, laminar flow activates PPARγ activity and activation of PPARγ exerts anti-inflammatory effects since PPARγ ligands reduce TNF induced VCAM-1 expression [29
]. We have shown previously that flow-induced PPARγ activation is dependent upon ERK5, and is functionally important because expression of dominant negative PPARγ decreased the ability of ERK5 to decrease VCAM-1 expression [30
The mechanisms by which MEK5-BMK1 inhibit TNF signaling remain to be elucidated fully. Intriguingly, both MEK5 and PKC-ζ are PB1 domain containing proteins. Thus it is possible that competition between MEK5 and PKC-ζ influences the relative activation of these pathways. Future studies of PB1 domain interacting proteins and MEK5-BMK1 kinase substrates would appear indicated.
The data in this study confirm the selectivity of the novel MEK5 inhibitor, BIX02188 in EC. We also confirmed the selectivity of PD183452 for MEK1 in EC. BIX02188 may be a useful reagent to investigate the role of MEK5-BMK1 signaling in the cardiovascular system, bypassing the mortality associated with MEK5 and BMK1 deficiency. This reagent should enable long term studies of the role of MEK5-BMK1 in cardiovascular diseases such as heart failure, aortic aneurysm and atherosclerosis.