KLF11 Modulates Arterial Thrombosis: New Insights into Transcriptional Control of Tissue Factor Expression in Smooth Muscle Cells

Tissue Factor (TF, coagulation factor III), encoded by F3 in human (Cf3 in mouse), is an integral membrane protein that serves as an obligatory cofactor for the serine protease fVII(a). TF/fVIIa complex (“extrinsic tenase”) triggers the coagulation cascade;¹ it can also promote cell signaling events via a variety of mechanisms, most notably the cleavage of protease-activated receptors.² TF expression in the intima and media is upregulated in response to arterial injury.³ F3 genomic structure was described in 1989, leading to the identification of multiple cis-acting elements in the F3 promoter including Sp1, Egr-1, AP-1, and NFκB that were subsequently shown to be functional.^4,5 Further work identified multiple agonists that promote the expression of TF in vascular smooth muscle cells (SMC) and monocytic cells including LPS, phorbol esters, serum, thrombin and cytokines including TNFα, IL-1β, IL-17, and MCP-1.^5–7

In contrast to well-studied transcription factors that promote TF expression, relatively little is known concerning those that suppress it. However, new insights are emerging that establish Krüppel-like factors (KLFs), a family of transcription factors containing a C-terminal Cys2/His2 zinc-finger DNA-binding domain, as modulators of TF expression in the vasculature. Previous studies have demonstrated that KLF2 suppresses TNFα-mediated upregulation of TF expression in the vascular endothelium,⁸ and increased KLF4 may be responsible for the lower TF levels observed in the aorta of prekallikrein-deficient mice, which have a delayed thrombotic response to FeCl₃-induced carotid injury.⁹ In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Liang et al convincingly demonstrate that KLF11 is an anti-thrombotic factor in the vascular wall: KLF11 acts, at least in part, by serving as a transcriptional regulator of TF expression in SMC, likely via competing with AP-1 for binding to the F3 promoter.¹⁰ Importantly, Liang et al demonstrate KLF11-mediated TF suppression in vivo using Klf11 knockout mice (Figure, panel A), and in vitro using human aortic SMC. Further, they show that F3 promoter contains a transforming growth factor-β-inducible early gene (TIEG) site recognized by KLF11 that is also present in mouse, rat, and rabbit. Deletion of this TIEG site in an F3 promoter-driven luciferase reporter construct diminishes sensitivity to KLF11 when assayed in rat SMC. This raises the possibility that enhancing KLF11 expression in the vessel wall could be translationally explored to reduce TF in such disease settings as atherosclerosis.¹¹

Figure. KLF1-TF axis in arterial thrombosis.

A. Compared to wildtype (WT) mice, conventional Klf11 knockout mice and vascular SMC-specific Klf11 knockout mice have higher levels of TF in the arterial wall; steady-state levels of circulating TF+ MVs are similar in WT and conventional Klf11 knockout mice. Upon exposure of the adventitia to FeCl₃, carotids lacking KLF11 occlude faster than WT carotids; whether SMC-derived TF⁺ MVs enter the circulation following FeCl₃ injury is unknown. B. Two possible mechanisms for how SMC-TF contributes to thrombosis in the FeCl₃ model of arterial injury: FeCl₃ may promote the release of SMC-derived TF⁺ MVs, which are endocytosed by endothelial cells that then display SMC-TF on their luminal surface (left). FeCl₃ may also increase the permeability of tight junctions between endothelial cells, thus facilitating the release of SMC-derived TF⁺ MVs into the circulation (right).

Extending earlier observations on the important contribution of SMC-TF to arterial thrombosis,^12,13 Liang et al demonstrate that the deletion of KLF11 in murine SMC yields higher TF levels in the vessel wall, and it also aggravates FeCl₃–induced thrombosis of carotid arteries. In this light, Barr et al¹⁴ explored FeCl₃–induced thrombosis of carotid arteries using scanning electron and brightfield intravital microscopy, finding that FeCl₃ does not cause endothelial denudation; thus, the mechanism(s) by which SMC-TF impacts luminal thrombus formation in this setting remains to be defined. One intriguing possibility is that SMC-derived microvesicles (MVs) deliver TF to the endothelial cells, enabling its exposure on the luminal surface. Another possibility is that FeCl₃ may increase endothelial tight junction permeability, thus facilitating the release of SMC-derived TF⁺ MVs (Figure, panel B). Further elucidation of the KLF11-TF axis in the vascular setting is highly likely to be of clinical significance, and it will also expand our understanding of the pathophysiology underlying injury-triggered arterial thrombosis.