The stimulation of thrombosis by hypoxia

Highlights

• Hypoxia is a stimulus for thrombus formation.

• Hypoxia-inducible factors control the vascular response to hypoxia.

• Thrombus formation can be regulated via HIF-dependent or -independent pathways.

Abstract

Thrombus formation is increased under conditions of hypoxia in animal models of thrombosis and in human populations, but current therapies for thrombosis do not directly target hypoxia-responsive signaling pathways. The vascular response to hypoxia is controlled primarily by the hypoxia-inducible transcription factors (HIFs), whose target genes include several factors that regulate thrombus formation. In this article, we review the HIF-dependent and HIF-independent signaling pathways that regulate thrombus formation under hypoxic conditions. A better understanding of hypoxia-induced thrombus formation could lead to the development of novel prophylactic therapies for thrombosis.

Introduction

Venous thromboembolism (VTE), comprising of deep vein thrombosis (DVT) and pulmonary embolism (PE), remains a major health burden [1,2]. Risk factors for VTE include surgery, pregnancy, and systemic hypoxia (i.e. reduced oxygenation) [3,4]. In Europe, ~1.1 million VTE events occur per year, causing >500,000 deaths annually [5]. In USA, VTE incidences of ~60% are found in patients undergoing orthopedic surgery [1] and total deaths from VTE are >500,000 per year [6]. The global incidence of VTE has been estimated at ~2 per 1000 people per year, and thromboembolic conditions are estimated to account for ~1 in 4 deaths worldwide [7]. Despite progress in the development of effective treatments for thrombosis, current therapies still possess limitations, including increased risk of bleeding [8,9]. Direct oral anticoagulants are often contraindicated in elderly patients (>80 years) and in patients with impaired renal function or advanced cancer [10,11]. The lack of safe and effective treatments for VTE raises an urgent need to better understand the mechanisms that regulate thrombus formation, which could lead to the identification of novel therapeutic targets, and eventually to the development of effective prophylactic therapies. In this review, we describe the regulation of thrombus formation by hypoxia and hypoxia-responsive signaling pathways.

Thrombus formation occurs under conditions of increased coagulation, endothelial injury, and venous stasis (i.e. Virchow's Triad). Risk factors for thrombosis are directly or indirectly associated with one of these conditions and commonly known risk factors include immobilization and trauma [12]. Reduced oxygenation (i.e. hypoxia) is also a risk factor for thrombosis, since the incidence of thrombosis is increased under systemic or local hypoxia [[13], [14], [15], [16]]. Hypoxia occurs when oxygen demand is greater than oxygen supply, for example when blood flow is reduced by immobility or disrupted by trauma. Reductions in oxygenation trigger a myriad of molecular and cellular signaling pathways that can contribute to the regulation of thrombus formation [17,18]. In other words, hypoxia is not only a consequence of vascular occlusion, but also stimulates thrombogenesis [19,20] (Fig. 1). Investigations of human populations and animal models of hypoxia and thrombosis suggest that hypoxia and its downstream signaling promote thrombus formation and propagation. These studies indicate that hypoxia-responsive signaling pathways could be therapeutically targeted to reduce VTE burden.

The vascular response to hypoxia is controlled primarily by the hypoxia-inducible transcription factors (HIFs). HIFs are heterodimeric nuclear transcription factors consisting of α and β subunits, which together regulate transcription of genes that mediate the homeostatic responses to reduced oxygenation [21]. The α sub-units of HIF1 and HIF2 (i.e. HIF1α and HIF2α respectively) are hypoxia-dependent, but the HIFβ sub-unit is constitutively expressed in all nucleated cell types. Under normoxic conditions, oxygen-dependent hydroxylation of the HIFα sub-units occurs at distinct proline residues, driven by prolyl-hydroxylase domain (PHD) enzymes 1–3. HIFα hydroxylation facilitates binding with the von Hippel-Lindau protein, which interacts with elongin C and recruits the ubiquitin ligase complex causing ubiquitination and rapid proteosomal degradation [21,22]. Expression of the HIFα sub-units is suppressed under normoxia, but expression increases exponentially as oxygen concentration declines. Under hypoxic conditions, hydroxylation activity of the PHD enzymes is suppressed, allowing the HIFα sub-units to accumulate in the nucleus, dimerize with HIFβ and bind to the hypoxia-responsive element (HRE) in the promotor or enhancer region of its target genes to activate their transcription. HIF targets include factors that promote thrombosis, such as plasminogen activator inhibitor (PAI) 1, but not all hypoxia-induced factors are pro-thrombotic, and not all hypoxia-induced factors that enhance thrombus formation contain an HRE. In other words, hypoxia-induced changes in the expression of pro- or anti-thrombotic factors can be controlled directly via HIFs or HIF target genes or indirectly via HIF-independent mechanisms. For example, hypoxia also activates early growth response (EGR) 1, which is known to regulate thrombus formation [23,24]. Hypoxia-responsive signaling pathways can also regulate thrombogenesis indirectly through the induction of pro-inflammatory mediators such as tumor necrosis factor (TNF) α and interleukin (IL) 1 [19]. The identification of hypoxia-responsive transcription factors, target genes, or signaling responses that control thrombus formation could represent an important step towards the development of novel and safe prophylactic therapies that reduce thrombosis.