Impact of soluble Flt-1 on endothelial cell activation and its implications in leukocyte adhesion and microvascular dysfunction in ischemia-reperfusion injury

急性肾脏病（阿基）是一种常见、复杂且可能危及生命的疾病。肾缺血/再灌注（IR）损伤是缺血性阿基的主要原因，其结果是整体或局部肾血流减少。内皮细胞的活化和炎症，从而导致肾微循环障碍，似乎是IR相关阿基中涉及的进一步过程的关键作用。缺乏血管修复的可能解释是肾IR导致表达因子的转变，有利于抗血管生成相对于血管生成/血管稳定。我们已经证明可溶性血管内皮生长因子（VEGF）受体1通过中和VEGF（血管生成和内皮细胞维持的关键细胞因子），升高的sFlt-1水平可能导致内皮修复受损，长期损害内皮功能和微血管系统。然而，如果sFlt-1在引起内皮功能障碍中的作用被我们和其他人很好地表征，则其在内皮活化中的作用（白细胞粘附和炎症中的重要步骤）探索得很差。我们的初步结果显示重组sFlt-1在体外增加了内皮细胞向白细胞的转化。在动物中，通过渗透微泵递送的重组sFlt-1诱导白细胞粘附于提睾肌内皮细胞，并通过活体显微镜评估增加白细胞移行。此外，它还导致肾脏中白细胞-内皮细胞相互作用增加;促炎介质表达增加和局部血容量减少。换句话说，它导致与缺血性阿基相似的内皮损伤。相反，在Tx患者中，sFlt-1与内皮细胞活化/功能障碍（sVCAM）和炎症（C-反应蛋白）的标志物相关。此外，高sFlt-1水平与肾活检中毛细血管密度降低相关，并且与这些患者中与缺血性阿基相关的所有结局相关，即移植物功能延迟、急性排斥、晚期移植物功能受损和死亡率。在这种情况下，我们假设1）sFlt-1通过促进内皮功能障碍/活化，并因此促进白细胞-内皮细胞粘附而参与缺血/再灌注损伤;以及2）白细胞粘附是sFlt-1引起内皮损伤/血管脱落的可能的额外机制。在这种情况下，抑制sFlt-1将通过减少白细胞募集和/或提高维持肾微血管完整性的能力来最小化IR损伤。了解sFlt-1和炎症之间的联系可能有助于阐明sFlt-1在IR相关阿基中的作用。

Acute kidney disease (AKI) is a common, complex and potentially life-threatening disease. Renal ischemia/reperfusion (IR) injury is a leading cause of ischemic AKI, having as consequence the reduction of the global or local renal blood flow. Activation of endothelial cells and inflammation, thus resulting in disturbance of the renal microcirculation, appear to be a key role for the further processes involved in IR-associated AKI. A possible explanation for the lack of vascular repair is that renal IR results in a shift of expressed factors in favor of anti-angiogenesis vs. angiogenesis/vascular stabilization. We have shown that the soluble vascular endothelial growth factor (VEGF) receptor 1 (known as sFlt-1) is increased in a murine model of IR and throughout the first weeks after renal transplantation (Tx), a unique human/clinical model of IR. By neutralizing VEGF, a crucial cytokine for angiogenesis and endothelial cell maintenance, elevated sFlt-1 levels may contribute to the impairment of endothelial repair, chronically compromising endothelial function and the microvasculature. However, if the role of sFlt-1 in causing endothelial dysfunction is well characterized by us and others, its role in endothelial activation, an important step in leukocyte adhesion and inflammation, is poorly explored. Our preliminary results show that recombinant sFlt-1 increases the adhesiveness of endothelial cells to leukocytes in vitro. In animals, recombinant sFlt-1 delivered by osmotic minipumps induces leukocyte adhesion to cremaster endothelial cells and increases leukocyte transmigration as assessed by intravital microscopy. In addition, it leads to increased leukocyte-endothelial interactions in the kidney; increased expression of proinflammatory mediators and decreased regional blood volume. In other words, it leads to similar endothelial injury as in ischemic AKI. Conversely, in Tx patients, sFlt-1 correlates with markers of endothelial cell activation/dysfunction (sVCAM) and inflammation (C-reactive protein). Moreover, high sFlt-1 levels are associated with decreased capillary density in renal biopsies, and with all outcomes associated with ischemic AKI in these patients, namely delayed graft function, acute rejection, impaired late graft function, and mortality. In this context, we hypothesized that 1) sFlt-1 participates in ischemia/reperfusion injury by contributing to endothelial dysfunction/activation, and, consequently, to leukocyte-endothelial cell adhesion; and that 2) leukocyte adhesion is a possible additional mechanism by which sFlt-1 causes endothelial damage/vessel dropout. In this context, inhibition of sFlt-1 would minimize the IR injury by reducing leukocyte recruitment; and/or improving the capacity to maintain renal microvascular integrity. Understanding the link between sFlt-1 and inflammation may be helpful to unravel sFlt-1 role in IR-associated AKI.