Targeting Hypoxia-Induced Inflammation

In the current issue of Anesthesiology, a paper by Kim et al. from the research laboratory of Dr. H. Thomas Lee investigates strategies to dampen ischemia-driven inflammation of the kidneys and subsequent multi-organ failure utilizing experimental models of acute kidney injury (AKI).¹ In fact, one of the leading causes of morbidity and mortality in surgical patients is perioperative organ failure, such as occurs in the context of acute kidney injury, liver failure, intestinal or myocardial ischemia, stroke or acute lung injury. A common feature of these perioperative diseases is the presence of hypoxia-induced inflammation.^2,3 The relationship between hypoxia and inflammation under these conditions is inter-dependent. For example, exposure to ambient hypoxia - as seen during high-altitude mountaineering - is associated with edema of the lungs or the brain, and systemic inflammatory responses in humans.^4,5 Similarly, acute exposure of mice to ambient hypoxia (e.g. 8% of oxygen over 4–8h) leads to increased inflammatory cytokine levels and pulmonary edema.⁶ Moreover, prolonged donor organ exposure to ischemia during organ transplantation is known to enhance graft inflammation and early graft failure.^7,8 For example, the endotoxin-receptor TLR4 is expressed in the donor kidney during kidney transplantation, and TLR4 expression levels increase with prolonged ischemia time. Donor kidneys with a loss-of-function mutation of the TLR4 receptor show attenuated kidney inflammation, and an increased rate of immediate graft function.⁷ Together, these studies indicate that hypoxia represents an inflammatory stimulus (Figure 1) and suggest that targeting hypoxia-elicited inflammation could represent an important therapeutic approach in perioperative medicine.

Figure 1. Interdependent Relationship between Hypoxia and Inflammation.

Exposure to ambient hypoxia triggers an acute inflammatory response in different organs, including the kidney, the intestine, the heart or the lungs. Hypoxia-induced inflammation can manifest itself as increased vascular leakage, accumulation of inflammatory cells in hypoxic organs, or release of inflammatory mediators, for example tumor necrosis factor (TNF)-α, IL6 or IL8. At the same time – and as shown here for acute lung injury - inflammation is associated with tissue hypoxia caused by dramatic increases in the demand for metabolites and oxygen by resident cells (e.g. pulmonary epithelia, vascular endothelia) and recruited inflammatory cells (e.g. neutrophils). At the same time, oxygen supply is attenuated due to pulmonary edema, airway plugging, or micro thrombi.

While hypoxia can trigger inflammatory responses, inflammation itself is also a cause for tissue hypoxia. During active inflammatory disease, metabolic shifts towards hypoxia are severe.² An example for an inflammatory disease characterized by severe tissue hypoxia is acute lung injury. Several factors contribute to pulmonary hypoxia in this context, including attenuated oxygen supply due to airway atelectasis, or diminished blood flow to ventilated areas within the lungs. Moreover, the increased metabolic demand by resident cells or recruited inflammatory cells results in profound shifts in the supply and demand ratio of metabolites and oxygen (Figure 1), and hypoxia-driven signaling pathways are activated.^9,10 It is important to point out that tissue hypoxia during inflammation is more than simply a bystander effect but can greatly impact upon the development or attenuation of inflammation through the regulation of oxygen-dependent gene expression.¹¹ In fact, studies that target transcriptionally regulated tissue adaptation to hypoxia are currently an area of intense investigation to prevent hypoxia-induced inflammation and organ failure. For instance, experimental strategies that target hypoxia-driven inflammation have been proposed in the treatment of intestinal, myocardial, hepatic ischemia, acute lung injury or acute kidney injury.¹²

An area where hypoxia-driven inflammation greatly impacts perioperative outcomes is acute kidney injury.¹³ Acute kidney injury is characterized by a decrease in the glomerular filtration rate, occurring over a period of minutes to days.¹⁴. In hospitalized patients, over 50% of cases of acute kidney injury are related to conditions of renal ischemia, or more than 80% in the critical care setting.^14,15 A recent study of hospitalized patients revealed that only a mild increase in the serum creatinine level (0.3 to 0.4 mg/dl) is associated with a 70% greater risk of death than in persons without any increase.¹⁴ Moreover, surgical procedures requiring cross-clamping of the aorta and renal vessels are associated with a renal failure rates of up to 30%. Similarly, AKI after cardiac surgery occurs in over 10% of patients under normal circumstances and is associated with dramatic increases in mortality. AKI and chronic kidney disease re also common complications after liver transplantation.¹⁶ For example, the incidence of AKI following liver transplantation is at least 50%, and 8–17% of patients end up requiring renal replacement therapy.¹⁷ Delayed graft function during kidney transplantation is frequently related to ischemia-associated acute kidney injury.¹⁸ In addition, AKI occurs in approximately 20% of patients suffering from sepsis.^15,19

As shown in the present study by Kim et al, ischemia-induced acute kidney injury triggers a down-ward spiral leading to subsequent multi-organ failure.¹ Here, the authors elegantly demonstrate that exposure to renal ischemia with concomitant inflammatory activation of the kidneys will result in subsequent gut injury, including breakdown of the intestinal epithelial barrier, and massive intestinal inflammation. The intestinal injury will eventually spill over to the liver, and also cause severe hepatic disease.¹ It is amazing that this downward spiral is initiated by a local injury to the kidneys (Figure 2). As such, it will be of critical importance to understand mechanistic aspects of the crosstalk pathway that connects the kidneys and the intestine during hypoxia-driven renal inflammation, and how gut inflammation triggers subsequent hepatic injury and multi-organ failure. The present studies demonstrate that volatile anesthetics dampen kidney inflammation and multi-organ failure initiated by acute kidney injury.¹ In fact, Kim et al. found that isoflurane protected against acute kidney injury, and reduced hepatic and intestinal injury via induction of sphingosine kinase 1 in the gut. By combining pharmacological studies with sphingosine kinase inhibitors, and genetic approaches using knockout mice for sphingosine kinase, they elegantly demonstrated a functional role of this pathway in kidney protection. This is consistent with their previous studies showing kidney protection via sphingosine kinase and sphingosine-1-phosphate-dependent pathways.^20–22 Interestingly, other experimental studies from Dr. Lee’s laboratory indicate that infusion of local anesthetics (lidocaine, bupivacaine or tetracaine) have an opposite effect by potentiating renal dysfunction and kidney inflammation following ischemia-reperfusion injury.²³

Figure 2. Proposed crosstalk pathways involved in multi-organ failure elicited by acute kidney injury (see Kim et al. Anesthesiology 2010)¹.

The present studies are consistent with previous research work from Dr. Lee’s group demonstrating a protective role of extracellular adenosine signaling during acute kidney injury. In the extracellular compartment, adenosine stems from phosphohydrolysis of precursor nucleotides such as ATP and AMP. In fact, conditions of hypoxia - such as occur during acute kidney injury - significantly enhance the production of extracellular adenosine.^24–26 Extracellular adenosine generation and signaling has been strongly implicated in attenuation of hypoxia-driven inflammation.¹² As such, genetically altered mice that have defects in the enzymatic generation of extracellular adenosine are more prone to develop kidney inflammation and organ dysfunction when exposed to renal ischemia.^27,28 Extracellular adenosine can signal through four individual adenosine receptors (ARs) – the A1AR, A2AAR, A2BAR, and A3AR. In this context, several studies from Dr. Lee’s research team have demonstrated an important role for the A1AR in attenuating renal inflammation and preserving organ function during ischemia. For instance, mice with genetic deletion of the A1AR exhibit increased renal injury following ischemia and reperfusion injury.²⁹ Moreover, A1AR^−/− mice also show increased mortality, renal dysfunction, and hepatic injury when exposed to murine septic peritonitis.³⁰ In addition, a very elegant proof-of-principle study utilizing a viral over-expression technique revealed that kidney-specific reconstitution of the A1AR in A1AR^−/− mice reduces renal ischemia-reperfusion injury.³¹ Consistent with the present studies demonstrating a cross-talk pathway between the kidneys, the intestine and the liver,¹ Dr. Lee’s research team was also able to demonstrated that protection against acute kidney injury via A1AR-mediated Akt activation also reduces liver injury.³² Other studies have implicated the hypoxia-inducible A2BAR in kidney protection from ischemia.³³ Taken together these studies highlight that endogenous anti-inflammatory pathways that are activeted by hypoxia-dependent signaling pathways can be targeted to treat renal inflammation induced by ischemia.

While many experimental studies – including the exciting contributions of Dr. Lee’s research team – have provided new insight into preventing or treating hypoxia-induced inflammation of the kidney, it remains a challenge to implement those approaches into a clinical setting. In fact, treatment approaches to prevent or treat acute kidney injury in the perioperative setting are extremely limited. For instance, several clinical trials involving numerous drugs have shown little or no protective effects. Studies investigating the role of dopamine in prevention of acute kidney injury in a perioperative setting have shown no reduction in mortality or renal outcome. Moreover, studies investigating loop diuretics - such as furosemide - as a means of kidney protection from acute kidney injury - observed no reduction in overall mortality.³⁴ Similarly, studies on the protective effects of the selective D1-dopamine receptor agonist fenoldopam failed to justify its routine clinical use for perioperative kidney protection.³⁵ Therefore, future challenges will include systematic approaches to help translate novel experimental approaches of kidney protection from bench to bedside and from mice to man. Moreover, additional mechanistic insights into transcriptionally regulated pathways that dampen hypoxia-elicited kidney inflammation hold the promise for effectively treating hypoxia-driven inflammation. By tapping into endogenous mechanisms of hypoxia-elicited tissue adaptation, such studies could address hypoxia-dependent changes in renal gene expression regulated by micro-RNAs, or could address the direct role of hypoxia sensing and signaling mechanisms in kidney diseases. In fact, I hope that such studies will allow us to further develop therapeutic strategies to dampen inflammation caused by hypoxia, while simultaneously increasing ischemia resistance. These therapeutic approaches could be applicable in many perioperative scenarios that involve hypoxia-elicited organ dysfunction, including acute lung injury, organ ischemia or solid organ transplantation.