Reprogramming metabolism to enhance kidney tolerance during sepsis:The role of fatty acid oxidation, aerobic glycolysis and epithelial de-differentiation

Abstract

Background:

The recognition that sepsis induces acute kidney injury in the absence of overt necrosis or apoptosis, and even in the presence of increased renal blood flow has led to the consideration that kidney tubular epithelial cells (TEC) may deploy defense mechanisms to survive the insult.

Summary:

This concept dovetails well with the notion that the defense against infection not only depends on the capacity of the immune system to limit the microbial burden or Resistance capacity, but also on the capacity of the host to limit tissue injury, collectively known as tolerance. To sustain the high energy requirement that ion transport mandates, kidney TEC use fatty acid oxidation (FAO) as one of the preferred sources of energy. Inflammatory processes like endotoxemia and sepsis decrease mitochondrial FAO and hinder mitochondrial respiration. Impaired FAO is associated with TEC de-differentiation, loss kidney function and TEC injury through lipotoxicity and oxidative stress in the acute setting, and with maladaptive repair and fibrosis after AKI in latter stages. AMP activated protein kinase (AMPK) is a master regulator of energy and promoter of FAO, that can be activated pharmacologically to protect against AKI and death during experimental sepsis, operating through a tolerance mechanism.

Key Messages:

Organ dysfunction during sepsis is the expression of tissue injury and adaptive defense mechanisms operating through resistance or tolerance, that prioritize cell survival over organ function. Metabolic reprogramming away from FAO/OXPHOS seems to be a common pathologic denominator throughout the AKI continuum, that may be targeted through the activation of AMPK.

Introduction

Three disruptive notions have changed the way we conceive the relationship between tissue injury and organ dysfunction during sepsis. First, Langenberg et al. induced sepsis in sheep by injection of Escherichia coli, and demonstrated that acute kidney injury (AKI) occurred despite increments in renal blood flow.[1] Takasu and Hotchkiss et al. demonstrated that during sepsis, organ dysfunction occurs in the absence of significant apoptosis or necrosis.[2] The third notion was discovered by plant ecologists and evolutionary biologists, who recognized initially in plants, that the activation of the immune response to limit the microbial burden, also known as resistance capacity, was not the only defense mechanism against infection. They demonstrated that independently of their Resistance capacity, plants were capable of decreasing their own susceptibility to injury inflicted by pathogens or by their own immune response, through a conglomerate of intrinsic mechanisms collectively known as tolerance capacity.[3, 4] This is highly relevant to clinical practice because resistance and tolerance have been demonstrated to operate in mammals and humans.[3]

Overall, these data suggest that in response to the sublethal injury induced by sepsis, cells are capable of prioritizing survival over function. In this framework, we submit that one of the key ‘knobs’ that cells can turn in order to launch resistance and tolerance defense mechanisms is metabolic reprogramming. The aim of this review is to discuss the importance of TEC metabolic reprogramming as a defense strategy during sepsis, the association of impaired FAO and de-differentiation, and the promising role of promoters of FAO like AMP activated protein kinase (AMPK) as potential novel therapeutics to treat sepsis-associated AKI.

Sepsis induces a shift away from oxidative phosphorylation and fatty acid oxidation in the kidney

The primary function of kidney proximal tubular epithelial cells (TEC) is to transport sodium filtered by the glomerulus, an energetically expensive task that consumes up to 70% of the total energy used by the kidney. To sustain this high energy requirement, TEC use fatty acid oxidation (FAO) as one of the preferred sources of energy and kidney tubules are equipped with the second highest density of mitochondria after cardiac myocytes.[5, 6] Inflammatory processes like endotoxemia and sepsis decrease mitochondrial FAO as much as 40%,[7] shift metabolism toward glycolysis, and hinder mitochondrial respiration.[8, 9] This metabolic shift is associated with increased expression of glycolytic enzymes, a reduction in functional glycolysis,[9] the internalization of the Na⁺K⁺ATPase pump, loss of kidney function, and TEC injury through lipotoxicity and oxidative stress.

Impaired FAO is a common denominator in AKI, a hallmark of TEC that undergo maladaptive repair, atrophy and fibrosis, and a common, final metabolic phenotype in kidney biopsies of animals and humans with established chronic kidney disease.[10] Support for a mechanistic role of FAO in preventing tissue injury and maladaptive repair comes from the following findings: First, once injury is established, tubules that fail to restore FAO undergo maladaptive repair, atrophy and fibrosis.[11] Second, genetic deletion of upstream FAO regulators, like Liver Kinase B1 (LKB1), promote maladaptive repair and tubular fibrosis after nephrotoxic AKI.[12] Third, pharmacologic activation of FAO regulators PGC-lα and AMPK,[8,13] or genetic overexpression of TEC CPT1A,[14] prevents fibrosis in models of nephrotoxic AKI. Together, these data suggest that impaired FAO is a potential unifying mechanism driving the transition through the AKI continuum, from acute dysfunction through persistent AKI to progression to acute kidney disease.

Sepsis induced de-differentiation in kidney tubular epithelial cells

Cells that respond to injury with downregulation of non-vital processes have the capacity to regress morphologically and metabolically to a more immature version, a process termed de-differentiation.[15] Metabolically, this may be relevant to the proximal TEC because de-differentiation may be associated with a shift away from FAO and OXPHOS. Like ischemia-reperfusion and drug-induced toxicity,[16] our preliminary data suggests that sepsis also induces kidney TEC de-differentiation based on two findings. First, sepsis induced the internalization of the Na⁺K⁺ATPase pump from the basolateral membrane toward the cytoplasm in kidney TEC as early as 24h after cecal ligation and puncture. Second, these same TEC express Sox9. Sox9 is rarely expressed in un-injured adult kidneys, and within its multiple functions, is thought to be a marker of de-differentiation and TEC proliferation.[17] Sox9 has been shown to be expressed after ischemia-reperfusion injury models.[18] These changes have important consequences on tubular and kidney function. Internalization is a known mechanism of Na⁺K⁺ATPase pump deactivation, which leads to the arrest of tubular ion transport. An increase in Na⁺ and Cl⁻content in tubular fluid triggers the tubule-glomerular feedback at the level of the macula densa, resulting in vasoconstriction of the afferent arteriole, a decrease in glomerular hydrostatic pressure and a decline in glomerular filtration rate (GFR), which manifests clinically as AKI.

Enhancement of OXPHOS and FAO may be protective during sepsis

Multiple groups have demonstrated that pharmacologic activation of promoters of oxidative phosphorylation (OXPHOS) and FAO like the NAD⁺-dependent deacetylase, Sirtuin 1 (Sirtl) or PGC-lα improve survival during sepsis.[13,19] These data provided grounds to investigate the effects of activating the adenosine monophosphate activated protein kinase (AMPK), a key master energy regulator and promoter of FAO and OXPHOS that is highly expressed in kidney proximal TEC. AMPK is highly relevant to the tubular epithelium for several reasons. First, AMPK is a potent promoter OXPHOS. Second, by inhibiting acetyl Co-A carboxylase, AMPK decreases production of Malonyl-CoA, the natural repressor of carnitine palmitoyl transferase 1A (CPT1A), the rate limiting enzyme for FAO. Therefore, AMPK activation indirectly promotes the activation of CPT1A,[20] and promotes transcription factors that increase the expression of FAO-related enzymes. Third, AMPK regulates mitochondrial quality and promotes the synthesis of new functional mitochondria through biogenesis. Based on this, we hypothesized that AMPK activation would protect the TEC from sepsis-induced injury, and proved that treatment with the AMPK activators AICAR or metformin prevented AKI and decreased mortality during CLP-induced sepsis.[8, 21]

Possible protective mechanisms include the direct activation of TEC FAO and/or OXPHOS, limiting the extent of TEC de-differentiation and/or protection of TEC mitochondrial function. Because AMPK is ubiquitous, it is also possible that pharmacologic activation of systemic AMPK protects independently of kidney TEC AMPK expression. For instance, we showed that the use of the AMPK activator AICAR decreased the expression of ICAM-1 in the kidney microvasculature, microvascular fluid leak and neutrophil adhesion, suggesting protection of peritubular microvascular function.[21] In addition, systemic AMPK activation may lead to improved myocardial contractility and systemic hemodynamics, thereby ameliorating the inflammatory and injurious consequences of distributive shock. More mechanistic studies will need to dissect the underpinnings that drive the protective effect we have demonstrated during sepsis.

AMPK activation as a therapeutic target to invoke tolerance

If the protection achieved using a pharmacologic AMPK activator is associated with a decrease in the bacterial burden, then such protection may be enhancing the capacity of the immune system to eliminate bacteria and thus would be operating through a Resistance mechanism. However, if protection occurs in the absence of decreased bacterial burden, this strongly suggests that the protective effect is operating through a tolerance mechanism, independent of resistance. To understand whether AMPK activation protects through resistance or tolerance, we compared the peritoneal bacterial burden of animals exposed to CLP treated with or without the AMPK activator, AICAR. While AICAR protected against AKI and death, it did not decrease the peritoneal bacterial burden when compared to animals exposed only to CLP, suggesting that AMPK activation operated through a tolerance mechanism.[22]

Conclusion

Organ dysfunction during sepsis is the expression of the interaction between tissue injury and adaptive defense mechanisms operating through resistance or tolerance, that prioritize cell survival over organ function. Metabolic reprogramming away from FAO and OXPHOS seems to be a common pathologic denominator throughout the AKI continuum, which in the acute phase of sepsis induced AKI, is associated with TEC de-differentiation. Pharmacologic activation of the FAO promoter AMPK is promising as a potential therapeutic target as it protects from AKI and death through a tolerance mechanism.

Data Availability Statement:

All data generated or analysed during this study are included in this article. Further enquiries can be directed to the corresponding author.