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Protective potential of 3HB against critical illness-induced muscle weakness

Boek - Dissertatie

Critical illness is a complex interplay of metabolic, endocrine and inflammatory changes within the human body. Overall, critical illness is defined as any acute life threatening medical condition that requires vital organ support, without which death would ensue. A myriad of events can elicit critical illness, ranging from serious trauma to major surgery or severe sepsis. Although the initial cause can be diverse, critical illness generally follows a uniform disease pattern hallmarked by hyperinflammation, endocrine alterations and hypercatabolism. Despite a decreasing mortality in the acute phase due to medical advances, implementation of best clinical practice guidelines and early initiation of supportive care, an increasing number of patients progress into a prolonged phase of critical illness that still requires supportive care in order to survive. During this chronic phase of critical illness, patient are at higher risk of developing ICU-acquired weakness (ICUAW). Generalized muscle weakness, solely attributed to acute illness or the treatment thereof is a frequent occurring pathology among prolonged critically ill patients. Despite current preventive measures, such as aggressive sepsis treatment, withholding parenteral nutrition for a minimum of 7 days, tight glycemic control and early mobilization, effective treatment options are still lacking.Interestingly, the Leuven Laboratory of Intensive Care Medicine showed that premorbid obesity, but not nutrition, prevented loss of muscle mass and function during critical illness in ICU-patients and septic mice. Additionally, these septic mice presented with increased markers of fatty acid mobilization and ketogenesis. Furthermore, parenteral administration of high doses of lipids to lean septic mice attenuated muscle weakness, but not the wasting. Although this nutritional strategy resulted in unfavorable liver steatosis, it clearly upregulated fatty acid oxidation and ketogenesis, similarly as in the obese. Remarkably, the Leuven laboratory next demonstrated that supplementation of mixed parenteral nutrition (PN) with the ketone body 3-hydroxybutyrate (3HB) by itself was also able to protect against loss of muscle strength in lean critically ill mice, although loss of muscle mass was not prevented.Overall, ketone bodies are best known as an alternative fuel source for energy production in glucose-deprived situations like fasting and intensive exercise. During such metabolic challenging conditions, ketone bodies serve as a glucose sparing carbon source that can be readily oxidized in most tissues to provide energy through ATP production, while preserving gluconeogenic reserves and muscle protein stores. In addition to their energetic potential 3HB also can induce a myriad of signaling and regulatory effects and this at physiologically relevant concentrations. As such, ketone bodies have been reported to exert anti-inflammatory and autophagy-stimulating properties and to induce mTOR-mediated protein synthesis and muscle regeneration.The overall goal of this thesis is to gain more insight into the role of ketone bodies as protection against ICU acquired weakness. The first aim, studied in the objective 1 and 2, is to identify the underlying pathways explaining the protection against ICU-acquired weakness with 3HB supplementation. The second aim, studied in objective 3 and 4, is to translate the protective effect of ketone bodies to a usable therapy for the human patient.In our first objective, we investigated whether subcutaneous bolus injections of the ketone body D,L-3HB sodium salt (3HB-Na) rather acts as an energy substrate or as a signaling molecule, when supplemented to septic mice. We documented that the protective effect of 3HB against muscle weakness coincided with enhanced markers of muscle regeneration, and not with markers of substrate handling. With this experiment, we showed that 3HB supplementation rather acts as a signaling molecule rather than to serve as a super fuel during critical illness. The observation that 3HB enhanced markers of muscle regeneration suggests that this intervention could possibly also contribute to the attenuation of muscle weakness on the long term.In a second objective, we assessed whether cholesterol homeostasis plays a role in the development of ICU-acquired weakness and whether the protective effect of 3HB supplementation on weakness is related to its effects on cholesterol homeostasis. Serum cholesterol concentrations were lower in weak than in non-weak critically ill patients, and serum cholesterol was inversely correlated with weakness. In addition, plasma cholesterol correlated positively with muscle force in septic mice, and exogenous 3-hydroxybutyrate supplementation increased plasma cholesterol and altered cholesterol homeostasis, by normalization of plasma mevalonate (cholesterol precursor) and elevation of muscular, but not hepatic, expression of cholesterol synthesis genes. Furthermore, tracer technology revealed that 3-hydroxybutyrate in septic mice was preferentially taken up by muscle and metabolized into cholesterol-precursor mevalonate, rather than TCA metabolites. The 3-hydroxybutyrate protection against weakness was not related to ubiquinone or downstream myofiber mitochondrial function, whereas cholesterol content in myofibers was increased. These findings point to a role for low cholesterol in critical illness-induced muscle weakness and to a protective mechanism-of-action for 3-hydroxybutyrate-supplementation.In a third objective, we assessed efficacy versus toxicity of stepwise escalating doses of 3HB-Na in septic mice. In this experiment, we used a 3HB-Na dose of 40 mmol/kg/day as a reference dose as this dose was previously shown to prevent muscle weakness in septic mice without obvious signs of toxicity. By doubling the reference dose of 40 mmol/kg/day to 80 mmol/kg/day 3HB-Na, illness severity scores doubled and mortality increased from 30.4% to 87.5%. De-escalating this dose to 60 mmol/kg/day still increased mortality and reducing the dose to 48 mmol/kg/day still increased illness severity. Doses of 48 mmol/kg/day and higher caused more pronounced metabolic alkalosis and hypernatremia and increased markers of kidney damage. Doses of 60 mmol/kg/day 3HB-Na and higher caused dehydration of brain and lungs and increased markers of hippocampal neuronal damage and inflammation. Among survivors, 40 mmol/kg/day and 48 mmol/kg/day increased muscle force compared with placebo up to healthy control levels. This study indicated that 40 mmol/kg/day 3HB-Na supplementation prevented sepsis-induced muscle weakness in mice. However, this dose appeared maximally effective though close to the toxic threshold, possibly in part explained by excessive Na+ intake with 3HB-Na. Therefore, we concluded that the clinical use of ketone salts in human critically ill patients is precluded.In a fourth and final objective, we investigated 3-hydroxybutyl-3-hydroxybutanoate (3HHB) esters as a potential alternative for 3HB-Na. These esters are metabolized by the body to form two 3HB molecules, and thus would avoid excessive Na+ supplementation. In septic mice, we assessed efficacy versus toxicity of stepwise escalating doses of 3HHB, potential stereospecific characteristics of 3HHB and compared bolus versus continuous administration of 3HHB to identify the most usable therapy for human patients. In septic mice, as compared with placebo, severity of illness and mortality was increased by bolus injections of L-3HHB at 40 mmol/kg/day and by all other 3HHB formulations at a dose of 80 mmol/kg/day. Compared with placebo, muscle force was increased at 20 mmol/kg/day L-3HHB and at 40 mmol/kg/day D- and D,L-3HHB. Up to a dose of 40 mmol/kg/day 3HHB, signs of organ damage or liver toxicity were absent. However, septic mice showed a higher peak plasma concentration and slower 3HB- clearance than healthy controls. Unlike bolus injections, continuous infusion at doses up to 80 mmol/kg/day D,L-3HHB did not increase severity of illness or mortality as compared with placebo, whereas muscle force was increased at 40 mmol/kg/day. Based on our findings, we concluded that treatment of septic mice with pure and racemic mixtures of the ketone ester 3HHB partly prevented muscle weakness. Doubling the effective ester bolus dose to 80 mmol/kg/day in septic mice was 100% lethal, whereas this toxicity was completely avoided by continuous infusion of the same dose.The studies performed in this doctoral thesis have highlighted the protective potential of 3HB against the loss of muscle force during critical illness. Furthermore, these experiments have shed a light on the underlying mechanisms of the muscle force protective effect of ketone body supplementation during critical illness and provided valuable information on formulation and mode of administration, which is essential for further development of this treatment for future clinical use in the human patient.
Jaar van publicatie:2021
Toegankelijkheid:Closed