NF-κB-Inducing Kinase Maintains Mitochondrial Efficiency and Systemic Metabolic Homeostasis
Authors: Pflug Kathryn M. , Lee Dong W. , Keeney Justin , Sitcheran Raquel 
Published: August 2021

AbstractBackgroundNF-κB-inducing kinase (NIK) is a critical regulator of immunity and inflammation and NIK loss-of-function mutations have recently been described in patients with primary immunodeficiency disease. Based on our previous work showing that NIK regulates adaptive metabolic responses in glucose-starved cancer cells, we investigated whether NIK is required for mitochondrial functions in bioenergetic processes and metabolic responses to nutritional stress in NIK knockout (KO) mice, which recapitulate the clinical presentation of NIK PID patients.MethodsWe performed whole body composition analysis of wild type (WT) and NIK KO mice using EchoMRI and DEXA imaging. Seahorse extracellular flux analyses were used to monitor oxidative phosphorylation and glycolysis through oxygen consumption rates (OCR) and extracellular acidification rates (ECAR) in preadipocyte cells and in ex vivo adipose tissue. NIK regulation of systemic metabolic output was measured by indirect calorimetry using TSE Phenomaster metabolic chambers under basal conditions as well as in response to nutritional stress induced by a prolonged high-fat diet (HFD). Finally, we analyzed a role for NIK in adipocyte differentiation, as well as the contributions of canonical and noncanonical NF-κB signaling to adipose development and metabolic output.ResultsWe observed that in adipose cells, NIK is required for maintaining efficient mitochondrial membrane potential and spare respiratory capacity (SRC), indicators of mitochondrial fitness. NIK KO preadipocytes and ex vivo adipose tissue exhibited diminished SRC, increased proton leak, with compensatory upregulation of glycolysis. Systemically, NIK KO mice exhibited increased glucose utilization, increased energy expenditure, and reduced adiposity, which persisted under the stress of HFD. Finally, while NIK controlled adipocyte differentiation through activation of RelB and the noncanonical NF-κB pathway, NIK regulation of metabolism in preadipocytes was NF-κB/RelB-independent.ConclusionOur results demonstrate that NIK is required for metabolic homeostasis both locally, on a cellular and tissue level, as well as systemically, on an organismal level. Collectively, the data suggest that NIK KO cells upregulate glycolytic metabolism as a compensatory response to impaired mitochondrial fitness (diminished SRC) and mitochondrial efficiency (increased proton leak). To meet changes in bioenergetic demands, NIK KO mice undergo metabolic rewiring through increased glucose utilization and glycolysis, which persists under the stress of overnutrition with a HFD. Moreover, while NIK regulation of metabolism is RelB-independent, NIK regulation of adipocyte development requires RelB and activation of the noncanonical NF-κB pathway. Our findings establish NIK as an important regulator of cellular and systemic metabolic homeostasis, suggesting that metabolic dysfunction may be an important component of primary immunodeficiency diseases arising from loss of NIK function.