mTORC1 activation is not sufficient to suppress hepatic PPARα signaling or ketogenesis

Mechanisms of hepatic fatty acid oxidation and ketogenesis during fasting

Fasting is part of many weight management and health-boosting regimens. Fasting causes substantial metabolic adaptations in the liver that include the stimulation of fatty acid oxidation and ketogenesis. The induction of fatty acid oxidation and ketogenesis during fasting is mainly driven by interrelated changes in plasma levels of various hormones and an increase in plasma nonesterified fatty acid (NEFA) levels and is mediated transcriptionally by the peroxisome proliferator-activated receptor (PPAR)α, supported by CREB3L3 (cyclic AMP-responsive element-binding protein 3 like 3). Compared with men, women exhibit higher ketone levels during fasting, likely due to higher NEFA availability, suggesting that the metabolic response to fasting shows sexual dimorphism. Here, we synthesize the current molecular knowledge on the impact of fasting on hepatic fatty acid oxidation and ketogenesis.

Increasing glutathione levels by a novel posttranslational mechanism inhibits neuronal hyperexcitability

Glutathione (GSH) depletion, and impaired redox homeostasis have been observed in experimental animal models and patients with epilepsy. Pleiotropic strategies that elevate GSH levels via transcriptional regulation have been shown to significantly decrease oxidative stress and seizure frequency, increase seizure threshold, and rescue certain cognitive deficits. Whether elevation of GSH per se alters neuronal hyperexcitability remains unanswered. We previously showed that thiols such as dimercaprol (DMP) elevate GSH via post-translational activation of glutamate cysteine ligase (GCL), the rate limiting GSH biosynthetic enzyme. Here, we asked if elevation of cellular GSH by DMP altered neuronal hyperexcitability in-vitro and in-vivo. Treatment of primary neuronal-glial cerebrocortical cultures with DMP elevated GSH and inhibited a voltage-gated potassium channel blocker (4-aminopyridine, 4AP) induced neuronal hyperexcitability. DMP increased GSH in wildtype (WT) zebrafish larvae and significantly attenuated convulsant pentylenetetrazol (PTZ)-induced acute 'seizure-like' swim behavior. DMP treatment increased GSH and inhibited convulsive, spontaneous 'seizure-like' swim behavior in the Dravet Syndrome (DS) zebrafish larvae (scn1Lab). Furthermore, DMP treatment significantly decreased spontaneous electrographic seizures and associated seizure parameters in scn1Lab zebrafish larvae. We investigated the role of the redox-sensitive mammalian target of rapamycin (mTOR) pathway due to the presence of several cysteine-rich proteins and their involvement in regulating neuronal excitability. Treatment of primary neuronal-glial cerebrocortical cultures with 4AP or l-buthionine-(S,R)-sulfoximine (BSO), an irreversible inhibitor of GSH biosynthesis, significantly increased mTOR complex I (mTORC1) activity which was rescued by pre-treatment with DMP. Furthermore, BSO-mediated GSH depletion oxidatively modified the tuberous sclerosis protein complex (TSC) consisting of hamartin (TSC1), tuberin (TSC2), and TBC1 domain family member 7 (TBC1D7) which are critical negative regulators of mTORC1. In summary, our results suggest that DMP-mediated GSH elevation by a novel post-translational mechanism can inhibit neuronal hyperexcitability both in-vitro and in-vivo and a plausible link is the redox sensitive mTORC1 pathway.

p21-activated kinase 4 suppresses fatty acid β-oxidation and ketogenesis by phosphorylating NCoR1

PPARα corepressor NCoR1 is a key regulator of fatty acid β-oxidation and ketogenesis. However, its regulatory mechanism is largely unknown. Here, we report that oncoprotein p21-activated kinase 4 (PAK4) is an NCoR1 kinase. Specifically, PAK4 phosphorylates NCoR1 at T1619/T2124, resulting in an increase in its nuclear localization and interaction with PPARα, thereby repressing the transcriptional activity of PPARα. We observe impaired ketogenesis and increases in PAK4 protein and NCoR1 phosphorylation levels in liver tissues of high fat diet-fed mice, NAFLD patients, and hepatocellular carcinoma patients. Forced overexpression of PAK4 in mice represses ketogenesis and thereby increases hepatic fat accumulation, whereas genetic ablation or pharmacological inhibition of PAK4 exhibites an opposite phenotype. Interestingly, PAK4 protein levels are significantly suppressed by fasting, largely through either cAMP/PKA- or Sirt1-mediated ubiquitination and proteasome degradation. In this way, our findings provide evidence for a PAK4-NCoR1/PPARα signaling pathway that regulates fatty acid β-oxidation and ketogenesis.

Molecular Mechanisms for Ketone Body Metabolism, Signaling Functions, and Therapeutic Potential in Cancer

The ketone bodies (KBs) β-hydroxybutyrate and acetoacetate are important alternative energy sources for glucose during nutrient deprivation. KBs synthesized by hepatic ketogenesis are catabolized to acetyl-CoA through ketolysis in extrahepatic tissues, followed by the tricarboxylic acid cycle and electron transport chain for ATP production. Ketogenesis and ketolysis are regulated by the key rate-limiting enzymes, 3-hydroxy-3-methylglutaryl-CoA synthase 2 and succinyl-CoA:3-oxoacid-CoA transferase, respectively. KBs participate in various cellular processes as signaling molecules. KBs bind to G protein-coupled receptors. The most abundant KB, β-hydroxybutyrate, regulates gene expression and other cellular functions by inducing post-translational modifications. KBs protect tissues by regulating inflammation and oxidative stress. Recently, interest in KBs has been increasing due to their potential for treatment of various diseases such as neurological and cardiovascular diseases and cancer. Cancer cells reprogram their metabolism to maintain rapid cell growth and proliferation. Dysregulation of KB metabolism also plays a role in tumorigenesis in various types of cancer. Targeting metabolic changes through dietary interventions, including fasting and ketogenic diets, has shown beneficial effects in cancer therapy. Here, we review current knowledge of the molecular mechanisms involved in the regulation of KB metabolism and cellular signaling functions, and the therapeutic potential of KBs and ketogenic diets in cancer.

Circadian clock controls rhythms in ketogenesis by interfering with PPARα transcriptional network

Ketone bodies are energy-rich metabolites and signaling molecules whose production is mainly regulated by diet. Caloric restriction (CR) is a dietary intervention that improves metabolism and extends longevity across the taxa. We found that CR induced high-amplitude daily rhythms in blood ketone bodies (beta-hydroxybutyrate [βOHB]) that correlated with liver βOHB level. Time-restricted feeding, another periodic fasting-based diet, also led to rhythmic βOHB but with reduced amplitude. CR induced strong circadian rhythms in the expression of fatty acid oxidation and ketogenesis genes in the liver. The transcriptional factor peroxisome-proliferator-activated-receptor α (PPARα) and its transcriptional target hepatokine fibroblast growth factor 21 (FGF21) are primary regulators of ketogenesis. Fgf21 expression and the PPARα transcriptional network became highly rhythmic in the CR liver, which implicated the involvement of the circadian clock. Mechanistically, the circadian clock proteins CLOCK, BMAL1, and cryptochromes (CRYs) interfered with PPARα transcriptional activity. Daily rhythms in the blood βOHB level and in the expression of PPARα target genes were significantly impaired in circadian clock-deficient Cry1,2^-/- mice. These data suggest that blood βOHB level is tightly controlled and that the circadian clock is a regulator of diet-induced ketogenesis.

Hepatic non-parenchymal S100A9-TLR4-mTORC1 axis normalizes diabetic ketogenesis

Unrestrained ketogenesis leads to life-threatening ketoacidosis whose incidence is high in patients with diabetes. While insulin therapy reduces ketogenesis this approach is sub-optimal. Here, we report an insulin-independent pathway able to normalize diabetic ketogenesis. By generating insulin deficient male mice lacking or re-expressing Toll-Like Receptor 4 (TLR4) only in liver or hepatocytes, we demonstrate that hepatic TLR4 in non-parenchymal cells mediates the ketogenesis-suppressing action of S100A9. Mechanistically, S100A9 acts extracellularly to activate the mechanistic target of rapamycin complex 1 (mTORC1) in a TLR4-dependent manner. Accordingly, hepatic-restricted but not hepatocyte-restricted loss of Tuberous Sclerosis Complex 1 (TSC1, an mTORC1 inhibitor) corrects insulin-deficiency-induced hyperketonemia. Therapeutically, recombinant S100A9 administration restrains ketogenesis and improves hyperglycemia without causing hypoglycemia in diabetic mice. Also, circulating S100A9 in patients with ketoacidosis is only marginally increased hence unveiling a window of opportunity to pharmacologically augment S100A9 for preventing unrestrained ketogenesis. In summary, our findings reveal the hepatic S100A9-TLR4-mTORC1 axis in non-parenchymal cells as a promising therapeutic target for restraining diabetic ketogenesis.

Excess ketogenesis can lead to ketoacidosis, a serious complication in patients with diabetes. Here the authors report an insulin independent pathway, the hepatic nonparenchymal S100A9-TLR4-mTORC1 axis, that is able to normalize diabetic ketogenesis and pre-clinical data to suggest potential for development of S100A9 based adjunctive therapy to insulin.

Nutraceuticals/Drugs Promoting Mitophagy and Mitochondrial Biogenesis May Combat the Mitochondrial Dysfunction Driving Progression of Dry Age-Related Macular Degeneration

In patients with age-related macular degeneration (AMD), the crucial retinal pigment epithelial (RPE) cells are characterized by mitochondria that are structurally and functionally defective. Moreover, deficient expression of the mRNA-editing enzyme Dicer is noted specifically in these cells. This Dicer deficit up-regulates expression of Alu RNA, which in turn damages mitochondria—inducing the loss of membrane potential, boosting oxidant generation, and causing mitochondrial DNA to translocate to the cytoplasmic region. The cytoplasmic mtDNA, in conjunction with induced oxidative stress, triggers a non-canonical pathway of NLRP3 inflammasome activation, leading to the production of interleukin-18 that acts in an autocrine manner to induce apoptotic death of RPE cells, thereby driving progression of dry AMD. It is proposed that measures which jointly up-regulate mitophagy and mitochondrial biogenesis (MB), by replacing damaged mitochondria with “healthy” new ones, may lessen the adverse impact of Alu RNA on RPE cells, enabling the prevention or control of dry AMD. An analysis of the molecular biology underlying mitophagy/MB and inflammasome activation suggests that nutraceuticals or drugs that can activate Sirt1, AMPK, Nrf2, and PPARα may be useful in this regard. These include ferulic acid, melatonin urolithin A and glucosamine (Sirt1), metformin and berberine (AMPK), lipoic acid and broccoli sprout extract (Nrf2), and fibrate drugs and astaxanthin (PPARα). Hence, nutraceutical regimens providing physiologically meaningful doses of several or all of the: ferulic acid, melatonin, glucosamine, berberine, lipoic acid, and astaxanthin, may have potential for control of dry AMD.

Functional loss of ketogenesis in odontocete cetaceans

Odontocete cetaceans exhibit genomic mutations in key ketogenesis genes. In order to validate an inferred lack of ketogenesis made by observations from genome sequencing, we biochemically analyzed tissues from several odontocete cetacean species and demonstrate that they indeed do not exhibit appreciable hepatic β-hydroxybutyrate (βHB) or its carnitine ester. Furthermore, liver tissue exhibited significantly lower long chain acylcarnitines and increased odd chain acylcarnitines indicative of a decreased reliance on hepatic long chain fatty acid oxidation in these carnivorous mammals. Finally, we performed single molecule, real-time next generation sequencing of liver and brain RNA of Tursiops truncatus and demonstrate that the succinyl-CoA transferase required for acetoacetate catabolism is expressed in the nervous system. These data show that odontocete cetaceans have lost the ability to perform ketogenesis and suggest a hepatocentric coenzyme A recycling function rather than a predominantly systemic-bioenergetic role for ketogenesis in other ketogenic competent mammals such as humans.