Metabolic control by S6 kinases depends on dietary lipids

Neural Circuits Underlying Reciprocal Cardiometabolic Crosstalk: 2023 Arthur C. Corcoran Memorial Lecture

The interplay of various body systems, encompassing those that govern cardiovascular and metabolic functions, has evolved alongside the development of multicellular organisms. This evolutionary process is essential for the coordination and maintenance of homeostasis and overall health by facilitating the adaptation of the organism to internal and external cues. Disruption of these complex interactions contributes to the development and progression of pathologies that involve multiple organs. Obesity-associated cardiovascular risks, such as hypertension, highlight the significant influence that metabolic processes exert on the cardiovascular system. This cardiometabolic communication is reciprocal, as indicated by substantial evidence pointing to the ability of the cardiovascular system to affect metabolic processes, with pathophysiological implications in disease conditions. In this review, I outline the bidirectional nature of the cardiometabolic interaction, with special emphasis on the impact that metabolic organs have on the cardiovascular system. I also discuss the contribution of the neural circuits and autonomic nervous system in mediating the crosstalk between cardiovascular and metabolic functions in health and disease, along with the molecular mechanisms involved.

Protein translation paradox: Implications in translational regulation of aging

Protein translation is an essential cellular process playing key roles in growth and development. Protein translation declines over the course of age in multiple animal species, including nematodes, fruit flies, mice, rats, and even humans. In all these species, protein translation transiently peaks in early adulthood with a subsequent drop over the course of age. Conversely, lifelong reductions in protein translation have been found to extend lifespan and healthspan in multiple animal models. These findings raise the protein synthesis paradox: age-related declines in protein synthesis should be detrimental, but life-long reductions in protein translation paradoxically slow down aging and prolong lifespan. This article discusses the nature of this paradox and complies an extensive body of work demonstrating protein translation as a modulator of lifespan and healthspan.

S6K1 controls adiponectin expression by inducing a transcriptional switch: BMAL1-to-EZH2

Adiponectin (encoded by Adipoq), a fat-derived hormone, alleviates risk factors associated with metabolic disorders. Although many transcription factors are known to control adiponectin expression, the mechanism underlying its fluctuation with regard to metabolic status remains unclear. Here, we show that ribosomal protein S6 kinase 1 (S6K1) controls adiponectin expression by inducing a transcriptional switch between two transcriptional machineries, BMAL1 and EZH2. Active S6K1 induced a suppressive histone code cascade, H2BS36p-EZH2-H3K27me3, leading to suppression of adiponectin expression. Moreover, active S6K1 phosphorylated BMAL1, an important transcription factor regulating the circadian clock system, at serine 42, which led to its dissociation from the Adipoq promoter region. This response resulted in EZH2 recruitment and subsequent H3K27me3 modification of the Adipoq promoter. Upon fasting, inactivation of S6K1 induced the opposite transcriptional switch, EZH2-to-BMAL1, promoting adiponectin expression. Consistently, S6K1-depleted mice exhibited lower H3K27me3 levels and elevated adiponectin expression. These findings identify a novel epigenetic switch system by which S6K1 controls the production of adiponectin, which displays beneficial effects on metabolism.

Non-hematopoietic IL-4Rα expression contributes to fructose-driven obesity and metabolic sequelae

OBJECTIVE

The risks of excess sugar intake in addition to high-fat diet consumption on immunopathogenesis of obesity-associated metabolic diseases are poorly defined. Interleukin-4 (IL-4) and IL-13 signaling via IL-4Rα regulates adipose tissue lipolysis, insulin sensitivity, and liver fibrosis in obesity. However, the contribution of IL-4Rα to sugar rich diet-driven obesity and metabolic sequelae remains unknown.

METHODS

WT, IL-4Rα-deficient (IL-4Rα^-/-) and STAT6-deficient mice (STAT6^-/-) male mice were fed low-fat chow, high fat (HF) or HF plus high carbohydrate (HC/fructose) diet (HF + HC). Analysis included quantification of: (i) body weight, adiposity, energy expenditure, fructose metabolism, fatty acid oxidation/synthesis, glucose dysmetabolism and hepatocellular damage; (ii) the contribution of the hematopoietic or non-hematopoietic IL-4Rα expression; and (iii) the relevance of IL-4Rα downstream canonical STAT6 signaling pathway in this setting.

RESULTS

We show that IL-4Rα regulated HF + HC diet-driven weight gain, whole body adiposity, adipose tissue inflammatory gene expression, energy expenditure, locomotor activity, glucose metabolism, hepatic steatosis, hepatic inflammatory gene expression and hepatocellular damage. These effects were potentially, and in part, dependent on non-hematopoietic IL-4Rα expression but were independent of direct STAT6 activation. Mechanistically, hepatic ketohexokinase-A and C expression was dependent on IL-4Rα, as it was reduced in IL-4Rα-deficient mice. KHK activity was also affected by HF + HC dietary challenge. Further, reduced expression/activity of KHK in IL-4Rα mice had a significant effect on fatty acid oxidation and fatty acid synthesis pathways.

CONCLUSION

Our findings highlight potential contribution of non-hematopoietic IL-4Rα activation of a non-canonical signaling pathway that regulates the HF + HC diet-driven induction of obesity and severity of obesity-associated sequelae.

Type I interferon sensing unlocks dormant adipocyte inflammatory potential

White adipose tissue inflammation, in part via myeloid cell contribution, is central to obesity pathogenesis. Mechanisms regulating adipocyte inflammatory potential and consequent impact of such inflammation in disease pathogenesis remain poorly defined. We show that activation of the type I interferon (IFN)/IFNα receptor (IFNAR) axis amplifies adipocyte inflammatory vigor and uncovers dormant gene expression patterns resembling inflammatory myeloid cells. IFNβ-sensing promotes adipocyte glycolysis, while glycolysis inhibition impeded IFNβ-driven intra-adipocyte inflammation. Obesity-driven induction of the type I IFN axis and activation of adipocyte IFNAR signaling contributes to obesity-associated pathogenesis in mice. Notably, IFNβ effects are conserved in human adipocytes and detection of the type I IFN/IFNAR axis-associated signatures positively correlates with obesity-driven metabolic derangements in humans. Collectively, our findings reveal a capacity for the type I IFN/IFNAR axis to regulate unifying inflammatory features in both myeloid cells and adipocytes and hint at an underappreciated contribution of adipocyte inflammation in disease pathogenesis.

White adipose inflammation can occur in obesity and is at least in part mediated by inflammatory immune cells. Here the authors show that the Type I Interferon/Interferon alpha-beta receptor axis promotes an inflammatory, glycolysis associated adipocyte phenotype.

Mammalian EAK-7 activates alternative mTOR signaling to regulate cell proliferation and migration

Nematode EAK-7 (enhancer-of-akt-1-7) regulates dauer formation and controls life span; however, the function of the human ortholog mammalian EAK-7 (mEAK-7) is unknown. We report that mEAK-7 activates an alternative mechanistic/mammalian target of rapamycin (mTOR) signaling pathway in human cells, in which mEAK-7 interacts with mTOR at the lysosome to facilitate S6K2 activation and 4E-BP1 repression. Despite interacting with mTOR and mammalian lethal with SEC13 protein 8 (mLST8), mEAK-7 does not interact with other mTOR complex 1 (mTORC1) or mTOR complex 2 (mTORC2) components; however, it is essential for mTOR signaling at the lysosome. This phenomenon is distinguished by S6 and 4E-BP1 activity in response to nutrient stimulation. Conventional S6K1 phosphorylation is uncoupled from S6 phosphorylation in response to mEAK-7 knockdown. mEAK-7 recruits mTOR to the lysosome, a crucial compartment for mTOR activation. Loss of mEAK-7 results in a marked decrease in lysosomal localization of mTOR, whereas overexpression of mEAK-7 results in enhanced lysosomal localization of mTOR. Deletion of the carboxyl terminus of mEAK-7 significantly decreases mTOR interaction. mEAK-7 knockdown decreases cell proliferation and migration, whereas overexpression of mEAK-7 enhances these cellular effects. Constitutively activated S6K rescues mTOR signaling in mEAK-7-knocked down cells. Thus, mEAK-7 activates an alternative mTOR signaling pathway through S6K2 and 4E-BP1 to regulate cell proliferation and migration.

Absence of the kinase S6k1 mimics the effect of chronic endurance exercise on glucose tolerance and muscle oxidative stress

Objective

Ribosomal protein S6 Kinase-1 (S6K1) has been linked to resistance exercise-mediated improvements in glycemia. We hypothesized that S6K1 may also play a role in regulating glycemic control in response to endurance exercise training.

Methods

S6k1-knockout (S6K1KO) and WT mice on a 60 cal% high-fat diet were trained for 4 weeks on treadmills, metabolically phenotyped, and compared to sedentary controls.

Results

WT mice showed improved glucose tolerance after training. In contrast, S6K1KO mice displayed equally high glucose tolerance already in the sedentary state with no further improvement after training. Similarly, training decreased mitochondrial ROS production in skeletal muscle of WT mice, whereas ROS levels were already low in the sedentary S6K1KO mice with no further decrease after training. Nevertheless, trained S6K1KO mice displayed an increased running capacity compared to trained WT mice, as well as substantially reduced triglyceride contents in liver and skeletal muscle. The improvements in glucose handling and running endurance in S6K1KO mice were associated with markedly increased ketogenesis and a higher respiratory exchange ratio.

Conclusions

In high-fat fed mice, loss of S6K1 mimics endurance exercise training by reducing mitochondrial ROS production and upregulating oxidative utilization of ketone bodies. Pharmacological targeting of S6K1 may improve the outcome of exercise-based interventions in obesity and diabetes.

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Absence of S6k1 upregulates oxidative substrate utilization under HFD consumption.

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S6k1 knockout mice show enhanced running performance and improved glycemia already in the sedentary state.

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Aerobic endurance exercise training of S6k1 knockout mice further improves running performance but not glycemia.

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Metabolic improvements are associated with lower rates of TCA-linked mitochondrial H₂O₂ production and increased ketogenesis.

Thermoneutral housing exacerbates nonalcoholic fatty liver disease in mice and allows for sex-independent disease modeling

Nonalcoholic fatty liver disease (NAFLD), a common prelude to cirrhosis and hepatocellular carcinoma, is the most common chronic liver disease worldwide. Defining the molecular mechanisms underlying the pathogenesis of NAFLD has been hampered by a lack of animal models that closely recapitulate the severe end of the disease spectrum in humans, including bridging hepatic fibrosis. Here we demonstrate that a novel experimental model employing thermoneutral housing, as opposed to standard housing, resulted in lower stress-driven production of corticosterone, augmented mouse proinflammatory immune responses and markedly exacerbated high-fat diet (HFD)-induced NAFLD pathogenesis. Disease exacerbation at thermoneutrality was conserved across multiple mouse strains and was associated with augmented intestinal permeability, an altered microbiome and activation of inflammatory pathways that are associated with the disease in humans. Depletion of Gram-negative microbiota, hematopoietic cell deletion of Toll-like receptor 4 (TLR4) and inactivation of the IL-17 axis resulted in altered immune responsiveness and protection from thermoneutral-housing-driven NAFLD amplification. Finally, female mice, typically resistant to HFD-induced obesity and NAFLD, develop full disease characteristics at thermoneutrality. Thus, thermoneutral housing provides a sex-independent model of exacerbated NAFLD in mice and represents a novel approach for interrogation of the cellular and molecular mechanisms underlying disease pathogenesis.

Modulation of ambient temperature promotes inflammation and initiates atherosclerosis in wild type C57BL/6 mice

Objectives

Obesity and obesity-associated inflammation is central to a variety of end-organ sequelae including atherosclerosis, a leading cause of death worldwide. Although mouse models have provided important insights into the immunopathogenesis of various diseases, modeling atherosclerosis in mice has proven difficult. Specifically, wild-type (WT) mice are resistant to developing atherosclerosis, while commonly used genetically modified mouse models of atherosclerosis are poor mimics of human disease. The lack of a physiologically relevant experimental model of atherosclerosis has hindered the understanding of mechanisms regulating disease development and progression as well as the development of translational therapies. Recent evidence suggests that housing mice within their thermoneutral zone profoundly alters murine physiology, including both metabolic and immune processes. We hypothesized that thermoneutral housing would allow for augmentation of atherosclerosis induction and progression in mice.

Methods

ApoE^−/− and WT mice were housed at either standard (T_S) or thermoneutral (T_N) temperatures and fed either a chow or obesogenic “Western” diet. Analysis included quantification of (i) obesity and obesity-associated downstream sequelae, (ii) the development and progression of atherosclerosis, and (iii) inflammatory gene expression pathways related to atherosclerosis.

Results

Housing mice at T_N, in combination with an obesogenic “Western” diet, profoundly augmented obesity development, exacerbated atherosclerosis in ApoE^−/− mice, and initiated atherosclerosis development in WT mice. This increased disease burden was associated with altered lipid profiles, including cholesterol levels and fractions, and increased aortic plaque size. In addition to the mild induction of atherosclerosis, we similarly observed increased levels of aortic and white adipose tissue inflammation and increased circulating immune cell expression of pathways related to adverse cardiovascular outcome.

Conclusions

In sum, our novel data in WT C57Bl/6 mice suggest that modulation of a single environmental variable, temperature, dramatically alters mouse physiology, metabolism, and inflammation, allowing for an improved mouse model of atherosclerosis. Thus, thermoneutral housing of mice shows promise in yielding a better understanding of the cellular and molecular pathways underlying the pathogenesis of diverse diseases.

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Thermoneutral housing augments atherosclerosis in ApoE^−/− and WT mice.

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Thermoneutral housing increases serum LDL levels in obese WT mice.

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Thermoneutral housing increases inflammatory potential in lean and obese mice.

[Role of the mTOR pathway in the central regulation of energy balance]

The pathway of the mammalian (or mechanistic) target of rapamycin kinase (mTOR) responds to different signals such as nutrients and hormones and regulates many cellular functions as the synthesis of proteins and lipids, mitochondrial activity and the organization of the cytoskeleton. At the cellular level, mTOR forms two distinct complexes: mTORC1 and mTORC2. This review intends to summarize the various recent advances on the role of these two protein complexes in the central regulation of energy balance. mTORC1 activity modulates energy balance and metabolic responses by regulating the activity of neuronal populations, such as those located in the arcuate nucleus of the hypothalamus. Recent studies have shown that activity of the hypothalamic mTORC1 pathway varies according to cell and stimulus types, and that this signaling cascade regulates food intake and body weight in response to nutrients, such as leucine, and hormones like leptin, ghrelin and triiodothyronine. On the other hand, mTORC2 seems to be involved in the regulation of neuronal morphology and synaptic activity. However, its function in the central regulation of the energy balance is less known. Dysregulation of mTORC1 and mTORC2 is described in obesity and type 2 diabetes. Therefore, a better understanding of the molecular mechanisms involved in the regulation of energy balance by mTOR may lead to the identification of new therapeutic targets for the treatment of these metabolic pathologies.

Molecular Characterization and Expression Analysis of S6K1 in Cashmere Goats (Capra hircus)

p70 ribosomal S6 kinase (p70S6K) can integrate nutrient and growth factor signals to promote cell growth and survival. We report our molecular characterization of the complementary DNA (cDNA) that encodes the goat p70S6K gene 40S ribosomal S6 kinase 1 (S6K1) (GenBank accession GU144017) and its 3′ noncoding sequence in Inner Mongolia Cashmere goats (Capra hircus). Goat S6K1 cDNA was 2,272 bp and include an open reading frame (ORF) of 1,578 bp, corresponding to a polypeptide of 525 amino acids, and a 694-residue 3′ noncoding sequence with a polyadenylation signal at nucleotides 2,218 to 2,223. The relative abundance of S6K1 mRNA was measured by real-time PCR in 6 tissues, and p70S6K expression was examined by immunohistochemistry in heart and testis. The phosphorylation of p70S6K is regulated by mitogen-activated protein kinase (MAPK) signaling in fetal fibroblasts.

Ghrelin-induced food intake and adiposity depend on central mTORC1/S6K1 signaling

Signaling through the mammalian target of rapamycin complex 1 (mTORC1) and its effectors the S6-kinases (S6K) in the hypothalamus is thought to be involved in nutrient sensing and control of food intake. Given the anatomical proximity of this pathway to circuits for the hormone ghrelin, we investigated the potential role of the mTORC1/S6K pathway in mediating the metabolic effects of ghrelin. We found that ghrelin promoted phosphorylation of S6K1 in the mouse hypothalamic cell line N-41 and in the rat hypothalamus after intracerebroventricular administration. Rapamycin, an inhibitor of mTORC1, suppressed ghrelin-induced phosphorylation of hypothalamic S6K1 and increased food intake and insulin in rats. Chronic peripheral administration of ghrelin induced a significant increase in body weight, fat mass and food efficiency in wild-type and S6K2-knockout but not in S6K1-knockout mice. We therefore propose that ghrelin-induced hyperphagia, adiposity and insulin secretion are controlled by a central nervous system involving the mTORC1/S6K1 pathway.

Signal integration by mTORC1 coordinates nutrient input with biosynthetic output

Flux through metabolic pathways is inherently sensitive to the levels of specific substrates and products, but cellular metabolism is also managed by integrated control mechanisms that sense the nutrient and energy status of a cell or organism. The mechanistic target of rapamycin complex 1 (mTORC1), a protein kinase complex ubiquitous to eukaryotic cells, has emerged as a critical signalling node that links nutrient sensing to the coordinated regulation of cellular metabolism. Here, we discuss the role of mTORC1 as a conduit between cellular growth conditions and the anabolic processes that promote cell growth. The emerging network of signalling pathways through which mTORC1 integrates systemic signals (secreted growth factors) with local signals (cellular nutrients - amino acids, glucose and oxygen - and energy, ATP) is detailed. Our expanding understanding of the regulatory network upstream of mTORC1 provides molecular insights into the integrated sensing mechanisms by which diverse cellular signals converge to control cell physiology.

Diet controls Drosophila follicle stem cell proliferation via Hedgehog sequestration and release

Dietary cholesterol levels control follicle stem cell proliferation in the Drosophila ovary via regulation of Hedgehog protein localization.

A healthy diet improves adult stem cell function and delays diseases such as cancer, heart disease, and neurodegeneration. Defining molecular mechanisms by which nutrients dictate stem cell behavior is a key step toward understanding the role of diet in tissue homeostasis. In this paper, we elucidate the mechanism by which dietary cholesterol controls epithelial follicle stem cell (FSC) proliferation in the fly ovary. In nutrient-restricted flies, the transmembrane protein Boi sequesters Hedgehog (Hh) ligand at the surface of Hh-producing cells within the ovary, limiting FSC proliferation. Upon feeding, dietary cholesterol stimulates S6 kinase–mediated phosphorylation of the Boi cytoplasmic domain, triggering Hh release and FSC proliferation. This mechanism enables a rapid, tissue-specific response to nutritional changes, tailoring stem cell divisions and egg production to environmental conditions sufficient for progeny survival. If conserved in other systems, this mechanism will likely have important implications for studies on molecular control of stem cell function, in which the benefits of low calorie and low cholesterol diets are beginning to emerge.

Potential role of FoxO1 and mTORC1 in the pathogenesis of Western diet-induced acne

Acne in adolescents of developed countries is an epidemic skin disease and has currently been linked to the Western diet (WD). It is the intention of this viewpoint to discuss the possible impact of WD-mediated nutrient signalling in the pathogenesis of acne. High glycaemic load and dairy protein consumption both increase insulin/insulin-like growth factor-1 (IGF-1) signalling (IIS) that is superimposed on elevated IGF-1 signalling of puberty. The cell's nutritional status is primarily sensed by the forkhead box transcription factor O1 (FoxO1) and the serine/threonine kinase mammalian target of rapamycin complex 1 (mTORC1). Increased IIS extrudes FoxO1 into the cytoplasm, whereas nuclear FoxO1 suppresses hepatic IGF-1 synthesis and thus impairs somatic growth. FoxO1 attenuates androgen signalling, interacts with regulatory proteins important for sebaceous lipogenesis, regulates the activity of innate and adaptive immunity, antagonizes oxidative stress and most importantly functions as a rheostat of mTORC1, the master regulator of cell growth, proliferation and metabolic homoeostasis. Thus, FoxO1 links nutrient availability to mTORC1-driven processes: increased protein and lipid synthesis, cell proliferation, cell differentiation including hyperproliferation of acroinfundibular keratinocytes, sebaceous gland hyperplasia, increased sebaceous lipogenesis, insulin resistance and increased body mass index. Enhanced androgen, TNF-α and IGF-1 signalling due to genetic polymorphisms promoting the risk of acne all converge in mTORC1 activation, which is further enhanced by nutrient signalling of WD. Deeper insights into the molecular interplay of FoxO1/mTORC1-mediated nutrient signalling are thus of critical importance to understand the impact of WD on the promotion of epidemic acne and more serious mTORC1-driven diseases of civilization.

PTEN in cancer, metabolism, and aging

Recent reports on mice with systemic overexpression of the tumor-suppressor PTEN (phosphatase and tensin homolog) have expanded our understanding of its physiological functions. Pten transgenic mice present increased energy expenditure, decreased adiposity, improved insulin sensitivity upon high-fat feeding or with aging, and extended lifespan. This has led to new mechanistic insights about the role of PTEN in metabolism. Interestingly, PTEN promotes oxidative phosphorylation and decreases glycolysis, thus preventing the metabolic reprogramming characteristic of cancer cells, which might be relevant to PTEN-mediated cancer protection. PTEN also upregulates UCP1 expression in brown adipocytes, which enhances their nutrient burning capacity and decreases adiposity and associated pathologies. The newly discovered effects of PTEN on metabolism open new avenues for exploration relevant to cancer, obesity, diabetes, and aging.

Coupling nutrient sensing to metabolic homoeostasis: the role of the mammalian target of rapamycin complex 1 pathway

The mammalian target of rapamycin complex 1 (mTORC1) pathway is known to couple different environmental cues to the regulation of several energy-demanding functions within the cell, spanning from protein translation to mitochondrial activity. As a result, at the organism level, mTORC1 activity affects energy balance and general metabolic homoeostasis by modulating both the activity of neuronal populations that play key roles in the control of food intake and body weight, as well as by determining storage and use of fuel substrates in peripheral tissues. This review focuses on recent advances made in understanding the role of the mTORC1 pathway in the regulation of energy balance. More particularly, it aims at providing an overview of the status of knowledge regarding the mechanisms underlying the ability of certain amino acids, glucose and fatty acids, to affect mTORC1 activity and in turn illustrates how the mTORC1 pathway couples nutrient sensing to the hypothalamic regulation of the organisms' energy homoeostasis and to the control of intracellular metabolic processes, such as glucose uptake, protein and lipid biosynthesis. The evidence reviewed pinpoints the mTORC1 pathway as an integrator of the actions of nutrients on metabolic health and provides insight into the relevance of this intracellular pathway as a potential target for the therapy of metabolic diseases such as obesity and type-2 diabetes.