Fibroblast Growth Factor-21 Controls Dietary Protein Intake in Male Mice

Mice Regulate Dietary Amino Acid Balance and Energy Intake by Selecting between Complementary Protein Sources

BACKGROUND

A balanced intake of protein and constituent amino acids (AAs) requires adjustments to total food intake (protein leverage [PL]) and food selection to balance deficits and excesses (complementary feeding). We provided mice with choices of casein and whey, 2 protein sources that are complementary in AA balance, across a range of protein concentrations (P%) of digestible energy (DE).

OBJECTIVES

We aimed to determine if: 1) PL operates similarly for casein and whey; 2) one protein source is preferred at control P%; 3) the preference changes as P% falls; and 4) AA intakes under control and low P% levels identify AAs that drive changes in protein selection.

METHODS

Food intake and plasma fibroblast growth factor-21 (FGF21) concentrations were measured in mice at various P% (P7.5%-P33%). For direct comparisons, defined diets were used in which the protein source was either casein or whey. In food choice studies, mice had access to foods in which both casein and whey were provided at the same P% level at the same time.

RESULTS

PL operated at different P% thresholds in casein (13%)- and whey (10%)-based diets, and the magnitude of PL was greater for casein. Although mice preferred casein under control conditions (P23%), a pronounced preference shift to whey occurred as P% fell to P13% and P10%. At low P%, increases in food intake were accompanied by increases in plasma FGF21, a protein hunger signal. Among AAs deficient in casein and enriched in whey, the intake of Cys was the most invariant as P% changed between P23% and P10%, appearing to drive the switch in protein preference.

CONCLUSIONS

Mice selected between complementary protein sources, casein and whey, achieving stable total energy intake and regulated intake of AAs as P% varied. Supplementation of low P% casein diets with one whey-enriched AA, Cys, suppressed plasma FGF21 and total food intake.

Context matters for addressing controversies in FGF21 biology

Recent discoveries by Solon-Biet and colleagues highlight the importance of nutritional context for addressing current controversies in Fibroblast Growth Factor 21 (FGF21) biology. Through a series of complex studies, the authors explored the physiological and pharmacological effects of FGF21 on feeding behavior and energy balance under differing nutritional and metabolic statuses.

FGF21 Induces Skeletal Muscle Atrophy and Increases Amino Acids in Female Mice: A Potential Role for Glucocorticoids

Fibroblast growth factor-21 (FGF21) is an intercellular signaling molecule secreted by metabolic organs, including skeletal muscle, in response to intracellular stress. FGF21 crosses the blood-brain barrier and acts via the nervous system to coordinate aspects of the adaptive starvation response, including increased lipolysis, gluconeogenesis, fatty acid oxidation, and activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Given its beneficial effects for hepatic lipid metabolism, pharmaceutical FGF21 analogues are used in clinical trials treatment of fatty liver disease. We predicted pharmacologic treatment with FGF21 increases HPA axis activity and skeletal muscle glucocorticoid signaling and induces skeletal muscle atrophy in mice. Here we found a short course of systemic FGF21 treatment decreased muscle protein synthesis and reduced tibialis anterior weight; this was driven primarily by its effect in female mice. Similarly, intracerebroventricular FGF21 reduced tibialis anterior muscle fiber cross-sectional area; this was more apparent among female mice than male littermates. In agreement with the reduced muscle mass, the topmost enriched metabolic pathways in plasma collected from FGF21-treated females were related to amino acid metabolism, and the relative abundance of plasma proteinogenic amino acids was increased up to 3-fold. FGF21 treatment increased hypothalamic Crh mRNA, plasma corticosterone, and adrenal weight, and increased expression of glucocorticoid receptor target genes known to reduce muscle protein synthesis and/or promote degradation. Given the proposed use of FGF21 analogues for the treatment of metabolic disease, the study is both physiologically relevant and may have important clinical implications.

Toward reconciling the roles of FGF21 in protein appetite, sweet preference, and energy expenditure

Fibroblast growth factor 21 (FGF21), an endocrine signal robustly increased by protein restriction independently of an animal's energy status, exerts profound effects on feeding behavior and metabolism. Here, we demonstrate that considering the nutritional contexts within which FGF21 is elevated can help reconcile current controversies over its roles in mediating macronutrient preference, food intake, and energy expenditure. We show that FGF21 is primarily a driver of increased protein intake in mice and that the effect of FGF21 on sweet preference depends on the carbohydrate balance of the animal. Under no-choice feeding, FGF21 infusion either increased or decreased energy expenditure depending on whether the animal was fed a high- or low-energy diet, respectively. We show that while the role of FGF21 in mediating feeding behavior is complex, its role in promoting protein appetite is robust and that the effects on sweet preference and energy expenditure are macronutrient-state-dependent effects of FGF21.

Dietary protein restriction modulates 'dessert' intake after a meal, via fibroblast growth factor 21 (FGF21)

Pharmacological administration of fibroblast growth factor 21 (FGF21) alters food choice, including that it decreases the consumption of sucrose and other sweet tastants. Conversely, endogenous secretion of FGF21 by the liver is modulated by diet, such that plasma FGF21 is increased after eating foods that have a low dietary protein: total energy (P: E) ratio. Together, these findings suggest a strategy to promote healthy eating, in which the macronutrient content of a pre-load diet could reduce the consumption of sweet desserts in sated mice. Here, we tested the prediction that individuals maintained on a low P: E diet, and offered a highly palatable sweet 'dessert' following a pre-load meal, would eat less of the sugary snack compared to controls-due to increased FGF21 signaling. In addition to decreasing sweet intake, FGF21 increases the consumption of dietary protein. Thus, we predicted that individuals maintained on the low P: E diet, and offered a very high-protein pellet as 'dessert' or snack after a meal, would eat more of the high protein pellet compared to controls, and that this depends on FGF21. We tested this in C57Bl/6J, and liver-specific FGF21-null (FGF21ΔL) null male and female mice and littermate controls. Contrary to expectation, eating a low protein pre-load did not reduce the later consumption of a sweet solution in either males or females, despite robustly increasing plasma FGF21. Rather, eating the low protein pre-load increased later consumption of a high protein pellet. This was more apparent among males and was abrogated in the FGF21ΔL mice. We conclude that physiologic induction of hepatic FGF21 by a low protein pre-load diet is not sufficient to reduce the consumption of sweet desserts, though it effectively increases the subsequent intake of dietary protein in male mice.

Meal sugar-protein balance determines postprandial FGF21 response in humans

Biological mechanisms to promote dietary balance remain unclear. Fibroblast growth factor 21 (FGF21) has been suggested to contribute to such potential regulation considering that FGF21 1) is genetically associated with carbohydrate/sugar and protein intake in opposite directions, 2) is secreted after sugar ingestion and protein restriction, and 3) pharmacologically reduces sugar and increases protein intake in rodents. To gain insight of the nature of this potential regulation, we aimed to study macronutrient interactions in the secretory regulation of FGF21 in healthy humans. We conducted a randomized, double-blinded, crossover meal study (NCT05061485), wherein healthy volunteers consumed a sucrose drink, a sucrose + protein drink, and a sucrose + fat drink (matched sucrose content), and compared postprandial FGF21 responses between the three macronutrient combinations. Protein suppressed the sucrose-induced FGF21 secretion [incremental area under the curve (iAUC) for sucrose 484 ± 127 vs. sucrose + protein -35 ± 49 pg/mL × h, P < 0.001]. The same could not be demonstrated for fat (iAUC 319 ± 102 pg/mL × h, P = 203 for sucrose + fat vs. sucrose). We found no indications that regulators of glycemic homeostasis could explain this effect. This indicates that FGF21 responds to disproportionate intake of sucrose relative to protein acutely within a meal, and that protein outweighs sucrose in FGF21 regulation. Together with previous findings, our results suggests that FGF21 might act to promote macronutrient balance and sufficient protein intake.NEW & NOTEWORTHY Here we test the interactions between sugar, protein, and fat in human FGF21 regulation and demonstrate that protein, but not fat, suppresses sugar-induced FGF21 secretion. This indicates that protein outweighs the effects of sugar in the secretory regulation of FGF21, and could suggest that the nutrient-specific appetite-regulatory actions of FGF21 might prioritize ensuring sufficient protein intake over limiting sugar intake.

Molecular mechanisms of metabolic disease-associated hepatic inflammation in non-alcoholic fatty liver disease and non-alcoholic steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is the leading chronic liver disease worldwide, with a progressive form of non-alcoholic steatohepatitis (NASH). It may progress to advanced liver diseases, including liver fibrosis, cirrhosis, and hepatocellular carcinoma. NAFLD/NASH is a comorbidity of many metabolic disorders such as obesity, insulin resistance, type 2 diabetes, cardiovascular disease, and chronic kidney disease. These metabolic diseases are often accompanied by systemic or extrahepatic inflammation, which plays an important role in the pathogenesis and treatment of NAFLD or NASH. Metabolites, such as short-chain fatty acids, impact the function, inflammation, and death of hepatocytes, the primary parenchymal cells in the liver tissue. Cholangiocytes, the epithelial cells that line the bile ducts, can differentiate into proliferative hepatocytes in chronic liver injury. In addition, hepatic non-parenchymal cells, including liver sinusoidal endothelial cells, hepatic stellate cells, and innate and adaptive immune cells, are involved in liver inflammation. Proteins such as fibroblast growth factors, acetyl-coenzyme A carboxylases, and nuclear factor erythroid 2-related factor 2 are involved in liver metabolism and inflammation, which are potential targets for NASH treatment. This review focuses on the effects of metabolic disease-induced extrahepatic inflammation, liver inflammation, and the cellular and molecular mechanisms of liver metabolism on the development and progression of NAFLD and NASH, as well as the associated treatments.

Protein appetite as an integrator in the obesity system: the protein leverage hypothesis

Despite the large volume and extensive range of obesity research, there is substantial disagreement on the causes and effective preventative strategies. We suggest the field will benefit from greater emphasis on integrative approaches that examine how various potential contributors interact, rather than regarding them as competing explanations. We demonstrate the application of nutritional geometry, a multi-nutrient integrative framework developed in the ecological sciences, to obesity research. Such studies have shown that humans, like many other species, regulate protein intake more strongly than other dietary components, and consequently if dietary protein is diluted there is a compensatory increase in food intake-a process called protein leverage. The protein leverage hypothesis (PLH) proposes that the dilution of protein in modern food supplies by fat and carbohydrate-rich highly processed foods has resulted in increased energy intake through protein leverage. We present evidence for the PLH from a variety of sources (mechanistic, experimental and observational), and show that this mechanism is compatible with many other findings and theories in obesity research. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

Fluctuations of serum fibroblast growth factor-21 levels in prolonged fasting in male adults

BACKGROUND & AIMS

Fibroblast Growth Factor-21 (FGF21) is a significant protein in the process of adaptation to fasting. The aim of this cross-sectional study to examine the effects of fasting on serum FGF21 levels, nutritional status, and anthropometric measurements of adult males during prolonged Ramadan fasting.

METHODS

This study included a total of 12 healthy individuals with normal body mass index, between the ages of 18-35. All individuals fasted for 16 h each day for 29 days of Ramadan. A questionnaire was administered to determine individuals' general characteristics and nutritional habits. Anthropometric measurements, 24-h dietary recall, and physical activities were recorded, and blood samples were collected four times in the following periods: before Ramadan, 1st week of Ramadan, 3rd week of Ramadan and 4 weeks after Ramadan.

RESULTS

It was observed that serum FGF21 levels increased until the 1st week of fasting. Remarkably, FGF21 levels declined in most subjects during the late phase of fasting and continued to decrease until 4 weeks after fasting. There was a positive correlation between serum FGF21 levels and protein intake at the 3rd week of fasting. Individuals' PAL and energy expenditure increased during fasting and continued to increase until 4 weeks after fasting. Energy intake, on the other hand, decreased from baseline until 4 weeks after fasting.

CONCLUSION

FGF21 serum levels may show an increase during fasting, however in long-term fasting periods like Ramadan it may decrease back to the beginning levels, therefore, FGF21 serum levels must be evaluated carefully.

The potential effect of metformin on fibroblast growth factor 21 in type 2 diabetes mellitus (T2DM)

Fibroblast growth factor 21 (FGF21) is a peptide hormone mainly synthesized and released from the liver. FGF21 acts on FGF21 receptors (FGFRs) and β-Klotho, which is a transmembrane co-receptor. In type 2 diabetes mellitus (T2DM), inflammatory disorders stimulate the release of FGF21 to overcome insulin resistance (IR). FGF21 improves insulin sensitivity and glucose homeostasis. Metformin which is used in the management of T2DM may increase FGF21 expression. Accordingly, the objective of this review was to clarify the metformin effect on FGF21 in T2DM. FGF21 level and expression of FGF2Rs are dysregulated in T2DM due to the development of FGF21 resistance. Metformin stimulates the hepatic expression of FGF21/FGF2Rs by different signaling pathways. Besides, metformin improves the expression of β-Klotho which improves FGF21 sensitivity. In conclusion, metformin advances FGF21 signaling and decreases FGF21 resistance in T2DM, and this might be an innovative mechanism for metformin in the enhancement of glucose homeostasis and metabolic disorders in T2DM patients.

A human laboratory study on the link between alcohol administration and circulating fibroblast growth factor 21 (FGF21) in individuals with alcohol use disorder

Growing evidence indicates that the crosstalk between the central nervous system and the periphery plays an important role in the pathophysiology of neuropsychiatric conditions, including addictive disorders. Fibroblast growth factor 21 (FGF21) is part of the liver-brain axis and regulates energy homeostasis, metabolism, and macronutrient intake. In addition, FGF21 signaling modulates alcohol intake and preference, and changes in FGF21 levels are observed following alcohol consumption. To further elucidate the relationship between alcohol use and FGF21, we assessed serum FGF21 concentrations in 16 non-treatment seeking individuals with alcohol use disorder (AUD) in a naturalistic outpatient setting, as well as a controlled laboratory experiment that included alcohol cue-reactivity, alcohol priming, and alcohol self-administration in a bar-like setting. FGF21 levels were stable during the outpatient phase when participants received placebo and had no significant lifestyle changes. During the bar-like laboratory experiment, a robust increase in serum FGF21 concentrations was found after the 2-hr alcohol self-administration session (F3, 49 = 23.39, p < 0.001). Percent change in FGF21 levels positively correlated with the amount of alcohol self-administered but did not reach statistical significance. No significant changes in FGF21 levels were found after exposure to alcohol cues or consuming the priming drink. Given the bidirectional link between FGF21 and alcohol, targeting the FGF21 system may be further examined as a potential pharmacotherapy for AUD.

Macronutrient intake: Hormonal controls, pathological states, and methodological considerations

A plethora of studies to date has examined the roles of feeding-related peptides in the control of food intake. However, the influence of these peptides on the intake of particular macronutrient constituents of food - carbohydrate, fat, and protein - has not been as extensively addressed in the literature. Here, the roles of several feeding-related peptides in controlling macronutrient intake are reviewed. Next, the relationship between macronutrient intake and diseases including diabetes mellitus, obesity, and eating disorders are examined. Finally, some key considerations in macronutrient intake research are discussed. We hope that this review will shed light onto this underappreciated topic in ingestive behavior research and will help to guide further scientific investigation in this area.

Liver Brain Interactions: Focus on FGF21 a Systematic Review

Fibroblast growth factor 21 is a pleiotropic hormone secreted mainly by the liver in response to metabolic and nutritional challenges. Physiologically, fibroblast growth factor 21 plays a key role in mediating the metabolic responses to fasting or starvation and acts as an important regulator of energy homeostasis, glucose and lipid metabolism, and insulin sensitivity, in part by its direct action on the central nervous system. Accordingly, pharmacological recombinant fibroblast growth factor 21 therapies have been shown to counteract obesity and its related metabolic disorders in both rodents and nonhuman primates. In this systematic review, we discuss how fibroblast growth factor 21 regulates metabolism and its interactions with the central nervous system. In addition, we also state our vision for possible therapeutic uses of this hepatic-brain axis.

Suppressive Effect of Autocrine FGF21 on Autophagy-Deficient Hepatic Tumorigenesis

Mice with hepatocyte-specific deletion of autophagy-related 7 (Atg7ΔHep mice) develop hepatoma, suggesting that autophagy deficiency could be a factor in the initiation of tumorigenesis. We have shown that FGF21 is induced as a ‘mitokine’ when Atg7 is disrupted in insulin target tissues such as the liver, which could affect systemic metabolism through endocrine activity. Since FGF21 or other endocrine FGF such as FGF19 can affect tumor growth, we hypothesized that FGF21 produced by Atg7-knockout (KO) hepatocytes may affect the behavior of Atg7-KO hepatoma in an autocrine manner. We, thus, crossed Atg7ΔHep mice with systemic Fgf21-KO (Fgf21−/−) mice to generate Atg7ΔHepFgf21−/− mice. The number and size of hepatoma of Atg7ΔHep mice were significantly increased by additional Fgf21 KO. The proliferation of Atg7-KO hepatocyte was significantly increased by Fgf21 KO. pYAP1/YAP1 representing YAP1 degradation was significantly decreased in the liver of Atg7ΔHepFgf21−/− mice compared to Atg7ΔHepFgf21+/+ mice. Consistently, expression of YAP1/TAZ downstream genes was significantly increased in the liver of Atg7ΔHepFgf21−/− mice compared to Atg7ΔHepFgf21+/+ mice, which could explain the increased size of hepatoma in Atg7ΔHepFgf21−/− mice. Accumulation of ROS and ROS-mediated DNA damage were increased in the liver of Atg7ΔHepFgf21+/+ mice, which was further aggravated by additional Fgf21 KO probably due to the absence of positive effect of FGF21 on mitochondrial function, explaining the increased number of hepatoma in Atg7ΔHepFgf21−/− mice compared to Atg7ΔHepFgf21+/+ mice. These results show that FGF21 produced by autophagy-deficient hepatocytes could have autocrine or paracrine effects on the number and proliferation of autophagy-deficient hepatoma, suggesting that hormones or factors released from autophagy-deficient tumors can influence the behavior or prognosis of the tumor in addition to the effects on host metabolism.

Hepatocyte-specific fibroblast growth factor 21 overexpression ameliorates high-fat diet-induced obesity and liver steatosis in mice

Fibroblast growth factor (FGF) 21 is an endocrine growth factor mainly secreted by the liver in response to a ketogenic diet and alcohol consumption. FGF21 signaling requires co-receptor β-klotho (KLB) co-acting with FGF receptors, which has pleiotropic metabolic effects, including induced hepatic fatty acid oxidation and ketogenesis, in human and animal models of obesity. We examined the hepatocyte-specific enhancer/promoter of FGF21 expression plasmids in high-fat diet-fed mice for 12 weeks. Hydrodynamic injection for FGF21 delivery every 6 weeks sustained high circulating levels of FGF21, resulting in marked reductions in body weight, epididymal fat mass, insulin resistance, and liver steatosis. FGF21-induced lipolysis in the adipose tissue enabled the liver to be flooded with fat-derived FFAs. The hepatic expression of Glut2 and Bdh1 was upregulated, whereas that of gluconeogenesis-related genes, G6p and Pepck, and lipogenesis-related genes, Srebp-1 and Srebp-2, was significantly suppressed. FGF21 induced the phosphorylation of AMPK at Thr172 and Raptor at ser792 and suppressed that of mTOR at ser2448, which downregulated mTORC1 signaling and reduced IRS-1 phosphorylation at ser1101. Finally, in the skeletal muscle, FGF21 increased Glut4 and Mct2, a membrane protein that acts as a carrier for ketone bodies. Enzymes for ketone body catabolism (Scot) and citrate cycle (Cs, Idh3a), and a marker of regenerating muscle (myogenin) were also upregulated via increased KLB expression. Thus, FGF21-induced lipolysis was continuously induced by a high-fat diet and fat-derived FFAs might cause liver damage. Hepatic fatty acid oxidation and ketone body synthesis may act as hepatic FFAs' disposal mechanisms and contribute to improved liver steatosis. Liver-derived ketone bodies might be used for energy in the skeletal muscle. The potential FGF21-related crosstalk between the liver and extraliver organs is a promising strategy to prevent and treat metabolic syndrome-related nonalcoholic steatohepatitis.

Fibroblast Growth Factor 21 Facilitates the Homeostatic Control of Feeding Behavior

Fibroblast growth factor 21 (FGF21) is a stress hormone that is released from the liver in response to nutritional and metabolic challenges. In addition to its well-described effects on systemic metabolism, a growing body of literature now supports the notion that FGF21 also acts via the central nervous system to control feeding behavior. Here we review the current understanding of FGF21 as a hormone regulating feeding behavior in rodents, non-human primates, and humans. First, we examine the nutritional contexts that induce FGF21 secretion. Initial reports describing FGF21 as a ‘starvation hormone’ have now been further refined. FGF21 is now better understood as an endocrine mediator of the intracellular stress response to various nutritional manipulations, including excess sugars and alcohol, caloric deficits, a ketogenic diet, and amino acid restriction. We discuss FGF21’s effects on energy intake and macronutrient choice, together with our current understanding of the underlying neural mechanisms. We argue that the behavioral effects of FGF21 function primarily to maintain systemic macronutrient homeostasis, and in particular to maintain an adequate supply of protein and amino acids for use by the cells.

A gene-diet interaction controlling relative intake of dietary carbohydrates and fats

OBJECTIVE

Preference for dietary fat vs. carbohydrate varies markedly across free-living individuals. It is recognized that food choice is under genetic and physiological regulation, and that the central melanocortin system is involved. However, how genetic and dietary factors interact to regulate relative macronutrient intake is not well understood.

METHODS

We investigated how the choice for food rich in carbohydrate vs. fat is influenced by dietary cholesterol availability and agouti-related protein (AGRP), the orexigenic component of the central melanocortin system. We assessed how macronutrient intake and different metabolic parameters correlate with plasma AGRP in a cohort of obese humans. We also examined how both dietary cholesterol levels and inhibiting de novo cholesterol synthesis affect carbohydrate and fat intake in mice, and how dietary cholesterol deficiency during the postnatal period impacts macronutrient intake patterns in adulthood.

RESULTS

In obese human subjects, plasma levels of AGRP correlated inversely with consumption of carbohydrates over fats. Moreover, AgRP-deficient mice preferred to consume more calories from carbohydrates than fats, more so when each diet lacked cholesterol. Intriguingly, inhibiting cholesterol biosynthesis (simvastatin) promoted carbohydrate intake at the expense of fat without altering total caloric consumption, an effect that was remarkably absent in AgRP-deficient mice. Finally, feeding lactating C57BL/6 dams and pups a cholesterol-free diet prior to weaning led the offspring to prefer fats over carbohydrates as adults, indicating that altered cholesterol metabolism early in life programs adaptive changes to macronutrient intake.

CONCLUSIONS

Together, our study illustrates a specific gene-diet interaction in modulating food choice.

Influence of leptin administration to pregnant female mice on obesity development, taste preferences, and gene expression in the liver and muscles of their male and female offspring

. The consumption of food rich in sugar and fat provokes obesity. Prenatal conditions have an impact on taste preferences and metabolism in the adult offspring, and this impact may manifest differently in different sexes. An increase in blood leptin level in pregnant females reduces the risk of obesity and insulin resistance in the offspring, although the mechanisms mediating this effect are unknown. Neither is it known whether maternal leptin affects taste preferences. In this study, we investigated the effect of leptin administration to pregnant mice on the development of diet-induced obesity, food choice, and gene expression in the liver and muscles of the offspring with regard to sex. Leptin was administered to female mice on days 11, 12, and 13 of pregnancy. In male and female offspring, growth rate and intake of standard chow after weaning, obesity development, gene expression in the liver and muscles, and food choice when kept on a high-calorie diet (standard chow, lard, sweet cookies) were recorded. Leptin administration to pregnant females reduced body weight in the female offspring fed on the standard diet. When the offspring were given a high-calorie diet, leptin administration inhibited obesity development and reduced the consumption of cookies only in males. It also increased the consumption of standard chow and the mRNA levels of genes for the insulin receptor and glucose transporter type 4 in the muscles of both male and female offspring. The results demonstrate that an increase in blood leptin levels in pregnant females has a sex-specif ic effect on the metabolism of the offspring increasing resistance to obesity only in male offspring. The mechanism underlying this effect includes a shift in food preference in favor of a balanced diet and maintenance of insulin sensitivity in muscle tissues.

Protein Appetite at the Interface between Nutrient Sensing and Physiological Homeostasis

Feeding behavior is guided by multiple competing physiological needs, as animals must sense their internal nutritional state and then identify and consume foods that meet nutritional needs. Dietary protein intake is necessary to provide essential amino acids and represents a specific, distinct nutritional need. Consistent with this importance, there is a relatively strong body of literature indicating that protein intake is defended, such that animals sense the restriction of protein and adaptively alter feeding behavior to increase protein intake. Here, we argue that this matching of food consumption with physiological need requires at least two concurrent mechanisms: the first being the detection of internal nutritional need (a protein need state) and the second being the discrimination between foods with differing nutritional compositions. In this review, we outline various mechanisms that could mediate the sensing of need state and the discrimination between protein-rich and protein-poor foods. Finally, we briefly describe how the interaction of these mechanisms might allow an animal to self-select between a complex array of foods to meet nutritional needs and adaptively respond to changes in either the external environment or internal physiological state.

Protein-carbohydrate interaction effects on energy balance, FGF21, IGF-1, and hypothalamic gene expression in rats

Amino acids are involved in energy homeostasis, just as are carbohydrates and lipids. Therefore, mechanisms controlling protein intake should operate independently and in combination with systems controlling overall energy intake to coordinate appropriate metabolic and behavioral responses. The objective of this study was to quantify the respective roles of dietary protein and carbohydrate levels on energy balance, plasma fibroblast growth factor 21 (FGF21) and insulin growth factor 1 (IGF-1) concentrations, and hypothalamic neurotransmitters (POMC, NPY, AgRP, and CART). In a simplified geometric framework, 7-wk-old male Wistar rats were fed 12 diets containing 3%-30% protein for 3 wk, in which carbohydrates accounted for 30%-75% of the carbohydrate and fat part of the diet. As a result of this study, most of the studied parameters (body composition, energy expenditure, plasma FGF21 and IGF-1 concentrations, and Pomc/Agrp ratio) responded mainly to the protein content and to a lesser extent to the carbohydrate content in the diet.NEW & NOTEWORTHY As mechanisms controlling protein intake can operate independently and in combination with those controlling energy intakes, we investigated the metabolic and behavioral effects of the protein-carbohydrate interaction. With a simplified geometric framework, we showed that body composition, energy balance, plasma FGF21 and IGF-1 concentrations, and hypothalamic Pomc/Agrp ratio were primarily responsive to protein content and, to a lesser extent, to carbohydrate content of the diet.

The Same Metabolic Response to FGF21 Administration in Male and Female Obese Mice Is Accompanied by Sex-Specific Changes in Adipose Tissue Gene Expression

The preference for high-calorie foods depends on sex and contributes to obesity development. Fibroblast growth factor 21 (FGF21) beneficially affects taste preferences and obesity, but its action has mainly been studied in males. The aim of this study was to compare the effects of FGF21 on food preferences and glucose and lipid metabolism in C57Bl/6J male and female mice with diet-induced obesity. Mice were injected with FGF21 or vehicle for 7 days. Body weight, choice between standard (SD) and high-fat (HFD) diets, blood parameters, and gene expression in white (WAT) and brown (BAT) adipose tissues, liver, muscles, and the hypothalamus were assessed. Compared to males, females had a greater preference for HFD; less WAT; lower levels of cholesterol, glucose, and insulin; and higher expression of Fgf21, Insr, Ppara, Pgc1, Acca and Accb in the liver and Dio2 in BAT. FGF21 administration decreased adiposity; blood levels of cholesterol, glucose, and insulin; hypothalamic Agrp expression, increased SD intake, decreased HFD intake independently of sex, and increased WAT expression of Pparg, Lpl and Lipe only in females. Thus, FGF21 administration beneficially affected mice of both sexes despite obesity-associated sex differences in metabolic characteristics, and it induced female-specific activation of gene expression in WAT.

FGF21 prevents low-protein diet-induced renal inflammation in aged mice

Low-protein (LP) diets extend lifespan through a comprehensive improvement in metabolic health across multiple tissues and organs. Many of these metabolic responses to protein restriction are secondary to transcriptional activation and release of FGF21 from the liver. However, the effects of an LP diet on the kidney in the context of aging has not been examined. Therefore, the goal of the current study was to investigate the impact of chronic consumption of an LP diet on the kidney in aging mice lacking FGF21. Wild-type (WT; C57BL/6J) and FGF21 knockout (KO) mice were fed a normal protein diet (20% casein) or an LP (5% casein) diet ad libitum from 3 to 22 mo of age. The LP diet led to a decrease in kidney weight and urinary albumin-to-creatinine ratio in both WT and FGF21 KO mice. Although the LP diet produced only mild fibrosis and infiltration of leukocytes in WT kidneys, the effects were significantly exacerbated by the absence of FGF21. Accordingly, transcriptomic analysis showed that inflammation-related pathways were significantly enriched and upregulated in response to LP diet in FGF21 KO mice but not WT mice. Collectively, these data demonstrate that the LP diet negatively affected the kidney during aging, but in the absence of FGF21, the LP diet-induced renal damage and inflammation were significantly worse, indicating a protective role of FGF21 in the kidney.NEW & NOTEWORTHY Long-term protein restriction is not advantageous for an otherwise healthy, aging kidney, as it facilitates the development of renal tubular injury and inflammatory cell infiltration. We provide evidence using FGF21 knockout animals that FGF21 is essential to counteract the renal injury and inflammation during aging on a low-protein diet.

Estrogen-Wnt signaling cascade regulates expression of hepatic fibroblast growth factor 21

We have generated the transgenic mouse line LTCFDN in which dominant negative TCF7L2 (TCF7L2DN) is specifically expressed in the liver during adulthood. Male but not female LTCFDN mice showed elevated hepatic and plasma triglyceride (TG) levels, indicating the existence of estrogen-β-cat/TCF signaling cascade that regulates hepatic lipid homeostasis. We show here that hepatic fibroblast growth factor 21 (FGF21) expression was reduced in male but not in female LTCFDN mice. The reduction was not associated with altered hepatic expression of peroxisome proliferator-activated receptor α (PPARα). In mouse primary hepatocytes (MPH), Wnt-3a treatment increased FGF21 expression in the presence of PPARα inhibitor. Results from our luciferase-reporter assay and chromatin immunoprecipitation suggest that evolutionarily conserved TCF binding motifs (TCFBs) on Fgf21 promoter mediate Wnt-3a-induced Fgf21 transactivation. Female mice showed reduced hepatic FGF21 production and circulating FGF21 level following ovariectomy (OVX), associated with reduced hepatic TCF expression and β-catenin S675 phosphorylation. Finally, in MPH, estradiol (E2) treatment enhanced FGF21 expression, as well as binding of TCF7L2 and ribonucleic acid (RNA) polymerase II to the Fgf21 promoter; and the enhancement can be attenuated by the G-protein-coupled estrogen receptor 1 (GPER) antagonist G15. Our observations hence indicate that hepatic FGF21 is among the effectors of the newly recognized E2-β-cat/TCF signaling cascade.NEW & NOTEWORTHY FGF21 is mainly produced in the liver. Therapeutic effect of FGF21 analogues has been demonstrated in clinical trials on reducing hyperlipidemia. We show here that Fgf21 transcription is positively regulated by Wnt pathway effector β-cat/TCF. Importantly, hepatic β-cat/TCF activity can be regulated by the female hormone estradiol, involving GPER. The investigation enriched our understanding on hepatic FGF21 hormone production, and expanded our view on metabolic functions of the Wnt pathway in the liver.

Pharmacological effects of fibroblast growth factor 21 are sex-specific in mice with the lethal yellow (Ay) mutation

Hypothalamic melanocortin 4 receptors (MC4R) regulate energy balance. Mutations in the MC4R gene are the most common cause of monogenic obesity in humans. Fibroblast growth factor 21 (FGF21) is a promising antiobesity agent, but its effects on melanocortin obesity are unknown. Sex is an important biological variable that must be considered when conducting preclinical studies; however, in laboratory animal models, the pharmacological effects of FGF21 are well documented only for male mice. We aimed at investigating whether FGF21 affects metabolism in male and female mice with the lethal yellow (A^y) mutation, which results in MC4R blockage and obesity development. Obese C57Bl-A^y male and female mice were administered subcutaneously for 10 days with vehicle or FGF21 (1 mg per 1 kg). Food intake (FI), body weight (BW), blood parameters, and gene expression in the liver, muscles, brown adipose tissue, subcutaneous and visceral white adipose tissues, and hypothalamus were measured. FGF21 action strongly depended on the sex of the animals. In the males, FGF21 decreased BW and insulin blood levels without affecting FI. In the females, FGF21 increased FI and liver weight, but did not affect BW. In control A^y-mice, expression of genes involved in lipid and glucose metabolism (Ppargc1a, Cpt1, Pck1, G6p, Slc2a2) in the liver and genes involved in lipogenesis (Pparg, Lpl, Slc2a4) in visceral adipose tissue was higher in females than in males, and FGF21 administration inhibited the expression of these genes in females. FGF21 administration decreased hypothalamic POMC mRNA only in males. Thus, the pharmacological effect of FGF21 were significantly different in male and female A^y-mice; unlike males, females were resistant to catabolic effects of FGF21.

The Nuanced Metabolic Functions of Endogenous FGF21 Depend on the Nature of the Stimulus, Tissue Source, and Experimental Model

Fibroblast growth factor 21 (FGF21) is a hormone that is involved in the regulation of lipid, glucose, and energy metabolism. Pharmacological FGF21 administration promotes weight loss and improves insulin sensitivity in rodents, non-human primates, and humans. However, pharmacologic effects of FGF21 likely differ from its physiological effects. Endogenous FGF21 is produced by many cell types, including hepatocytes, white and brown adipocytes, skeletal and cardiac myocytes, and pancreatic beta cells, and acts on a diverse array of effector tissues such as the brain, white and brown adipose tissue, heart, and skeletal muscle. Different receptor expression patterns dictate FGF21 function in these target tissues, with the primary effect to coordinate responses to nutritional stress. Moreover, different nutritional stimuli tend to promote FGF21 expression from different tissues; i.e., fasting induces hepatic-derived FGF21, while feeding promotes white adipocyte-derived FGF21. Target tissue effects of FGF21 also depend on its capacity to enter the systemic circulation, which varies widely from known FGF21 tissue sources in response to various stimuli. Due to its association with obesity and non-alcoholic fatty liver disease, the metabolic effects of endogenously produced FGF21 during the pathogenesis of these conditions are not well known. In this review, we will highlight what is known about endogenous tissue-specific FGF21 expression and organ cross-talk that dictate its diverse physiological functions, with particular attention given to FGF21 responses to nutritional stress. The importance of the particular experimental design, cellular and animal models, and nutritional status in deciphering the diverse metabolic functions of endogenous FGF21 cannot be overstated.

Current understanding and controversies on the clinical implications of fibroblast growth factor 21

Metabolic functions of the hepatic hormone fibroblast growth factor 21 (FGF21) have been recognized for more than a decade in studying the responses of human subjects and rodent models to nutritional stresses such as fasting, high-fat diet or ketogenic diet consumption, and ethanol intake. Our interest in the beneficial metabolic effects of FGF21 has risen due to its potential ability to serve as a therapeutic agent for various metabolic disorders, including type 2 diabetes, obesity, and fatty liver diseases, as well as its potential to act as a diagnostic or prognostic biomarker for metabolic and other disorders. Here, we briefly review the FGF21 gene and protein structures, its expression pattern, and cellular signaling cascades that mediate FGF21 production and function. We mainly focus on discussing experimental and clinical literature pertaining to FGF21 as a therapeutic agent. Furthermore, we present several lines of investigation, including a few studies conducted by our team, suggesting that FGF21 expression and function can be regulated by dietary polyphenol interventions. Finally, we discuss the literature debating FGF21 as a potential biomarker in various disorders.

FGF21 signaling in glutamatergic neurons is required for weight loss associated with dietary protein dilution

Alterations in macronutrient intake can have profound effects on energy intake and whole-body metabolism. For example, reducing protein intake increases energy expenditure, increases insulin sensitivity and decreases body weight in rodents. Fibroblast growth factor 21 (FGF21) signaling in the brain is necessary for the metabolic effects of dietary protein restriction and has more recently been proposed to promote protein preference. However, the neuron populations through which FGF21 elicits these effects are unknown. Here, we demonstrate that deletion of β-klotho in glutamatergic, but not GABAergic, neurons abrogated the effects of dietary protein restriction on reducing body weight, but not on improving insulin sensitivity in both diet-induced obese and lean mice. Specifically, FGF21 signaling in glutamatergic neurons is necessary for protection against body weight gain and induction of UCP1 in adipose tissues associated with dietary protein restriction. However, β-klotho expression in glutamatergic neurons was dispensable for the effects of dietary protein restriction to increase insulin sensitivity. In addition, we report that FGF21 administration does not alter protein preference, but instead promotes the foraging of other macronutrients primarily by suppressing simple sugar consumption. This work provides important new insights into the neural substrates and mechanisms behind the endocrine control of metabolism during dietary protein dilution.

High-protein and low-calorie diets improved the anti-aging Klotho protein in the rats' brain: the toxic role of high-fat diet

BACKGROUND

In the current study, our specific aim was to characterize the Klotho protein and expression levels in the hippocampus and prefrontal cortex of old rats treated with different diets (high-fat, high-protein, low-calorie, high-protein and low-calorie).

METHODS

Rats were treated with high-fat, high-protein, low-calorie, low-calorie high-protein diets for 10 weeks and then behavioral and molecular assessments were evaluated.

RESULTS

Statistical analysis showed the percentage of open arm time was increased in the high-protein, low-calorie and low-calorie high-protein groups compared with old control (old-C) rats. The percentage of open arm entries was increased in the low-calorie and low-calorie high-protein group compared with old-C rats. The body weight and serum triglyceride were decreased in the low-calorie and low-calorie high-protein groups in comparison to control old rats. Low-calorie and low-calorie high-protein treatments statistically enhanced caspase-3 level compared with old-C rats in the hippocampus and prefrontal cortex. Treatment of old rats with high-protein, low-calorie and low-calorie high-protein could increase Klotho-α level compared with control old rats. The levels of Klotho-α, c-fos and brain-derived neurotrophic factors were decreased in the low-calorie high-protein group in Klotho inhibitor's presence compared with the low-calorie high-protein group.

CONCLUSION

According to our findings, Klotho-α level was reduced in old rats. Low-calorie, high-protein and particularly low-calorie high-protein diets increased this protein level and consequently increased neuronal plasticity and improved memory function.

GRAPHIC ABSTRACT

FGF21 Signals Protein Status to the Brain and Adaptively Regulates Food Choice and Metabolism

Reduced dietary protein intake induces adaptive physiological changes in macronutrient preference, energy expenditure, growth, and glucose homeostasis. We demonstrate that deletion of the FGF21 co-receptor βKlotho (Klb) from the brain produces mice that are unable to mount a physiological response to protein restriction, an effect that is replicated by whole-body deletion of FGF21. Mice forced to consume a low-protein diet exhibit reduced growth, increased energy expenditure, and a resistance to diet-induced obesity, but the loss of FGF21 signaling in the brain completely abrogates that response. When given access to a higher protein alternative, protein-restricted mice exhibit a shift toward protein-containing foods, and central FGF21 signaling is essential for that response. FGF21 is an endocrine signal linking the liver and brain, which regulates adaptive, homeostatic changes in metabolism and feeding behavior during protein restriction.

FGF21 and the Physiological Regulation of Macronutrient Preference

The ability to respond to variations in nutritional status depends on regulatory systems that monitor nutrient intake and adaptively alter metabolism and feeding behavior during nutrient restriction. There is ample evidence that the restriction of water, sodium, or energy intake triggers adaptive responses that conserve existing nutrient stores and promote the ingestion of the missing nutrient, and that these homeostatic responses are mediated, at least in part, by nutritionally regulated hormones acting within the brain. This review highlights recent research that suggests that the metabolic hormone fibroblast growth factor 21 (FGF21) acts on the brain to homeostatically alter macronutrient preference. Circulating FGF21 levels are robustly increased by diets that are high in carbohydrate but low in protein, and exogenous FGF21 treatment reduces the consumption of sweet foods and alcohol while alternatively increasing the consumption of protein. In addition, while control mice adaptively shift macronutrient preference and increase protein intake in response to dietary protein restriction, mice that lack either FGF21 or FGF21 signaling in the brain fail to exhibit this homeostatic response. FGF21 therefore mediates a unique physiological niche, coordinating adaptive shifts in macronutrient preference that serve to maintain protein intake in the face of dietary protein restriction.

FGF19 and FGF21 for the Treatment of NASH-Two Sides of the Same Coin? Differential and Overlapping Effects of FGF19 and FGF21 From Mice to Human

FGF19 and FGF21 analogues are currently in clinical development for the potential treatment of NASH. In Phase 2 clinical trials analogues of FGF19 and FGF21 decrease hepatic steatosis with up to 70% (MRI-PDFF) after 12 weeks and as early as 12-16 weeks of treatment an improvement in NASH resolution and fibrosis has been observed. Therefore, this class of compounds is currently of great interest in the field of NASH. FGF19 and FGF21 belong to the endocrine FGF19 subfamily and both require the co-receptor beta-klotho for binding and signalling through the FGF receptors. FGF19 is expressed in the ileal enterocytes and is released into the enterohepatic circulation in response to bile acids stimuli and in the liver FGF19 inhibits hepatic bile acids synthesis by transcriptional regulation of Cyp7A1, which is the rate limiting enzyme. FGF21 is, on the other hand, highly expressed in the liver and is released in response to high glucose, high free-fatty acids and low amino-acid supply and regulates energy, glucose and lipid homeostasis by actions in the CNS and in the adipose tissue. FGF19 and FGF21 are differentially expressed, have distinct target tissues and separate physiological functions. It is therefore of peculiar interest to understand why treatment with both FGF19 and FGF21 analogues have strong beneficial effects on NASH parameters in mice and human and whether the mode of action is overlapping This review will highlight the physiological and pharmacological effects of FGF19 and FGF21. The potential mode of action behind the anti-steatotic, anti-inflammatory and anti-fibrotic effects of FGF19 and FGF21 will be discussed. Finally, development of drugs is always a risk benefit analysis and the human relevance of adverse effects observed in pre-clinical species as well as findings in humans will be discussed. The aim is to provide a comprehensive overview of the current understanding of this drug class for the potential treatment of NASH.

Protein Leverage: Theoretical Foundations and Ten Points of Clarification

Much attention has been focused on fats and carbohydrates as the nutritional causes of energy overconsumption and obesity. In 2003, a model of intake regulation was proposed in which the third macronutrient, protein, is not only involved but is a primary driver of calorie intake via its interactions with carbohydrates and fats. This model, called protein leverage, posits that the strong regulation of protein intake causes the overconsumption of fats and carbohydrates (hence total energy) on diets with a low proportion of energy from protein and their underconsumption on diets with a high proportion of protein. Protein leverage has since been demonstrated in a range of animal studies and in several studies of human macronutrient regulation, and its potential role in contributing to the obesity epidemic is increasingly attracting discussion. Over recent years, however, several misconceptions about protein leverage have arisen. Our aim in this paper is to briefly outline some key aspects of the underlying theory and clarify 10 points of misunderstanding that have the potential to divert attention from the substantive issues.

Dietary Sulfur Amino Acid Restriction and the Integrated Stress Response: Mechanistic Insights

Dietary sulfur amino acid restriction, also referred to as methionine restriction, increases food intake and energy expenditure and alters body composition in rodents, resulting in improved metabolic health and a longer lifespan. Among the known nutrient-responsive signaling pathways, the evolutionary conserved integrated stress response (ISR) is a lesser-understood candidate in mediating the hormetic effects of dietary sulfur amino acid restriction (SAAR). A key feature of the ISR is the concept that a family of protein kinases phosphorylates eukaryotic initiation factor 2 (eIF2), dampening general protein synthesis to conserve cellular resources. This slowed translation simultaneously allows for preferential translation of genes with special sequence features in the 5' leader. Among this class of mRNAs is activating transcription factor 4 (ATF4), an orchestrator of transcriptional control during nutrient stress. Several ATF4 gene targets help execute key processes affected by SAAR such as lipid metabolism, the transsulfuration pathway, and antioxidant defenses. Exploration of the canonical ISR demonstrates that eIF2 phosphorylation is not necessary for ATF4-driven changes in the transcriptome during SAAR. Additional research is needed to clarify the regulation of ATF4 and its gene targets during SAAR.

Fibroblast growth factor 21: an endocrine inhibitor of sugar and alcohol appetite

Fibroblast growth factor 21 (FGF21) is a liver-derived hormone with pleiotropic metabolic effects. Its production is induced by various dietary imbalances in mice (including low-protein and ketogenic diets, fructose feeding and ethanol), hinting that it might influence food preference given the role of the liver in maintaining homeostatic levels of circulating nutrients. In 2016, it was shown that FGF21 selectively inhibits consumption of sugars and the primary product of their fermentation, ethanol, but not intake of fat, protein or complex carbohydrates. Since then, studies have sought to unravel this selectivity, its physiological purpose and translational relevance, as well as delineate the neural mechanisms involved. Initially found to impact ingestive behaviours in mice and non-human primates, FGF21 is also induced in humans by sugars and, far more dramatically, by acute alcohol intake. Genetic studies have revealed that patterns of weekly candy and alcohol consumption are associated with genetic variants in FGF21 and its co-receptor β-klotho (KLB), suggesting that liking for sugar, and fermented sugar, may be influenced by natural variation in FGF21 signal strength in humans. Herein, we discuss our nascent understanding of FGF21 as a selective negative regulator of sugar and alcohol appetite as well as reasons why such a peculiar system may have evolved in mammals. Uncovering the regulatory network governing sugar, and fermented sugar, intake could provide new opportunities to improve dietary choices in a population suffering from Western diet-induced diseases fuelled in part by a runaway sweet - and alcohol - tooth.