The GDF15-GFRAL Pathway in Health and Metabolic Disease: Friend or Foe?

GDF15 is a cell activation and stress response cytokine of the glial cell line-derived neurotrophic factor family within the TGF-ß superfamily. It acts through a recently identified orphan member of the GFRα family called GFRAL and signals through the Ret coreceptor. Cell stress and disease lead to elevated GDF15 serum levels, causing anorexia, weight loss, and alterations to metabolism, largely by actions on regions of the hindbrain. These changes restore homeostasis and, in the case of obesity, cause a reduction in adiposity. In some diseases, such as advanced cancer, serum GDF15 levels can rise by as much as 10-100-fold, leading to an anorexia-cachexia syndrome, which is often fatal. This review discusses how GDF15 regulates appetite and metabolism, the role it plays in resistance to obesity, and how this impacts diseases such as diabetes, nonalcoholic fatty liver disease, and anorexia-cachexia syndrome. It also discusses potential therapeutic applications of targeting the GDF15-GFRAL pathway and lastly suggests some potential unifying hypotheses for its biological role.

GDF15 mediates adiposity resistance through actions on GFRAL neurons in the hindbrain AP/NTS.

BACKGROUND: Elevated circulating levels of the divergent transforming growth factor-beta (TGFb) family cytokine, growth differentiation factor 15 (GDF15), acting through its CNS receptor, glial-derived neurotrophic factor receptor alpha-like (GFRAL), can cause anorexia and weight loss leading to anorexia/cachexia syndrome of cancer and other diseases. Preclinical studies suggest that administration of drugs based on recombinant GDF15 might be used to treat severe obesity. However, the role of the GDF15-GFRAL pathway in the physiological regulation of body weight and metabolism is unclear. The critical site of action of GFRAL in the CNS has also not been proven beyond doubt. To investigate these two aspects, we have inhibited the actions of GDF15 in mice started on high-fat diet (HFD). METHODS: The actions of GDF15 were inhibited using two methods: (1) Groups of 8 mice under HFD had their endogenous GDF15 neutralised by monoclonal antibody treatment, (2) Groups of 15 mice received AAV-shRNA to knockdown GFRAL at its hypothesised major sites of action, the hindbrain area postrema (AP) and the nucleus of the solitary tract (NTS). Metabolic measurements were determined during both experiments. CONCLUSIONS: Treating mice with monoclonal antibody to GDF15 shortly after commencing HFD results in more rapid gain of body weight, adiposity and hepatic lipid deposition than the control groups. This is accompanied by reduced glucose and insulin tolerance and greater expression of pro-inflammatory cytokines in adipose tissue. Localised AP and NTS shRNA-GFRAL knockdown in mice commencing HFD similarly caused an increase in body weight and adiposity. This effect was in proportion to the effectiveness of GFRAL knockdown, indicated by quantitative analysis of hindbrain GFRAL staining. We conclude that the GDF15-GFRAL axis plays an important role in resistance to obesity in HFD-fed mice and that the major site of action of GDF15 in the CNS is GFRAL-expressing neurons in the AP and NTS.

Intracellular chloride channel protein CLIC1 regulates macrophage function through modulation of phagosomal acidification.

Intracellular chloride channel protein 1 (CLIC1) is a 241 amino acid protein of the glutathione S transferase fold family with redox- and pH-dependent membrane association and chloride ion channel activity. Whilst CLIC proteins are evolutionarily conserved in Metazoa, indicating an important role, little is known about their biology. CLIC1 was first cloned on the basis of increased expression in activated macrophages. We therefore examined its subcellular localisation in murine peritoneal macrophages by immunofluorescence confocal microscopy. In resting cells, CLIC1 is observed in punctate cytoplasmic structures that do not colocalise with markers for endosomes or secretory vesicles. However, when these macrophages phagocytose serum-opsonised zymosan, CLIC1 translocates onto the phagosomal membrane. Macrophages from CLIC1(-/-) mice display a defect in phagosome acidification as determined by imaging live cells phagocytosing zymosan tagged with the pH-sensitive fluorophore Oregon Green. This altered phagosomal acidification was not accompanied by a detectable impairment in phagosomal-lysosomal fusion. However, consistent with a defect in acidification, CLIC1(-/-) macrophages also displayed impaired phagosomal proteolytic capacity and reduced reactive oxygen species production. Further, CLIC1(-/-) mice were protected from development of serum transfer induced K/BxN arthritis. These data all point to an important role for CLIC1 in regulating macrophage function through its ion channel activity and suggest it is a suitable target for the development of anti-inflammatory drugs.

Tumor-induced anorexia and weight loss are mediated by the TGF-beta superfamily cytokine MIC-1.

Anorexia and weight loss are part of the wasting syndrome of late-stage cancer, are a major cause of morbidity and mortality in cancer, and are thought to be cytokine mediated. Macrophage inhibitory cytokine-1 (MIC-1) is produced by many cancers. Examination of sera from individuals with advanced prostate cancer showed a direct relationship between MIC-1 abundance and cancer-associated weight loss. In mice with xenografted prostate tumors, elevated MIC-1 levels were also associated with marked weight, fat and lean tissue loss that was mediated by decreased food intake and was reversed by administration of antibody to MIC-1. Additionally, normal mice given systemic MIC-1 and transgenic mice overexpressing MIC-1 showed hypophagia and reduced body weight. MIC-1 mediates its effects by central mechanisms that implicate the hypothalamic transforming growth factor-beta receptor II, extracellular signal-regulated kinases 1 and 2, signal transducer and activator of transcription-3, neuropeptide Y and pro-opiomelanocortin. Thus, MIC-1 is a newly defined central regulator of appetite and a potential target for the treatment of both cancer anorexia and weight loss, as well as of obesity.

GDF15 analogs as obesity therapeutics.

There is increasing interest in GDF15 analogs as therapeutic agents for obesity. In this issue of Cell Metabolism, Benichou et al. report the first clinical trial of such a drug in obese humans.

GDF15 enhances body weight and adiposity reduction in obese mice by leveraging the leptin pathway.

GDF15 regulates its anorexic effects through the hindbrain area postrema (AP) and nucleus of the solitary tract (NTS) neurons where its receptor, glial-derived neurotrophic factor receptor alpha-like (GFRAL), is expressed. The actions of GDF15 may interact with other appetite regulators elevated in obesity, such as leptin. Here, we report that in mice with high-fat-diet-induced obesity (HFD), the combined infusion of GDF15 and leptin causes significantly greater weight and adiposity loss than either treatment alone, indicating potentiation between GDF15 and leptin. Furthermore, obese, leptin-deficient ob/ob mice are less responsive to GDF15, as are normal mice treated with a competitive leptin antagonist. GDF15 and leptin induce more hindbrain neuronal activation in HFD mice than either treatment alone does. We report extensive connections between GFRAL- and LepR-expressing neurons and find LepR knockdown in the NTS to reduce the GDF15-mediated activation of AP neurons. Overall, these findings suggest that leptin signaling pathways in the hindbrain increase GDF15's metabolic actions.

Macrophage inhibitory cytokine-1 (MIC-1/GDF15) and mortality in end-stage renal disease.

BACKGROUND: Elevated macrophage inhibitory cytokine-1 (MIC-1/GDF15) levels in serum mediate anorexia and weight loss in some cancer patients and similarly elevated levels occur in chronic kidney disease (CKD). Serum MIC-1/GDF15 is also elevated in chronic inflammatory diseases and predicts atherosclerotic events independently of traditional risk factors. The relationship between chronic inflammation, decreasing body mass index (BMI) and increased mortality in CKD is not well understood and is being actively investigated. MIC-1/GDF15 may link these features of CKD. METHODS: Cohorts of incident dialysis patients from Sweden (n = 98) and prevalent hemodialysis patients from the USA (n = 381) had serum MIC-1/GDF15, C-reactive protein (CRP) levels and BMI measured at study entry. Additional surrogate markers of nutritional adequacy, body composition and inflammation were assessed in Swedish patients. Patients were followed for all-cause mortality. RESULTS: In the Swedish cohort, serum MIC-1/GDF15 was associated with decreasing BMI, measures of nutrition and markers of oxidative stress and inflammation. Additionally, high serum MIC-1/GDF15 levels identified patients with evidence of protein-energy wasting who died in the first 3 years of dialysis. The ability of serum MIC-1/GDF15 to predict mortality in the first 3 years of dialysis was confirmed in the USA cohort. In both cohorts, serum MIC-1/GDF15 level was an independent marker of mortality when adjusted for age, CRP, BMI, history of diabetes mellitus and/or cardiovascular disease and glomerular filtration rate or length of time on dialysis at study entry. CONCLUSIONS: MIC-1/GDF15 is a novel independent serum marker of mortality in CKD capable of significantly improving the mortality prediction of other established markers. MIC-1/GDF15 may mediate protein-energy wasting in CKD and represent a novel therapeutic target for this fatal complication.

Growth differentiation factor-15 slows the growth of murine prostate cancer by stimulating tumor immunity.

Growth Differentiation Factor-15 (GDF15) is a divergent TGF-beta superfamily cytokine that is overexpressed by most cancers and is induced by anticancer therapy. Transgenic and induced animal models suggest that it protects from cancer development but the mechanisms are uncertain. We investigated the role of immunity in GDF15 induced reduction in prostate cancer (PCa) growth. The C57BL/6 transgenic TRAMP prostate cancer prone mice were bred with mice that were immunodeficient and/or systemically overexpressed GDF15. We developed a novel orthotopic TRAMP PCa model in which primary TRAMP tumor cells were implanted into prostates of mice to reduce the study time. These mice were administered recombinant mouse GDF15, antibody to CD8, PD1 or their respective controls. We found that GDF15 induced protection from tumor growth was reversed by lack of adaptive immunity. Flow cytometric evaluation of lymphocytes within these orthotopic tumors showed that GDF15 overexpression was associated with increased CD8 T cell numbers and an increased number and proportion of recently activated CD8+CD11c+ T cells and a reduced proportion of "exhausted" CD8+PD1+ T cells. Further, depletion of CD8 T cells in tumor bearing mice abolished the GDF15 induced protection from tumor growth. Infusion of GDF15 into mice bearing orthotopic TRAMP tumor, substantially reduced tumor growth that was further reduced by concurrent PD1 antibody administration. GDF15 overexpression or recombinant protein protects from TRAMP tumor growth by modulating CD8 T cell mediated antitumor immunity and augments the positive effects of anti-PD1 blockers.

Targeting the divergent TGFß superfamily cytokine MIC-1/GDF15 for therapy of anorexia/cachexia syndromes.

PURPOSE OF REVIEW: To review recent finding on MIC-1/GDF15 and re-evaluate it as a potential target for the therapy of anorexia/cachexia syndromes. RECENT FINDINGS: MIC-1/GDF15 consistently induces anorexia/cachexia in animal models. Its actions on brainstem feeding centers leads to anorexia, inducing prolonged undernutrition and consequent loss of both lean and fat mass. Epidemiological studies by multiple groups have linked substantially elevated serum levels of this cytokine to anorexia/cachexia syndromes in diverse diseases such as cancer, chronic renal and cardiac failure, and chronic obstructive lung disease. These elevated serum levels are similar to those required to induce this syndrome in animals. Recent identifications of its previously elusive receptor as GFRAL, has enhanced understanding of its biology and suggests that modulating the MIC-1/GDF15-GFRAL pathway may be a therapeutic target for anorexia/cachexia syndrome. SUMMARY: Inhibiting MIC-1/GDF15 or its receptor GFRAL are high-value potential targets for treatment of anorexia/cachexia syndrome in patients whose elevated serum levels may justify its use.

Targeting Obesity and Cachexia: Identification of the GFRAL Receptor-MIC-1/GDF15 Pathway.

Macrophage inhibitory cytokine-1/growth differentiation factor 15 (MIC-1/GDF15) is a divergent transforming growth factor (TGFß) superfamily cytokine implicated in biological and disease processes including metabolism, cancer, and chronic inflammation, but whose receptor has remained elusive. Four laboratories have recently identified GFRAL, an orphan receptor of the glial-derived neurotrophic factor (GDNF) receptor α family, as the receptor for MIC-1/GDF15, signaling though the coreceptor Ret. These data identify a new systemic to central nervous system (CNS) circuit that regulates metabolism in response to stress and which could be targeted to treat both severe obesity and anorexia/cachexia syndrome.

Macrophage inhibitory cytokine-1 (MIC-1/GDF15) gene deletion promotes cancer growth in TRAMP prostate cancer prone mice.

The divergent TGF-ß superfamily member, macrophage inhibitory cytokine-1 (MIC-1/GDF15), is overexpressed by most cancers, including prostate cancer (PCa). Whilst its circulating levels are linked to cancer outcome, the role MIC-1/GDF15 plays in cancer development and progression is incompletely understood. To investigate its effect on PCa development and spread, we have used TRAMP prostate cancer prone mice bearing a germline deletion of MIC-1/GDF15 (TRAMPMIC-/-). On average TRAMPMIC-/- mice died about 5 weeks earlier and had larger prostatic tumors compared with TRAMP mice that were wild type for MIC-1/GDF15 (TRAMPMIC+/+). Additionally, at the time of death or ethical end point, even when adjusted for lifespan, there were no significant differences in the number of mice with metastases between the TRAMPMIC+/+ and TRAMPMIC-/- groups. However, consistent with our previous data, more than twice as many TRAMP mice overexpressing MIC-1/GDF15 (TRAMPfmsmic-1) had metastases than TRAMPMIC+/+ mice (p<0.0001). We conclude that germ line gene deletion of MIC-1/GDF15 leads to increased local tumor growth resulting in decreased survival consistent with an overall protective role for MIC-1/GDF15 in early primary tumor development. However, in advancing disease, as we have previously noted, MIC-1/GDF15 overexpression may promote local invasion and metastatic spread.

Serum Levels of Human MIC-1/GDF15 Vary in a Diurnal Pattern, Do Not Display a Profile Suggestive of a Satiety Factor and Are Related to BMI.

The TGF-b superfamily cytokine MIC-1/GDF15 circulates in the blood of healthy humans. Its levels rise substantially in cancer and other diseases and this may sometimes lead to development of an anorexia/cachexia syndrome. This is mediated by a direct action of MIC-1/GDF15 on feeding centres in the hypothalamus and brainstem. More recent studies in germline gene deleted mice also suggest that this cytokine may play a role in physiological regulation of energy homeostasis. To further characterize the role of MIC-1/GDF15 in physiological regulation of energy homeostasis in man, we have examined diurnal and food associated variation in serum levels and whether variation in circulating levels relate to BMI in human monozygotic twin pairs. We found that the within twin pair differences in serum MIC-1/GDF15 levels were significantly correlated with within twin pair differences in BMI, suggesting a role for MIC-1/GDF15 in the regulation of energy balance in man. MIC-1/GDF15 serum levels altered slightly in response to a meal, but comparison with variation its serum levels over a 24 hour period suggested that these changes are likely to be due to bimodal diurnal variation which can alter serum MIC-1/GDF15 levels by about plus or minus 10% from the mesor. The lack of a rapid and substantial postprandial increase in MIC-1/GDF15 serum levels suggests that MIC1/GDF15 is unlikely to act as a satiety factor. Taken together, our findings suggest that MIC-1/GDF15 may be a physiological regulator of energy homeostasis in man, most probably due to actions on long-term regulation of energy homeostasis.

The anorectic actions of the TGFß cytokine MIC-1/GDF15 require an intact brainstem area postrema and nucleus of the solitary tract.

Macrophage inhibitory cytokine-1 (MIC-1/GDF15) modulates food intake and body weight under physiological and pathological conditions by acting on the hypothalamus and brainstem. When overexpressed in disease, such as in advanced cancer, elevated serum MIC-1/GDF15 levels lead to an anorexia/cachexia syndrome. To gain a better understanding of its actions in the brainstem we studied MIC-1/GDF15 induced neuronal activation identified by induction of Fos protein. Intraperitoneal injection of human MIC-1/GDF15 in mice activated brainstem neurons in the area postrema (AP) and the medial (m) portion of the nucleus of the solitary tract (NTS), which did not stain with tyrosine hydroxylase (TH). To determine the importance of these brainstem nuclei in the anorexigenic effect of MIC-1/GDF15, we ablated the AP alone or the AP and the NTS. The latter combined lesion completely reversed the anorexigenic effects of MIC-1/GDF15. Altogether, this study identified neurons in the AP and/or NTS, as being critical for the regulation of food intake and body weight by MIC-1/GDF15.

TGF-b superfamily cytokine MIC-1/GDF15 is a physiological appetite and body weight regulator.

The TGF-b superfamily cytokine MIC-1/GDF15 circulates in all humans and when overproduced in cancer leads to anorexia/cachexia, by direct action on brain feeding centres. In these studies we have examined the role of physiologically relevant levels of MIC-1/GDF15 in the regulation of appetite, body weight and basal metabolic rate. MIC-1/GDF15 gene knockout mice (MIC-1(-/-)) weighed more and had increased adiposity, which was associated with increased spontaneous food intake. Female MIC-1(-/-) mice exhibited some additional alterations in reduced basal energy expenditure and physical activity, possibly owing to the associated decrease in total lean mass. Further, infusion of human recombinant MIC-1/GDF15 sufficient to raise serum levels in MIC-1(-/-) mice to within the normal human range reduced body weight and food intake. Taken together, our findings suggest that MIC-1/GDF15 is involved in the physiological regulation of appetite and energy storage.

Macrophage inhibitory cytokine 1 (MIC-1/GDF15) decreases food intake, body weight and improves glucose tolerance in mice on normal & obesogenic diets.

Food intake and body weight are controlled by a variety of central and peripheral factors, but the exact mechanisms behind these processes are still not fully understood. Here we show that that macrophage inhibitory cytokine-1 (MIC-1/GDF15), known to have anorexigenic effects particularly in cancer, provides protection against the development of obesity. Both under a normal chow diet and an obesogenic diet, the transgenic overexpression of MIC-1/GDF15 in mice leads to decreased body weight and fat mass. This lean phenotype was associated with decreased spontaneous but not fasting-induced food intake, on a background of unaltered energy expenditure and reduced physical activity. Importantly, the overexpression of MIC-1/GDF15 improved glucose tolerance, both under normal and high fat-fed conditions. Altogether, this work shows that the molecule MIC-1/GDF15 might be beneficial for the treatment of obesity as well as perturbations in glucose homeostasis.

Macrophage inhibitory cytokine-1 (MIC-1/GDF15) slows cancer development but increases metastases in TRAMP prostate cancer prone mice.

Macrophage inhibitory cytokine-1 (MIC-1/GDF15), a divergent member of the TGF-ß superfamily, is over-expressed by many common cancers including those of the prostate (PCa) and its expression is linked to cancer outcome. We have evaluated the effect of MIC-1/GDF15 overexpression on PCa development and spread in the TRAMP transgenic model of spontaneous prostate cancer. TRAMP mice were crossed with MIC-1/GDF15 overexpressing mice (MIC-1(fms)) to produce syngeneic TRAMP(fmsmic-1) mice. Survival rate, prostate tumor size, histopathological grades and extent of distant organ metastases were compared. Metastasis of TC1-T5, an androgen independent TRAMP cell line that lacks MIC-1/GDF15 expression, was compared by injecting intravenously into MIC-1(fms) and syngeneic C57BL/6 mice. Whilst TRAMP(fmsmic-1) survived on average 7.4 weeks longer, had significantly smaller genitourinary (GU) tumors and lower PCa histopathological grades than TRAMP mice, more of these mice developed distant organ metastases. Additionally, a higher number of TC1-T5 lung tumor colonies were observed in MIC-1(fms) mice than syngeneic WT C57BL/6 mice. Our studies strongly suggest that MIC-1/GDF15 has complex actions on tumor behavior: it limits local tumor growth but may with advancing disease, promote metastases. As MIC-1/GDF15 is induced by all cancer treatments and metastasis is the major cause of cancer treatment failure and cancer deaths, these results, if applicable to humans, may have a direct impact on patient care.