Fibroblast growth factor 21 (FGF21) and bone: is there a relationship in humans?

FGF-based drug discovery: advances and challenges

The fibroblast growth factor (FGF) family comprises 15 paracrine-acting and 3 endocrine-acting polypeptides, which govern a multitude of processes in human development, metabolism and tissue homeostasis. Therapeutic endocrine FGFs have recently advanced in clinical trials, with FGF19 and FGF21-based therapies on the cusp of approval for the treatment of primary sclerosing cholangitis and metabolic syndrome-associated steatohepatitis, respectively. By contrast, while paracrine FGFs were once thought to be promising drug candidates for wound healing, burns, tissue repair and ischaemic ailments based on their potent mitogenic and angiogenic properties, repeated failures in clinical trials have led to the widespread perception that the development of paracrine FGF-based drugs is not feasible. However, the observation that paracrine FGFs can exert FGF hormone-like metabolic activities has restored interest in these FGFs. The recent structural elucidation of the FGF cell surface signalling machinery and the formulation of a new threshold model for FGF signalling specificity have paved the way for therapeutically harnessing paracrine FGFs for the treatment of a range of metabolic diseases.

Associations of Serum Irisin and Fibroblast Growth Factor-21 Levels With Bone Mineral Characteristics in Eumenorrheic Adolescent Athletes With Different Training Activity Patterns

PURPOSE

To describe serum irisin and fibroblast growth factor-21 (FGF-21) concentrations in healthy female adolescents with different training activity patterns and their associations with bone mineral properties and metabolic markers.

METHODS

A total of 62 adolescent girls aged 14-18 years were recruited: 22 rhythmic gymnasts, 20 swimmers, and 20 untrained controls. Bone mineral characteristics by dual-energy X-ray absorptiometry, daily energy intake by dietary recall, serum irisin, FGF-21, undercarboxylated osteocalcin, and C-terminal telopeptide of type I collagen were measured in all girls.

RESULTS

Whole body and lumbar spine areal bone mineral density and lumbar spine bone mineral content were higher in the rhythmic gymnasts group compared with swimmers and untrained controls groups (P < .05). Serum irisin, FGF-21, undercarboxylated osteocalcin, and C-terminal telopeptide of type I collagen levels were not significantly different between the groups. In the rhythmic gymnasts group, serum FGF-21 concentration was positively correlated with lumbar spine areal bone mineral density independently of confounding factors (r = .51; P = .027).

CONCLUSIONS

Serum irisin and FGF-21 levels were not different between adolescent eumenorrheic girls with different training activity patterns. FGF-21 was positively associated with lumbar spine areal bone mineral density, which predominantly consists of trabecular bone in adolescent rhythmic gymnasts.

Effect of Fibroblast Growth Factor (FGF) 19 and 21 on Hip Geometry and Strength in Post-menopausal Osteoporosis (PMO)

Fibroblast Growth Factor (FGF) receptor signalling is important for skeletal development. The FGF19 subfamily which includes FGF19 and FGF21 are involved in bone metabolism, although their effects on bone mineral density (BMD) and bone strength remain unclear. To further characterise the influence of these two factors on the skeleton, we studied the association between circulating concentrations of FGF19 and 21 with BMD and parameters of hip geometry and strength in post-menopausal osteoporosis (PMO). The study cohort consisted of 374 women aged (mean [SD]) 68.7[12.3] years with PMO. FGF19 and FGF21 were measured in serum by ELISA. BMD was measured at the lumbar spine (LS), total hip (TH) and femoral neck (FN) (n = 277) by dual energy X-ray absorptiometry (DXA) and hip structural analysis (HSA) parameters (n = 263) at the narrow neck of the femur (NN), Intertrochanter (IT) and Femoral shaft (FS) were derived from DXA scans. FGF19 and 21 were not associated with prevalent fractures or BMD when corrected for covariates; age, BMI, smoking habits and alcohol intake. Log-transformed FGF 21 was negatively associated with HSA parameters including Outer Diameter (OD) (p = 0.019), Cross-sectional area (CSA) (p = 0.01), cross-sectional moment of inertia (CSMI) (p = 0.011), Section modulus (Z) (p = 0.002) and cortical thickness (Co Th) (p = 0.026) at the IT only. CSA, CSMI, Z and Co Th were significantly lower (p < 0.05) in women with FGF21 concentrations greater than the median (> 103.5 pg/ml). Our data suggest that FGF 21 may have potentially adverse effects on the skeleton. Further characterisation is needed, particularly as FGF 21 analogues or agonists may be used to treat obesity-related metabolic disorders.

Effects of FGF21 overexpression in osteoporosis and bone mineral density: a two-sample, mediating Mendelian analysis

Objective

Fibroblast growth factor 21 (FGF21) is a secreted protein that regulates body metabolism. In recent years, many observational studies have found that FGF21 is closely related to bone mineral density and osteoporosis, but the causal relationship between them is still unclear. Therefore, this study used two-sample, mediated Mendelian randomization (MR) analysis to explore the causal relationship between FGF21 and osteoporosis and bone mineral density.

Methods

We conducted a two-sample, mediator MR Analysis using genetic data from publicly available genome-wide association studies (GWAS) that included genetic variants in the inflammatory cytokine FGF21, and Total body bone mineral density, Heel bone mineral density, Forearm bone mineral density, Femoral neck bone mineral density, osteoporosis. The main analysis method used was inverse variance weighting (IVW) to investigate the causal relationship between exposure and outcome. In addition, weighted median, simple median method, weighted median method and MR-Egger regression were used to supplement the explanation, and sensitivity analysis was performed to evaluate the reliability of the results.

Results

MR Results showed that FGF21 overexpression reduced bone mineral density: Total body bone mineral density (OR=0.920, 95%CI: 0.876-0.966), P=0.001), Heel bone mineral density (OR=0.971, 95%CI (0.949-0.993); P=0.01), Forearm bone mineral density (OR=0.882, 95%CI(0.799-0.973); P=0.012), Femoral neck bone mineral density (OR=0.952, 95%CI(0.908-0.998), P=0.039); In addition, it also increased the risk of osteoporosis (OR=1.003, 95%CI (1.001-1.005), P=0.004). Sensitivity analysis supported the reliability of these results. The effect of FGF21 overexpression on osteoporosis may be mediated by type 2 diabetes mellitus and basal metabolic rate, with mediating effects of 14.96% and 12.21%, respectively.

Conclusions

Our study suggests that the overexpression of FGF21 may lead to a decrease in bone mineral density and increase the risk of osteoporosis, and the effect of FGF21 on osteoporosis may be mediated through type 2 diabetes and basal metabolic rate. This study can provide a reference for analyzing the potential mechanism of osteoporosis and is of great significance for the prevention and treatment of osteoporosis.

Fibroblast growth factor 21 and bone homeostasis

Fibroblast growth factor 21 (FGF21), a member of the FGF subfamily, is produced primarily in the liver and adipose tissue. The main function of FGF21 is to regulate energy metabolism of carbohydrates and lipids in the body through endocrine and other means, making FGF21 have potential clinical value in the treatment of metabolic disorders. Although FGF21 and its receptors play a role in the regulation of bone homeostasis through a variety of signaling pathways, a large number of studies have reported that the abuse of FGF21 and its analogues and the abnormal expression of FGF21 in vivo may be associated with bone abnormalities. Due to limited research information on the effect of FGF21 on bone metabolism regulation, the role of FGF21 in the process of bone homeostasis regulation and the mechanism of its occurrence and development have not been fully clarified. Certainly, the various roles played by FGF21 in the regulation of bone homeostasis deserve increasing attention. In this review, we summarize the basic physiological knowledge of FGF21 and the effects of FGF21 on metabolic homeostasis of the skeletal system in animal and human studies. The information provided in this review may prove beneficial for the intervention of bone diseases.

The roles of hepatokine and osteokine in liver-bone crosstalk: Advance in basic and clinical aspects

Both the liver and bone are important secretory organs in the endocrine system. By secreting organ factors (hepatokines), the liver regulates the activity of other organs. Similarly, bone-derived factors, osteokines, are created during bone metabolism and act in an endocrine manner. Generally, the dysregulation of hepatokines is frequently accompanied by changes in bone mass, and osteokines can also disrupt liver metabolism. The crosstalk between the liver and bone, particularly the function and mechanism of hepatokines and osteokines, has increasingly gained notoriety as a topic of interest in recent years. Here, based on preclinical and clinical evidence, we summarize the potential roles of hepatokines and osteokines in liver-bone interaction, discuss the current shortcomings and contradictions, and make recommendations for future research.

Investigation of the Proliferative Potential of FGF21 or FGF19 in Liver-Specific FGFR4-Deficient Mice

Fibroblast growth factor 21 (FGF21) and FGF15/FGF19 belong to the same subgroup of FGFs and are believed to have therapeutic potential in the treatment of type 2 diabetes and associated metabolic dysfunctionalities and pathological conditions. FGF19 has been proposed to induce hyperplasia and liver tumors in FVB mice (named after its susceptibility to Friend leukemia virus B), mediated by the FGF receptor 4 (FGFR4). The goal of this work was to investigate whether FGF21 might also have a potential proliferative effect mediated via FGFR4 using liver-specific Fgfr4 knockout (KO) mice. We conducted a mechanistic 7-day study involving female Fgfr4 fl/fl and Fgfr4 KO mice with a treatment regimen of twice daily or daily subcutaneous injections of FGF21 or FGF19 (positive control), respectively. The Ki-67 liver labeling index (LI) was evaluated by a semi-automated bioimaging analysis. The results showed a statistically significant increase in FGF21- and FGF19-treated Fgfr4 fl/fl mice. Interestingly, in Fgfr4 KO mice, this effect was absent following both treatments of FGF19 and FGF21, indicating that not only the FGFR4 receptor is pivotal for the mediation of hepatocellular proliferation by FGF19 leading finally to liver tumors but it seems also that FGFR4/FGF21 signaling has an impact on the hepatocellular proliferative activity, which does not promote the formation of hepatocellular liver tumors based on the current knowledge.

FGF2 positively regulates osteoclastogenesis via activating the ERK-CREB pathway

Fibroblast growth factor 2 (FGF2) plays crucial roles in the growth and development of several tissues. However, its function in bone homeostasis remains controversial. Here, we found that exogenous FGF2 supplementation inhibited the mineralization of bone marrow stromal cells (BMSCs), at least partially, via up-regulating the gene expression of osteoclastogenesis. The FGF receptor (FGFR) allosteric antagonist SSR128129E modestly, whereas the FGFR tyrosine kinase inhibitor AZD4547 significantly antagonized the effects of FGF2. Mechanistically, FGF2 stimulated ERK phosphorylation, and the ERK signaling inhibitor PD325901 strongly blocked FGF2 enhancement of osteoclastogenesis. Moreover, the phosphorylation of CREB was also activated in response to FGF2, thereby potentiating the interaction of p-CREB with the promoter region of Rankl gene. Notably, FGF2-deficient BMSCs exhibited higher mineralization capability and lower osteoclastogenic gene expression. Correspondingly, FGF2-knockout mice showed increased bone mass and attenuated expression of osteoclast-related markers, which were associated with moderate inhibition of the ERK signaling. In conclusion, FGF2 positively regulates osteoclastogenesis via stimulating the ERK-CREB pathway. These findings establish the importance of FGF2 in bone homeostasis, hinting the potential use of FGF2/ERK/CREB specific inhibitors to fight against bone-related disorders, such as osteoporosis.

The Impact of Bariatric Surgery on Bone Health: State of the Art and New Recognized Links

Bariatric surgery (BS) is the most effective therapy for severe obesity, which improves several comorbidities (such as diabetes, hypertension, dyslipidemia, among others) and results in marked weight loss. Despite these consensual beneficial effects, sleeve gastrectomy and Roux-en-Y gastric bypass (the two main bariatric techniques) have also been associated with changes in bone metabolism and progressive bone loss. The objective of this literature review is to examine the impact of bariatric surgery on bone and its main metabolic links, and to analyze the latest findings regarding the risk of fracture among patients submitted to bariatric surgery.

Skeletal Muscle and Bone - Emerging Targets of Fibroblast Growth Factor-21

Fibroblast growth factor 21 (FGF21) is an atypical member of the FGF family, which functions as a powerful endocrine and paracrine regulator of glucose and lipid metabolism. In addition to liver and adipose tissue, recent studies have shown that FGF21 can also be produced in skeletal muscle. As the most abundant tissue in the human body, skeletal muscle has become increasingly recognized as a major site of metabolic activity and an important modulator of systemic metabolic homeostasis. The function and mechanism of action of muscle-derived FGF21 have recently gained attention due to the findings of considerably increased expression and secretion of FGF21 from skeletal muscle under certain pathological conditions. Recent reports regarding the ectopic expression of FGF21 from skeletal muscle and its potential effects on the musculoskeletal system unfolds a new chapter in the story of FGF21. In this review, we summarize the current knowledge base of muscle-derived FGF21 and the possible functions of FGF21 on homeostasis of the musculoskeletal system with a focus on skeletal muscle and bone.

Potential role of fibroblast growth factor 21 in the deterioration of bone quality in impaired glucose tolerance

PURPOSE

Findings on trabecular bone score (TBS), an index of bone quality, have been reported in prediabetes defined by impaired fasting glucose or HbA1c. Here, we assessed the bone mineral density (BMD) and TBS in prediabetes individuals with impaired glucose tolerance (IGT), and investigated the association of these bone parameters with serum levels of fibroblast growth factor 21 (FGF21), a hormone implicated in bone metabolism and with higher levels in IGT.

METHODS

Chinese postmenopausal women aged 55-80 years, without diabetes, were recruited from the Hong Kong Cardiovascular Risk Factor Prevalence Study in 2016-2018. Normal glucose tolerance (NGT) was defined by fasting glucose < 5.6 mmol/L and 2-h plasma glucose (2hG) < 7.8 mmol/L, and IGT by 2hG 7.8-11 mmol/L. Serum levels of FGF21 and other bone metabolism regulators were measured. Insulin sensitivity was assessed by the Matsuda index. Independent determinants of TBS were evaluated using multivariable stepwise linear regression.

RESULTS

173 individuals with NGT and 73 with IGT were included. TBS was lower in those with IGT compared to those with NGT, while BMD was comparable. Individuals with IGT had significantly higher serum FGF21 levels, which in turn showed an independent inverse relationship with TBS, attenuated after inclusion of the Matsuda index. Serum FGF21 levels, however, did not correlate with BMD.

CONCLUSION

Among Chinese postmenopausal women, bone quality was worse in IGT, despite comparable bone density. FGF21 levels showed a significant independent inverse relationship with TBS, partly attributed to insulin resistance. Whether FGF21 contributes to the impaired bone quality in IGT remains speculative.

Brown Adipose Tissue Rescues Bone Loss Induced by Cold Exposure

Cold temperature activates the sympathetic nervous system (SNS) to induce bone loss by altering bone remodeling. Brown adipose tissue (BAT) is influenced by the SNS in cold environments. Many studies have confirmed a positive relationship between BAT volume and bone mass, but the influence and mechanism of BAT on bone in vivo and in vitro is still unknown. Two-month-old C57/BL6j male mice were exposed to cold temperature (4°C) to induce BAT generation. BAT volume, bone remodeling and microstructure were assessed after 1 day, 14 days and 28 days of cold exposure. CTX-1, P1NP and IL-6 levels were detected in the serum by ELISA. To determine the effect of BAT on osteoclasts and osteoblasts in vitro, brown adipocyte conditional medium (BAT CM) was collected and added to the differentiation medium of bone marrow-derived macrophages (BMMs) and bone marrow mesenchymal stem cells (BMSCs). Micro-CT results showed that the bone volume fraction (BV/TV, %) significantly decreased after 14 days of exposure to cold temperature but recovered after 28 days. Double labeling and TRAP staining in vivo showed that bone remodeling was altered during cold exposure. BAT volume enlarged after 14 days of cold stimulation, and IL-6 increased. BAT CM promoted BMSC mineralization by increasing osteocalcin (Ocn), RUNX family transcription factor 2 (Runx2) and alkaline phosphatase (Alp) expression, while bone absorption was inhibited by BAT CM. In conclusion, restoration of bone volume after cold exposure may be attributed to enlarged BAT. BAT has a beneficial effect on bone mass by facilitating osteogenesis and suppressing osteoclastogenesis.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

FGF21, not GCN2, influences bone morphology due to dietary protein restrictions

BACKGROUND

Dietary protein restriction is emerging as an alternative approach to treat obesity and glucose intolerance because it markedly increases plasma fibroblast growth factor 21 (FGF21) concentrations. Similarly, dietary restriction of methionine is known to mimic metabolic effects of energy and protein restriction with FGF21 as a required mechanism. However, dietary protein has been shown to be required for normal bone growth, though there is conflicting evidence as to the influence of dietary protein restriction on bone remodeling. The purpose of the current study was to evaluate the effect of dietary protein and methionine restriction on bone in lean and obese mice, and clarify whether FGF21 and general control nonderepressible 2 (GCN2) kinase, that are part of a novel endocrine pathway implicated in the detection of protein restriction, influence the effect of dietary protein restriction on bone.

METHODS

Adult wild-type (WT) or Fgf21 KO mice were fed a normal protein (18 kcal%; CON) or low protein (4 kcal%; LP) diet for 2 or 27 weeks. In addition, adult WT or Gcn2 KO mice were fed a CON or LP diet for 27 weeks. Young New Zealand obese (NZO) mice were placed on high-fat diets that provided protein at control (16 kcal%; CON), low levels (4 kcal%) in a high-carbohydrate (LP/HC) or high-fat (LP/HF) regimen, or on high-fat diets (protein, 16 kcal%) that provided methionine at control (0.86%; CON-MR) or low levels (0.17%; MR) for up to 9 weeks. Long bones from the hind limbs of these mice were collected and evaluated with micro-computed tomography (μCT) for changes in trabecular and cortical architecture and mass.

RESULTS

In WT mice the 27-week LP diet significantly reduced cortical bone, and this effect was enhanced by deletion of Fgf21 but not Gcn2. This decrease in bone did not appear after 2 weeks on the LP diet. In addition, Fgf21 KO mice had significantly less bone than their WT counterparts. In obese NZO mice dietary protein and methionine restriction altered bone architecture. The changes were mediated by FGF21 due to methionine restriction in the presence of cystine, which did not increase plasma FGF21 levels and did not affect bone architecture.

CONCLUSIONS

This study provides direct evidence of a reduction in bone following long-term dietary protein restriction in a mouse model, effects that appear to be mediated by FGF21.

Bone Health Following Bariatric Surgery: An Update

Obesity rates are increasing rapidly, and bariatric surgery is currently the most effective tool for weight loss. Recently, bariatric surgery induced bone loss has gained attention. Such detrimental effect on bone is multifactorial and causes may include nutrient deficiencies, gut and gonadal hormonal changes, mechanical unloading, loss of lean mass, increased bone marrow fat, and increased risk of fall. This review describes the available evidence on bone loss and fracture risk following bariatric surgery and summarizes the guidelines on the topic. Increased bone resorption starts early postsurgery, and bone markers peak at 1-2 yr. Across studies, the drop in areal bone mineral density is inconsistent at the lumbar spine, while a 2%-5% drop at 6 mo and a 6%-10.5% at 9-12 mo are observed at the total hip. Conversely, studies using quantitative CT showed a 6%-7% decrease in volumetric bone mineral density at the lumbar spine at 6-12 mo postsurgery. These studies also report significant bone loss at the radius and tibia, in addition to alteration in bone microarchitecture. Fracture risk increases 2 yr after surgery, more so following malabsorptive procedures. Fractures were reported at axial, weight bearing sites and at appendicular sites. The available evidence is very heterogeneous, and mostly derived from studies on Roux-en-y gastric bypass in premenopausal women. Data on restrictive procedures is scarce. Our findings suggest that the early postoperative phase represents the "golden window" to intervene and promote bone health. More research is needed to determine the effect of different bariatric procedures on bone, to identify optimal interventions to prevent bone loss and to characterize high risk individuals who should be targeted.

Increased Expression of FGF-21 Negatively Affects Bone Homeostasis in Dystrophin/Utrophin Double Knockout Mice

Duchenne muscular dystrophy (DMD) is the most common muscular dystrophy seen in children. In addition to skeletal muscle, DMD also has a significant impact on bone. The pathogenesis of bone abnormalities in DMD is still unknown. Recently, we have identified a novel bone-regulating cytokine, fibroblast growth factor-21 (FGF-21), which is dramatically upregulated in skeletal muscles from DMD animal models. We hypothesize that muscle-derived FGF-21 negatively affects bone homeostasis in DMD. Dystrophin/utrophin double-knockout (dKO) mice were used in this study. We found that the levels of circulating FGF-21 were significantly higher in dKO mice than in age-matched WT controls. Further tests on FGF-21 expressing tissues revealed that both FGF-21 mRNA and protein expression were dramatically upregulated in dystrophic skeletal muscles, whereas FGF-21 mRNA expression was downregulated in liver and white adipose tissue (WAT) compared to WT controls. Neutralization of circulating FGF-21 by i.p. injection of anti-FGF-21 antibody significantly alleviated progressive bone loss in weight-bearing (vertebra, femur, and tibia) and non-weight bearing bones (parietal bones) in dKO mice. We also found that FGF-21 directly promoted RANKL-induced osteoclastogenesis from bone marrow macrophages (BMMs), as well as promoted adipogenesis while concomitantly inhibiting osteogenesis of bone marrow mesenchymal stem cells (BMMSCs). Furthermore, fibroblast growth factor receptors (FGFRs) and co-receptor β-klotho (KLB) were expressed in bone cells (BMM-derived osteoclasts and BMMSCs) and bone tissues. KLB knockdown by small interfering RNAs (siRNAs) significantly inhibited the effects of FGF21 on osteoclast formation of BMMs and on adipogenic differentiation of BMMSCs, indicating that FGF-21 may directly affect dystrophic bone via the FGFRs-β-klotho complex. In conclusion, this study shows that dystrophic skeletal muscles express and secrete significant levels of FGF-21, which negatively regulates bone homeostasis and represents an important pathological factor for the development of bone abnormalities in DMD. The current study highlights the importance of muscle/bone cross-talk via muscle-derived factors (myokines) in the pathogenesis of bone abnormalities in DMD. © 2019 American Society for Bone and Mineral Research.

Lower Serum Fibroblast Growth Factor 21 Levels are Associated with Normal Lumbar Spine Bone Mineral Density in Hemodialysis Patients

Recent evidence has indicated that fibroblast growth factor 21 (FGF21) regulates longitudinal bone growth, with increased FGF21 levels leading to bone loss. The present study evaluated the relationship between bone mineral density (BMD) and serum FGF21 levels in patients undergoing hemodialysis (HD). We analyzed blood samples from 95 patients undergoing HD and measured BMD using dual-energy X-ray absorptiometry of the lumbar vertebrae (L2–L4). Serum FGF21 concentrations were determined using a commercially available enzyme-linked immunosorbent assay kit. Thirteen (11.6%) patients were found to have osteoporosis, 27 (28.4%) osteopenia, and 57 patients had normal BMD. Advanced age and decreased body mass index, height, body weight, waist circumference, and triglyceride level were associated with lower lumbar T-scores, as were increased alkaline phosphatase, urea reduction rate, fractional clearance index for urea, and FGF21 levels. Low log-FGF21, increased body mass index, increased pre-HD body weight, and increased logarithmically transformed triglycerides (log-TG) were found to be significantly and independently associated with lumbar BMD by multivariate forward stepwise linear regression analysis with adjustment for significant confounders. We conclude that high serum FGF21 level is negatively associated with BMD in patients undergoing HD.

Age-related bone loss is associated with FGF21 but not IGFBP1 in healthy adults

What is the central question of this study? Fibroblast growth factor 21 (FGF21) plays important therapeutic roles in metabolic diseases but is associated with bone loss, through insulin-like growth factor binding protein 1 (IGFBP1), in animals. However, the effect of the FGF21-IGFBP1 axis on age-related bone loss has not been explored in humans. What is the main finding and its importance? Using 'genetically linked' parent and child family pairs, we show that the FGF21 concentration, but not the IGFBP1 concentration, is higher in older than in younger adults. Our results suggest that age-associated decline in bone mineral density is associated with FGF21 and increased bone turnover but not likely to involve IGFBP1 in healthy humans. ABSTRACT: Bone fragility increases with age. The fibroblast growth factor 21 (FGF21)-insulin-like growth factor binding protein 1 (IGFBP1) axis regulates bone loss in animals. However, the role of FGF21 in mediating age-associated bone fragility in humans remains unknown. The purpose of this study was to explore the FGF21-regulatory axis in bone turnover and the age-related decline in bone mineral density (BMD). Twenty 'genetically linked' family (parent and child) pairs were recruited. Younger adults were 22-39 years old and older adults 60-71 years old. The BMD and serum concentrations of FGF21, IGFBP1, receptor activator of nuclear factor-κB ligand (RANKL), tartrate-resistant acid phosphatase 5b (TRAP5b) and bone-specific alkaline phosphatase (BAP) were measured. Older adults had 10-18% lower BMD at the hip and spine (P < 0.008) and a twofold higher FGF21 concentration (P < 0.001). The IGFBP1 concentration was similar in younger and older adults (P = 0.961). The RANKL concentration was 44% lower (P = 0.006), whereas TRAP5b and BAP concentrations were 36 and 31% higher (P = 0.01 and P = 0.004), respectively, in older adults than in younger adults. Adjusting for sex did not affect these results. The FGF21 concentration was negatively correlated with BMD at the spine (r = -0.460, P = 0.003), but not with the IGFBP1 concentration (r = -0.144, P = 0.374). The IGFBP1 concentration was not correlated with BMD at the hip or spine (all P > 0.05). In humans, FGF21 might be involved in the age-associated decline in BMD, especially at the spine, through increased bone turnover. IGFBP1 is unlikely to be the downstream effector of FGF21 in driving the age-associated decline in BMD and in RANKL-associated osteoclast differentiation.

Myokines: The endocrine coupling of skeletal muscle and bone

Bone and skeletal muscle are integrated organs and their coupling has been considered mainly a mechanical one in which bone serves as attachment site to muscle while muscle applies load to bone and regulates bone metabolism. However, skeletal muscle can affect bone homeostasis also in a non-mechanical fashion, i.e., through its endocrine activity. Being recognized as an endocrine organ itself, skeletal muscle secretes a panel of cytokines and proteins named myokines, synthesized and secreted by myocytes in response to muscle contraction. Myokines exert an autocrine function in regulating muscle metabolism as well as a paracrine/endocrine regulatory function on distant organs and tissues, such as bone, adipose tissue, brain and liver. Physical activity is the primary physiological stimulus for bone anabolism (and/or catabolism) through the production and secretion of myokines, such as IL-6, irisin, IGF-1, FGF2, beside the direct effect of loading. Importantly, exercise-induced myokine can exert an anti-inflammatory action that is able to counteract not only acute inflammation due to an infection, but also a condition of chronic low-grade inflammation raised as consequence of physical inactivity, aging or metabolic disorders (i.e., obesity, type 2 diabetes mellitus). In this review article, we will discuss the effects that some of the most studied exercise-induced myokines exert on bone formation and bone resorption, as well as a brief overview of the anti-inflammatory effects of myokines during the onset pathological conditions characterized by the development a systemic low-grade inflammation, such as sarcopenia, obesity and aging.

Fibroblast Growth Factor 21 Is Associated With Bone Mineral Density, but not With Bone Turnover Markers and Fractures in Chinese Postmenopausal Women

Fibroblast growth factor 21 (FGF21) is a member of the endocrine FGF subfamily and an important metabolic regulator that has multiple beneficial effects on glucose homeostasis and lipid metabolism. However, it was unclear whether FGF21 would induce bone defects in humans. This study evaluated the associations of FGF21 levels, bone mineral density (BMD), osteoporotic fracture, and bone turnover marks (BTMs) in postmenopausal women. A total of 1342 postmenopausal Chinese Han women (511 cases of fragility fracture in the case group and 831 cases in nonfragility fracture group) were enrolled. Serum FGF21 concentration was measured by ELISA (Quantikine), serum calcium (Ca), phosphate (P), alkaline phosphatase, 25-hydroxyvitamin D, parathyroid hormone, β-crosslinked C-telopeptide of type l collagen, were measured using an automated Roche electro-chemiluminescence system. BMD was measured using dual-energy X-ray absorptiometry. The association with age, BMD, 25-hydroxyvitamin D, parathyroid hormone, β-crosslinked C-telopeptide of type l collagen, and FGF21 levels were also evaluated in postmenopausal women. In nonfracture group and fragility fracture group, postmenopausal women's FGF21 level was 226.57pg/mL (149.11-354.43 pg/mL) and 219.43pg/mL (147.21-323.74 pg/mL), respectively. There is no significant difference in serum FGF21 levels between the fragility fracture group and the nonfracture group (p = 0.160). There was a significant statistical difference in BMD between the fragility fracture group and the nonfracture group (p = 0.000). In multiple linear regression analysis, FGF21 levels were significantly positive associated with lumbar BMD in postmenopausal women (L1-4, p = 0.007), independent of other factors, especially in fragility fracture group (L1-4, p = 0.001). In addition, a significant positive association was also observed between serum FGF21 levels and age in postmenopausal women (p < 0.05). We reveal a positive correlation between serum FGF21 concentrations with lumbar BMD in Chinese Han postmenopausal women. No significant correlations are present between serum FGF21 and bone turnover marks or serum FGF21 and fragility fracture in our study.

MicroRNA‑100‑5p inhibits osteoclastogenesis and bone resorption by regulating fibroblast growth factor 21

MicroRNAs (miRNAs/miRs) are post‑transcriptional regulators that serve important roles in osteoclastogenesis and bone metabolism; however, the roles of miRNAs have not been completely clarified. The present study aimed to investigate the effects of miR‑100‑5p on the mechanism of liver‑bone endocrine metabolism. A miRNA microarray analysis was conducted to evaluate the miRNA expression profile during receptor activator of nuclear factor‑κB ligand‑stimulated osteoclastogenesis. Hematoxylin and eosin and tartrate‑resistant acid phosphatase staining were performed to analyze the trabecular bone microstructure and osteoclast differentiation. The mRNA and protein expression levels were assessed by reverse transcription‑quantitative polymerase chain reaction and western blotting, respectively. The results revealed that in vitro osteoclast differentiation and in vivo bone resorption were suppressed by miR‑100‑5p overexpression. In vivo, a decrease in miR‑100‑5p and an increase in FGF21 were simultaneously observed in mice following ovariectomy (OVX). Bioinformatics analysis and experimental data confirmed that FGF21 was a direct target of miR‑100‑5p. Conversely, augmentation of miR‑100‑5p using a specific agomir in OVX‑operated mice decreased the levels of FGF21 in the serum and liver, and prevented osteoclastogenesis and bone loss. The present study revealed that FGF21 may be a signal molecule associated with the mechanism of liver‑bone endocrine metabolism and may be targeted by miR‑100‑5p. In addition, miR‑100‑5p may serve an important role in protecting against OVX‑induced osteoporosis.

Association between Serum Fibroblast Growth Factor 21 Levels and Bone Mineral Density in Postmenopausal Women

BACKGROUND

Despite the beneficial effect of fibroblast growth factor 21 (FGF21) on metabolic disease, there are concerns about adverse effects on bone metabolism, supported by animal studies. However, a recent human study showed the positive association between serum FGF21 level and bone mineral density (BMD) in healthy premenopausal women. We undertook this study to examine the association between FGF21 level and BMD in healthy postmenopausal Korean women who are susceptible to osteoporosis.

METHODS

We used data of 115 participants from a cohort of healthy postmenopausal women (>50 years old) to examine the association between serum FGF21 level and BMD. The clinical characteristics were obtained from the participants, and blood testing and serum FGF21 testing were undertaken. BMD of the lumbar spine, femoral neck and total hip area, and bone markers were used in the analyses.

RESULTS

The mean age of the participants was 60.2±7.2 years. Serum FGF21 levels showed negative correlation with BMD and T-scores in all three areas, but there were no statistically significant differences. Multivariate analyses with adjustment for age and body mass index also did not show significant association between serum FGF21 level and BMD. In addition, serum FGF21 level also showed no correlation with osteocalcin and C-telopeptide levels.

CONCLUSION

In our study, serum FGF21 level showed no significant correlation with BMD and T-scores.

Association between fibroblast growth factor 21 and bone mineral density in adults

PURPOSE

Animal-based studies have reported a decrease in bone mass resulting from high level of fibroblast growth factor 21 (FGF21). However, the correlation between plasma FGF21 levels and bone mineral density (BMD) is paradoxical in previous human-based studies, and the associations between FGF21 gene polymorphisms and BMD haven't been reported yet. Therefore, here, we evaluated plasma FGF21 levels with sufficient study samples, and performed genetic association test to reveal the physiological and genetic role of FGF21 on BMD in adults.

METHODS

Plasma and genetic samples containing 168 and 569 Han Chinese subjects, respectively, were employed in this study. Fasting plasma FGF21 levels were determined using enzyme-linked immunosorbent assay (ELISA). Regional BMD values were measured by dual energy X-ray absorptiometry (DXA). Five variants of FGF21 gene were successfully genotyped.

RESULTS

Physiological association suggested that plasma FGF21 levels were inversely correlated with BMD in femoral neck (Neck-BMD: P = 0.039) and Ward's triangle (Ward's-BMD: P = 0.002) of hip region. A FGF21 gene variant, rs490942, was significantly associated with the increase of Ward's-BMD in total (P = 0.027) and female (P = 0.016) cohorts, as well as Neck-BMD in female cohort (P = 7.45 × 10^-3). Meanwhile, eQTL results indicated that this SNP was related to the decreased level of FGF21 gene expression.

CONCLUSIONS

Taking together from both physiological and genetic levels, we suggest that FGF21 is inversely associated with regional BMD. And we haven't observed sex-specific effect in this study.

Sex-specific association between fibroblast growth factor 21 and type 2 diabetes: a nested case-control study in Singapore Chinese men and women

Background

Fibroblast growth factor 21 (FGF-21) is mainly secreted by liver and has been reported to be involved in the pathogenesis of type 2 diabetes. Some prospective studies have shown a positive association between FGF-21 and diabetes risk. However, no study has examined whether the association differed by sex, which has been reported between FGF-21 and atherosclerosis. Therefore, we prospectively evaluated the sex-specific association between FGF-21 and diabetes in a Chinese population.

Methods

Serum FGF-21 concentration was measured in a case-control study comprising of 251 incident diabetes cases and 251 age-sex-matched controls nested within a prospective population-based cohort, the Singapore Chinese Health Study. At blood collection between 1999 and 2004, participants were free of diagnosed diabetes, cardiovascular disease, and cancer. Incident self-reported diabetes cases were identified at follow-up II interview (2006–2010). Odds ratio (OR) and 95% confidence interval (CI) were calculated using multivariable logistic regression models.

Results

After adjustment for risk biomarkers of diabetes including lipids, liver enzymes and inflammatory marker, the OR of type 2 diabetes with per one unit increment in log FGF-21 concentration was 1.16 (95% CI 0.90–1.50). Significant interaction was found with sex (P-interaction = 0.029): the OR (95% CI) was 1.50 (1.00-2.25) in women and 0.89 (0.52–1.53) in men.

Conclusions

Higher serum FGF-21 level was associated with an increased risk of diabetes in Chinese women but not in men. The sex difference in the association between FGF-21 and diabetes risk deserves further investigation and replication in other populations.

Electronic supplementary material

The online version of this article (10.1186/s12986-017-0216-0) contains supplementary material, which is available to authorized users.

Fibroblast Growth Factor 21 Mimetics for Treating Atherosclerosis

Fibroblast growth factor 21 (FGF21) is an atypical member of the FGF family. Acting in an endocrine fashion, it increases glucose uptake, modulates lipid metabolism, and sensitizes insulin response in metabolically active organs, including the liver and adipose tissue. Emerging evidence shows a strong correlation between circulating FGF21 levels and the incidence and severity of atherosclerosis. Animal studies have demonstrated a beneficial role of FGF21 in protecting against aberrant lipid profile, while recent development in FGF21 mimetics has provided further insight into the lipid-lowering effects of FGF21 signaling. The present review summarizes the physiological roles of FGF21, and discusses major breakthroughs and limitations of FGF21 mimetic-based therapeutic strategies for treating atherosclerosis.

High FGF21 levels are associated with altered bone homeostasis in HIV-1-infected patients

BACKGROUND

Fibroblast growth factor-21 (FGF21) has emerged as an important regulator of glucose, lipid, and body weight homeostasis. However, recent experimental studies have reported that increased FGF21 levels may lead to bone loss.

OBJECTIVE

To assess the relationship of serum FGF21 levels and altered bone homeostasis in HIV-1-infected patients.

DESIGN

Cross-sectional study of 137 HIV-1-infected patients and 35 healthy controls conducted at the Hospital de la Santa Creu i Sant Pau, Barcelona. Among HIV-1-infected patients, 35 were untreated (naïve), 43 were treated with antiretrovirals (HIV-1/ART) with no lipodystrophy, and 59 patients were HIV-1/ART and experienced lipodystrophy. Bone mineral density (BMD) and content (BMC) were assessed using dual-energy X-ray absorptiometry. Serum levels of FGF21, receptor activator of nuclear factor (NF)-KB ligand (RANKL), and C-telopeptide of type-I collagen (CTX-1) were measured by enzyme-linked immunosorbent assays. Serum levels of osteocalcin, osteoprotegerin, leptin, tumor necrosis factor-α, interleukin-6, interleukin-8, and monocyte chemoattractant protein-1 were determined using an antibody-linked, fluorescently labeled microsphere bead-based multiplex analysis system.

RESULTS

Alterations in bone parameters and bone homeostasis marker levels were consistent with higher turnover and bone loss in HIV-1 infected patients. FGF21 correlated negatively with BMD and BMC. FGF21 correlated positively with serum levels of osteoprotegerin and CTX-1, as well as with the CTX-1/osteocalcin ratio.

CONCLUSIONS

Elevated FGF21 levels are associated with poor bone homeostasis in HIV-1-infected patients. Increases in FGF21 serum level may be an indicator not only of metabolic derangement but it may also serve as a biomarker of altered bone homeostasis in HIV-1 infected patients.

FGF21 Is Not a Major Mediator for Bone Homeostasis or Metabolic Actions of PPARα and PPARγ Agonists

Results of prior studies suggest that fibroblast growth factor 21 (FGF21) may be involved in bone turnover and in the actions of peroxisome proliferator-activated receptor (PPAR) α and γ in mice. We have conducted independent studies to examine the effects of FGF21 on bone homeostasis and the role of FGF21 in PPARα and γ actions. High-fat-diet-induced obesity (DIO) mice were administered vehicle or recombinant human FGF21 (rhFGF21) intraperitoneally at 0 (vehicle), 0.1, 1, and 3 mg/kg daily for 2 weeks. Additional groups of DIO mice received water or 10 mg/kg rosiglitazone daily. Mice treated with rhFGF21 or rosiglitazone showed expected metabolic improvements in glucose, insulin, and lipid levels. However, bone loss was not detected in rhFGF21-treated mice by dual-energy X-ray absorptiometry (DXA), micro-CT, and histomorphometric analyses. Mineral apposition rate, a key bone formation parameter, was unchanged by rhFGF21, while significantly decreased by rosiglitazone in DIO mice. Bone resorption markers, OPG/RANKL mRNA expression, and histological bone resorption indices were unchanged by rhFGF21 or rosiglitazone. Bone marrow fat was unchanged by rhFGF21, while increased by rosiglitazone. Furthermore, FGF21 knockout mice did not show high bone mass phenotype. Treatment with PPARα or PPARγ agonists caused similar metabolic effects in FGF21 knockout and wild-type mice. These results contrast with previous findings and suggest that FGF21 is not critical for bone homeostasis or actions of PPARα and PPARγ. © 2016 American Society for Bone and Mineral Research.

Assessed and Emerging Biomarkers in Stroke and Training-Mediated Stroke Recovery: State of the Art

Since the increasing update of the biomolecular scientific literature, biomarkers in stroke have reached an outstanding and remarkable revision in the very recent years. Besides the diagnostic and prognostic role of some inflammatory markers, many further molecules and biological factors have been added to the list, including tissue derived cytokines, growth factor-like molecules, hormones, and microRNAs. The literatures on brain derived growth factor and other neuroimmune mediators, bone-skeletal muscle biomarkers, cellular and immunity biomarkers, and the role of microRNAs in stroke recovery were reviewed. To date, biomarkers represent a possible challenge in the diagnostic and prognostic evaluation of stroke onset, pathogenesis, and recovery. Many molecules are still under investigation and may become promising and encouraging biomarkers. Experimental and clinical research should increase this list and promote new discoveries in this field, to improve stroke diagnosis and treatment.

Associations between FGF21, osteonectin and bone turnover markers in type 2 diabetic patients with albuminuria

AIM

We measured the levels of bone turnover markers (BTMs) in patients with early diabetic nephropathy from type 2 diabetes mellitus (T2DM), and investigated the associations of BTMs with adipokines, serum fibroblast growth factor-21 (FGF21) and osteonectin.

METHODS

We included 159 males and 300 females with T2DM in this cross-sectional study. Clinical characteristics, BTMs and adipokines levels were measured.

RESULTS

One-hundred and ninety-two (41.8%) patients presented with albuminuria. Patients with albuminuria had significantly higher levels of serum osteonectin (P<0.0001) and FGF21 (P=0.0125) than those with normoalbuminuria. Serum levels of P1NP were slightly lower among patients with albuminuria (P=0.031), but the difference disappeared after adjusting for FBG, PBG, and HbA1c. Serum FGF21 levels were independently and negatively related to eGFR (overall β=-0.161, P=0.001; albuminuria group β=-0.240, P=0.001) but not related to uACR. While Osteonectin was independently and positively related to uACR (overall β=0.209, P=0.001; albuminuria group β=0.170, P=0.021). The levels of serum FGF21 were independently inversely related with P1NP (overall β=-0.192, P<0.0001; albuminuria group β=-0.195, P=0.031).

CONCLUSIONS

Our results suggest that persistent hyperglycemia may inhibit bone formation. Both osteonectin and FGF21 were associated with early nephropathy in T2DM patients, albeit with different patterns.

Fibroblast growth factor 21 night watch: advances and uncertainties in the field

Fibroblast growth factor (FGF) 21 belongs to a hormone-like subgroup within the FGF superfamily. The members of this subfamily, FGF19, FGF21 and FGF23, are characterized by their reduced binding affinity for heparin that enables them to be transported in the circulation and function in an endocrine manner. It is likely that FGF21 also acts in an autocrine and paracrine fashion, as multiple organs can produce this protein and its plasma concentration seems to be below the level necessary to induce a pharmacological effect. FGF21 signals via FGF receptors, but for efficient receptor engagement it requires a cofactor, membrane-spanning βKlotho (KLB). The regulation of glucose uptake in adipocytes was the initial biological activity ascribed to FGF21, but this hormone is now recognized to stimulate many other pathways in vitro and display multiple pharmacological effects in metabolically compromised animals and humans. Understanding of the precise physiology of FGF21 and its potential medicinal role has evolved exponentially over the last decade, yet numerous aspects remain to be defined and others are a source of debate. Here we provide a historical overview of the advances in FGF21 biology focusing on the uncertainties in the mechanism of action as well as the differing viewpoints relating to this intriguing protein.

Adiponectin alters renal calcium and phosphate excretion through regulation of klotho expression

The kidney controls systemic calcium and phosphate levels and disturbances of its control mechanisms can lead to a variety of diseases. The insulin-sensitizing adipokine adiponectin is renoprotective and accelerates functional recovery following renal injury. However, unlike other adipokines, adiponectin is reduced in obesity. High adiponectin levels are also correlated with bone loss, suggestive of an additional action in mineral metabolism. Using knockout, wild-type, and adiponectin-overexpressing transgenic mice, we sought to identify the mechanistic basis for adiponectin's ability to regulate calcium and phosphate balance at the level of the kidney. Adiponectin knockout mice exhibited lower serum calcium, lower urinary calcium excretion, and markedly lower serum fibroblast growth factor 23 (FGF23) levels, although circulating klotho concentrations were significantly higher than in wild-type littermates. The transgenic mice exhibited lower bone mass and strength, particularly compared to adiponectin knockout mice. The transgenic mice were hyper-responsive to a 2% phosphate-enriched diet, exhibiting 2-fold higher serum FGF23 and concomitantly higher fractional phosphate excretion. These mice also excreted more calcium with calcium-enriched diet and had less renal klotho protein expression. In contrast, the knockout mice exhibited a smaller increase in FGF23 and maintained elevated klotho levels on both mineral challenges. Kidney-specific adiponectin expression in doxycycline-inducible adiponectin mice and adiponectin addition in vitro confirmed adiponectin's ability to reduce tubular epithelial cell klotho secretion. Thus, adiponectin alters calcium and phosphate balance and renal mineral excretion, in part, through klotho. This work highlights the profound effects of adipose tissue on renal function and has identified a new mechanism by which adiponectin may regulate bone mass.

Targeting adipose tissue in the treatment of obesity-associated diabetes

Adipose tissue regulates numerous physiological processes, and its dysfunction in obese humans is associated with disrupted metabolic homeostasis, insulin resistance and type 2 diabetes mellitus (T2DM). Although several US-approved treatments for obesity and T2DM exist, these are limited by adverse effects and a lack of effective long-term glucose control. In this Review, we provide an overview of the role of adipose tissue in metabolic homeostasis and assess emerging novel therapeutic strategies targeting adipose tissue, including adipokine-based strategies, promotion of white adipose tissue beiging as well as reduction of inflammation and fibrosis.

Fibroblast Growth Factor 21 As an Emerging Therapeutic Target for Type 2 Diabetes Mellitus

Fibroblast growth factor (FGF) 21 is a distinctive member of the FGF family that functions as an endocrine factor. It is expressed predominantly in the liver, but is also found in adipose tissue and the pancreas. Pharmacological studies have shown that FGF21 normalizes glucose and lipid homeostasis, thereby preventing the development of metabolic disorders, such as obesity and diabetes. Despite growing evidence for the therapeutic potential of FGF21, paradoxical increases of FGF21 in different disease conditions point to the existence of FGF21 resistance. In this review, we give a critical appraisal of recent advances in the understanding of the regulation of FGF21 production under various physiological conditions, its antidiabetic actions, and the clinical implications. We also discuss recent preclinical and clinical trials using engineered FGF21 analogs in the management of diabetes, as well as the potential side effects of FGF21 therapy.

A Long-Acting FGF21 Molecule, PF-05231023, Decreases Body Weight and Improves Lipid Profile in Non-human Primates and Type 2 Diabetic Subjects

FGF21 plays a central role in energy, lipid, and glucose homeostasis. To characterize the pharmacologic effects of FGF21, we administered a long-acting FGF21 analog, PF-05231023, to obese cynomolgus monkeys. PF-05231023 caused a marked decrease in food intake that led to reduced body weight. To assess the effects of PF-05231023 in humans, we conducted a placebo-controlled, multiple ascending-dose study in overweight/obese subjects with type 2 diabetes. PF-05231023 treatment resulted in a significant decrease in body weight, improved plasma lipoprotein profile, and increased adiponectin levels. Importantly, there were no significant effects of PF-05231023 on glycemic control. PF-05231023 treatment led to dose-dependent changes in multiple markers of bone formation and resorption and elevated insulin-like growth factor 1. The favorable effects of PF-05231023 on body weight support further evaluation of this molecule for the treatment of obesity. Longer studies are needed to assess potential direct effects of FGF21 on bone in humans.

Mapping the response of human fibroblast growth factor 21 (FGF21) promoter to serum availability and lipoic acid in HepG2 hepatoma cells

The hormone-like polypeptide, fibroblast growth factor 21 (FGF21), is a major modulator of lipid and glucose metabolism and an exploratory treatment strategy for obesity related metabolic disorders. The costs of recombinant FGF21 and mode of delivery by injection are important constraints to its wide therapeutic use. The stimulation of endogenous FGF21 production through diet is being explored as an alternative approach. To that end, we examined the mechanism(s) by which serum manipulation and lipoic acid (a dietary activator of FGF21) induce FGF21 in human hepatocellular carcinoma HepG2 cells. Serum withdrawal markedly induced FGF21 mRNA levels (88 fold) and FGF21 secreted in the media (19 fold). Lipoic acid induced FGF21 mRNA 7 fold above DMSO-treated control cells and FGF21 secretion 3 fold. These effects were several-fold greater than those of PPARα agonist, Wy14643, which failed to induce FGF21 above and beyond the induction seen with serum withdrawal. The use of transcription inhibitor, actinomycin D, revealed that de novo mRNA synthesis drives FGF21 secretion in response to serum starvation. Four previously unrecognized loci in FGF21 promoter were nucleosome depleted and enriched in acetylated histone H3 revealing their role as transcriptional enhancers and putative transcription factor binding sites. FGF21 did not accumulate to a significant degree in induced HepG2 cells, which secreted FGF21 time dependently in media. We conclude that lipoic acid cell signaling connects with the transcriptional upregulation of FGF21 and it may prove to be a safe and affordable means to stimulate FGF21 production.

Understanding the Physiology of FGF21

Fibroblast growth factor 21 (FGF21) is a peptide hormone that is synthesized by several organs and regulates energy homeostasis. Excitement surrounding this relatively recently identified hormone is based on the documented metabolic beneficial effects of FGF21, which include weight loss and improved glycemia. The biology of FGF21 is intrinsically complicated owing to its diverse metabolic functions in multiple target organs and its ability to act as an autocrine, paracrine, and endocrine factor. In the liver, FGF21 plays an important role in the regulation of fatty acid oxidation both in the fasted state and in mice consuming a high-fat, low-carbohydrate ketogenic diet. FGF21 also regulates fatty acid metabolism in mice consuming a diet that promotes hepatic lipotoxicity. In white adipose tissue (WAT), FGF21 regulates aspects of glucose metabolism, and in susceptible WAT depots, it can cause browning. This peptide is highly expressed in the pancreas, where it appears to play an anti-inflammatory role in experimental pancreatitis. It also has an anti-inflammatory role in cardiac muscle. Although typically not expressed in skeletal muscle, FGF21 is induced in situations of muscle stress, particularly mitochondrial myopathies. FGF21 has been proposed as a novel therapeutic for metabolic complications such as diabetes and fatty liver disease. This review aims to interpret and delineate the ever-expanding complexity of FGF21 physiology.

Serum FGF-21 levels are associated with worsened radial trabecular bone microarchitecture and decreased radial bone strength in women with anorexia nervosa

BACKGROUND

Anorexia nervosa (AN) is a psychiatric disorder characterized by self-induced starvation and low body weight. Women with AN have impaired bone formation, low bone mass and an increased risk of fracture. FGF-21 is a hormone secreted by the liver in starvation and FGF-21 transgenic mice have significant bone loss due to an uncoupling of bone resorption and bone formation. We hypothesized that FGF-21 may contribute to the low bone mass state of AN.

SUBJECTS AND METHODS

We studied 46 women: 20 with AN (median age [interquartile range]: 27.5 [25, 30.75] years) and 26 normal-weight controls (NWC) of similar age (25 [24, 28.5] years). We investigated associations between serum FGF-21 and 1) aBMD measured by dual energy X-ray absorptiometry, 2) parameters of bone microarchitecture in the distal radius and tibia measured by high-resolution peripheral quantitative CT and 3) bone strength, estimated by microfinite element analysis.

RESULTS

FGF-21 levels were similar in AN and NWC (AN: 33.1 [18.1, 117.0] pg/ml vs. NWC: 57.4 [23.8, 107.1] pg/ml; p = 0.54). There was a significant inverse association between log FGF-21 and trabecular number in the radius in both AN (R = -0.57, p < 0.01) and NWC (R=-0.53, p < 0.01) and a significant positive association between log FGF-21 and trabecular separation in the radius in AN (R = 0.50, p < 0.03) and NWC (R = 0.52, p < 0.01). Estimates of radial bone strength were inversely associated with log FGF-21 in AN (R = -0.50, p < 0.03 for both stiffness and failure load). There were no associations between FGF-21 and aBMD, cortical parameters or tibial parameters in the AN or NWC groups.

CONCLUSIONS

FGF-21 may be an important determinant of trabecular skeletal homeostasis in AN.

High bone mass in adult mice with diet-induced obesity results from a combination of initial increase in bone mass followed by attenuation in bone formation; implications for high bone mass and decreased bone quality in obesity

Obesity is generally recognized as a condition which positively influences bone mass and bone mineral density (BMD). Positive effect of high body mass index (BMI) on bone has been recognized as a result of increased mechanical loading exerted on the skeleton. However, epidemiologic studies indicate that obesity is associated with increased incidence of fractures. The results presented here offer a new perspective regarding the mechanisms which may be responsible for the increase of bone mass and concurrent decrease in bone quality. Two groups of 12 week old C57BL/6 males were fed either high fat diet (HFD) or regular diet (RD) for 11 weeks. Metabolic profile, bone parameters and gene expression were assessed in these groups at the end of the experiment. Additionally, bone status was evaluated in a third group of 12 week old animals corresponding to animals at the start of the feeding period. Administration of HFD resulted in development of a diet-induced obesity (DIO), glucose intolerance, alteration in energy metabolism, and impairment in WAT function, as compared to the age-matched control animals fed RD. The expression of adiponectin, FABP4/aP2, DIO2 and FoxC2 were decreased in WAT of DIO animals, as well as transcript levels for IGFBP2, the cytokine regulating both energy metabolism and bone mass. At the end of experiment, DIO mice had higher bone mass than both control groups on RD, however they had decreased bone formation, as assessed by calcein labeling, and increased marrow adipocyte content. This study suggests that the bone mass acquired in obesity is a result of a two-phase process. First phase would consist of either beneficial effect of fat expansion to increase bone mass by increased mechanical loading and/or increased production of bone anabolic adipokines and/or nutritional effect of fatty acids. This is followed by a second phase characterized by decreased bone formation and bone turnover resulting from development of metabolic impairment.

The role of fibroblast growth factor 21 in diabetes and its complications: A review from clinical perspective

Fibroblast growth factor 21 (FGF21) has been well-recognized as a metabolic hormone and a promising target for treatment of metabolic diseases. The level of endogenous FGF21 is elevated in patients with impaired glucose tolerance and progressively increased from patients with overt type 2 diabetes to those with micro- and macro-vascular complications, presumably as a compensation or response to the deterioration of metabolic imbalance. A few exploratory in vivo studies, including a recent clinical trial, showed that exogenous FGF21 mimetics targeting FGF21 signaling can attain beneficial metabolic effects not with-standing the already elevated ambient FGF21 levels. In addition, some clinically available pharmacologic agents such as fenofibrates and metformin may modulate energy and macronutrients metabolism by acting through FGF21. This review mainly focuses on the role of FGF21 in development, progression and treatment of type 2 diabetes from a clinical perspective.

Fibroblast growth factor-21, body composition, and insulin resistance in pre-pubertal and early pubertal males and females

OBJECTIVE

Accumulating evidence derived primarily from animal models suggests that fibroblast growth factor-21 (FGF-21) may affect the musculoskeletal system via effects on the capacity of tissues to respond to insulin. A proportion of musculoskeletal properties and underpinnings of promoting/preventing insulin resistance are established early in the pubertal transition. Thus, the objective of this study was to test the hypothesis that insulin resistance and/or obesity will promote greater FGF-21 concentration which will be inversely associated with musculoskeletal parameters [lean mass and bone mineral content (BMC)] in pre-/early pubertal children. Given the sexual dimorphic nature of musculoskeletal development of fat mass accrual, differences by obesity status and sex were also investigated.

DESIGN

Cross-sectional.

PATIENTS

Children ages 7-12 years (n = 69, 38% male, 48% non-Hispanic black, 45% obese).

MEASUREMENTS

Fasting FGF-21, glucose and insulin measures were obtained. An estimate of insulin resistance was derived using the homoeostatic model assessment of insulin resistance (HOMA-IR). Body composition (BMC, lean mass and fat mass) was assessed by DXA. Multivariate regression analysis was used to evaluate the influence of FGF-21 on BMC, lean mass and HOMA-IR as dependent variables. Obesity status was established based on BMI z-score.

RESULTS

FGF-21 concentrations did not differ by obesity status or by sex. There was an inverse association between FGF-21 and BMC among nonobese individuals (P = 0·01) and an inverse association between FGF-21 and lean mass among females (P = 0·02), which were both independent of fat mass. FGF-21 was inversely associated with HOMA-IR in males, but not females (P = 0·04).

CONCLUSIONS

The existence of relationships of FGF-21 with musculoskeletal parameters and insulin resistance raises the possibility of crosstalk between these systems. These findings suggest that circulating FGF-21 may differ in its association with bone, lean mass and insulin resistance depending on sex and weight status.

Basal and postprandial change in serum fibroblast growth factor-21 concentration in type 1 diabetic mellitus and in healthy controls

Fibroblast growth factor-21 (FGF-21) appears to have an important role in glucose and lipid metabolism. FGF-21 secretion is mainly determined by nutritional status. The aim of this study was to measure basal and postprandial FGF-21 and postprandial change of FGF-21 concentration in type 1 diabetes mellitus (T1DM) patients and in healthy controls, and to investigate the differences between the groups. The cross-sectional study included 30 C-peptide negative T1DM patients, median age 37 years (20-59), disease duration 22 years (3-45), and nine healthy controls, median age 30 years (27-47). Basal and postprandial FGF-21 concentrations were measured by ELISA. The associations of FGF-21 with glucose, lipids, and insulin were analyzed. Individuals with T1DM showed significantly lower basal FGF-21 concentration (P=0.046) when compared with healthy controls (median value 28.2 vs 104 pg/mL) and had significantly different postprandial change (∆ 30'-0') of FGF-21 (P=0.006) in comparison with healthy controls (median value -1.1 vs -20.5 pg/mL). The glucose and lipid status did not correlate with FGF-21. In healthy controls, postprandial insulin level correlated with basal FGF-21 (ρ=0.7, P=0.036). Multiple regression analysis showed that they are independently associated after adjustment for confounding factors (β=1.824, P=0.04). We describe the pathological pattern of basal and postprandial change of FGF-21 secretion not associated with glucose, lipid levels, or insulin therapy in patients with T1DM. Since FGF-21 has numerous protective metabolic effects in the experimental model, the lower basal FGF-21 concentration in T1DM patients opens the question about the potential role of recombinant FGF-21 therapy.

Current and Emerging Treatment Options in Diabetes Care

Diabetes constitutes an increasing threat to human health, particularly in newly industrialized and densely populated countries. Type 1 and type 2 diabetes arise from different etiologies but lead to similar metabolic derangements constituted by an absolute or relative lack of insulin that results in elevated plasma glucose. In the last three decades, a set of new medicines built upon a deeper understanding of physiology and diabetic pathology have emerged to enhance the clinical management of the disease and related disorders. Recent insights into insulin-dependent and insulin-independent molecular events have accelerated the generation of a series of novel medicinal agents, which hold the promise for further advances in the management of diabetes. In this chapter, we provide a historical context for what has been accomplished to provide perspective for future research and novel emerging treatment options.

Fibroblast growth factor-21 concentration in serum and synovial fluid is associated with radiographic bone loss of knee osteoarthritis

We aimed to evaluate whether FGF-21 concentration in serum and synovial fluid (SF) is associated with radiographic bone loss of knee osteoarthritis (OA). A total of 186 OA patients and 108 controls were recruited. The radiographic bone loss of knee OA was assessed by the Ahlbäck grading scale. FGF-21 concentration in serum and SF was measured by enzyme-linked immunosorbent assay (ELISA). We demonstrated that OA patients had significantly higher serum FGF-21 concentration compared with controls (204.30 [range 158.25-279.16] ng/L vs. 130.72 [range 94.93-218.03] ng/L, p < 0.01). FGF-21 concentration in serum was well correlated with that in paired SF samples (r = 0.668, p < 0.001). In OA patients, those with a higher Ahlbäck grade had significantly higher serum and SF FGF-21 concentration (p < 0.001 for both). FGF-21 concentration in serum and SF was significantly and independently associated with the Ahlbäck grade (r = 0.403, p < 0.001 and r = 0.410, p < 0.001; respectively). These findings indicated that FGF-21 might be a potential biomarker for predicting bone loss of OA. Therapeutic interventions by blocking FGF-21 signaling pathways to delay the degenerative process of OA warrants further investigations.

Irisin levels are lower in young amenorrheic athletes compared with eumenorrheic athletes and non-athletes and are associated with bone density and strength estimates

Irisin and FGF21 are novel hormones implicated in the “browning” of white fat, thermogenesis, and energy homeostasis. However, there are no data regarding these hormones in amenorrheic athletes (AA) (a chronic energy deficit state) compared with eumenorrheic athletes (EA) and non-athletes. We hypothesized that irisin and FGF21 would be low in AA, an adaptive response to low energy stores. Furthermore, because (i) brown fat has positive effects on bone, and (ii) irisin and FGF21 may directly impact bone, we hypothesized that bone density, structure and strength would be positively associated with these hormones in athletes and non-athletes. To test our hypotheses, we studied 85 females, 14–21 years [38 AA, 24 EA and 23 non-athletes (NA)]. Fasting serum irisin and FGF21 were measured. Body composition and bone density were assessed using dual energy X-ray absorptiometry, bone microarchitecture using high resolution peripheral quantitative CT, strength estimates using finite element analysis, resting energy expenditure (REE) using indirect calorimetry and time spent exercising/week by history. Subjects did not differ for pubertal stage. Fat mass was lowest in AA. AA had lower irisin and FGF21 than EA and NA, even after controlling for fat and lean mass. Across subjects, irisin was positively associated with REE and bone density Z-scores, volumetric bone mineral density (total and trabecular), stiffness and failure load. FGF21 was negatively associated with hours/week of exercise and cortical porosity, and positively with fat mass and cortical volumetric bone density. Associations of irisin (but not FGF21) with bone parameters persisted after controlling for potential confounders. In conclusion, irisin and FGF21 are low in AA, and irisin (but not FGF21) is independently associated with bone density and strength in athletes.

FGF21-based pharmacotherapy--potential utility for metabolic disorders

Currently available therapies for diabetes or obesity produce modest efficacy and are usually used in combination with agents targeting cardiovascular risk factors. Fibroblast growth factor 21 (FGF21) is a circulating protein with pleiotropic metabolic actions; pharmacological doses of FGF21 produce anti-diabetic, lipid-lowering, and weight-reducing effects in rodents. Several potential benefits have translated to non-human primates and obese humans with type 2 diabetes (T2D). Accumulating results point to a specific receptor complex and actions in adipose tissue, liver, and brain; several pathways lead to enhanced fatty acid oxidation, increased insulin sensitivity, and augmented energy expenditure. A range of strategies are being explored to derive potent, safe, and convenient therapies which could potentially represent novel approaches to prevent and treat a variety of metabolic disorders.

Inventing new medicines: The FGF21 story

Since the discovery of insulin in 1921, protein therapeutics have become vital tools in the treatment of diabetes mellitus. This heritage has been extended with the comparatively recent introduction of recombinant and re-engineered insulins, in addition to the advent of GLP1 agonists. FGF21 represents an example of a novel experimental protein therapy which is able to induce favorable metabolic effects in various species ranging from rodents to man.

The aim of this review is to communicate the story of the FGF21 drug discovery path from identification in a functional in vitro screen, to the eventual evaluation of its utility in patients. Given that the development of FGF21 advanced hand-in-hand with rapidly evolving scientific research around this target, we have also attempted to describe our view of recent developments regarding the mechanistic understanding of FGF21 biology.