Erratum: Author correction to 'FGF4 protects the liver from immune-mediated injury by activating CaMKKß-PINK1 signal pathway to inhibit hepatocellular apoptosis' [Acta Pharm Sin B 14 (2024) 1605-1623].

[This corrects the article DOI: 10.1016/j.apsb.2023.12.012.].

FGF4 protects the liver from immune-mediated injury by activating CaMKKß-PINK1 signal pathway to inhibit hepatocellular apoptosis.

Immune-mediated liver injury (ILI) is a condition where an aberrant immune response due to various triggers causes the destruction of hepatocytes. Fibroblast growth factor 4 (FGF4) was recently identified as a hepatoprotective cytokine; however, its role in ILI remains unclear. In patients with autoimmune hepatitis (type of ILI) and mouse models of concanavalin A (ConA)- or S-100-induced ILI, we observed a biphasic pattern in hepatic FGF4 expression, characterized by an initial increase followed by a return to basal levels. Hepatic FGF4 deficiency activated the mitochondria-associated intrinsic apoptotic pathway, aggravating hepatocellular apoptosis. This led to intrahepatic immune hyper-reactivity, inflammation accentuation, and subsequent liver injury in both ILI models. Conversely, administration of recombinant FGF4 reduced hepatocellular apoptosis and rectified immune imbalance, thereby mitigating liver damage. The beneficial effects of FGF4 were mediated by hepatocellular FGF receptor 4, which activated the Ca2+/calmodulin-dependent protein kinasekinase 2 (CaMKKß) and its downstream phosphatase and tensin homologue-induced putative kinase 1 (PINK1)-dependent B-cell lymphoma 2-like protein 1-isoform L (Bcl-XL) signalling axis in the mitochondria. Hence, FGF4 serves as an early response factor and plays a protective role against ILI, suggesting a therapeutic potential of FGF4 and its analogue for treating clinical immune disorder-related liver injuries.

Early elevations of RAS protein level and activity are critical for the development of PDAC in the context of inflammation.

The KRASG12D mutation was believed to be locked in a GTP-bound form, rendering it fully active. However, recent studies have indicated that the presence of mutant KRAS alone is insufficient; it requires additional activation through inflammatory stimuli to effectively drive the development of pancreatic ductal adenocarcinoma (PDAC). It remains unclear to what extent RAS activation occurs during the development of PDAC in the context of inflammation. Here, in a mouse model with the concurrent expression of KrasG12D/+ and inflammation mediator IKK2 in pancreatic acinar cells, we showed that, compared to KRASG12D alone, the cooperative interaction between KRASG12D and IKK2 rapidly elevated both the protein level and activity of KRASG12D and NRAS in a short term. This high level was sustained throughout the rest phase of PDAC development. These results suggest that inflammation not only rapidly augments the activity but also the protein abundance, leading to an enhanced total amount of GTP-bound RAS (KRASG12D and NRAS) in the early stage. Notably, while KRASG12D could be further activated by IKK2, not all KRASG12D proteins were in the GTP-bound state. Overall, our findings suggest that although KRASG12D is not fully active in the context of inflammation, concurrent increases in both the protein level and activity of KRASG12D as well as NRAS at the early stage by inflammation contribute to the rise in total GTP-bound RAS.

Targeting the FGF19-FGFR4 pathway for cholestatic, metabolic, and cancerous diseases.

Human fibroblast growth factor 19 (FGF19, or FGF15 in rodents) plays a central role in controlling bile acid (BA) synthesis through a negative feedback mechanism. This process involves a postprandial crosstalk between the BA-activated ileal farnesoid X receptor and the hepatic Klotho beta (KLB) coreceptor complexed with fibrobalst growth factor receptor 4 (FGFR4) kinase. Additionally, FGF19 regulates glucose, lipid, and energy metabolism by coordinating responses from functional KLB and FGFR1-3 receptor complexes on the periphery. Pharmacologically, native FGF19 or its analogs decrease elevated BA levels, fat content, and collateral tissue damage. This makes them effective in treating both cholestatic diseases such as primary biliary or sclerosing cholangitis (PBC or PSC) and metabolic abnormalities such as nonalcoholic steatohepatitis (NASH). However, chronic administration of FGF19 drives oncogenesis in mice by activating the FGFR4-dependent mitogenic or hepatic regenerative pathway, which could be a concern in humans. Agents that block FGF19 or FGFR4 signaling have shown great potency in preventing FGF19-responsive hepatocellular carcinoma (HCC) development in animal models. Recent phase 1/2 clinical trials have demonstrated promising results for several FGF19-based agents in selectively treating patients with PBC, PSC, NASH, or HCC. This review aims to provide an update on the clinical development of both analogs and antagonists targeting the FGF19-FGFR4 signaling pathway for patients with cholestatic, metabolic, and cancer diseases. We will also analyze potential safety and mechanistic concerns that should guide future research and advanced trials.

Engineered Apoptosis-Bioinspired Nanoparticles Initiate Immune Cascade for Cancer Immunotherapy of Malignant Ascites.

Malignant ascites (MA) is a common symptom of peritoneal metastasis in liver cancer. Cancer immunotherapy can modulate immune cells to induce antitumor immune efficiency. Reprogramming tumor immune microenvironment (TIME) is a momentous strategy to overcome immunosuppression and achieve immune functional normalization. Inspired by the inherent apoptotic bodies and vesicles, we proposed and systematically studied engineered apoptosis-bioinspired nanoparticles (EBN) for cancer immunotherapy of MA. Using both in vitro and in vivo experimental validations, we elucidated that EBN could be efficiently engulfed by the tumor-associated macrophages (TAMs) and manipulate their polarization. Moreover, a boosted immune cascade response as a result of heightening cytotoxic T-lymphocytes (CTLs) activity was investigated. Based on these results, EBN was confirmed to have strong immune cascade activation capability. Remarkably, the injection of EBN further reduced ascites volume and reformed immune cell subtypes, compared to the injection of either PBS or free TMP195 alone. In short, this novel nanodrug delivery system (NDDS) represents a prospective immunotherapeutic approach for clinical therapeutics of hepatoma ascites and other malignant effusion.

FGF4 Promotes Skin Wound Repair through p38 MAPK and GSK3ß-Mediated Stabilization of Slug.

Cutaneous wound healing is an orderly and intricate process that restores the barrier function and integrity of injured skin. Re-epithelialization, which involves the proliferation and migration of keratinocytes to cover the denuded surface, is essential for successful wound closure. There are many members of the FGF family, of which the paracrine-acting FGF1 and FGF7 subfamily members have been identified as positive regulators of wound repair. However, the role and underlying mechanisms of some other paracrine FGFs in wound repair still remain obscure. In this report, we found that paracrine FGF4 localized predominantly to the epidermal keratinocytes and was markedly upregulated at the wound edges in response to re-epithelialization in human and mouse wound models. Blockade of FGF4 resulted in delayed re-epithelialization of human ex vivo skin wounds, whereas recombinant FGF4 treatment promoted re-epithelialization and wound repair. Mechanistically, recombinant FGF4 promotes p38 MAPK‒GSK3ß‒mediated stabilization of Slug by reducing its ubiquitination, which triggers epithelial-to-mesenchymal transition and promotes the migration and proliferation of keratinocytes and thus wound re-epithelialization. Our findings uncover FGF4 as an important regulator of wound healing, highlighting a promising therapeutic avenue for skin injury.

A non-mitogenic FGF4 analog alleviates non-alcoholic steatohepatitis through an AMPK-dependent pathway.

BACKGROUND & AIMS: Non-alcoholic fatty liver disease (NAFLD) has emerged as a major liver disease increasingly in association with non-alcoholic steatohepatitis (NASH), cirrhosis and hepatocellular carcinoma (HCC). However, there are currently no approved therapies for treating NAFLD and NASH. Fibroblast growth factor 4 (FGF4) has recently been shown as a promising drug candidate for several metabolic diseases. METHODS: Mice fed a high-fat diet with high fructose/glucose drinking water (HF/HFG, Western-like diet) for 21 weeks were intraperitoneally injected with non-mitogenic recombinant FGF4△NT (rFGF4△NT, 1.0 mg/kg body weight) every other day for 8 weeks. Primary mouse hepatocytes cultured in medium containing high glucose/palmitic acid (HG/PA) or TNFα/cyclohexane (TNFα/CHX) were treated with 1.0 µg/ml rFGF4△NT. Changes in parameters for histopathology, lipid metabolism, inflammation, hepatocellular apoptosis and fibrosis were determined. The Caspase6 activity and AMPK pathway were assessed. RESULTS: Administration of rFGF4△NT significantly attenuated the Western-like diet-induced hepatic steatosis, inflammation, liver injury and fibrosis in mice. rFGF4△NT treatment reduced fatty acid-induced lipid accumulation and lipotoxicity-induced hepatocyte apoptosis, which were associated with inhibition of Caspase6 cleavage and activation. Inhibition of AMP-activated protein kinase (AMPK) by Compound C or deficiency of Ampk abrogated rFGF4△NT-induced hepatoprotection in primary hepatocytes and in mice with NASH. CONCLUSION: rFGF4△NT exerts significant protective effects on NASH via an AMPK-dependent signaling pathway. Our study indicates that FGF4 analogs may have therapeutic potential for the Western-like diet induced NASH.

Differential Effects of Dietary Macronutrients on the Development of Oncogenic KRAS-Mediated Pancreatic Ductal Adenocarcinoma.

KRAS mutations are prevalent in patients with pancreatic ductal adenocarcinoma (PDAC) and are critical to fostering tumor growth in part by aberrantly rewiring glucose, amino acid, and lipid metabolism. Obesity is a modifiable risk factor for pancreatic cancer. Corroborating this epidemiological observation, mice harboring mutant KRAS are highly vulnerable to obesogenic high-fat diet (HFD) challenges leading to the development of PDAC with high penetrance. However, the contributions of other macronutrient diets, such as diets rich in carbohydrates that are regarded as a more direct source to fuel glycolysis for cancer cell survival and proliferation than HFD, to pancreatic tumorigenesis remain unclear. In this study, we compared the differential effects of a high-carbohydrate diet (HCD), an HFD, and a high-protein diet (HPD) in PDAC development using a mouse model expressing an endogenous level of mutant KRASG12D specifically in pancreatic acinar cells. Our study showed that although with a lower tumorigenic capacity than chronic HFD, chronic HCD promoted acinar-to-ductal metaplasia (ADM) and pancreatic intraepithelial neoplasia (PanIN) lesions with increased inflammation, fibrosis, and cell proliferation compared to the normal diet (ND) in KrasG12D/+ mice. By contrast, chronic HPD showed no significant adverse effects compared to the ND. Furthermore, ablation of pancreatic acinar cell cyclooxygenase 2 (Cox-2) in KrasG12D/+ mice abrogated the adverse effects induced by HCD, suggesting that diet-induced pancreatic inflammation is critical for promoting oncogenic KRAS-mediated neoplasia. These results indicate that diets rich in different macronutrients have differential effects on pancreatic tumorigenesis in which the ensuing inflammation exacerbates the process. Management of macronutrient intake aimed at thwarting inflammation is thus an important preventive strategy for patients harboring oncogenic KRAS.

Roles of the fibroblast growth factor signal transduction system in tissue injury repair.

Following injury, tissue autonomously initiates a complex repair process, resulting in either partial recovery or regeneration of tissue architecture and function in most organisms. Both the repair and regeneration processes are highly coordinated by a hierarchy of interplay among signal transduction pathways initiated by different growth factors, cytokines and other signaling molecules under normal conditions. However, under chronic traumatic or pathological conditions, the reparative or regenerative process of most tissues in different organs can lose control to different extents, leading to random, incomplete or even flawed cell and tissue reconstitution and thus often partial restoration of the original structure and function, accompanied by the development of fibrosis, scarring or even pathogenesis that could cause organ failure and death of the organism. Ample evidence suggests that the various combinatorial fibroblast growth factor (FGF) and receptor signal transduction systems play prominent roles in injury repair and the remodeling of adult tissues in addition to embryonic development and regulation of metabolic homeostasis. In this review, we attempt to provide a brief update on our current understanding of the roles, the underlying mechanisms and clinical application of FGFs in tissue injury repair.

FGF4 protects the liver from nonalcoholic fatty liver disease by activating the AMP-activated protein kinase-Caspase 6 signal axis.

BACKGROUND AND AIMS: NAFLD represents an increasing health problem in association with obesity and diabetes with no effective pharmacotherapies. Growing evidence suggests that several FGFs play important roles in diverse aspects of liver pathophysiology. Here, we report a previously unappreciated role of FGF4 in the liver. APPROACH AND RESULTS: Expression of hepatic FGF4 is inversely associated with NAFLD pathological grades in both human patients and mouse models. Loss of hepatic Fgf4 aggravates hepatic steatosis and liver damage resulted from an obesogenic high-fat diet. By contrast, pharmacological administration of recombinant FGF4 mitigates hepatic steatosis, inflammation, liver damage, and fibrogenic markers in mouse livers induced to develop NAFLD and NASH under dietary challenges. Such beneficial effects of FGF4 are mediated predominantly by activating hepatic FGF receptor (FGFR) 4, which activates a downstream Ca2+ -Ca2+ /calmodulin-dependent protein kinase kinase beta-dependent AMP-activated protein kinase (AMPK)-Caspase 6 signal axis, leading to enhanced fatty acid oxidation, reduced hepatocellular apoptosis, and mitigation of liver damage. CONCLUSIONS: Our study identifies FGF4 as a stress-responsive regulator of liver pathophysiology that acts through an FGFR4-AMPK-Caspase 6 signal pathway, shedding light on strategies for treating NAFLD and associated liver pathologies.

Sulfarotene, a synthetic retinoid, overcomes stemness and sorafenib resistance of hepatocellular carcinoma via suppressing SOS2-RAS pathway.

BACKGROUND: Recurrent hepatocellular carcinoma (HCC) shows strong resistance to sorafenib, and the tumor-repopulating cells (TRCs) with cancer stem cell-like properties are considered a driver for its high recurrent rate and drug resistance. METHODS: Suppression of TRCs may thus be an effective therapeutic strategy for treating this fatal disease. We evaluated the pharmacology and mechanism of sulfarotene, a new type of synthetic retinoid, on the cancer stem cell-like properties of HCC TRCs, and assessed its preclinical efficacy in models of HCC patient-derived xenografts (PDXs). RESULTS: Sulfarotene selectively inhibited the growth of HCC TRCs in vitro and significantly deterred TRC-mediated tumor formation and lung metastasis in vivo without apparent toxicity, with an IC50 superior to that of acyclic retinoid and sorafenib, to which the recurrent HCC exhibits significant resistance at advanced stage. Sulfarotene promoted the expression and activation of RARα, which down-regulated SOS2, a key signal mediator associated with RAS activation and signal transduction involved in multiple downstream pathways. Moreover, sulfarotene selectively inhibited tumorigenesis of HCC PDXs with high expression for SOS2. CONCLUSIONS: Our study identified sulfarotene as a selective inhibitor for the TRCs of HCC, which targets a novel RARα-SOS2-RAS signal nexus, shedding light on a new, promising strategy of target therapy for advanced liver cancer.

Pancreatic Tumorigenesis: Oncogenic KRAS and the Vulnerability of the Pancreas to Obesity.

Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal malignancies and KRAS (Kirsten rat sarcoma 2 viral oncogene homolog) mutations have been considered a critical driver of PDAC initiation and progression. However, the effects of mutant KRAS alone do not recapitulate the full spectrum of pancreatic pathologies associated with PDAC development in adults. Historically, mutant KRAS was regarded as constitutively active; however, recent studies have shown that endogenous levels of mutant KRAS are not constitutively fully active and its activity is still subject to up-regulation by upstream stimuli. Obesity is a metabolic disease that induces a chronic, low-grade inflammation called meta-inflammation and has long been recognized clinically as a major modifiable risk factor for pancreatic cancer. It has been shown in different animal models that obesogenic high-fat diet (HFD) and pancreatic inflammation promote the rapid development of mutant KRAS-mediated PDAC with high penetrance. However, it is not clear why the pancreas with endogenous levels of mutant KRAS is vulnerable to chronic HFD and inflammatory challenges. Recently, the discovery of fibroblast growth factor 21 (FGF21) as a novel anti-obesity and anti-inflammatory factor and as a downstream target of mutant KRAS has shed new light on this problem. This review is intended to provide an update on our knowledge of the vulnerability of the pancreas to KRAS-mediated invasive PDAC in the context of challenges engendered by obesity and associated inflammation.

Selective killing of cancer cells harboring mutant RAS by concomitant inhibition of NADPH oxidase and glutathione biosynthesis.

Oncogenic RAS is a critical driver for the initiation and progression of several types of cancers. However, effective therapeutic strategies by targeting RAS, in particular RASG12D and RASG12V, and associated downstream pathways have been so far unsuccessful. Treatment of oncogenic RAS-ravaged cancer patients remains a currently unmet clinical need. Consistent with a major role in cancer metabolism, oncogenic RAS activation elevates both reactive oxygen species (ROS)-generating NADPH oxidase (NOX) activity and ROS-scavenging glutathione biosynthesis. At a certain threshold, the heightened oxidative stress and antioxidant capability achieve a higher level of redox balance, on which cancer cells depend to gain a selective advantage on survival and proliferation. However, this prominent metabolic feature may irrevocably render cancer cells vulnerable to concurrent inhibition of both NOX activity and glutathione biosynthesis, which may be exploited as a novel therapeutic strategy. In this report, we test this hypothesis by treating the HRASG12V-transformed ovarian epithelial cells, mutant KRAS-harboring pancreatic and colon cancer cells of mouse and human origins, as well as cancer xenografts, with diphenyleneiodonium (DPI) and buthionine sulfoximine (BSO) combination, which inhibit NOX activity and glutathione biosynthesis, respectively. Our results demonstrate that concomitant targeting of NOX and glutathione biosynthesis induces a highly potent lethality to cancer cells harboring oncogenic RAS. Therefore, our studies provide a novel strategy against RAS-bearing cancers that warrants further mechanistic and translational investigation.

FGF21 in obesity and cancer: New insights.

The endocrine FGF21 was discovered as a novel metabolic regulator in 2005 with new functions bifurcating from the canonic heparin-binding FGFs that directly promote cell proliferation and growth independent of a co-receptor. Early studies have demonstrated that FGF21 is a stress sensor in the liver and possibly, several other endocrine and metabolic tissues. Hepatic FGF21 signals via endocrine routes to quench episodes of metabolic derangements, promoting metabolic homeostasis. The convergence of mouse and human studies shows that FGF21 promotes lipid catabolism, including lipolysis, fatty acid oxidation, mitochondrial oxidative activity, and thermogenic energy dissipation, rather than directly regulating insulin and appetite. The white and brown adipose tissues and, to some extent, the hypothalamus, all of which host a transmembrane receptor binary complex of FGFR1 and co-receptor KLB, are considered the essential tissue and molecular targets of hepatic or pharmacological FGF21. On the other hand, a growing body of work has revealed that pancreatic acinar cells form a constitutive high-production site for FGF21, which then acts in an autocrine or paracrine mode. Beyond regulation of macronutrient metabolism and physiological energy expenditure, FGF21 appears to function in forestalling the development of fatty pancreas, steato-pancreatitis, fatty liver, and steato-hepatitis, thereby preventing the development of advanced pathologies such as pancreatic ductal adenocarcinoma or hepatocellular carcinoma. This review is intended to provide updates on these new discoveries that illuminate the protective roles of FGF21-FGFR1-KLB signal pathway in metabolic anomalies-associated severe tissue damage and malignancy, and to inform potential new preventive or therapeutic strategies for obesity-inflicted cancer patients via reducing metabolic risks and inflammation.

The Significance of Mitochondrial Dysfunction in Cancer.

As an essential organelle in nucleated eukaryotic cells, mitochondria play a central role in energy metabolism, maintenance of redox balance, and regulation of apoptosis. Mitochondrial dysfunction, either due to the TCA cycle enzyme defects, mitochondrial DNA genetic mutations, defective mitochondrial electron transport chain, oxidative stress, or aberrant oncogene and tumor suppressor signaling, has been observed in a wide spectrum of human cancers. In this review, we summarize mitochondrial dysfunction induced by these alterations that promote human cancers.

Oncogenic KRAS Reduces Expression of FGF21 in Acinar Cells to Promote Pancreatic Tumorigenesis in Mice on a High-Fat Diet.

BACKGROUND & AIMS: Obesity is a risk factor for pancreatic cancer. In mice, a high-fat diet (HFD) and expression of oncogenic KRAS lead to development of invasive pancreatic ductal adenocarcinoma (PDAC) by unknown mechanisms. We investigated how oncogenic KRAS regulates the expression of fibroblast growth factor 21, FGF21, a metabolic regulator that prevents obesity, and the effects of recombinant human FGF21 (rhFGF21) on pancreatic tumorigenesis. METHODS: We performed immunohistochemical analyses of FGF21 levels in human pancreatic tissue arrays, comprising 59 PDAC specimens and 45 nontumor tissues. We also studied mice with tamoxifen-inducible expression of oncogenic KRAS in acinar cells (KrasG12D/+ mice) and fElasCreERT mice (controls). KrasG12D/+ mice were placed on an HFD or regular chow diet (control) and given injections of rhFGF21 or vehicle; pancreata were collected and analyzed by histology, immunoblots, quantitative polymerase chain reaction, and immunohistochemistry. We measured markers of inflammation in the pancreas, liver, and adipose tissue. Activity of RAS was measured based on the amount of bound guanosine triphosphate. RESULTS: Pancreatic tissues of mice expressed high levels of FGF21 compared with liver tissues. FGF21 and its receptor proteins were expressed by acinar cells. Acinar cells that expressed KrasG12D/+ had significantly lower expression of Fgf21 messenger RNA compared with acinar cells from control mice, partly due to down-regulation of PPARG expression-a transcription factor that activates Fgf21 transcription. Pancreata from KrasG12D/+ mice on a control diet and given injections of rhFGF21 had reduced pancreatic inflammation, infiltration by immune cells, and acinar-to-ductal metaplasia compared with mice given injections of vehicle. HFD-fed KrasG12D/+ mice given injections of vehicle accumulated abdominal fat, developed extensive inflammation, pancreatic cysts, and high-grade pancreatic intraepithelial neoplasias (PanINs); half the mice developed PDAC with liver metastases. HFD-fed KrasG12D/+ mice given injections of rhFGF21 had reduced accumulation of abdominal fat and pancreatic triglycerides, fewer pancreatic cysts, reduced systemic and pancreatic markers of inflammation, fewer PanINs, and longer survival-only approximately 12% of the mice developed PDACs, and none of the mice had metastases. Pancreata from HFD-fed KrasG12D/+ mice given injections of rhFGF21 had lower levels of active RAS than from mice given vehicle. CONCLUSIONS: Normal acinar cells from mice and humans express high levels of FGF21. In mice, acinar expression of oncogenic KRAS significantly reduces FGF21 expression. When these mice are placed on an HFD, they develop extensive inflammation, pancreatic cysts, PanINs, and PDACs, which are reduced by injection of FGF21. FGF21 also reduces the guanosine triphosphate binding capacity of RAS. FGF21 might be used in the prevention or treatment of pancreatic cancer.

Obesogenic high-fat diet heightens aerobic glycolysis through hyperactivation of oncogenic KRAS.

Oncogenic KRAS plays a vital role in controlling tumor metabolism by enhancing aerobic glycolysis. Obesity driven by chronic consumption of high-fat diet (HFD) is a major risk factor for oncogenic KRAS-mediated pancreatic ductal adenocarcinoma (PDAC). However, the role of HFD in KRAS-mediated metabolic reprogramming has been obscure. Here, by using genetically engineered mouse models expressing an endogenous level of KRASG12D in pancreatic acinar cells, we demonstrate that hyperactivation of KRASG12D by obesogenic HFD, as compared to carbohydrate-rich diet, is responsible for enhanced aerobic glycolysis that associates with critical pathogenic responses in the path towards PDAC. Ablation of Cox-2 attenuates KRAS hyperactivation leading to the reversal of both aggravated aerobic glycolysis and high-grade dysplasia under HFD challenge. Our data highlight a pivotal role of the cooperative interaction between obesity-ensuing HFD and oncogenic KRAS in driving the heightened aerobic glycolysis during pancreatic tumorigenesis and suggest that in addition to directly targeting KRAS and aerobic glycolysis pathway, strategies to target the upstream of KRAS hyperactivation may bear important therapeutic value.

Emerging Structure-Function Paradigm of Endocrine FGFs in Metabolic Diseases.

Endocrine fibroblast growth factors (eFGFs) control pathways that are crucial for maintaining metabolic homeostasis of lipids, glucose, energy, bile acids, and minerals. Unlike the heparin-binding paracrine FGFs, eFGFs require a unique Klotho family protein to form a productive triad complex, but the structural and mechanistical details of this complex have remained obscure since the beginning of the eFGF field. However, recent breakthroughs in resolving the 3D structures of eFGF signaling complexes have now unveiled the atomic details of multivalent interactions among eFGF, FGFR, and Klotho. We provide here a timely review on the architecture and the structure-function relationships of these complexes, and highlight how the structural knowledge opens a new door to structure-based drug design against a repertoire of eFGF-associated metabolic diseases.