Klotho Family Research Articles

Klotho, FOXO1 and cytokines associations in patients with coronary artery disease

IntroductionAtherosclerosis is one of the main reasons for adult mortality in advanced populations and countries with high stress levels. Klotho family are single-pass trans-membrane proteins that involve in the genesis and progression of various diseases, including acardiovascular disease, apoptosis and stress oxidative imbalance.Present study, investigates the pattern of changes in Klotho and FOXO1 gene expressions and levels in atherosclerosis. MethodsPresent case control study consisted of 79 patients with atherosclerosis and 78 healthy controls. PBMC (peripheral mono-nuclear blood cells) expression levels of Klotho and FOXO1 were assayed, using qPCR method. Serum concentration of Klotho and FOXO1 were measured by ELISA method. ResultsA significant reduction was found in PBMC genes expression levels of Klotho (P < 0.01) of patients as comparison with controls. PBMC Gene expression of FOXO1 in patients was increased significantly (P < 0.01) when compared with controls. Pearson analysis showed a positive correlation between PBMC Klotho gene expression and Klotho levels of patients (P < 0.01). The correlation between serum concentrations of Klotho and FOXO1 of patients was also positive significantly (P < 0.01). AUC of ROC for gene expression and serum concentration of Klotho in patients were 0.701 and 0.737 respectively. ConclusionInvestigating the PBMC gene expression and serum concentration of Klotho in patients with atherosclerosis is suggested could be a convenient novel biomarker for predicting, prognosis, monitoring the disease progression and designing a suitable drug for patients with atherosclerosis.

Soluble \u03b1Klotho downregulates Orai1-mediated store-operated Ca2+ entry via PI3K-dependent signaling

αKlotho is a type 1 transmembrane anti-aging protein. αKlotho-deficient mice have premature aging phenotypes and an imbalance of ion homeostasis including Ca2+ and phosphate. Soluble αKlotho is known to regulate multiple ion channels and growth factor-mediated phosphoinositide-3-kinase (PI3K) signaling. Store-operated Ca2+ entry (SOCE) mediated by pore-forming subunit Orai1 and ER Ca2+ sensor STIM1 is a ubiquitous Ca2+ influx mechanism and has been implicated in multiple diseases. However, it is currently unknown whether soluble αKlotho regulates Orai1-mediated SOCE via PI3K-dependent signaling. Among the Klotho family, αKlotho downregulates SOCE while βKlotho or γKlotho does not affect SOCE. Soluble αKlotho suppresses serum-stimulated SOCE and Ca2+ release-activated Ca2+ (CRAC) channel currents. Serum increases the cell-surface abundance of Orai1 via stimulating vesicular exocytosis of the channel. The serum-stimulated SOCE and cell-surface abundance of Orai1 are inhibited by the preincubation of αKlotho protein or PI3K inhibitors. Moreover, the inhibition of SOCE and cell-surface abundance of Orai1 by pretreatment of brefeldin A or tetanus toxin or PI3K inhibitors prevents further inhibition by αKlotho. Functionally, we further show that soluble αKlotho ameliorates serum-stimulated SOCE and cell migration in breast and lung cancer cells. These results demonstrate that soluble αKlotho downregulates SOCE by inhibiting PI3K-driven vesicular exocytosis of the Orai1 channel and contributes to the suppression of SOCE-mediated tumor cell migration.

Expression of α-Klotho Is Downregulated and Associated with Oxidative Stress in the Lens in Streptozotocin-induced Diabetic Rats

ABSTRACT Purpose: Oxidative stress, an imbalance between the production of reactive oxygen species and antioxidant defenses, plays an important role in the pathogenesis of diabetic cataract. The lens in diabetes mellitus (DM) has been shown to exhibit impaired antioxidant defenses, but the underlying mechanisms remain poorly understood. Accumulating evidence reveals that Klotho family genes can regulate antioxidant defenses and prevent oxidative stress in multiple tissues. Here, we examined whether DM alters Klotho expression in the lens and if so, whether altered Klotho expression is associated with oxidative stress in the lens in DM. Methods: Male Wistar rats were divided into DM and control groups. DM was induced by injection of streptozotocin (STZ, 60 mg/kg ip) and control rats were injected with vehicle. Twelve weeks after DM induction, levels of α-Klotho in plasma, expression of α- and γ-Klotho, and nuclear factor erythroid 2-related factor 2 (Nrf2), and levels of antioxidants superoxide dismutase (SOD), glutathione peroxidase (GPX), glutathione (GSH) and oxidative stress marker malondialdehyde (MDA) in the lens were measured. Results: Diabetic rats had markedly higher blood glucose concentrations and lower plasma α-Klotho levels than control rats. Both α- and γ-Klotho were expressed in the lens in diabetic and control rats. The expression of α-Klotho but not γ-Klotho in the lens was downregulated in diabetic rats, which was accompanied by reduced expression of nuclear Nrf2 and levels of all antioxidants and increased levels of MDA. Moreover, expression of α-Klotho in the lens was positively correlated with expression of nuclear Nrf2 and levels of all antioxidants, but negatively correlated with levels of MDA. Conclusions: These findings suggest that DM selectively reduces α-Klotho levels in the circulation and lens, which may attenuate transcriptional activity of Nrf2 and impair antioxidant defenses in response to oxidative insults, contributing to oxidative stress and cataract formation in DM.

γ-Klotho is correlated with resistance to docetaxel in castration-resistant prostate cancer.

The Klotho (KL) gene was first identified as a potent aging suppressor. The KL family currently comprises of three proteins: α-Klotho (KLA), β-Klotho (KLB), and γ-Klotho (KLG). Many studies have shown that KLA and KLB participate in tumor progression or suppression, depending on the type of cancer; however, the relationship between KLG and prostate cancer has not yet been studied. Some studies have claimed that KL is correlated to sensitivity to chemotherapy. Here, we investigated the oncogenic potential of KLG in castration-resistant prostate cancer (CRPC). Immunohistochemical analysis using prostate biopsy specimens revealed that patients with high KLG expression in primary prostate cancer tissue had a significantly poor prognosis for overall survival. In addition, the prostate-specific antigen response rate after docetaxel (DTX) therapy in patients with high KLG expression was lower than that in patients with low KLG expression. To evaluate the potential of KLG as a therapeutic target in human prostate cancer, we generated a xenograft model of human CRPC cell line (PC-3) in male athymic mice. The animals were randomly divided into four groups as follows: i) control group (vehicle only); ii) DTX group (intraperitoneal administration); iii) small interfering RNA targeting KLG (KLG siRNA) group (intratumoral administration); and iv) a combination group (DTX plus KLG siRNA). After 3 weeks of treatment, the tumor weight and tumor Ki-67 labeling index were significantly lower in the KLG siRNA group and the combination group than in the control group. Sensitivity to DTX was increased upon treatment with KLG siRNA. These findings suggest that KLG expression in primary prostate cancer lesions is associated with resistance to DTX in CRPC and has potential as a diagnostic and therapeutic target for patients with CRPC.

The Role of The βklotho Gene, Fgf21 and Fgfr1 in Cancerogenesis

Klotho was discovered in 1997 as an anti-aging gene that, when overexpressed, may extend the life span, but when it is disrupted, it may be a factor responsible for premature aging syndrome. The structure and the role of αKlotho and βKlotho genes from Klotho family in malignant tumors is described. The expression profile of the βKlotho gene is significantly different from the expression of the αKlotho gene. Analysis of Klotho expression in breast cancer, cervical cancer as well as endometrial cancer are discussed. The available data indicate the involvement of βKlotho in the neoplastic transformation of the endometrium. More advanced disease is related to negative expression of βKlotho gene. Fibroblast growth factors (FGFs) are a large family of proteins characterized by different functions in the cell development and metabolism. The FGF signaling is also associated with cancerogenesis. The relation between some FGF subfamilies and endometrial cancer clinical data is reported. The interaction between FGF subfamilies and the Klotho subfamily proteins acting as a co-receptor is stressed. Disorders in signaling of the FGF / FGFR pathway have been confirmed in gynecology. It can be assumed that increased expression of FGF21 might be a suppressor factor in endometrial cancer. The FGF21 factor, like the βKlotho protein, achieves its biological effect via the FGFR1 receptor. High expression of the FGFR1 gene inhibits further tumor growth. FGFR1 has the potential to perform both a suppressor and promoter role in the oncogenesis process.

Role of αKlotho and FGF23 in regulation of type II Na-dependent phosphate co-transporters.

Alpha-Klotho is a member of the Klotho family consisting of two other single-pass transmembrane proteins: βKlotho and γKlotho; αKlotho has been shown to circulate in the blood. Fibroblast growth factor (FGF)23 is a member of the FGF superfamily of 22 genes/proteins. αKlotho serves as a co-receptor with FGF receptors (FGFRs) to provide a receptacle for physiological FGF23 signaling including regulation of phosphate metabolism. The extracellular domain of transmembrane αKlotho is shed by secretases and released into blood circulation (soluble αKlotho). Soluble αKlotho has both FGF23-independent and FGF23-dependent roles in phosphate homeostasis by modulating intestinal phosphate absorption, urinary phosphate excretion, and phosphate distribution into bone in concerted interaction with other calciophosphotropic hormones such as PTH and 1,25-(OH)2D. The direct role of αKlotho and FGF23 in the maintenance of phosphate homeostasis is partly mediated by modulation of type II Na+-dependent phosphate co-transporters in target organs. αKlotho and FGF23 are principal phosphotropic hormones, and the manipulation of the αKlotho-FGF23 axis is a novel therapeutic strategy for genetic and acquired phosphate disorders and for conditions with FGF23 excess and αKlotho deficiency such as chronic kidney disease.

The klotho-related protein KLPH (lctl) has preferred expression in lens and is essential for expression of clic5 and normal lens suture formation

KLPH/lctl belongs to the Klotho family of proteins. Expressed sequence tag analyses unexpectedly revealed that KLPH is highly expressed in the eye lens while northern blots showed that expression is much higher in the eye than in other tissues. In situ hybridization in mouse localized mRNA to the lens, particularly in the equatorial epithelium. Immunofluorescence detected KLPH in lens epithelial cells with highest levels in the germinative/differentiation zone. The gene for KLPH in mouse was deleted by homologous recombination. Littermate knockout (KO) and wild type (WT) mice were compared in a wide panel of pathology examinations and were all grossly normal, showing no systemic effects of the deletion. However, the lens, while superficially normal at young ages, had focusing defects and exhibited age-related cortical cataract by slit lamp examination. Whole-lens imaging showed that KO mice had disorganized lens sutures, forming a loose double-y or x instead of the tight y formation of WT. RNA-seq profiles for KO and WT littermates confirmed the absence of KLPH mRNA in KO lens and also showed complete absence of transcripts for Clic5, a protein associated with cilium/basal body related auditory defects in a mouse model. Immunofluorescence of lens epithelial flat mounts showed that Clic5 localized to cilia/centrosomes. Mice mutant for Clic5 (jitterbug) also had defective sutures. These results suggest that KLPH is required for lens-specific expression of Clic5 and that Clic5 has an important role in the machinery that controls lens fiber cell extension and organization.

The Expressions of Klotho Family Genes in Human Ocular Tissues and in Anterior Lens Capsules of Age-Related Cataract

ABSTRACTPurpose: Klotho genes are expressed in limited tissues and have been found to be a pathogenic factor of age-related diseases in mammals, but their expressions and functions in human eyes are yet to be defined. This study was performed to investigate the expression of Klotho family genes in human ocular tissues and anterior lens capsules of age-related cataract.Methods: Individual tissues were isolated from human eyeballs collected from Henan Eye Bank. Human anterior lens capsules were collected from cortical cataract patients during phacoemulsification with the informed consent. Real-time quantitative reverse transcription was performed to detect the mRNA expressions of α-Klotho, β-Klotho, γ-Klotho, IGFR1, AKT, and ERK; Western blot analysis for detecting the expression of ERK1/2 and phosphor-ERK1/2 protein; and Pearson correlation analysis was performed to detect the relationship between Klotho genes and IGFR1, AKT or ERK genes.Results: The expressions of α-, β-, and γ-Klotho mRNA were detected in human retina and optic nerve, but were not detected in iris, ciliary body, sclera, and choroid. Only α-and γ-Klotho mRNA expressions were detected in the lens, and the expression levels were higher than that in the retina and optic nerve. The mRNA expressions of IGFR1 and AKT were not changed among the transparent and the opaque lens capsules. The mRNA expression of ERK was significantly decreased in the opaque lens capsule. Correlation analysis showed that the ERK mRNA expression was positively correlated with the expression of the γ-Klotho gene. Western blot results showed a significant decrease of ERK1/2 and phosphor-ERK1/2 protein in opaque lens capsules.Conclusions: α- and γ-Klotho genes express in the human retina, optic nerve, and lens. The β-Klotho gene expresses only in retina and optic nerve. The γ-Klotho gene may regulate human lens epithelial cells’ function mainly by the MAPK/ERK1/2 signaling pathway.

Klotho-Related Protein KLrP: Structure and Functions.

Klotho (KL) family proteins share one or two glycoside hydrolase (GH) motifs homologous to GH family 1. However, the biological significance of GH motifs in KL family proteins remains elusive. We describe here that KL-related protein (KLrP), which is composed of a single GH motif, is a cytosolic β-glucocerebrosidase (GCase, EC 3.2.1.145). We detected a neutral conduritol B epoxide (CBE)-insensitive glucosylceramide (GlcCer)-degrading activity in the cytosol fractions of human fibroblasts, rat brains, and zebrafish embryos. KL family proteins emerged as a potent candidate for the neutral GCase using a bioinformatics approach. Recombinant human KLrP, but not α-KL, β-KL, or KLPH, exhibited GCase activity with a neutral pH optimum in the presence of CBE. We solved the crystal structures of KLrP and a KLrP mutant (E165Q) in complex with glucose, which indicate that KLrP forms a (β/α)8TIM barrel structure with the double-displacement mechanism of the retaining β-glycosidase. Furthermore, knockdown of endogenous KLrP in CHOP cells using small interfering RNA (siRNA) decreased the CBE-insensitive neutral GCase activity and increased the cellular levels of GlcCer, which suggests that KLrP is involved in a novel GlcCer catabolism pathway. A KLrP D106N mutant was discovered in patients with severe Gaucher disease; however, this mutation did not affect the GCase activity of KLrP.

The FGF21 Receptor Signaling Complex: Klothoβ, FGFR1c, and Other Regulatory Interactions.

Scientific evidence is quickly growing that establishes FGF21 as a cytokine that signals both locally and systemically to induce metabolic effects. The focus of this chapter is the receptor/co-receptor signaling complex formed by endocrine FGF21. We provide an introduction to the major components of the complex including the Klotho family of co-receptors, fibroblast growth factor receptors (FGFRs), and the fibroblast growth factor ligands, placing each in the context of its own family members while emphasizing structural features that drive interaction. We subsequently focus specifically on FGF21 signaling through FGFR1c and KLB, describing what is known about each protein's structure and how this drives protein interaction and formation of the signaling complex at the plasma membrane. We subsequently explore the stoichiometry of FGFR1c and KLB at the plasma membrane before and after the addition of FGF21 ligand, comparing how unique features of the interaction could potentially affect signaling intensity. Finally, we discuss how formation of the signaling complex is potentially regulated by other regulatory interactions, including galectins, the extracellular matrix, and co-expression of FGFR5.

Alpha Klotho and phosphate homeostasis.

The Klotho family consists of three single-pass transmembrane proteins—αKlotho, βKlotho and γKlotho. Each of them combines with fibroblast growth factor (FGF) receptors (FGFRs) to form receptor complexes for various FGF’s. αKlotho is a co-receptor for physiological FGF23 signaling and appears essential for FGF23-mediated regulation of mineral metabolism. αKlotho protein also plays a FGF23-independent role in phosphate homeostasis. Animal experimental studies and clinical observations have demonstrated that αKlotho deficiency leads to severe hyperphosphatemia; moderate elevation of αKlotho reduces serum phosphate and extremely high αKlotho induces hypophosphatemia and high-FGF23. αKlotho maintains circulating phosphate in a narrow range by modulating intestinal phosphate absorption, urinary phosphate excretion by the kidney, and phosphate distribution into bone rather than soft tissue in concerted interaction with other calciophosphotropic hormones such as PTH, FGF23, and 1,25-(OH)2 vitamin D. The role of αKlotho in maintenance of phosphate homeostasis is mediated by direct suppression of Na-dependent phosphate cotransporters in target organs. Therefore, αKlotho manipulation may be a novel strategy for genetic and acquired phosphate disorders and for medical conditions with αKlotho deficiency such as chronic kidney disease in future.

Retinitis Pigmentosa: over‐expression of anti‐ageing protein Klotho in degenerating photoreceptors

Retinitis Pigmentosa involves a hereditary degeneration of photoreceptors by as yet unresolved mechanisms. The secretable protein α-Klotho has a function related to ageing processes, and α-Klotho-deficient mice have reduced lifespan and declining functions in several tissues. Here, we studied Klotho in connection with inherited photoreceptor degeneration. Increased nuclear immunostaining for α-Klotho protein was seen in degenerating photoreceptors in four different Retinitis Pigmentosa models (rd1, rd2 mice; P23H, S334ter rhodopsin mutant rats). Correspondingly, in rd1 retina α-Klotho mRNA expression was significantly up-regulated. Moreover, immunostaining for another Klotho family protein, β-Klotho, also co-localized with degenerating rd1 photoreceptors. The rd1 retina displayed reduced levels of fibroblast growth factor 15, a member of the fibroblast growth factor subfamily for which Klotho acts as a co-receptor. Exogenous α-Klotho protein added to retinal explant cultures did not affect cell death in rd1 retinae, but caused a severe layer disordering in wild-type retinae. Our study suggests Klotho as a novel player in the retina, with a clear connection to photoreceptor cell death as well as with an influence on retinal organization.

Klotho in health and disease

The klotho gene was originally identified as a putative aging-suppressor gene in mice that extended life span when overexpressed and induced a premature aging syndrome when disrupted. Subsequently, it became clear that the Klotho family of membrane proteins function as obligate co-receptors for endocrine fibroblast growth factors (FGFs) that regulate various metabolic processes. This review focuses on the Klotho-FGF23 endocrine system that maintains phosphate (Pi) homeostasis, and discusses the mechanism of action and the potential contribution of Klotho deficiency to acute kidney injury (AKI), chronic kidney disease (CKD) and cancer. Klotho functions as a receptor for the phosphaturic hormone FGF23. Klotho deficiency induces resistance to FGF23 and predisposition to Pi retention, which represents a critical feature of pathophysiology of CKD. The extracellular domain of Klotho protein is subject to ectodomain shedding and released into the blood and urine. Secreted Klotho functions as a humoral factor that inhibits AKI, vascular calcification, renal fibrosis, and cancer metastasis in an FGF23-independent manner. Various factors that affect Klotho expression have been identified. Prevention of Klotho decline and supplementation of Klotho can be a novel therapeutic strategy for many age-related diseases.

Short Hairpin RNA Screen Indicates That Klotho Beta/FGF19 Protein Overcomes Stasis in Human Colonic Epithelial Cells

Normal human colonic epithelial cells (HCECs) are not immortalized by telomerase alone but also require CDK4. Some human cell types growth-arrest due to stress- or aberrant signaling-induced senescence (stasis). Stasis represents the consequences of growth conditions culture that are inadequate to maintain long-term proliferation. Overexpressed CDK4 titers out p16 and allows cells to ignore the growth arrest signals produced by stasis. To identify factors contributing to the inadequate culture environment, we used a 62,000-member shRNA library to knock down factors cooperating with human telomerase reverse transcriptase (hTERT) in the immortalization of HCECs. Knockdown of Klotho gamma (KLG; also known as KLPH and LCTL) allowed hTERT to immortalize HCECs. KLG is one isoform of the Klotho family of factors that coordinate interaction between different FGF ligands and the FGF receptor. We also found that knockdown of KLG induced another member of the Klotho family, Klotho beta (KLB). Induction of KLB was maintained and could activate ERK1/2 in immortalized cells. Supplementation of the culture medium with the KLB ligand FGF19 had a similar effect on hTERT-expressing HCECs as knockdown of KLG regarding both immortalization and down-regulation of the tumor suppressor Klotho alpha. Together, these data suggest that KLB is an important regulator in the immortalization of HCECs by facilitating FGF19 growth factor signaling.

Research resource: Comprehensive expression atlas of the fibroblast growth factor system in adult mouse.

Although members of the fibroblast growth factor (FGF) family and their receptors have well-established roles in embryogenesis, their contributions to adult physiology remain relatively unexplored. Here, we use real-time quantitative PCR to determine the mRNA expression patterns of all 22 FGFs, the seven principal FGF receptors (FGFRs), and the three members of the Klotho family of coreceptors in 39 different mouse tissues. Unsupervised hierarchical cluster analysis of the mRNA expression data reveals that most FGFs and FGFRs fall into two groups the expression of which is enriched in either the central nervous system or reproductive and gastrointestinal tissues. Interestingly, the FGFs that can act as endocrine hormones, including FGF15/19, FGF21, and FGF23, cluster in a third group that does not include any FGFRs, underscoring their roles in signaling between tissues. We further show that the most recently identified Klotho family member, Lactase-like, is highly and selectively expressed in brown adipose tissue and eye and can function as an additional coreceptor for FGF19. This FGF atlas provides an important resource for guiding future studies to elucidate the physiological functions of FGFs in adult animals.

Endocrine fibroblast growth factors as regulators of metabolic homeostasis

Endocrine fibroblast growth factors (FGFs) function as hormones that maintain specific metabolic states by controlling homeostasis of bile acid, glucose, fatty acid, phosphate, and vitamin D. Endocrine FGFs exert their biological activity through a common design of coreceptor system consisting of the Klotho gene family of transmembrane proteins and cognate FGF receptors. Moreover, expression of endocrine FGFs is regulated by nuclear receptors whose lipophilic ligands are generated under the control of these hormones in the target organs. Thus, novel endocrine axes have emerged that regulate diverse metabolic processes through feedback loops composed of the FGF, Klotho, FGF receptor, and nuclear receptor gene families. This review summarizes the role of Klotho family proteins in the regulation of metabolic activity and expression of the endocrine FGFs.

The Klotho gene family as a regulator of endocrine fibroblast growth factors

The Klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends lifespan when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates phosphate and vitamin D homeostasis. It has been shown that Klotho-deficient mice and Fgf23 knockout mice exhibit identical phenotypes. This observation led to the identification of Klotho as a cofactor essential for interactions between FGF23 and FGF receptors. In addition to the Klotho-FGF23 axis, recent studies has shown that betaKlotho, a Klotho family protein, also functions as a cofactor required for FGF19 and FGF21 signaling and determines the tissue-specific metabolic activities of FGF19 and FGF21. This review summarizes recent progress in understanding of Klotho and betaKlotho function in the regulation of tissue-specific metabolic activity of the endocrine fibroblast growth factors (FGF19, FGF21, and FGF23).

C-terminal Tail of FGF19 Determines Its Specificity toward Klotho Co-receptors

FGF19 subfamily proteins (FGF19, FGF21, and FGF23) are unique members of fibroblast growth factors (FGFs) that regulate energy, bile acid, glucose, lipid, phosphate, and vitamin D homeostasis in an endocrine fashion. Their activities require the presence of alpha or betaKlotho, two related single-pass transmembrane proteins, as co-receptors in relevant target tissues. We previously showed that FGF19 can bind to both alpha and betaKlotho, whereas FGF21 and FGF23 can bind only to either betaKlotho or alphaKlotho, respectively in vitro. To determine the mechanism regulating the binding and specificity among FGF19 subfamily members to Klotho family proteins, chimeric proteins between FGF19 subfamily members or chimeric proteins between Klotho family members were constructed to probe the interaction between those two families. Our results showed that a chimera of FGF19 with the FGF21 C-terminal tail interacts only with betaKlotho and a chimera with the FGF23 C-terminal tail interacts only with alphaKlotho. FGF signaling assays also reflected the change of specificity we observed for the chimeras. These results identified the C-terminal tail of FGF19 as a region necessary for its recognition of Klotho family proteins. In addition, chimeras between alpha and betaKlotho were also generated to probe the regions in Klotho proteins that are important for signaling by this FGF subfamily. Both FGF23 and FGF21 require intact alpha or betaKlotho for signaling, respectively, whereas FGF19 can signal through a Klotho chimera consisting of the N terminus of alphaKlotho and the C terminus of betaKlotho. Our results provide the first glimpse of the regions that regulate the binding specificity between this unique family of FGFs and their co-receptors.

The Klotho gene family and the endocrine fibroblast growth factors

This review summarizes recent progress in understanding of Klotho and betaKlotho function in the regulation of tissue-specific metabolic activity of the endocrine fibroblast growth factors (FGF19, FGF21, and FGF23). The Klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends lifespan when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. FGF23 is a bone-derived hormone that regulates phosphate and vitamin D metabolism. It has been shown that Klotho-deficient mice and FGF23 knockout mice exhibit identical phenotypes. This observation led to the identification of Klotho as a cofactor essential for the interaction between FGF23 and fibroblast growth factor receptors. In addition to the Klotho-FGF23 axis, recent studies have shown that betaKlotho, a Klotho family protein, also functions as a cofactor required for FGF19 and FGF21 signaling and determines tissue-specific metabolic activities of FGF19 and FGF21. Recent mouse genetic studies have broadened our understanding of molecular pathways involved in mineral and bile acid homeostasis regulated by FGF23 and FGF19, respectively. The FGF19 subfamily of ligands requires the Klotho gene family of transmembrane proteins for their tissue-specific bioactivity. Further investigations on endocrine axes mediated by the Klotho family and FGF19 subfamily members are expected to provide new insights into the molecular mechanisms by which the endocrine fibroblast growth factors regulate bile acid, energy, and phosphate/vitamin D metabolism.

Co-receptor Requirements for Fibroblast Growth Factor-19 Signaling

FGF19 is a unique member of the fibroblast growth factor (FGF) family of secreted proteins that regulates bile acid homeostasis and metabolic state in an endocrine fashion. Here we investigate the cell surface receptors required for signaling by FGF19. We show that betaKlotho, a single-pass transmembrane protein highly expressed in liver and fat, induced ERK1/2 phosphorylation in response to FGF19 treatment and significantly increased the interactions between FGF19 and FGFR4. Interestingly, our results show that alphaKlotho, another Klotho family protein related to betaKlotho, also induced ERK1/2 phosphorylation in response to FGF19 treatment and increased FGF19-FGFR4 interactions in vitro, similar to the effects of betaKlotho. In addition, heparin further enhanced the effects of both alphaKlotho and betaKlotho in FGF19 signaling and interaction experiments. These results suggest that a functional FGF19 receptor may consist of FGF receptor (FGFR) and heparan sulfate complexed with either alphaKlotho or betaKlotho.