Since its discovery in December 2019 in Wuhan City, China, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has swiftly propagated worldwide, resulting in over 769 million cases and 6.9 million deaths.^1,2 The symptoms of coronavirus disease 2019 (COVID-19) span a spectrum ranging from asymptomatic to critical, encompassing manifestations like fever, cough, breath shortness, fatigue, muscle pain, diarrhea, dizziness, and loss of smell and taste. In severe cases, COVID-19 can lead to pneumonia, acute respiratory distress syndrome, sepsis, and multiple organ failure, contributing critically to mortality.^1,2 Among the diverse factors influencing susceptibility to severe forms of COVID-19 such as older age, metabolic disorders, and some types of cancers, obesity has emerged as a major risk determinant.^3-9 However, the pathological mechanisms underlying this association have remained elusive.

SARS-CoV-2, like all viruses, relies on the host environment for survival and utilizes host cell surface proteins for the infection.^10,11 While angiotensin-converting enzyme 2 (ACE2) has been acknowledged as the primary host receptor for SARS-CoV-2, the low expression level and limited cell distribution of ACE2 have prompted inquiries into the involvement of other host factors.¹² In this pursuit, glucose-regulated protein 78 (GRP78) has emerged as a novel host factor. GRP78 (heat shock protein family A [Hsp70] member 5 [HSPA5]; binding immunoglobulin protein [BiP]) is a molecular chaperone, maintaining the protein homeostasis within the endoplasmic reticulum (ER). However, cumulative studies have unveiled new functional facets of GRP78 by identifying its presence on the cell surface.^13,14 This expanded its role beyond the ER to participation in diverse biological processes such as cellular signaling, inflammatory responses, cell death (apoptosis), and even viral infections.^13,14

Recent findings, independently reported by our team and another research group, highlight GRP78 as a host binding partner and/or co-receptor for SARS-CoV-2 infection.^12,15 Given the multifaceted roles of GRP78 spanning the ER, cell surface, and circulation,^13,14,16-19 it is conceivable that GRP78’s influence extends across various stages of the virus’s life cycle and the intricacies of SARS-CoV-2 pathology. Moreover, the expression of GRP78 has significant associations with COVID-19 risk factors, including older age, obesity, diabetes, and some types of cancer.^12,20-22 Consequently, gaining insights into GRP78’s roles from diverse viewpoints promises a better comprehension of COVID-19’s disease processes, while also introducing innovative perspectives for potential therapeutic interventions.

Through this review, we endeavor to not only enhance our understanding of the pathogenesis of COVID-19, particularly in relation to obesity, but also to unravel the pathological roles of GRP78 in the context of COVID-19 to pave the way for novel therapeutic interventions and strategies that could prevent the current and future pandemics.

Obesity is a complex medical condition characterized by the accumulation of excess body fat. It is not merely a cosmetic concern but a significant health issue with far-reaching effects on various body systems and an increased risk for several diseases. The number of obese people has nearly tripled since 1975 worldwide and is still increasing (Fig. 1);^23,24 39% of adults were estimated as overweight (body mass index [BMI] ≥25.0 to 29.9 kg/m²) and 13% were obese (BMI ≥30.0 kg/m²) globally in a 2016 report. Trends are more prominent in Western countries;²³ 40% of adults have obesity and another 32% are overweight in the United States;²⁵ 29% of adults have obesity and a further 36% are overweight in England.²⁶ Obesity is recognized as a major risk factor for COVID-19 and contributes significantly to various pathological outcomes, such as hospitalization, severity, and mortality.^27,28

Obesity can adversely affect the respiratory system and systemic immune response, all of which play crucial roles in the progression of COVID-19.²⁹ Individuals with obesity often exhibit impaired lung function and reduced lung capacity, which are associated with their ability to effectively combat respiratory infections.³⁰ Moreover, obesity can lead to a state of chronic low-grade inflammation, characterized by elevated levels of pro-inflammatory cytokines.³¹ This chronic inflammation can impact the body’s immune defense mechanisms, making it more difficult for the immune system to mount a robust response against viral infections. Furthermore, obesity is often associated with underlying health conditions such as diabetes, hypertension, and cardiovascular and kidney diseases, which also increase the risk of severe COVID-19.³²

In addition to these pathological aspects, host factors could affect the higher susceptibility of individuals with obesity to COVID-19. ACE2 and transmembrane serine protease 2 (TMPRSS2) are two key proteins involved in the entry of the SARS-CoV-2 virus into host cells.³³ ACE2 serves as the receptor that the virus uses to bind and enter host cells, while TMPRSS2 facilitates the priming of the virus spike protein, allowing it to fuse with the cell membrane and enter the cell.³³ In the context of obesity, there is evidence to suggest that the expression of ACE2 and TMPRSS2 may be altered,^34-37 potentially influencing the susceptibility and severity of COVID-19. Some studies have suggested that the expression of ACE2 may be upregulated in individuals with obesity.^34-37 This upregulation could create more entry points for the virus, potentially increasing the viral load and severity of infection. The relationship between ACE2, TMPRSS2, obesity, and COVID-19 is complex and not fully understood. Further clinical studies are needed to establish a clear link between these host factors, obesity, and COVID-19 outcomes.

The relationship between COVID-19 and obesity is multifaceted. The impact of obesity on lung function, inflammation, immune response, underlying health conditions, and host factors collectively contribute to the increased risk of severe illness in COVID-19. Understanding and addressing the molecular basis is crucial for developing effective strategies to protect vulnerable populations and mitigate the impact of COVID-19.

Although ACE2 has been recognized as the primary receptor for SARS-CoV-2, its low expression and confined cell distribution suggest the possibility of other host factors that might augment the interaction between the virus and its host. Recent studies have suggested that GRP78 functions as a host binding partner and/or co-receptor for SARS-CoV-2 infection.^12,15 GRP78, a molecular chaperone within the ER, assumes a pivotal role in regulating protein homeostasis such as protein folding, assembly, stability, and degradation.^13,14 However, under stress, GRP78 becomes overexpressed and translocated to the cell surface, acting as a binding partner for diverse ligands and influencing the pathology of various conditions like infections and cancers.^13,14

The cellular localization of GRP78 is associated with intrinsic and extrinsic elements.^38-40 GRP78 features an N-terminal signal peptide, guiding its ER localization during translation.³⁸ The C-terminal KDEL sequence interacts with the KDEL receptor, governing retrograde transport from Golgi to the ER.³⁹ Stress-induced GRP78 overexpression disrupts ER retention, leading to KDEL receptor saturation and GRP78 relocation to the cell surface.^41-43 Cellular stress such as viral infection also increases the GRP78 expression and relocation from the ER to cell surface and circulation,^15,44,45 which possibly affects the pathology and lifecycle of SARS-CoV-2.

Cell surface GRP78 (csGRP78) exhibits binding affinity for endogenous and exogenous ligands, including viral proteins.^13,14 Recent studies have established csGRP78 as a binding partner and/or co-receptor with ACE2 for SARS-CoV-2 infection.^12,15 While csGRP78 alone does not promote SARS-CoV-2 binding, its co-expression with ACE2 enhances viral protein accumulation on cell surfaces.¹² Mechanistically, csGRP78 directly interacts with SARS-CoV-2 spike protein and forms a protein complex with host cell receptor ACE2 on the cell surface, which facilitates entry into the target cells;^12,15 the host factor efficacy of csGRP78 for SARS-CoV-2 infection was associated with the stabilization of ACE2 protein expression on the cell surface.¹⁵ This co-receptor role mirrors its involvement in other coronaviruses and viruses such as dengue virus, coxsackievirus A9, Japanese encephalitis virus (JEV), and Tembusu virus.^19,45-51

In summary, cell surface-localized GRP78 assumes significance as a co-receptor or binding partner in SARS-CoV-2 infection, mediating direct interactions between spike protein and ACE2 (Figs. 1 and 2). In addition to that, GRP78 can control the protein homeostasis of membrane proteins, such as ACE2, potentially enhancing the stability and presence of ACE2 on the cell surface.^12-15 GRP78’s binding affinities for both endogenous ACE2 and viral proteins probably contribute to the increased entry of SARS-CoV-2 into target cells on the cell surface.

Since viruses cannot self-replicate, they need to exploit host cells to produce their proteins.^11,19,52-54 SARS-CoV-2 is a type of coronavirus enclosed within a lipid membrane, with numerous viral proteins forming its structure.^54-56 These proteins encompass four essential ones: spike (S), envelope (E), membrane (M), and nucleocapsid (N); an additional 16 non-structural proteins (NSP1-16), derived from open reading frame 1a (ORF1a) and ORF1ab; and 11 accessory proteins: ORF3a, ORF3b, ORF3c, ORF3d, ORF6, ORF7a, ORF7b, ORF8, ORF9b, ORF9c, and ORF10. These components are critical for the viral life cycle and infection, though how they are produced and replicated remains largely unknown.

The ER is a hub for the production, folding and assembly of membrane and secretary proteins.^13-15 GRP78, as a molecular chaperone in the ER, plays a critical role in regulating protein folding, assembly, and homeostasis not just for host cell proteins but also for external viral proteins.^13-16 Many viruses, including dengue virus, JEV, human cytomegalovirus, Ebola virus, and hepatitis B virus, rely on GRP78 to assemble their viral proteins.^48,50,57-60 The spike protein of SARS-CoV-2 directly interacts with GRP78 in host cells.^12,15 Ablation of GRP78 decreases the production of viral protein and SARS-CoV-2 replication.^18,45,61

The cumulative protein interactome data have shown that GRP78 also interacts with other viral proteins of SARS-CoV-2, such as E, N, NSP2, NSP4, NSP14, ORF7a, and ORF8.⁶² The S, E, and N proteins are indispensable elements for the virus’s structure and infectivity;^55,56 NSP2, NSP4, and NSP14 form a replication-transcription complex that actively participates in genome replication and the regulation of early transcription processes;⁵⁶ ORF7a plays a role in suppressing the type I interferon response, amplifying immune and cytokine reactions, and contributing to SARS-CoV-2’s replication.^63-65 Additionally, ORF8 is intricately connected to the evasion of the immune system and the elicitation of inflammatory responses in the context of SARS-CoV-2 infection.^66-68

Taken together, GRP78, acting as a host chaperone for viral SARS-CoV-2 proteins, assists in the proper folding, assembly, and maturation of viral components like S, E, N, NSPs, and ORFs, contributing to the pathology of COVID-19 (Figs. 1 and 2). The diverse range of protein binding capabilities exhibited by GRP78 can be advantageous for viruses. This suggests that viruses may have evolved to use GRP78 as a viral chaperone, aiding in viral replication.

GRP78 is present not only within cellular compartments such as the ER and cell surface but also in circulation in a soluble form.^20,21,44,69 Notably, the level of soluble GRP78 increases with SARS-CoV-2 infection, particularly in patients with pneumonia.⁴⁴ This circulating GRP78 has strong links to metabolic disorders such as obesity and diabetes.⁶⁹ Similar elevation of GRP78 is also observed in patients with lung cancers.^20,22 These pathological conditions are recognized contributors to the severity and mortality risk of COVID-19.^6,70

In our recent study, we treated ACE2-expressing human lung epithelial cells with soluble GRP78—mimicking circulating GRP78 in vitro—which resulted in facilitated binding and accumulation of SARS-CoV-2 spike protein in the cells. These results suggested that the soluble form of GRP78 in circulation possibly binds to SARS-CoV-2 via the interaction with spike protein on the viral membrane, enhances its stability, and facilitates the infection into host cells.¹² Previous research indicates that GRP78 enhances the stability and expression of binding proteins; for instance, csGRP78 stabilizes ACE2 or ADAM metallopeptidase domain 17 (ADMA17) protein on the cell surface, influencing virus infectivity.^15,71 Additionally, GRP78 promotes the entry of binding substances into cells through the endocytosis pathway.^72,73

It is plausible that soluble GRP78 might directly bind to SARS-CoV-2 spike protein in circulation (Figs. 1 and 2). This complex formation of soluble GRP78 with viral particles could enhance virus stability and facilitate attachment and endocytosis in target cells. These properties might contribute to SARS-CoV-2’s ability to infect multiple organs beyond the respiratory tract, potentially leading to conditions like ‘long COVID’ characterized by prolonged symptoms after infection.^74-80 Further experimental and clinical studies are required to validate these hypotheses.

GRP78 has traditionally been recognized as a chaperone protein that maintains protein balance and manages stress within the ER. However, accumulating research demonstrates its broader role as a signaling mediator on the cell surface, impacting various disease-related processes like inflammation, cell death, survival, and cell growth.^13,81-84 This signaling role of csGRP78 is facilitated by its interactions with other cell membrane proteins and molecules, which in turn trigger diverse cellular responses and gene activities.

For instance, when csGRP78 interacts with α2-macroglobulin, it triggers signaling pathways involving mitogen-activated protein kinase (MAPK), protein kinase B (AKT), and nuclear factor-κB (NF-κB) that promote cell proliferation while decreasing cell death.^81,85 This interaction also regulates other pathways, such as pyruvate dehydrogenase kinase 1 (PDK1) signaling and cellular MYC (c-MYC) activity, linked to cell growth.⁸⁶ Furthermore, csGRP78’s involvement in controlling transcriptional coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ), which are crucial for cell mechanics and movement, influences the mobility of cancer cells.⁸³ Moreover, the complex formation of GRP78 with Cripto on the cell surface impacts various cellular processes by activating specific pathways such as MAPK/phosphoinositide 3-kinase (PI3K) and SMAD2/3, influencing cell adhesion and growth while also activating the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway linked to cell survival.^82,87 These cellular responses associated with csGRP78’s interactions are key contributors to the development of various diseases.

Notably, well-known pathways like NF-κB and JAK/STAT3 are associated with inflammatory and apoptotic responses, while SMAD2/3 pathways are established in fibrosis formation (Figs. 1 and 2).^13,81,82 Additionally, the PI3K/AKT/MAPK and YAP/TAZ pathways are essential for cell survival and growth.^13,82-84 The activation of csGRP78, either through direct binding with SARS-CoV-2 or related signals, has the potential to modify these signaling and transcriptional pathways, affecting the development of COVID-19 pathology. Further experiments are necessary to fully comprehend how csGRP78 influences cell signaling and gene activity during SARS-CoV-2 infection and its contribution to COVID-19 pathological development, including inflammation, cytokine storms, cell death, and tissue damage.

Not everyone infected with SARS-CoV-2 experiences severe symptoms, but certain groups are more susceptible to developing serious outcomes. Cumulative studies have found that individuals with older age, obesity, diabetes, lung cancer, and other metabolic abnormalities are more vulnerable to COVID-19.^3-7

Recently, we have demonstrated that GRP78 expression is prominent in adipose tissues, particularly within the visceral regions.¹² This expression is further heightened under conditions of older age, obesity, and diabetes. The elevated expression of GRP78 in adipocytes is attributed to hyperinsulinemia, a characteristic often observed in older age, obesity, and diabetes.¹² Of note, the expression of GRP78 in adipocytes remained unaffected by glucose levels, underscoring that the primary driver of GRP78 overexpression in adipose tissue is hyperinsulinemia.¹² Mechanistically, hyperinsulinemia in obesity triggers cellular stress and activates the stress-responsive transcription factor the spliced form of X-box binding protein 1 (XBP-1s), resulting in the overexpression of both the gene and protein expression of GRP78 in adipocytes.¹² Notably, in human adipose tissue, there is a significant correlation between the gene expression levels of XBP-1 and GRP78,¹² thus confirming the regulatory mechanism underlying this process.

The circulating level of GRP78 is also significantly correlated with obesity, diabetes, and other metabolic syndromes.^21,69 Moreover, specific types of lung cancer are linked to the overexpression of GRP78, with its circulating levels showing a marked increase in concurrence with disease progression and severity.^20,22 The soluble GRP78 in circulation could possibly facilitate the stability and infectivity of SARS-CoV-2 into host cells.¹² Collectively, underlying health conditions, such as older age, obesity, diabetes, and specific cancer types, potentially cultivate environments that stimulate the induction and activation of GRP78 across cellular, extracellular, and circulatory contexts (Fig. 1). As a result, individuals with such conditions might be more susceptible to the impact of COVID-19.

In recent years, it has become apparent that SARS-CoV-2 infection can have harmful effects far beyond the lungs; COVID-19 is well known for causing respiratory disease but can also trigger metabolic abnormalities.^70,77 However, the molecular mechanisms remain largely elusive. SARS-CoV-2 infection elevates the expression of GRP78 in the lung and circulation,^44,88 and presumably in other infected organs and cells, such as liver, adipose tissue, pancreatic, and immune cells via cellular stress and unfolded protein responses (UPRs).⁷⁷

The pathological significance of GRP78 overexpression in metabolic diseases has been well established in numerous prior studies (Table 1). The expression of GRP78 is significantly increased in the adipose tissue of patients with obesity and diabetes.¹² The circulating level of GRP78 serves as a molecular marker for several metabolic diseases, encompassing obesity, diabetes, and atherosclerosis.^21,69 Heterozygosity of the Grp78 gene increases energy expenditure and offers protection against hyperinsulinemia, hyperglycemia, liver steatosis, and adipose inflammation induced by a high-fat diet through adaptive UPR mechanisms.⁸⁹ Macrophage-selective ablation of the Grp78 gene enhances insulin sensitivity and improves glucose metabolism in muscle, while simultaneously reducing inflammation in adipose tissue.⁹⁰ Furthermore, covalent inhibition of GRP78 using celastrol curbs lipid accumulation in the liver and adipose tissue, concurrently mitigating ER stress and inflammation.⁹¹ The overexpression of Vaspin, an endogenous antagonist for the csGRP78 and DnaJ-like protein 1 (MTJ-1) complex, ameliorates diet-induced obesity, glucose intolerance, and hepatic steatosis while enhancing ER stress.⁹²

Collectively, these studies have suggested that the expression of GRP78 induced by infection and stress could potentially contribute to the metabolic abnormalities associated with COVID-19 (Fig. 1). The molecular mechanisms underlying metabolic disorders following SARS-CoV-2 infection remain largely unexplored. Therefore, it is imperative to conduct detailed investigations into this relationship in future studies.

There are various other pathological dimensions of GRP78 associated with the life cycle of SARS-CoV-2 and the progression of COVID-19 symptoms. GRP78 is overexpressed when the ER faces stress conditions, such as an overwhelming burden of protein synthesis.^13,14 This induction of GRP78, coupled with the UPR, not only prevents cell apoptosis but also fosters pro-survival mechanisms within the cells.^13,14,16 Additionally, elevated levels of GRP78 are detected in various viral infections.^16,57,60 Importantly, the upregulation of GRP78 is also evident in instances of SARS-CoV-2 infection, possibly linked to ER stress induced by the excessive synthesis of viral proteins; elevated levels of GRP78 have been identified in pneumocytes, macrophages, and within the circulating system.^44,88 Collectively, these findings suggest its potential involvement in the prolonged presence of SARS-CoV-2, resulting in an extended and sustained duration of the virus within the host.

The expression of GRP78 plays a crucial role in influencing apoptotic, immune, and fibrotic responses within the lung.^93,94 Mice with Grp78 heterozygosity (Grp78^+/−) have shown remarkable protection against bleomycin-induced fibrosis, accompanied by improved lung function.⁹³ Furthermore, the deactivation of GRP78 has been linked to reduced endothelial inflammation and enhanced barrier integrity in the context of acute lung injury,⁹⁴ suggesting its potential implication in the lung pathology and function of individuals affected by COVID-19. Further comprehensive research is needed on the relationship between GRP78 and lung pathology, encompassing aspects such as cell death, immune responses, and fibrotic reactions in COVID-19.

GRP78 plays a pivotal role in stabilizing ADAM17 on the cell surface, a function that has been reported to enhance the activity of the previous coronavirus strain, SARS-CoV, by facilitating the shedding of the viral receptor ACE2.⁷¹ Moreover, GRP78 likely participates in the regulation of ACE2 protein expression on the cell surface, potentially through direct binding and stabilization mechanisms.¹⁵ Following infection, GRP78 is released alongside replicated SARS-CoV-2 viruses from host cells through the lysosomal exocytic pathway, hinting at its involvement in virus replication, egress, and stabilization processes.¹⁷ Additionally, GRP78 can interact with major histocompatibility class one molecules on the cell surface,⁹⁵ suggesting a potential contribution to antigen presentation and subsequent immune responses during SARS-CoV-2 infection.

Co-infections and secondary bacterial or fungal infections have been reported in COVID-19 patients, exacerbating the severity of the disease and leading to worse outcomes.^96-98 Intriguingly, GRP78 exhibits remarkable conservation across different organisms, ranging from bacteria (Dna K: bacterial GRP78 homolog) to humans (GRP78/BiP/HSPA5).⁹⁹ Bacterial GRP78 plays an essential role in promoting proper bacterial growth and mRNA/protein regulation.¹⁰⁰ It has become a therapeutic target for addressing both bacterial and viral infections.¹⁰¹

Importantly, viable SARS-CoV-2 has been detected in fecal specimens from COVID-19 patients,^102,103 and the virus demonstrates bacteriophage-like behavior by replicating within bacteria.¹⁰⁴ Additionally, SARS-CoV-2 infection disrupts gastrointestinal (GI) microbiota and is linked to the severity of COVID-19.¹⁰⁵ These observations indicate a potential interplay between GRP78 and SARS-CoV-2, which might contribute to virus replication/transmission via GI/fecal bacteria, perturb the microbiota, and potentially influence the outcome of COVID-19.

Furthermore, GRP78 functions as a host receptor for fungal pathogens such as Rhizopus oryzae, a common species within the Mucorales order, facilitating invasion and damage to human endothelial cells.¹⁰⁶ GRP78 also interacts with Pneumocystis carinii on the surface of lung epithelial cells, facilitating attachment.¹⁰⁷ These discoveries suggest that GRP78 could play a role in scenarios involving co-infections and secondary infections among COVID-19 patients, contributing to the outcome of COVID-19 pathology (Fig. 1).

Given the pivotal role of GRP78 in diverse biological processes, its interaction with SARS-CoV-2 proteins, and its link to COVID-19-associated pathological risks, a strategic focus on modulating either the activity or expression of GRP78 is a promising therapeutic and preventative avenue. This approach holds the potential to effectively mitigate various stages of the SARS-CoV-2 life cycle, encompassing binding, entry, replication, egress, and stability. Encouragingly, evidence from studies indicates that neutralizing csGRP78 with specific antibodies can curtail the infectivity of related viruses, including SARS-CoV-2.^15,16,19,47 Similarly, the targeted suppression of GRP78 expression using small interfering RNA not only impedes virus entry but also attenuates the production of viral proteins within host cells.^{16,19,45,50,51} Other inhibitory molecules related to the expression and/or activity of GRP78, such as subtilase cytotoxin (SubAB), AR12 (direct GRP78 inhibitor), HA15 (potent GRP78 inhibitor; OSU-03012), and epigallocatechin gallate (EGCG), have shown analogous potential by inhibiting virus entry and replication processes within host cells.^{16,18,45,48,50}

Strategies that mitigate metabolic stress, whether through pharmacological interventions or lifestyle changes, offer an avenue to curtail GRP78 expression. Notably, anti-diabetic medications like metformin and sodium-glucose cotransporter 2 inhibitors, as well as lifestyle modifications such as calorie restriction and exercise, have demonstrated efficacy in reducing GRP78 expression in adipose tissues.¹² Managing metabolic abnormalities via diabetic drugs may also hold the potential to ameliorate the severity of COVID-19 outcomes.^12,108,109 Collectively, directing interventions toward GRP78 under conditions of stress holds potential as a promising therapeutic strategy to prevent or inhibit SARS-CoV-2 infection and the progression of COVID-19 pathology.

In conclusion, GRP78 appears to play a versatile role as a host factor influencing multiple stages of the SARS-CoV-2 life cycle. It acts as a co-receptor, chaperone, protein stabilizer, and mediator of cellular signaling and transcription (Figs. 1 and 2). These emerging functions of GRP78 likely contribute to various aspects of COVID-19 pathology, encompassing not only infection but also inflammation, fibrosis, egress, organ tropism, long-term COVID symptoms, co-infections, secondary infections, and metabolic disorders. Its expression is notably linked to a heightened risk of severe symptoms and outcomes in COVID-19 patients, particularly among the elderly, obese, diabetic, and those with lung cancer. Consequently, targeting GRP78 could prove effective as both a preventive and therapeutic approach against COVID-19-related pathology. This review aims to stimulate further research on GRP78 across both the clinical and basic science domains. In-depth investigations are warranted to fully comprehend GRP78’s potential roles and its associations with COVID-19 pathology, risk factors, and metabolic implications.

The authors declare no conflict of interest.

We would like to thank all the members of the Department of Metabolic Medicine, Graduate School of Medicine, Osaka University; Division of Endocrinology, Diabetes and Metabolism, Beth Israel Deaconess Medical Center and Harvard Medical School.

Study concept and design: JS; drafting of the manuscript: JS and IS; and critical revision of the manuscript: IS.