Recombinant Human Adiponectin receptor protein 1 (ADIPOR1)

What is the basic structure and function of ADIPOR1?

ADIPOR1 is a seven-transmembrane domain receptor with an unusual topology featuring an internal N-terminus and external C-terminus, which is opposite to the configuration of G protein-coupled receptors. The receptor belongs to the progestin and adipoQ receptor family (PAQR family) and functions as a primary receptor for adiponectin, an essential hormone secreted by adipocytes that regulates glucose and lipid metabolism. ADIPOR1 is required for maintaining normal glucose and fat homeostasis and body weight regulation. The receptor structure can exist in both closed and open conformational states, with significant differences in the positioning of helices IV and V between these states. In the closed form, the seven transmembrane helices form an internal cavity, while the open form features large openings in this internal cavity, with the intracellular ends of helices IV and V shifted by approximately 4 and 11 Å, respectively. This conformational flexibility is likely crucial to the receptor's signaling mechanisms.

How does ADIPOR1 signaling work at the molecular level?

ADIPOR1 primarily activates the AMP-activated protein kinase (AMPK) pathway upon binding adiponectin. The signaling cascade proceeds as follows: adiponectin (particularly the globular form, for which ADIPOR1 has high affinity) binds to ADIPOR1, triggering conformational changes that likely involve the transition between closed and open states observed in crystal structures. This activation leads to increased AMPK activity, which subsequently enhances fatty acid oxidation, increases glucose uptake in peripheral tissues, and decreases hepatic gluconeogenesis. The interconversion between closed and open conformations may represent a key aspect of the receptor's activation mechanism, allowing it to transduce the adiponectin binding signal across the membrane to initiate intracellular signaling cascades. The functional significance of ADIPOR1's unique topology appears related to its ability to recognize specific forms of adiponectin and engage with appropriate downstream effectors.

What are the known variants and alternative names for ADIPOR1?

ADIPOR1 is known by several alternative names in the scientific literature, which is important for comprehensive literature searches and database queries:

• Progestin and adipoQ receptor family member 1 (PAQR1)

• TESBP1A

• CGI-45

• Adiponectin receptor protein 1

The protein exists in various recombinant forms for research purposes, including human fragment proteins that span specific amino acid ranges (e.g., 72 to 136 aa) expressed in systems such as wheat germ. These recombinant versions are suitable for various experimental applications including ELISA and Western blotting. The amino acid sequence of a typical fragment includes: QAHHAMEKMEEFVYKVWEGRWRVIPYDVLPDWLKDNDYLLHGHRPPMPSFRACFKSIFRIHTETG, which represents a portion of the full receptor.

How does ADIPOR1 expression vary across different cancer types and what are the implications for cancer research?

ADIPOR1 exhibits remarkable heterogeneity in expression patterns across various cancer types, which has important implications for understanding its role in cancer biology. Comprehensive pan-cancer analyses using TIMER2.0 and GEPIA2 databases have revealed that ADIPOR1 is significantly upregulated in numerous cancer types including breast invasive carcinoma (BRCA), cervical squamous cell carcinoma (CESC), cholangiocarcinoma (CHOL), colon adenocarcinoma (COAD), esophageal carcinoma (ESCA), head and neck squamous cell carcinoma (HNSC), liver hepatocellular carcinoma (LIHC), lung adenocarcinoma (LUAD), lung squamous cell carcinoma (LUSC), pancreatic adenocarcinoma (PAAD), pheochromocytoma and paraganglioma (PCPG), prostate adenocarcinoma (PRAD), rectum adenocarcinoma (READ), stomach adenocarcinoma (STAD), and uterine corpus endometrial carcinoma (UCEC). Conversely, ADIPOR1 is downregulated in kidney chromophobe (KICH), kidney renal papillary cell carcinoma (KIRP), lymphoid neoplasm diffuse large B-cell lymphoma (DLBC), acute myeloid leukemia (LAML), and thymoma (THYM).

These differential expression patterns suggest tissue-specific roles of ADIPOR1 in cancer development and progression. Research focusing on the mechanisms driving these expression changes could provide valuable insights into cancer metabolism and potential therapeutic targets. The correlation between ADIPOR1 expression and cancer stemness in multiple cancer types (including ACC, BRCA, ESCA, GBM, KICH, LAML, LGG, LIHC, LUSC, PCPG, PRAD, SKCM, STAD, TGCT, THCA, and THYM) further underscores its potential importance in cancer stem cell biology, which is critical for understanding tumor initiation, progression, and therapy resistance.

What is the prognostic value of ADIPOR1 across different cancer types?

The prognostic significance of ADIPOR1 varies substantially across cancer types, highlighting the context-dependent nature of its function in cancer biology. Univariate Cox analysis has revealed that ADIPOR1 functions as a risk factor in adrenocortical carcinoma (ACC), bladder urothelial carcinoma (BLCA), cervical squamous cell carcinoma (CESC), kidney chromophobe (KICH), kidney renal papillary cell carcinoma (KIRP), and brain lower grade glioma (LGG). In these cancers, higher ADIPOR1 expression correlates with poorer patient outcomes. In contrast, ADIPOR1 serves as a favorable prognostic factor in kidney renal clear cell carcinoma (KIRC) and sarcoma (SARC), where higher expression is associated with better survival outcomes.

How is ADIPOR1 genetically altered across different cancers, and what are the implications?

ADIPOR1 genetic alterations have been studied through comprehensive pan-cancer analyses using genomic databases. ADIPOR1 polymorphisms, particularly rs1342387, have been associated with cancer risk in multiple studies. A meta-analysis focused on colorectal cancer (CRC) risk revealed significant associations between the rs1342387 polymorphism and CRC susceptibility. The GG genotype shows a notably increased risk (OR 1.45, 95%CI 1.19-1.77) compared to the AA genotype, while the G allele generally confers higher risk than the A allele (OR 1.25, 95%CI 1.12-1.40).

This table demonstrates the significant association between ADIPOR1 rs1342387 polymorphism and colorectal cancer risk, with population-based (PB) controls showing no heterogeneity (I² = 0%). The consistent association suggests this genetic variant may serve as a potential biomarker for CRC risk assessment and warrants further functional investigation to elucidate its mechanistic role in carcinogenesis.

What are the optimal expression systems for producing recombinant ADIPOR1 protein for structural and functional studies?

The production of high-quality recombinant ADIPOR1 protein is critical for structural and functional studies. Based on current literature, wheat germ cell-free expression systems have proven effective for generating functional ADIPOR1 fragments. For instance, a recombinant human ADIPOR1 fragment spanning amino acids 72-136 has been successfully expressed in wheat germ systems and validated for applications including ELISA and Western blotting. This approach is particularly valuable for producing protein fragments that maintain proper folding and functional epitopes.

For full-length ADIPOR1 production, which is more challenging due to its seven transmembrane domains, insect cell expression systems (particularly Sf9 cells) have been used successfully for crystallography studies. These systems allow for proper membrane protein folding and post-translational modifications. Alternative approaches include mammalian expression systems (HEK293 or CHO cells) for studies requiring mammalian glycosylation patterns. For structural studies, protein engineering approaches such as T4 lysozyme fusion constructs or thermostabilizing mutations have proven beneficial for enhancing protein stability and crystallization propensity. The choice of detergents (typically mild non-ionic detergents like DDM or LMNG) is crucial for maintaining proper folding during membrane protein extraction and purification.

What methods are most effective for studying ADIPOR1 conformational changes between closed and open states?

Understanding the conformational dynamics of ADIPOR1 between closed and open states requires sophisticated biophysical and structural biology approaches. X-ray crystallography has been instrumental in capturing static snapshots of these conformational states, as demonstrated in studies of wild-type ADIPOR1 and the D208A variant. The wild-type receptor was found to exist as a mixture of closed (44%) and open (56%) forms, while specific mutations can shift this equilibrium.

For dynamic studies, hydrogen-deuterium exchange mass spectrometry (HDX-MS) provides valuable information on protein dynamics and solvent accessibility changes. Single-molecule FRET (smFRET) techniques are particularly powerful for monitoring real-time conformational changes by strategically introducing fluorescent labels at positions that undergo significant distance changes between conformational states (such as the intracellular ends of helices IV and V, which shift by ~4 and ~11 Å, respectively). Molecular dynamics simulations complement experimental approaches by providing atomic-level insights into conformational transition pathways and energetics. For functional correlation, site-directed mutagenesis targeting residues at the hinge regions between conformational states, combined with downstream signaling assays (e.g., AMPK activation), can help establish structure-function relationships.

What are the recommended approaches for investigating ADIPOR1's role in cancer using patient samples?

When investigating ADIPOR1's role in cancer using patient samples, a multi-omics approach is recommended to capture the complexity of its involvement. Immunohistochemistry (IHC) on tissue microarrays allows for protein expression profiling across large patient cohorts, with careful attention to antibody validation and quantitative scoring systems. This should be complemented by mRNA expression analysis through qRT-PCR or RNA-seq to account for post-transcriptional regulation.

Specialized analyses like single-cell RNA-seq can reveal ADIPOR1 expression heterogeneity within tumors and identify specific cell populations where ADIPOR1 signaling may be most relevant. For functional validation, patient-derived xenografts (PDX) or organoids offer systems to test the effects of ADIPOR1 modulation in patient-specific contexts, potentially informing personalized therapeutic approaches.

How does ADIPOR1 contribute to metabolic disorders and what are the implications for therapeutic development?

ADIPOR1 plays a central role in metabolic homeostasis through its mediation of adiponectin signaling. Upon activation by adiponectin, ADIPOR1 initiates signaling cascades that increase AMPK activity, leading to enhanced fatty acid oxidation, increased glucose uptake in peripheral tissues, and decreased hepatic gluconeogenesis. These metabolic effects position ADIPOR1 as a critical regulator of whole-body energy metabolism. In obesity and type 2 diabetes, plasma adiponectin levels are typically reduced, compromising ADIPOR1 signaling and contributing to metabolic dysregulation.

Therapeutic strategies targeting ADIPOR1 offer promising approaches for metabolic disorders. The small-molecule ADIPOR agonist AdipoRon has shown remarkable efficacy in preclinical models, ameliorating diabetes, increasing exercise endurance, and even prolonging the shortened lifespan associated with obesity. These findings suggest that ADIPOR1 activation could address multiple aspects of metabolic syndrome simultaneously. For therapeutic development, structural insights into ADIPOR1's conformational states (closed versus open) provide valuable information for rational drug design. Compounds that stabilize the active conformation of ADIPOR1 could potentially mimic adiponectin's beneficial effects while offering advantages in terms of pharmacokinetics, tissue penetration, and manufacturing.

What is the relationship between ADIPOR1 and immune function in cancer microenvironments?

ADIPOR1 exhibits complex relationships with immune function in cancer microenvironments, suggesting its potential role in cancer immunobiology. Database analyses have revealed correlations between ADIPOR1 expression and immune cell infiltration across various cancer types. The receptor's expression varies across normal immune cell populations, as demonstrated by The Human Protein Atlas (THPA) database findings.

Functional analyses using the TIMER2.0 database with EPIC algorithm have identified significant correlations between ADIPOR1 expression and specific immune cell populations within tumors, though these relationships are highly cancer-type specific. Additionally, ADIPOR1 shows associations with microsatellite instability (MSI) in multiple cancers including HNSC, KIRC, LUSC, PCPG, READ, and UCEC, suggesting potential involvement in DNA mismatch repair mechanisms that influence tumor immunogenicity.

These findings highlight the potential immunomodulatory functions of ADIPOR1 in cancer contexts. Understanding these relationships could inform combination therapies that leverage both ADIPOR1 targeting and immunotherapeutic approaches. Further research is needed to elucidate the mechanistic basis of these associations and determine whether ADIPOR1 directly influences immune cell function and recruitment or whether these correlations reflect broader metabolic effects that indirectly shape the tumor immune microenvironment.

How might ADIPOR1 polymorphisms influence cancer susceptibility and treatment response?

ADIPOR1 genetic polymorphisms, particularly rs1342387, demonstrate significant associations with cancer susceptibility. Meta-analysis data for colorectal cancer show that individuals carrying the GG genotype have a 45% increased risk (OR 1.45, 95%CI 1.19-1.77) compared to those with the AA genotype. Similarly, the G allele generally confers a 25% higher risk than the A allele (OR 1.25, 95%CI 1.12-1.40). These consistent associations across population-based studies suggest ADIPOR1 genetic variation may serve as a valuable biomarker for cancer risk stratification.

The mechanistic basis for these associations may involve alterations in ADIPOR1 expression, protein structure, or signaling efficiency, potentially disrupting metabolic homeostasis and cellular growth regulation. From a clinical perspective, identifying individuals with high-risk ADIPOR1 genotypes could inform personalized screening protocols and preventive interventions. For treatment considerations, ADIPOR1 polymorphisms may influence response to metabolic-targeting therapies, including potential ADIPOR1 agonists under development.

Future research should explore how these polymorphisms affect ADIPOR1 function at the molecular level, their interaction with environmental factors, and their potential utility as predictive biomarkers for response to specific therapeutic approaches. Integration of ADIPOR1 genotyping into clinical trials of metabolic-targeting agents would be particularly valuable for establishing pharmacogenetic relationships.

What are the emerging areas of interest in ADIPOR1 research at the intersection of metabolism and cancer?

The intersection of metabolism and cancer represents a particularly promising frontier in ADIPOR1 research. Emerging evidence suggests that ADIPOR1's metabolic regulatory functions may directly influence cancer cell metabolism, potentially offering novel therapeutic approaches. Future research should investigate how ADIPOR1-mediated AMPK activation affects cancer cell metabolic reprogramming, including effects on glycolysis, oxidative phosphorylation, and lipid metabolism. The differential expression of ADIPOR1 across cancer types suggests context-specific metabolic functions that warrant deeper investigation using metabolomic approaches coupled with ADIPOR1 modulation.

Another emerging area concerns the potential interaction between ADIPOR1 signaling and established oncogenic pathways. Preliminary evidence suggests connections between ADIPOR1 and cancer stemness in multiple cancer types, but the molecular mechanisms underlying these associations remain poorly understood. Additionally, the relationship between ADIPOR1 polymorphisms and cancer risk points to potential germline genetic effects that may predispose individuals to metabolic dysfunction and subsequent cancer development. Understanding these connections could inform preventive strategies targeting metabolic health for cancer risk reduction.

How might new structural insights into ADIPOR1 inform drug discovery efforts?

Recent structural discoveries regarding ADIPOR1's closed-open conformational states provide valuable foundations for structure-based drug design efforts. The elucidation of the D208A variant structure, showing distinct conformational states among three molecules in the asymmetric unit (two in closed form, one in open form), offers critical insights into the receptor's activation mechanism. The significant movement of helices IV and V between these states (with intracellular ends shifting by approximately 4 and 11 Å, respectively) highlights potential allosteric sites that could be targeted to modulate receptor activity.

Future drug discovery efforts should focus on compounds that can selectively stabilize either the open or closed conformations, depending on the desired therapeutic outcome. Virtual screening campaigns targeting the internal cavity and potential binding sites at the interfaces of mobile helices could identify novel chemical scaffolds. Fragment-based drug discovery approaches, particularly using biophysical methods like surface plasmon resonance (SPR) or nuclear magnetic resonance (NMR), might identify small molecular binders that can be optimized into lead compounds. Additional structural studies capturing ADIPOR1 in complex with natural ligands or synthetic modulators would further inform rational drug design efforts aimed at precisely controlling ADIPOR1 signaling for therapeutic benefit.