FKBP4 Human

What is the genomic organization of human FKBP4?

Human FKBP4 consists of 10 exons and 9 introns spanning approximately 10 kb of genomic DNA, and maps to chromosome 12p13.33. The gene organization is highly conserved among chordates, with identical exon-intron boundaries. Eight pseudogenes for FKBP4 have been identified, seven on chromosome 9 and one on chromosome 4. Unlike its related protein FKBP51 (encoded by FKBP5), which contains non-coding exons 1-3, FKBP4 lacks these elements but otherwise maintains similar organization to FKBP5 .

What are the key structural domains of FKBP4 protein and their functions?

FKBP4 protein contains two main types of functional domains:

• PPIase domains: These domains exhibit peptidyl-prolyl isomerase activity critical for protein folding and are involved in specific protein-protein interactions, such as the FKBP4/IKKγ interaction.

• TPR domains: Tetratricopeptide repeat domains mediate interactions with heat shock proteins, particularly Hsp90, which is essential for FKBP4's role in steroid receptor regulation and IKK complex formation .

These distinct domains allow FKBP4 to serve as a molecular scaffold, integrating multiple signaling pathways through protein-protein interactions.

How did FKBP4 evolve and how conserved is it across species?

FKBP4 likely evolved from an ancestral invertebrate gene through a gene duplication event that occurred before the emergence of fishes. Orthologues of FKBP4 are found in chordates and also in Drosophila melanogaster and Caenorhabditis elegans, although the exon-intron boundary organization is conserved only among chordates. Phylogenetic analysis suggests that FKBP4 and FKBP5 evolved from a common ancestral gene, possibly fkb-6, with FKBP4 showing greater evolutionary conservation across species than FKBP5, which is found only in chordates .

How is FKBP4 expression altered in hepatocellular carcinoma (HCC)?

FKBP4 is significantly upregulated in HCC tissues compared to normal liver tissues, as confirmed by bioinformatics analysis of TCGA LIHC datasets, ICGC LIRI-JP datasets, and GEO datasets. This elevated expression has been validated in clinical HCC samples and multiple HCC cell lines through quantitative RT-PCR. Importantly, elevated FKBP4 expression correlates with poor prognosis in HCC patients. The upregulation of FKBP4 is strongly related to HCC staging, suggesting its involvement in disease progression .

What is the relationship between FKBP4 expression and clinical outcomes in cancer patients?

High FKBP4 expression is associated with worse clinical outcomes in multiple cancer types:

How does FKBP4 regulate glycolysis in hepatocellular carcinoma?

FKBP4 promotes glycolysis in HCC through the p53-mediated HK2 signaling pathway. Specifically:

• FKBP4 knockdown suppresses glucose uptake, lactic acid production, and 18F-FDG uptake in HCC cells.

• Mechanistically, FKBP4 knockdown promotes both the expression and stability of p53 protein without affecting its transcription.

• p53 then negatively regulates HK2 (hexokinase 2), a key glycolytic enzyme.

• Rescue experiments demonstrated that simultaneous knockdown of both FKBP4 and p53 partially reversed the inhibitory effects on HK2 protein levels and 18F-FDG uptake.

This FKBP4-p53-HK2 axis represents a novel mechanism by which FKBP4 promotes the Warburg effect (aerobic glycolysis) in HCC development .

What is the role of FKBP4 in NF-κB signaling?

FKBP4 potentiates IKK/NF-κB signaling through two integrated mechanisms:

• FKBP4/Hsp90/IKK complex: FKBP4 enhances IKK kinase activity by:

  • Interacting directly with Hsp90 and IKK subunits

  • Promoting Hsp90/IKK association

  • Facilitating IKK complex assembly by promoting the binding of IKKγ to IKKβ

• FKBP4/Hsp70/RelA complex: FKBP4 associates with Hsp70 and RelA (p65), facilitating the transport of RelA toward the nucleus.

These mechanisms integrate to potentiate both the transcriptional activity and nuclear translocation of NF-κB. The TPR domains of FKBP4 are essential for FKBP4/IKK interaction through Hsp90 association, while the PPIase domains are involved in FKBP4/IKKγ interaction .

How does FKBP4 interact with steroid hormone receptors compared to FKBP51?

Despite 75% sequence similarity, FKBP4 and FKBP51 have diverse and often opposite actions on steroid hormone receptors:

• Binding mechanism: Both FKBP4 and FKBP51 bind to Hsp90 through their C-terminal TPR domains and compete for association with Hsp90 complexes, especially those associated with steroid hormone receptors.

• Functional effects:

  • FKBP4 generally promotes steroid receptor activity for glucocorticoid (GR), mineralocorticoid (MR), androgen (AR), progesterone (PR), and estrogen receptors (ER).

  • FKBP51 typically inhibits or modulates these same receptors.

• Cancer relevance: The differential regulation of hormone receptors by these proteins is particularly important for hormone-dependent cancers such as breast and prostate cancer .

What experimental techniques are most effective for studying FKBP4 protein interactions?

Several complementary techniques have proven effective for studying FKBP4 protein interactions:

• BioID technique: This proximity-dependent biotinylation approach has been successfully used to identify the FKBP4 interactome, revealing proximal proteins including Hsp90 and components of the IKK complex. This technique is particularly valuable for detecting transient or weak interactions that might be missed by traditional co-immunoprecipitation.

• Co-immunoprecipitation (Co-IP): Used to confirm direct interactions between FKBP4 and other proteins such as Hsp90, IKKβ, and IKKγ. This approach has been instrumental in mapping the interaction network of FKBP4.

• Domain truncation experiments: Creating truncated mutants of FKBP4 (e.g., PPIase domain-truncated or TPR domain-truncated mutants) has been effective in defining the specific domains responsible for protein-protein interactions. These experiments revealed that TPR domains are essential for FKBP4/Hsp90 interaction while PPIase domains mediate FKBP4/IKKγ interaction .

What are the most reliable methods for assessing FKBP4's role in glycolysis?

To assess FKBP4's role in glycolysis, researchers have employed multiple complementary approaches:

• Glucose uptake assays: Measuring the cellular uptake of glucose after FKBP4 knockdown or overexpression.

• Lactic acid production measurement: Quantifying lactic acid in cell culture medium as an indicator of aerobic glycolysis.

• 18F-FDG uptake: Using radioactively labeled glucose analog (18F-fluorodeoxyglucose) to measure glucose metabolism in cells with modified FKBP4 expression.

• Key glycolytic enzyme assessment: Monitoring expression and activity of hexokinase 2 (HK2) and other glycolytic enzymes.

• Rescue experiments: Simultaneous knockdown of both FKBP4 and p53 to verify the mechanistic pathway, observing partial reversal of effects on HK2 protein levels and 18F-FDG uptake .

How can researchers effectively study the epigenetic regulation of FKBP4?

While the search results provide more information about epigenetic regulation of FKBP5 than FKBP4, researchers can apply similar approaches to study FKBP4 epigenetic regulation:

• DNA methylation analysis: Site-specific methylation analysis can be performed using bisulfite sequencing, particularly focusing on promoter regions and potential regulatory intronic sequences.

• Histone modification assessment: Chromatin immunoprecipitation (ChIP) assays can detect changes in histone density and modifications at the FKBP4 gene locus in response to hormones or other treatments.

• Single nucleotide polymorphism (SNP) analysis: Identifying SNPs in non-coding regions of FKBP4 that may influence basal and hormone-stimulated expression of the gene.

• Long-term treatment studies: Examining how prolonged exposure to hormones like corticosteroids affects DNA methylation patterns and expression of FKBP4, similar to studies done with FKBP5 .

How might FKBP4 serve as a prognostic biomarker in cancer?

FKBP4 shows significant potential as a prognostic biomarker in multiple cancer types:

These findings suggest that FKBP4 expression analysis could be incorporated into prognostic algorithms for cancer patients, potentially improving risk stratification and treatment planning .

What therapeutic strategies could target FKBP4 in cancer?

Based on the molecular mechanisms of FKBP4 in cancer, several therapeutic strategies could be developed:

• Direct FKBP4 inhibitors: Developing small molecule inhibitors targeting the PPIase or TPR domains to disrupt specific protein interactions.

• Targeting the FKBP4-p53-HK2 axis: Since FKBP4 promotes glycolysis through p53-mediated HK2 signaling, combination therapy with glycolysis inhibitors might be effective in FKBP4-overexpressing cancers.

• Disrupting FKBP4-Hsp90 interaction: Compounds that interfere with the FKBP4-Hsp90 complex could inhibit IKK activation and subsequent NF-κB signaling.

• Hormone therapy modulation: In hormone-dependent cancers, targeting FKBP4 could enhance response to anti-hormone therapies by altering steroid receptor activity.

• FKBP4-specific antibody therapies: Developing antibodies that selectively target FKBP4-overexpressing cancer cells.

Given the critical role of aerobic glycolysis and NF-κB signaling in cancer progression, targeting FKBP4 may offer a novel therapeutic strategy for treating malignancies where it is overexpressed .

What are the challenges in targeting FKBP4 therapeutically?

Developing FKBP4-targeted therapies faces several challenges:

• Functional redundancy: FKBP4 functions may overlap with other immunophilins, particularly FKBP51, which could compensate for FKBP4 inhibition.

• Tissue-specific effects: FKBP4 has diverse roles in different tissues and cancer types, requiring careful consideration of potential off-target effects.

• Dual roles in signaling: FKBP4 integrates multiple signaling pathways, including both pro-oncogenic (glycolysis promotion, NF-κB activation) and potentially tumor-suppressive functions (depending on context).

• Selectivity issues: Developing inhibitors that selectively target FKBP4 without affecting related immunophilins presents a pharmaceutical challenge.

• Context-dependent functions: FKBP4's roles may vary depending on the specific cellular context, hormone levels, and genetic background, necessitating personalized therapeutic approaches .

What are emerging areas of FKBP4 research beyond cancer?

While cancer research dominates current FKBP4 studies, several emerging areas show promise:

• Neurodegenerative diseases: FKBP4 regulates the microtubule-associated protein tau and microtubule assembly, with potential implications for tauopathies and related neurodegenerative disorders.

• Transient receptor potential (TRP) channel regulation: FKBP4 interacts with and influences the transient receptor potential canonical subfamily of ion channel proteins, suggesting roles in calcium signaling and related physiological processes.

• Amyloid beta toxicity and copper homeostasis: Studies in Drosophila indicate FKBP4 may control amyloid beta toxicity and copper homeostasis, relevant to Alzheimer's disease pathology.

• Alpha-synuclein aggregation: FKBP4 has been implicated in modulating α-synuclein aggregation, relevant to Parkinson's disease and other synucleinopathies.

• Pregnancy and oxidative stress: FKBP4 may protect pregnancy from oxidative stress through regulation of peroxiredoxin-6 levels .

How do FKBP4 and FKBP51 function as a regulatory pair in cellular processes?

FKBP4 and FKBP51 function as a regulatory pair with often antagonistic roles:

• Competitive binding: Both proteins compete for binding to Hsp90 complexes through their TPR domains, creating a dynamic equilibrium that regulates downstream signaling.

• Opposing effects on steroid receptors: Generally, FKBP4 potentiates while FKBP51 inhibits steroid receptor activity, creating a balanced regulatory system for hormone response.

• Differential expression patterns: Despite structural similarities, FKBP4 and FKBP51 show distinct expression patterns across tissues and in response to various stimuli.

• Evolutionary conservation: Their antagonistic behavior appears to be a fundamental characteristic conserved through evolution, suggesting important biological roles for this regulatory pair.

The interplay between these proteins likely provides cells with fine-tuned control over various signaling pathways, with the ratio of FKBP4 to FKBP51 potentially determining cellular responses to hormones and other stimuli .

What technologies are advancing our understanding of FKBP4 function?

Several cutting-edge technologies are enhancing our understanding of FKBP4:

• Proximity labeling techniques: BioID and related approaches are revealing the full interactome of FKBP4 in different cellular contexts.

• CRISPR-Cas9 gene editing: Precise modification of FKBP4 and interacting partners is enabling detailed functional studies.

• Structural biology advances: Cryo-electron microscopy and X-ray crystallography are providing insights into the structural basis of FKBP4 interactions.

• Multi-omics integration: Combining transcriptomics, proteomics, and metabolomics data is revealing FKBP4's role in coordinating cellular responses.

• Patient-derived organoids and xenografts: These models better recapitulate human disease and allow for testing of FKBP4-targeting approaches in physiologically relevant systems.

• Single-cell analysis: Examining FKBP4 expression and function at the single-cell level is uncovering cell type-specific roles within heterogeneous tissues .