IL 24 Human

What is the molecular structure of human IL-24?

Human IL-24 is a 206 amino acid protein with a molecular weight of approximately 24 kDa. It contains an IL-10 signature motif at amino acids 101-121, which is shared by other IL-10 family cytokines. The structure revealed through crystallization shows a notable lack of disulfide bonds, which likely contributes to IL-24's relative instability compared to other interleukins. To study IL-24's structure experimentally, researchers have used a fusion approach with flexible linkers connecting IL-24 to its receptors (IL-22R1 and IL-20R2) to facilitate crystallization .

What are the genomic characteristics of human IL-24?

Human IL-24 is encoded by the IL24 gene located on chromosome 1q32-33, clustered with several other IL-10 family cytokine genes. The gene encompasses seven exons and six introns. The complete cDNA is 1,718 base pairs in length, encoding the 206-amino acid IL-24 protein. For genomic analysis, researchers can utilize PCR-based approaches targeting this region, with particular attention to regulatory elements that control IL-24 expression in different cell types .

Which cell types express IL-24 in humans under physiological conditions?

IL-24 is produced by several immune cells, including:

• Peripheral blood mononuclear cells (PBMCs), particularly monocytes

• T helper 2 (Th2) lymphocytes

• B lymphocytes

• Macrophages

Non-immune cells that express physiological levels of IL-24 include:

• Cultured melanocytes

• Dermal keratinocytes

• IL-1 stimulated human colonic subepithelial myofibroblasts

For experimental verification of cell-specific expression, researchers should combine flow cytometry with intracellular staining and cell sorting techniques, followed by RT-qPCR or protein analysis .

What stimuli induce IL-24 expression in human immune cells?

Multiple stimuli can induce IL-24 expression:

• In monocytes: concanavalin A, lipopolysaccharide (LPS), and specific cytokines

• In T cells: TCR stimulation aided by anti-CD3 and CD28 or PMA and Ionomycin

• In B cells: B cell receptor signaling (anti-IgM plus CD40-L)

• In PBMCs: individual cytokines including IL-2, IL-7, IL-15, TNF-α, GM-CSF, and IL-1β

For experimental induction of IL-24, researchers should note the timing dynamics: in PHA-stimulated PBMCs, IL-24 mRNA reaches peak levels at 8-12 hours post-stimulation, while protein expression peaks at approximately 24 hours .

How is IL-24 expression regulated at the post-transcriptional level?

IL-24 expression in human PBMCs involves significant post-transcriptional regulation. When LPS- or PHA-stimulated cells are treated with Actinomycin D (which inhibits transcription), IL-24 mRNA persists at high levels over at least 4 hours, suggesting regulation through mRNA stabilization mechanisms. This indicates that post-transcriptional processes are crucial for controlling IL-24 expression. Researchers investigating these mechanisms should employ mRNA stability assays with transcription inhibitors, followed by RNA immunoprecipitation to identify RNA-binding proteins involved in stabilizing IL-24 transcripts .

What receptor complexes mediate IL-24 signaling?

IL-24 signals through two distinct heterodimeric receptor complexes:

• IL-20R1/IL-20R2

• IL-22R1/IL-20R2

Both receptor complexes are essential for full IL-24 signaling. Experimental evidence shows that the lack of either subunit is insufficient to trigger STAT3 phosphorylation in response to IL-24. To study receptor specificity, researchers can use receptor knockout or knockdown approaches, combined with phospho-STAT detection assays .

What are the primary downstream signaling cascades activated by IL-24?

The primary downstream signaling pathway activated by IL-24 is the JAK-STAT pathway, particularly STAT1 and STAT3. When testing various reporter constructs in HEK293T cells (including AP-1, CREB, IFN-β, ISRE, m67-SIE, and NF-κB), rhIL-24 treatment resulted in significant increase in m67-SIE reporter activity (indicating STAT1/3 activation) without notable activation of other reporters. Both human and mouse recombinant IL-24 can trigger STAT1/3 signaling, indicating conservation of this mechanism across species. For experimental analysis of signaling pathways, researchers should employ reporter assays, phospho-protein detection, and pathway inhibitors .

How does IL-24 induce apoptosis in specific cell types?

IL-24 has been demonstrated to induce apoptosis in various cell types, particularly cancer cells and renal tubular epithelial cells. In renal epithelial cells, IL-24-induced apoptosis is accompanied by increased endoplasmic reticulum (ER) stress response. The mechanism appears to involve both extracellular signaling (through receptor binding) and intracellular targets. To investigate these mechanisms, researchers should examine markers of ER stress (such as BiP/GRP78, CHOP, XBP1 splicing) alongside classical apoptosis markers (caspase activation, mitochondrial membrane potential, phosphatidylserine externalization) using a combination of biochemical assays and microscopy techniques .

What is the role of IL-24 in atopic dermatitis pathogenesis?

IL-24 plays a critical role in atopic dermatitis (AD)-like inflammation:

• IL-24 is induced in keratinocytes by MRSA infection

• Keratinocyte-specific deletion of IL-24 alleviates AD symptoms, including:

  • Reduced ear thickness

  • Mitigated chronic spontaneous itch and acute itch flare

  • Lower levels of serum and tissue IgE

  • Dampened allergic inflammation with lower tissue IL-4

  • Decreased epidermal thickness

• IL-24 enhances IL-33 production in keratinocytes, which drives type 2 immune responses

• IL-24 downregulates filaggrin (encoded by Flg) expression while upregulating Tslp and Postn, suggesting IL-24 impairs skin barrier function

These findings indicate IL-24 as a potential therapeutic target for AD management. For experimental investigation, researchers should use conditional knockout models (e.g., Krt14Cre;Il24fl/fl) combined with established AD models such as MC903 treatment with or without microbial challenge .

How does IL-24 contribute to acute kidney injury?

IL-24 plays a significant role in renal ischemia-reperfusion injury (IRI), a major cause of acute kidney injury:

• IL-24 is upregulated in the kidney after renal IRI

• Tubular epithelial cells and infiltrating inflammatory cells are the source of IL-24

• Mice lacking IL-24 show protection from renal injury and inflammation

• IL-24 induces apoptosis in renal tubular epithelial cells

• IL-24-induced apoptosis is associated with increased endoplasmic reticulum stress

• IL-24 promotes expression of endogenous IL-24 in tubular cells, creating a self-amplifying loop

IL-24 has potential as both a biomarker and therapeutic target in ischemia-induced acute kidney injury. To investigate this experimentally, researchers should use IL-24 knockout models, combined with IRI procedures and assessment of renal function, inflammatory markers, and cell death indicators .

What is the evidence for IL-24's tumor suppressor function in humans?

IL-24 was originally identified as a tumor suppressor molecule (initially named melanoma differentiation-associated gene 7 or mda-7). Multiple studies have demonstrated cell death in cancer cells and cell lines following exposure to IL-24. The tumor-suppressing effects appear to be selective, affecting malignant cells while sparing normal cells.

The mechanisms of tumor suppression likely involve:

• Induction of apoptosis through ER stress

• Regulation of cell cycle and proliferation

• Anti-angiogenic effects

• Immune-mediated anti-tumor responses

For experimental investigation of IL-24's tumor suppressor functions, researchers should use a combination of in vitro studies with various cancer cell lines, in vivo xenograft models, and mechanistic analyses focusing on cell death pathways and tumor microenvironment interactions .

What methods are optimal for detecting IL-24 expression at protein and mRNA levels?

For comprehensive IL-24 detection, researchers should employ multiple complementary techniques:

mRNA Detection:

• RT-qPCR: The gold standard for quantifying IL-24 transcript levels

• RNA-Seq: For global expression analysis and identifying alternative splice variants

• In situ hybridization: For tissue localization of IL-24 mRNA

Protein Detection:

• ELISA: For quantification of secreted IL-24 in biological fluids and cell culture supernatants

• Western blotting: For detecting IL-24 protein in cell/tissue lysates

• Intracellular flow cytometry: For cell-specific IL-24 expression analysis (peak detection at 24h post-stimulation)

• Immunofluorescence/immunohistochemistry: For tissue localization and co-localization studies

When designing experiments, researchers should be aware that IL-24 expression is often dynamic and transient, with mRNA peaking at 8-12 hours and protein at 24 hours post-stimulation in PBMCs .

What experimental models are most appropriate for studying IL-24 functions?

Depending on the research question, several experimental models are valuable for IL-24 research:

In vitro models:

• Primary human cell cultures (PBMCs, monocytes, T cells, keratinocytes)

• Cell lines expressing IL-24 receptors (for signaling studies)

• HEK293T cells with reporter constructs (for pathway analysis)

• Normal rat kidney (NRK) cells (for renal function studies)

In vivo models:

• Global IL-24 knockout mice

• Conditional tissue-specific IL-24 knockout mice (e.g., Krt14Cre;Il24fl/fl for keratinocyte-specific deletion)

• Disease-specific models:

  • MC903-induced AD model with MRSA treatment

  • Renal ischemia-reperfusion injury model

  • Tumor xenograft models

When selecting models, researchers should consider species-specific differences in IL-24 signaling and the expression of IL-24 receptors in target tissues .

What techniques are useful for manipulating IL-24 expression and signaling for functional studies?

Multiple approaches can be employed to manipulate IL-24 for functional studies:

Overexpression approaches:

• Recombinant human/mouse IL-24 protein administration

• Viral vector-mediated gene delivery (adenovirus, lentivirus)

• DNA-based expression constructs with inducible promoters

Inhibition approaches:

• Genetic: CRISPR/Cas9-mediated knockout, siRNA/shRNA knockdown

• Pharmacological: JAK-STAT inhibitors (for downstream signaling)

• Biological: Neutralizing antibodies against IL-24 or its receptors

• Conditional knockout models using Cre-loxP system

Reporter systems:

• Luciferase reporters (e.g., m67-SIE for STAT1/3 activation)

• Fluorescent protein-based reporters for real-time monitoring

For comprehensive functional assessment, researchers should combine multiple approaches and include appropriate controls for each manipulation strategy .

How does IL-24 integrate into the broader cytokine network during inflammatory responses?

IL-24 functions within a complex cytokine network, particularly in inflammatory conditions:

• Upstream regulators of IL-24:

  • Pro-inflammatory cytokines (IL-1β, TNF-α)

  • T cell-derived cytokines (IL-2, IL-7, IL-15)

  • Growth factors (GM-CSF)

  • Microbial components (LPS, S. aureus products)

• Downstream effects:

  • Induction of IL-33 in keratinocytes

  • Promotion of type 2 immune responses (increased IL-4, IgE)

  • Regulation of tissue barrier function (decreased filaggrin, increased TSLP)

• Integration with other pathways:

  • JAK-STAT signaling (primarily STAT1/3)

  • ER stress responses

  • Cell death pathways

For investigating these complex interactions, researchers should employ systems biology approaches including multiplex cytokine analysis, pathway mapping, and computational modeling of cytokine networks based on experimental data from relevant disease models .

What are the discrepancies between in vitro and in vivo IL-24 functions, and how might they be reconciled?

Several discrepancies between in vitro and in vivo IL-24 functions have been noted:

• Differential effects on cell viability:

  • In vitro: Direct apoptosis induction in certain cell types

  • In vivo: More complex outcomes influenced by microenvironmental factors

• Receptor utilization:

  • In vitro: Clear dependence on specific receptor combinations

  • In vivo: Potential compensation by alternative receptor complexes

• Kinetics of response:

  • In vitro: Rapid and direct effects

  • In vivo: More prolonged and integrated with other signaling systems

To reconcile these differences, researchers should:

• Use primary cells rather than cell lines when possible

• Develop 3D culture and organoid systems that better mimic in vivo conditions

• Perform parallel in vitro and in vivo experiments with matched readouts

• Consider microenvironmental factors (hypoxia, pH, neighboring cells)

• Validate key findings in multiple experimental systems and human samples .

What are the translational challenges in targeting IL-24 for therapeutic applications?

Several challenges exist in translating IL-24 research into therapeutic applications:

• Targeting specificity:

  • IL-24 shares receptors with other IL-10 family cytokines

  • Tissue-specific targeting is necessary to avoid systemic effects

• Therapeutic window:

  • Context-dependent functions (beneficial in cancer, detrimental in some inflammatory conditions)

  • Potential for triggering compensatory mechanisms

• Delivery challenges:

  • For protein-based approaches: stability issues due to lack of disulfide bonds

  • For gene therapy: efficient and specific delivery systems

• Biomarker development:

  • Need for reliable assays to measure IL-24 levels in patient samples

  • Identification of patient subgroups most likely to benefit

• Safety considerations:

  • Effects on normal tissues expressing IL-24 receptors

  • Potential immunogenicity of therapeutic proteins or vectors

For researchers pursuing translational applications, careful consideration of these challenges through preclinical efficacy and toxicity studies is essential, along with development of companion diagnostics to identify responsive patient populations .

Expression Kinetics of IL-24 in Stimulated Human PBMCs

Note: Data derived from PHA-stimulated human PBMCs

IL-24 Receptor Complexes and Their Signaling Outcomes

Note: Complete receptor complexes are required for effective signal transduction

Cytokine Induction of IL-24 in Human Immune Cells