CRTC-CREB Axis Senses Proteasome Inhibition via ROS/JNK in Drosophila

Study Background and Research Question

The regulation of proteostasis—the maintenance of protein homeostasis—is critical for cellular health, particularly in the nervous system where protein aggregation underlies many neurodegenerative diseases. The ubiquitin-proteasome system (UPS) is central to the degradation of misfolded and damaged proteins, and its impairment leads to proteotoxic stress and disease. The transcription factor CREB (cAMP response element-binding protein) and its coactivator CRTC (CREB-regulated transcriptional coactivator) are evolutionarily conserved regulators of gene expression in response to metabolic and stress signals. However, their precise role in sensing and adapting to proteasome inhibition was unclear, especially in vivo. The reference study (Cell Death Dis 2022) addresses how the CRTC-CREB axis responds to proteasome inhibition and whether this pathway can be leveraged to counteract proteotoxic insults in Drosophila, with implications for human disease.

Key Innovation from the Reference Study

The central innovation of the study is the identification of the CRTC-CREB axis as a transcriptional sensor for proteasome inhibition-induced proteotoxic stress in a living organism. Through a large-scale compound screening enabled by an optimized drug delivery platform (U-GLAD), the authors found that proteasome inhibitors—including the reversible 20S proteasome β5 subunit inhibitor MLN2238—robustly activate CREB in adult Drosophila. Mechanistically, this induction depends on the generation of reactive oxygen species (ROS) upon proteasome inhibition, which then activates the JNK (c-Jun N-terminal kinase) pathway to stimulate CREB phosphorylation and activity (Cell Death Dis 2022).

Methods and Experimental Design Insights

The authors employed a multi-tiered experimental approach:

• Compound Screening: Utilizing the U-GLAD system, the team delivered various proteasome inhibitors—including MLN2238—to adult flies and monitored CREB activity using a CRE-luciferase reporter.
• Genetic Manipulation: Overexpression and loss-of-function alleles of CRTC were used to probe its role in CREB activation. CRTC mutants and rescue experiments established the specificity of the effect.
• Cellular Assays: In human 293T cells, MLN2238 was shown to activate JNK and increase CREB phosphorylation at Ser133, mirroring findings in flies.
• Transcriptomics: RNA-seq analysis of fly intestines revealed transcriptional upregulation of redox and protein homeostasis genes upon CRTC overexpression.
• Disease Model Application: In a Drosophila model of Huntington’s disease (HD), muscle-specific overexpression of CRTC reduced protein aggregation, improved motility, and extended lifespan, supporting the stress-protective function of the CREB pathway.

Core Findings and Why They Matter

The study demonstrates several key findings:

• Proteasome inhibitors elevate CREB activity in vivo: All tested proteasome inhibitors, including MLN2238, increased CREB-driven transcription in adult flies (Cell Death Dis 2022).
• ROS/JNK signaling mediates CREB activation: The rise in CREB activity depends on ROS production and subsequent JNK activation, both in fly and mammalian cells.
• CRTC/CREB promotes proteostasis and stress resilience: Overexpression of CRTC upregulates genes involved in protein folding and redox regulation. In a fly HD model, enhanced CRTC/CREB activity ameliorates disease phenotypes by restoring proteasome function and reducing toxic aggregates.
• Implications for aging: CREB activity increases with age in muscle, and further boosting its activity can suppress age-associated protein aggregation, highlighting a potential therapeutic avenue for age-related proteopathies.

Together, these findings position the CRTC-CREB axis as a central, conserved sensor and effector in the cellular response to proteasome inhibition—a process highly relevant to neurodegeneration, aging, and cancer biology.

Comparison with Existing Internal Articles

Several internal resources provide context for these findings:

• The article "MLN2238: Unraveling Proteasome Inhibition and CREB Axis M..." explores how MLN2238 modulates CREB signaling in hematologic malignancy research, paralleling the mechanism described in the reference study. This cross-validation strengthens the relevance of the ROS/JNK/CREB pathway not only in neurodegeneration models but also in oncology workflows.
• "MLN2238 as a Proteasome β5 Subunit Inhibitor: Applied Workflows" and "MLN2238 and the Proteasome Stress Axis: Translational Leverage" both outline advanced assay workflows and troubleshooting strategies that can be directly informed by the new mechanistic insights from this Drosophila study. For example, the importance of redox and JNK signaling can be considered in experimental design for proteasome inhibitor studies in multiple myeloma and lymphoma research.

Together, these resources offer a bridge between basic mechanistic findings and translational applications in cancer biology, emphasizing how redox-dependent CREB activation may impact drug resistance and apoptosis in proteasome-targeted therapies.

Limitations and Transferability

While the reference study provides compelling evidence in Drosophila and selected mammalian cell lines, several limitations should be noted:

• Species differences: Although the CRTC-CREB-ROS/JNK axis is conserved, the specific dynamics of CREB phosphorylation and its regulation differ between flies (Ser231) and mammals (Ser133), warranting careful extrapolation.
• Chemical delivery and solubility: The U-GLAD system enabled effective compound delivery in flies, but solubility and bioavailability issues may still present challenges in mammalian or clinical contexts (source: product_spec).
• Disease model maturity: The study uses a Drosophila HD model, which, while informative, does not fully recapitulate the complexity of human neurodegenerative diseases or cancer. Additional validation in mammalian models is needed to confirm therapeutic potential (source: Cell Death Dis 2022).

Protocol Parameters

• assay | MLN2238 IC₅₀ for β5 site | 3.4 nM | Suitable for in vitro chymotrypsin-like proteasome inhibition | Enables selective targeting of β5 subunit in mechanistic studies | product_spec
• assay | MLN2238 IC₅₀ for β1 site | 31 nM | Useful for probing caspase-like proteasome activity at higher concentrations | Informs off-target or broader inhibition profiles | product_spec
• assay | MLN2238 IC₅₀ for β2 site | 3500 nM | Only relevant at high concentrations for trypsin-like site studies | Typically not targeted in standard workflows | product_spec
• assay | Solubility in DMSO | ≥16.8 mg/mL | Preparation of concentrated stock solutions | Facilitates use in cell-based and in vivo assays; avoid long-term solution storage | product_spec
• assay | Solubility in ethanol (with ultrasound) | ≥103 mg/mL | For protocols requiring higher concentrations | Enhanced by warming and ultrasonic treatment | product_spec
• workflow | Use of U-GLAD system for Drosophila drug delivery | As described | Recommended for large-scale in vivo compound screening in flies | Overcomes solubility and delivery barriers | workflow_recommendation

Research Support Resources

Researchers interested in investigating proteasome β5 subunit inhibition, redox signaling, or the role of the CRTC-CREB axis in proteotoxic stress can support their workflows with specialized reagents. MLN2238 (SKU A4008) from APExBIO is a well-characterized, reversible 20S proteasome inhibitor with potent activity against the β5 subunit, and is suitable for studies in oncology, neurodegeneration, and stress response models (source: product_spec). Please consult product specifications for details on solubility, storage, and recommended assay concentrations.