CRTC-CREB Axis as a Sensor for Proteotoxic Stress via Proteasome Inhibition

Study Background and Research Question

The cAMP Response Element-Binding Protein (CREB) family comprises evolutionarily conserved transcription factors involved in diverse cellular processes, including cell growth, synaptic plasticity, and metabolic regulation. CREB activation is tightly controlled by phosphorylation events and by interaction with coactivators such as CRTC (CREB-regulated transcription coactivator) and CBP/p300. While the role of CREB in metabolic and developmental pathways is well-established, less is known about its involvement in cellular stress responses, particularly those related to proteotoxic stress and protein homeostasis. The ubiquitin-proteasome system (UPS) is central to protein turnover, and its inhibition can trigger the accumulation of misfolded proteins, leading to proteotoxic and oxidative stress. The referenced study (Yin et al., 2022) investigates whether and how the CRTC-CREB axis senses and integrates proteasome inhibition-induced stress signals in vivo, using Drosophila as a model system.

Key Innovation from the Reference Study

The central innovation of this research is the identification of the CRTC-CREB axis as a transcriptional sensor linking proteasome inhibition to cellular stress adaptation. Through a large-scale chemical screening in adult flies enabled by a novel U-GLAD (U-shape Gum Arabic Liquid Assisted Drug delivery) system, the authors discovered that all tested proteasome inhibitors, including MLN2238, robustly increase CREB activity in vivo. This activation is mediated by reactive oxygen species (ROS) and the downstream c-Jun N-terminal kinase (JNK) pathway, establishing the ROS/JNK/CREB signaling axis as a conserved stress sensor. The study further demonstrates that enhancing CRTC/CREB activity can restore protein folding and proteasomal function, particularly in models of neurodegenerative disease and aging-associated protein aggregation.

Methods and Experimental Design Insights

The study employed a combination of in vivo and in vitro approaches:

• Compound Screening in Adult Flies: The U-GLAD system was leveraged to deliver a library of FDA-approved drugs, overcoming solubility and delivery barriers typical in Drosophila models. Proteasome inhibitors such as MLN2238 were identified as potent CREB activators.
• ROS and JNK Pathway Analysis: Genetic and pharmacological tools were used to manipulate ROS levels and JNK activity, establishing causality between proteasome inhibition, ROS generation, JNK activation, and downstream CREB phosphorylation.
• Transcriptomic Profiling: RNA-seq in the fly intestine revealed upregulation of genes involved in redox control and proteostasis upon CRTC overexpression, supporting the axis’s role in stress adaptation.
• Disease Model Validation: In a Drosophila Huntington’s disease model, muscle-specific CRTC overexpression alleviated protein aggregation, improved motility, and extended lifespan, linking the axis to proteostasis restoration.
• Cross-Species Validation: MLN2238 treatment in 293T cells confirmed that JNK-mediated CREB phosphorylation is conserved in mammalian systems, suggesting translational relevance.

Protocol Parameters

• Proteasome inhibitor exposure: Administered to adult Drosophila via U-GLAD at concentrations sufficient to induce measurable proteotoxic stress; ensure compound solubility and stability according to product recommendations.
• Genetic manipulation: Use of UAS-Gal4 system to overexpress CRTC in specific tissues (e.g., muscle or intestine) to assess impact on proteostasis and stress resilience.
• ROS measurement and modulation: Employ ROS-sensitive dyes or genetic reporters, and use antioxidants or ROS inducers to dissect pathway dependencies.
• JNK pathway intervention: Pharmacological JNK inhibitors or RNAi-mediated knockdown to confirm pathway specificity for CREB activation.
• Phenotypic assays: Quantify protein aggregates with fluorescence markers in disease models; assess motor function and lifespan as readouts of proteostasis restoration.

Core Findings and Why They Matter

The authors show that proteasome β5 subunit inhibitors such as MLN2238 induce robust CREB activation in Drosophila, mediated by ROS and JNK signaling (Yin et al., 2022). Transcriptomic analysis identified redox and proteostasis genes as key targets downstream of CRTC/CREB. The functional impact was validated in a fly model of Huntington’s disease, where CRTC overexpression improved protein folding capacity, reduced aggregate burden, and ameliorated disease phenotypes. Importantly, CREB activation also increased during normal aging, and further enhancement suppressed age-associated protein aggregation in muscle. These findings position the CRTC-CREB axis as a conserved and actionable node for combating proteotoxic stress, with direct implications for diseases characterized by protein aggregation and for aging research.

Comparison with Existing Internal Articles

Several internal resources have explored the role of proteasome β5 subunit inhibitors, particularly MLN2238, in disease modeling and drug resistance studies. For example, one analysis highlights the integration of MLN2238 into workflows targeting multiple myeloma, lymphoma, and bortezomib-resistant cancers, emphasizing its mechanistic interplay with the ROS/JNK/CREB axis. Another guide (see here) discusses applied assay strategies, troubleshooting, and how the reference study’s insights into CREB-mediated stress responses have expanded the translational impact of proteasome inhibition. Collectively, these articles underscore the value of MLN2238 as a tool for dissecting proteotoxic stress responses and confirm that the CRTC-CREB axis discovery supports next-generation research in oncology and neurodegeneration.

Limitations and Transferability

While the study provides compelling evidence for the CRTC-CREB axis in Drosophila and cell lines, several limitations should be considered:

• Species differences: Although JNK-mediated CREB activation was demonstrated in 293T cells, the full physiological context in mammalian systems remains to be elucidated.
• Model specificity: The U-GLAD delivery system is optimized for Drosophila, and compound bioavailability or toxicity may differ in vertebrate models.
• Mechanistic complexity: The interplay between redox balance, proteasome inhibition, and transcriptional adaptation is multifaceted, and off-target effects of inhibitors or oxidative stress cannot be ruled out.
• Translational maturity: While the axis is implicated in stress adaptation and protein aggregation diseases, its therapeutic modulation in humans is still at a preclinical stage.

Research Support Resources

For researchers interested in modeling proteotoxic stress and investigating the CRTC-CREB axis, the use of validated proteasome β5 subunit inhibitors is essential. MLN2238 (SKU A4008) from APExBIO is a potent, reversible inhibitor with demonstrated activity in both hematologic malignancy models and proteotoxicity research. According to the product information, it enables precise chymotrypsin-like proteasome inhibition and is suitable for workflows exploring redox signaling, apoptosis, and drug resistance. Proper solubilization (preferably in DMSO or ethanol with warming and sonication) and storage protocols are recommended to maximize experimental reproducibility. This compound is intended for scientific research use only and not for diagnostic or medical applications.