Proteasome Inhibition and the CRTC-CREB Axis: Mechanistic Insights from Drosophila

Study Background and Research Question

The ubiquitin-proteasome system (UPS) is essential for regulated protein degradation and cellular homeostasis. Disruption of this system, particularly via selective inhibition of the proteasome β5 subunit, has significant implications for oncology and neurodegenerative disease research. The cAMP response element-binding protein (CREB) is a conserved transcription factor implicated in cell survival, synaptic plasticity, and metabolic regulation. Previous work established the role of CREB and its co-activator CRTC (CREB-regulated transcriptional coactivator) in various stress responses; however, the precise mechanistic connection linking proteasome inhibition, CREB activation, and cellular adaptation to proteotoxic stress remained unclear.

The central question addressed by the reference study was: How do proteasome inhibitors influence CREB-mediated transcriptional responses in vivo, and what are the underlying signaling mechanisms in Drosophila?

Key Innovation from the Reference Study

The study presents a novel finding: proteasome inhibitors such as MLN2238 robustly activate the CRTC-CREB transcriptional axis in adult Drosophila. This activation is mediated via reactive oxygen species (ROS) generation and subsequent c-Jun N-terminal kinase (JNK) signaling. The research defines CRTC/CREB as a transcriptional sensor capable of detecting and responding to proteotoxic and oxidative stress, highlighting its conserved role in protein quality control and cellular defense. The authors also demonstrate that boosting CRTC/CREB activity can alleviate protein aggregation and related phenotypes in a fly Huntington’s disease (HD) model, suggesting broader implications for neurodegeneration and aging research.

Methods and Experimental Design Insights

To systematically identify compounds that modulate CREB activity in vivo, the researchers utilized a large-scale chemical screening approach in adult flies. They overcame previous solubility and delivery limitations by employing the U-GLAD (U shape Gum Arabic Liquid Assisted Drug delivery) system, enabling consistent administration of a diverse compound library, including FDA-approved proteasome inhibitors such as MLN2238. CREB transcriptional activity was monitored using a CRE-luciferase reporter system in whole animals.

Further mechanistic interrogation involved genetic manipulations and chemical perturbations to dissect the signaling cascade downstream of proteasome inhibition. ROS involvement was confirmed using antioxidant treatments and by measuring oxidative stress markers. JNK signaling was evaluated through both pharmacological inhibition and genetic knockdown. Transcriptomic profiling (RNA-seq) in the Drosophila intestine was conducted to identify CRTC/CREB-regulated gene networks. In the context of neurodegeneration, the team utilized a Drosophila Huntington’s disease model characterized by polyglutamine-expanded huntingtin, assessing the impact of CRTC overexpression on protein aggregation, motility, and lifespan.

Core Findings and Why They Matter

Several key findings emerged from the study:

• Proteasome inhibitors activate CREB in vivo: Administration of MLN2238 and related compounds led to a pronounced increase in CREB activity in adult flies, as measured by CRE-luciferase output. This establishes a direct link between chymotrypsin-like proteasome inhibition and transcriptional adaptation.
• ROS/JNK signaling underpins CREB activation: Proteasome inhibition induced substantial ROS production, which was both necessary and sufficient for CREB activation. JNK, a stress-activated MAPK, was identified as the critical signaling intermediary. In human 293T cells, MLN2238 treatment also enhanced CREB phosphorylation at Ser133 via JNK, underscoring the evolutionary conservation of this pathway.
• CRTC/CREB regulates a redox-proteostasis gene network: Transcriptome analysis revealed that overexpression of CRTC in fly intestines upregulated genes involved in oxidative stress response and protein folding, suggesting that the CRTC-CREB axis orchestrates a multifaceted defense against proteotoxicity.
• Therapeutic relevance in models of neurodegeneration and aging: Muscle-specific CRTC overexpression in Drosophila HD models restored proteasome activity, reduced protein aggregates, improved locomotor function, and extended lifespan. Furthermore, age-related increases in CREB activity were observed, and further augmenting this pathway suppressed protein aggregation in aged muscles. These results implicate the CRTC-CREB axis as a potential therapeutic node in protein aggregation diseases.

Together, these findings provide a mechanistic framework linking proteasome β5 subunit inhibition to adaptive transcriptional responses via ROS/JNK/CREB signaling. They highlight a conserved cellular program that senses and mitigates proteotoxic stress, with relevance for studies in oncology, neurodegeneration, and redox biology.

Comparison with Existing Internal Articles

Several internal resources provide complementary perspectives and workflow guidance for researchers exploring proteasome inhibition and redox stress:

• The article CRTC-CREB Axis Senses Proteasome Inhibition via ROS/JNK in Drosophila closely parallels the reference study, underscoring the role of MLN2238 and related compounds in activating CREB-dependent transcription through redox signaling in invertebrate models.
• MLN2238: Precision Proteasome β5 Inhibition in Redox Stress Research expands on the translational potential of MLN2238 for dissecting protein homeostasis mechanisms, particularly in the context of multiple myeloma research and oxidative stress adaptation.
• Applied workflow guides such as MLN2238 as a Proteasome β5 Subunit Inhibitor: Applied Workflows and Optimizing Hematologic Assays with MLN2238 offer practical assay designs and troubleshooting strategies for using MLN2238 in cell-based studies, including applications in bortezomib-resistant lymphoma and multiple myeloma lines.

These resources collectively reinforce the reference study’s mechanistic insights and provide actionable protocols for researchers interested in leveraging MLN2238 as a tool for proteostasis and redox signaling research.

Limitations and Transferability

While the findings in Drosophila and 293T cells robustly support the CRTC-CREB axis as a sensor for proteasome inhibition-induced stress, several limitations should be considered:

• Species differences may impact the direct translatability of these mechanisms to mammalian systems, especially in the context of complex tissue architecture and immune responses.
• The study primarily used genetic overexpression strategies to boost CRTC/CREB function; pharmacological modulation in mammalian models warrants further validation.
• The focus on acute and subacute stress responses leaves open questions about the long-term consequences of sustained CREB activation, particularly in oncologic versus neurodegenerative disease settings.

Nonetheless, the conserved nature of the ROS/JNK/CREB pathway and its responsiveness to proteasome β5 subunit inhibitors such as MLN2238 suggest broad relevance across research domains.

Protocol Parameters

• Compound delivery in Drosophila: The U-GLAD system enabled efficient oral administration of poorly soluble compounds like MLN2238 in adult flies; this may inspire analogous approaches in other model organisms.
• CRE-luciferase reporter assay: Quantitative measurement of CREB activity was achieved via whole-animal or tissue-specific luciferase readouts after compound exposure.
• ROS/JNK pathway interrogation: Use of antioxidants (e.g., N-acetylcysteine) and JNK inhibitors, as well as genetic knockdowns, distinguished pathway dependencies.
• Transcriptomic profiling: RNA-seq performed on dissected intestines after CRTC overexpression provided gene network insights relevant to redox and proteostasis regulation.
• Disease modeling: Polyglutamine-expanded huntingtin transgenic flies were used to assess the impact of CRTC/CREB modulation on protein aggregation and lifespan.
• For in vitro studies: MLN2238 is recommended to be dissolved in DMSO or ethanol, with ultrasonic treatment and warming to 37°C improving solubility, as the product information indicates optimal stock storage at -20°C and avoidance of long-term solution storage.

Research Support Resources

Researchers interested in investigating chymotrypsin-like proteasome inhibition, CRTC-CREB signaling, or redox/proteostasis adaptation can employ MLN2238 (SKU A4008), a reversible proteasome β5 subunit inhibitor with demonstrated utility in both oncology models and redox stress research. For robust assay development and troubleshooting, APExBIO provides detailed solubility guidance and application notes. Integrating these resources supports advanced workflows in multiple myeloma, lymphoma, and neurodegeneration studies.