CRTC-CREB Axis Activation by Proteasome Inhibitors: Mechanisms and Implications

Study Background and Research Question

The ubiquitin-proteasome system (UPS) is essential for regulated protein degradation, ensuring cellular proteostasis. Disruption of this pathway via proteasome inhibition leads to accumulation of misfolded proteins, triggering cellular stress responses that are particularly relevant in neurodegeneration and cancer. The cAMP response element-binding protein (CREB) is a conserved transcription factor regulating diverse cellular processes, including growth, metabolism, and stress responses. Its coactivator, CRTC (CREB-regulated transcriptional coactivator), further modulates CREB activity in response to cytosolic signaling changes. However, the molecular mechanisms that connect proteasome inhibition to CREB activity, and the role of this axis in handling proteotoxic stress, have remained unclear.

Key Innovation from the Reference Study

The reference study (Yin et al., 2022) provides a novel mechanistic link between proteasome inhibition and CREB-driven transcriptional adaptation. Through a large-scale compound screen in adult Drosophila, the authors unexpectedly found that all tested proteasome inhibitors, including MLN2238, robustly increased CREB activity. The study delineates a pathway where proteasome inhibition generates reactive oxygen species (ROS), which in turn activate c-Jun N-terminal kinase (JNK). This ROS/JNK signaling cascade is both necessary and sufficient to promote CREB phosphorylation and nuclear activity, ultimately driving transcriptional programs that mitigate proteotoxic stress.

Methods and Experimental Design Insights

The research leveraged a high-throughput in vivo screening approach using adult Drosophila and a sustainable compound delivery method (U-GLAD system) to overcome solubility challenges. CREB activity was monitored using established reporter assays. Key experimental strategies included:

• Compound Screening: FDA-approved proteasome inhibitors, notably MLN2238, were administered to adult flies. CREB activation was quantitatively assessed.
• Genetic Manipulation: Overexpression and loss-of-function mutants for CRTC and CREB were utilized to dissect their roles in stress response.
• ROS and JNK Pathway Analysis: Pharmacological and genetic tools were employed to modulate ROS levels and JNK activity, establishing causality between proteasome inhibition, ROS, JNK signaling, and CREB activation.
• Transcriptomic Profiling: RNA-seq analysis of fly intestines identified CREB/CRTC target genes upregulated during proteotoxic stress.
• Disease Model Application: The Drosophila Huntington’s disease (HD) model was used to assess the functional consequences of modulating CRTC-CREB activity on protein aggregation, motility, and lifespan.

Core Findings and Why They Matter

The study's major findings reveal a conserved adaptive mechanism in which the CRTC-CREB axis senses and counters proteotoxic stress:

• Proteasome inhibitors activate CREB in vivo: MLN2238 and other chymotrypsin-like proteasome β5 subunit inhibitors markedly increased CREB activity in adult flies (reference study).
• ROS/JNK pathway as mediator: Proteasome inhibition-induced ROS is both necessary and sufficient for JNK activation, which subsequently enhances CREB phosphorylation (Ser133 in mammalian systems, the functional equivalent in Drosophila).
• Transcriptional reprogramming: Transcriptome analysis identifies upregulation of genes involved in redox homeostasis and proteostasis upon CRTC overexpression, supporting the axis's protective role.
• Functional rescue in neurodegeneration: Muscle-specific overexpression of CRTC in a Drosophila HD model restores protein folding capacity, reduces aggregate burden, improves motility, and extends lifespan.
• Regulation during aging: CREB activity naturally rises with age in muscle tissues, and further enhancement suppresses age-associated protein aggregation.

Collectively, these findings highlight the CRTC-CREB axis as a transcriptional sensor that leverages redox and stress kinase signaling to restore proteome integrity during proteotoxic challenges. This mechanism is particularly relevant for understanding and potentially mitigating protein aggregation diseases, such as Huntington’s, and offers a basis for exploring similar strategies in other models of proteostasis failure.

Comparison with Existing Internal Articles

Recent internal articles have examined the molecular and translational aspects of MLN2238 as a reversible 20S proteasome β5 subunit inhibitor in oncology. For example, one review emphasized MLN2238's robust engagement of ROS/JNK/CREB stress signaling in bortezomib-resistant multiple myeloma and lymphoma models, paralleling the reference study's mechanistic insights. Another article (see here) explored advanced applications of MLN2238 for dissecting CREB signaling and apoptosis, reinforcing the translational bridge between fundamental signaling mechanisms and therapeutic targeting in hematologic malignancies. The current reference study extends these themes by providing direct in vivo evidence of the CRTC-CREB axis as a conserved sensor responding to proteasome inhibition, with implications that reach beyond cancer to aging and neurodegenerative disease models.

Limitations and Transferability

While the study robustly demonstrates the CRTC-CREB axis function in Drosophila, several limitations should be considered:

• Species Specificity: Although CREB and CRTC are highly conserved, the signaling dynamics and transcriptional targets may differ in mammalian systems. Direct extrapolation to human disease contexts requires further validation.
• Model Constraints: The Huntington’s disease model in flies recapitulates key aspects of protein aggregation, but does not fully capture the complexity of human neurodegeneration.
• Pharmacological Dosing: The concentrations and delivery methods for proteasome inhibitors in Drosophila may not directly translate to mammalian or clinical settings.
• Temporal Resolution: The study primarily assesses acute responses; the long-term adaptations and potential side effects of sustained CREB activation remain to be explored.

Despite these caveats, the work provides a compelling framework for investigating proteostasis and stress adaptation in a cross-species context, with potential for informing translational strategies in oncology, neurodegeneration, and aging research.

Protocol Parameters

• MLN2238 administration in Drosophila: Delivered via U-GLAD system, allowing for sustained compound exposure despite solubility challenges (see reference study).
• ROS modulation: Pharmacological ROS scavengers or genetic knockdown (e.g., SOD overexpression) used to dissect the requirement for oxidative signaling in CREB activation.
• JNK pathway manipulation: Utilized inhibitors or RNAi-mediated knockdown to confirm dependency of CREB activation on JNK function.
• Reporter assay for CREB activity: CREB-driven luciferase or GFP reporters quantified to monitor transcriptional response.
• Assessment of proteotoxic phenotypes: Protein aggregation visualized by immunofluorescence; motility and lifespan evaluated in disease models.

Research Support Resources

Researchers interested in investigating proteasome inhibition, CREB signaling, or proteotoxic stress responses can utilize MLN2238 (SKU A4008), a dipeptidyl boronic acid derivative that reversibly inhibits the β5 subunit of the 20S proteasome with nanomolar potency, as reported in the product information. MLN2238 is suitable for workflows studying chymotrypsin-like proteasome inhibition, apoptosis induction, and stress signaling in preclinical models, including those relevant to multiple myeloma, lymphoma, or neurodegenerative disease research. For further technical guidance, see related protocol and troubleshooting insights as discussed in internal articles, such as this review.