BaxΔ2 sensitizes colorectal cancer cells to proteasome inhibitor-induced cell death

Exploring the Mechanisms, Biomarkers, and Therapeutic Targets of TRIM Family in Gastrointestinal Cancer

Gastrointestinal region (GI) cancers are closely linked to the ubiquitination system, with the E3 ubiquitin ligase playing a crucial role by targeting various substrates. As E3 ubiquitin ligases, proteins of tripartite motif (TRIM) family play a role in cancer signaling, development, apoptosis, and formation. These proteins regulate diverse biological activities and signaling pathways. This study comprehensively outlines the functions of TRIM proteins in gastrointestinal physiology, contributing to our knowledge of the molecular pathways involved in gastrointestinal tumors. Gastrointestinal region (GI) cancers are closely linked to the ubiquitination system, with the E3 ubiquitin ligase playing a crucial role by targeting various substrates.

Alternative production of pro-death Bax∆2 protein via ribosomal frameshift in Alzheimer's disease

Pro-death Bax family member, Bax∆2, forms protein aggregates in Alzheimer's disease neurons and causes stress granule-mediated neuronal death. Production of Bax∆2 originated from two events: alternative splicing of Bax exon 2 and a microsatellite mutation (a deletion from poly guanines, G8 to G7). Each event alone leads to a reading frameshift and premature termination. While Bax exon 2 alternative splicing is common in Alzheimer's brains, the G7 mutation is not, which is inconsistent with the high Bax∆2 protein levels detected in clinical samples. Here, we report an alternative mechanism to produce Bax∆2 protein in the absence of the G7 mutation. Using dual-tag systems, we showed that a ribosomal frameshift (RFS) can compensate the lack of G7 mutation in translating Bax∆2 protein. Intriguingly, the microsatellite poly G repeat is neither essential nor the site for the RFS. However, disruption of the poly G sequence impaired the RFS, potentially due to alteration of the local RNA structure. Using immunoprecipitation-mass spectrometry, we pinpointed the RFS site at 15 base pairs upstream of the microsatellite. These results uncover an alternative mechanism for generating functional Bax∆2-subfamily isoforms, highlighting the production plasticity of Bax family isoforms and unveiling potential new therapeutic targets for Alzheimer's disease.

Unconventional Source of Neurotoxic Protein Aggregation from Organelle Off-Target Bax∆2 in Alzheimer's Disease

Protein aggregates are a hallmark of Alzheimer's disease (AD). Extensive studies have focused on β-amyloid plaques and Tau tangles. Here, we illustrate a novel source of protein aggregates in AD neurons from organelle off-target proteins. Bax is a mitochondrial pore-forming pro-death protein. What happens to Bax if it fails to target mitochondria? We previously showed that a mitochondrial target-deficient alternatively spliced variant, Bax∆2, formed large cytosolic protein aggregates and triggered caspase 8-mediated cell death. Bax∆2 protein levels were low in most normal organs and the proteins were quickly degraded in cancer. Here, we found that 85% of AD patients had Bax∆2 required alternative splicing. Increased Bax∆2 proteins were mostly accumulated in neurons of AD-susceptible brain regions. Intracellularly, Bax∆2 aggregates distributed independently of Tau tangles. Interestingly, Bax∆2 aggregates triggered the formation of stress granules (SGs), a large protein-RNA complex involved in AD pathogenesis. Although the functional domains required for aggregation and cell death are the same as in cancer cells, Bax∆2 relied on SGs, not caspase 8, for neuronal cell death. These results imply that the aggregation of organelle off-target proteins, such as Bax∆2, broadens the scope of traditional AD pathogenic proteins that contribute to the neuronal stress responses and AD pathogenesis.

The anti-tumor effect of proteasome inhibitor MG132 for human adenoid cystic carcinoma: correlate with the emerging role of Nrf2/Keap1 signaling pathway

BACKGROUND

Adenoid cystic carcinoma (ACC) is one of the most common malignant salivary gland tumors. Moreover, the unique biological characteristics and complex structures of ACC contribute to its poor survival rates. Recently, proteasome inhibitors have been shown to elicit satisfactory therapeutic effects in the treatment of certain solid tumors, but few studies have been implemented to investigate the effects of proteasome inhibitor therapy for ACC.

METHODS

In this present study, cell counting kit-8 assay and flow cytometry assay were performed to examine the effects of proteasome inhibitor (MG132) on cell viability and apoptosis. We applied western blot and immunofluorescence staining to explore the expression of the Nrf2/Keap1 signaling pathway and P62, additionally Nrf2 inhibitor (ML385) was utilized to evaluate the role of Nrf2/Keap1 signaling pathway in MG132-induced cell apoptosis.

RESULTS

Our data indicated that MG132 significantly suppressed the growth of ACC-83 cells(MG132 10µM P = 0.0046; 40µM P = 0.0033; 70µM P = 0.0007 versus control) and induced apoptosis (MG132 10µM P = 0.0458; 40µM P = 0.0018; 70µM P = 0.0087 versus control). The application of MG132 induced the up-regulation of Nrf2/Keap1 signaling pathway. Furthermore, inhibition of Nrf2 attenuated the therapeutic effects of MG132 for ACC (both ML385 + MG132 10µM P = 0.0013; 40µM P = 0.0057; 70µM P = 0.0003 versus MG132). P < 0.05 was considered statistically significant.

CONCLUSIONS

Our results revealed that proteasome inhibitors MG132 could inhibit the cell viability and induce the apoptosis of ACC through Nrf2/Keap1 signaling pathway.

Indocyanine green loaded pH-responsive bortezomib supramolecular hydrogel for synergistic chemo-photothermal/photodynamic colorectal cancer therapy

Colorectal cancer is with high incidence worlwide.. Because of the heterogeneity of the tumor, combination therapy is probably of great significance to improve the prognosis of colorectal cancer patients. Herein, the pH-responsive supramolecular hydrogels mPEG-luteolin-BTZ@ICG based on bortezomib (BTZ) and indocyanine green (ICG) were prepared, and the colorectal cancer was treated with mPEG-luteolin-BTZ@ICG through the combination of photothermal/photodynamic and chemotherapy. BTZ performed drug therapy, meanwhile ICG wrapped in supramolecular hydrogels possessed higher light stability than free ICG to perform photothermal/photodynamic therapy. In vitro and in vivo assays showed excellent inhibition of tumor cells due to the combined effect of BTZ and ICG. The mPEG-luteolin-BTZ@ICG combined with laser therapy possessed exceptional biological safety and provided new candidates for advanced colon cancer therapy and important references for other tumor therapies.

Alternative splicing of mRNA in colorectal cancer: new strategies for tumor diagnosis and treatment

Alternative splicing (AS) is an important event that contributes to posttranscriptional gene regulation. This process leads to several mature transcript variants with diverse physiological functions. Indeed, disruption of various aspects of this multistep process, such as cis- or trans- factor alteration, promotes the progression of colorectal cancer. Therefore, targeting some specific processes of AS may be an effective therapeutic strategy for treating cancer. Here, we provide an overview of the AS events related to colorectal cancer based on research done in the past 5 years. We focus on the mechanisms and functions of variant products of AS that are relevant to malignant hallmarks, with an emphasis on variants with clinical significance. In addition, novel strategies for exploiting the therapeutic value of AS events are discussed.

A Structural Model for Bax∆2-Mediated Activation of Caspase 8-Dependent Apoptosis

Bax∆2 is a pro-apoptotic anti-tumor protein in the Bax family. While most of the Bax family causes cell death by targeting mitochondria, Bax∆2 forms cytosolic aggregates and activates caspase 8-dependent cell death. We previously showed that the Bax∆2 helix α9 is critical for caspase 8 recruitment. However, the interaction between these two proteins at the structural level is unknown. In this in silico study, we performed molecular dynamics (MD) simulations and protein-protein docking on Bax∆2 variants. The results suggest that the Bax∆2 variants have different stable states. Mutating the Baxα mitochondria-targeting signal [L26P/L27P] appears to introduce a kink into helix α1. Protein-protein docking suggests that helices α9 of both wild-type Bax∆2 and Bax∆2 caspase 8 binding-deficient mutant [L164P] can fit in the same caspase 8 binding site, but the mutant is unable to fit as well as wild-type Bax∆2. Together, these data point to a structural basis for explaining Bax∆2 function in caspase 8-dependent cell death.

Immunohistochemical detection of the pro-apoptotic Bax∆2 protein in human tissues

The pro-apoptotic Bax isoform Bax∆2 was originally discovered in cancer patients with a microsatellite guanine deletion (G8 to G7). This deletion leads to an early stop codon; however, when combined with the alternative splicing of exon 2, the reading frame is restored allowing production of a full-length protein (Bax∆2). Unlike the parental Baxα, Bax∆2 triggers apoptosis through a non-mitochondrial pathway and the expression in human tissues was unknown. Here, we analyzed over 1000 tissue microarray samples from 13 different organs using immunohistochemistry. Bax∆2-positive cells were detected in all examined organs at low rates (1-5%) and mainly scattered throughout the connective tissues. Surprisingly, over 70% of normal colon samples scored high for BaxΔ2-positive staining. Only 7% of malignant colon samples scored high, with most high-grade tumors being negative. A similar pattern was observed in most organs examined. We also showed that both Baxα and Bax∆2 can co-exist in the same cells. Genotyping showed that the majority of Bax∆2-positive normal tissues contain no G7 mutation, but an unexpected high rate of G9 was observed. Although the underlying mechanism remains to be explored, the inverse correlation of Bax∆2 expression with tissue malignancy suggests that it may have a clinical implication in cancer development and treatment.

Detection of pro-apoptotic Bax∆2 proteins in the human cerebellum

Bax∆2 is a pro-apoptotic protein originally discovered in colon cancer patients with high microsatellite instability. Unlike most pro-apoptotic Bax family members, Bax∆2 mediates cell death through a non-mitochondrial caspase 8-dependent pathway. In the scope of analyzing the distribution of Bax∆2 expression in human tissues, we examined a panel of human brain samples. Here, we report four cerebellar cases in which the subjects had no neurological disorder or disease documented. We found Bax∆2 positive cells scattered in all areas of the cerebellum, but most strikingly concentrated in Purkinje cell bodies and dendrites. Two out the four subjects tested had strong Bax∆2-positive staining in nearly all Purkinje cells; one was mainly negative; and one had various levels of positive staining within the same sample. Further genetic analysis of the Purkinje cell layer, collected by microdissection from two subjects, showed that the samples contained G7 and G9 Bax microsatellite mutations. Both subjects were young and had no diseases reported at the time of death. As the distribution of Bax∆2 is consistent with that known for Baxα, but in a less ubiquitous manner, these results may imply a potential function of Bax∆2 in Purkinje cells.