Adaptor FADD is recruited by RTN3/HAP in ER-bound signaling complexes

The diagnostic or prognostic values of FADD in cancers based on pan‑cancer analysis

Previous studies have determined that aberrant expression of the fas-associated death domain (FADD) contributes to the development of cancer. However, no pan-cancer analysis has been reported to explore the relationship between FADD and various cancers. Multiple databases were screened to identify cancer datasets for the present study and to validate the expression of FADD in various tumors. The association of FADD alteration with cancer prognosis, clinical features and tumor immunity was also evaluated. Reverse transcription-quantitative PCR (RT-qPCR) was utilized to confirm the expression of FADD in breast, colon, liver and gastric cancer cells. Analysis of Gene Expression Omnibus database and The Cancer Genome Atlas database indicated that FADD was highly expressed in breast invasive carcinoma (BRCA), cervical squamous cell carcinoma and endocervical adenocarcinoma, cholangiocarcinoma, colon adenocarcinoma (COAD), esophageal carcinoma (ESCA), kidney renal clear cell carcinoma, kidney renal papillary cell carcinoma, liver hepatocellular carcinoma (LIHC), lung adenocarcinoma (LUAD) and prostate adenocarcinoma, whereas RT-qPCR results revealed that FADD was highly expressed in breast cancer and colon cancer. Further analyses demonstrated that FADD expression was significantly altered in ESCA, head and neck squamous cell carcinoma (HNSC), lung squamous cell carcinoma and BRCA. FADD expression was observed to be a risk factor of the overall survival in patients with HNSC, LIHC and LUAD as demonstrated by Kaplan-Meier and Cox regression analyses. The results of the present study demonstrated that FADD is highly expressed in numerous malignancies and can be utilized as a biomarker for the diagnosis of BRCA, COAD, LIHC and stomach adenocarcinoma. Moreover, FADD expression is a predictive risk factor for the development of HNSC, LIHC and LUAD and can potentially be used as a prognostic marker for these cancers.

Study on the protective effect of RNA-binding motif protein 3 in mild hypothermia oxygen-glucose deprivation/reoxygenation cell model

Mild hypothermia is proven neuroprotective in clinical practice. While hypothermia leads to the decrease of global protein synthesis rate, it upregulates a small subset of protein including RNA-binding motif protein 3 (RBM3). In this study, we treated mouse neuroblastoma cells (N2a) with mild hypothermia before oxygen-glucose deprivation/reoxygenation (OGD/R) and discovered the decrease of apoptosis rate, down-regulation of apoptosis-associated protein and enhancement of cell viability. Overexpression of RBM3 via plasmid exerted similar effect while silencing RBM3 by siRNAs partially reversed the protective effect exerted by mild hypothermia pretreatment. The protein level of Reticulon 3(RTN3), a downstream gene of RBM3, also increased after mild hypothermia pretreatment. Silencing RTN3 weakened the protective effect of mild hypothermia pretreatment or RBM3 overexpression. Also, the protein level of autophagy gene LC3B increased after OGD/R or RBM3 overexpression while silencing RTN3 decreased this trend. Furthermore, immunofluorescence observed enhanced fluorescence signal of LC3B and RTN3 as well as a large number of overlaps after RBM3 overexpressing. In conclusion, RBM3 plays a cellular protective role by regulating apoptosis and viability via its downstream gene RTN3 in the hypothermia OGD/R cell model and autophagy may participate in it.

Reticulon 3-mediated Chk2/p53 activation suppresses hepatocellular carcinogenesis and is blocked by hepatitis B virus

OBJECTIVE

Dysfunction of endoplasmic reticulum (ER) proteins is closely related to homeostasis disturbance and malignant transformation of hepatocellular carcinoma (HCC). Reticulons (RTN) are a family of ER-resident proteins critical for maintaining ER function. Nevertheless, the precise roles of RTN in HCC remain largely unclear. The aim of the study is to examine the effect of reticulon family member RTN3 on HCC development and explore the underlying mechanisms.

DESIGN

Clinical HCC samples were collected to assess the relationship between RTN3 expression and patients' outcome. HCC cell lines were employed to examine the effects of RTN3 on cellular proliferation, apoptosis and signal transduction in vitro. Nude mice model was used to detect the role of RTN3 in modulating tumour growth in vivo.

RESULTS

We found that RTN3 was highly expressed in normal hepatocytes but frequently downregulated in HCC. Low RTN3 expression predicted poor outcome in patients with HCC in TP53 gene mutation and HBV infection status-dependent manner. RTN3 restrained HCC growth and induced apoptosis by activating p53. Mechanism studies indicated that RTN3 facilitated p53 Ser392 phosphorylation via Chk2 and enhanced subsequent p53 nuclear localisation. RTN3 interacted with Chk2, recruited it to ER and promoted its activation in an ER calcium-dependent manner. Nevertheless, the tumour suppressive effects of RTN3 were abrogated in HBV-positive cells. HBV surface antigen competed with Chk2 for RTN3 binding and blocked RTN3-mediated Chk2/p53 activation.

CONCLUSION

The findings suggest that RTN3 functions as a novel suppressor of HCC by activating Chk2/p53 pathway and provide more clues to better understand the oncogenic effects of HBV.

The endogenous caspase-8 inhibitor c-FLIPL regulates ER morphology and crosstalk with mitochondria

Components of the death receptor-mediated pathways like caspase-8 have been identified in complexes at intracellular membranes to spatially restrict the processing of local targets. In this study, we report that the long isoform of the cellular FLICE-inhibitory protein (c-FLIP(L)), a well-known inhibitor of the extrinsic cell death initiator caspase-8, localizes at the endoplasmic reticulum (ER) and mitochondria-associated membranes (MAMs). ER morphology was disrupted and ER Ca(2+)-release as well as ER-mitochondria tethering was decreased in c-FLIP(-/-) mouse embryonic fibroblasts (MEFs). Mechanistically, c-FLIP ablation resulted in enhanced basal caspase-8 activation and in caspase-mediated processing of the ER-shaping protein reticulon-4 (RTN4) that was corrected by re-introduction of c-FLIP(L) and caspase inhibition, resulting in the recovery of a normal ER morphology and ER-mitochondria juxtaposition. Thus, the caspase-8 inhibitor c-FLIP(L) emerges as a component of the MAMs signaling platforms, where caspases appear to regulate ER morphology and ER-mitochondria crosstalk by impinging on ER-shaping proteins like the RTN4.

The potential role of RTN3 in monocyte recruitment and atherosclerosis

The recruitment of monocytes to arterial wall and their transformation into macrophages are generally accepted as important early events in the pathogenesis of atherosclerosis (AS). Our research group found Reticulon3 (RTN3), a member of the reticulon family, may be a candidate pathogenic element in the progress of AS. But it is virtually unknown in which process RTN3 may participate in and regulate the pathogenesis of AS. Here, we hypothesis that RTN3 may participate in the continuous process of circulating monocyte recruitment in AS including: (1) monocyte spreading and adhesion to luminal endothelium; (2) transendothelial migration and may also contribute to the conversion of monocyte to macrophage in subendothelium.

RTN3 inducing apoptosis is modulated by an adhesion protein CRELD1

Reticulon3 (RTN3), as a member of the reticulon family, is generally regarded as a novel human apoptosis-inducing protein. But the extensional role of RTN3 remains virtually unknown. Herein, we showed that cysteine rich with EGF like domains 1(CRELD1), a cell adhesion molecule played a critical role in atrioventricular septal defects and it had mutual effect with RTN3 in vitro. Furthermore, we discovered that ectopic CRELD1 could interact with ectopic or endogenous RTN3. CRELD1 bound with RTN3 so as to increase the localization of RTN3 on the plasma membrane and decreased the apoptotic activity of RTN3 moderately. Moreover, the tunicamycin-inducing cell apoptosis was partly suppressed by this kind of interaction mentioned above. These results suggested that CRELD1 could partly change the localization of RTN3 from the endoplasmic reticulum to the plasma membrane and modulate the apoptotic activity of RTN3 through binding with it.

Reticulon 3 mediates Bcl-2 accumulation in mitochondria in response to endoplasmic reticulum stress

Reticulon3 (RTN3), firstly isolated from the retina and widely expressed in human tissues with the highest expression in the brain, is presumed to play an important role in the developing axons through the transport of liquids and proteins. We have identified and characterized RTN3 as a RTN4B/ASY interaction protein. Here we demonstrated that ER-stress activated RTN3 expression. CHOP and ATF6 were sufficient to up-regulate the expression of RTN3. The down-regulation of RTN3 would induce apoptosis and attenuate the anti-apoptotic activity of Bcl-2, indicating RTN3 was required for the cellular survival and optimal anti-apoptotic activity of Bcl-2. Our present studies also indicated ER-stress induced RTN3 up-regulation could trigger Bcl-2 translocation from ER to mitochondria. Moreover, the previous studies showed that RTN4B was also a Bcl-2-interacted protein. We found that RTN3 and RTN4B could block the access of Bcl-2 to each other and thereafter determined the Bcl-2 subcellular distribution. Taken together, our findings indicate that RTN3 is directly involved in the ER-constituents trafficking events through dually acting as an essential and important ER-stress sensor, and a trigger for the Bcl-2 translocation.