BCL-XL directly retrotranslocates the monomeric BAK

Live-cell imaging analysis on the anti-apoptotic function of the Bcl-xL transmembrane carboxyl terminal domain

The Transmembrane Carboxyl Terminal Domain (TMD) of some Bcl-2 family proteins has been demonstrated to play a key role in modulating apoptosis. We here ustilzed live-cell fluorescence imaging to evaluate how the Bcl-xL TMD (XT) regulate apoptosis. Cell viability assay revealed that XT had strong anti-apoptotic ability similarly to the full-length Bcl-xL. Fluorescence images of living cells co-expressing CFP-XT and Bad-YFP or YFP-Bax revealed that XT recruited Bad to mitochondria but prevented Bax translocation to mitochondria, and also significantly suppressed Bad/Bax-mediated apoptosis, indicating that XT prevents the pro-apoptotic function of Bad and Bax. Fluorescence Resonance Energy Transfer (FRET) analyses determined that XT directly interacted with Bad and Bax, and deletion of XT completely eliminated the mitochondrial localization and homo-oligomerization of Bcl-xL. Fluorescence images of living cells co-expressing CFP-XT and YFP-Bax revealed that XT significantly prevented mitochondrial Bax oligomerization, resulting in cytosolic Bax distribution. Collectively, XT is necessary for the mitochondrial localization and anti-apoptotic capacity of Bcl-xL, and XT, similarly to the full-length Bcl-xL, forms homo-oligomers on mitochondria to directly interact with Bad and Bax to inhibit their apoptotic functions.

Asiaticoside Prevents Oxidative Stress and Apoptosis in Endothelial Cells by Activating ROS-dependent p53/Bcl-2/Caspase-3 Signaling Pathway

BACKGROUND

Asiaticoside (AC) is a triterpenoid saponin found in Centella asiatica (L.) urban extract that has a wide range of pharmacological properties. Our previous study demonstrated that AC could promote angiogenesis in diabetic wounds, but the specific mechanisms remain unknown.

OBJECTIVE

This study aimed to examine the effectiveness and mechanism of AC on human umbilical vein endothelial cells (HUVECs) exposed to tert-butyl hydroperoxide (t-BHP) toxicity.

METHODS

Senescence was confirmed using senescence-associated betagalactosidase (SA-β-gal) activity and expression of the cell cycle phase markers p16 and p21. The levels of SOD, NO, MDA, GSH-Px, and ROS were tested. Furthermore, several cell death-related genes and proteins (p53, Bax, Bcl-2 and Caspase-3) were assessed with RT-qPCR and Western blotting.

RESULTS

AC significantly reduced SA-β-gal activity, with both the suppression of cellcycle inhibitors p16 and p21. We also found that the induced oxidative stress and apoptosis caused by t-BHP treatment resulted in the decrease of antioxidant enzymes activities, the surge of ROS and MDA, the up-regulation of p53, Bax and caspase-3, and the decrease of SOD, NO, GSH-Px and Bcl-2. These biochemical changes were all reversed by treatment with varying doses of AC.

CONCLUSION

AC alleviates t-BHP-induced oxidative injury and apoptosis in HUVECs through the ROS-dependent p53/Bcl-2/Caspase-3 signaling pathway. It may be a potential antioxidant applied in metabolic disorders and pharmaceutical products.

Bim- and Bax-mediated mitochondrial pathway dominates abivertinib-induced apoptosis and ferroptosis

Abivertinib (AC) is a novel epidermal growth factor receptor tyrosine kinase inhibitor with highly efficient antitumor activity. Here, we report the capacity of AC to induce both reactive oxygen species (ROS)-dependent apoptosis and ferroptosis in tumor cells. Our data showed that AC induced iron- and ROS-dependent cytotoxicity in MCF7, HeLa, and A549 cell lines. Flow cytometry analyses showed that AC increased ferrous ions and ROS and induced ferroptosis in MCF-7 cells. This was confirmed by the findings that AC not only decreased solute carrier family 7 member 11 (SLC7A11) and glutathione peroxidase 4 (GPX4) expression but also induced iron- and ROS-dependent aggrandized lipid ROS accumulation and plasma membrane damage. Meanwhile, AC induced nuclear condensation and increased ROS-dependent phosphatidylserine (PS) eversion, caspase-3 activation, and cleaved-PARP expression, suggesting that AC also induced ROS-dependent apoptosis. In addition, mitochondrial depletion significantly inhibited AC-induced cytotoxicity, including ferroptosis and apoptosis, indicating the key role of mitochondria in AC-induced ferroptosis and apoptosis. Moreover, knockout of Bim or Bax not only remarkably inhibited AC-induced apoptosis, but also markedly inhibited AC-triggered downregulation of SLC711 and GPX4, accumulation of lipid ROS, and damage to the plasma membrane. This suggests that Bim and Bax act upstream of SLC7A11 and GPX4 to mediate AC-induced ferroptosis. Collectively, AC induces ferroptosis and apoptosis, in which the Bim- and Bax-mediated mitochondrial pathways play a dominant role.

Bcl-xL Reduces Chinese Giant Salamander Iridovirus-Induced Mitochondrial Apoptosis by Interacting with Bak and Inhibiting the p53 Pathway

Chinese giant salamander iridovirus (GSIV) infection could lead to mitochondrial apoptosis in this animal, a process that involves B-cell lymphoma-2 (BCL-2) superfamily molecules. The mRNA expression level of Bcl-xL, a crucial antiapoptotic molecule in the BCL-2 family, was reduced in early infection and increased in late infection. However, the molecular mechanism remains unknown. In this study, the function and regulatory mechanisms of Chinese giant salamander (Andrias davidianus) Bcl-xL (AdBcl-xL) during GSIV infection were investigated. Western blotting assays revealed that the level of Bcl-xL protein was downregulated markedly as the infection progressed. Plasmids expressing AdBcl-xL or AdBcl-xL short interfering RNAs were separately constructed and transfected into Chinese giant salamander muscle cells. Confocal microscopy showed that overexpressed AdBcl-xL was translocated to the mitochondria after infection with GSIV. Additionally, flow cytometry analysis demonstrated that apoptotic progress was reduced in both AdBcl-xL-overexpressing cells compared with those in the control, while apoptotic progress was enhanced in cells silenced for AdBcl-xL. A lower number of copies of virus major capsid protein genes and a reduced protein synthesis were confirmed in AdBcl-xL-overexpressing cells. Moreover, AdBcl-xL could bind directly to the proapoptotic molecule AdBak with or without GSIV infection. In addition, the p53 level was inhibited and the mRNA expression levels of crucial regulatory molecules in the p53 pathway were regulated in AdBcl-xL-overexpressing cells during GSIV infection. These results suggest that AdBcl-xL plays negative roles in GSIV-induced mitochondrial apoptosis and virus replication by binding to AdBak and inhibiting p53 activation.

Evaluating the inhibitory priority of Bcl-xL to Bad, tBid and Bax by using live-cell imaging assay

Molecular regulatory network among the B cell leukemia-2 (Bcl-2) family proteins is a research hotspot on apoptosis. The inhibitory priority of anti-apoptotic Bcl-2 family proteins (such as Bcl-xL) to pro-apoptotic Bcl-2 family proteins (such as Bad, tBid and Bax) determines the outcome of their interactions. Based on over-expression model system, we here evaluate the inhibitory priority of Bcl-xL to Bad, tBid and Bax by using live-cell imaging assay on cell viability. Fluorescence images of living cells co-expressing CFP-Bcl-xL and YFP-Bad or YFP-tBid or YFP-Bax showed that Bcl-xL markedly inhibited Bad/tBid/Bax-mediated apoptosis, revealing that Bcl-xL inhibits the proapoptotic function of Bad, tBid and Bax. In the case of equimolar co-expression of Bad and CFP-Bcl-xL, the inhibition of Bcl-xL on tBid/Bax mediate-apoptosis was completely relieved. Moreover, co-expression of tBid-P2A-CFP-Bcl-xL significantly relieved the inhibition of Bcl-xL on the pro-apoptotic ability Bax, suggesting that Bcl-xL preferentially inhibits the pro-apoptotic ability of Bad over tBid, subsequently to Bax.

Co-expression of Mcl-1 and Bak induces mitochondrial swelling

We here used fluorescence imaging to explore the effect of co-overexpression of Mcl-1 and Bak/BH3-only proteins on mitochondrial morphology. The cells co-expressing CFP-Mcl-1 and YFP-Bak/BimL/Puma/tBid showed co-localization of Mcl-1 with Bak/Puma/BimL/tBid and also showed the inhibitory action of Mcl-1 on the Bak-, BimL-, Puma- or tBid-mediated cell death. Co-expression of Mcl-1 and Bak but not BH3-only proteins induced time-dependent mitochondrial swelling. Fluorescence resonance energy transfer (FRET) imaging proved the direct binding of Mcl-1 to Bak, BimL, Puma and tBid, respectively. In addition, Mcl-1 prevented Bak oligomerization by retrotranslocating Bak from mitochondria into cytoplasm. Moreover, Mcl-1-Bak complex exhibited a good co-localization with mitochondria, and co-expression of Mcl-1 and Bak for more than 24 h not only induced mitochondrial swelling but also impaired mitochondrial membrane potential. Collectively, co-expression of Mcl-1 and Bak but not BH3-only proteins significantly induced mitochondrial swelling and subsequent loss of mitochondrial membrane potential.

Anti-apoptotic capacity of Mcl-1Δ127

The anti-apoptotic ability of Mcl-1Δ127, a caspase cleavage product of Mcl-1, is debated. We here used fluorescence imaging to assess the anti-apoptotic capacity of Mcl-1Δ127 in living cells. Fluorescence imaging of living cells expressing CFP-Mcl-1Δ127 showed that Mcl-1Δ127 existed mainly in cytoplasm. Fluorescence imaging of living cells co-expressing CFP-Mcl-1Δ127 and YFP-Bak, CFP-Mcl-1Δ127 and YFP-BimL, CFP-Mcl-1Δ127 and YFP-Puma or CFP-Mcl-1Δ127 and YFP-tBid showed that Mcl-1Δ127 markedly inhibited the oligomerization of Bak, BimL, Puma and tBid on mitochondria and also inhibited the Bak-, BimL-, Puma- or tBid-mediated cell death, resulting in their partial localization in cytoplasm. Fluorescence resonance energy transfer (FRET) imaging proved that Mcl-1Δ127 bound to Bak, BimL, Puma and tBid, respectively. Fluorescence loss in photobleaching (FLIP) analyses showed that Mcl-1Δ127 did prevent Bak oligomerization by retrotranslocating Bak from mitochondria into cytoplasm. Collectively, Mcl-1Δ127 has the same anti-apoptotic capacity as Mcl-1, and prevents apoptosis by sequestering BH3-only or Bak proteins, thus inhibiting their oligomerization on mitochondria.

Mcl-1 inhibits Mff-mediated mitochondrial fragmentation and apoptosis

Myeloid cell leukemia-1 (Mcl-1) is involved in the regulation of mitochondrial fission and fusion. This report aims to explore whether Mcl-1 can interact with mitochondrial fission factor (Mff) and regulate Mff-mediated mitochondrial fragmentation and apoptosis. Fluorescence images of living cells coexpressing YFP-Mff and CFP-Mcl-1 showed that Mcl-1 markedly inhibited Mff-mediated mitochondrial fragmentation and apoptosis, suggesting that Mcl-1 played a key role in inhibiting mitochondrial fission. The cells coexpressing YFP-Mff and CFP-Mcl-1 exhibited consistent fluorescence resonance energy transfer (FRET) efficiency with that of the cells coexpressing CFP-Mcl-1 and YFP, demonstrating that Mcl-1 did not directly bind to Mff on mitochondria. Collectively, Mcl-1 inhibits Mff-mediated mitochondrial fission and apoptosis not via directly binding to Mff on mitochondria.

Bcl-xL inhibits PINK1/Parkin-dependent mitophagy by preventing mitochondrial Parkin accumulation

This report aims to explore how Bcl-xL, a Bcl-2 family protein, regulates PINK1/Parkin-dependent mitophagy. Compared with the Hela cells expressing Parkin alone, co-expression of Bcl-xL significantly inhibited CCCP (Carbonyl cyanide 3- chlorophenylhydrazone)-induced mitochondrial Parkin accumulation and mitophagy. Western blotting analysis illustrated that over-expressed Bcl-xL inhibited CCCP-induced decrease of mitochondrial proteins in Parkin over-expressed cells. Fluorescence loss in photobleaching (FLIP) analyses demonstrated that Bcl-xL inhibited the CCCP-induced translocation of Parkin into mitochondria not by retrotranslocating Parkin from mitochondria to cytoplasm. Fluorescence resonance energy transfer (FRET) imaging revealed in Hela cells that Bcl-xL physically bound with Parkin to form oligomer in cytoplasm, and that Bcl-xL also directly interacted with PINK1 on mitochondria. analysis for HEK293 T cells verified that endogenous Bcl-xL interacted with both endogenous Parkin and PINK1. Collectively, Bcl-xL inhibits PINK1/Parkin- dependent mitophagy by preventing the accumulation of Parkin on mitochondria via two regulation ways: directly binds to Parkin in cytoplasm to prevent the translocation of Parkin from cytoplasm to mitochondria and directly binds to PINK1 on mitochondria to inhibit the Parkin from cytoplasm to mitochondria by PINK1.