bims-lymeca 2023-11-12 papers

Issue of 2023‒11‒12
four papers selected by
Harilaos Filippakis, University of New England

Dev Cell. 2023 Oct 29. pii: S1534-5807(23)00526-9. [Epub ahead of print]

V-ATPase recruitment to ER exit sites switches COPII-mediated transport to lysosomal degradation.

Yiwei Sun, Xi'e Wang, Xiaotong Yang, Lei Wang, Jingjin Ding, Chih-Chen Wang, Hong Zhang, Xi Wang.

Endoplasmic reticulum (ER)-phagy is crucial to regulate the function and homeostasis of the ER via lysosomal degradation, but how it is initiated is unclear. Here we discover that Z-AAT, a disease-causing mutant of α1-antitrypsin, induces noncanonical ER-phagy at ER exit sites (ERESs). Accumulation of misfolded Z-AAT at the ERESs impairs coat protein complex II (COPII)-mediated ER-to-Golgi transport and retains V0 subunits that further assemble V-ATPase at the arrested ERESs. V-ATPase subsequently recruits ATG16L1 onto ERESs to mediate in situ lipidation of LC3C. FAM134B-II is then recruited by LC3C via its LIR motif and elicits ER-phagy leading to efficient lysosomal degradation of Z-AAT. Activation of this ER-phagy mediated by the V-ATPase-ATG16L1-LC3C axis (EVAC) is also triggered by blocking ER export. Our findings identify a pathway which switches COPII-mediated transport to lysosomal degradation for ER quality control.

Keywords: ATG16L1; COPII; ER exit sites; ER-phagy; LC3C; Sec24C; V-ATPase; autophagy; protein quality control; α1 antitrypsin

DOI: https://doi.org/10.1016/j.devcel.2023.10.007

J Biol Chem. 2023 Nov 08. pii: S0021-9258(23)02483-3. [Epub ahead of print] 105455

CBAP regulates the function of Akt-associated TSC protein complexes to modulate mTORC1 signaling.

Wei-Ting Liao, Yun-Jung Chiang, Hsin-Fang Yang-Yen, Li-Chung Hsu, Zee-Fen Chang, Jeffrey Jy Yen.

The Akt-Rheb-mTORC1 pathway plays a crucial role in regulating cell growth, but the mechanisms underlying the activation of Rheb-mTORC1 by Akt remain unclear. In our previous study, we found that CBAP was highly expressed in human T-ALL cells and primary tumors, and its deficiency led to reduced phosphorylation of TSC2/S6K1 signaling proteins, as well as impaired cell proliferation and leukemogenicity. We also demonstrated that CBAP was required for Akt-mediated TSC2 phosphorylation in vitro. In response to insulin, CBAP was also necessary for phosphorylation of TSC2/S6K1 and the dissociation of TSC2 from the lysosomal membrane. Here we report that CBAP interacts with AKT and TSC2, and knockout of CBAP or serum starvation leads to an increase in TSC1 in the Akt/TSC2 immunoprecipitation complexes. Lysosomal-anchored CBAP was found to override serum starvation and promote S6K1 and 4EBP1 phosphorylation and c-Myc expression in a TSC2-dependent manner. Additionally, recombinant CBAP inhibited the GAP activity of TSC2 complexes in vitro, leading to increased Rheb-GTP loading, likely due to the competition between TSC1 and CBAP for binding to the HBD domain of TSC2. Overexpression of the N26 region of CBAP, which is crucial for binding to TSC2, resulted in a decrease in mTORC1 signaling and an increase in TSC1 association with the TSC2/AKT complex, ultimately leading to increased GAP activity toward Rheb and impaired cell proliferation. Thus, we propose that CBAP can modulate the stability of TSC1-TSC2 as well as promote translocation of TSC1/TSC2 complexes away from lysosomes to regulate Rheb-mTORC1 signaling.

Keywords: Akt; Rheb; cell growth; mTORC1 activation; small GTPase; tumor cell biology

DOI: https://doi.org/10.1016/j.jbc.2023.105455

Autophagy. 2023 Nov 05. 1-3

SREBF1/SREBP-1 concurrently regulates lipid synthesis and lipophagy to maintain lipid homeostasis and tumor growth.

Feng Geng, Deliang Guo.

Cholesterol is an essential structural component of the cell membrane, whereas excess cholesterol can be toxic and thus is stored in intracellular lipid droplets (LDs). Malignant tumor cells grow rapidly and require abundant cholesterol to build new membranes. How they maintain cholesterol homeostasis is largely unknown. We recently revealed that SREBF1/SREBP-1 (sterol regulatory element binding transcription factor 1), a key lipogenic transcription factor, plays a critical role in maintaining cholesterol homeostasis in tumor cells. We found that in addition to activation of de novo lipid synthesis and cholesterol uptake, SREBF1 also upregulates macroautophagy/autophagy to hydrolyze LDs, and increases the expression of NPC2, a lysosome cholesterol transporter, actively mobilizing LD-stored cholesterol and fatty acids to promote tumor growth. Our study demonstrates that SREBF1 controls the balance of lipid synthesis, uptake, storage and liberation to maintain lipid homeostasis for rapid tumor growth, while suggesting it as a very promising molecular target for cancer treatment.

Keywords: Autophagy; cancer; cholesterol; glioblastoma; lipid droplets; lipophagy

DOI: https://doi.org/10.1080/15548627.2023.2275501

J Biomed Sci. 2023 Nov 07. 30(1): 91

Spatiotemporal roles of AMPK in PARP-1- and autophagy-dependent retinal pigment epithelial cell death caused by UVA.

Anthony Yan-Tang Wu, Ponarulselvam Sekar, Duen-Yi Huang, Shu-Hao Hsu, Chi-Ming Chan, Wan-Wan Lin.

BACKGROUND: Although stimulating autophagy caused by UV has been widely demonstrated in skin cells to exert cell protection, it remains unknown the cellular events in UVA-treated retinal pigment epithelial (RPE) cells.METHODS: Human ARPE-19 cells were used to measure cell viability, mitochondrial reactive oxygen species (ROS), mitochondrial membrane potential (MMP), mitochondrial mass and lysosomal mass by flow cytometry. Mitochondrial oxygen consumption rate (OCR) was recorded using Seahorse XF flux analyzer. Confocal microscopic images were performed to indicate the mitochondrial dynamics, LC3 level, and AMPK translocation after UVA irradiation.
RESULTS: We confirmed mitochondrial ROS production and DNA damage are two major features caused by UVA. We found the cell death is prevented by autophagy inhibitor 3-methyladenine and gene silencing of ATG5, and UVA induces ROS-dependent LC3II expression, LC3 punctate and TFEB expression, suggesting the autophagic death in the UVA-stressed RPE cells. Although PARP-1 inhibitor olaparib increases DNA damage, ROS production, and cell death, it also blocks AMPK activation caused by UVA. Interestingly we found a dramatic nuclear export of AMPK upon UVA irradiation which is blocked by N-acetylcysteine and olaparib. In addition, UVA exposure gradually decreases lysosomal mass and inhibits cathepsin B activity at late phase due to lysosomal dysfunction. Nevertheless, cathepsin B inhibitor, CA-074Me, reverses the death extent, suggesting the contribution of cathepsin B in the death pathway. When examining the role of EGFR in cellular events caused by UVA, we found that UVA can rapidly transactivate EGFR, and treatment with EGFR TKIs (gefitinib and afatinib) enhances the cell death accompanied by the increased LC3II formation, ROS production, loss of MMP and mass of mitochondria and lysosomes. Although AMPK activation by ROS-PARP-1 mediates autophagic cell death, we surprisingly found that pretreatment of cells with AMPK activators (A769662 and metformin) reverses cell death. Concomitantly, both agents block UVA-induced mitochondrial ROS production, autophagic flux, and mitochondrial fission without changing the inhibition of cathepsin B.
CONCLUSION: UVA exposure rapidly induces ROS-PARP-1-AMPK-autophagic flux and late lysosomal dysfunction. Pre-inducing AMPK activation can prevent cellular events caused by UVA and provide a new protective strategy in photo-oxidative stress and photo-retinopathy.

Keywords: AMPK; Autophagic cell death; EGFR; Lysosome dysfunction; PARP; ROS; RPE; UVA

DOI: https://doi.org/10.1186/s12929-023-00978-4