bims-unfpre 2020-05-24 papers

Issue of 2020‒05‒24
five papers selected by
Susan Logue
University of Manitoba

Cell Rep. 2020 May 19. pii: S2211-1247(20)30612-4. [Epub ahead of print]31(7): 107659

ATF4-Induced Warburg Metabolism Drives Over-Proliferation in Drosophila.

Sebastian Sorge, Jonas Theelke, Kerem Yildirim, Helen Hertenstein, Ellen McMullen, Stephan Müller, Christian Altbürger, Stefanie Schirmeier, Ingrid Lohmann.

The mitochondrial electron transport chain (ETC) enables essential metabolic reactions; nonetheless, the cellular responses to defects in mitochondria and the modulation of signaling pathway outputs are not understood. We show that Notch signaling and ETC attenuation via knockdown of COX7a induces massive over-proliferation. The tumor-like growth is caused by a transcriptional response through the eIF2α-kinase PERK and ATF4, which activates the expression of metabolic enzymes, nutrient transporters, and mitochondrial chaperones. We find this stress adaptation to be beneficial for progenitor cell fitness, as it renders cells sensitive to proliferation induced by the Notch signaling pathway. Intriguingly, over-proliferation is not caused by transcriptional cooperation of Notch and ATF4, but it is mediated in part by pH changes resulting from the Warburg metabolism induced by ETC attenuation. Our results suggest that ETC function is monitored by the PERK-ATF4 pathway, which can be hijacked by growth-promoting signaling pathways, leading to oncogenic pathway activity.

Keywords: ATF4; Drosophila; ETC; ETC impairment; ISR; LDH; Notch pathway; PERK; UPR; lactate; mitochondrial electron transport chain; pH; proliferation

DOI: https://doi.org/10.1016/j.celrep.2020.107659

Sci Rep. 2020 May 20. 10(1): 8348

IRE1α and IGF signaling predict resistance to an endoplasmic reticulum stress-inducing drug in glioblastoma cells.

Jeffrey J Rodvold, Su Xian, Julia Nussbacher, Brian Tsui, T Cameron Waller, Stephen C Searles, Alyssa Lew, Pengfei Jiang, Ivan Babic, Natsuko Nomura, Jonathan H Lin, Santosh Kesari, Hannah Carter, Maurizio Zanetti.

To date current therapies of glioblastoma multiforme (GBM) are largely ineffective. The induction of apoptosis by an unresolvable unfolded protein response (UPR) represents a potential new therapeutic strategy. Here we tested 12ADT, a sarcoendoplasmic reticulum Ca2+ ATPase (SERCA) inhibitor, on a panel of unselected patient-derived neurosphere-forming cells and found that GBM cells can be distinguished into "responder" and "non-responder". By RNASeq analysis we found that the non-responder phenotype is significantly linked with the expression of UPR genes, and in particular ERN1 (IRE1) and ATF4. We also identified two additional genes selectively overexpressed among non-responders, IGFBP3 and IGFBP5. CRISPR-mediated deletion of the ERN1, IGFBP3, IGFBP5 signature genes in the U251 human GBM cell line increased responsiveness to 12ADT. Remarkably, >65% of GBM cases in The Cancer Genome Atlas express the non-responder (ERN1, IGFBP3, IGFBP5) gene signature. Thus, elevated levels of IRE1α and IGFBPs predict a poor response to drugs inducing unresolvable UPR and possibly other forms of chemotherapy helping in a better stratification GBM patients.

DOI: https://doi.org/10.1038/s41598-020-65320-6

iScience. 2020 Apr 29. pii: S2589-0042(20)30301-1. [Epub ahead of print]23(5): 101116

NADPH and Glutathione Redox Link TCA Cycle Activity to Endoplasmic Reticulum Homeostasis.

Erica R Gansemer, Kyle S McCommis, Michael Martino, Abdul Qaadir King-McAlpin, Matthew J Potthoff, Brian N Finck, Eric B Taylor, D Thomas Rutkowski.

Many metabolic diseases disrupt endoplasmic reticulum (ER) homeostasis, but little is known about how metabolic activity is communicated to the ER. Here, we show in hepatocytes and other metabolically active cells that decreasing the availability of substrate for the tricarboxylic acid (TCA) cycle diminished NADPH production, elevated glutathione oxidation, led to altered oxidative maturation of ER client proteins, and attenuated ER stress. This attenuation was prevented when glutathione oxidation was disfavored. ER stress was also alleviated by inhibiting either TCA-dependent NADPH production or Glutathione Reductase. Conversely, stimulating TCA activity increased NADPH production, glutathione reduction, and ER stress. Validating these findings, deletion of the Mitochondrial Pyruvate Carrier-which is known to decrease TCA cycle activity and protect the liver from steatohepatitis-also diminished NADPH, elevated glutathione oxidation, and alleviated ER stress. Together, our results demonstrate a novel pathway by which mitochondrial metabolic activity is communicated to the ER through the relay of redox metabolites.

Keywords: biological sciences; cell biology; functional aspects of cell biology

DOI: https://doi.org/10.1016/j.isci.2020.101116

Cell Death Dis. 2020 May 21. 11(5): 385

Inhibition of soluble epoxide hydrolase attenuates renal tubular mitochondrial dysfunction and ER stress by restoring autophagic flux in diabetic nephropathy.

Xu-Shun Jiang, Xing-Yang Xiang, Xue-Mei Chen, Jun-Ling He, Ting Liu, Hua Gan, Xiao-Gang Du.

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease (ESRD), and renal tubular cell dysfunction contributes to the pathogenesis of DN. Soluble epoxide hydrolase (sEH) is an enzyme that can hydrolyze epoxyeicosatrienoic acids (EETs) and other epoxy fatty acids (EpFAs) into the less biologically active metabolites. Inhibition of sEH has multiple beneficial effects on renal function, however, the exact role of sEH in hyperglycemia-induced dysfunction of tubular cells is still not fully elucidated. In the present study, we showed that human proximal tubular epithelial (HK-2) cells revealed an upregulation of sEH expression accompanied by the impairment of autophagic flux, mitochondrial dysfunction, ubiquitinated protein accumulation and enhanced endoplasmic reticulum (ER) stress after high glucose (HG) treatment. Furthermore, dysfunctional mitochondria accumulated in the cytoplasm, which resulted in excessive reactive oxygen species (ROS) generation, Bax translocation, cytochrome c release, and apoptosis. However, t-AUCB, an inhibitor of sEH, partially reversed these negative outcomes. Moreover, we also observed increased sEH expression, impaired autophagy flux, mitochondrial dysfunction and enhanced ER stress in the renal proximal tubular cells of db/db diabetic mice. Notably, inhibition of sEH by treatment with t-AUCB attenuated renal injury and partially restored autophagic flux, improved mitochondrial function, and reduced ROS generation and ER stress in the kidneys of db/db mice. Taken together, these results suggest that inhibition of sEH by t-AUCB plays a protective role in hyperglycemia-induced proximal tubular injury and that the potential mechanism of t-AUCB-mediated protective autophagy is involved in modulating mitochondrial function and ER stress. Thus, we provide new evidence linking sEH to the autophagic response during proximal tubular injury in the pathogenesis of DN and suggest that inhibition of sEH can be considered a potential therapeutic strategy for the amelioration of DN.

DOI: https://doi.org/10.1038/s41419-020-2594-x

J Biol Chem. 2020 May 19. pii: jbc.RA120.012979. [Epub ahead of print]

The prohibitin-binding compound fluorizoline affects multiple components of the translational machinery and inhibits protein synthesis.

Xin Jin, Jianling Xie, Michael Zabolocki, Xuemin Wang, Tao Jiang, Dong Wang, Laurent Desaubry, Cedric Bardy, Christopher G Proud.

Fluorizoline (FLZ) binds to prohibitin-1 and -2 (PHB1/2), which are pleiotropic scaffold proteins known to affect signaling pathways involved in several intracellular processes. However, it is not yet clear how FLZ exerts its effect. Here, we show that exposure of three different human cancer cell lines to FLZ increases the phosphorylation of key translation factors, particularly of initiation factor 2 (eIF2) and elongation factor 2 (eEF2), modifications that inhibit their activities. FLZ also impaired signaling through mTOR complex 1, which also regulates the translational machinery, e.g. through the eIF4E-binding protein 4E-BP1. In line with these findings, FLZ potently inhibited protein synthesis. We noted that the first phase of this inhibition involves very rapid eEF2 phosphorylation, which is catalyzed by a dedicated Ca2+-dependent protein kinase, eEF2 kinase (eEF2K). We also demonstrate that FLZ induces a swift and marked rise in intracellular Ca2+ levels, likely explaining the effects on eEF2. Disruption of normal Ca2+ homeostasis can also induce endoplasmic reticulum stress, and our results suggest that induction of this stress response contributes to the increased phosphorylation of eIF2, likely due to activation of the eIF2-modifying kinase PKR-like endoplasmic reticulum kinase (PERK). We show that FLZ induces cancer cell death and that this effect involves contributions from the phosphorylation of both eEF2 and eIF2. Our findings provide important new insights into the biological effects of FLZ and thus the roles of PHBs, specifically in regulating Ca2+ levels, cellular protein synthesis, and cell survival.

Keywords: calcium; cancer; cell death; eEF2; endoplasmic reticulum stress (ER stress); eukaryotic initiation factor 2 (eIF2); eukaryotic translation initiation; prohibitin; protein synthesis; translation elongation factor

DOI: https://doi.org/10.1074/jbc.RA120.012979