Search

Summary: ARF suppresses aberrant cell growth upon c-Myc overexpression by activating p53 responses. Nevertheless, the precise mechanism by which ARF specifically restrains the oncogenic potential of c-Myc without affecting its normal physiological function is not well understood. Here, we show that low levels of c-Myc expression stimulate cell proliferation, whereas high levels inhibit by activating the ARF/p53 response. Although the mRNA levels of ARF are induced in both scenarios, the accumulation of ARF protein occurs only when ULF-mediated degradation of ARF is inhibited by c-Myc overexpression. Moreover, the levels of ARF are reduced through ULF-mediated ubiquitination upon DNA damage. Blocking ARF degradation by c-Myc overexpression dramatically stimulates the apoptotic responses. Our study reveals that ARF stability control is crucial for differentiating normal (low) versus oncogenic (high) levels of c-Myc expression and suggests that differential effects on ULF- mediated ARF ubiquitination by c-Myc levels act as a barrier in oncogene-induced stress responses. [Copyright &y& Elsevier]

Summary: Cell-cycle arrest, apoptosis, and senescence are widely accepted as the major mechanisms by which p53 inhibits tumor formation. Nevertheless, it remains unclear whether they are the rate-limiting steps in tumor suppression. Here, we have generated mice bearing lysine to arginine mutations at one (p53K117R ) or three (p533KR ; K117R+K161R+K162R) of p53 acetylation sites. Although p53K117R/K117R cells are competent for p53-mediated cell-cycle arrest and senescence, but not apoptosis, all three of these processes are ablated in p533KR/3KR cells. Surprisingly, unlike p53 null mice, which rapidly succumb to spontaneous thymic lymphomas, early-onset tumor formation does not occur in either p53K117R/K117R or p533KR/3KR animals. Notably, p533KR retains the ability to regulate energy metabolism and reactive oxygen species production. These findings underscore the crucial role of acetylation in differentially modulating p53 responses and suggest that unconventional activities of p53, such as metabolic regulation and antioxidant function, are critical for suppression of early-onset spontaneous tumorigenesis. [ABSTRACT FROM AUTHOR]

Summary: Brown adipose tissue (BAT) can disperse stored energy as heat. Promoting BAT-like features in white adipose (WAT) is an attractive, if elusive, therapeutic approach to staunch the current obesity epidemic. Here we report that gain of function of the NAD-dependent deacetylase SirT1 or loss of function of its endogenous inhibitor Deleted in breast cancer-1 (Dbc1) promote “browning” of WAT by deacetylating peroxisome proliferator-activated receptor (Ppar)-γ on Lys268 and Lys293. SirT1-dependent deacetylation of Lys268 and Lys293 is required to recruit the BAT program coactivator Prdm16 to Pparγ, leading to selective induction of BAT genes and repression of visceral WAT genes associated with insulin resistance. An acetylation-defective Pparγ mutant induces a brown phenotype in white adipocytes, whereas an acetylated mimetic fails to induce “brown” genes but retains the ability to activate “white” genes. We propose that SirT1-dependent Pparγ deacetylation is a form of selective Pparγ modulation of potential therapeutic import. [Copyright &y& Elsevier]

Our previous study showed that ubiquitination of p53 is reversible and that the ubiquitin hydrolase HAUSP can stabilize p53 by deubiquitination. Here, we found that partial reduction of endogenous HAUSP levels by RNAi indeed destabilizes endogenous p53; surprisingly, however, nearly complete ablation of HAUSP stabilizes and activates p53. We further show that this phenomenon occurs because HAUSP stabilizes Mdm2 in a p53-independent manner, providing an interesting feedback loop in p53 regulation. Notably, HAUSP is required for Mdm2 stability in normal cells; in HAUSP-ablated cells, self-ubiquitinated-Mdm2 becomes extremely unstable, leading to indirect p53 activation. Furthermore, this feedback regulation is specific to Mdm2; in HeLa cells, where p53 is preferentially degraded by viral E6-dependent ubiquitination, depletion of HAUSP fails to activate p53. This study provides an example of an ubiquitin ligase (Mdm2) that is directly regulated by a deubiquitinase (HAUSP) and also reveals a dynamic role of HAUSP in the p53-Mdm2 pathway. [Copyright &y& Elsevier]

Although the role of ferroptosis in killing tumor cells is well established, recent studies indicate that ferroptosis inducers also sabotage anti-tumor immunity by killing neutrophils and thus unexpectedly stimulate tumor growth, raising a serious issue about whether ferroptosis effectively suppresses tumor development in vivo. Through genome-wide CRISPR-Cas9 screenings, we discover a pleckstrin homology-like domain family A member 2 (PHLDA2)-mediated ferroptosis pathway that is neither ACSL4-dependent nor requires common ferroptosis inducers. PHLDA2-mediated ferroptosis acts through the peroxidation of phosphatidic acid (PA) upon high levels of reactive oxygen species (ROS). ROS-induced ferroptosis is critical for tumor growth in the absence of common ferroptosis inducers; strikingly, loss of PHLDA2 abrogates ROS-induced ferroptosis and promotes tumor growth but has no obvious effect in normal tissues in both immunodeficient and immunocompetent mouse tumor models. These data demonstrate that PHLDA2-mediated PA peroxidation triggers a distinct ferroptosis response critical for tumor suppression and reveal that PHLDA2-mediated ferroptosis occurs naturally in vivo without any treatment from ferroptosis inducers., Competing Interests: Declaration of interests B.R.S. is an inventor on patents and patent applications involving small molecule drug discovery and ferroptosis; holds equity in Sonata Therapeutics; holds equity in and serves as a consultant to Exarta Therapeutics and ProJenX, Inc.; and serves as a consultant to Weatherwax Biotechnologies Corporation and Akin Gump Strauss Hauer & Feld LLP. X.J. is an inventor on patents related to autophagy and cell death and holds equity of and consults for Exarta Therapeutics and Lime Therapeutics., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Ferroptosis, an iron-dependent programmed cell death triggered by excessive lipid peroxidation, has shown promising therapeutic potentials in human diseases. Here, we describe a protocol of a CRISPR-Cas9 loss-of-function screen to identify regulators in response to different inducers of ferroptosis. We emphasize the steps of library amplification, drug treatment, high-throughput sequencing preparation, and bioinformatics analysis using model-based analysis of genome-wide CRISPR-Cas9 knockout (MAGeCK). We also present a method to discover the regulators of ferroptosis and verify the potential targets efficiently. For complete details on use and execution of this protocol, please refer to Yang et al. (2023). 1 ., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Non-alcoholic fatty liver disease (NAFLD) is the most pervasive liver pathology worldwide. Here, we demonstrate that the ubiquitin E3 ligase Huwe1 is vital in NAFLD pathogenesis. Using mass spectrometry and RNA sequencing, we reveal that liver-specific deletion of Huwe1 ( Huwe1 LKO ) in 1-year-old mice (approximately middle age in humans) elicits extensive lipid metabolic reprogramming that involves downregulation of de novo lipogenesis and fatty acid uptake, upregulation of fatty acid β-oxidation, and increased oxidative phosphorylation. ChEA transcription factor prediction analysis inferred these changes result from attenuated PPARɑ, LXR, and RXR activity in Huwe1 LKO livers. Consequently, Huwe1 LKO mice fed chow diet exhibited significantly reduced hepatic steatosis and superior glucose tolerance compared to wild-type mice. Huwe1 LKO also conferred protection from high-fat diet-induced hepatic steatosis by 6-months of age, with increasingly robust differences observed as mice reached middle age. Together, we present evidence that Huwe1 plays a critical role in the development of age- and diet-induced NAFLD., Competing Interests: The authors declare no competing interests., (© 2023 The Authors.)

Here, we identified vitamin K epoxide reductase complex subunit 1 like 1 (VKORC1L1) as a potent ferroptosis repressor. VKORC1L1 protects cells from ferroptosis by generating the reduced form of vitamin K, a potent radical-trapping antioxidant, to counteract phospholipid peroxides independent of the canonical GSH/GPX4 mechanism. Notably, we found that VKORC1L1 is also a direct transcriptional target of p53. Activation of p53 induces downregulation of VKORC1L1 expression, thus sensitizing cells to ferroptosis for tumor suppression. Interestingly, a small molecular inhibitor of VKORC1L1, warfarin, is widely prescribed as an FDA-approved anticoagulant drug. Moreover, warfarin represses tumor growth by promoting ferroptosis in both immunodeficient and immunocompetent mouse models. Thus, by downregulating VKORC1L1, p53 executes the tumor suppression function by activating an important ferroptosis pathway involved in vitamin K metabolism. Our study also reveals that warfarin is a potential repurposing drug in cancer therapy, particularly for tumors with high levels of VKORC1L1 expression., Competing Interests: Declaration of interests B.R.S. is an inventor on patents and patent applications involving small-molecule drug discovery and ferroptosis; holds equity in Sonata Therapeutics; holds equity in and serves as a consultant to Exarta Therapeutics and ProJenX, Inc.; and serves as a consultant to Weatherwax Biotechnologies Corporation and Akin Gump Strauss Hauer & Feld LLP. X.J. is an inventor on patents related to autophagy and cell death and holds equity of and consults for Exarta Therapeutics and Lime Therapeutics., (Copyright © 2023 Elsevier Inc. All rights reserved.)

HUWE1 is a HECT-domain ubiquitin E3 ligase expressed in various tissues. Although HUWE1 is known to promote degradation of the tumor suppressor p53, given a growing list of its substrates, in vivo functions of HUWE1 remain elusive. Here, we investigated the role of HUWE1 in the female reproductive system. Homozygous deletion of Huwe1 in mouse oocytes of primary follicles caused oocyte death and female infertility, whereas acute depletion of HUWE1 protein by Trim-Away technology did not impact oocytes from antral follicles. Interestingly, oocytes from Huwe1 heterozygous females matured and fertilized normally, but the majority of embryos that lacked maternal Huwe1 were arrested at the morula stage after fertilization. Consequently, Huwe1 heterozygous females only produced wild-type pups. Concomitant knockout of p53 did not recover fertility of the Huwe1 knockout females. These findings make HUWE1 a unique and critical maternal factor indispensable for maintaining the quality of oocytes and embryos., Competing Interests: The authors declare no competing interests., (© 2020 The Author(s).)

Obesity is associated with higher incidence of cancer, but the predisposing mechanisms remain poorly understood. The NAD(+)-dependent deacetylase SirT1 orchestrates metabolism, cellular survival, and growth. However, there is no unifying mechanism to explain the metabolic and tumor-related effects of SirT1. In this work, we demonstrate that genetic ablation of the endogenous inhibitor of SirT1, Deleted-in-Breast-Cancer-1 (Dbc1), unexpectedly results in obesity and insulin resistance. Dbc1 deficiency promoted SirT1-dependent gain of function of stearoyl-coenzyme A desaturase 1 (Scd1), increasing plasma and tissue levels of unsaturated fatty acids. The metabolic abnormalities in Dbc1(-/-) mice were reversed by ablation of hepatic SirT1 or by inhibition of Scd1 activity. Furthermore, loss of Dbc1 impaired activation of the master tumor suppressor p53 and treatment with an Scd1 inhibitor extended survival of tumor-prone TP53(-/-) mice by decreasing tumor-related death. Together, our findings illustrate a shared mechanism of obesity and tumor progression mediated by hepatic SirT1 and resulting in the activation of a key lipid synthetic enzyme, with potential therapeutic implications., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

In yeast, worms, and flies, an extra copy of the gene encoding the Sirtuin Sir2 increases metabolic efficiency, as does administration of polyphenols like resveratrol, thought to act through Sirtuins. But evidence that Sirtuin gain of function results in increased metabolic efficiency in mammals is limited. We generated transgenic mice with moderate overexpression of SirT1, designed to mimic the Sirtuin gain of function that improves metabolism in C. elegans. These mice exhibit normal insulin sensitivity but decreased food intake and locomotor activity, resulting in decreased energy expenditure. However, in various models of insulin resistance and diabetes, SirT1 transgenics display improved glucose tolerance due to decreased hepatic glucose production and increased adiponectin levels, without changes in body weight or composition. We conclude that SirT1 gain of function primes the organism for metabolic adaptation to insulin resistance, increasing hepatic insulin sensitivity and decreasing whole-body energy requirements. These findings have important implications for Sirtuin-based therapies in humans.

Although the importance of the ARF tumor suppressor in p53 regulation is well established, numerous studies indicate that ARF also suppresses cell growth in a p53/Mdm2-independent manner. To understand the mechanism of ARF-mediated tumor suppression, we identified a ubiquitin ligase, ARF-BP1, as a key factor associated with ARF in vivo. ARF-BP1 harbors a signature HECT motif, and its ubiquitin ligase activity is inhibited by ARF. Notably, inactivation of ARF-BP1, but not Mdm2, suppresses the growth of p53 null cells in a manner reminiscent of ARF induction. Surprisingly, in p53 wild-type cells, ARF-BP1 directly binds and ubiquitinates p53, and inactivation of endogenous ARF-BP1 is crucial for ARF-mediated p53 stabilization. Thus, our study modifies the current view of ARF-mediated p53 activation and reveals that ARF-BP1 is a critical mediator of both the p53-independent and p53-dependent tumor suppressor functions of ARF. As such, ARF-BP1 may serve as a potential target for therapeutic intervention in tumors regardless of p53 status.