Normoxic destabilization of ATF-4 depends on proteasomal degradation

Chloroquine attenuates chronic hypoxia-induced testicular damage via suppressing endoplasmic reticulum stress and apoptosis in experimental rat model

Chronic hypoxia negatively affects male fertility by causing pathological changes in male reproductive system. However, underlying mechanisms of this damage are unknown. Chloroquine (CLQ) is an anti-inflammatory agent which is widely used in the treatment of inflammation-related diseases such as malaria and rheumatoid arthritis. This study aimed to investigate the therapeutic effects of CLQ in the hypoxia-induced testicular damage via assessment of hypoxic response, endoplasmic reticulum stress and apoptosis. For this purpose, 32 Wistar albino rats were divided into 4 groups as Control (given %20-21 O₂ , no treatment), CLQ (given 50 mg/kg and %20-21 O₂ for 28 days), HX (given %10 O₂ for 28 days) and HX + CLQ (given 50 mg/kg and %10 O₂ for 28 days). After experiment, blood samples and testicular tissues were taken. Histopathological evaluation was performed on testicular tissues and HIF1-α, HSP70, HSP90 and GADD153 expression levels were detected via immunohistochemistry. Moreover, apoptotic cells were detected via TUNEL staining and serum testosterone levels were determined by ELISA assay. Histopathological changes, apoptotic cell numbers and HIF1-α, HSP70, HSP90 and GADD153 expressions significantly increased in HX group (p < 0.05). Moreover, serum testosterone levels decreased in this group (p > 0.05). However, CLQ exerted a strong ameliorative effect on all parameters in HX + CLQ group. According to our results, we suggested that CLQ can be considered as an alternative protective agent for eliminating the negative effects of hypoxic conditions on male fertility. This article is protected by copyright. All rights reserved.

HIF Prolyl Hydroxylase Inhibitors for COVID-19 Treatment: Pros and Cons

The review analyzes the potential advantages and problems associated with using HIF prolyl hydroxylase inhibitors as a treatment for COVID-19. HIF prolyl hydroxylase inhibitors are known to boost endogenous erythropoietin (Epo) and activate erythropoiesis by stabilizing and activating the hypoxia inducible factor (HIF). Recombinant Epo treatment has anti-inflammatory and healing properties, and thus, very likely, will be beneficial for moderate to severe cases of COVID-19. However, HIF PHD inhibition may have a significantly broader effect, in addition to stimulating the endogenous Epo production. The analysis of HIF target genes reveals that some HIF-targets, such as furin, could play a negative role with respect to viral entry. On the other hand, HIF prolyl hydroxylase inhibitors counteract ferroptosis, the process recently implicated in vessel damage during the later stages of COVID-19. Therefore, HIF prolyl hydroxylase inhibitors may serve as a promising treatment of COVID-19 complications, but they are unlikely to aid in the prevention of the initial stages of infection.

FIH Regulates Cellular Metabolism through Hydroxylation of the Deubiquitinase OTUB1

The asparagine hydroxylase, factor inhibiting HIF (FIH), confers oxygen-dependence upon the hypoxia-inducible factor (HIF), a master regulator of the cellular adaptive response to hypoxia. Studies investigating whether asparagine hydroxylation is a general regulatory oxygen-dependent modification have identified multiple non-HIF targets for FIH. However, the functional consequences of this outside of the HIF pathway remain unclear. Here, we demonstrate that the deubiquitinase ovarian tumor domain containing ubiquitin aldehyde binding protein 1 (OTUB1) is a substrate for hydroxylation by FIH on N22. Mutation of N22 leads to a profound change in the interaction of OTUB1 with proteins important in cellular metabolism. Furthermore, in cultured cells, overexpression of N22A mutant OTUB1 impairs cellular metabolic processes when compared to wild type. Based on these data, we hypothesize that OTUB1 is a target for functional hydroxylation by FIH. Additionally, we propose that our results provide new insight into the regulation of cellular energy metabolism during hypoxic stress and the potential for targeting hydroxylases for therapeutic benefit.

The oxygen-dependent asparagine hydroxylase FIH regulates the transcription factor HIF during the cellular response to hypoxia. This study suggests that FIH may also contribute to the hypoxia response by affecting cellular metabolism via altered deubiquitinase targeting.

Hypoxia is a commonly encountered physiologic and pathophysiologic stress to which mammalian cells have evolved an effective adaptive response. This response is governed by a transcription factor termed the hypoxia-inducible factor (HIF). The mechanisms linking the cellular sensing of oxygen levels to HIF activation have been elucidated and involve oxygen-dependent hydroxylation of HIF on proline and asparagine residues by a family of hydroxylases. A key question that remains unclear is the extent to which oxygen-dependent hydroxylation occurs as a functional post-translational modification outside of the HIF pathway. This is key to developing our understanding of whether hydroxylation is a general regulatory modification or one which has specifically evolved for the regulation of HIF. Here, we demonstrate that the deubiquitinase ovarian tumor domain containing ubiquitin aldehyde binding protein 1 (OTUB1) is a target for functional hydroxylation by the FIH hydroxylase. Hydroxylation of OTUB1 by FIH on asparagine residue N22 results in a restriction in its interactome, leading us to hypothesize a possible role for hydroxylation in substrate targeting. Of interest, interactions of OTUB1 with a number of proteins involved in metabolism are altered upon removal of the hydroxylation site—implicating OTUB1 as a possible link between oxygen sensing and the regulation of metabolism.

Prolyl-4-hydroxylase domain 3 (PHD3) is a critical terminator for cell survival of macrophages under stress conditions

On a molecular level, cells sense changes in oxygen availability through the PHDs, which regulate the protein stability of the α-subunit of the transcription factor HIF. Especially, PHD3 has been additionally associated with apoptotic cell death. We hypothesized that PHD3 plays a role in cell-fate decisions in macrophages. Therefore, myeloid-specific PHD3(-/-) mice were created and analyzed. PHD3(-/-) BMDM showed no altered HIF-1α or HIF-2α stabilization or increased HIF target gene expression in normoxia or hypoxia. Macrophage M1 and M2 polarization was unchanged likewise. Compared with macrophages from WT littermates, PHD3(-/-) BMDM exhibited a significant reduction in TUNEL-positive cells after serum withdrawal or treatment with stauro and SNAP. Under the same conditions, PHD3(-/-) BMDM also showed less Annexin V staining, which is representative for membrane disruption, and indicated a reduced early apoptosis. In an unbiased transcriptome screen, we found that Angptl2 expression was reduced in PHD3(-/-) BMDM under stress conditions. Addition of rAngptl2 rescued the antiapoptotic phenotype, demonstrating that it is involved in the PHD3-mediated response toward apoptotic stimuli in macrophages.

Prolyl 4-hydroxylases, master regulators of the hypoxia response

A decrease in oxygenation is a life-threatening situation for most organisms. An evolutionarily conserved efficient and rapid hypoxia response mechanism activated by a hypoxia-inducible transcription factor (HIF) is present in animals ranging from the simplest multicellular phylum Placozoa to humans. In humans, HIF induces the expression of more than 100 genes that are required to increase oxygen delivery and to reduce oxygen consumption. As its name indicates HIF is found at protein level only in hypoxic cells, whereas in normoxia, it is degraded by the proteasome pathway. Prolyl 4-hydroxylases, enzymes that require oxygen in their reaction, are the cellular oxygen sensors regulating the stability of HIF. In normoxia, 4-hydroxyproline residues formed in the α-subunit of HIF by these enzymes lead to its ubiquitination by the von Hippel-Lindau E3 ubiquitin ligase and immediate destruction in proteasomes thus preventing the formation of a functional HIF αβ dimer. Prolyl 4-hydroxylation is inhibited in hypoxia, facilitating the formation of the HIF dimer and activation of its target genes, such as those for erythropoietin and vascular endothelial growth factor. This review starts with a summary of the molecular and catalytic properties and individual functions of the four HIF prolyl 4-hydroxylase isoenzymes. Induction of the hypoxia response via inhibition of the HIF prolyl 4-hydroxylases may provide a novel therapeutic target in the treatment of hypoxia-associated diseases. The current status of studies aiming at such therapeutic approaches is introduced in the final part of this review.

ATF4 interacts with Abro1/KIAA0157 scaffold protein and participates in a cytoprotective pathway

Abro1 (Abraxas brother 1), also known as KIAA0157, is a scaffold protein that recruits various polypeptides to assemble the BRISC (BRCC36 isopeptide) deubiquitinating enzyme (DUB) complex. The BRISC enzyme has a Lys63-linked deubiquitinating activity and is comprised of four known subunits: MERIT40 (mediator of Rap80 interactions and targeting 40kDa), BRE (brain and reproductive organ-expressed), BRCC36 (BRCA1/BRCA2-containing complex, subunit 3) and Abro1. We have previously shown that Abro1 has a cytoprotective role that involves the BRISC DUB complex acting on specific Lys63-linked polyubiquitinated substrates. In this report we identify three members of the AP-1 (activating protein-1) family, the ATF4, ATF5 (activating transcription factor) and JunD proteins, as specific interactors of Abro1. The function of ATF4-Abro1 interaction was investigated under normal conditions as well as under cellular stress. Abro1 is predominantly cytoplasmic, but during cellular stress it enters the nucleus and co-localizes with ATF4. Furthermore, this interaction with ATF4 is necessary and essential for the cytoprotective function of Abro1 following oxidative stress. The ability of Abro1 to specifically interact with a number of transcription factors suggests a new mechanism of regulation of the BRISC DUB complex. This regulation involves the participation of at least three known members of the AP-1 family of transcription factors.

The 26S proteasome complex: an attractive target for cancer therapy

The 26S proteasome complex engages in an ATP-dependent proteolytic degradation of a variety of oncoproteins, transcription factors, cell cycle specific cyclins, cyclin-dependent kinase inhibitors, ornithine decarboxylase, and other key regulatory cellular proteins. Thus, the proteasome regulates either directly or indirectly many important cellular processes. Altered regulation of these cellular events is linked to the development of cancer. Therefore, the proteasome has become an attractive target for the treatment of numerous cancers. Several proteasome inhibitors that target the proteolytic active sites of the 26S proteasome complex have been developed and tested for anti-tumor activities. These proteasome inhibitors have displayed impressive anti-tumor functions by inducing apoptosis in different tumor types. Further, the proteasome inhibitors have been shown to induce cell cycle arrest, and inhibit angiogenesis, cell-cell adhesion, cell migration, immune and inflammatory responses, and DNA repair response. A number of proteasome inhibitors are now in clinical trials to treat multiple myeloma and solid tumors. Many other proteasome inhibitors with different efficiencies are being developed and tested for anti-tumor activities. Several proteasome inhibitors currently in clinical trials have shown significantly improved anti-tumor activities when combined with other drugs such as histone deacetylase (HDAC) inhibitors, Akt (protein kinase B) inhibitors, DNA damaging agents, Hsp90 (heat shock protein 90) inhibitors, and lenalidomide. The proteasome inhibitor bortezomib is now in the clinic to treat multiple myeloma and mantle cell lymphoma. Here, we discuss the 26S proteasome complex in carcinogenesis and different proteasome inhibitors with their potential therapeutic applications in treatment of numerous cancers.

PHD1 interacts with ATF4 and negatively regulates its transcriptional activity without prolyl hydroxylation

Cellular response to hypoxia plays an important role in both circulatory and pulmonary diseases and cancer. Hypoxia-inducible factors (HIFs) are major transcription factors regulating the response to hypoxia. The α-subunits of HIFs are hydroxylated by members of the prolyl-4-hydroxylase domain (PHD) family, PHD1, PHD2, and PHD3, in an oxygen-dependent manner. Here, we report on the identification of ATF4 as a protein interacting with PHD1 as well as PHD3, but not with PHD2. The central region of ATF4 including the Zipper II domain, ODD domain and β-TrCP recognition motif were involved in the interaction with PHD1. Coexistence of PHD1 stabilized ATF4, as opposed to the destabilization of ATF4 by PHD3. Moreover, coexpression of ATF4 destabilized PHD3, whereas PHD1 stability was not affected by the presence of ATF4. Mutations to alanine of proline residues in ATF4 that satisfied hydroxylation consensus by PHDs did not affect binding activity of ATF4 to PHD1 and PHD3. Furthermore, in vitro prolyl hydroxylation assay clearly indicated that ATF4 did not serve as a substrate of both PHD1 and PHD3. Coexpression of PHD1 or PHD3 with ATF4 repressed the transcriptional activity of ATF4. These results suggest that PHD1 and PHD3 control the transactivation activity of ATF4.