The Four Homeostasis Knights: In Balance upon Post-Translational Modifications

A cancer outcome is a multifactorial event that comes from both exogenous injuries and an endogenous predisposing background. The healthy state is guaranteed by the fine-tuning of genes controlling cell proliferation, differentiation, and development, whose alteration induces cellular behavioral changes finally leading to cancer. The function of proteins in cells and tissues is controlled at both the transcriptional and translational level, and the mechanism allowing them to carry out their functions is not only a matter of level. A major challenge to the cell is to guarantee that proteins are made, folded, assembled and delivered to function properly, like and even more than other proteins when referring to oncogenes and onco-suppressors products. Over genetic, epigenetic, transcriptional, and translational control, protein synthesis depends on additional steps of regulation. Post-translational modifications are reversible and dynamic processes that allow the cell to rapidly modulate protein amounts and function. Among them, ubiquitination and ubiquitin-like modifications modulate the stability and control the activity of most of the proteins that manage cell cycle, immune responses, apoptosis, and senescence. The crosstalk between ubiquitination and ubiquitin-like modifications and post-translational modifications is a keystone to quickly update the activation state of many proteins responsible for the orchestration of cell metabolism. In this light, the correct activity of post-translational machinery is essential to prevent the development of cancer. Here we summarize the main post-translational modifications engaged in controlling the activity of the principal oncogenes and tumor suppressors genes involved in the development of most human cancers.

Hops/Tmub1 Heterozygous Mouse Shows Haploinsufficiency Effect in Influencing p53-Mediated Apoptosis

HOPS is a ubiquitin-like protein implicated in many aspects of cellular function including the regulation of mitotic activity, proliferation, and cellular stress responses. In this study, we focused on the complex relationship between HOPS and the tumor suppressor p53, investigating both transcriptional and non-transcriptional p53 responses. Here, we demonstrated that Hops heterozygous mice and mouse embryonic fibroblasts exhibit an impaired DNA-damage response to etoposide-induced double-strand breaks when compared to wild-type genes. Specifically, alterations in HOPS levels caused significant defects in the induction of apoptosis, including a reduction in p53 protein level and percentage of apoptotic cells. We also analyzed the effect of reduced HOPS levels on the DNA-damage response by examining the transcript profiles of p53-dependent genes, showing a suggestive deregulation of the mRNA levels for a number of p53-dependent genes. Taken together, these results show an interesting haploinsufficiency effect mediated by Hops monoallelic deletion, which appears to be enough to destabilize the p53 protein and its functions. Finally, these data indicate a novel role for Hops as a tumor-suppressor gene in DNA damage repair in mammalian cells.

Anakinra Activates Superoxide Dismutase 2 to Mitigate Inflammasome Activity

Inflammasomes are powerful cytosolic sensors of environmental stressors and are critical for triggering interleukin-1 (IL-1)-mediated inflammatory responses. However, dysregulation of inflammasome activation may lead to pathological conditions, and the identification of negative regulators for therapeutic purposes is increasingly being recognized. Anakinra, the recombinant form of the IL-1 receptor antagonist, proved effective by preventing the binding of IL-1 to its receptor, IL-1R1, thus restoring autophagy and dampening NLR family pyrin domain containing 3 (NLRP3) activity. As the generation of mitochondrial reactive oxidative species (ROS) is a critical upstream event in the activation of NLRP3, we investigated whether anakinra would regulate mitochondrial ROS production. By profiling the activation of transcription factors induced in murine alveolar macrophages, we found a mitochondrial antioxidative pathway induced by anakinra involving the manganese-dependent superoxide dismutase (MnSOD) or SOD2. Molecularly, anakinra promotes the binding of SOD2 with the deubiquitinase Ubiquitin Specific Peptidase 36 (USP36) and Constitutive photomorphogenesis 9 (COP9) signalosome, thus increasing SOD2 protein longevity. Functionally, anakinra and SOD2 protects mice from pulmonary oxidative inflammation and infection. On a preclinical level, anakinra upregulates SOD2 in murine models of chronic granulomatous disease (CGD) and cystic fibrosis (CF). These data suggest that protection from mitochondrial oxidative stress may represent an additional mechanism underlying the clinical benefit of anakinra and identifies SOD2 as a potential therapeutic target.

The Circadian Protein PER1 Modulates the Cellular Response to Anticancer Treatments

The circadian clock driven by the daily light–dark and temperature cycles of the environment regulates fundamental physiological processes and perturbations of these sophisticated mechanisms may result in pathological conditions, including cancer. While experimental evidence is building up to unravel the link between circadian rhythms and tumorigenesis, it is becoming increasingly apparent that the response to antitumor agents is similarly dependent on the circadian clock, given the dependence of each drug on the circadian regulation of cell cycle, DNA repair and apoptosis. However, the molecular mechanisms that link the circadian machinery to the action of anticancer treatments is still poorly understood, thus limiting the application of circadian rhythms-driven pharmacological therapy, or chronotherapy, in the clinical practice. Herein, we demonstrate the circadian protein period 1 (PER1) and the tumor suppressor p53 negatively cross-regulate each other’s expression and activity to modulate the sensitivity of cancer cells to anticancer treatments. Specifically, PER1 physically interacts with p53 to reduce its stability and impair its transcriptional activity, while p53 represses the transcription of PER1. Functionally, we could show that PER1 reduced the sensitivity of cancer cells to drug-induced apoptosis, both in vitro and in vivo in NOD scid gamma (NSG) mice xenotransplanted with a lung cancer cell line. Therefore, our results emphasize the importance of understanding the relationship between the circadian clock and tumor regulatory proteins as the basis for the future development of cancer chronotherapy.

Functional expression and localisation of HOPS/TMUB1 in mouse lens

Transparency represents the functional phenotype of eye lens. A number of defined steps including quiescence, proliferation, migration and cell differentiation culminates in cell elongation and organelle degradation, allowing the light to reach the retina. HOPS (Hepatocyte Odd Protein Shuttling)/TMUB1 (Trans Membrane Ubiquitin-like containing protein 1) is a nucleo-cytoplasmic shuttling protein, highly expressed both in vivo and in vitro proliferating systems, bearing a ubiquitin-like domain. The present study shows HOPS expression during the phases of lens cell proliferation and fiber differentiation, and its localisation in lens compartments. In lens, HOPS localises mainly in the nucleus of central epithelial cells. During mitosis, HOPS/TMUB1 shuttles to the cytoplasm and returns to the nucleus at the end of mitosis. The differentiating cells share distinct HOPS/TMUB1 localisation in transitional zone depending on the differentiation phases. HOPS/TMUB1 is observed in lens cortex and nucleus. Here, it is attached to fibers, having a structural function with crystallin proteins, probably acting in the ubiquitin–proteasome system.

HOPS and p53: thick as thieves in life and death

The oncosuppressor protein p53 plays a major role in transcriptionally controlling the expression of a number of genes, which in turn regulates many functions in response to DNA damage, oncogene triggering, oxidative, and additional cell stresses. A developing area of interest in p53 is the studies related to its cytoplasmic function(s). Many investigations revealed the significant role of p53 in the cytoplasm, acting in a transcriptional-independent manner in important processes related to cell homeostasis such as; apoptosis, autophagy, metabolism control, drug, and oxidative stress response. The studies on cytoplasmic p53 have shown intricate mechanisms by which posttranslational modifications allow p53 to perform its cytoplasmic functions. A number of ubiquitins, deubiquitins, and small ubiquitin-like proteins, have a pivotal role in controlling cytoplasmic stability and localization. Recently, HOPS/TMUB1 a novel small ubiquitin-like protein has been described as a vital molecule stabilizing p53 half-life, directing it to the mitochondria and favoring p53-mediated apoptosis. Furthermore, HOPS/TMUB1 competing with importin-α lessens p53 nuclear localization, thereby increasing cytoplasmic concentration. HOPS/TMUB1 as p53 modifiers could be attractive candidates to elucidate apoptosis or other important transcriptional-independent functions which are key in cancer research in order to develop new therapeutic approaches.

HOPS/Tmub1 involvement in the NF-kB-mediated inflammatory response through the modulation of TRAF6

HOPS/Tmub1 is a ubiquitously expressed transmembrane ubiquitin-like protein that shuttles between nucleus and cytoplasm during cell cycle progression. HOPS causes cell cycle arrest in G₀/G₁ phase, an event associated to stabilization of p19^Arf, an important tumor suppressor protein. Moreover, HOPS plays an important role in driving centrosomal assembly and maintenance, mitotic spindle proper organization, and ultimately a correct cell division. Recently, HOPS has been described as an important regulator of p53, which acts as modifier, stabilizing p53 half-life and playing a key role in p53 mediating apoptosis after DNA damage. NF-κB is a transcription factor with a central role in many cellular events, including inflammation and apoptosis. Our experiments demonstrate that the transcriptional activity of the p65/RelA NF-κB subunit is regulated by HOPS. Importantly, Hops^−/− cells have remarkable alterations of pro-inflammatory responses. Specifically, we found that HOPS enhances NF-κB activation leading to increase transcription of inflammatory mediators, through the reduction of IκBα stability. Notably, this effect is mediated by a direct HOPS binding to the E3 ubiquitin ligase TRAF6, which lessens TRAF6 stability ultimately leading increased IKK complex activation. These findings uncover a previously unidentified function of HOPS/Tmub1 as a novel modulator of TRAF6, regulating inflammatory responses driven by activation of the NF-κB signaling pathway. The comprehension on how HOPS/Tmub1 takes part to the inflammatory processes in vivo and whether this function is important in the control of proliferation and tumorigenesis could establish the basis for the development of novel pharmacological strategies.

Histone Deacetylase SIRT1 Controls Proliferation, Circadian Rhythm, and Lipid Metabolism during Liver Regeneration in Mice

Liver regeneration offers a distinctive opportunity to study cell proliferation in vivo Mammalian silent information regulator 1 (SIRT1), a NAD⁺-dependent histone deacetylase, is an important regulator of various cellular processes, including proliferation, metabolism, and circadian rhythms. In the liver, SIRT1 coordinates the circadian oscillation of clock-controlled genes, including genes that encode enzymes involved in metabolic pathways. We performed partial hepatectomy in WT and liver-specific Sirt1-deficient mice and analyzed the expression of cell cycle regulators in liver samples taken at different times during the regenerative process, by real time PCR, Western blotting analysis, and immunohistochemistry. Lipidomic analysis was performed in the same samples by MS/HPLC. We showed that G₁/S progression was significantly affected by absence of SIRT1 in the liver, as well as circadian gene expression. This was associated to lipid accumulation due to defective fatty acid beta-oxidation. Our study revealed for the first time the importance of SIRT1 in the regulation of hepatocellular proliferation, circadian rhythms, and lipid metabolism during liver regeneration in mice. These results represent an additional step toward the characterization of SIRT1 function in the liver.

Impaired cell proliferation in regenerating liver of 3 β-hydroxysterol Δ14-reductase (TM7SF2) knock-out mice

The liver is the most important organ in cholesterol metabolism, which is instrumental in regulating cell proliferation and differentiation. The gene Tm7sf2 codifies for 3 β-hydroxysterol-Δ14-reductase (C14-SR), an endoplasmic reticulum resident protein catalyzing the reduction of C14-unsaturated sterols during cholesterol biosynthesis from lanosterol. In this study we analyzed the role of C14-SR in vivo during cell proliferation by evaluating liver regeneration in Tm7sf2 knockout (KO) and wild-type (WT) mice. Tm7sf2 KO mice showed no alteration in cholesterol content. However, accumulation and delayed catabolism of hepatic triglycerides was observed, resulting in persistent steatosis at all times post hepatectomy. Moreover, delayed cell cycle progression to the G1/S phase was observed in Tm7sf2 KO mice, resulting in reduced cell division at the time points examined. This was associated to abnormal ER stress response, leading to alteration in p53 content and, consequently, induction of p21 expression in Tm7sf2 KO mice. In conclusion, our results indicate that Tm7sf2 deficiency during liver regeneration alters lipid metabolism and generates a stress condition, which, in turn, transiently unbalances hepatocytes cell cycle progression.

NEDD4 controls the expression of GUCD1, a protein upregulated in proliferating liver cells

Liver regeneration is a unique means of studying cell proliferation in vivo. Screening of a large cDNA library from regenerating liver has previously allowed us to identify and characterize a cluster of genes encoding proteins with important roles in proliferative processes. Here, by examining different rat and human tissues as well as cell lines, we characterized a highly conserved gene, guanylyl cyclase domain containing 1 (GUCD1), whose modulation occurs in liver regeneration and cell cycle progression in vitro. High-level expression of GUCD1 transcripts was observed in livers from patients with hepatocellular carcinoma. A yeast two-hybrid interaction assay, aimed at identifying any relevant interaction partners of GUCD1, revealed direct interactions with NEDD4-1 (E3 ubiquitin protein ligase neural precursor cell expressed, developmentally downregulated gene 4), resulting in control of GUCD1 stability. Thus, we have characterized expression and function of a ubiquitous protein, GUCD1, which might have a role in regulating normal and abnormal cell growth in the liver.

Hepatocyte odd protein shuttling (HOPS) is a bridging protein in the nucleophosmin-p19 Arf network

Nucleophosmin (NPM), a ubiquitously and abundantly expressed protein, occurs in the nucleolus, shuttling between the nucleoplasm and cytoplasm. The NPM gene is mutated in almost 30% of human acute myeloid leukemia cells. NPM interacts with p53 and p19(Arf), directs localization of p19(Arf) in the nucleolus and protects the latter from degradation. Hepatocyte odd protein shuttling (HOPS) is also a ubiquitously expressed protein that moves between the nucleus and cytoplasm. Within the nucleus of resting cells, HOPS overexpression causes cell cycle arrest in G0/G1. HOPS knockdown causes centrosome hyperamplification leading to multinucleated cells and the formation of micronuclei. We demonstrate a direct interaction of HOPS with NPM and p19(Arf), resulting in a functionally active trimeric complex. NPM appeared to regulate HOPS half-life, which, in turn, stabilized p19(Arf) and controlled its localization in the nucleolus. These findings suggest that HOPS acts as a functional bridge in the interaction between NPM and p19(Arf), providing new mechanistic insight into how NPM and p19(Arf) will oppose tumor cell proliferation.