Subcellular localization and mechanisms of nucleocytoplasmic distribution of p202, an interferon-inducible candidate for lupus susceptibility

Mitochondrial dysfunction, a weakest link of network of aging, relation to innate intramitochondrial immunity of DNA recognition receptors

Aging probably is the most complexed process in biology. It is manifested by a variety of hallmarks. These hallmarks weave a network of aging; however, each hallmark is not uniformly strong for the network. It is the weakest link determining the strengthening of the network of aging, or the maximum lifespan of an organism. Therefore, only improvement of the weakest link has the chance to increase the maximum lifespan but not others. We hypothesize that mitochondrial dysfunction is the weakest link of the network of aging. It may origin from the innate intramitochondrial immunity related to the activities of pathogen DNA recognition receptors. These receptors recognize mtDNA as the PAMP or DAMP to initiate the immune or inflammatory reactions. Evidence has shown that several of these receptors including TLR9, cGAS and IFI16 can be translocated into mitochondria. The potentially intramitochondrial presented pathogen DNA recognition receptors have the capacity to attack the exposed second structures of the mtDNA during its transcriptional or especially the replicational processes, leading to the mtDNA mutation, deletion, heteroplasmy colonization, mitochondrial dysfunction, and alterations of other hallmarks, as well as aging. Pre-consumption of the intramitochondrial presented pathogen DNA recognition receptors by medical interventions including development of mitochondrial targeted small molecule which can neutralize these receptors may retard or even reverse the aging to significantly improve the maximum lifespan of the organisms.

The Role of Mitochondria in Systemic Lupus Erythematosus: A Glimpse of Various Pathogenetic Mechanisms

BACKGROUND

Systemic Lupus Erythematosus (SLE) is a polysystem autoimmune disease that adversely affects human health. Various organs can be affected, including the kidney or brain. Traditional treatment methods for SLE primarily rely on glucocorticoids and immunosuppressors. Unfortunately, these therapeutic agents cannot prevent a high recurrence rate after SLE remission. Therefore, novel therapeutic targets are urgently required.

METHODS

A systematic search of the published literature regarding the abnormal structure and function of mitochondria in SLE and therapies targeting mitochondria was performed in several databases.

RESULTS

Accumulating evidence indicates that mitochondrial dysfunction plays important roles in the pathogenesis of SLE, including influencing mitochondrial DNA damage, mitochondrial dynamics change, abnormal mitochondrial biogenesis and energy metabolism, mitophagy, oxidative stress, inflammatory reactions, apoptosis and NETosis. Further investigation of mitochondrial pathophysiological roles will result in further clarification of SLE. Specific lupus-induced organ damage also exhibits characteristic mitochondrial changes.

CONCLUSION

This review aimed to summarize the current research on the role of mitochondrial dysfunction in SLE, which will necessarily provide potential novel therapeutic targets for SLE.

Absent in Melanoma 2 proteins in SLE

Type I interferons (IFN-α/β)-inducible PYRIN and HIN domain-containing protein family includes Absent in Melanoma 2 (murine Aim2 and human AIM2), murine p202, and human PYRIN-only protein 3 (POP3). The generation of Aim2-deficient mice indicated that the Aim2 protein is essential for inflammasome activation, resulting in the secretion of interleukin-1β (IL-1β) and IL-18 and cell death by pyroptosis. Further, Aim2-deficiency also increased constitutive expression of the IFN-β and expression of the p202 protein. Notably, an increased expression of p202 protein in female mice associated with the development of systemic lupus erythematosus (SLE). SLE in patients is characterized by a constitutive increase in serum levels of IFN-α and an increase in the expression IFN-stimulated genes. Recent studies indicate that p202 and POP3 proteins inhibit activation of the Aim2/AIM2 inflammasome and promote IFN-β expression. Therefore, we discuss the role of Aim2/AIM2 proteins in the suppression of type I IFNs production and lupus susceptibility.

Absent in melanoma 2 proteins in the development of cancer

Recent studies utilizing chemical-induced colitis-associated and sporadic colon cancer in mouse models indicated a protective role for absent in melanoma 2 (Aim2) in colon epithelial cells. Accordingly, mutations in the human AIM2 gene have been found in colorectal cancer (CRC), and reduced expression of AIM2 in CRC is associated with its progression. Furthermore, the overexpression of AIM2 protein in human cancer cell lines inhibits cell proliferation. Interferon-inducible Aim2 and AIM2 are members of the PYHIN (PYRIN and HIN domain-containing) protein family and share ~57 % amino acid identity. The family also includes murine p202, human PYRIN-only protein 3, and IFI16, which negatively regulate Aim2/AIM2 functions. Because the CRC incidence and mortality rates are higher among men compared with women and the expression of Aim2/AIM2 proteins and their regulators is dependent upon age, gender, and sex hormones, we discuss the potential roles of Aim2/AIM2 in the development of cancer. An improved understanding of the biological functions of the AIM2 in the development of CRC will likely identify new therapeutic approaches to treat patients.

Interferon-inducible Ifi200-family genes as modifiers of lupus susceptibility

Both genetic and environmental factors contribute to the development and progression of systemic lupus erythematosus (SLE), a complex autoimmune disease. The disease exhibits a strong gender bias and develops predominantly in females. Additionally, most SLE patients exhibit increased serum levels of interferon-α (IFN-α) and the "IFN signature". Studies using the mouse models of lupus have identified several lupus susceptibility loci, including the New Zealand Black (NZB)-derived autoimmunity 2 (Nba2) interval on the chromosome 1. The interval, which is syntenic to the human chromosome 1q region, harbors the FcγR family, SLAM/CD2-family, and the IFN-inducible Ifi200-family genes (encoding for the p200-family proteins). Studies involving the B6.Nba2 congenic mice revealed that the development of antinuclear autoantibodies (ANAs) depends on the age, gender, and activation of type I IFN-signaling. Interestingly, recent studies involving the generation of Nba2 subcongenic mouse lines and generation of mice deficient for the Fcgr2b or Aim2 gene within the interval have provided evidence that epistatic interactions among the Nba2 genes contribute to increased lupus susceptibility. Given that the expression of some of the p200-family proteins is differentially regulated by sex hormones and these proteins differentially regulate cytosolic DNA-induced production of type I IFN and proinflammatory cytokines (IL-1β and IL-18), the major known contributors of SLE-associated inflammation, we discuss the recent advancements in our understanding of the role of p200-family proteins in lupus susceptibility modification. An improved understanding of the role of p200-family proteins in the development of autoimmunity is likely to identify new approaches to treat SLE patients.

DNA-responsive inflammasomes and their regulators in autoimmunity

Upon sensing microbial and self-derived DNA, DNA sensors initiate innate immune responses. These sensors include the interferon (IFN)-inducible Toll-like receptor 9 (TLR9) and PYHIN proteins. Upon sensing DNA, cytosolic (murine Aim2 and human AIM2) and nuclear (IFI16) PYHIN proteins recruit an adaptor protein (ASC) and pro-caspase-1 to form an inflammasome, which activates caspase-1. The activated caspase-1 cleaves pro-IL-1β and pro-IL-18 to generate active forms. However, upon sensing cytosolic DNA, the IFI16 protein recruits STING to induce the expression of type I IFN. Recognition of self DNA by innate immune cells contributes to the production of increased levels of type I IFN. Given that the type I IFNs modulate the expression of inflammasome proteins and that the IFN-inducible proteins inhibit the activity of DNA-responsive inflammasomes, an improved understanding of the molecular mechanisms that regulate the activity of DNA-responsive inflammasomes is likely to identify new therapeutic targets to treat autoimmune diseases.

Cell type and gender-dependent differential regulation of the p202 and Aim2 proteins: implications for the regulation of innate immune responses in SLE

Upon sensing cytosolic double-stranded DNA (dsDNA), the murine Aim2 (encoded by the Aim2 gene) protein forms an inflammasome and promotes the secretion of proinflammatory cytokines, such as IL-1β and IL-18. In contrast, the p202 protein (encoded by the Ifi202 gene) does not form an inflammasome. Previously, we have reported that the interferon (IFN) and female sex hormone-induced increased nuclear levels of p202 protein in immune cells are associated with increased susceptibility to develop a lupus-like disease. However, signaling pathways that regulate the expression of Aim2 protein remain unknown. Here we report that the expression of Aim2 gene is induced in bone marrow-derived macrophages (BMDMs) by IFN-α treatment and the expression is, in part, STAT1-dependent. However, treatment of splenic T or B cells with IFN-α or their stimulation, which induced the expression of Ifi202 gene, did not induce the expression of Aim2 gene. Furthermore, treatment of cells with the male hormone androgen increased levels of Aim2 mRNA and protein. Moreover, treatment of murine macrophage cell lines (RAW264.7 and J774A.1) with IFN-α differentially induced the expression of Aim2 and p202 proteins and regulated their sub-cellular localization. Additionally, activation of Toll-like receptors (TLR3, 4, and 9) in BMDMs and cell lines also differentially regulated the expression of Aim2 and Ifi202 genes. Our observations demonstrate that cell type and gender-dependent factors differentially regulate the expression of the Aim2 and p202 proteins, thus, suggesting opposing roles for these two proteins in innate immune responses in lupus disease.

Emerging roles for the interferon-inducible p200-family proteins in sex bias in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a complex autoimmune disease involving multiple organs. The disease is characterized by the production of pathogenic autoantibodies to DNA and certain nuclear antigens, chronic inflammation, and immune dysregulation. Genetic studies involving SLE patients and mouse models have indicated that multiple lupus susceptible genes contribute to the disease phenotype. Notably, the development of SLE in patients and in certain mouse models exhibits a strong sex bias. In addition, several lines of evidence indicates that activation of interferon-α (IFN-α) signaling in immune cells and alterations in the expression of certain immunomodulatory cytokines contribute to lupus pathogenesis. Studies have implicated factors, such as the X chromosomal gene dosage effect and the sex hormones, in gender bias in SLE. However, the molecular mechanisms remain unclear. Additionally, it remains unclear whether these factors influence the "IFN-signature," which is associated with SLE. In this regard, a mutually positive regulatory feedback loop between IFNs and estrogen receptor-α (ERα) has been identified in immune cells. Moreover, studies indicate that the expression of certain IFN-inducible p200-family proteins that act as innate immune sensors for cytosolic DNA is differentially regulated by sex hormones. In this review, we discuss how the modulation of the expression of the p200-family proteins in immune cells by sex hormones and IFNs contributes to sex bias in SLE. An improved understanding of the regulation and roles of the p200-family proteins in immune cells is critical to understand lupus pathogenesis as well as response (or the lack of it) to various therapies.

Interferon-inducible p200-family proteins as novel sensors of cytoplasmic DNA: role in inflammation and autoimmunity

Deregulated innate immune responses that result in increased levels of type I interferons (IFNs) and stimulation of IFN-inducible genes are thought to contribute to chronic inflammation and autoimmunity. One family of IFN-inducible genes is the Ifi200 family, which includes the murine (eg, Ifi202a, Ifi202b, Ifi203, Ifi204, Mndal, and Aim2) and human (eg, IFI16, MNDA, IFIX, and AIM2) genes. Genes in the family encode structurally related proteins (the p200-family proteins), which share at least one partially conserved repeat of 200-amino acid (200-AA) residues. Consistent with the presence of 2 consecutive oligonucleotide/oligosaccharide-binding folds in the repeat, the p200-family proteins can bind to DNA. Additionally, these proteins (except the p202 proteins) also contain a pyrin (PYD) domain in the N-terminus. Increased expression of p202 proteins in certain strains of female mice is associated with lupus-like disease. Interestingly, only the Aim2 protein is conserved between the mouse and humans. Several recent studies have provided evidence that the Aim2 and p202 proteins can recognize DNA in cytoplasm and the Aim2 protein upon sensing DNA can form a caspase-1-activating inflammasome. In this review, we discuss how the ability of p200-family proteins to sense cytoplasmic DNA may contribute to the development of chronic inflammation and associated diseases.

Aim2 deficiency stimulates the expression of IFN-inducible Ifi202, a lupus susceptibility murine gene within the Nba2 autoimmune susceptibility locus

Murine Aim2 and p202 proteins (encoded by the Aim2 and Ifi202 genes) are members of the IFN-inducible p200 protein family. Both proteins can sense dsDNA in the cytoplasm. However, upon sensing dsDNA, only the Aim2 protein through its pyrin domain can form an inflammasome to activate caspase-1 and induce cell death. Given that the p202 protein has been predicted to inhibit the activation of caspase-1 by the Aim2 protein and that increased levels of the p202 protein in female mice of certain strains are associated with lupus susceptibility, we compared the expression of Aim2 and Ifi202 genes between Aim2-deficient and age-matched wild-type mice. We found that the Aim2 deficiency in immune cells stimulated the expression of Ifi202 gene. The increased levels of the p202 protein in cells were associated with increases in the expression of IFN-β, STAT1, and IFN-inducible genes. Moreover, after knockdown of Aim2 expression in the murine macrophage cell line J774.A1, IFN-β treatment of cells robustly increased STAT1 protein levels (compared with those of control cells), increased the activating phosphorylation of STAT1 on Tyr-701, and stimulated the activity of an IFN-responsive reporter. Notably, the expression of Aim2 in non-lupus-prone (C57BL/6 and B6.Nba2-C) and lupus-prone (B6.Nba2-ABC) splenic cells and in a murine macrophage cell line that overexpressed p202 protein was found to be inversely correlated with Ifi202. Collectively, our observations demonstrate an inverse correlation between Aim2 and p202 expressions. We predict that defects in Aim2 expression within immune cells contribute to increased susceptibility to lupus.

Systemic lupus erythematosus and increased risk to develop B cell malignancies: role of the p200-family proteins

Systemic lupus erythematosus (SLE), an autoimmune disease, develops at a female-to-male ratio of 10:1. Increased serum levels of type I interferons (IFN-alpha/beta) and induction of "IFN-signature" genes are associated with an active SLE disease in patients. Moreover, SLE patients exhibit three- to four-fold increase in the risk of developing malignancies involving B cells, including non-Hodgkin lymphoma (NHL) and Hodgkin's lymphoma (HL). Interestingly, homozygous mice expressing a deletion mutant (the proline-rich domain deleted) of the p53 develop various types of spontaneous tumors, particularly of B cell origin upon aging. The deletion is associated with defects in transcriptional activation of genes by p53 and inhibition of DNA damage-induced apoptosis. Notably, increased levels of the p202 protein, which is encoded by the p53-repressible interferon-inducible Ifi202 gene, in B cells of female mice are associated with defects in B cell apoptosis, inhibition of the p53-mediated transcription of pro-apoptotic genes, and increased lupus susceptibility. In this review we discuss how increased levels of the p202 protein (and its human functional homologue IFI16 protein) in B cells increase lupus susceptibility and are likely to increase the risk of developing certain B cell malignancies. A complete understanding of the molecular mechanisms that regulate B cell homeostasis is necessary to identify SLE patients with an increased risk to develop B cell malignancies.

Interferon-inducible Ifi200-family genes in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is the prototype of complex autoimmune diseases. Studies have suggested that genetic, hormonal, and environmental factors contribute to the development of the disease. Interestingly, several recent studies involving SLE patients and mouse models of the disease have suggested a role for interferon (IFN)-stimulated genes (ISGs) in the development of SLE. One family of ISGs is the Ifi200-family, which includes mouse (Ifi202a, Ifi202b, Ifi203, Ifi204, and Ifi205) and human (IFI16, MNDA, AIM2, and IFIX) genes. The mouse genes cluster between serum amyloid P-component (Apcs) and alpha-spectrin (Spna-1) genes on chromosome 1 and the human genes cluster in syntenic region 1q23. The Ifi200-family genes encode structurally and functionally related proteins (the p200-family proteins). Increased expression of certain p200-family proteins in cells is associated with inhibition of cell proliferation, modulation of apoptosis, and cell differentiation. Our studies involving generation of B6.Nba2 congenic mice, coupled with gene expression analyses, identified the Ifi202 as a candidate lupus-susceptibility gene. Importantly, recent studies using different mouse models of SLE have suggested that increased expression of Ifi202 gene (encoding p202 protein) in immune cells contributes to lupus susceptibility. Consistent with a functional role for the p202 protein in lupus susceptibility, increased levels of IFI16 protein in human SLE patients are associated with the diseases. This review summarizes recent findings concerning the regulation and role of p200-family proteins in the development of SLE.

Cloning and characterisation of Ifi206: a new murine HIN-200 family member

HIN-200 proteins are interferon-inducible proteins capable of regulating cell growth, senescence, differentiation and death. Using a combination of in silico analysis of NCBI EST databases and screening of murine C57BL/6 cDNA libraries we isolated novel murine HIN-200 cDNAs designated Ifi206S and Ifi206L encoding two putative mRNA splice variants. The p206S and p206L protein isoforms have a modular domain structure consisting of an N-terminal PAAD/DAPIN/Pyrin domain, a region rich in serine, threonine and proline residues and a C-terminal 200 B domain characteristic of other HIN-200 proteins. Ifi206 mRNA was detected only in the spleen and lung of BALB/c and C57BL/6 mice and expression was up-regulated by both types I and II IFN subtypes. p206 protein was predominantly expressed in the cytoplasm and addition of LMB, a CRM1 dependent nuclear export inhibitor, caused p206 to accumulate in the nucleus. Unlike other human and mouse HIN-200 proteins that contain only a single 200 amino acid domain, overexpression of p206 impaired the clonogenic growth of tumour cell lines. Thus, p206 represents the newest HIN-200 family member discovered. It has distinct and restricted pattern of expression however maintains many of the hallmarks of HIN-200 proteins including the presence of a characteristic 200 X domain, induction by interferon and an ability to suppress tumour cell growth.

Increased expression of Ifi202, an IFN-activatable gene, in B6.Nba2 lupus susceptible mice inhibits p53-mediated apoptosis

Increased expression of p202 protein (encoded by the Ifi202 gene) in splenocytes derived from B6.Nba2 mice (congenic for the Nba2 interval derived from the New Zealand Black mice) was correlated with defects in apoptosis of splenic B cells and increased susceptibility to develop systemic lupus erythematosus. We have now investigated the molecular mechanisms by which increased expression of p202 in B6.Nba2 cells contributes to defects in apoptosis. In this study, we report that increased expression of p202 in the B6.Nba2 splenocytes, as compared with cells derived from the parental C57BL/6 (B6) mice, was correlated with increased levels of p53 protein and inhibition of p53-mediated transcription of target genes that encode proapoptotic proteins. Conversely, knockdown of p202 expression in B6.Nba2 cells resulted in stimulation of p53-mediated transcription. We found that p202 bound to p53 in the N-terminal region (aa 44-83) comprising the proline-rich region that is important for p53-mediated apoptosis. Consistent with the binding of p202 to p53, increased expression of p202 in B6.Nba2 mouse embryonic fibroblasts inhibited UV-induced apoptosis. Taken together, our observations support the idea that increased expression of p202 in B6.Nba2 mice increases the susceptibility to develop lupus, in part, by inhibiting p53-mediated apoptosis.