A20 deficiency sensitizes pancreatic beta cells to cytokine-induced apoptosis in vitro but does not influence type 1 diabetes development in vivo

Protective effect of TNFAIP3 on testosterone production in Leydig cells under an aging inflammatory microenvironment

BACKGROUND

The aging inflammatory microenvironment surrounding Leydig cells is linked to reduced testosterone levels in males. Tumor necrosis factor alpha-induced protein 3 (TNFAIP3) acts as a critical anti-inflammatory factor in various aging-related diseases. This study aims to investigate the protective effect of TNFAIP3 on testosterone production in Leydig cells under an aging inflammatory microenvironment.

METHODS

Bioinformatics analysis examined TNFAIP3 expression differences in aging rat testes and validated the findings in aging mouse testes. In vitro models of inflammation were established using two Leydig cell lines, with tumor necrosis factor alpha (TNF-α) as the inflammatory factor. Lentiviral transduction was utilized to manipulate TNFAIP3 expression in these cell lines. Transcriptomic sequencing identified differentially expressed genes in TNFAIP3-overexpressing cells.

RESULTS

Bioinformatics analysis and validation experiments revealed increased inflammatory signaling and elevated TNFAIP3 expression in aging rat and mouse testes. TNFAIP3 knockdown worsened testosterone synthesis inhibition and apoptosis in cells, while TNFAIP3 overexpression reversed these effects. Transcriptome analysis identified alterations in the P38MAPK pathway following TNFAIP3 overexpression. TNFAIP3 knockdown enhanced TNF-induced P38MAPK signaling, whereas its overexpression attenuated this effect. TNFAIP3 was found to regulate testosterone synthesis by upregulating CEBPB expression.

CONCLUSIONS

TNFAIP3 exhibits inhibitory effects on apoptosis and promotes testosterone production in Leydig cells. The protective influence of TNFAIP3 on Leydig cells within an inflammatory microenvironment is likely mediated through by inhibiting the P38MAPK pathway and upregulating CEBPB expression.

NF-κB-inducing kinase (NIK) is activated in pancreatic β-cells but does not contribute to the development of diabetes

The transcription factor nuclear factor-κB (NF-κB) has a key role in the pathogenesis of diabetes and its complications. Although activation of the canonical NF-κB pathway in β-cells is generally deleterious, little is known about the role of the non-canonical NF-κB signalling and its main regulator, the NF-κB-inducing kinase (NIK), on pancreatic β-cell survival and function. Previous studies based on models of NIK overexpression in pancreatic islet cells showed that NIK induced either spontaneous β-cell death due to islet inflammation or glucose intolerance during diet-induced obesity (DIO) in mice. Therefore, NIK has been proposed as a potential target for diabetes therapy. However, no clear studies showed whether inhibition of NIK improves diabetes development. Here we show that genetic silencing of NIK in pancreatic β-cells neither modifies diabetes incidence nor inflammatory responses in a mouse model of immune-mediated diabetes. Moreover, NIK silencing in DIO mice did not influence body weight gain, nor glucose metabolism. In vitro studies corroborated the in vivo findings in terms of β-cell survival, function, and downstream gene regulation. Taken together, our data suggest that NIK activation is dispensable for the development of diabetes.

The dual roles of A20 in cancer

A20 is a prototypical anti-inflammatory molecule that is linked to multiple human diseases, including cancers. The role of A20 as a tumor suppressor was first discovered in B cell lymphomas. Subsequent studies revealed the dual roles of A20 in solid cancers. This review focuses on the roles of A20 in different cancer types to demonstrate that the effects of A20 are cancer type-dependent. A20 plays antitumor roles in colorectal carcinomas and hepatocellular carcinomas, whereas A20 acts as an oncogene in breast cancers, gastric cancers and melanomas. Moreover, the roles of A20 in the setting of glioma therapy are context-dependent. The action mechanisms of A20 in different types of cancer are summarized. Additionally, the role of A20 in antitumor immunity is discussed. Furthermore, some open questions in this rapidly advancing field are proposed. Exploration of the actions and molecular mechanisms of A20 in cancer paves the way for the application of A20-targeting approaches in future cancer therapy.

Innate immune receptors in type 1 diabetes: the relationship to cell death-associated inflammation

The importance of innate immunity in host defense and inflammatory responses has been clearly demonstrated after the discovery of innate immune receptors such as Toll-like receptors (TLRs) or Nucleotide-binding oligomerization domain-containing protein (Nod)-like receptors (NLRs). Innate immunity also plays a critical role in diverse pathological conditions including autoimmune diseases such as type 1 diabetes (T1D). In particular, the role of a variety of innate immune receptors in T1D has been demonstrated using mice with targeted disruption of such innate immune receptors. Here, we discuss recent findings showing the role of innate immunity in T1D that were obtained mostly from studies of genetic mouse models of innate immune receptors. In addition, the role of innate immune receptors involved in the pathogenesis of T1D in sensing death-associated molecular patterns (DAMPs) released from dead cells or pathogen-associated molecular patterns (PAMPs) will also be covered. Elucidation of the role of innate immune receptors in T1D and the nature of DAMPs sensed by such receptors may lead to the development of new therapeutic modalities against T1D.

Type 1 diabetes: genes associated with disease development

Type 1 diabetes (T1D) is the third most common autoimmune disease which develops due to genetic and environmental risk factors. Based on the World Health Organization (WHO) report from 2014 the number of people suffering from all types of diabetes ascended to 422 million, compared to 108 million in 1980. It was calculated that this number will double by the end of 2030. In 2015 American Diabetes Association (ADA) announced that 30.3 million Americans (that is 9.4% of the overall population) had diabetes of which only approximately 1.25 million had T1D. Nowadays, T1D represents roughly 10% of adult diabetes cases total. Multiple genetic abnormalities at different loci have been found to contribute to type 1 diabetes development. The analysis of genome-wide association studies (GWAS) of T1D has identified over 50 susceptible regions (and genes within these regions). Many of these regions are defined by single nucleotide polymorphisms (SNPs) but molecular mechanisms through which they increase or lower the risk of diabetes remain unknown. Genetic factors (in existence since birth) can be detected long before the emergence of immunological or clinical markers. Therefore, a comprehensive understanding of the multiple genetic factors underlying T1D is extremely important for further clinical trials and development of personalized medicine for diabetic patients. We present an overview of current studies and information about regions in the human genome associated with T1D. Moreover, we also put forward information about epigenetic modifications, non-coding RNAs and environmental factors involved in T1D development and onset.

The Impact of Acute or Chronic Alcohol Intake on the NF-κB Signaling Pathway in Alcohol-Related Liver Disease

Ethanol misuse is frequently associated with a multitude of profound medical conditions, contributing to health-, individual- and social-related damage. A particularly dangerous threat from this classification is coined as alcoholic liver disease (ALD), a liver condition caused by prolonged alcohol overconsumption, involving several pathological stages induced by alcohol metabolic byproducts and sustained cellular intoxication. Molecular, pathological mechanisms of ALD principally root in the innate immunity system and are especially associated with enhanced functionality of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway. NF-κB is an interesting and convoluted DNA transcription regulator, promoting both anti-inflammatory and pro-inflammatory gene expression. Thus, the abundancy of studies in recent years underlines the importance of NF-κB in inflammatory responses and the mechanistic stimulation of inner molecular motifs within the factor components. Hereby, in the following review, we would like to put emphasis on the correlation between the NF-κB inflammation signaling pathway and ALD progression. We will provide the reader with the current knowledge regarding the chronic and acute alcohol consumption patterns, the molecular mechanisms of ALD development, the involvement of the NF-κB pathway and its enzymatic regulators. Therefore, we review various experimental in vitro and in vivo studies regarding the research on ALD, including the recent active compound treatments and the genetic modification approach. Furthermore, our investigation covers a few human studies.

Preserving immune homeostasis with A20

A20/TNFAIP3 is a TNF induced gene that plays a profound role in preserving cellular and organismal homeostasis (Lee, et al., 2000; Opipari etal., 1990). This protein has been linked to multiple human diseases via genetic, epigenetic, and an emerging series of patients with mono-allelic coding mutations. Diverse cellular functions of this pleiotropically expressed protein include immune-suppressive, anti-inflammatory, and cell protective functions. The A20 protein regulates ubiquitin dependent cell signals; however, the biochemical mechanisms by which it performs these functions is surprisingly complex. Deciphering these cellular and biochemical facets of A20 dependent biology should greatly improve our understanding of murine and human disease pathophysiology as well as unveil new mechanisms of cell and tissue biology.

A20 and Cell Death-driven Inflammation

A20 is a potent anti-inflammatory molecule, and mutations in TNFAIP3, the gene encoding A20, are associated with a wide panel of inflammatory pathologies, both in human and mouse. The anti-inflammatory properties of A20 are commonly attributed to its ability to suppress inflammatory NF-κB signaling by functioning as a ubiquitin-editing enzyme. However, A20 also protects cells from death, independently of NF-κB regulation, and recent work has demonstrated that cell death may drive some of the inflammatory conditions caused by A20 deficiency. Adding to the fact that the protective role of A20 does not primarily rely on its catalytic activities, these findings shed new light on A20 biology.

A20 Restricts Inflammatory Response and Desensitizes Gingival Keratinocytes to Apoptosis

The pathophysiology of periodontal disease involves a perturbed immune system to a dysbiotic microflora leading to unrestrained inflammation, collateral tissue damage, and various systemic complications. Gingival epithelial cells function as an important part of immunity to restrict microbial invasion and orchestrate the subsequent innate responses. A20 (TNFAIP3), an ubiquitin-editing enzyme, is one of the key regulators of inflammation and cell death in numerous tissues including gastrointestinal tract, skin, and lungs. Emerging evidence indicates A20 as an essential molecule in the oral mucosa as well. In this study, we characterized the role of A20 in human telomerase immortalized gingival keratinocytes (TIGKs) through loss and gain of function assays in preclinical models of periodontitis. Depletion of A20 through gene editing in TIGKs significantly increased IL-6 and IL-8 secretion in response to Porphyromonas gingivalis infection while A20 over-expression dampened the cytokine production compared to A20 competent cells through modulating NF-κB signaling pathway. In the subsequent experiments which assessed apoptosis, A20 depleted TIGKs displayed increased levels of cleaved caspase 3 and DNA fragmentation following P. gingivalis infection and TNF/CHX challenge compared to A20 competent cells. Consistently, there was reduced apoptosis in the cells overexpressing A20 compared to the control cells expressing GFP further substantiating the role of A20 in regulating gingival epithelial cell fate in response to exogenous insult. Collectively, our findings reveal first systematic evidence and demonstrate that A20 acts as a regulator of inflammatory response in gingival keratinocytes through its effect on NF-κB signaling and desensitizes cells to bacteria and cytokine induced apoptosis in the oral mucosa. As altered A20 levels can have profound effect on different cellular responses, future studies will determine whether A20-targeted therapies can be exploited to restrain periodontal inflammation and maintain oral mucosa tissue homeostasis.

A20 at the Crossroads of Cell Death, Inflammation, and Autoimmunity

A20 is a potent anti-inflammatory protein, acting by inhibiting nuclear factor κB (NF-κB) signaling and inflammatory gene expression and/or by preventing cell death. Mutations in the A20/TNFAIP3 gene have been associated with a plethora of inflammatory and autoimmune pathologies in humans and in mice. Although the anti-inflammatory role of A20 is well accepted, fundamental mechanistic questions regarding its mode of action remain unclear. Here, we review new findings that further clarify the molecular and cellular mechanisms by which A20 controls inflammatory signaling and cell death, and discuss new evidence for its involvement in inflammatory and autoimmune disease development.

MCL-1 Is a Key Antiapoptotic Protein in Human and Rodent Pancreatic β-Cells

Induction of endoplasmic reticulum stress and activation of the intrinsic apoptotic pathway is widely believed to contribute to β-cell death in type 1 diabetes (T1D). MCL-1 is an antiapoptotic member of the BCL-2 protein family, whose depletion causes apoptosis in rodent β-cells in vitro. Importantly, decreased MCL-1 expression was observed in islets from patients with T1D. We report here that MCL-1 downregulation is associated with cytokine-mediated killing of human β-cells, a process partially prevented by MCL-1 overexpression. By generating a β-cell-specific Mcl-1 knockout mouse strain (βMcl-1KO), we observed that, surprisingly, MCL-1 ablation does not affect islet development and function. β-Cells from βMcl-1KO mice were, however, more susceptible to cytokine-induced apoptosis. Moreover, βMcl-1KO mice displayed higher hyperglycemia and lower pancreatic insulin content after multiple low-dose streptozotocin treatment. We found that the kinase GSK3β, the E3 ligases MULE and βTrCP, and the deubiquitinase USP9x regulate cytokine-mediated MCL-1 protein turnover in rodent β-cells. Our results identify MCL-1 as a critical prosurvival protein for preventing β-cell death and clarify the mechanisms behind its downregulation by proinflammatory cytokines. Development of strategies to prevent MCL-1 loss in the early stages of T1D may enhance β-cell survival and thereby delay or prevent disease progression.

Connectivity mapping (ssCMap) to predict A20-inducing drugs and their antiinflammatory action in cystic fibrosis

Cystic fibrosis (CF) lung disease is characterized by chronic and exaggerated inflammation in the airways. Despite recent developments to therapeutically overcome the underlying functional defect in the cystic fibrosis transmembrane conductance regulator, there is still an unmet need to also normalize the inflammatory response. The prolonged and heightened inflammatory response in CF is, in part, mediated by a lack of intrinsic down-regulation of the proinflammatory NF-κB pathway. We have previously identified reduced expression of the NF-κB down-regulator A20 in CF as a key target to normalize the inflammatory response. Here, we have used publicly available gene array expression data together with a statistically significant connections' map (sscMap) to successfully predict drugs already licensed for the use in humans to induce A20 mRNA and protein expression and thereby reduce inflammation. The effect of the predicted drugs on A20 and NF-κB(p65) expression (mRNA) as well as proinflammatory cytokine release (IL-8) in the presence and absence of bacterial LPS was shown in bronchial epithelial cells lines (16HBE14o-, CFBE41o-) and in primary nasal epithelial cells from patients with CF (Phe508del homozygous) and non-CF controls. Additionally, the specificity of the drug action on A20 was confirmed using cell lines with tnfαip3 (A20) knockdown (siRNA). We also show that the A20-inducing effect of ikarugamycin and quercetin is lower in CF-derived airway epithelial cells than in non-CF cells.

A20 Inhibits β-Cell Apoptosis by Multiple Mechanisms and Predicts Residual β-Cell Function in Type 1 Diabetes

Activation of the transcription factor nuclear factor kappa B (NFkB) contributes to β-cell death in type 1 diabetes (T1D). Genome-wide association studies have identified the gene TNF-induced protein 3 (TNFAIP3), encoding for the zinc finger protein A20, as a susceptibility locus for T1D. A20 restricts NF-κB signaling and has strong antiapoptotic activities in β-cells. Although the role of A20 on NF-κB inhibition is well characterized, its other antiapoptotic functions are largely unknown. By studying INS-1E cells and rat dispersed islet cells knocked down or overexpressing A20 and islets isolated from the β-cell-specific A20 knockout mice, we presently demonstrate that A20 has broader effects in β-cells that are not restricted to inhibition of NF-κB. These involves, suppression of the proapoptotic mitogen-activated protein kinase c-Jun N-terminal kinase (JNK), activation of survival signaling via v-akt murine thymoma viral oncogene homolog (Akt) and consequently inhibition of the intrinsic apoptotic pathway. Finally, in a cohort of T1D children, we observed that the risk allele of the rs2327832 single nucleotide polymorphism of TNFAIP3 predicted lower C-peptide and higher hemoglobin A1c (HbA1c) levels 12 months after disease onset, indicating reduced residual β-cell function and impaired glycemic control. In conclusion, our results indicate a critical role for A20 in the regulation of β-cell survival and unveil novel mechanisms by which A20 controls β-cell fate. Moreover, we identify the single nucleotide polymorphism rs2327832 of TNFAIP3 as a possible prognostic marker for diabetes outcome in children with T1D.