Pathologic effects of RNase-L dysregulation in immunity and proliferative control

IFN-γ restores the impaired function of RNase L and induces mitochondria-mediated apoptosis in lung cancer

RNase L is an essential component in interferon (IFN)-mediated antiviral signaling that showed antitumor effects in cancer. Cancer immunotherapy based on interferon has achieved encouraging results that indicate an applicable potential for cancer therapy. Here we showed that function of RNase L, though highly upregulated, was functionally impaired both in nuclear and cytoplasm in lung cancer cells. In normal lung epithelial cells, RNase L activation induced by 2–5A promoted nuclear condensation, DNA cleavage, and cell apoptosis, while in lung cancer cells, these processes were inhibited and RNase L-mediated downregulation of fibrillarin, Topo I and hnRNP A1 was also impaired in lung cancer cells. Moreover, the impairment of RNase L in lung cancer cells was due to the elevated expression of RLI. Application of IFN-γ to lung cancer cells led to enhanced expression of RNase L that compensated the RLI inhibition and restored the cytoplasmic and nuclear function of RNase L, leading to apoptosis of lung cancer cells. Thus, the present study discovered the impaired function and mechanism of RNase L in lung cancer cells and proved the efficacy of IFN-γ in restoring RNase L function and inducing apoptosis in the lung cancer cell. These results indicated the RNase L as a therapeutic target in lung cancer cells and immunotherapy of IFN-γ may serve as an adjuvant to enhance the efficacy.

The RNASEL -1385G/A polymorphism is associated with risk of prostate cancer in Africans

The RNASEL –1385G/A (rs486907) variant has been reported to be associated with increased risk of prostate cancer. However, these associations are not consistent among studies. To address this issue, we performed a meta-analysis to evaluate the association between RNASEL –1385G/A polymorphism and prostate cancer risk. The PubMed, Embase, and Web of Science databases were searched for relevant papers published in the past 20 years from 1997 to 2017. Odds ratios (ORs) and 95% confidence intervals (CIs) were used to assess the strength of associations. Based on our search for manuscripts reporting prostate cancer susceptibility related to the rs486907 polymorphism, 16 case–control studies from 13 different publications were retrieved. No significantly positive associations were found for the polymorphism and prostate cancer susceptibility in the total population. When stratified by ethnicity, the results demonstrated that the –1385G/A polymorphism was associated with a decreased cancer risk in Africans (GG vs AA: OR =0.371, 95% CI =0.176–0.783; GG/GA vs AA: OR =0.368, 95% CI =0.175–0.776). We also found that the rs486907 polymorphism was associated with a decreased cancer risk in hospital-based controls (GG vs AA: OR =0.697, 95% CI =0.488–0.996; GG + GA vs AA: OR =0.701, 95% CI =0.502–0.978). Our meta-analysis suggests that polymorphism in the RNASEL gene is a protective factor against prostate cancer in Africans. Further studies using larger sample sizes should be conducted to elucidate the role of gene polymorphism in prostate cancer risk.

Chemical Biology Probes from Advanced DNA-encoded Libraries

The identification of bioactive compounds is a crucial step toward development of probes for chemical biology studies. Screening of DNA-encoded small molecule libraries (DELs) has emerged as a validated technology to interrogate vast chemical space. DELs consist of chimeric molecules composed of a low-molecular weight compound that is conjugated to a DNA identifier tag. They are screened as pooled libraries using selection to identify "hits." Screening of DELs has identified numerous bioactive compounds. Some of these molecules were instrumental in gaining a deeper understanding of biological systems. One of the main challenges in the field is the development of synthesis methodology for DELs.

The Roles of RNase-L in Antimicrobial Immunity and the Cytoskeleton-Associated Innate Response

The interferon (IFN)-regulated endoribonuclease RNase-L is involved in multiple aspects of the antimicrobial innate immune response. It is the terminal component of an RNA cleavage pathway in which dsRNA induces the production of RNase-L-activating 2-5A by the 2′-5′-oligoadenylate synthetase. The active nuclease then cleaves ssRNAs, both cellular and viral, leading to downregulation of their expression and the generation of small RNAs capable of activating retinoic acid-inducible gene-I (RIG-I)-like receptors or the nucleotide-binding oligomerization domain-like receptor 3 (NLRP3) inflammasome. This leads to IFNβ expression and IL-1β activation respectively, in addition to broader effects on immune cell function. RNase-L is also one of a growing number of innate immune components that interact with the cell cytoskeleton. It can bind to several cytoskeletal proteins, including filamin A, an actin-binding protein that collaborates with RNase-L to maintain the cellular barrier to viral entry. This antiviral activity is independent of catalytic function, a unique mechanism for RNase-L. We also describe here the interaction of RNase-L with the E3 ubiquitin ligase and scaffolding protein, ligand of nump protein X (LNX), a regulator of tight junction proteins. In order to better understand the significance and context of these novel binding partners in the antimicrobial response, other innate immune protein interactions with the cytoskeleton are also discussed.

Family History of Cancer in Relation to Breast Cancer Subtypes in African American Women

BACKGROUND

The evidence on the relation of family history of cancers other than breast cancer to breast cancer risk is conflicting, and most studies have not assessed specific breast cancer subtypes.

METHODS

We assessed the relation of first-degree family history of breast, prostate, lung, colorectal, ovarian, and cervical cancer and lymphoma or leukemia, to the risk of estrogen receptor-positive (ER(+)), ER(-), and triple-negative breast cancer in data from the African American Breast Cancer Epidemiology and Risk Consortium. Multivariable logistic regression models were used to calculate ORs and 95% confidence intervals (CI).

RESULTS

There were 3,023 ER(+) and 1,497 ER(-) breast cancer cases (including 696 triple-negative cases) and 17,420 controls. First-degree family history of breast cancer was associated with increased risk of each subtype: OR = 1.76 (95% CI, 1.57-1.97) for ER(+), 1.67 (1.42-1.95) for ER(-), and 1.72 (1.38-2.13) for triple-negative breast cancer. Family history of cervical cancer was associated with increased risk of ER(-) (OR = 2.39; 95% CI, 1.36-4.20), but not ER(+) cancer. Family history of both breast and prostate cancer was associated with increased risk of ER(+) (3.40; 2.42-4.79) and ER(-) (2.09; 1.21-3.63) cancer, but family history of both breast and lung cancer was associated only with ER(-) cancer (2.11; 1.29-3.46).

CONCLUSIONS

A family history of cancers other than breast may influence the risk of breast cancer, and associations may differ by subtype.

IMPACT

Greater surveillance and counseling for additional screening may be warranted for women with a family history of cancer.

Cardif (MAVS) Regulates the Maturation of NK Cells

Cardif, also known as IPS-1, VISA, and MAVS, is an intracellular adaptor protein that functions downstream of the retinoic acid-inducible gene I family of pattern recognition receptors. Cardif is required for the production of type I IFNs and other inflammatory cytokines after retinoic acid-inducible gene I-like receptors recognize intracellular antigenic RNA. Studies have recently shown that Cardif may have other roles in the immune system in addition to its role in viral immunity. In this study, we find that the absence of Cardif alters normal NK cell development and maturation. Cardif(-/-) mice have a 35% loss of mature CD27(-)CD11b(+) NK cells in the periphery. In addition, Cardif(-/-) NK cells have altered surface marker expression, lower cytotoxicity, decreased intracellular STAT1 levels, increased apoptosis, and decreased proliferation compared with wild-type NK cells. Mixed chimeric mice revealed that the defective maturation and increased apoptotic rate of peripheral Cardif(-/-) NK cells is cell intrinsic. However, Cardif(-/-) mice showed enhanced control of mouse CMV (a DNA β-herpesvirus) by NK cells, commensurate with increased activation and IFN-γ production by these immature NK cell subsets. These results indicate that the skewed differentiation and altered STAT expression of Cardif(-/-) NK cells can result in their hyperresponsiveness in some settings and support recent findings that Cardif-dependent signaling can regulate aspects of immune cell development and/or function distinct from its well-characterized role in mediating cell-intrinsic defense to RNA viruses.

The Role of Phosphodiesterase 12 (PDE12) as a Negative Regulator of the Innate Immune Response and the Discovery of Antiviral Inhibitors

2',5'-Oligoadenylate synthetase (OAS) enzymes and RNase-L constitute a major effector arm of interferon (IFN)-mediated antiviral defense. OAS produces a unique oligonucleotide second messenger, 2',5'-oligoadenylate (2-5A), that binds and activates RNase-L. This pathway is down-regulated by virus- and host-encoded enzymes that degrade 2-5A. Phosphodiesterase 12 (PDE12) was the first cellular 2-5A- degrading enzyme to be purified and described at a molecular level. Inhibition of PDE12 may up-regulate the OAS/RNase-L pathway in response to viral infection resulting in increased resistance to a variety of viral pathogens. We generated a PDE12-null cell line, HeLaΔPDE12, using transcription activator-like effector nuclease-mediated gene inactivation. This cell line has increased 2-5A levels in response to IFN and poly(I-C), a double-stranded RNA mimic compared with the parental cell line. Moreover, HeLaΔPDE12 cells were resistant to viral pathogens, including encephalomyocarditis virus, human rhinovirus, and respiratory syncytial virus. Based on these results, we used DNA-encoded chemical library screening to identify starting points for inhibitor lead optimization. Compounds derived from this effort raise 2-5A levels and exhibit antiviral activity comparable with the effects observed with PDE12 gene inactivation. The crystal structure of PDE12 complexed with an inhibitor was solved providing insights into the structure-activity relationships of inhibitor potency and selectivity.

RNase-L control of cellular mRNAs: roles in biologic functions and mechanisms of substrate targeting

RNase-L is a mediator of type 1 interferon-induced antiviral activity that has diverse and critical cellular roles, including the regulation of cell proliferation, differentiation, senescence and apoptosis, tumorigenesis, and the control of the innate immune response. Although RNase-L was originally shown to mediate the endonucleolytic cleavage of both viral and ribosomal RNAs in response to infection, more recent evidence indicates that RNase-L also functions in the regulation of cellular mRNAs as an important mechanism by which it exerts its diverse biological functions. Despite this growing body of work, many questions remain regarding the roles of mRNAs as RNase-L substrates. This review will survey known and putative mRNA substrates of RNase-L, propose mechanisms by which it may selectively cleave these transcripts, and postulate future clinical applications.

RNase L attenuates mitogen-stimulated gene expression via transcriptional and post-transcriptional mechanisms to limit the proliferative response

The cellular response to mitogens is tightly regulated via transcriptional and post-transcriptional mechanisms to rapidly induce genes that promote proliferation and efficiently attenuate their expression to prevent malignant growth. RNase L is an endoribonuclease that mediates diverse antiproliferative activities, and tristetraprolin (TTP) is a mitogen-induced RNA-binding protein that directs the decay of proliferation-stimulatory mRNAs. In light of their roles as endogenous proliferative constraints, we examined the mechanisms and functional interactions of RNase L and TTP to attenuate a mitogenic response. Mitogen stimulation of RNase L-deficient cells significantly increased TTP transcription and the induction of other mitogen-induced mRNAs. This regulation corresponded with elevated expression of serum-response factor (SRF), a master regulator of mitogen-induced transcription. RNase L destabilized the SRF transcript and formed a complex with SRF mRNA in cells providing a mechanism by which RNase L down-regulates SRF-induced genes. TTP and RNase L proteins interacted in cells suggesting that RNase L is directed to cleave TTP-bound RNAs as a mechanism of substrate specificity. Consistent with their concerted function in RNA turnover, the absence of either RNase L or TTP stabilized SRF mRNA, and a subset of established TTP targets was also regulated by RNase L. RNase L deficiency enhanced mitogen-induced proliferation demonstrating its functional role in limiting the mitogenic response. Our findings support a model of feedback regulation in which RNase L and TTP target SRF mRNA and SRF-induced transcripts. Accordingly, meta-analysis revealed an enrichment of RNase L and TTP targets among SRF-regulated genes suggesting that the RNase L/TTP axis represents a viable target to inhibit SRF-driven proliferation in neoplastic diseases.

Expression of mRNA and protein-protein interaction of the antiviral endoribonuclease RNase L in mouse spleen

The interferon-inducible, 2',5'-oligoadenylate (2-5A)-dependent endoribonuclease, RNase L is a unique antiviral RNA-degrading enzyme involved in RNA-metabolism, translational regulation, stress-response besides its anticancer/tumor-suppressor and antibacterial functions. RNase L represents complex cellular RNA-regulations in mammalian cells but diverse functions of RNase L are not completely explained by its 2-5A-regulated endoribonuclease activity. We hypothesized that RNase L has housekeeping function(s) through interaction with cellular proteins. We investigated RNase L mRNA expression in mouse tissues by RT-PCR and its protein-protein interaction in spleen by GST-pulldown and immunoprecipitation assays followed by proteomic analysis. RNase L mRNA is constitutively and differentially expressed in nine different mouse tissues, its level is maximum in immunological tissues (spleen, thymus and lungs), moderate in reproductive tissues (testis and prostate) and low in metabolic tissues (kidney, brain, liver and heart). Cellular proteins from mouse spleen [fibronectin precursor, β-actin, troponin I, myosin heavy chain 9 (non-muscle), growth-arrest specific protein 11, clathrin light chain B, a putative uncharacterized protein (Ricken cDNA 8030451F13) isoform (CRA_d) and alanyl tRNA synthetase] were identified as cellular RNase L-interacting proteins. Thus our results suggest for more general cellular functions of RNase L through protein-protein interactions in the spleen for immune response in mammals.

Enteropathogenic Escherichia coli inhibits type I interferon- and RNase L-mediated host defense to disrupt intestinal epithelial cell barrier function

Enteropathogenic Escherichia coli (EPEC) primarily infects children in developing countries and causes diarrhea that can be deadly. EPEC pathogenesis occurs through type III secretion system (T3SS)-mediated injection of effectors into intestinal epithelial cells (IECs); these effectors alter actin dynamics, modulate the immune response, and disrupt tight junction (TJ) integrity. The resulting compromised barrier function and increased gastrointestinal (GI) permeability may be responsible for the clinical symptoms of infection. Type I interferon (IFN) mediates anti-inflammatory activities and serves essential functions in intestinal immunity and homeostasis; however, its role in the immune response to enteric pathogens, such as EPEC, and its impact on IEC barrier function have not been examined. Here, we report that IFN-β is induced following EPEC infection and regulates IEC TJ proteins to maintain barrier function. The EPEC T3SS effector NleD counteracts this protective activity by inhibiting IFN-β induction and enhancing tumor necrosis factor alpha to promote barrier disruption. The endoribonuclease RNase L is a key mediator of IFN induction and action that promotes TJ protein expression and IEC barrier integrity. EPEC infection inhibits RNase L in a T3SS-dependent manner, providing a mechanism by which EPEC evades IFN-induced antibacterial activities. This work identifies novel roles for IFN-β and RNase L in IEC barrier functions that are targeted by EPEC effectors to escape host defense mechanisms and promote virulence. The IFN-RNase L axis thus represents a potential therapeutic target for enteric infections and GI diseases involving compromised barrier function.

RNASEL and MIR146A SNP-SNP interaction as a susceptibility factor for non-melanoma skin cancer

Immunity and inflammatory pathways are important in the genesis of non-melanoma skin cancers (NMSC). Functional genetic variation in immune modulators has the potential to affect disease etiology. We investigated associations between common variants in two key regulators, MIR146A and RNASEL, and their relation to NMSCs. Using a large population-based case-control study of basal cell (BCC) and squamous cell carcinoma (SCC), we investigated the impact of MIR146A SNP rs2910164 on cancer risk, and interaction with a SNP in one of its putative targets (RNASEL, rs486907). To examine associations between genotype and BCC and SCC, occurrence odds ratios (OR) and 95% confidence intervals (95%CI) were calculated using unconditional logistic regression, accounting for multiple confounding factors. We did not observe an overall change in the odds ratios for SCC or BCC among individuals carrying either of the RNASEL or MIR146A variants compared with those who were wild type at these loci. However, there was a sex-specific association between BCC and MIR146A in women (ORGC = 0.73, [95%CI = 0.52-1.03]; ORCC = 0.29, [95% CI = 0.14-0.61], p-trend<0.001), and a reduction in risk, albeit not statistically significant, associated with RNASEL and SCC in men (ORAG = 0.88, [95%CI = 0.65-1.19]; ORAA = 0.68, [95%CI = 0.43-1.08], p-trend = 0.10). Most striking was the strong interaction between the two genes. Among individuals carrying variant alleles of both rs2910164 and rs486907, we observed inverse relationships with SCC (ORSCC = 0.56, [95%CI = 0.38-0.81], p-interaction = 0.012) and BCC (ORBCC = 0.57, [95%CI = 0.40-0.80], p-interaction = 0.005). Our results suggest that genetic variation in immune and inflammatory regulators may influence susceptibility to NMSC, and novel SNP-SNP interaction for a microRNA and its target. These data suggest that RNASEL, an enzyme involved in RNA turnover, is controlled by miR-146a and may be important in NMSC etiology.

Regulation of human RNase-L by the miR-29 family reveals a novel oncogenic role in chronic myelogenous leukemia

The endoribonuclease RNase-L is the terminal component of an interferon-regulated RNA decay pathway known as the 2'-5'-oligoadenylate (2-5A) system, whose established functions include antimicrobial and tumor suppressive activities. RNase-L activity requires binding of the small molecule 2-5A, leading to RNase-L dimerization and cleavage of single-stranded RNA. RNase-L expression is controlled post-transcriptionally by its 3'-untranslated region (3' UTR), which exerts a strong negative effect on RNase-L levels. MicroRNAs (miRNAs) are a class of small noncoding RNAs that repress expression of target genes by binding to regions of complementarity often in the 3' UTR. The miR-29 family acts as a tumor suppressor in several cancers, including acute and chronic myelogenous leukemia (CML), and has many oncogenic targets. We report that the miR-29 family represses RNase-L protein expression across several cell types. Using a luciferase reporter, we showed that miR-29 acts via 4 target sites within the RNASEL 3' UTR. Mutation of all sites is required for abrogation of miR-29 repression. In light of the reported tumor suppressive role of miR-29 in K562 CML cells and miR-29 repression of RNase-L in these cells, we generated K562 cells with stable RNase-L knockdown and demonstrated that loss of RNase-L inhibits proliferation in vitro as well as tumor growth in a xenograft model. Our findings identify a previously unknown miRNA regulator of RNase-L expression and support a novel oncogenic role for RNase-L in CML and potentially other hematopoietic malignancies.