Inverse correlation between TP53 gene status and PD-L1 protein levels in a melanoma cell model depends on an IRF1/SOX10 regulatory axis

BACKGROUND

PD-L1 expression on cancer cells is an important mechanism of tumor immune escape, and immunotherapy targeting the PD-L1/PD1 interaction is a common treatment option for patients with melanoma. However, many patients do not respond to treatment and novel predictors of response are emerging. One suggested modifier of PD-L1 is the p53 pathway, although the relationship of p53 pathway function and activation is poorly understood.

METHODS

The study was performed on human melanoma cell lines with various p53 status. We investigated PD-L1 and proteins involved in IFNγ signaling by immunoblotting and mRNA expression, as well as membrane expression of PD-L1 by flow cytometry. We evaluated differences in the ability of NK cells to recognize and kill target tumor cells on the basis of p53 status. We also investigated the influence of proteasomal degradation and protein half-life, IFNγ signaling and p53 activation on biological outcomes, and performed bioinformatic analysis using available data for melanoma cell lines and melanoma patients.

RESULTS

We demonstrate that p53 status changes the level of membrane and total PD-L1 protein through IRF1 regulation and show that p53 loss influences the recently discovered SOX10/IRF1 regulatory axis. Bioinformatic analysis identified a dependency of SOX10 on p53 status in melanoma, and a co-regulation of immune signaling by both transcription factors. However, IRF1/PD-L1 regulation by p53 activation revealed complicated regulatory mechanisms that alter IRF1 mRNA but not protein levels. IFNγ activation revealed no dramatic differences based on TP53 status, although dual p53 activation and IFNγ treatment confirmed a complex regulatory loop between p53 and the IRF1/PD-L1 axis.

CONCLUSIONS

We show that p53 loss influences the level of PD-L1 through IRF1 and SOX10 in an isogenic melanoma cell model, and that p53 loss affects NK-cell cytotoxicity toward tumor cells. Moreover, activation of p53 by MDM2 inhibition has a complex effect on IRF1/PD-L1 activation. These findings indicate that evaluation of p53 status in patients with melanoma will be important for predicting the response to PD-L1 monotherapy and/or dual treatments where p53 pathways participate in the overall response.

Roles of Interferon Regulatory Factor 1 in Tumor Progression and Regression: Two Sides of a Coin

IRF1 is a transcription factor well known for its role in IFN signaling. Although IRF1 was initially identified for its involvement in inflammatory processes, there is now evidence that it provides a function in carcinogenesis as well. IRF1 has been shown to affect several important antitumor mechanisms, such as induction of apoptosis, cell cycle arrest, remodeling of tumor immune microenvironment, suppression of telomerase activity, suppression of angiogenesis and others. Nevertheless, the opposite effects of IRF1 on tumor growth have also been demonstrated. In particular, the "immune checkpoint" molecule PD-L1, which is responsible for tumor immune evasion, has IRF1 as a major transcriptional regulator. These and several other properties of IRF1, including its proposed association with response and resistance to immunotherapy and several chemotherapeutic drugs, make it a promising object for further research. Numerous mechanisms of IRF1 regulation in cancer have been identified, including genetic, epigenetic, transcriptional, post-transcriptional, and post-translational mechanisms, although their significance for tumor progression remains to be explored. This review will focus on the established tumor-suppressive and tumor-promoting functions of IRF1, as well as the molecular mechanisms of IRF1 regulation identified in various cancers.

SPOP targets the immune transcription factor IRF1 for proteasomal degradation

Adaptation of the functional proteome is essential to counter pathogens during infection, yet precisely timed degradation of these response proteins after pathogen clearance is likewise key to preventing autoimmunity. Interferon regulatory factor 1 (IRF1) plays an essential role as a transcription factor in driving the expression of immune response genes during infection. The striking difference in functional output with other IRFs is that IRF1 also drives the expression of various cell cycle inhibiting factors, making it an important tumor suppressor. Thus, it is critical to regulate the abundance of IRF1 to achieve a 'Goldilocks' zone in which there is sufficient IRF1 to prevent tumorigenesis, yet not too much which could drive excessive immune activation. Using genetic screening, we identified the E3 ligase receptor speckle type BTB/POZ protein (SPOP) to mediate IRF1 proteasomal turnover in human and mouse cells. We identified S/T-rich degrons in IRF1 required for its SPOP MATH domain-dependent turnover. In the absence of SPOP, elevated IRF1 protein levels functionally increased IRF1-dependent cellular responses, underpinning the biological significance of SPOP in curtailing IRF1 protein abundance.

Chaperone-assisted E3 ligase CHIP: A double agent in cancer

The carboxy-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin ligase and co-chaperone belonging to Ubox family that plays a crucial role in the maintenance of cellular homeostasis by switching the equilibrium of the folding-refolding mechanism towards the proteasomal or lysosomal degradation pathway. It links molecular chaperones viz. HSC70, HSP70 and HSP90 with ubiquitin proteasome system (UPS), acting as a quality control system. CHIP contains charged domain in between N-terminal tetratricopeptide repeat (TPR) and C-terminal Ubox domain. TPR domain interacts with the aberrant client proteins via chaperones while Ubox domain facilitates the ubiquitin transfer to the client proteins for ubiquitination. Thus, CHIP is a classic molecule that executes ubiquitination for degradation of client proteins. Further, CHIP has been found to be indulged in cellular differentiation, proliferation, metastasis and tumorigenesis. Additionally, CHIP can play its dual role as a tumor suppressor as well as an oncogene in numerous malignancies, thus acting as a double agent. Here, in this review, we have reported almost all substrates of CHIP established till date and classified them according to the hallmarks of cancer. In addition, we discussed about its architectural alignment, tissue specific expression, sub-cellular localization, folding-refolding mechanisms of client proteins, E4 ligase activity, normal physiological roles, as well as involvement in various diseases and tumor biology. Further, we aim to discuss its importance in HSP90 inhibitors mediated cancer therapy. Thus, this report concludes that CHIP may be a promising and worthy drug target towards pharmaceutical industry for drug development.

XAF1 Protects Host against Emerging RNA Viruses by Stabilizing IRF1-Dependent Antiviral Immunity

XIAP-associated factor 1 (XAF1) is an interferon (IFN)-stimulated gene (ISG) that enhances IFN-induced apoptosis. However, it is unexplored whether XAF1 is essential for the host fighting against invaded viruses. Here, we find that XAF1 is significantly upregulated in the host cells infected with emerging RNA viruses, including influenza, Zika virus (ZIKV), and SARS-CoV-2. IFN regulatory factor 1 (IRF1), a key transcription factor in immune cells, determines the induction of XAF1 during antiviral immunity. Ectopic expression of XAF1 protects host cells against various RNA viruses independent of apoptosis. Knockout of XAF1 attenuates host antiviral innate immunity in vitro and in vivo, which leads to more severe lung injuries and higher mortality in the influenza infection mouse model. XAF1 stabilizes IRF1 protein by antagonizing the CHIP-mediated degradation of IRF1, thus inducing more antiviral IRF1 target genes, including DDX58, DDX60, MX1, and OAS2. Our study has described a protective role of XAF1 in the host antiviral innate immunity against RNA viruses. We have also elucidated the molecular mechanism that IRF1 and XAF1 form a positive feedback loop to induce rapid and robust antiviral immunity. IMPORTANCE Rapid and robust induction of antiviral genes is essential for the host to clear the invaded viruses. In addition to the IRF3/7-IFN-I-STAT1 signaling axis, the XAF1-IRF1 positive feedback loop synergistically or independently drives the transcription of antiviral genes. Moreover, XAF1 is a sensitive and reliable gene that positively correlates with the viral infection, suggesting that XAF1 is a potential diagnostic marker for viral infectious diseases. In addition to the antitumor role, our study has shown that XAF1 is essential for antiviral immunity. XAF1 is not only a proapoptotic ISG, but it also stabilizes the master transcription factor IRF1 to induce antiviral genes. IRF1 directly binds to the IRF-Es of its target gene promoters and drives their transcriptions, which suggests a unique role of the XAF1-IRF1 loop in antiviral innate immunity, particularly in the host defect of IFN-I signaling such as invertebrates.

Transient Increases in Inflammation and Proapoptotic Potential Are Associated with the HESN Phenotype Observed in a Subgroup of Kenyan Female Sex Workers

Interferon (IFN) -stimulated genes (ISGs) are critical effectors of IFN response to viral infection, but whether ISG expression is a correlate of protection against HIV infection remains elusive. A well-characterized subcohort of Kenyan female sex workers, who, despite being repeatedly exposed to HIV-1 remain seronegative (HESN), exhibit reduced baseline systemic and mucosal immune activation. This study tested the hypothesis that regulation of ISGs in the cells of HESN potentiates a robust antiviral response against HIV. Transcriptional profile of a panel of ISGs with antiviral function in PBMC and isolated CD4+ T cells from HESN and non-HESN sex worker controls were defined following exogenous IFN-stimulation using relative RT-qPCR. This study identified a unique profile of proinflammatory and proapoptotic ISGs with robust but transient responses to exogenous IFN-γ and IFN-α2 in HESN cells. In contrast, the non-HESN cells had a strong and prolonged proinflammatory ISG profile at baseline and following IFN challenge. Potential mechanisms may include augmented bystander apoptosis due to increased TRAIL expression (16-fold), in non-HESN cells. The study also identified two negative regulators of ISG induction associated with the HESN phenotype. Robust upregulation of SOCS-1 and IRF-1, in addition to HDM2, could contribute to the strict regulation of proinflammatory and proapoptotic ISGs in HESN cells. As reducing IRF-1 in the non-HESN cells resulted in the identified HESN ISG profile, and decreased HIV susceptibility, the unique HESN ISG profile could be a correlate of protection against HIV infection.

CHIP-dependent regulation of the actin cytoskeleton is linked to neuronal cell membrane integrity

CHIP is an E3-ubiquitin ligase that contributes to healthy aging and has been characterized as neuroprotective. To elucidate dominant CHIP-dependent changes in protein steady-state levels in a patient-derived human neuronal model, CHIP function was ablated using gene-editing and an unbiased proteomic analysis conducted to compare knock-out and wild-type isogenic induced pluripotent stem cell (iPSC)-derived cortical neurons. Rather than a broad effect on protein homeostasis, loss of CHIP function impacted on a focused cohort of proteins from actin cytoskeleton signaling and membrane integrity networks. In support of the proteomics, CHIP knockout cells had enhanced sensitivity to induced membrane damage. We conclude that the major readout of CHIP function in cortical neurons derived from iPSC of a patient with elevate α-synuclein, Parkinson's disease and dementia, is the modulation of substrates involved in maintaining cellular "health". Thus, regulation of the actin cytoskeletal and membrane integrity likely contributes to the neuroprotective function(s) of CHIP.

Negative Regulation of the Innate Immune Response through Proteasomal Degradation and Deubiquitination

The rapid and dynamic activation of the innate immune system is achieved through complex signaling networks regulated by post-translational modifications modulating the subcellular localization, activity, and abundance of signaling molecules. Many constitutively expressed signaling molecules are present in the cell in inactive forms, and become functionally activated once they are modified with ubiquitin, and, in turn, inactivated by removal of the same post-translational mark. Moreover, upon infection resolution a rapid remodeling of the proteome needs to occur, ensuring the removal of induced response proteins to prevent hyperactivation. This review discusses the current knowledge on the negative regulation of innate immune signaling pathways by deubiquitinating enzymes, and through degradative ubiquitination. It focusses on spatiotemporal regulation of deubiquitinase and E3 ligase activities, mechanisms for re-establishing proteostasis, and degradation through immune-specific feedback mechanisms vs. general protein quality control pathways.

Ubiquitination modification: critical regulation of IRF family stability and activity

Interferon regulatory factors (IRFs) play pivotal and critical roles in innate and adaptive immune responses; thus, precise and stringent regulation of the stability and activation of IRFs in physiological processes is necessary. The stability and activities of IRFs are directly or indirectly targeted by endogenous and exogenous proteins in an ubiquitin-dependent manner. However, few reviews have summarized how host E3 ligases/DUBs or viral proteins regulate IRF stability and activity. Additionally, with recent technological developments, details about the ubiquitination of IRFs have been continuously revealed. As knowledge of how these proteins function and interact with IRFs may facilitate a better understanding of the regulation of IRFs in immune responses or other biological processes, we summarized current studies on the direct ubiquitination of IRFs, with an emphasis on how these proteins interact with IRFs and affect their activities, which may provide exciting targets for drug development by regulating the functions of specific E3 ligases.

IκB kinase-ε-mediated phosphorylation triggers IRF-1 degradation in breast cancer cells

Interferon Regulatory Factors (IRFs) are key regulators of immunity, cell survival and apoptosis. IRF transcriptional activity and subcellular localization are tightly regulated by posttranscriptional modifications including phosphorylation. The IκB kinase family member IKK-ε is essential in regulating antiviral innate immunity mediated by IRFs but is now also recognized as an oncoprotein amplified and overexpressed in breast cancer cell lines and patient-derived tumors. In the present study, we report that the tumor suppressor IRF-1 is a specific target of IKK-ε in breast cancer cells. IKK-ε-mediated phosphorylation of IRF-1 dramatically decreases IRF-1 protein stability, accelerating IRF-1 degradation and quenching IRF-1 transcriptional activity. Chemical inhibition of IKK-ε activity, fully restores IRF-1 levels and function and positively correlates with inhibition of cell growth and proliferation of breast cancer cells. By using a breast cancer cell line stably expressing a dominant negative version of IRF-1 we were able to demonstrate that IKK-ε preferentially exerts its oncogenic potential in breast cancer through the regulation of IRF-1 and point to the IKK-ε-mediated phosphorylation of IRF-1 as a therapeutic target to overcome IKK-ε-mediated tumorigenesis.

GSK3β-SCFFBXW7α mediated phosphorylation and ubiquitination of IRF1 are required for its transcription-dependent turnover

IRF1 (Interferon Regulatory Factor-1) is the prototype of the IRF family of DNA binding transcription factors. IRF1 protein expression is regulated by transient up-regulation in response to external stimuli followed by rapid degradation via the ubiquitin-proteasome system. Here we report that DNA bound IRF1 turnover is promoted by GSK3β (Glycogen Synthase Kinase 3β) via phosphorylation of the T181 residue which generates a phosphodegron for the SCF (Skp-Cul-Fbox) ubiquitin E3-ligase receptor protein Fbxw7α (F-box/WD40 7). This regulated turnover is essential for IRF1 activity, as mutation of T181 results in an improperly stabilized protein that accumulates at target promoters but fails to induce RNA-Pol-II elongation and subsequent transcription of target genes. Consequently, the anti-proliferative activity of IRF1 is lost in cell lines expressing T181A mutant. Further, cell lines with dysfunctional Fbxw7 are less sensitive to IRF1 overexpression, suggesting an important co-activator function for this ligase complex. As T181 phosphorylation requires both DNA binding and RNA-Pol-II elongation, we propose that this event acts to clear ‘spent’ molecules of IRF1 from transcriptionally engaged target promoters.

Anti-apoptotic effect by the suppression of IRF1 as a downstream of Wnt/β-catenin signaling in colorectal cancer cells

Impaired Wnt signaling pathway plays a crucial role in the development of colorectal cancer through activation of the β-catenin/TCF7L2 complex. Although genes upregulated by Wnt/β-catenin signaling have been intensively studied, the roles of downregulated genes are poorly understood. Previously, we reported that interferon-induced proteins with tetratricopeptide repeats 2 (IFIT2) was downregulated by the Wnt/β-catenin signaling, and that the suppressed expression of IFIT2 conferred antiapoptotic property to colorectal cancer (CRC) cells. However, the mechanisms underlying how Wnt/β-catenin signaling regulates IFIT2 remain to be elucidated. In this study, we have uncovered that the expression of IFIT2 is induced by IRF1, which is negatively regulated by the Wnt/β-catenin signaling. In addition, we found that downregulation of IRF1 is mediated by its degradation through the ubiquitination-proteasome pathway, and that decreased activity of a deubiquitinase complex containing USP1 and UAF1 is involved in the degradation of IRF1 by Wnt/β-catenin signaling. These data should provide better understanding of the Wnt signaling pathway and human carcinogenesis.

A model of the three-dimensional structure of human interferon responsive factor 1 and its modifications upon phosphorylation or phosphorylation-mimicking mutations

Interferon responsive factor 1 (IRF-1) is a pleiotropic transcription factor, possessing non-redundant biological activities that depend on its interaction with different protein partners and multiple post-translational modifications including phosphorylation. In particular, a 5'-SXXXSXS-3' motif of the protein represents the target of the IκB-related kinases, TANK-binding kinase (TBK)-1 and inhibitor of nuclear factor kappa-B kinase (IKK)-ε. Here, a 3D model of human IRF-1 was determined by using multi-template comparative modeling and molecular dynamics approaches. Models obtained through either phosphorylation or aspartate mutation of residues 215, 219 and 221 were also calculated and compared to the wild type. Calculations indicated that each of these modifications mainly induces a rigidification of the protein structure and only slightly changes in electrostatics and hydrophobicity of IRF-1 surface, resulting in the impairment of the capacity of IRF-1 containing as partate mutations (S221D and S215D/S219D/S221D) to synergize with tumour necrosis factor (TNF)-α stimulation in inducing interferon (IFN) promoter-mediated reporter gene activation. Therefore, these changes are qualitatively correlated to the amount of negative charge located on the 215-221 segments of IRF-1 by phosphorylation or aspartate mutation. Hypotheses on the structural mechanism that governs the phosphorylation-related damping of IRF-1 activity were also drawn. Communicated by Ramaswamy H. Sarma.

Regulation of transcriptional activators by DNA-binding domain ubiquitination

Ubiquitin is a key component of the regulatory network that maintains gene expression in eukaryotes, yet the molecular mechanism(s) by which non-degradative ubiquitination modulates transcriptional activator (TA) function is unknown. Here endogenous p53, a stress-activated transcription factor required to maintain health, is stably monoubiquitinated, following pathway activation by IR or Nutlin-3 and localized to the nucleus where it becomes tightly associated with chromatin. Comparative structure-function analysis and in silico modelling demonstrate a direct role for DNA-binding domain (DBD) monoubiquitination in TA activation. When attached to the DBD of either p53, or a second TA IRF-1, ubiquitin is orientated towards, and makes contact with, the DNA. The contact is made between a predominantly cationic surface on ubiquitin and the anionic DNA. Our data demonstrate an unexpected role for ubiquitin in the mechanism of TA-activity enhancement and provides insight into a new level of transcriptional regulation.

HIV-1 Tat Recruits HDM2 E3 Ligase To Target IRF-1 for Ubiquitination and Proteasomal Degradation

In addition to its ability to regulate HIV-1 promoter activation, the viral transactivator Tat also functions as a determinant of pathogenesis and disease progression by directly and indirectly modulating the host anti-HIV response, largely through the capacity of Tat to interact with and modulate the activities of multiple host proteins. We previously demonstrated that Tat modulated both viral and host transcriptional machinery by interacting with the cellular transcription factor interferon regulatory factor 1 (IRF-1). In the present study, we investigated the mechanistic basis and functional significance of Tat−IRF-1 interaction and demonstrate that Tat dramatically decreased IRF-1 protein stability. To accomplish this, Tat exploited the cellular HDM2 (human double minute 2 protein) ubiquitin ligase to accelerate IRF-1 proteasome-mediated degradation, resulting in a quenching of IRF-1 transcriptional activity during HIV-1 infection. These data identify IRF-1 as a new target of Tat-induced modulation of the cellular protein machinery and reveal a new strategy developed by HIV-1 to evade host immune responses.

Current therapies have dramatically reduced morbidity and mortality associated with HIV infection and have converted infection from a fatal pathology to a chronic disease that is manageable via antiretroviral therapy. Nevertheless, HIV-1 infection remains a challenge, and the identification of useful cellular targets for therapeutic intervention remains a major goal. The cellular transcription factor IRF-1 impacts various physiological functions, including the immune response to viral infection. In this study, we have identified a unique mechanism by which HIV-1 evades IRF-1-mediated host immune responses and show that the viral protein Tat accelerates IRF-1 proteasome-mediated degradation and inactivates IRF-1 function. Restoration of IRF-1 functionality may thus be regarded as a potential strategy to reinstate both a direct antiviral response and a more broadly acting immune regulatory circuit.

Functional role of lysine 12 in Leishmania major AQP1

Leishmania major aquaglyceroporin (AQP1) is an adventitious metalloid channel that allows the bidirectional movement of arsenite and antimonite. Here we demonstrate that AQP1 is subjected to proteasome-dependent degradation. Treatment of Leishmania promastigotes with the proteasome inhibitor MG132 resulted in increased AQP1 accumulation. Site-directed mutagenesis in AQP1 revealed that alteration of lysine 12 to either alanine or arginine improves protein stability. AQP1 expression is stabilized by mitogen-activated protein kinase 2 (MPK2). Cells expressing a dominant-negative MPK2 mutant exhibited severely reduced AQP1 expression, which could be reversed upon addition of MG132. Interestingly, the dominant-negative MPK2 mutant could not destabilize either AQP1K12A or AQP1K12R. While stabilization of AQP1 by MPK2 leads to its relocalization from flagellum to the entire surface of the parasite, altered AQP1K12A or AQP1K12R was restricted to flagellum only. Our data demonstrate that lysine 12 is targeted for proteasomal degradation of AQP1 and plays an integral role in subcellular localization of AQP1 as well as its interaction with MPK2. This work also raises the possibility that a strategy combining antimonial with a proteasome inhibitor may be an effective combination regimen against diverse forms of leishmaniasis.

Identification of IRF1 as critical dual regulator of Smac mimetic-induced apoptosis and inflammatory cytokine response

Smac (second mitochondria-derived activator of caspase) mimetics are considered as promising anticancer therapeutics and used to induce apoptosis by antagonizing inhibitor of apoptosis proteins, which are often abundantly expressed in cancer cells. Here, we identify interferon regulatory factor 1 (IRF1) as a novel critical regulator of Smac mimetic BV6-induced apoptosis and proinflammatory cytokine secretion with impact on the immune response. IRF1 knockdown rescues cells from BV6-induced apoptosis and attenuates BV6-stimulated upregulation of tumor necrosis factor-α (TNFα), indicating that IRF1 mediates BV6-triggered cell death, at least in part, by inducing TNFα. This notion is supported by data showing that exogenous supply of TNFα restores BV6-induced cell death in IRF-knockdown cells. Interestingly, IRF1 selectively controls the induction of nuclear factor-κB (NF-κB) target genes, as IRF1 depletion attenuates BV6-stimulated upregulation of TNFα and interleukin-8 (IL-8) but not p100 and RelB. Concomitant knockdown of IRF1 and p65 cooperate to inhibit BV6-induced cell death, implying a cooperative interaction of IRF1 and NF-κB. In addition, IRF1 silencing hampers TNFα induction by TNFα itself as an another prototypical NF-κB stimulus. Importantly, IRF1 depletion impedes BV6-stimulated secretion of additional proinflammatory cytokines such as granulocyte–macrophage colony-stimulating factor (GM-CSF), IL-8, IL-6 and monocyte chemoattractant protein-1, and migration of primary monocytes to BV6-treated tumor cells. In conclusion, this identification of IRF1 as a dual regulator of BV6-induced apoptosis and inflammatory cytokine secretion provides novel insights into determinants of sensitivity towards Smac mimetic and possible implications of Smac mimetic treatment on tumor microenvironment and immune response.

A CHIPotle in physiology and disease

The carboxy-terminus of Hsc70 interacting protein (CHIP) is known to function as a chaperone associated E3 ligase for several proteins and regulates a variety of physiological processes. Being a connecting link between molecular chaperones and 26S proteasomes, it is widely regarded as the central player in the cellular protein quality control system. Recent analyses have provided new insights on the biochemical and functional dynamics of CHIP. In this review article, we give a comprehensive account of our current knowledge on the biology of CHIP, which apart from shedding light on fundamental biological questions promises to provide a potential target for therapeutic intervention.

Two-way communications between ubiquitin-like modifiers and DNA

Many aspects of nucleic acid metabolism, such as DNA replication, repair and transcription, are regulated by the post-translational modifiers ubiquitin and SUMO. Not surprisingly, DNA itself plays an integral part in determining the modification of most chromatin-associated targets. Conversely, ubiquitination or SUMOylation of a protein can impinge on its DNA-binding properties. This review describes mechanistic principles governing the mutual interactions between DNA and ubiquitin or SUMO.

The E3 ligase CHIP: insights into its structure and regulation

The carboxy-terminus of Hsc70 interacting protein (CHIP) is a cochaperone E3 ligase containing three tandem repeats of tetratricopeptide (TPR) motifs and a C-terminal U-box domain separated by a charged coiled-coil region. CHIP is known to function as a central quality control E3 ligase and regulates several proteins involved in a myriad of physiological and pathological processes. Recent studies have highlighted varied regulatory mechanisms operating on the activity of CHIP which is crucial for cellular homeostasis. In this review article, we give a concise account of our current knowledge on the biochemistry and regulation of CHIP.

Interferon regulatory factor-1 (IRF-1) interacts with regulated in development and DNA damage response 2 (REDD2) in the cytoplasm of mouse bone marrow cells

IRF-1 is a critical hematopoietic transcription factor, which regulates cell growth, development of immune cells, immune response, tumor suppression, apoptosis and autophagy in mammalian cells. Protein-protein interactions of IRF-1 in mouse bone marrow cells (BMCs) by GST-IRF-1 pull-down followed by mass spectrometry, coimmunoprecipitation, immunoblotting and colocalization show that regulated in development and DNA damage response 2 (REDD2) is an IRF-1-interacting protein. REDD2 is a highly conserved mammalian regulatory protein of the TSC2/mTOR pathway. It is structurally similar to REDD1 but has a distinct loop region. Cellular IRF-1 and REDD2 complex is present in the cytoplasm of BMCs as distinct speckles in punctate pattern. In vitro interaction of recombinant IRF-1 and REDD2 shows their physical interaction. Taken together, our results suggest that IRF-1 physically interacts with REDD2 in the large cytoplasmic protein complex, which may function as cellular signaling proteins for 'cross-talk' of mTOR and cytokine pathways during regulation of cell growth/proliferation, apoptosis and autophagy of mammalian bone marrow cells during health and disease.

Digested disorder: Quarterly intrinsic disorder digest (January/February/March, 2013)

The current literature on intrinsically disordered proteins is blooming. A simple PubMed search for “intrinsically disordered protein OR natively unfolded protein” returns about 1,800 hits (as of June 17, 2013), with many papers published quite recently. To keep interested readers up to speed with this literature, we are starting a “Digested Disorder” project, which will encompass a series of reader’s digest type of publications aiming at the objective representation of the research papers and reviews on intrinsically disordered proteins. The only two criteria for inclusion in this digest are the publication date (a paper should be published within the covered time frame) and topic (a paper should be dedicated to any aspect of protein intrinsic disorder). The current digest covers papers published during the period of January, February and March of 2013. The papers are grouped hierarchically by topics they cover, and for each of the included paper a short description is given on its major findings.