Raf kinase inhibitory protein function is regulated via a flexible pocket and novel phosphorylation-dependent mechanism. Mol Cell Biol. 2009;29:1306-1320. [PMID: 19103740 DOI: 10.1128/mcb.01271-08]

Cross-Talks between Raf Kinase Inhibitor Protein and Programmed Cell Death Ligand 1 Expressions in Cancer: Role in Immune Evasion and Therapeutic Implications

Innovations in cancer immunotherapy have resulted in the development of several novel immunotherapeutic strategies that can disrupt immunosuppression. One key advancement lies in immune checkpoint inhibitors (ICIs), which have shown significant clinical efficacy and increased survival rates in patients with various therapy-resistant cancers. This immune intervention consists of monoclonal antibodies directed against inhibitory receptors (e.g., PD-1) on cytotoxic CD8 T cells or against corresponding ligands (e.g., PD-L1/PD-L2) overexpressed on cancer cells and other cells in the tumor microenvironment (TME). However, not all cancer cells respond-there are still poor clinical responses, immune-related adverse effects, adaptive resistance, and vulnerability to ICIs in a subset of patients with cancer. This challenge showcases the heterogeneity of cancer, emphasizing the existence of additional immunoregulatory mechanisms in many patients. Therefore, it is essential to investigate PD-L1's interaction with other oncogenic genes and pathways to further advance targeted therapies and address resistance mechanisms. Accordingly, our aim was to investigate the mechanisms governing PD-L1 expression in tumor cells, given its correlation with immune evasion, to uncover novel mechanisms for decreasing PD-L1 expression and restoring anti-tumor immune responses. Numerous studies have demonstrated that the upregulation of Raf Kinase Inhibitor Protein (RKIP) in many cancers contributes to the suppression of key hyperactive pathways observed in malignant cells, alongside its broadening involvement in immune responses and the modulation of the TME. We, therefore, hypothesized that the role of PD-L1 in cancer immune surveillance may be inversely correlated with the low expression level of the tumor suppressor Raf Kinase Inhibitor Protein (RKIP) expression in cancer cells. This hypothesis was investigated and we found several signaling cross-talk pathways between the regulations of both RKIP and PD-L1 expressions. These pathways and regulatory factors include the MAPK and JAK/STAT pathways, GSK3β, cytokines IFN-γ and IL-1β, Sox2, and transcription factors YY1 and NFκB. The pathways that upregulated PD-L1 were inhibitory for RKIP expression and vice versa. Bioinformatic analyses in various human cancers demonstrated the inverse relationship between PD-L1 and RKIP expressions and their prognostic roles. Therefore, we suspect that the direct upregulation of RKIP and/or the use of targeted RKIP inducers in combination with ICIs could result in a more targeted anti-tumor immune response-addressing the therapeutic challenges related to PD-1/PD-L1 monotherapy alone.

Raf kinase inhibitor protein combined with phosphorylated extracellular signal-regulated kinase offers valuable prognosis in gastrointestinal stromal tumor

BACKGROUND

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. Tyrosine kinase inhibitors, such as imatinib, have been used as first-line therapy for the treatment of GISTs. Although these drugs have achieved considerable efficacy in some patients, reports of resistance and recurrence have emerged. Extracellular signal-regulated kinase 1/2 (ERK1/2) protein, as a member of the mitogen-activated protein kinase (MAPK) family, is a core molecule of this signaling pathway. Nowadays, research reports on the important clinical and prognostic value of phosphorylated-ERK (P-ERK) and phosphorylated-MAPK/ERK kinase (P-MEK) proteins closely related to raf kinase inhibitor protein (RKIP) have gradually emerged in digestive tract tumors such as gastric cancer, colon cancer, and pancreatic cancer. However, literature on the expression of these downstream proteins combined with RKIP in GIST is scarce. This study will focus on this aspect and search for answers to the problem.

AIM

To detect the expression of RKIP, P-ERK, and P-MEK protein in GIST and to analyze their relationship with clinicopathological characteristics and prognosis of this disease. Try to establish a new prognosis evaluation model using RKIP and P-ERK in combination with analysis and its prognosis evaluation efficacy.

METHODS

The research object of our experiment was 66 pathologically diagnosed GIST patients with complete clinical and follow-up information. These patients received surgical treatment at China Medical University Affiliated Hospital from January 2015 to January 2020. Immunohistochemical method was used to detect the expression of RKIP, P-ERK, and P-MEK proteins in GIST tissue samples from these patients. Kaplan-Meier method was used to calculate the survival rate of 63 patients with complete follow-up data. A Nomogram was used to represent the new prognostic evaluation model. The Cox multivariate regression analysis was conducted separately for each set of risk evaluation factors, based on two risk classification systems [the new risk grade model vs the modified National Institutes of Health (NIH) 2008 risk classification system]. Receiver operating characteristic (ROC) curves were used for evaluating the accuracy and efficiency of the two prognostic evaluation systems.

RESULTS

In GIST tissues, RKIP protein showed positive expression in the cytoplasm and cell membrane, appearing as brownish-yellow or brown granules. The expression of RKIP was related to GIST tumor size, NIH grade, and mucosal invasion. P-ERK protein exhibited heterogeneous distribution in GIST cells, mainly in the cytoplasm, with occasional presence in the nucleus, and appeared as brownish-yellow granules, and the expression of P-ERK protein was associated with GIST tumor size, mitotic count, mucosal invasion, and NIH grade. Meanwhile, RKIP protein expression was negatively correlated with P-ERK expression. The results in COX multivariate regression analysis showed that RKIP protein expression was not an independent risk factor for tumor prognosis. However, RKIP combined with P-ERK protein expression were identified as independent risk factors for prognosis with statistical significance. Furthermore, we establish a new prognosis evaluation model using RKIP and P-ERK in combination and obtained the nomogram of the new prognosis evaluation model. ROC curve analysis also showed that the new evaluation model had better prognostic performance than the modified NIH 2008 risk classification system.

CONCLUSION

Our experimental results showed that the expression of RKIP and P-ERK proteins in GIST was associated with tumor size, NIH 2008 staging, and tumor invasion, and P-ERK expression was also related to mitotic count. The expression of the two proteins had a certain negative correlation. The combined expression of RKIP and P-ERK proteins can serve as an independent risk factor for predicting the prognosis of GIST patients. The new risk assessment model incorporating RKIP and P-ERK has superior evaluation efficacy and is worth further practical application to validate.

RKIP localizes to the nucleus through a bipartite nuclear localization signal and interaction with importin α to regulate mitotic progression

Raf kinase inhibitor protein (RKIP) is a multifunctional modulator of intracellular signal transduction. Although most of its functions have been considered cytosolic, we show here that the localization of RKIP is primarily nuclear in both growing and quiescent Madin-Darby canine kidney epithelial cells and in Cal-51 and BT-20 human breast cancer cells. We have identified a putative bipartite nuclear localization signal (NLS) in RKIP that maps to the surface of the protein surrounding a known regulatory region. Like classical NLS sequences, the putative NLS of RKIP is rich in arginine and lysine residues. Deletion of and point mutations in the putative NLS lead to decreased nuclear localization. Point mutation of all the basic residues in the putative NLS of RKIP particularly strongly reduces nuclear localization. We found consistent results in reexpression experiments with wildtype or mutant RKIP in RKIP-silenced cells. A fusion construct of the putative NLS of RKIP alone to a heterologous reporter protein leads to nuclear localization of the fusion protein, demonstrating that this sequence alone is sufficient for import into the nucleus. We found that RKIP interacts with the nuclear transport factor importin α in BT-20 and MDA-MB-231 human breast cancer cells, suggesting importin-mediated active nuclear translocation. Taken together, these findings suggest that a bipartite NLS in RKIP interacts with importin α for active transport of RKIP into the nucleus and that this process may be involved in the regulation of mitotic progression. Evaluating the biological function of nuclear localization of RKIP, we found that the presence of the putative NLS is important for the role of RKIP in mitotic checkpoint regulation in MCF-7 human breast cancer cells.

Photoaffinity labelling-based chemoproteomic strategy identifies PEBP1 as the target of ethyl gallate against macrophage activation

Ulcerative colitis (UC) is an inflammatory disease of the colon with an unmet need for therapeutic targets. Ethyl gallate (EG) is a natural small molecule for UC treatment, but its cellular target is unknown. By labelling EG with a diazirine photocrosslinker and a click chemistry handle, we identified phosphatidyl-ethanolamine binding protein1 (PEBP1) as a direct cellular target of EG by forming hydrogen bonds with Asp70 and Tyr120. In particular, hydrogen/deuterium exchange mass spectrometry indicated that EG induced the sequence (residues 141-153) embedding to inhibit S153 phosphorylation of PEBP1. Additionally, the EG-mediated sequence (residues 108-122) exposure significantly enhanced PEBP1-Raf-1 interaction to block the downstream NF-κB inflammatory pathway in macrophages. Moreover, PEBP1 siRNA substantially reversed the EG-dependent down-regulation of the phosphorylation of IKKβ, IκBα and NF-κB, demonstrating that the NF-κB signal functioned as an essential anti-inflammation mechanism of PEBP1. Collectively, we revealed PEBP1 as a previously undescribed cellular target in macrophages for UC therapy and identified a new allosteric site for PEBP1 biology study using EG as a chemical probe.

Cefotetan-bound human RKIP involves in Ras/Raf1/MEK/ERK signaling pathway

Cefotetan is widely used to treat bacterial infections in the clinic owing to its broad spectrum of antibacterial activity. In the present study, we demonstrate that cefotetan can bind to the conserved ligand-binding pocket of human Raf1 kinase inhibitory protein (hRKIP), which acts as a negative regulator of the Ras/Raf1/MEK/ERK signaling pathway. The cefotetan-bound hRKIP adopts a rigid structure with insufficient space for binding Raf1 kinase, thereby reliving the inhibitory activity of hRKIP in the Ras/Raf1/MEK/ERK signaling pathway and enhancing the phosphorylation level of ERK. Both NMR titration and molecular docking approaches show that several residues (P74, Y81, W84, P111, P112, K113, S142, G143, D144, W173, P178, Y181 and L184) play crucial roles in hRKIP binding cefotetan. NMR dynamics analysis reveals that the binding of cefotetan with hRKIP promotes ps-ns internal motion but reduces μs-ms conformational exchange for residues in the cefotetan-binding pocket of hRKIP. Our results not only disclose the structural basis of cefotetan upregulating the Ras/Raf1/MEK/ERK signaling pathway but also benefit developing novel drugs against diseases caused by the impaired Ras/Raf1/MEK/ERK pathway.

Sphingosine kinases regulate ER contacts with late endocytic organelles and cholesterol trafficking

Membrane contact sites (MCS), close membrane apposition between organelles, are platforms for interorganellar transfer of lipids including cholesterol, regulation of lipid homeostasis, and co-ordination of endocytic trafficking. Sphingosine kinases (SphKs), two isoenzymes that phosphorylate sphingosine to the bioactive sphingosine-1-phosphate (S1P), have been implicated in endocytic trafficking. However, the physiological functions of SphKs in regulation of membrane dynamics, lipid trafficking and MCS are not known. Here, we report that deletion of SphKs decreased S1P with concomitant increases in its precursors sphingosine and ceramide, and markedly reduced endoplasmic reticulum (ER) contacts with late endocytic organelles. Expression of enzymatically active SphK1, but not catalytically inactive, rescued the deficit of these MCS. Although free cholesterol accumulated in late endocytic organelles in SphK null cells, surprisingly however, cholesterol transport to the ER was not reduced. Importantly, deletion of SphKs promoted recruitment of the ER-resident cholesterol transfer protein Aster-B (also called GRAMD1B) to the plasma membrane (PM), consistent with higher accessible cholesterol and ceramide at the PM, to facilitate cholesterol transfer from the PM to the ER. In addition, ceramide enhanced in vitro binding of the Aster-B GRAM domain to phosphatidylserine and cholesterol liposomes. Our study revealed a previously unknown role for SphKs and sphingolipid metabolites in governing diverse MCS between the ER network and late endocytic organelles versus the PM to control the movement of cholesterol between distinct cell membranes.

A Negative Regulatory Role for RKIP in Breast Cancer Immune Response

Simple Summary

Breast cancer is the second most common cancer in women worldwide. Regulation of breast cancer metastasis remains an elusive phenomenon. Elucidating the mechanistic pathway of metastatic signaling may identify targets for regulating cancer metastatic potential. Raf-1 kinase inhibitor protein (RKIP) has been shown to negatively regulate signaling pathways involved in cancer progression and metastasis. RKIP may suppress metastasis of breast cancer cells by downregulating elements of the immune system.

Abstract

Raf-1 kinase inhibitor protein was first identified as a negative regulator of the Raf signaling pathway. Subsequently, it was shown to have a causal role in containing cancer progression and metastasis. Early studies suggested that RKIP blocks cancer progression by inhibiting the Raf-1 pathway. However, it is not clear if the RKIP tumor and metastasis suppression function involve other targets. In addition to the Raf signaling pathway, RKIP has been found to modulate several other signaling pathways, affecting diverse biological functions including immune response. Recent advances in medicine have identified both positive and negative roles of immune response in cancer initiation, progression and metastasis. It is possible that one way that RKIP exerts its effect on cancer is by targeting an immune response mechanism. Here, we provide evidence supporting the causal role of tumor and metastasis suppressor RKIP in downregulating signaling pathways involved with immune response in breast cancer cells and discuss its potential ramification on cancer therapy.

Raf kinase inhibitory protein reduces bradykinin receptor desensitization

Inflammatory hyperalgesia represents a nociceptive phenotype that can become persistent in nature through dynamic protein modifications. However, a large gap in knowledge exists concerning how the integration of intracellular signaling molecules coordinates a persistent inflammatory phenotype. Herein, we demonstrate that Raf Kinase Anchoring Protein (RKIP) interrupts a vital canonical desensitization pathway to maintain bradykinin (BK) receptor activation in primary afferent neurons. Biochemical analyses of primary neuronal cultures indicate bradykinin-stimulated PKC phosphorylation of RKIP at Ser153. Furthermore, BK exposure increases G-protein Receptor Kinase 2 (GRK2) binding to RKIP, inhibiting pharmacological desensitization of the BK receptor. Additional studies found that molecular RKIP down-regulation increases BK receptor desensitization in real-time imaging of primary afferent neurons, identifying a key pathway integrator in the desensitization process that controls multiple GRK2-sensitive G-protein coupled receptors. Therefore, RKIP serves as an integral scaffolding protein that inhibits BK receptor desensitization.

Raf Kinase Inhibitory Protein regulates the cAMP-dependent protein kinase signaling pathway through a positive feedback loop

Raf Kinase Inhibitory Protein (RKIP) maintains cellular robustness and prevents the progression of diseases such as cancer and heart disease by regulating key kinase cascades including MAP kinase and protein kinase A (PKA). Phosphorylation of RKIP at S153 by Protein Kinase C (PKC) triggers a switch from inhibition of Raf to inhibition of the G protein coupled receptor kinase 2 (GRK2), enhancing signaling by the β-adrenergic receptor (β-AR) that activates PKA. Here we report that PKA-phosphorylated RKIP promotes β-AR-activated PKA signaling. Using biochemical, genetic, and biophysical approaches, we show that PKA phosphorylates RKIP at S51, increasing S153 phosphorylation by PKC and thereby triggering feedback activation of PKA. The S51V mutation blocks the ability of RKIP to activate PKA in prostate cancer cells and to induce contraction in primary cardiac myocytes in response to the β-AR activator isoproterenol, illustrating the functional importance of this positive feedback circuit. As previously shown for other kinases, phosphorylation of RKIP at S51 by PKA is enhanced upon RKIP destabilization by the P74L mutation. These results suggest that PKA phosphorylation at S51 may lead to allosteric changes associated with a higher-energy RKIP state that potentiates phosphorylation of RKIP at other key sites. This allosteric regulatory mechanism may have therapeutic potential for regulating PKA signaling in disease states.

The tobacco phosphatidylethanolamine-binding protein NtFT4 increases the lifespan of Drosophila melanogaster by interacting with the proteostasis network

Proteostasis reflects the well-balanced synthesis, trafficking and degradation of cellular proteins. This is a fundamental aspect of the dynamic cellular proteome, which integrates multiple signaling pathways, but it becomes increasingly error-prone during aging. Phosphatidylethanolamine-binding proteins (PEBPs) are highly conserved regulators of signaling networks and could therefore affect aging-related processes. To test this hypothesis, we expressed PEPBs in a heterologous context to determine their ectopic activity. We found that heterologous expression of the tobacco (Nicotiana tabacum) PEBP NtFT4 in Drosophila melanogaster significantly increased the lifespan of adult flies and reduced age-related locomotor decline. Similarly, overexpression of the Drosophila ortholog CG7054 increased longevity, whereas its suppression by RNA interference had the opposite effect. In tobacco, NtFT4 acts as a floral regulator by integrating environmental and intrinsic stimuli to promote the transition to reproductive growth. In Drosophila, NtFT4 engaged distinct targets related to proteostasis, such as HSP26. In older flies, it also prolonged Hsp26 gene expression, which promotes longevity by maintaining protein integrity. In NtFT4-transgenic flies, we identified deregulated genes encoding proteases that may contribute to proteome stability at equilibrium. Our results demonstrate that the expression of NtFT4 influences multiple aspects of the proteome maintenance system via both physical interactions and transcriptional regulation, potentially explaining the aging-related phenotypes we observed.

Florigen and its homologs of FT/CETS/PEBP/RKIP/YbhB family may be the enzymes of small molecule metabolism: review of the evidence

BACKGROUND

Flowering signals are sensed in plant leaves and transmitted to the shoot apical meristems, where the formation of flowers is initiated. Searches for a diffusible hormone-like signaling entity ("florigen") went on for many decades, until a product of plant gene FT was identified as the key component of florigen in the 1990s, based on the analysis of mutants, genetic complementation evidence, and protein and RNA localization studies. Sequence homologs of FT protein are found throughout prokaryotes and eukaryotes; some eukaryotic family members appear to bind phospholipids or interact with the components of the signal transduction cascades. Most FT homologs are known to share a constellation of five charged residues, three of which, i.e., two histidines and an aspartic acid, are located at the rim of a well-defined cavity on the protein surface.

RESULTS

We studied molecular features of the FT homologs in prokaryotes and analyzed their genome context, to find tentative evidence connecting the bacterial FT homologs with small molecule metabolism, often involving substrates that contain sugar or ribonucleoside moieties. We argue that the unifying feature of this protein family, i.e., a set of charged residues conserved at the sequence and structural levels, is more likely to be an enzymatic active center than a catalytically inert ligand-binding site.

CONCLUSIONS

We propose that most of FT-related proteins are enzymes operating on small diffusible molecules. Those metabolites may constitute an overlooked essential ingredient of the florigen signal.

Insights of RKIP-Derived Suppression of Prostate Cancer

Simple Summary

Despite an intensive research effort in the past few decades, prostate cancer (PC) remains a top cause of cancer death in men, particularly in the developed world. The major cause of fatality is the progression of local prostate cancer to metastasis disease. Treatment of patients with metastatic prostate cancer (mPC) is generally ineffective. Based on the discovery of mPC relying on androgen for growth, many patients with mPC show an initial response to the standard of care: androgen deprivation therapy (ADT). However, lethal castration resistant prostate cancers (CRPCs) commonly develop. It is widely accepted that intervention of metastatic progression of PC is a critical point of intervention to reduce PC death. Accumulative evidence reveals a role of RKIP in suppression of PC progression towards mPC. We will review current evidence and discuss the potential utilization of RKIP in preventing mPC progression.

Abstract

Prostate cancer (PC) is a major cause of cancer death in men. The disease has a great disparity in prognosis. Although low grade PCs with Gleason scores ≤ 6 are indolent, high-risk PCs are likely to relapse and metastasize. The standard of care for metastatic PC (mPC) remains androgen deprivation therapy (ADT). Resistance commonly occurs in the form of castration resistant PC (CRPC). Despite decades of research efforts, CRPC remains lethal. Understanding of mechanisms underpinning metastatic progression represents the overarching challenge in PC research. This progression is regulated by complex mechanisms, including those regulating PC cell proliferation, epithelial–mesenchymal transition (EMT), and androgen receptor (AR) signaling. Among this PC metastatic network lies an intriguing suppressor of PC metastasis: the Raf kinase inhibitory protein (RKIP). Clinically, the RKIP protein is downregulated in PC, and showed further reduction in mPC. In xenograft mouse models for PC, RKIP inhibits metastasis. In vitro, RKIP reduces PC cell invasion and sensitizes PC cells to therapeutic treatments. Mechanistically, RKIP suppresses Raf-MEK-ERK activation and EMT, and modulates extracellular matrix. In return, Snail, NFκB, and the polycomb protein EZH2 contribute to inhibition of RKIP expression. In this review, we will thoroughly analyze RKIP’s tumor suppression actions in PC.

RKIP Pleiotropic Activities in Cancer and Inflammatory Diseases: Role in Immunity

Simple Summary

The human body consists of tissues and organs formed by cells. In each cell there is a switch that allows the cell to divide or not. In contrast, cancer cells have their switch on which allow them to divide and invade other sites leading to death. Over two decades ago, Doctor Kam Yeung, University of Toledo, Ohio, has identified a factor (RKIP) that is responsible for the on/off switch which functions normally in healthy tissues but is inactive or absent in cancers. Since this early discovery, many additional properties have been ascribed to RKIP including its role in inhibiting cancer metastasis and resistance to therapeutics and its role in modulating the normal immune response. This review describes all of the above functions of RKIP and suggesting therapeutics to induce RKIP in cancers to inhibit their growth and metastases as well as inhibit its activity to treat non-cancerous inflammatory diseases.

Abstract

Several gene products play pivotal roles in the induction of inflammation and the progression of cancer. The Raf kinase inhibitory protein (RKIP) is a cytosolic protein that exerts pleiotropic activities in such conditions, and thus regulates oncogenesis and immune-mediated diseases through its deregulation. Herein, we review the general properties of RKIP, including its: (i) molecular structure; (ii) involvement in various cell signaling pathways (i.e., inhibition of the Raf/MEK/ERK pathway; the NF-kB pathway; GRK-2 or the STAT-3 pathway; as well as regulation of the GSK3Beta signaling; and the spindle checkpoints); (iii) regulation of RKIP expression; (iv) expression’s effects on oncogenesis; (v) role in the regulation of the immune system to diseases (i.e., RKIP regulation of T cell functions; the secretion of cytokines and immune mediators, apoptosis, immune check point inhibitors and RKIP involvement in inflammatory diseases); and (vi) bioinformatic analysis between normal and malignant tissues, as well as across various immune-related cells. Overall, the regulation of RKIP in different cancers and inflammatory diseases suggest that it can be used as a potential therapeutic target in the treatment of these diseases.

The tobacco phosphatidylethanolamine-binding protein NtFT4 simultaneously improves vitality, growth, and protein yield in human cells

The production of biopharmaceutical proteins in mammalian cells by transient expression or stable transformation requires robust and viable cells. Cell line engineering must therefore balance improved cell growth and viability with high productivity. We tested the ability of nonmammalian phosphatidylethanolamine-binding proteins to enhance cell proliferation in monolayers and suspension cultures. The tobacco protein NtFT4 improved the proliferation of multiple human cell lines. Viable cell density is usually impaired by efficient transfection, but we found that the number of HEK-293TNtFT4 cells at the peak of protein expression was twice that of standard HEK-293T cells, and the antibody yield increased by approximately one-third. Improved growth and viability were observed in different cell lines, in different culture media, and also after transient transfection, suggesting the beneficial trait is consistent and transferable. Additional modifications could boost the productivity of high-density HEK-293TNtFT4 cells even further as we showed for a fluorescent marker protein and recombinant antibody expressed in monolayer cultures. The HEK-293TNtFT4 cell line provides a new human model platform that increases cell proliferation, also achieving a fundamental improvement in recombinant protein expression.

Suramin Targets the Conserved Ligand-Binding Pocket of Human Raf1 Kinase Inhibitory Protein

Suramin was initially used to treat African sleeping sickness and has been clinically tested to treat human cancers and HIV infection in the recent years. However, the therapeutic index is low with numerous clinical side-effects, attributed to its diverse interactions with multiple biological macromolecules. Here, we report a novel binding target of suramin, human Raf1 kinase inhibitory protein (hRKIP), which is an important regulatory protein involved in the Ras/Raf1/MEK/ERK (MAPK) signal pathway. Biolayer interference technology showed that suramin had an intermediate affinity for binding hRKIP with a dissociation constant of 23.8 µM. Both nuclear magnetic resonance technology and molecular docking analysis revealed that suramin bound to the conserved ligand-binding pocket of hRKIP, and that residues K113, W173, and Y181 play crucial roles in hRKIP binding suramin. Furthermore, suramin treatment at 160 µM could profoundly increase the ERK phosphorylation level by around 3 times. Our results indicate that suramin binds to hRKIP and prevents hRKIP from binding with hRaf1, thus promoting the MAPK pathway. This work is beneficial to both mechanistically understanding the side-effects of suramin and efficiently improving the clinical applications of suramin.

NF-κB sub-pathways and HIV cure: A revisit

HIV cure is thwarted by the presence of quiescent yet replication competent HIV-1 (HIV). Antiretroviral therapy (ART) is unable to eradicate reservoirs, and upon cessation of ART, HIV will rebound. This review encompasses the curative strategies of HIV in the context of NF-κB sub-pathways that are currently exploited and demonstrate promise in the disruption of latent HIV. Canonical NF-κB signaling has long been established to drive HIV proviral expression while noncanonical NF-κB signaling, a novel and perhaps more desirable mechanism of latency reversal due to its unique characteristics, has recently been shown to also promote HIV expression from latency. Furthermore, we discuss the previously unrecognized upstream signaling of NF-κB as a new avenue for exploration of a functional cure of HIV.

PEBP1 suppresses HIV transcription and induces latency by inactivating MAPK/NF-κB signaling

The latent HIV‐1 reservoir is a major barrier to viral eradication. However, our understanding of how HIV‐1 establishes latency is incomplete. Here, by performing a genome‐wide CRISPR‐Cas9 knockout library screen, we identify phosphatidylethanolamine‐binding protein 1 (PEBP1), also known as Raf kinase inhibitor protein (RKIP), as a novel gene inducing HIV latency. Depletion of PEBP1 leads to the reactivation of HIV‐1 in multiple models of latency. Mechanistically, PEBP1 de‐phosphorylates Raf1/ERK/IκB and IKK/IκB signaling pathways to sequestrate NF‐κB in the cytoplasm, which transcriptionally inactivates HIV‐1 to induce latency. Importantly, the induction of PEBP1 expression by the green tea compound epigallocatechin‐3‐gallate (EGCG) prevents latency reversal by inhibiting nuclear translocation of NF‐κB, thereby suppressing HIV‐1 transcription in primary CD4⁺ T cells isolated from patients receiving antiretroviral therapy (ART). These results suggest a critical role for PEBP1 in the regulation of upstream NF‐κB signaling pathways governing HIV transcription. Targeting of this pathway could be an option to control HIV reservoirs in patients in the future.

Resveratrol binds and activates RKIP protein in colorectal cancer

Raf-1 kinase inhibitory protein (RKIP) acts as a tumor cell metastasis suppressor and prognostic indicator for survival in various cancers. Its use is predicted to improve therapy for various malignancies, including colorectal cancer (CRC). RKIP, frequently denoted as phosphatidylethanolamine-binding protein 1, is expressed in all normal mammalian tissues. RKIP functions as an inhibitor of the Raf-1, PI-3K, and MAP kinase (MAPK) pathways. In this study, we found that resveratrol induced the expression of RKIP at protein levels. To elucidate the structural basis of the interaction between resveratrol and RKIP, we performed computational studies that explore the binding affinity and ligand efficacy of resveratrol against RKIP. This study reveals the prognostic significance of RKIP metastasis suppressor activity against CRC and its structural arrangements during drug-target interactions.

Anti-leprosy drug Clofazimine binds to human Raf1 kinase inhibitory protein and enhances ERK phosphorylation

Human Raf1 kinase inhibitory protein (hRKIP) is an important modulator of the Ras/Raf1/MEK/ERK signaling pathway. Here, we demonstrated that anti-leprosy drug Clofazimine can bind to hRKIP with a significantly stronger affinity than the endogenous substrate phosphatidylethanolamine (PE) by using Biolayer interference technology. Moreover, we identified that residues P74, S75, K80, P111, P112, V177, and P178 play crucial roles in the binding of hRKIP to Clofazimine by using a combination of Nuclear Magnetic Resonance spectroscopy and molecular docking approach. These residues are located at the conserved ligand-binding pocket of hRKIP. Furthermore, we found that 3.2 μM Clofazimine could significantly increase the ERK phosphorylation level by about 37%. Our results indicate that Clofazimine can enhance Ras/Raf1/MEK/ERK signaling transduction pathway via binding to hRKIP. This work provides valuable hints for exploiting Clofazimine as a potential lead compound to efficiently treat the diseases related to RKIP or the Ras/Raf/MEK/ERK pathway.

RKIP: A Key Regulator in Tumor Metastasis Initiation and Resistance to Apoptosis: Therapeutic Targeting and Impact

RAF-kinase inhibitor protein (RKIP) is a well-established tumor suppressor that is frequently downregulated in a plethora of solid and hematological malignancies. RKIP exerts antimetastatic and pro-apoptotic properties in cancer cells, via modulation of signaling pathways and gene products involved in tumor survival and spread. Here we review the contribution of RKIP in the regulation of early metastatic steps such as epithelial–mesenchymal transition (EMT), migration, and invasion, as well as in tumor sensitivity to conventional therapeutics and immuno-mediated cytotoxicity. We further provide updated justification for targeting RKIP as a strategy to overcome tumor chemo/immuno-resistance and suppress metastasis, through the use of agents able to modulate RKIP expression in cancer cells.

Clinical and prognostic significance of Raf kinase inhibitory protein expression in gastrointestinal stromal tumors

AIM

To detect the expression of Raf kinase inhibitory protein (RKIP) in gastrointestinal stromal tumors (GISTs) and to analyze its relationship with clinicopatholgical characteristics and prognosis of this disease.

METHODS

Sixty-three patients with pathologically diagnosed GISTs who underwent surgical resection at the Shengjing Hospital of China Medical University from January 2011 to January 2015 and had complete clinical, pathological, and follow-up data were included. Immunohistochemical method was used to detect the expression of RKIP in GIST tissue samples from these patients. Kaplan-Meier method was used to calculate the survival rate of 60 patients with complete follow-up data, and Cox regression analysis was performed to identify factors affecting the prognosis of patients GISTs to evaluate further the diagnostic and prognostic value of RKIP in GISTs.

RESULTS

In GIST tissues, RKIP positive signals, manifesting as brownish yellow or brown granules, were located in the cytoplasm or on the membrane. Of 63 tissue samples included in this study, 34 (54%) were positive and 29 (46%) were negative for RKIP expression. Statistical analysis showed that RKIP expression in GISTs was significantly associated with tumor size, National Institutes of Health (NIH) risk grade, and mucosal invasion, but had no significant association with age, gender, tumor location, or the number of mitotic figures. Univariate Kaplan-Meier analysis revealed that the 1-, 3-, and 5-year survival rates were 94.4%, 89.2%, and 80.5% for patients with positive RKIP expression, and 88.6%, 68.2%, and 48.2% for patients with negative RKIP expression, suggesting that patients with high RKIP expression had significantly higher survival rates than those with low expression (Log-rank test, P = 0.0015). Cox regression analysis demonstrated that NIH risk grade was significantly associated with the prognosis of GISTs (P = 0.037), suggesting that NIH risk grade is a significant predictor of the prognosis of GISTs. RKIP expression had a tendency to predict the survival of GISTs (P = 0.122), suggesting that RKIP expression may have appreciated value to predict the prognosis of GISTs.

CONCLUSION

This study demonstrated that: (1) RKIP expression in GISTs is associated with tumor size, NIH risk grade, and mucosal invasion, and low or no expression of RKIP predicts a high malignancy potential; (2) high RKIP correlates positively with the survival of patients with GISTs; and (3) RKIP expression has appreciated value for predicting the survival of patients with GISTs, although it is not an independent prognostic factor in GISTs.

Conserved salt-bridge competition triggered by phosphorylation regulates the protein interactome

Phosphorylation is a major regulator of protein interactions; however, the mechanisms by which regulation occurs are not well understood. Here we identify a salt-bridge competition or "theft" mechanism that enables a phospho-triggered swap of protein partners by Raf Kinase Inhibitory Protein (RKIP). RKIP transitions from inhibiting Raf-1 to inhibiting G-protein-coupled receptor kinase 2 upon phosphorylation, thereby bridging MAP kinase and G-Protein-Coupled Receptor signaling. NMR and crystallography indicate that a phosphoserine, but not a phosphomimetic, competes for a lysine from a preexisting salt bridge, initiating a partial unfolding event and promoting new protein interactions. Structural elements underlying the theft occurred early in evolution and are found in 10% of homo-oligomers and 30% of hetero-oligomers including Bax, Troponin C, and Early Endosome Antigen 1. In contrast to a direct recognition of phosphorylated residues by binding partners, the salt-bridge theft mechanism represents a facile strategy for promoting or disrupting protein interactions using solvent-accessible residues, and it can provide additional specificity at protein interfaces through local unfolding or conformational change.

Raf kinase inhibitor protein: lessons of a better way for β-adrenergic receptor activation in the heart

Stimulation of β-adrenergic receptors (βARs) provides the most efficient physiological mechanism to enhance contraction and relaxation of the heart. Activation of βARs allows rapid enhancement of myocardial function in order to fuel the muscles for running and fighting in a fight-or-flight response. Likewise, βARs become activated during cardiovascular disease in an attempt to counteract the restrictions of cardiac output. However, long-term stimulation of βARs increases the likelihood of cardiac arrhythmias, adverse ventricular remodelling, decline of cardiac performance and premature death, thereby limiting the use of βAR agonists in the treatment of heart failure. Recently the endogenous Raf kinase inhibitor protein (RKIP) was found to activate βAR signalling of the heart without adverse effects. This review will summarize the current knowledge on RKIP-driven compared to receptor-mediated signalling in cardiomyocytes. Emphasis is given to the differential effects of RKIP on β1 - and β2 -ARs and their downstream targets, the regulation of myocyte calcium cycling and myofilament activity.

Crosstalks between Raf-kinase inhibitor protein and cancer stem cell transcription factors (Oct4, KLF4, Sox2, Nanog)

Raf-kinase inhibitor protein has been reported to inhibit both the Raf/mitogen extracellular signal-regulated kinase/extracellular signal-regulated kinase and nuclear factor kappa-light-chain of activated B cells pathways. It has also been reported in cancers that Raf-kinase inhibitor protein behaves as a metastatic suppressor as well as a chemo-immunosensitizing factor to drug/immune-mediated apoptosis. The majority of cancers exhibit low or no levels of Raf-kinase inhibitor protein. Hence, the activities of Raf-kinase inhibitor protein contrast, in part, to those mediated by several cancer stem cell transcription factors for their roles in resistance and metastasis. In this review, the existence of crosstalks in the signaling pathways between Raf-kinase inhibitor protein and several cancer stem cell transcription factors (Oct4, KLF4, Sox2 and Nanog) was assembled. Oct4 is induced by Lin28, and Raf-kinase inhibitor protein inhibits the microRNA binding protein Lin28. The expression of Raf-kinase inhibitor protein inversely correlates with the expression of Oct4. KLF4 does not interact directly with Raf-kinase inhibitor protein, but rather interacts indirectly via Raf-kinase inhibitor protein's regulation of the Oct4/Sox2/KLF4 complex through the mitogen-activated protein kinase pathway. The mechanism by which Raf-kinase inhibitor protein inhibits Sox2 is via the inhibition of the mitogen-activated protein kinase pathway by Raf-kinase inhibitor protein. Thus, Raf-kinase inhibitor protein's relationship with Sox2 is via its regulation of Oct4. Inhibition of extracellular signal-regulated kinase by Raf-kinase inhibitor protein results in the upregulation of Nanog. The inhibition of Oct4 by Raf-kinase inhibitor protein results in the failure of the heterodimer formation of Oct4 and Sox2 that is necessary to bind to the Nanog promoter for the transcription of Nanog. The findings revealed that there exists a direct correlation between the expression of Raf-kinase inhibitor protein and the expression of each of the above transcription factors. Based on these analyses, we suggest that the expression level of Raf-kinase inhibitor protein may be involved in the regulation of the cancer stem cell phenotype.

Enrichment of Metabolite-Binding Proteins by Affinity Elution in Tandem Hydrophobic Interaction Chromatography (AETHIC) Reveals RKIP Regulating ERK Signaling in an ATP-Dependent Manner

To elucidate the molecular mechanisms underlying the action of bioactive compounds such as metabolites, identification of their binding targets is essential. However, available techniques for enriching metabolite-binding proteins are practically restrained by special equipment requirements and laborious efforts. Here we have developed a novel method, affinity elution in tandem hydrophobic interaction chromatography (AETHIC), which enables enrichment of metabolite-binding proteins from a crude tissue extract. AETHIC constitutes two major steps, protein fractionation and affinity elution. The basic strategy of AETHIC uses a series of HIC matrices encompassing aliphatic chains of different length and thus provides a wide range of hydrophobicity for interactions with most proteins. Thereafter, target proteins are eluted selectively by a given ligand. As our first proof-of-principle, we demonstrated that AETHIC was able to enrich ATP-binding proteins from porcine brain extract. In addition, we have demonstrated that raf kinase inhibitory protein (RKIP) is an ATP-binding protein and ATP attenuates the interaction between RKIP and Raf-1. In parallel, short-term ATP depletion in cultured HEK293 cells augments interaction between RKIP and Raf-1, resulting in decreased activation of the downstream ERK signaling. Therefore, the ATP-binding function renders RKIP's inhibition on Raf-1 modulated by cellular ATP concentrations. These data shed light on how energy levels affect the propagation of cellular signaling. Taken together, the enclosed results advocate the potential of AETHIC in the study of metabolite-protein interactions.

Using NMR and molecular dynamics to link structure and dynamics effects of the universal base 8-aza, 7-deaza, N8 linked adenosine analog

A truly universal nucleobase enables a host of novel applications such as simplified templates for PCR primers, randomized sequencing and DNA based devices. A universal base must pair indiscriminately to each of the canonical bases with little or preferably no destabilization of the overall duplex. In reality, many candidates either destabilize the duplex or do not base pair indiscriminatingly. The novel base 8-aza-7-deazaadenine (pyrazolo[3,4-d]pyrimidin- 4-amine) N⁸-(2'deoxyribonucleoside), a deoxyadenosine analog (UB), pairs with each of the natural DNA bases with little sequence preference. We have utilized NMR complemented with molecular dynamic calculations to characterize the structure and dynamics of a UB incorporated into a DNA duplex. The UB participates in base stacking with little to no perturbation of the local structure yet forms an unusual base pair that samples multiple conformations. These local dynamics result in the complete disappearance of a single UB proton resonance under native conditions. Accommodation of the UB is additionally stabilized via heightened backbone conformational sampling. NMR combined with various computational techniques has allowed for a comprehensive characterization of both structural and dynamic effects of the UB in a DNA duplex and underlines that the UB as a strong candidate for universal base applications.

Interactions of RKIP with inflammatory signaling pathways

The inflammatory response plays an important role in host defense and maintenance of homeostasis, while imbalances in these responses can also lead to pathologic disease processes. Emerging data show that RKIP interacts with multiple signaling molecules that may potentiate multiple functions during inflammatory processes. Here, we review the interaction of RKIP with both the MAPK and NF-κB pathways in relation to chronic inflammatory diseases. In these settings, it can both inhibit inflammatory pathways as well contribute to pro-inflammatory signaling, often depending on the interactions with multiple proteins and perhaps lipids. The interactions of RKIP with proteins, phospholipids, fatty acids, and their enzymes thus could play a substantial role in diseases like asthma and diabetes. Targeting interactions of RKIP with these pathways could lead to novel approaches to treatment.

A General Network Pharmacodynamic Model-Based Design Pipeline for Customized Cancer Therapy Applied to the VEGFR Pathway

A unified approach to optimize multidrug chemotherapy using a pharmacokinetic (PK)/enhanced pharmacodynamic model was developed using the vascular endothelial growth factor receptor (VEGFR) signaling system. The base VEGFR network model, characterized by ligand–receptor interactions, enzyme recruitment (Grb2-Sos, phospholipase C γ (PLCγ), and phosphoinositide-3 kinase (PI3K)), and downstream mitogen-activated protein kinase and Akt cascade activation, was linked to a sunitinib (VEGFR inhibitor) PK model and underwent Sobol sensitivity analysis that revealed potential sunitinib-enhancing mechanisms. Drugs targeting these mechanisms (a VEGF inhibitor, a PI3K inhibitor, a PLCγ inhibitor, and a mitogen-activated protein kinase inhibitor) and sunitinib were input to optimization-based control analyses to design multidrug regimens that maintained 80% pERK and pAkt inhibition for 28 days while minimizing drug dose. The resultant combination regimens contained both continuous and discontinuous schedules, mostly at low doses, and were altered by oncogenic mutations. This pipeline of computational analyses demonstrates how model-based methods can capture the complexities of drug action, tailor cancer chemotherapy, and empower personalized medicine.

Raf kinase inhibitory protein (RKIP) as a metastasis suppressor: regulation of signaling networks in cancer

Cancer is one of the deadliest diseases worldwide, accounting for about 8 million deaths a year. For solid tumors, cancer patients die as a result of the metastatic spread of the tumor to the rest of the body. Therefore, there is a clinical need for understanding the molecular and cellular basis of metastasis, identifying patients whose tumors are more likely to metastasize, and developing effective therapies against metastatic progression. Over the years, Raf kinase inhibitory protein (RKIP) has emerged as a natural suppressor of the metastatic process, constituting a tool for studying metastasis and its clinical outcomes. Here, we review RKIP's role as a metastasis suppressor and the signaling networks and genes regulated by RKIP in metastatic, triple-negative breast cancer. We also highlight the clinical implications and power of building gene signatures based on RKIP-regulated signaling modules in identifying cancer patients that are at higher risk for metastases. Finally, we highlight the potential of RKIP as a tool for developing new therapeutic strategies in cancer treatment.

RKIP structure drives its function: a three-state model for regulation of RKIP

Raf kinase inhibitory protein (RKIP) is a highly conserved regulator of many signaling networks whose loss or inactivation can lead to a variety of disease states. The multifaceted roles played by RKIP are enabled by an allosteric structure that is controlled through phosphorylation of RKIP and dynamics in the RKIP pocket loop. Perhaps the most striking feature of RKIP is that it can assume multiple functional states. Specifically, phosphorylation redirects RKIP from a state that binds and inhibits Raf-1 to a state that binds and inhibits GRK2. Recent evidence suggests the presence of a third functional state that facilitates RKIP phosphorylation. Here, we present a three-state model to explain the RKIP functional switch and discuss the role of the pocket loop in regulating RKIP activity.

RKIP: much more than Raf kinase inhibitory protein

From its discovery as a phosphatidylethanolamine-binding protein in bovine brain to its designation as a physiological inhibitor of Raf kinase protein, RKIP has emerged as a critical molecule for maintaining subdued, well-orchestrated cellular responses to stimuli. The disruption of RKIP in a wide range of pathologies, including cancer, Alzheimer's disease, and pancreatitis, makes it an exciting target for individualized therapy and disease-specific interventions. This review attempts to highlight recent advances in the RKIP field underscoring its potential role as a master modulator of many pivotal intracellular signaling cascades that control cellular growth, motility, apoptosis, genomic integrity, and therapeutic resistance. Specific biological and functional niches are highlighted to focus future research towards an enhanced understanding of the multiple roles of RKIP in health and disease.

Raf-1 kinase inhibitory protein (RKIP) mediates ethanol-induced sensitization of secretagogue signaling in pancreatic acinar cells

Excessive alcohol consumption is associated with most cases of chronic pancreatitis, a progressive necrotizing inflammatory disease that can result in pancreatic insufficiency due to acinar atrophy and fibrosis and an increased risk of pancreatic cancer. At a cellular level acute alcohol exposure can sensitize pancreatic acinar cells to secretagogue stimulation, resulting in dysregulation of intracellular Ca(2+) homeostasis and premature digestive enzyme activation; however, the molecular mechanisms by which ethanol exerts these toxic effects have remained undefined. In this study we identify Raf-1 kinase inhibitory protein as an essential mediator of ethanol-induced sensitization of cholecystokinin- and carbachol-regulated Ca(2+) signaling in pancreatic acinar cells. We show that exposure of rodent acinar cells to ethanol induces protein kinase C-dependent Raf-1 kinase inhibitory protein phosphorylation, sensitization of cholecystokinin-stimulated Ca(2+) signaling, and potentiation of both basal and cholecystokinin-stimulated extracellular signal-regulated kinase activation. Furthermore, we show that either suppression of Raf-1 kinase inhibitory protein expression using short hairpin RNA or gene ablation prevented the sensitizing effects of ethanol on cholecystokinin- and carbachol-stimulated Ca(2+) signaling and intracellular chymotrypsin activation in pancreatic acinar cells, suggesting that the modulation of Raf-1 inhibitory protein expression may have future therapeutic utility in the prevention or treatment of alcohol-associated pancreatitis.

Raf kinase inhibitor protein (RKIP) dimer formation controls its target switch from Raf1 to G protein-coupled receptor kinase (GRK) 2

Proteins controlling cellular networks have evolved distinct mechanisms to ensure specificity in protein-protein interactions. Raf kinase inhibitor protein (RKIP) is a multifaceted kinase modulator, but it is not well understood how this small protein (21 kDa) can coordinate its diverse signaling functions. Raf1 and G protein-coupled receptor kinase (GRK) 2 are direct interaction partners of RKIP and thus provide the possibility to untangle the mechanism of its target specificity. Here, we identify RKIP dimer formation as an important mechanistic feature in the target switch from Raf1 to GRK2. Co-immunoprecipitation and cross-linking experiments revealed RKIP dimerization upon phosphorylation of RKIP at serine 153 utilizing purified proteins as well as in cells overexpressing RKIP. A functional phosphomimetic RKIP mutant had a high propensity for dimerization and reproduced the switch from Raf1 to GRK2. RKIP dimerization and GRK2 binding, but not Raf1 interaction, were prevented by a peptide comprising amino acids 127-146 of RKIP, which suggests that this region is critical for dimer formation. Furthermore, a dimeric RKIP mutant displayed a higher affinity to GRK2, but a lower affinity to Raf1. Functional analyses of phosphomimetic as well as dimeric RKIP demonstrated that enhanced dimerization of RKIP translates into decreased Raf1 and increased GRK2 inhibition. The detection of RKIP dimers in a complex with GRK2 in murine hearts implies their physiological relevance. These findings represent a novel mechanistic feature how RKIP can discriminate between its different interaction partners and thus advances our understanding how specific inhibition of kinases can be achieved.

Ligand binding study of human PEBP1/RKIP: interaction with nucleotides and Raf-1 peptides evidenced by NMR and mass spectrometry

Background

Human Phosphatidylethanolamine binding protein 1 (hPEBP1) also known as Raf kinase inhibitory protein (RKIP), affects various cellular processes, and is implicated in metastasis formation and Alzheimer's disease. Human PEBP1 has also been shown to inhibit the Raf/MEK/ERK pathway. Numerous reports concern various mammalian PEBP1 binding ligands. However, since PEBP1 proteins from many different species were investigated, drawing general conclusions regarding human PEBP1 binding properties is rather difficult. Moreover, the binding site of Raf-1 on hPEBP1 is still unknown.

Methods/Findings

In the present study, we investigated human PEBP1 by NMR to determine the binding site of four different ligands: GTP, FMN, and one Raf-1 peptide in tri-phosphorylated and non-phosphorylated forms. The study was carried out by NMR in near physiological conditions, allowing for the identification of the binding site and the determination of the affinity constants K_D for different ligands. Native mass spectrometry was used as an alternative method for measuring K_D values.

Conclusions/Significance

Our study demonstrates and/or confirms the binding of hPEBP1 to the four studied ligands. All of them bind to the same region centered on the conserved ligand-binding pocket of hPEBP1. Although the affinities for GTP and FMN decrease as pH, salt concentration and temperature increase from pH 6.5/NaCl 0 mM/20°C to pH 7.5/NaCl 100 mM/30°C, both ligands clearly do bind under conditions similar to what is found in cells regarding pH, salt concentration and temperature. In addition, our work confirms that residues in the vicinity of the pocket rather than those within the pocket seem to be required for interaction with Raf-1.

15-Lipoxygenase 1 interacts with phosphatidylethanolamine-binding protein to regulate MAPK signaling in human airway epithelial cells

Epithelial 15-lipoxygenase 1 (15LO1) and activated ERK are increased in asthma despite modest elevations in IL-13. MAPK kinase (MEK)/ERK activation is regulated by interactions of Raf-1 with phosphatidylethanolamine-binding protein 1 (PEBP1). Epithelial 15LO1 generates intracellular 15-hydroxyeicosatetraenoic acid (15HETE) conjugated to phosphatidylethanolamine (PE) (15HETE-PE). We hypothesized that (i) 15LO1 and its product 15HETE-PE serve as signaling molecules interacting with PEBP1 to activate Raf-1/MEK/ERK and that (ii) this 15LO1-15HETE-PE-regulated ERK activation amplifies IL-4Rα downstream pathways. Our results demonstrate that high epithelial 15LO1 levels correlate with ERK phosphorylation ex vivo. In vitro, IL-13 induces 15LO1, which preferentially binds to PEBP1, causing PEBP1 to dissociate from Raf-1 and activate ERK. Exogenous 15HETE-PE similarly induces dissociation of PEBP1 from Raf-1 independently of IL-13/15LO1. siRNA knockdown of 15LO1 decreases the dissociation of Raf-1 from PEBP1, and the resulting lower ERK activation leads to lower downstream IL-4Rα-related gene expression. Identical protein-protein interactions are observed in endobronchial biopsies and fresh epithelial cells from asthmatics ex vivo. Colocalization of Raf-1 to PEBP1 is low in asthmatic tissue and cells compared with normals, whereas there is striking colocalization of 15LO1 with PEBP1 in asthma. Low 15LO1 levels in normals limit its colocalization with PEBP1. The results confirm a previously unknown signaling role for 15LO1 and its PE-conjugated eicosanoid product in human airway epithelial cells. This pathway enhances critical inflammatory pathways integral to asthma pathogenesis.

Expression of phosphorylated raf kinase inhibitor protein (pRKIP) is a predictor of lung cancer survival

Background

Raf-1 kinase inhibitor protein (RKIP) has been reported to negatively regulate signal kinases of major survival pathways. RKIP activity is modulated in part by phosphorylation on Serine 153 by protein kinase C, which leads to dissociation of RKIP from Raf-1. RKIP expression is low in many human cancers and represents an indicator of poor prognosis and/or induction of metastasis. The prognostic power has typically been based on total RKIP expression and has not considered the significance of phospho-RKIP.

Methods

The present study examined the expression levels of both RKIP and phospho-RKIP in human lung cancer tissue microarray proteomics technology.

Results

Total RKIP and phospho-RKIP expression levels were similar in normal and cancerous tissues. phospho-RKIP levels slightly decreased in metastatic lesions. However, the expression levels of phospho-RKIP, in contrast to total RKIP, displayed significant predictive power for outcome with normal expression of phospho-RKIP predicting a more favorable survival compared to lower levels (P = 0.0118); this was even more pronounced in more senior individuals and in those with early stage lung cancer.

Conclusions

This study examines for the first time, the expression profile of RKIP and phospho-RKIP in lung cancer. Significantly, we found that phospho-RKIP was a predictive indicator of survival.

Locostatin Disrupts Association of Raf Kinase Inhibitor Protein With Binding Proteins by Modifying a Conserved Histidine Residue in the Ligand-Binding Pocket

Raf kinase inhibitor protein (RKIP) interacts with a number of different proteins and regulates multiple signaling pathways. Here, we show that locostatin, a small molecule that covalently binds RKIP, not only disrupts interactions of RKIP with Raf-1 kinase, but also with G protein-coupled receptor kinase 2. In contrast, we found that locostatin does not disrupt binding of RKIP to two other proteins: inhibitor of κB kinase α and transforming growth factor β-activated kinase 1. These results thus imply that different proteins interact with different regions of RKIP. Locostatin's mechanism of action involves modification of a nucleophilic residue on RKIP. We observed that after binding RKIP, part of locostatin is slowly hydrolyzed, leaving a smaller RKIP-butyrate adduct. We identified the residue alkylated by locostatin as His86, a highly conserved residue in RKIP's ligand-binding pocket. Computational modeling of the binding of locostatin to RKIP suggested that the recognition interaction between small molecule and protein ensures that locostatin's electrophilic site is poised to react with His86. Furthermore, binding of locostatin would sterically hinder binding of other ligands in the pocket. These data provide a basis for understanding how locostatin disrupts particular interactions of RKIP with RKIP-binding proteins and demonstrate its utility as a probe of specific RKIP interactions and functions.

Phosphatidylenthanolamine Binding Protein aka Raf Kinase Inhibitor Protein: A Brief History of Its Discovery and the Remarkable Diversity of Biological Functions

Phosphatidylethanolamine-binding protein (PEBP) was identified almost three decades ago as an abundant protein in bovine brain. PEBP is the prototype of a highly conserved family of proteins represented in all three major phylogenetic divisions, eukaryota, bacteria, and archaea, with no significant sequence homology to other proteins. PEBP proteins have been studied in many species. The most thoroughly explored biological role of PEBP is that of a modulator of intracellular signaling pathways, which is mediated by its ability to bind and inhibit a number of protein kinases. The first such interaction that came to light was with the Raf1 kinase, and PEBP is thus widely referred to in the literature under its alternate name RKIP (Raf kinase inhibitory protein). The activity of RKIP itself is subject to regulation by phosphorylation. Intriguingly, PEBP has also been reported to possess additional, and diverse, biological functions unrelated to protein kinase networks that remain to be investigated in detail. Recent findings that RKIP may function as a suppressor of cancer metastasis are of great interest and importance. Prognostic and therapeutic applications of RKIP in human cancer were the subject of the first international workshop on RKIP that was held at the University of California, Los Angeles, in March 2010. This paper was presented at the workshop as a summary of the history of this still small but rapidly evolving field.

Characterization of the Raf kinase inhibitory protein (RKIP) binding pocket: NMR-based screening identifies small-molecule ligands

BACKGROUND

Raf kinase inhibitory protein (RKIP), also known as phoshaptidylethanolamine binding protein (PEBP), has been shown to inhibit Raf and thereby negatively regulate growth factor signaling by the Raf/MAP kinase pathway. RKIP has also been shown to suppress metastasis. We have previously demonstrated that RKIP/Raf interaction is regulated by two mechanisms: phosphorylation of RKIP at Ser-153, and occupation of RKIP's conserved ligand binding domain with a phospholipid (2-dihexanoyl-sn-glycero-3-phosphoethanolamine; DHPE). In addition to phospholipids, other ligands have been reported to bind this domain; however their binding properties remain uncharacterized.

METHODS/FINDINGS

In this study, we used high-resolution heteronuclear NMR spectroscopy to screen a chemical library and assay a number of potential RKIP ligands for binding to the protein. Surprisingly, many compounds previously postulated as RKIP ligands showed no detectable binding in near-physiological solution conditions even at millimolar concentrations. In contrast, we found three novel ligands for RKIP that specifically bind to the RKIP pocket. Interestingly, unlike the phospholipid, DHPE, these newly identified ligands did not affect RKIP binding to Raf-1 or RKIP phosphorylation. One out of the three ligands displayed off target biological effects, impairing EGF-induced MAPK and metabolic activity.

CONCLUSIONS/SIGNIFICANCE

This work defines the binding properties of RKIP ligands under near physiological conditions, establishing RKIP's affinity for hydrophobic ligands and the importance of bulky aliphatic chains for inhibiting its function. The common structural elements of these compounds defines a minimal requirement for RKIP binding and thus they can be used as lead compounds for future design of RKIP ligands with therapeutic potential.

Raf Kinase Inhibitory Protein protects cells against locostatin-mediated inhibition of migration

BACKGROUND

Raf Kinase Inhibitory Protein (RKIP, also PEBP1), a member of the Phosphatidylethanolamine Binding Protein family, negatively regulates growth factor signaling by the Raf/MAP kinase pathway. Since an organic compound, locostatin, was reported to bind RKIP and inhibit cell migration by a Raf-dependent mechanism, we addressed the role of RKIP in locostatin function.

METHODS/FINDINGS

We analyzed locostatin interaction with RKIP and examined the biological consequences of locostatin binding on RKIP function. NMR studies show that a locostatin precursor binds to the conserved phosphatidylethanolamine binding pocket of RKIP. However, drug binding to the pocket does not prevent RKIP association with its inhibitory target, Raf-1, nor affect RKIP phosphorylation by Protein Kinase C at a regulatory site. Similarly, exposure of wild type, RKIP-depleted HeLa cells or RKIP-deficient (RKIP(-/-)) mouse embryonic fibroblasts (MEFs) to locostatin has no effect on MAP kinase activation. Locostatin treatment of wild type MEFs causes inhibition of cell migration following wounding. RKIP deficiency impairs migration further, indicating that RKIP protects cells against locostatin-mediated inhibition of migration. Locostatin treatment of depleted or RKIP(-/-) MEFs reveals cytoskeletal disruption and microtubule abnormalities in the spindle.

CONCLUSIONS/SIGNIFICANCE

These results suggest that locostatin's effects on cytoskeletal structure and migration are caused through mechanisms independent of its binding to RKIP and Raf/MAP kinase signaling. The protective effect of RKIP against drug inhibition of migration suggests a new role for RKIP in potentially sequestering toxic compounds that may have deleterious effects on cells.