C-end Rule Research Articles

Protease-activated CendR peptides targeting tenascin-C: mitigating off-target tissue accumulation.

To achieve precision and selectivity, anticancer compounds and nanoparticles (NPs) can be targeted with affinity ligands that engage with malignancy-associated molecules in the blood vessels. While tumor-penetrating C-end Rule (CendR) peptides hold promise for precision tumor delivery, C-terminally exposed CendR peptides can accumulate undesirably in non-malignant tissues expressing neuropilin-1 (NRP-1), such as the lungs. One example of such promiscuous peptides is PL3 (sequence: AGRGRLVR), a peptide that engages with NRP-1 through its C-terminal CendR element, RLVR.Here, we report thedevelopment of PL3 derivatives that bind to NRP-1 only after proteolytic processing by urokinase-type plasminogen activator (uPA), while maintaining binding to the other receptor of the peptide, the C-domain of tenascin-C (TNC-C). Through a rational design approach and screening of a uPA-treated peptide-phage library (PL3 peptide followed by four random amino acids) on the recombinant NRP-1, derivatives of the PL3 peptide capable of binding to NRP-1 only post-uPA processing were successfully identified. In vitro cleavage, binding, and internalization assays, along with in vivo biodistribution studies in orthotopic glioblastoma-bearing mice, confirmed the efficacy of two novel peptides, PL3uCendR (AGRGRLVR↓SAGGSVA) and SKLG (AGRGRLVR↓SKLG), which exhibit uPA-dependent binding to NRP-1, reducing off-target binding to healthy NRP-1-expressing tissues. Our study not only unveils novel uPA-dependent TNC-C targeting CendR peptides but also introduces a broader paradigm and establishes a technology for screening proteolytically activated tumor-penetrating peptides.

The herpesvirus UL49.5 protein hijacks a cellular C-degron pathway to drive TAP transporter degradation.

The transporter associated with antigen processing (TAP) is a key player in the major histocompatibility class I-restricted antigen presentation and an attractive target for immune evasion by viruses. Bovine herpesvirus 1 impairs TAP-dependent antigenic peptide transport through a two-pronged mechanism in which binding of the UL49.5 gene product to TAP both inhibits peptide transport and triggers its proteasomal degradation. How UL49.5 promotes TAP degradation has, so far, remained unknown. Here, we use high-content siRNA and genome-wide CRISPR-Cas9 screening to identify CLR2KLHDC3 as the E3 ligase responsible for UL49.5-triggered TAP disposal. We propose that the C terminus of UL49.5 mimics a C-end rule degron that recruits the E3 to TAP and engages the cullin-RING E3 ligase in endoplasmic reticulum-associated degradation.

The influence of a modified p53 C-terminal peptide by using a tumor-targeting sequence on cellular apoptosis and tumor treatment.

The restoration of the function of p53 in tumors is a therapeutic strategy for the highly frequent mutation of the TP53 tumor suppressor gene. P460 is a wild-type peptide derived from the p53 C-terminus and has been proven to be capable of restoring the tumor suppressor function of p53. The poor accumulation of drugs in tumors is a serious hindrance to tumor treatment. For enhancing the activity of P460, the tumor-targeting sequence Arg-Gly-Asp-Arg (RGDR, C-end rule peptide) was introduced into the C-terminus of P460 to generate the new peptide P462. P462 presented better activity than P460 in inhibiting the proliferation of cancer cells and increasing the number of tumor cells undergoing apoptosis. Cell adhesion analysis and tumor imaging results revealed that P462 showed more specific and extensive binding with tumor cells and greater accumulation in tumors than the wild-type peptide. Importantly, treatment with P462 was more efficacious than that with P460 in vivo and was associated with considerably improved tumor-homing activity. This study highlights the importance of the roles of the tumor-homing sequence RGDR in the enhancement in cell attachment and tumor accumulation. The results of this work indicate that P462 could be a novel drug candidate for tumor treatment.

C-end rule peptide-guided niosomes for prostate cancer cell targeting

The use of nanoformulations in drug delivery has a significant impact to increase the drug therapeutic efficacy and improve their safety in anticancer treatments. Non-ionic surfactant vesicles (NSVs), or niosomes, are a class of nanocarriers that are low-cost, physiochemical stable, and represent a promising alternative to liposomes. Tumor-penetrating C-end Rule (CendR) peptides, that target neuropilin-1 (NRP-1), a cell surface receptor commonly overexpressed on solid tumor cells, are widely used for precise delivery of diagnostic and therapeutic nanoparticles to malignant lesions. In this study, we investigated the impact of NSV functionalization with prototypic CendR peptide RPARPAR (abbr. RPAR) on NRP-1-dependent targeting of prostate cancer cells. Doxorubicin (DOX)-loaded NSVs, with or without RPAR conjugation, were prepared, physicochemical characterized, and finally tested for NRP-1 binding in human primary prostate carcinoma-1 and prostate carcinoma epithelial cell lines (PPC-1 and 22Rv1, respectively) that are NRP-1 positive, and melanoma (M21) cells that are NRP-1 negative. We demonstrated that the cellular uptake of RPAR-NSVs was NRP-1-dependent and the uptake rate of RPAR-NSVs was ca. ten times higher than un-targeted NSVs in PPC-1 cells; while no significant differences, between targeted and un-targeted nanocarriers, were obtained in M21 cell line. These data were further confirmed by using NRP-1 CendR-binding pocket-blocking (mAb7E8) antibody, which significantly inhibited the internalization of RPAR-NSVs in PPC-1 cells from ca. 80%–3%. The resulting data obtained of cytotoxic studies agreed the uptake results, thus showing a significant increase of cytotoxic effect for RPAR-NSVs@DOX in PPC-1 and 22Rv1 cells compared to NSVs@DOX. These results provide a proof of concept that the conjugation of CendR peptides on the surface of NSVs can increase the specific targeting of nanoparticles in NRP-1 positive cells, thus improving the therapeutic efficacy of resulting targeted nanocarriers for cancer treatment. Our results have important implications for the development of more efficient and selective cancer therapies and support further in vivo studies by using more complex targeting peptides, such as iRGD or other CendR peptides, that are also targeting primary recruitment receptors (i.e. integrins) and are then proteolytically activated into CendR peptides in the tumor microenvironment.

Intravitreal CendR peptides target laser-induced choroidal neovascularization sites in mice

Choroidal neovascularization (CNV) is a common ocular pathology that may be associated in a variety of eye diseases. Although intravitreal injection treatment of anti-vascular endothelial growth factor (anti-VEGF) drugs shows significant clinical benefits in CNV treatment, the limitations of the current therapy need to be addressed. The aim of our study was to investigate the potential utility of three C-end Rule (CendR) peptides (RPARPAR, PL3, iRGD) for CNV targeting and to evaluate the efficacy of peptides for treating experimental CNV in mice. We observed that the CendR peptides localize to the CNV lesion sites after intravitreal injection and were mainly found in the outer nuclear cell layer (ONL) of the mouse retina. Interestingly, experimental therapy with tenascin-C (TNC-C) and neuropilin-1 (NRP-1)-targeting PL3 peptide, reduced angiogenesis and decreased vascular leakage. The results suggest that PL3 and potentially other CendR peptides could serve as affinity targeting ligands and therapeutics for ocular diseases that involve pathological CNV.

A phase Ib/II, open-label, multicenter study to evaluate the safety, efficacy, and pharmacokinetics/pharmacodynamics of αvβ3/5 integrin and neuropilin-1 targeting peptide LSTA1 in patients with metastatic pancreatic ductal adenocarcinoma.

e16290 Background: LSTA1, formerly known as CEND-1, is a novel cyclic peptide which selectively binds to αv/β3 and β5 integrins, which are upregulated in solid tumors, where it undergoes protease cleavage producing a C-end rule peptide fragment (CendR) that selectively binds to and activates neuropilin-1 (NRP1). Once CendR binds NRP1, the CendR pathway is activated, leading to increased vascular permeability of tumors resulting in increased drug delivery and tumor accumulation of co-administered chemotherapeutic agents. Here we report the safety and efficacy results from a Phase Ib/II, multicenter study of LSTA1 plus (SoC) gemcitabine and locally produced nab-paclitaxel in patients with metastatic pancreatic ductal adenocarcinoma (mPDAC). Methods: The study included a 7-day safety run-in and dose expansion/extension parts. Previously untreated patients with histologically confirmed mPDAC and ≥1 target lesion were treated with systemic chemotherapy and received LSTA1 (1.6 mg/kg or 3.2 mg/kg), gemcitabine (1000 mg/m²) and nab-paclitaxel (125 mg/m2) on days 1, 8, 15, of each 28-day treatment cycle. LSTA1 alone was given during run-in to observe PK/PD. Treatment continued until disease progression or other discontinuation events. The primary endpoints were safety and preliminary efficacy. Results: As of 30 Nov 2022, 38 patients were enrolled at 10 sites in China (12 at 1.6 mg/kg; 26 at 3.2mg/kg). The median follow-up time was 6.28 months. 37 (97.4%) patients experienced TEAEs and 31 (81.6%) patients experienced grade≥3 TEAEs. The most common TEAEs were hematologic toxicity related to chemotherapy. The ORR was 42.9% (15/35) and DCR was 82.9% (29/35). The ORR in 1.6 mg/kg and 3.2 mg/kg groups was 45.5% and 41.7%, respectively. The mPFS in 1.6 mg/kg and 3.2 mg/kg groups was 7.43 months (95% CI: 0.59-NE) and 5.75 months (95% CI: 3.25-7.46) respectively. mOS was not reached. NRP1 expression levels were tested at baseline; six of whom had expression ≥40%. Of those, 4 had PRs and 2 had SD (ORR=66.7%; DCR=100%). The mPFS was 7.5 months. Conclusions: LSTA1 combined with SoC showed an acceptable safety profile in patients with mPDAC. Compared with historical SoC data, LSTA1 plus SoC numerically improved ORR and PFS. Higher NRP1 expression is associated with better ORR and PFS, suggesting NRP1 may be a potential biomarker of LSTA1 plus chemotherapy for mPDAC treatment. Further study is ongoing. Clinical trial information: NCT05052567 .

Targeting of Tomato Bushy Stunt Virus with a Genetically Fused C-End Rule Peptide.

Homing peptides are widely used to improve the delivery of drugs, imaging agents, and nanoparticles (NPs) to their target sites. Plant virus-based particles represent an emerging class of structurally diverse nanocarriers that are biocompatible, biodegradable, safe, and cost-effective. Similar to synthetic NPs, these particles can be loaded with imaging agents and/or drugs and functionalized with affinity ligands for targeted delivery. Here we report the development of a peptide-guided Tomato Bushy Stunt Virus (TBSV)-based nanocarrier platform for affinity targeting with the C-terminal C-end rule (CendR) peptide, RPARPAR (RPAR). Flow cytometry and confocal microscopy demonstrated that the TBSV-RPAR NPs bind specifically to and internalize in cells positive for the peptide receptor neuropilin-1 (NRP-1). TBSV-RPAR particles loaded with a widely used anticancer anthracycline, doxorubicin, showed selective cytotoxicity on NRP-1-expressing cells. Following systemic administration in mice, RPAR functionalization conferred TBSV particles the ability to accumulate in the lung tissue. Collectively, these studies show the feasibility of the CendR-targeted TBSV platform for the precision delivery of payloads.

Recent advances in the tumor-penetrating peptide internalizing RGDfor cancer treatment and diagnosis.

Cancer has become a prominent disease that seriously endangers human health. The complexity of the biological characteristics of the tumor makes it challenging for traditional therapeutic drugs to penetrate tumor tissues and exert their antitumor effects.Internalizing RGD peptide (iRGD) is a novel tumor-homing peptide that binds to αvβ3 and αvβ5 integrins on the surface of tumor vessels through the C-end rule (CendR) motif. The CendR motif binds to the neuropilin-1 (NRP-1) receptor on tumor cells, initiating NRP-1-mediated transcytosis to facilitate drug entry into the tumor tissue. Multiple studies demonstrated that iRGD improved the penetration and targeting of antitumor drugs, thereby enhancing their antitumor efficacy. In this review, we initially described the origins of iRGD and its penetration mechanism. Furthermore, we presented updates on the application of iRGD in cancer chemotherapy, photodynamic therapy, gene therapy, immunotherapy, treatment with antibodiesor protein-based biologics, and tumor imaging.

Novel Small-Molecule Inhibitors of the SARS-CoV-2 Spike Protein Binding to Neuropilin 1.

Furin cleavage of the SARS-CoV-2 spike protein results in a polybasic terminal sequence termed the C-end rule (CendR), which is responsible for the binding to neuropilin 1 (NRP1), enhancing viral infectivity and entry into the cell. Here we report the identification of 20 small-molecule inhibitors that emerged from a virtual screening of nearly 950,000 drug-like compounds that bind with high probability to the CendR-binding pocket of NRP1. In a spike NRP1 binding assay, two of these compounds displayed a stronger inhibition of spike protein binding to NRP1 than the known NRP1 antagonist EG00229, for which the inhibition of the CendR peptide binding to NRP1 was also experimentally confirmed. These compounds present a good starting point for the design of small-molecule antagonists against the SARS-CoV-2 viral entry.

A widespread viral entry mechanism: The C-end Rule motif–neuropilin receptor interaction

Many phylogenetically distant animal viruses, including the new coronavirus severe acute respiratory syndrome coronavirus 2, have surface proteins with polybasic sites that are cleaved by host furin and furin-like proteases. Other than priming certain viral surface proteins for fusion, cleavage generates a carboxy-terminal RXXR sequence. This C-end Rule (CendR) motif is known to bind to neuropilin (NRP) receptors on the cell surface. NRPs are ubiquitously expressed, pleiotropic cell surface receptors with important roles in growth factor signaling, vascular biology, and neurobiology, as well as immune homeostasis and activation. The CendR–NRP receptor interaction promotes endocytic internalization and tissue spreading of different cargo, including viral particles. We propose that the interaction between viral surface proteins and NRPs plays an underappreciated and prevalent role in the transmission and pathogenesis of diverse viruses and represents a promising broad-spectrum antiviral target.

Microfluidic preparation and in vitro evaluation of iRGD-functionalized solid lipid nanoparticles for targeted delivery of paclitaxel to tumor cells

Solid lipid nanoparticles (SLNs) can combine the advantages of different colloidal carriers and prevent some of their disadvantages. The production of nanoparticles by means of microfluidics represents a successful platform for industrial scale-up of nanoparticle manufacture in a reproducible way. The realisation of a microfluidic technique to obtain SLNs in a continuous and reproducible manner encouraged us to create surface functionalised SLNs for targeted drug release using the same procedure. A tumor homing peptide, iRGD, owning a cryptic C-end Rule (CendR) motif is responsible for neuropilin-1 (NRP-1) binding and for triggering extravasation and tumor penetration of the peptide. In this study, the Paclitaxel loaded-SLNs produced by microfluidics were functionalized with the iRGD peptide. The SLNs proved to be stable in aqueous medium andwere characterized by a Z-average under 150 nm, a polydispersity index below 0.2, a zeta-potential between −20 and −35 mV and a drug encapsulation efficiency around 40%. Moreover, in vitro cytotoxic effects and cellular uptake have been assessed using 2D and 3D tumour models of U87 glioblastoma cell lines. Overall, these results demonstrate that the surface functionalization of SLNs with iRGD allow better cellular uptake and cytotoxicity ability.

IRQC, a surveillance pathway for 40S ribosomal quality control during mRNA translation initiation.

Post-translational modification of ribosomal proteins enables rapid and dynamic regulation of protein biogenesis. Site-specific ubiquitylation of 40S ribosomal proteins uS10 and eS10 plays a key role during ribosome-associated quality control (RQC). Distinct, and previously functionally ambiguous, ubiquitylation events on the 40S proteins uS3 and uS5 are induced by diverse proteostasis stressors that impact translation activity. Here, we identify the ubiquitin ligase RNF10 and the deubiquitylating enzyme USP10 as the key enzymes that regulate uS3 and uS5 ubiquitylation. Prolonged uS3 and uS5 ubiquitylation results in 40S, but not 60S, ribosomal protein degradation in a manner independent of canonical autophagy. We show that blocking progression of either scanning or elongating ribosomes past the start codon triggers site-specific ubiquitylation events on ribosomal proteins uS5 and uS3. This study identifies and characterizes a distinct arm in the RQC pathway, initiation RQC (iRQC), that acts on 40S ribosomes during translation initiation to modulate translation activity and capacity.

Nutrient-Dependency of CendR Endocytosis Pathway is Uniquely Regulated by Mutant Kras and Targetable

Kras is the hallmark mutation of Pancreatic Ductal Adenocarcinoma (PDAC) and reprograms tumor metabolism towards glycolysis. Evolutionarily, PDAC instigates diabetes to increase glucose concentrations and fuel tumor growth. The C-end Rule (CendR)-pathway mediates nutrient endocytotic transcytosis and trans-tissue molecular transport. A discrepancy exists in that the CendR pathway is primed to high activity when glucose is low. Ongoing PDAC clinical trials utilize tumor-penetrating peptides (iRGD) to activate the CendR-pathway and dispense cargo. Here, we determine how glucose metabolism changes based on tumor Kras-status might affect the CendR pathway and hijack it to deliver therapy. Under diabetic-like conditions, iRGD increases its penetration and spread throughout patient-derived and mouse PDAC. Importantly, all CendR peptides delivered higher concentrations of co-administered therapeutic cargo in a glucose-dependent manner. Neuropilin-1 (NRP-1), the receptor for peptide-mediated activation of the CendR pathway, is expressed in greater abundance under high glucose concentrations in parallel with glycolytic machinery. Hypoglycemic Kras-mutant tumors, hyperglycemic Kras-wildtype tumors, and inhibition of the mitogen-activated protein kinase (MAPK) pathway all suppressed NRP-1, CendR-peptide transport, and cargo uptake. Overall, mutant Kras enhances CendR peptide-directed cargo delivery in glycolytic metabolic environments via NRP-1 expression. Manipulating patient blood glucose levels may enhance CendR-based targeting of therapeutics into Kras-mutant tumors.

Molecular basis of the new COVID-19 target neuropilin-1 in complex with SARS-CoV-2 S1 C-end rule peptide and small-molecule antagonists

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), responsible for causing the current coronavirus 2019 (COVID-19) pandemic, uses its spike (S1) protein for host cell attachment and entry. Apart from angiotensin-converting enzyme 2, neuropilin-1 (NRP1) has been recently found to serve as another host factor for SARS-CoV-2 infection; thus, blocking S1–NRP1 interaction can be a potential treatment for COVID-19. Herein, molecular recognition between SARS-CoV-2 S1 C-end rule (CendR) heptapeptide including small-molecule antagonists (EG00229 and EG01377) and the NRP1 was investigated using molecular dynamics simulations and binding free energy calculations based on MM-PBSA method. The binding affinity and the number of hot-spot residues of EG01377/NRP1 complex were higher than those of CendR/NRP1 and EG00229/NRP1 systems, in line with the reported experimental data as well as with the lower water accessibility at the ligand-binding site. The (i) T316, P317, and D320 and (ii) S346, T349, and Y353 residues of NRP1 were confirmed to respectively form H-bonds with the positively charged guanidinium group and the negatively charged carboxyl moiety of all studied ligands. Moreover, Rosetta protein design was employed to improve the binding affinity between CendR peptide and NRP1. The newly designed peptides, especially R683G and A684M, exhibited higher binding efficiency than the native CendR heptapeptide as well as the small-molecule EG00229 by forming more H-bonds and hydrophobic interactions with NPR1, suggesting that these designed peptides could be promising NRP1 inhibitors to combat SARS-CoV-2 infection.

Convergence of mammalian RQC and C-end rule proteolytic pathways via alanine tailing

Incompletely synthesized nascent chains obstructing large ribosomal subunits are targeted for degradation by ribosome-associated quality control (RQC). In bacterial RQC, RqcH marks the nascent chains with C-terminalalanine (Ala) tails that are directly recognized by proteasome-like proteases, whereas in eukaryotes, RqcH orthologs (Rqc2/NEMF [nuclear export mediator factor]) assist the Ltn1/Listerin E3 ligase in nascent chain ubiquitylation. Here, we study RQC-mediated proteolytic targeting of ribosome stalling products in mammalian cells. We show that mammalian NEMF has an additional, Listerin-independent proteolytic role, which, as in bacteria, is mediated by tRNA-Ala binding and Ala tailing. However, in mammalian cells Ala tails signal proteolysis indirectly, through a pathway that recognizes C-terminal degrons; we identify the CRL2KLHDC10 E3 ligase complex and the novel C-end rule E3, Pirh2/Rchy1, as bona fide RQC pathway components that directly bind to Ala-tailed ribosome stalling products and target them for degradation. As Listerin mutation causes neurodegeneration in mice, functionally redundant E3s may likewise be implicated in molecular mechanisms of neurodegeneration.

Homing Peptides for Cancer Therapy.

Tumor-homing peptides are widely used for improving tumor selectivity of anticancer drugs and imaging agents. The goal is to increase tumor uptake and reduce accumulation at nontarget sites. Here, we describe current approaches for tumor-homing peptide identification and validation, and provide comprehensive overview of classes of tumor-homing peptides undergoing preclinical and clinical development. We focus on unique mechanistic features and applications of a recently discovered class of tumor-homing peptides, tumor-penetrating C-end Rule (CendR) peptides, that can be used for tissue penetrative targeting of extravascular tumor tissue. Finally, we discuss unanswered questions and future directions in the field of development of peptide-guided smart drugs and imaging agents.

Exposed CendR Domain in Homing Peptide Yields Skin-Targeted Therapeutic in Epidermolysis Bullosa

Systemic skin-selective therapeutics would be a major advancement in the treatment of diseases affecting the entire skin, such as recessive dystrophic epidermolysis bullosa (RDEB), which is caused by mutations in the COL7A1 gene and manifests in transforming growth factor-β (TGF-β)-driven fibrosis and malignant transformation. Homing peptides containing a C-terminal R/KXXR/K motif (C-end rule [CendR] sequence) activate an extravasation and tissue penetration pathway for tumor-specific drug delivery. We have previously described a homing peptide CRKDKC (CRK) that contains a cryptic CendR motif and homes to angiogenic blood vessels in wounds and tumors, but it cannot penetrate cells or tissues. In this study, we demonstrate that removal of the cysteine from CRK to expose the CendR sequence confers the peptide novel ability to home to normal skin. Fusion of the truncated CRK (tCRK) peptide to the C terminus of an extracellular matrix protein decorin (DCN), a natural TGF-β inhibitor, resulted in a skin-homing therapeutic molecule (DCN-tCRK). Systemic DCN-tCRK administration in RDEB mice led to inhibition of TGF-β signaling in the skin and significant improvement in the survival of RDEB mice. These results suggest that DCN-tCRK has the potential to be utilized as a novel therapeutic compound for the treatment of dermatological diseases such as RDEB.

Tumor-penetrating peptide for systemic targeting of Tenascin-C

Extracellular matrix in solid tumors has emerged as a specific, stable, and abundant target for affinity-guided delivery of anticancer drugs. Here we describe the homing peptide that interacts with the C-isoform of Tenascin-C (TNC-C) upregulated in malignant tissues. TNC-C binding PL3 peptide (amino acid sequence: AGRGRLVR) was identified by in vitro biopanning on recombinant TNC-C. Besides TNC-C, PL3 interacts via its C-end Rule (CendR) motif with cell-and tissue penetration receptor neuropilin-1 (NRP-1). Functionalization of iron oxide nanoworms (NWs) and metallic silver nanoparticles (AgNPs) with PL3 peptide increased tropism of systemic nanoparticles towards glioblastoma (GBM) and prostate carcinoma xenograft lesions in nude mice (eight and five-fold respectively). Treatment of glioma-bearing mice with proapoptotic PL3-guided NWs improved the survival of the mice, whereas treatment with untargeted particles had no effect. PL3-coated nanoparticles were found to accumulate in TNC-C and NRP-1-positive areas in clinical tumor samples, suggesting a translational relevance. The systemic tumor-targeting properties and binding of PL3-NPs to the clinical tumor sections, suggest that the PL3 peptide may have applications as a targeting moiety for the selective delivery of imaging and therapeutic agents to solid tumors.

Cul5-type Ubiquitin Ligase KLHDC1 Contributes to the Elimination of Truncated SELENOS Produced by Failed UGA/Sec Decoding.

SummaryThe UGA codon signals protein translation termination, but it can also be translated into selenocysteine (Sec, U) to produce selenocysteine-containing proteins (selenoproteins) by dedicated machinery. As Sec incorporation can fail, Sec-containing longer and Sec-lacking shorter proteins co-exist. Cul2-type ubiquitin ligases were recently shown to destabilize such truncated proteins; however, which ubiquitin ligase targets truncated proteins for degradation remained unclear. We report that the Cul5-type ubiquitin ligase KLHDC1 targets truncated SELENOS, a selenoprotein, for proteasomal degradation. SELENOS is involved in endoplasmic reticulum (ER)-associated degradation, which is linked to reactive oxygen species (ROS) production, and the knockdown of KLHDC1 in U2OS cells decreased ER stress-induced cell death. Knockdown of SELENOS increased the cell population with lower ROS levels. Our findings reveal that, in addition to Cul2-type ubiquitin ligases, KLHDC1 is involved in the elimination of truncated oxidoreductase-inactive SELENOS, which would be crucial for maintaining ROS levels and preventing cancer development.

IRGD: A Promising Peptide for Cancer Imaging and a Potential Therapeutic Agent for Various Cancers.

Poor penetration into the tumor parenchyma and the reduced therapeutic efficacy of anticancer drugs and other medications are the major problems in tumor treatment. A new tumor-homing and penetrating peptide, iRGD (CRGDK/RGPD/EC), can be effectively used to combine and deliver imaging agents or anticancer drugs into tumors. The different “vascular zip codes” expressed in different tissues can serve as targets for docking-based (synaptic) delivery of diagnostic and therapeutic molecules. αv-Integrins are abundantly expressed in the tumor vasculature, where they are recognized by peptides containing the RGD integrin recognition motif. The iRGD peptide follows a multistep tumor-targeting process: First, it is proteolytically cleaved to generate the CRGDK fragment by binding to the surface of cells expressing αv integrins (αvβ3 and αvβ5). Then, the fragment binds to neuropilin-1 and penetrates the tumor parenchyma more deeply. Compared with conventional RGD peptides, the affinity of iRGD for αv integrins is in the mid to low nanomolar range, and the CRGDK fragment has a stronger affinity for neuropilin-1 than that for αv integrins because of the C-terminal exposure of a conditional C-end Rule (CendR) motif (R/KXXR/K), whose receptor proved to be neuropilin-1. Consequently, these advantages facilitate the transfer of CRGDK fragments from integrins to neuropilin-1 and consequently deeper penetration into the tumor. Due to its specific binding and strong affinity, the iRGD peptide can deliver imaging agents and anticancer drugs into tumors effectively and deeply, which is useful in detecting the tumor, blocking tumor growth, and inhibiting tumor metastasis. This review aims to focus on the role of iRGD in the imaging and treatment of various cancers.