Targeting high circDNA2v levels in colorectal cancer induces cellular senescence and elicits an anti-tumor secretome

The efficacy of immunotherapy against colorectal cancer (CRC) is impaired by insufficient immune cell recruitment into the tumor microenvironment. Our study shows that targeting circDNA2v, a circular RNA commonly overexpressed in CRC, can be exploited to elicit cytotoxic T cell recruitment. circDNA2v functions through binding to IGF2BP3, preventing its ubiquitination, and prolonging the IGF2BP3 half-life, which in turn sustains mRNA levels of the protooncogene c-Myc. Targeting circDNA2v by gene silencing downregulates c-Myc to concordantly induce tumor cell senescence and the release of proinflammatory mediators. Production of CXCL10 and interleukin-9 by CRC cells is elicited through JAK-STAT1 signaling, in turn promoting the chemotactic and cytolytic activities of CD8+ T cells. Clinical evidence associates increased circDNA2v expression in CRC tissues with reductions in CD8+ T cell infiltration and worse outcomes. The regulatory relationship between circDNA2v, cellular senescence, and tumor-infiltrating lymphocytes thus provides a rational approach for improving immunotherapy in CRC.

Reciprocal regulation of lncRNA MEF and c-Myc drives colorectal cancer tumorigenesis

More than half of all cancers demonstrate aberrant c-Myc expression, making this arguably the most important human oncogene. Deregulated long non-coding RNAs (lncRNAs) are also commonly implicated in tumorigenesis, and some limited examples have been established where lncRNAs act as biological tuners of c-Myc expression and activity. Here, we demonstrate that the lncRNA denoted c-Myc Enhancing Factor (MEF) enjoys a cooperative relationship with c-Myc, both as a transcriptional target and driver of c-Myc expression. Mechanistically, MEF functions by binding to and stabilizing the expression of hnRNPK in colorectal cancer cells. The MEF-hnRNPK interaction serves to disrupt binding between hnRNPK and the E3 ubiquitin ligase TRIM25, which attenuates TRIM25-dependent hnRNPK ubiquitination and proteasomal destruction. In turn, the stabilization of hnRNPK through MEF enhances c-Myc expression by augmenting the translation c-Myc. Moreover, modulating the expression of MEF in shRNA-mediated knockdown and overexpression studies revealed that MEF expression is essential for colorectal cancer cell proliferation and survival, both in vitro and in vivo. From the clinical perspective, we show that MEF expression is differentially increased in colorectal cancer tissues compared to normal adjacent tissues. Further, correlations exist between MEF, c-Myc, and hnRNPK suggesting the MEF-c-Myc positive feedback loop is active in patients. Together these data demonstrate that MEF is a pivotal partner of the c-Myc network and propose MEF as a valuable therapeutic target for colorectal cancer.

The diapause-like colorectal cancer cells induced by SMC4 attenuation are characterized by low proliferation and chemotherapy insensitivity

In response to adverse environmental conditions, embryonic development may reversibly cease, a process termed diapause. Recent reports connect this phenomenon with the non-genetic responses of tumors to chemotherapy, but the mechanisms involved are poorly understood. Here, we establish a multifarious role for SMC4 in the switching of colorectal cancer cells to a diapause-like state. SMC4 attenuation promotes the expression of three investment phase glycolysis enzymes increasing lactate production while also suppressing PGAM1. Resultant high lactate levels increase ABC transporter expression via histone lactylation, rendering tumor cells insensitive to chemotherapy. SMC4 acts as co-activator of PGAM1 transcription, and the coordinate loss of SMC4 and PGAM1 affects F-actin assembly, inducing cytokinesis failure and polyploidy, thereby inhibiting cell proliferation. These insights into the mechanisms underlying non-genetic chemotherapy resistance may have significant implications for the field, advancing our understanding of aerobic glycolysis functions in tumor and potentially informing future therapeutic strategies.

NIPSNAP1 directs dual mechanisms to restrain senescence in cancer cells

BACKGROUND

Although the executive pathways of senescence are known, the underlying control mechanisms are diverse and not fully understood, particularly how cancer cells avoid triggering senescence despite experiencing exacerbated stress conditions within the tumor microenvironment.

METHODS

Mass spectrometry (MS)-based proteomic screening was used to identify differentially regulated genes in serum-starved hepatocellular carcinoma cells and RNAi employed to determine knockdown phenotypes of prioritized genes. Thereafter, gene function was investigated using cell proliferation assays (colony-formation, CCK-8, Edu incorporation and cell cycle) together with cellular senescence assays (SA-β-gal, SAHF and SASP). Gene overexpression and knockdown techniques were applied to examine mRNA and protein regulation in combination with luciferase reporter and proteasome degradation assays, respectively. Flow cytometry was applied to detect changes in cellular reactive oxygen species (ROS) and in vivo gene function examined using a xenograft model.

RESULTS

Among the genes induced by serum deprivation, NIPSNAP1 was selected for investigation. Subsequent experiments revealed that NIPSNAP1 promotes cancer cell proliferation and inhibits P27-dependent induction of senescence via dual mechanisms. Firstly, NIPSNAP1 maintains the levels of c-Myc by sequestering the E3 ubiquitin ligase FBXL14 to prevent the proteasome-mediated turnover of c-Myc. Intriguingly, NIPSNAP1 levels are restrained by transcriptional repression mediated by c-Myc-Miz1, with repression lifted in response to serum withdrawal, thus identifying feedback regulation between NIPSNAP1 and c-Myc. Secondly, NIPSNAP1 was shown to modulate ROS levels by promoting interactions between the deacetylase SIRT3 and superoxide dismutase 2 (SOD2). Consequent activation of SOD2 serves to maintain cellular ROS levels below the critical levels required to induce cell cycle arrest and senescence. Importantly, the actions of NIPSNAP1 in promoting cancer cell proliferation and preventing senescence were recapitulated in vivo using xenograft models.

CONCLUSIONS

Together, these findings reveal NIPSNAP1 as an important mediator of c-Myc function and a negative regulator of cellular senescence. These findings also provide a theoretical basis for cancer therapy where targeting NIPSNAP1 invokes cellular senescence.

Sestrin2-mediated disassembly of stress granules dampens aerobic glycolysis to overcome glucose starvation

Sestrins are a small gene family of pleiotropic factors whose actions promote cell adaptation to a range of stress conditions. In this report we disclose the selective role of Sestrin2 (SESN2) in dampening aerobic glycolysis to adapt to limiting glucose conditions. Removal of glucose from hepatocellular carcinoma (HCC) cells inhibits glycolysis associated with the downregulation of the rate-limiting glycolytic enzyme hexokinase 2 (HK2). Moreover, the accompanying upregulation of SESN2 through an NRF2/ATF4-dependent mechanism plays a direct role in HK2 regulation by destabilizing HK2 mRNA. We show SESN2 competes with insulin like growth factor 2 mRNA binding protein 3 (IGF2BP3) for binding with the 3'-UTR region of HK2 mRNA. Interactions between IGF2BP3 and HK2 mRNA result in their coalescence into stress granules via liquid-liquid phase separation (LLPS), a process which serves to stabilize HK2 mRNA. Conversely, the enhanced expression and cytoplasmic localization of SESN2 under glucose deprivation conditions favors the downregulation of HK2 levels via decreases in the half-life of HK2 mRNA. The resulting dampening of glucose uptake and glycolytic flux inhibits cell proliferation and protect cells from glucose starvation-induced apoptotic cell death. Collectively, our findings reveal an intrinsic survival mechanism allowing cancer cells to overcome chronic glucose shortages, also providing new mechanistic insights into SESN2 as an RNA-binding protein with a role in reprogramming of cancer cell metabolism.

Clinical implications of lncRNA LINC-PINT in cancer

Long noncoding RNAs (lncRNAs) possess the potential for therapeutic targeting to treat many disorders, including cancers. Several RNA-based therapeutics (ASOs and small interfering RNAs) have gained FDA approval over the past decade. And with their potent effects, lncRNA-based therapeutics are of emerging significance. One important lncRNA target is LINC-PINT, with its universalized functions and relationship with the famous tumor suppressor gene TP53. Establishing clinical relevance, much like p53, the tumor suppressor activity of LINC-PINT is implicated in cancer progression. Moreover, several molecular targets of LINC-PINT are directly or indirectly used in routine clinical practice. We further associate LINC-PINT with immune responses in colon adenocarcinoma, proposing the potential utility of LINC-PINT as a novel biomarker of immune checkpoint inhibitors. Collectively, current evidence suggests LINC-PINT can be considered for use as a diagnostic/prognostic marker for cancer and several other diseases.

Glycolytic Pfkp acts as a Lin41 protein kinase to promote endodermal differentiation of embryonic stem cells

Unveiling the principles governing embryonic stem cell (ESC) differentiation into specific lineages is critical for understanding embryonic development and for stem cell applications in regenerative medicine. Here, we establish an intersection between LIF-Stat3 signaling that is essential for maintaining murine (m) ESCs pluripotency, and the glycolytic enzyme, the platelet isoform of phosphofructokinase (Pfkp). In the pluripotent state, Stat3 transcriptionally suppresses Pfkp in mESCs while manipulating the cells to lift this repression results in differentiation towards the ectodermal lineage. Pfkp exhibits substrate specificity changes to act as a protein kinase, catalyzing serine phosphorylation of the developmental regulator Lin41. Such phosphorylation stabilizes Lin41 by impeding its autoubiquitination and proteasomal degradation, permitting Lin41-mediated binding and destabilization of mRNAs encoding ectodermal specification markers to favor the expression of endodermal specification genes. This provides new insights into the wiring of pluripotency-differentiation circuitry where Pfkp plays a role in germ layer specification during mESC differentiation.

Vanguard is a Glucose Deprivation-Responsive Long Non-Coding RNA Essential for Chromatin Remodeling-Reliant DNA Repair

Glucose metabolism contributes to DNA damage response pathways by regulating chromatin remodeling, double-strand break (DSB) repair, and redox homeostasis, although the underlying mechanisms are not fully established. Here, a previously uncharacterized long non-coding RNA is revealed that is call Vanguard which acts to promote HMGB1-dependent DNA repair in association with changes in global chromatin accessibility. Vanguard expression is maintained in cancer cells by SP1-dependent transcription according to glucose availability and cellular adenosine triphosphate (ATP) levels. Vanguard promotes complex formation between HMGB1 and HDAC1, with the resulting deacetylation of HMGB1 serving to maintain its nuclear localization and DSB repair function. However, Vanguard downregulation under glucose limiting conditions promotes HMGB1 translocation from the nucleus, increasing DNA damage, and compromising cancer cell growth and viability. Moreover, Vanguard silencing increases the effectiveness of poly (ADP-ribose) polymerase inhibitors against breast cancer cells with wild-type breast cancer gene-1 status, suggesting Vanguard as a potential therapeutic target.

P21-activated kinase 4 Pak4 maintains embryonic stem cell pluripotency via Akt activation

Exploiting the pluripotent properties of embryonic stem cells (ESCs) holds great promise for regenerative medicine. Nevertheless, directing ESC differentiation into specialized cell lineages requires intricate control governed by both intrinsic and extrinsic factors along with the actions of specific signaling networks. Here, we reveal the involvement of the p21-activated kinase 4 (Pak4), a serine/threonine kinase, in sustaining murine ESC (mESC) pluripotency. Pak4 is highly expressed in R1 ESC cells compared with embryonic fibroblast cells and its expression is progressively decreased during differentiation. Manipulations using knockdown and overexpression demonstrated a positive relationship between Pak4 expression and the clonogenic potential of mESCs. Moreover, ectopic Pak4 expression increases reprogramming efficiency of Oct4-Klf4-Sox2-Myc-induced pluripotent stem cells (iPSCs) whereas Pak4-knockdown iPSCs were largely incapable of generating teratomas containing mesodermal, ectodermal and endodermal tissues, indicative of a failure in differentiation. We further establish that Pak4 expression in mESCs is transcriptionally driven by the core pluripotency factor Nanog which recognizes specific binding motifs in the Pak4 proximal promoter region. In turn, the increased levels of Pak4 in mESCs fundamentally act as an upstream activator of the Akt pathway. Pak4 directly binds to and phosphorylates Akt at Ser473 with the resulting Akt activation shown to attenuate downstream GSK3β signaling. Thus, our findings indicate that the Nanog-Pak4-Akt signaling axis is essential for maintaining mESC self-renewal potential with further importance shown during somatic cell reprogramming where Pak4 appears indispensable for multi-lineage specification.

PINTology: A short history of the lncRNA LINC-PINT in different diseases

LINC-PINT is a p53-induced long intergenic noncoding transcript that plays a crucial role in many diseases, especially cancer. This long noncoding RNA (lncRNA) gene produces in total 102 (LNCipedia) alternatively spliced variants (LINC-PINT:1 to LINC-PINT:102). The functions of known variants include RNA transcripts, host transcripts for circular RNA (circRNA) generation and as sources for the translation of short peptides. In most human tumors, LINC-PINT is down-regulated where it serves as a tumor suppressor. However, the diversity of its functions in other maladies signifies its general clinical importance. Current LINC-PINT molecular functions include RNA-protein interactions, miRNA sponging and epigenetic modulation with these mechanisms operating in different cellular contexts to exert effects on biological processes ranging from DNA damage responses, cell cycle and growth arrest, senescence, cell migration and invasion, and apoptosis. Genetic polymorphisms in LINC-PINT have also been functionally associated with cancer and other pathologies including the autoimmune diseases pemphigus foliaceus and arthritis. Hence, LINC-PINT shows great potential as a clinical biomarker, especially for the diagnosis and prognosis of cancer. In this review, we explore the current knowledge highlighting the distinctive molecular functions of LINC-PINT in specific cancers and other disease states. This article is categorized under: RNA in Disease and Development > RNA in Disease.

DDIT3 Directs a Dual Mechanism to Balance Glycolysis and Oxidative Phosphorylation during Glutamine Deprivation

Extracellular glutamine represents an important energy source for many cancer cells and its metabolism is intimately involved in maintaining redox homeostasis. The heightened metabolic activity within tumor tissues can result in glutamine deficiency, necessitating metabolic reprogramming responses. Here, dual mechanisms involving the stress-responsive transcription factor DDIT3 (DNA damage induced transcript 3) that establishes an interrelationship between glycolysis and mitochondrial respiration are revealed. DDIT3 is induced during glutamine deprivation to promote glycolysis and adenosine triphosphate production via suppression of the negative glycolytic regulator TIGAR. In concert, a proportion of the DDIT3 pool translocates to the mitochondria and suppresses oxidative phosphorylation through LONP1-mediated down-regulation of COQ9 and COX4. This in turn dampens the sustained levels of reactive oxygen species that follow glutamine withdrawal. Together these mechanisms constitute an adaptive survival mechanism permitting tumor cells to survive metabolic stress induced by glutamine starvation.

lncRNA TRMP-S directs dual mechanisms to regulate p27-mediated cellular senescence

Long noncoding RNAs (lncRNAs) undergo extensive alternative splicing, but little is known about isoform functions. A prior investigation of lncRNA RP11-369C8.1 reported that its splice variant TRMP suppressed p27 translation through PTBP1. Here we characterize a second major splice variant, TRMP-S (short variant), whose enforced loss promotes cancer cell-cycle arrest and p27-dependent entry into cellular senescence. Remarkably, despite sharing a single common exon with TRMP, TRMP-S restrains p27 expression through distinct mechanisms. First, TRMP-S stabilizes UHRF1 protein levels, an epigenetic inhibitor of p27, by promoting interactions between UHRF1 and its deubiquitinating enzyme USP7. Alternatively, binding interactions between TRMP-S and FUBP3 prevent p53 mRNA interactions with RPL26 ribosomal protein, the latter essential for promoting p53 translation with ensuing suppression of p53 translation limiting p27 expression. Significantly, as TRMP-S is itself transactivated by p53, this identifies negative feedback regulation between p53 and TRMP-S. Different splicing variants of the RP11-369C8.1 gene thereby exert distinct roles that converge on the homeostatic control of p27 expression, providing an important precedent for understanding the actions of alternatively spliced lncRNAs.

LncRNA GIRGL drives CAPRIN1-mediated phase separation to suppress glutaminase-1 translation under glutamine deprivation

Glutamine constitutes an essential source of both carbon and nitrogen for numerous biosynthetic processes. The first and rate-limiting step of glutaminolysis involves the generation of glutamate from glutamine, catalyzed by glutaminase-1 (GLS1). Shortages of glutamine result in reductions in GLS1, but the underlying mechanisms are not fully known. Here, we characterize a long noncoding RNA, GIRGL (glutamine insufficiency regulator of glutaminase lncRNA), that is induced upon glutamine starvation. Manipulating GIRGL revealed a relationship between its expression and the translational suppression of GLS1. Cellular GIRGL levels are balanced by a combination of transactivation by c-JUN together with negative stability regulation via HuR/Ago2. Increased levels of GIRGL in the absence of glutamine drive formation of a complex between dimers of CAPRIN1 and GLS1 mRNA, serving to promote liquid-liquid phase separation of CAPRIN1 and inducing stress granule formation. Suppressing GLS1 mRNA translation enables cancer cells to survive under prolonged glutamine deprivation stress.

Antimicrobial Activity of Lemongrass Essential Oil (Cymbopogon flexuosus) and Its Active Component Citral Against Dual-Species Biofilms of Staphylococcus aureus and Candida Species

Compared to mono-species biofilm, biofilms formed by cross-kingdom pathogens are more refractory to conventional antibiotics, thus complicating clinical treatment and causing significant morbidity. Lemongrass essential oil and its bioactive component citral were previously demonstrated to possess strong antimicrobial efficacy against pathogenic bacteria and fungi. However, their effects on polymicrobial biofilms remain to be determined. In this study, the efficacy of lemongrass (Cymbopogon flexuosus) essential oil and its bioactive part citral against dual-species biofilms formed by Staphylococcus aureus and Candida species was evaluated in vitro. Biofilm staining and viability test showed both lemongrass essential oil and citral were able to reduce biofilm biomass and cell viability of each species in the biofilm. Microscopic examinations showed these agents interfered with adhesive characteristics of each species and disrupted biofilm matrix through counteracting nucleic acids, proteins and carbohydrates in the biofilm. Moreover, transcriptional analyses indicated citral downregulated hyphal adhesins and virulent factors of Candida albicans, while also reducing expression of genes involved in quorum sensing, peptidoglycan and fatty acids biosynthesis of S. aureus. Taken together, our results demonstrate the potential of lemongrass essential oil and citral as promising agents against polymicrobial biofilms as well as the underlying mechanisms of their activity in this setting.

LncRNA GUARDIN suppresses cellular senescence through a LRP130-PGC1α-FOXO4-p21-dependent signaling axis

The long noncoding RNA GUARDIN functions to protect genome stability. Inhibiting GUARDIN expression can alter cell fate decisions toward senescence or apoptosis, but the underlying molecular signals are unknown. Here, we show that GUARDIN is an essential component of a transcriptional repressor complex involving LRP130 and PGC1α. GUARDIN acts as a scaffold to stabilize LRP130/PGC1α heterodimers and their occupancy at the FOXO4 promotor. Destabilizing this complex by silencing of GUARDIN, LRP130, or PGC1α leads to increased expression of FOXO4 and upregulation of its target gene p21, thereby driving cells into senescence. We also found that GUARDIN expression was induced by rapamycin, an agent that suppresses cell senescence. FOS-like antigen 2 (FOSL2) acts as a transcriptional repressor of GUARDIN, and lower FOSL2 levels in response to rapamycin correlate with increased levels of GUARDIN. Together, these results demonstrate that GUARDIN inhibits p21-dependent senescence through a LRP130-PGC1α-FOXO4 signaling axis, and moreover, GUARDIN contributes to the anti-aging activities of rapamycin.

TP53LNC-DB, the database of lncRNAs in the p53 signalling network

The TP53 gene product, p53, is a pleiotropic transcription factor induced by stress, which functions to promote cell cycle arrest, apoptosis and senescence. Genome-wide profiling has revealed an extensive system of long noncoding RNAs (lncRNAs) that is integral to the p53 signalling network. As a research tool, we implemented a public access database called TP53LNC-DB that annotates currently available information relating lncRNAs to p53 signalling in humans.

Circulating CD36+ microparticles are not altered by docosahexaenoic or eicosapentaenoic acid supplementation

BACKGROUND AND AIMS

Circulating microparticles (MP) are the source of a plasma derived form of the scavenger receptor CD36, termed soluble (s)CD36, the levels of which correlate with markers of atherosclerosis and risk of cardiovascular disease. Long chain n-3 polyunsaturated fatty acids have cardioprotective effects that we have previously reported to be gender specific. The aim of this study was to determine if dietary docosahexaenoic acid (DHA) and/or eicosapentaenoic acid (EPA) supplementation affect circulating CD36 + MP levels, and if this occurs differentially in healthy men and women.

METHODS AND RESULTS

Participants (43M, 51F) aged 39.6 ± 1.7 years received 4 weeks of daily supplementation with DHA rich (200 mg EPA; 1000 mg DHA), EPA rich (1000 mg EPA; 200 mg DHA), or placebo (sunola) oil in a double-blinded, randomised, placebo controlled trial. Plasma CD36 + MP were enumerated by flow cytometry and differences between genders and treatments were evaluated by Student's or paired t-test and one way ANOVA. Males and females had similar levels of CD36 + MP at baseline (mean = 1018 ± 325 vs 980 ± 318; p = 0.577) and these were not significantly changed after DHA (M, p = 0.571; F, p = 0.444) or EPA (M, p = 0.361; F, p = 0.901) supplementation. Likewise, the overall percent change in these levels were not different between supplemented cohorts compared to placebo when all participants were combined (% change in CD36 + MP: DHA = 5.7 ± 37.5, EPA = -3.4 ± 35.4, placebo = -11.5 ± 32.9; p = 0.158) or stratified by gender (M, DHA = -2.6 ± 30.6, EPA = -15.1 ± 20.1, placebo = -21.4 ± 28.7, p = 0.187; F, DHA = 11.7 ± 41.5, EPA = 6.8 ± 42.9, placebo = -2.8 ± 34.7, p = 0.552).

CONCLUSION

The cardioprotective effects of DHA and EPA do not act through a CD36 + MP mechanism.

INPP4B is an oncogenic regulator in human colon cancer

Inositol polyphosphate 4-phosphatase type II (INPP4B) negatively regulates phosphatidylinositol 3-kinase signaling and is a tumor suppressor in some types of cancers. However, we have found that it is frequently upregulated in human colon cancer cells. Here we show that silencing of INPP4B blocks activation of Akt and serum- and glucocorticoid-regulated kinase 3 (SGK3), inhibits colon cancer cell proliferation and retards colon cancer xenograft growth. Conversely, overexpression of INPP4B increases proliferation and triggers anchorage-independent growth of normal colon epithelial cells. Moreover, we demonstrate that the effect of INPP4B on Akt and SGK3 is associated with inactivation of phosphate and tensin homolog through its protein phosphatase activity and that the increase in INPP4B is due to Ets-1-mediated transcriptional upregulation in colon cancer cells. Collectively, these results suggest that INPP4B may function as an oncogenic driver in colon cancer, with potential implications for targeting INPP4B as a novel approach to treat this disease.

Suppression of PP2A is critical for protection of melanoma cells upon endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress triggers apoptosis by activating Bim in diverse types of cells, which involves dephosphorylation of Bim_EL by protein phosphatase 2A (PP2A). However, melanoma cells are largely resistant to ER stress-induced apoptosis, suggesting that Bim activation is suppressed in melanoma cells undergoing ER stress. We show here that ER stress reduces PP2A activity leading to increased ERK activation and subsequent phosphorylation and proteasomal degradation of Bim_EL. Despite sustained upregulation of Bim at the transcriptional level, the Bim_EL protein expression was downregulated after an initial increase in melanoma cells subjected to pharmacological ER stress. This was mediated by increased activity of ERK, whereas the phosphatase activity of PP2A was reduced by ER stress in melanoma cells. The increase in ERK activation was, at least in part, due to reduced dephosphorylation by PP2A, which was associated with downregulation of the PP2A catalytic C subunit. Notably, instead of direct dephosphorylation of Bim_EL, PP2A inhibited its phosphorylation indirectly through dephosphorylation of ERK in melanoma cells. Taken together, these results identify downregualtion of PP2A activity as an important protective mechanism of melanoma cells against ER stress-induced apoptosis.

MicroRNA-497 targets insulin-like growth factor 1 receptor and has a tumour suppressive role in human colorectal cancer

Past studies have shown that amplified insulin-like growth factor 1 (IGF1)/IGF1 receptor (IGF1-R) signalling has an important role in colorectal cancer (CRC) development, progression and resistance to treatment. In this report, we demonstrate that downregulation of microRNA-497 (miR-497) as a result of DNA copy number reduction is involved in upregulation of IGF1-R in CRC cells. MiR-497 and miR-195 of the miR-15/16/195/424/497 family that share the same 3′ untranslated region (3′UTR) binding seed sequence and are predicted to target IGF1-R were concurrently downregulated in the majority of CRC tissues relative to paired adjacent normal mucosa. However, only overexpression of miR-497 led to suppression of the IGF1-R 3′UTR activity and downregulation of the endogenous IGF1-R protein in CRC cells. This was associated with inhibition of cell survival, proliferation and invasion, and increased sensitivity to apoptosis induced by various stimuli including the chemotherapeutic drugs cisplatin and 5-fluorouracil, and the death ligand tumour necrosis factor-related apoptosis-inducing ligand. The biological effect of miR-497 on CRC cells was largely mediated by inhibition of phosphatidylinositol 3-kinase/Akt signalling, as overexpression of an active form of Akt reversed its impact on cell survival and proliferation, recapitulating the effect of overexpression of IGF1-R. Downregulation of miR-497 and miR-195 appeared to associate with copy number loss of a segment of chromosome 17p13.1, where these miRs are located at proximity. Similarly to miR-195, the members of the same miR family, miR-424 that was upregulated, and miR-15a, miR-15b and miR-16 that were unaltered in expression in CRC tissues compared with paired adjacent normal mucosa, did not appear to have a role in regulating the expression of IGF1-R. Taken together, these results identify downregulation of miR-497 as an important mechanism of upregulation of IGF1-R in CRC cells that contributes to malignancy of CRC.

The putative diabetic plasma marker, soluble CD36, is non-cleaved, non-soluble and entirely associated with microparticles

BACKGROUND

CD36 is a widely expressed cell surface receptor that binds lipoproteins, and its function has been implicated in many complications of the metabolic syndrome. A cell-free form of CD36, soluble CD36 (sCD36), has been reported in human plasma, found to be elevated in obesity and diabetes, and claimed as a marker of insulin resistance.

OBJECTIVE

To determine the nature of sCD36; in particular, whether sCD36 is truly soluble or, as hypothesized, is found as a component of circulating microparticles (MPs).

METHODS

Lipoproteins were fractionated by density gradient centrifugation, and plasma MPs were isolated by ultracentrifugation, size exclusion, and immunoprecipitation with CD36 detected by immunoblotting. MPs from plasma and activated platelets were analyzed by multicolor flow cytometry, with a DyLight-488 anti-CD36 conjugate in combination with antibodies against different cellular markers.

RESULTS

Cell-free plasma CD36 was not observed associated with lipoproteins and was not a proteolytic fragment; rather, it was associated with the plasma MP fraction, suggesting that sCD36 in the plasma of normal subjects is a product of circulating MPs. Cytometric and immunoblotting analyses of plasma from normal donors showed that these MPs were derived mainly from platelets. Analysis of in vitro activated platelets also showed that CD36 to be secreted in the form of MPs.

CONCLUSIONS

sCD36 is not a proteolytic product, but rather is associated with a specific subset of circulating MPs that can readily be analysed. This finding will enable more specific investigations into the cellular source of the increased levels of plasma CD36 found in subjects with diabetes.

Directional sensing of a phorbol ester gradient requires CD44 and is regulated by CD44 phosphorylation

Cancer progression is associated with enhanced directional cell migration, both of the tumour cells invading into the stroma and stromal cells infiltrating the tumour site. In cell-based assays to study directional cell migration, phorbol esters are frequently used as a chemotactic agent. However, the molecular mechanism by which these activators of protein kinase C (PKC) result in the establishment of a polarized migratory phenotype is not known. Here we show that CD44 expression is essential for chemotaxis towards a phorbol ester gradient. In an investigation of CD44 phosphorylation kinetics in resting and stimulated cells, Ser316 was identified as a novel site of phosphorylation following activation of PKC. PKC does not phosphorylate Ser316 directly, but rather mediates the activation of downstream Ser316 kinase(s). In transfection studies, a phosphorylation-deficient Ser316 mutant was shown to act in a dominant-negative fashion to impair chemotaxis mediated by endogenous CD44 in response to a phorbol ester gradient. Importantly, this mutation had no effect on random cell motility or the ability of cells to migrate directionally towards a cocktail of chemoattractants. These studies demonstrate that CD44 functions to provide directional cues to migrating cells without affecting the motility apparatus.

The integrins alpha3beta1 and alpha6beta1 physically and functionally associate with CD36 in human melanoma cells. Requirement for the extracellular domain OF CD36

Lateral association between different transmembrane glycoproteins can serve to modulate integrin function. Here we characterize a physical association between the integrins alpha(3)beta(1) and alpha(6)beta(1) and CD36 on the surface of melanoma cells and show that ectopic expression of CD36 by CD36-negative MV3 melanoma cells increases their haptotactic migration on extracellular matrix components. The association was demonstrated by co-immunoprecipitation, reimmunoprecipitation, and immunoblotting of surface-labeled cells lysed in Brij 96 detergent. Confocal microscopy illustrated the co-association of alpha(3) and CD36 in cell membrane projections and ruffles. A requirement for the extracellular domain of CD36 in this association was shown by co-immunoprecipitation experiments using surface-labeled MV3 melanoma or COS-7 cells that had been transiently transfected with chimeric constructs between CD36 and intercellular adhesion molecule 1 (ICAM-1) or with a truncation mutant of CD36. CD36 is known to engage in signal transduction and to localize to membrane microdomains or rafts in several cell types. Toward a mechanistic explanation for the functional effects of CD36 expression, we demonstrate that in fractionated Triton X-100 lysates of the MV3 cells stably transfected with CD36, CD36 was greatly enriched with the detergent-insoluble fractions that represent plasma membrane rafts. Significantly, when these fractionated lysates were reprobed for endogenous beta(1) integrin, it was found that a 4-fold increase in the proportion of the mature protein was contained within the detergent-insoluble fractions when extracted from the CD36-transfected cells compared with MV3 cells transfected with vector only. These results suggest that in melanoma cells CD36 expression may induce the sequestration of certain integrins into membrane microdomains and promote cell migration.

Methionine 156 in the immunodominant domain of CD36 contributes to define the epitope recognized by the NL07 MoAb

CD36 is a membrane glycoprotein expressed by several cell types, and play a role as a receptor for different physiological and pathological ligands. An immunodominant domain of CD36 has been described in the amino acidic region 155-183, where many ligands and monoclonal antibodies (MoAbs) react. MoAbs against CD36 have proved useful in structural as well as functional studies. One of these antibodies, MoAb NL07, recognizes a conformational epitope that is acquired in the late steps of the CD36 maturation. The NL07 epitope appears to be functionally relevant and blocks CD36-mediated binding to red blood cells infected with the malaria parasite Plasmodium falciparum (IRBC). In this work a mutant COS-7 clone expressing NL07-negative CD36 molecules on the cell surface was investigated. In the mutant, the methionine in position 156 of the wild type CD36 sequence was replaced by a valine. It was determined that methionine 156 was essential for NL07 reactivity, mapping the NL07 epitope to the vicinity of the functionally important immunodominant domain (aa 155-183) of CD36. Although methionine 156 is located in this region, the CD36V156 mutated molecule was apparently functional and able to bind IRBC and oxidized LDL.

CD36 is a ditopic glycoprotein with the N-terminal domain implicated in intracellular transport

The CD36 receptor sequence predicts two hydrophobic domains located at the N- and C-termini of the protein, but there are conflicting reports as to whether the N-terminal uncleaved leader sequence functions as a transmembrane domain. To investigate the topology of CD36, we generated a panel of mutants lacking either one or both hydrophobic regions and analyzed their folding and transport in COS-7 cells. The N- and the C-terminal hydrophobic regions were both sufficient to anchor CD36 in the membrane, and a FLAG epitope inserted at the N-terminus was located intracellularly. These results indicate that CD36 adopts a ditopic configuration. Accordingly, neither N- nor C-terminal truncation mutants were secreted. Analysis with conformation-specific monoclonal antibodies showed that the N-terminal transmembrane domain truncated molecule was slowly transported through the exocytic pathway and largely accumulated intracellularly. Thus, dual membrane insertion dictates the correct topogenesis and seems to be necessary for efficient folding and intracellular transport.

Cytoplasmic interactions between decay-accelerating factor and intercellular adhesion molecule-1 are not required for coxsackievirus A21 cell infection

Coxsackievirus A21 (CAV-21) employs a cell receptor complex of decay-accelerating factor (DAF) and intercellular adhesion molecule-1 (ICAM-1) for cell infectivity. In this study, the nature of potential extra- and/or intracellular interactions between DAF and ICAM-1 involved in picornaviral cell entry was investigated. Firstly, it was shown that intracellular interplay between DAF and ICAM-1 is not required for CAV-21 infection, as CAV-21 lytic infection mediated via the DAF/ICAM-1 receptor complex is not inhibited by replacement of the transmembrane and cytoplasmic domains of ICAM-1 with those from an unrelated cell surface molecule, CD36. By immunoprecipitation, chemical cross-linking and picornaviral binding assays, the existence of a close spatial association between DAF and ICAM-1 on the surface of ICAM-1-transfected RD cells was confirmed. Furthermore, it was shown that potential extracellular DAF/ICAM-1 interactions are likely to occur in an area on or proximal to DAF SCR3 and may influence the route of CAV-21 cell entry.

Engagement of variant CD44 confers resistance to anti-integrin antibody-mediated apoptosis in a colon carcinoma cell line

The LIM 1863 colon carcinoma cell line grows as structured organoids around a central lumen, and we have previously demonstrated that the three-dimensional arrangement protects the individual cells from apoptosis induced by an anti-alpha v integrin antibody, 23C6 (Bates et al., 1994). Here we show that the intercellular forces which drive spheroid formation can be overcome by exposure of the cells to a collagen substrate, or more specifically through ligation of the CD44 receptor by a monoclonal antibody. Binding to immobilized anti-CD44 antibody induced a monolayer morphology which is accompanied by fibronectin production and secretion, and expression of the integrin alpha v beta 6. Significantly, the cells of the monolayer acquired resistance to 23C6 antibody-mediated apoptosis over time and this property was sustained even after removal from the monolayer. We provide data to show that this resistance is not dependent on monolayer morphology, constant engagement of the CD44 receptor, loss of the 23C6 antigen, or elevation of Bcl-2 or Bcl-XL protein. The CD44 expressed by LIM 1863 is shown to be the metastatic variant of the molecule therefore these results provide a possible explanation for the selective advantages conferred by expression of this variant for metastasizing colon cancer cells. Overall, the findings of this study support a model for the development of malignancy through the production of specific survival and growth signals as a direct consequence of a signaling event induced by stimulation of an epithelial variant of CD44.

Antibody binding to individual short consensus repeats of decay-accelerating factor enhances enterovirus cell attachment and infectivity

Decay-accelerating factor (DAF), a widely expressed membrane complement-regulatory protein, is utilized as a cellular receptor by many human enteric pathogens. We show here that the binding of two enteroviruses to individual short consensus repeats (SCR) of DAF on the cell surface is greatly augmented by mAb binding to an alternate SCR: Coxsackievirus A21 binding to the SCR1 of DAF is increased by Ab binding to SCR3 and, conversely, Echovirus 7 binding to SCR3 is enhanced severalfold by Ab binding to SCR1. These Ab-induced increases in viral binding also resulted in increased viral infectivity. Using purified soluble DAF in a solid phase assay it was found that Ab binding to SCR1 is increased greatly in the presence of an Ab against SCR3 and, reciprocally, Ab against SCR1 greatly increases Ab binding to SCR3. In contrast to the results obtained with the larger viral particles, however, this reciprocal Ab-induced enhancement of binding is not seen when measuring Ab binding to membrane-bound DAF SCR on the cell surface. These findings provide a possible explanation for functional differences between membrane-bound and soluble DAF with implications for a potential role for DAF-binding molecules in regulating DAF function. This is the first demonstration of enhancement of viral infectivity mediated by Ab against the viral receptor.

CD36 forms covalently associated dimers and multimers in platelets and transfected COS-7 cells

CD36 is a transmembrane glycoprotein expressed on the surface of a number of cell types. The analysis of CD36 from platelets using immunoblotting, gel filtration, and native PAGE indicated the presence of high molecular complexes exceeding the Mr of monomeric CD36. Experiments using transfected COS-7 cells revealed these complexes were homodimers and -multimers of CD36. The multimers could be dissociated by treatment with a reducing agent, indicating they were formed by intermolecular cysteine-bridging. Mutagenesis of the cDNA for CD36 implicated the cysteines in the extracellular domain of the molecule. The potential physiological roles of CD36 multimerisation are discussed.

Regulation of tumor cell motility and migration by CD63 in a human melanoma cell line

CD63 belongs to the transmembrane 4 superfamily of membrane proteins and is expressed in several normal tissues as well as in melanoma cells. Previous reports have suggested that CD63 may play an important role in inhibiting melanoma progression, and this was supported by our studies showing that CD63 was associated with suppression of the growth of melanoma in nude mice. Recently, we and others have shown that CD63 may form noncovalent associations with beta1 integrins, which suggests that the function of CD63 may be related to that of integrins. To further explore the role of CD63 in melanoma, we transfected CD63 into a highly motile, CD63-negative melanoma cell line, KM3, which was shown to express alpha(v)beta5 as the predominant integrin with only trace amounts of beta1 integrins. Following transfection, CD63 was shown to associate with beta1 integrins, and beta1 expression appeared to be up-regulated. Cell motility in serum-containing media was markedly suppressed following transfection of CD63. This inhibition was potentiated by mAbs to CD63, and correlated with the level of CD63 expression. The CD63-transfected, but not the untransfected, melanoma cells showed increased adhesion and migration on the beta1 substrates, fibronectin, laminin, and collagen, whereas rates of migration were similar on the beta5 substrate, vitronectin. These results show that CD63 is involved in regulation of the motility of melanoma cells and their adhesion and migration on substrates associated with beta1 integrins. We suggest they provide further insights into the role of CD63 in tumor progression.

Regulation of tumor cell motility and migration by CD63 in a human melanoma cell line

CD63 belongs to the transmembrane 4 superfamily of membrane proteins and is expressed in several normal tissues as well as in melanoma cells. Previous reports have suggested that CD63 may play an important role in inhibiting melanoma progression, and this was supported by our studies showing that CD63 was associated with suppression of the growth of melanoma in nude mice. Recently, we and others have shown that CD63 may form noncovalent associations with beta1 integrins, which suggests that the function of CD63 may be related to that of integrins. To further explore the role of CD63 in melanoma, we transfected CD63 into a highly motile, CD63-negative melanoma cell line, KM3, which was shown to express alpha(v)beta5 as the predominant integrin with only trace amounts of beta1 integrins. Following transfection, CD63 was shown to associate with beta1 integrins, and beta1 expression appeared to be up-regulated. Cell motility in serum-containing media was markedly suppressed following transfection of CD63. This inhibition was potentiated by mAbs to CD63, and correlated with the level of CD63 expression. The CD63-transfected, but not the untransfected, melanoma cells showed increased adhesion and migration on the beta1 substrates, fibronectin, laminin, and collagen, whereas rates of migration were similar on the beta5 substrate, vitronectin. These results show that CD63 is involved in regulation of the motility of melanoma cells and their adhesion and migration on substrates associated with beta1 integrins. We suggest they provide further insights into the role of CD63 in tumor progression.

Stimulation of platelet activation and aggregation by a carboxyl-terminal peptide from thrombospondin binding to the integrin-associated protein receptor

Thrombospondin, a major secretory product of the alpha-granules of activated platelets, is a large trimeric glycoprotein that plays an important role in platelet aggregation. On resting platelets, thrombospondin binds to a single receptor in a cation-independent manner, but upon platelet activation it binds at least two further, distinct receptors that are both dependent upon divalent cations. Each of these receptors on the platelet surface binds to different regions of the thrombospondin molecule, and such binding may be responsible for the multifunctional role of thrombospondin in aggregation. We show here that a peptide from the carboxyl terminus of thrombospondin, RFYVVMWK, directly and specifically induces the activation and aggregation of washed human platelets from different donors at concentrations of 5-25 microM. At lower concentrations the peptide synergizes with suboptimal concentrations of ADP to induce aggregation. Peptide affinity chromatography and immunoprecipitation with a monoclonal antibody were used to identify the receptor for the carboxyl-terminal peptide as the integrin-associated protein. The integrin-associated protein remained bound to the RFYVVMWK-containing peptide column when washed with a scrambled peptide in the presence of 5 mM EDTA, indicating a divalent cation-independent association. It is suggested that integrin-associated protein is the primary receptor for thrombospondin on the surface of resting platelets and is implicated in potentiating the platelet aggregation response.

CD63 associates with transmembrane 4 superfamily members, CD9 and CD81, and with beta 1 integrins in human melanoma

CD63 belongs to the Transmembrane 4 superfamily (TM4SF) of membrane proteins whose functions are largely unknown. Previous results have suggested that CD63 may play an important role in the regulation of melanoma progression. To explore the role of CD63 in melanoma we have examined its association with other molecules by immunoprecipitation of CD63 from detergent induced lysates of melanoma cells. These results are the first to demonstrate an association between CD63 and two other TM4SF members, CD9 and CD81 in 2 human melanoma cell lines. We are also able to identify an association between CD9 and CD63 with beta 1 integrins in melanoma. The results suggest that CD63 is capable of forming multicomponent complexes with TM4SF members and beta 1 integrins on the surface of melanoma. These findings provide further insights into the function of CD63.

Expression of natural killer (NK) cell-specific alloantigens on a mouse NK-like cell line

As part of the strategy for screening for natural killer (NK) cell-specific monoclonal antibodies (MoAb) we have raised a number of murine NK-like cell lines in media containing interleukin-2 (IL-2). The detection of specific NK cell alloantigens on a C57BL/6 cell line in long-term culture in IL-2 is the subject of this paper. The C57BL/6 cell line has the morphology of large granular lymphocytes (LGL) and exhibits strong cytolytic activity against the archetype NK cell target, YAC-1. Absorption of three anti-NK antiserum, NZB anti-BALB/c (anti-NK-2.1), BALB/c anti-DBA/2 (anti-NK-3.1) and CE anti-CBA (anti-NK-4.1), with the C57BL/6 cell line removed the anti-NK activity from these antisera. Flow cytometric studies of the C57BL/6 cell line demonstrated significant binding of the anti-NK-1.1 MoAb produced by hybridoma PK136. Our results suggest that the C57BL/6 NK-like cell line exhibits some of the properties of naive NK cells and expresses all the known NK cell-specific alloantigens, NK-1.1, NK-2.1, NK-3.1 and NK-4.1 and therefore is potentially useful in selecting NK specific hybridomas and in studying the biology of NK cells.

Effects of hormones and N6O2'-dibutyryl-adenosine 3' :5'-cyclic monophosphate, administered in vivo, on phosphate transport and metabolism in isolated rat liver mitochondria

1. The administration of glucagon or N6O2'-dibutyryl cyclic AMP to fed rats by intraperitoneal injection was associated with a 2-fold increase in the amounts of endogenous Pi and ATP, and an increase in the rate and extent of transport of exogenous Pi (measured in either the presence or the absence of Ca2+) in mitochondria subsequently isolated from the liver. No change was observed in either the maximum rate of transport of exogenous Pi or in the rate of 32Pi exchange. 2. The changes induced by glucagon and dibutyryl cyclic AMP were markedly decreased by the co-administration of cycloheximide. 3. The administration of insulin to rats resulted in an increase of about 1.3-fold in the concentration of endogenous mitochondrial Pi 4. The amounts of endogenous Pi in mitochondrial isolated from the livers of starved rats were 3 times those in mitochondria isolated from fed animals. 5. It is concluded that the liver mitochondrial phosphatetransport system may be an important site of hormone action. 6. In the course of these experiments, it was shown that Ca2+ markedly stimulates mitochondrial phosphate transports.

The role of mitochondria in modifying the cellular ionic environment. Calcium-induced respiratory activities in mitochondria isolated from various tumour cells

Cyclic stimulation by Ca(2+) of respiration in mitochondria isolated from Ehrlich ascites-tumour cells occurs only when low phosphate concentrations (approx. 0.5mm) are also included in the incubation system. Under these circumstances the extra oxygen consumed is related stoicheiometrically to the amount of Ca(2+) taken up by the mitochondria; the values are similar to those obtained with mitochondria from rat liver in the absence of added phosphate. In contrast with liver mitochondria, up to 280nmol of Ca(2+)/mg of protein can be added to ascites mitochondria without causing any deleterious effect. Respiration in mitochondria isolated from the Yoshida ascites hepatoma (HA 130) and from the Morris hepatomas 5123C and 9618A is also stimulated by Ca(2+) in a cyclic manner. However, that in mitochondria from regenerating rat liver responds to Ca(2+) in the same way as those from normal rat liver. ADP-stimulated respiration in mitochondria from Ehrlich ascites tumour cells, but not from rat liver, is inhibited by low amounts of Ca(2+).