Combined pharmacological, mutational and cell biology approaches indicate that p53-dependent caspase 3 activation triggered by cellular prion is dependent on its endocytosis

Design, synthesis and pharmacological evaluation of 2-arylurea-1,3,5-triazine derivative (XIN-9): A novel potent dual PI3K/mTOR inhibitor for cancer therapy

Blocking the PI3K/AKT/mTOR pathway has been widely recognized as an attractive cancer therapeutic strategy because of its crucial role in cell growth and survival. In this study, a novel series of 2-arylurea-1,3,5-triazine derivatives had been synthesized and evaluated as highly potent PI3K and mTOR inhibitors. The new compounds exhibited cytotoxic activities against MCF-7, Hela and A549 cancer cell lines (IC₅₀ = 0.03-36.54 μM). The most promising compound XIN-9 exhibited potent inhibition against PI3K and mTOR kinase (IC₅₀ = 23.8 and 10.9 nM). Mechanistic study using real-time PCR revealed the ability of XIN-9 to inhibit PI3K and mTOR. In addition, compound XIN-9 arrested the cell cycle of MCF-7 cells at the G0/G1 phase. XIN-9 also caused a significant dose-dependent increase of early and late apoptotic events. Molecular docking analysis revealed a high binding affinity for XIN-9 toward PI3K and mTOR. Following in vitro studies, XIN-9 was further evaluated in MCF-7 xenograft models to show significant in vivo anticancer efficacies with tumor growth inhibitions of 41.67% (po, 75 mg/kg). Overall, this work indicated that compound XIN-9 represents a potential anticancer targeting PI3K/AKT/mTOR pathway.

A New Take on Prion Protein Dynamics in Cellular Trafficking

The mobility of cellular prion protein (PrP^C) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrP^C works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrP^C is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrP^C through multiple pathways, in a well-orchestrated mechanism of intra and extracellular vesicular transport, defines its functional plasticity, and also assists the conversion and spreading of its infectious isoform associated with neurodegenerative diseases. In this review, we highlight how PrP^C traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

p53 in neurodegenerative diseases and brain cancers

More than thirty years elapsed since a protein, not yet called p53 at the time, was detected to bind SV40 during viral infection. Thousands of papers later, p53 evolved as the main tumor suppressor involved in growth arrest and apoptosis. A lot has been done but the protein has not yet revealed all its secrets. Particularly important is the observation that in totally distinct pathologies where apoptosis is either exacerbated or impaired, p53 appears to play a central role. This is exemplified for Alzheimer's and Parkinson's diseases that represent the two main causes of age-related neurodegenerative affections, where cell death enhancement appears as one of the main etiological paradigms. Conversely, in cancers, about half of the cases are linked to mutations in p53 leading to the impairment of p53-dependent apoptosis. The involvement of p53 in these pathologies has driven a huge amount of studies aimed at designing chemical tools or biological approaches to rescue p53 defects or over-activity. Here, we describe the data linking p53 to neurodegenerative diseases and brain cancers, and we document the various strategies to interfere with p53 dysfunctions in these disorders.

Two-steps control of cellular prion physiology by the extracellular regulated kinase-1 (ERK1)

Cellular prion (PrP(c)) undergoes a regulated α-secretase-like cleavage by the disintegrin ADAM17 similar to the one taking place on β-amyloid precursor protein (βAPP). Because these cleavages give rise to biologically active fragments, understanding their regulation could be of importance. We have established that the Extracellular Regulated Kinase-1 (ERK1) controls PrPc processing by modulating ADAM17 phosphorylation in a protein kinase C-dependent manner. Strikingly, we also demonstrated that ERK1 acts upstream to increase PrP(c) promoter transactivation in an AP-1 dependent manner. Therefore, ERK1 exerts a dual control of both PrP(c) metabolism and expression. Interestingly, α-secretase cleavage of βAPP appears to be independent of ERK1. I describe here similarities and differences in α-secretase-mediated PrP(c) and βAPP processing pathways and discuss putative physiopathological implications.

Prion proteins and copper ions. Biological and chemical controversies

The Prion protein (PrP(c)) involvement in some neurodegenerative diseases is well assessed although its "normal" biological role is not completely understood. It is known that PrP(C) can bind Cu(II) ions with high specificity but the order of magnitude of the corresponding affinity constant(s) is still highly debated. This perspective is an attempt to collect the current knowledge on these topics and to build up a bridge between the biological and the chemical points of view.

The alpha-secretase-derived N-terminal product of cellular prion, N1, displays neuroprotective function in vitro and in vivo

Cellular prion protein (PrP(c)) undergoes a disintegrin-mediated physiological cleavage, generating a soluble amino-terminal fragment (N1), the function of which remained unknown. Recombinant N1 inhibits staurosporine-induced caspase-3 activation by modulating p53 transcription and activity, whereas the PrP(c)-derived pathological fragment (N2) remains biologically inert. Furthermore, N1 protects retinal ganglion cells from hypoxia-induced apoptosis, reduces the number of terminal deoxynucleotidyltransferase-mediated biotinylated UTP nick end labeling-positive and p53-immunoreactive neurons in a pressure-induced ischemia model of the rat retina and triggers a partial recovery of b-waves but not a-waves of rat electroretinograms. Our work is the first demonstration that the alpha-secretase-derived PrP(c) fragment N1, but not N2, displays in vivo and in vitro neuroprotective function by modulating p53 pathway. It further demonstrates that distinct N-terminal cleavage products of PrP(c) harbor different biological activities underlying the various phenotypes linking PrP(c) to cell survival.

Alveolar macrophages in allergic asthma: an expression signature characterized by heat shock protein pathways

The implication of alveolar macrophages (AM) in asthma, a T_h2 disease, has not been well characterized. Thus, the goal of this study is to better characterize AM phenotype of allergic asthmatic compared with normal subjects using genomic expression analyses. Microarray analyses were performed with AM isolated from bronchoalveolar lavage. Robust multiarray analysis (RMA) normalization and Smyth's moderated t test were used to select differentially expressed genes. Fifty differentially expressed genes were identified. Nineteen have been classified in categories linked to stress or immune responses and among them; nine are part of the heat shock protein (HSP) family. Difference of expression for three (HSPD1, PRNP, SERPINH1) of the five selected genes were validated using real-time reverse transcription–polymerase chain reaction. Enzyme-linked immunosorbent assay was used to measure the protein level of heat shock protein 60 (HSP60), the protein encoded by HSPD1, and showed difference in AM protein level between allergic asthmatic and control subjects. In summary, this study suggests that HSP gene family, particularly HSP60, is involved in AM functions in a context of allergic asthma. These results also support the involvement of AM immune functions in the development of an allergic asthmatic response.

Prion protein: From physiology to cancer biology

Prion protein (PrPc) was originally viewed solely as being involved in prion disease, but now several intriguing lines of evidence have emerged indicating that it plays a fundamental role not only in the nervous system, but also throughout the human body. PrPc is expressed most abundantly in the brain, but has also been detected in other non-neuronal tissues as diverse as lymphoid cells, lung, heart, kidney, gastrointestinal tract, muscle, and mammary glands. Recent data indicate that PrPc may be implicated in biology of glioblastoma, breast cancer, prostate and gastric cancer. Over expression of PrPc is correlated to the acquisition by tumor cells of a phenotype for resistance to cell death induced by TNF alpha and TRAIL or antitumor drugs such as paclitaxel and anthracyclines. PrPc may promote tumorigenesis, proliferation and G1/S transition in gastric cancer cells. This review revisits the physiological functions of PrPc, and its possible implications for cancer biology.

Activation of p53-regulated pro-apoptotic signaling pathways in PrP-mediated myopathy

BACKGROUND

We have reported that doxycycline-induced over-expression of wild type prion protein (PrP) in skeletal muscles of Tg(HQK) mice is sufficient to cause a primary myopathy with no signs of peripheral neuropathy. The preferential accumulation of the truncated PrP C1 fragment was closely correlated with these myopathic changes. In this study we use gene expression profiling to explore the temporal program of molecular changes underlying the PrP-mediated myopathy.

RESULTS

We used DNA microarrays, and confirmatory real-time PCR and Western blot analysis to demonstrate deregulation of a large number of genes in the course of the progressive myopathy in the skeletal muscles of doxycycline-treated Tg(HQK) mice. These include the down-regulation of genes coding for the myofibrillar proteins and transcription factor MEF2c, and up-regulation of genes for lysosomal proteins that is concomitant with increased lysosomal activity in the skeletal muscles. Significantly, there was prominent up-regulation of p53 and p53-regulated genes involved in cell cycle arrest and promotion of apoptosis that paralleled the initiation and progression of the muscle pathology.

CONCLUSION

The data provides the first in vivo evidence that directly links p53 to a wild type PrP-mediated disease. It is evident that several mechanistic features contribute to the myopathy observed in PrP over-expressing mice and that p53-related apoptotic pathways appear to play a major role.

Endocytosis of prion protein is required for ERK1/2 signaling induced by stress-inducible protein 1

The secreted cochaperone STI1 triggers activation of protein kinase A (PKA) and ERK1/2 signaling by interacting with the cellular prion (PrP(C)) at the cell surface, resulting in neuroprotection and increased neuritogenesis. Here, we investigated whether STI1 triggers PrP(C) trafficking and tested whether this process controls PrP(C)-dependent signaling. We found that STI1, but not a STI1 mutant unable to bind PrP(C), induced PrP(C) endocytosis. STI1-induced signaling did not occur in cells devoid of endogenous PrP(C); however, heterologous expression of PrP(C) reconstituted both PKA and ERK1/2 activation. In contrast, a PrP(C) mutant lacking endocytic activity was unable to promote ERK1/2 activation induced by STI1, whereas it reconstituted PKA activity in the same condition, suggesting a key role of endocytosis in the former process. The activation of ERK1/2 by STI1 was transient and appeared to depend on the interaction of the two proteins at the cell surface or shortly after internalization. Moreover, inhibition of dynamin activity by expression of a dominant-negative mutant caused the accumulation and colocalization of these proteins at the plasma membrane, suggesting that both proteins use a dynamin-dependent internalization pathway. These results show that PrP(C) endocytosis is a necessary step to modulate STI1-dependent ERK1/2 signaling involved in neuritogenesis.

Physiology of the prion protein

Prion diseases are transmissible spongiform encephalopathies (TSEs), attributed to conformational conversion of the cellular prion protein (PrP(C)) into an abnormal conformer that accumulates in the brain. Understanding the pathogenesis of TSEs requires the identification of functional properties of PrP(C). Here we examine the physiological functions of PrP(C) at the systemic, cellular, and molecular level. Current data show that both the expression and the engagement of PrP(C) with a variety of ligands modulate the following: 1) functions of the nervous and immune systems, including memory and inflammatory reactions; 2) cell proliferation, differentiation, and sensitivity to programmed cell death both in the nervous and immune systems, as well as in various cell lines; 3) the activity of numerous signal transduction pathways, including cAMP/protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/Akt pathways, as well as soluble non-receptor tyrosine kinases; and 4) trafficking of PrP(C) both laterally among distinct plasma membrane domains, and along endocytic pathways, on top of continuous, rapid recycling. A unified view of these functional properties indicates that the prion protein is a dynamic cell surface platform for the assembly of signaling modules, based on which selective interactions with many ligands and transmembrane signaling pathways translate into wide-range consequences upon both physiology and behavior.

The cellular prion protein (PrP(C)): its physiological function and role in disease

Prion diseases are caused by conversion of a normal cell-surface glycoprotein (PrP(C)) into a conformationally altered isoform (PrP(Sc)) that is infectious in the absence of nucleic acid. Although a great deal has been learned about PrP(Sc) and its role in prion propagation, much less is known about the physiological function of PrP(C). In this review, we will summarize some of the major proposed functions for PrP(C), including protection against apoptotic and oxidative stress, cellular uptake or binding of copper ions, transmembrane signaling, formation and maintenance of synapses, and adhesion to the extracellular matrix. We will also outline how loss or subversion of the cytoprotective or neuronal survival activities of PrP(C) might contribute to the pathogenesis of prion diseases, and how similar mechanisms are probably operative in other neurodegenerative disorders.

Neonatal lethality in transgenic mice expressing prion protein with a deletion of residues 105-125

To identify sequence domains important for the neurotoxic and neuroprotective activities of the prion protein (PrP), we have engineered transgenic mice that express a form of murine PrP deleted for a conserved block of 21 amino acids (residues 105-125) in the unstructured, N-terminal tail of the protein. These mice spontaneously developed a severe neurodegenerative illness that was lethal within 1 week of birth in the absence of endogenous PrP. This phenotype was reversed in a dose-dependent fashion by coexpression of wild-type PrP, with five-fold overexpression delaying death beyond 1 year. The phenotype of Tg(PrPDelta105-125) mice is reminiscent of, but much more severe than, those described in mice that express PrP harboring larger deletions of the N-terminus, and in mice that ectopically express Doppel, a PrP paralog, in the CNS. The dramatically increased toxicity of PrPDelta105-125 is most consistent with a model in which this protein has greatly enhanced affinity for a hypothetical receptor that serves to transduce the toxic signal. We speculate that altered binding interactions involving the 105-125 region of PrP may also play a role in generating neurotoxic signals during prion infection.

The C-terminal products of cellular prion protein processing, C1 and C2, exert distinct influence on p53-dependent staurosporine-induced caspase-3 activation

The cellular prion protein (PrP(c)) undergoes various endopro-teolytic attacks within its N-terminal domain, leading to the production of C-terminal fragments (C) tethered to the plasma membrane and soluble N-terminal peptides (N). One of these cleavages occurs at position 110/111, thereby generating C1 and N1 products. We have reported that disintegrins ADAM-10, -9, and -17 participate either directly or indirectly to this proteolytic event. An alternative proteolytic event taking place around residue 90 yields C2 and N2 fragments. The putative function of these proteolytic fragments remained to be established. We have set up two novel human embryonic kidney 293 cell lines stably overexpressing either C1 or C2. We show that C1 potentiates staurosporine-induced caspase-3 activation through a p53-dependent mechanism. Thus, C1 positively controls p53 transcription and mRNA levels and increases p53-like immunoreactivity and activity. C1-induced caspase-3 activation remained unaffected by the blockade of endocytosis in HEK 293 cells and was abolished in p53-deficient fibroblasts. Conversely, overexpression of the C2 fragment did not significantly sensitize HEK 293 cells to apoptotic stimuli and did not modify p53 mRNA levels or activity. Therefore, the nature of the proteolytic cleavage taking place on PrP(c) yielded C-terminal catabolites with distinct function and could be seen as a switch mechanism controlling the function of the PrP(c) in cell survival.

Synthesis and structural characterization of a mimetic membrane-anchored prion protein

During pathogenesis of transmissible spongiform encephalopathies (TSEs) an abnormal form (PrP(Sc)) of the host encoded prion protein (PrP(C)) accumulates in insoluble fibrils and plaques. The two forms of PrP appear to have identical covalent structures, but differ in secondary and tertiary structure. Both PrP(C) and PrP(Sc) have glycosylphospatidylinositol (GPI) anchors through which the protein is tethered to cell membranes. Membrane attachment has been suggested to play a role in the conversion of PrP(C) to PrP(Sc), but the majority of in vitro studies of the function, structure, folding and stability of PrP use recombinant protein lacking the GPI anchor. In order to study the effects of membranes on the structure of PrP, we synthesized a GPI anchor mimetic (GPIm), which we have covalently coupled to a genetically engineered cysteine residue at the C-terminus of recombinant PrP. The lipid anchor places the protein at the same distance from the membrane as does the naturally occurring GPI anchor. We demonstrate that PrP coupled to GPIm (PrP-GPIm) inserts into model lipid membranes and that structural information can be obtained from this membrane-anchored PrP. We show that the structure of PrP-GPIm reconstituted in phosphatidylcholine and raft membranes resembles that of PrP, without a GPI anchor, in solution. The results provide experimental evidence in support of previous suggestions that NMR structures of soluble, anchor-free forms of PrP represent the structure of cellular, membrane-anchored PrP. The availability of a lipid-anchored construct of PrP provides a unique model to investigate the effects of different lipid environments on the structure and conversion mechanisms of PrP.