Current Medicinal Chemistry - Volume 20, Issue 1, 2013

The bulk structure of biological membranes consists of a bilayer of amphipathic lipids. According to the fluid mosaic model proposed by Singer and Nicholson, the glycerophospholipid bilayer is a two-dimensional fluid construct that allows the lateral movement of membrane components. Different types of lateral interactions among membrane components can take place, giving rise to multiple levels of lateral order that lead to highly organized structures. Early observations suggested that some of the lipid components of biological membranes may play active roles in the creation of these levels of order. In the late 1980s, a diverse series of experimental findings collectively gave rise to the lipid raft hypothesis. Lipid rafts were originally defined as membrane domains, i.e., ordered structures created as a consequence of the lateral segregation of sphingolipids and differing from the surrounding membrane in their molecular composition and properties. This definition was subsequently modified to introduce the notion that lipid rafts correspond to membrane areas stabilized by the presence of cholesterol within a liquid-ordered phase. During the past two decades, the concept of lipid rafts has become extremely popular among cell biologists, and these structures have been suggested to be involved in a great variety of cellular functions and biological events. During the same period, however, some groups presented experimental evidence that appeared to contradict the basic tenets that underlie the lipid raft concept. The concept is currently being re-defined, with greater consistency regarding the true nature and role of lipid rafts. In this article we will review the concepts, criticisms, and the novel confirmatory findings relating to the lipid raft hypothesis.

Functioning of G protein-coupled receptors (GPCRs) is tightly linked to the membrane environment, but a molecular level understanding of the modulation of GPCR by membrane lipids is not available. However, specific receptor-lipid interactions as well as unspecific effects mediated by the bulk properties of the membrane (thickness, curvature, etc.) have been proposed to be key regulators of GPCR modulation. In this review, we examine computational efforts made towards modeling and simulation of (i) the complex behavior of membrane lipids, (ii) membrane lipid-GPCR interactions as well as membrane lipid-mediated effects on GPCRs and (iii) GPCR oligomerization in a native-like membrane environment. We propose that, from the perspective of computational modeling, all three of these components need to be addressed in order to achieve a deeper understanding of GPCR functioning. Presently, we are able to simulate numerous lipid properties applying advanced computational techniques, although some barriers, such as the time-length of these simulations, need to be overcome. Implementing three-dimensional structures of GPCRs in such validated membrane systems can give novel insights in membrane-dependent receptor modulation and formation of higher order receptor complexes. Finally, more realistic GPCR-membrane models would provide a very useful tool in studying receptor behavior and its modulation by small drug-like ligands, a relevant issue for drug discovery.

G protein coupled-receptor (GPCR) kinases (GRKs) initiate the deactivation of GPCRs by phosphorylating their cytoplasmic loops and C-terminal tails. They are regulated not only by allosteric interactions with activated GPCRs, but also by the membrane environment itself. Herein we describe how the various GRKs are recruited to lipid bilayers and, where evident, how specific anionic phospholipids help regulate their activity. Using crystal structures representing each of the three vertebrate GRK subfamilies, we map the lipid binding sites in order to better understand how these enzymes are oriented at the cell surface. This analysis suggests that GRKs bind lipid and active GPCRs in a coordinated manner.

G-protein coupled receptors (GPCRs) are the largest class of molecules involved in signal transduction across membranes, and represent major targets in the development of novel drug candidates in all clinical areas. Since GPCRs are integral membrane proteins, interaction of membrane lipids such as cholesterol and sphingolipids with GPCRs constitutes an emerging area of research in contemporary biology. Cholesterol and sphingolipids represent important lipid components of eukaryotic membranes and play a crucial role in a variety of cellular functions. In this review, we highlight the role of these vital lipids in the function of two representative GPCRs, the serotonin1A receptor and the adrenergic receptor. We believe that development in deciphering molecular details of the nature of GPCR-lipid interaction would lead to better insight into our overall understanding of GPCR function in health and disease.

There is broad evidence that association of transmembrane receptors and signalling molecules with lipid rafts/caveolae provides an enriched environment for protein-protein interactions necessary for signal transduction, and a mechanism for the modulation of neurotransmitter and/or growth factor receptor function. Several receptors translocate into submembrane compartments after ligand binding, while others move in the opposite direction. The role of such a dynamic localization and functional facilitation is signalling modulation and receptor desensitization or internalization. Purine and pyrimidine nucleotides have been viewed as primordial precursors in the evolution of all forms of intercellular communication, and they are now regarded as fundamental extracellular signalling molecules. They propagate the purinergic signalling by binding to ionotropic and metabotropic receptors expressed on the plasma membrane of almost all cell types, tissues and organs. Here, we have illustrated the localization in lipid rafts/caveolae of G protein-coupled P1 receptors for adenosine and P2Y receptors for nucleoside tri- and di-phosphates. We have highlighted that microdomain partitioning of these purinergic GPCRs is cell-specific, as is the overall expression levels of these same receptors. Moreover, we have described that disruption of submembrane compartments can shift the purinergic receptors from raft/caveolar to non-raft/non-caveolar fractions, and then abolish their ability to activate lipid signalling pathways and to integrate with additional lipid-controlled signalling events. This modulates the biological response to purinergic ligands and most of all indicates that the topology of the various purinergic components at the cell surface not only organizes the signal transduction machinery, but also controls the final cellular response.

A number of integral membrane G protein-coupled receptors (GPCRs) share common structural features (including palmytoilated aminoacid residues and consensus sequences specific for interaction with cholesterol) that allow them to interact with lipid rafts, membrane cholesterol-rich microdomains able to regulate GPCR signalling and functions. Among GPCRs, type-1 and type-2 cannabinoid receptors, the molecular targets of endocannabinoids (eCBs), control many physiological and pathological processes through the activation of several signal transduction pathways. Recently, the orphan GPR55 receptor has been proved to be activated by many eCBs, thus leading to the hypothesis that it might be the “type-3” cannabinoid receptor. While the biological activity of eCBs and the influence of membrane lipids on their functions are rather well established, information regarding GPR55 is still scarce and often controversial. Based on this background, here we shall review current data about GPR55 pharmacology and signalling, highlighting its involvement in several pathophysiological conditions. We shall also outline the structural features that allow GPR55 to interact with cholesterol and to associate with lipid rafts; how the latter lipid microdomains impact the biological activity of GPR55 is also addressed, as well as their potential for the discovery of new therapeutics useful for the treatment of those human diseases that might be associated with alterations of GPR55 activity.

Thermosensitive hydrogels are very important biomaterials used in drug delivery systems (DDSs), which gained increasing attention of researchers. Thermosensitive hydrogels have great potential in various applications, such as drug delivery, cell encapsulation, tissue engineering, and etc. Especially, injectable thermosensitive hydrogels with lower sol-gel transition temperature around physiological temperature have been extensively studied. By in vivo injection, the hydrogels formed non-flowing gel at body temperature. Upon incorporation of pharmaceutical agents, the hydrogel systems could act as sustained drug release depot in situ. Injectable thermosensitive hydrogel systems have a number of advantages, including simplicity of drug formulation, protective environment for drugs, prolonged and localized drug delivery, and ease of application. The objective of this review is to summarize fundamentals, applications, and recent advances of injectable thermosensitive hydrogel as DDSs, including chitosan and related derivatives, poly(N-isopropylacrylamide)-based (PNIPAAM) copolymers, poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) copolymers and its derivatives, and poly(ethylene glycol)/ biodegradable polyester copolymers.

Drug repositioning, an innovative therapeutic application of an old drug, has received much attention as a particularly costeffective strategy in drug R&D. Recent work has indicated that repositioning can be promoted by utilizing a wide range of information sources, including medicinal chemical, target, mechanism, main and side-effect-related information, and also bibliometric and taxonomical fingerprints, signatures and knowledge bases. This article describes the adaptation of a conceptually novel, more efficient approach for the identification of new possible therapeutic applications of approved drugs and drug candidates, based on a kernel-based data fusion method. This strategy includes (1) the potentially multiple representation of information sources, (2) the automated weighting and statistically optimal combination of information sources, and (3) the automated weighting of parts of the query compounds. The performance was systematically evaluated by using Anatomical Therapeutic Chemical Classification System classes in a cross-validation framework. The results confirmed that kernel-based data fusion can integrate heterogeneous information sources significantly better than standard rank-based fusion can, and this method provides a unique solution for repositioning; it can also be utilized for de novo drug discovery. The advantages of kernel-based data fusion are illustrated with examples and open problems that are particularly relevant for pharmaceutical applications.

Sphingolipids are a class of lipids that have important functions in a variety of cellular processes such as, differentiation, proliferation, senescence, apoptosis and chemotherapeutic resistance. The most widely studied bioactive shingolipids include ceramides, dihydroceramide (dhCer), ceramide-1-phosphate (C1P), glucosyl-ceramide (GluCer), sphingosine and sphingosine-1-phosphate (S1P). Although the length of fatty acid chain affects the physiological role, ceramides and sphingosine are known to induce apoptosis whereas C1P, S1P and GluCer induce proliferation of cells, which causes the development of chemoresistance. Previous studies have implicated the significance of bioactive shingolipids in oncogenesis, cancer progression and drug- and radiation-resistance. Therefore, targeting the elements of sphingolipid metabolism appears important for the development of novel therapeutics or to increase the effectiveness of the current treatment strategies. Some approaches involve the development of synthetic ceramide analogs, small molecule inhibitors of enzymes such as sphingosine kinase, acid ceramidase or ceramide synthase that catalyze ceramide catabolism or its conversion to various molecular species and S1P receptor antagonists. These approaches mainly aim to up-regulate the levels of apoptotic shingolipids while the proliferative ones are down-regulated, or to directly deliver cytotoxic sphingolipids like short-chain ceramide analogs to tumor cells. It is suggested that a combination therapy with conventional cytotoxic approaches while preventing the conversion of ceramide to S1P and consequently increasing the ceramide levels would be more beneficial. This review compiles the current knowledge about sphingolipids, and mainly focuses on novel agents modulating sphingolipid pathways that represent recent therapeutic strategies for the treatment of cancer.

It has been reported that most tumor cells show an increased uptake of variety of amino acids specially methionine when compared with normal cells and amino acid transport is generally increased in malignant transformation. Based on the evidences, two novel nanosized analogues of methionine (Anionic Linear Globular Dendrimer G2, a biodigredabale anionic linear globular-Methionin, and DTPA-Methionine1 conjugates) were synthesized and labeled with 99mTc and used in tumor imaging/ therapy in vitro and in vivo. The results showed marked tumor SPECT molecular imaging liabilities for both compounds but with a better performance by administration of 99mTc-Dendrimer G2-Methionin. The results also showed a good anticancer activity for 99mTc-DTPA-Methionine. Based on the present study 99mTc-Dendrimer G2-Methionin or 99mTc-DTPA-(Methionine)1 have potentials to be used in tumor molecular imaging as well as cancer therapy in future.

Prevention of amyloidosis by chemical compounds is a potential therapeutic strategy in Alzheimer’s, prion and other neurodegenerative diseases. Regularly branched dendrimers and less regular hyperbranched polymers have been suggested as promising inhibitors of amyloid aggregation. As demonstrated in our previous studies, some widely used dendrimers (PAMAM, PPI) could not only inhibit amyloid aggregation in solution but also dissolve mature fibrils. In this study we have performed computer simulation of polylysine dendrimers of 3rd and 5th generations (D3 and D5) and analysed the effect of these dendrimers and some hyperbranched polymers on a lysine base (HpbK) on aggregation of amyloid peptide in solution. The effects of dendrimers on cell viability and their protective action against Aβ-induced cytotoxicity and alteration of K+channels was also analysed using human neuroblastoma SH-SY5Y cells. In addition, using fluorescence microscopy, we analysed uptake of FITC-conjugated D3 by SH-SY5Y cells and its distribution in the brain after intraventricular injections to rats. Our results demonstrated that dendrimers D3 and D5 inhibited amyloid aggregation in solution while HpbK enhanced amyloid aggregation. Cell viability and patch-clamp studies have shown that D3 can protect cells against Aβ-induced cytotoxicity and K+channel modulation. In contrast, HpbK had no protective effect against Aβ. Fluorescence microscopy studies demonstrated that FITC-D3 accumulates in the vacuolar compartments of the cells and can be detected in various brain structures and populations of cells after injections to the brain. As such, polylysine dendrimers D3 and D5 can be proposed as compounds for developing antiamyloidogenic drugs.

The mechanisms responsible for the switch of prostate cancer from androgen-sensitive (AS) to androgen-insensitive (AI) form are not well understood. Regulation of androgen receptor (AR), through which androgens control the expression of genes involved in prostate cells proliferation, migration and death also involves its cross-talk with the other signaling pathways, transcription factors and coregulatory proteins, such as β-catenin. With the aim to determine their possible contribution in triggering the switch from AS to AI form, which occurs upon androgen deprivation therapy - AR, Akt and β-catenin expression were knocked-down with respective siRNAs. Treatment of LNCaP prostate cells with siRNA for AR significantly reduced their proliferation (45-70%), expression of nuclear β- catenin, cyclin-D1, cyclin-G1, c-Myc as well as activity of metalloproteinases (MMPs) -2,-7,-9 and cell migration. Surprisingly, after longer (over 72 hrs) silencing of AR in LNCaP cells, elevated levels of p-Akt were detected and enhanced proliferation as well as expression of nuclear β-catenin, cyclin-D1, c-Myc and activity of MMPs were observed. Such effects were not observed in either PC-3 or DU145 AI cells. However, silencing of Akt and /or β-catenin in those as well as in LNCaP cells led to their decreased proliferation and migration. Our findings suggest that in prostate cancer cells, either AR or Akt signaling prevails, depending on their initial androgen sensitivity and its availability. In AI prostate cancer cells, Akt takes over the role of AR and more effectively contributes through the same signaling molecule, β-catenin, to AI cancer progression.