Sigma-2 receptors are specifically localized to lipid rafts in rat liver membranes

Interactions between 3,4-methylenedioxymethamphetamine and sigma1 receptors

Methamphetamine and 3,4-methylenedioxymethamphetamine (MDMA) are structurally similar and represent a serious and growing health threat. Earlier studies in our laboratory have shown that methamphetamine interacts with sigma receptors and that antagonism of these receptors can attenuate methamphetamine-induced locomotor stimulation and neurotoxicity. However, no research exists which characterizes the interaction between sigma receptors and MDMA. Therefore, the goal of the present study was to determine whether sigma receptors are involved in the actions of MDMA. In the first part of the study, competition and saturation binding assays were performed to measure the interaction of MDMA with sigma receptors. The receptor binding assays revealed that MDMA interacts preferentially with the sigma(1) subtype, as compared to the sigma(2) subtype, and that this interaction occurs in a competitive manner. The second part of the study focused on behavioral measurements in male, Swiss Webster mice to determine whether a selective sigma(1) receptor antagonist, BD1063 (1-[2-(3,4-dichlorophenyl)ethyl]-4-methylpiperazine, 0-30 mg/kg, i.p.) could attenuate the locomotor stimulant actions of MDMA (0-50 mg/kg, i.p.). BD1063 alone had no effect on locomotor activity, but dose-dependently attenuated the locomotor stimulant effects of MDMA and produced a significant shift to the right in the MDMA dose response curve. Together, the data support the functional relevance of the interaction of MDMA with sigma(1) receptors, and suggest that these receptors are involved in the stimulant actions of MDMA.

Effects of UMB24 and (+/-)-SM 21, putative sigma2-preferring antagonists, on behavioral toxic and stimulant effects of cocaine in mice

Earlier studies have demonstrated that antagonism of sigma1 receptors attenuates the convulsive, lethal, locomotor stimulatory and rewarding actions of cocaine in mice. In contrast, the contribution of sigma2 receptors is unclear because experimental tools to selectively target this subtype are unavailable. To begin addressing this need, we characterized UMB24 (1-(2-phenethyl)-4-(2-pyridyl)-piperazine) and (+/-)-SM 21 (3alpha-tropanyl-2-(4-chorophenoxy)butyrate) in receptor binding and behavioral studies. Receptor binding studies confirmed that UMB24 and (+/-)-SM 21 display preferential affinity for sigma2 over sigma1 receptors. In behavioral studies, pretreatment of Swiss Webster mice with UMB24 or (+/-)-SM 21 significantly attenuated cocaine-induced convulsions and locomotor activity, but not lethality. When administered alone, (+/-)-SM 21 produced no significant effects compared to control injections of saline, but UMB24 had locomotor depressant actions. Together, the data suggest that sigma2 receptor antagonists have the potential to attenuate some of the behavioral effects of cocaine, and further development of more selective, high affinity ligands are warranted.

The expression and functional characterization of sigma (sigma) 1 receptors in breast cancer cell lines

Sigma (sigma) receptors have been implicated in cancer. However, to date there is little molecular data demonstrating the role of sigma1 receptors in cancer. Expression of sigma1 receptors in various human cancer cell lines in comparison to non-cancerous cell lines was investigated, using real time RT-PCR and by western blotting with a sigma1 receptor specific antibody. Our results indicate that cancer cells express higher levels of sigma1 receptors than corresponding non-cancerous cells. Localization of the sigma1 receptor was investigated in MDA-MB-231 cells by immunocytochemistry and confocal microscopy, expression was visualized predominantly at the cell periphery. We have tested the effect of sigma1 and sigma2 drugs and a sigma1 receptor silencing construct on various aspects of the metastatic process on two breast cell lines of different metastatic potential and a normal breast cell line. Both sigma1 and sigma2 drugs and the sigma1 receptor silencing construct had effects on proliferation and adhesion for breast cancer cell lines, compared to a non-cancerous breast cell line. This data suggests sigma1 receptor plays a role in proliferation and adhesion of breast cancer cells. Therefore, it is likely to be a potential target for the diagnosis and therapy of breast cancer.

Cloning, characterization, expression and comparative analysis of pig Golgi membrane sphingomyelin synthase 1

Pig sphingomyelin synthase 1 (SMS1) cDNA was cloned, characterized and compared to the human ortholog. Porcine protein consists of 413 amino acids and displays a 97% sequence identity with human protein. A phylogenic tree of proteins reveals that porcine SMS1 is more closely related to bovine and rodent proteins than to human. Analysis of protein mass was higher than the theoretical prediction based on amino acid sequence suggesting a kind of posttranslational modification. Quantitative representation of tissue distribution obtained by real-time RT-PCR showed that it was widely expressed although important variations in levels were obtained among organs. Thus, the cardiovascular system, especially the heart, showed the highest value of all the tissues studied. Regional differences of expression were observed in the central nervous system and intestinal tract. Analysis of the hepatic mRNA and protein expressions of SMS1 following turpentine treatment revealed a progressive decrease in the former paralleled by a decrease in the protein concentration. These findings indicate the variation in expression in the different tissues might suggest a different requirement of Golgi sphingomyelin for the specific function in each organ and a regulation of the enzyme in response to turpentine-induced hepatic injury.

Is the σ2Receptor a Histone Binding Protein?

Starting from the high affinity sigma(2) receptor ligand 2, (PB28), we synthesized amino derivative 4 and coupled it to an NHS-ester activated sepharose stationary phase column to elute a crude protein prepared by lysed human SK-N-SH neuroblastoma cells. We characterized the SDS-PAGE gel electrophoresis stained bands by MALDI-MS and LC-MS-MS analysis. The MASCOT MS-MS ion search program led to the identification of the protein components. The six eluted proteins had a molecular weight ranging from 13 kDa to 26 kDa and were human histone proteins. A human 40S ribosomal protein S3 (SwissProt accession number: P23396) was also identified as a comigrated band. The human histone proteins that were characterized were H3.3A histone (NCBI accession number: 51859376), H2B histone (NCBI accession number: 1568557), H2A.5 histone (NCBI accession number: 70686), H1 (NCBI accession number: 22770677), and H2.1 histone (SwissProt accession number: P16403). These results disclosed a dual hypothesis about the sigma(2) receptor, that is, that it is formed by histones or that the sigma(2) ligands also bind histone proteins.

Sigma1 and sigma2 receptor binding affinity and selectivity of SA4503 and fluoroethyl SA4503

SA4503, a potent sigma(1) receptor agonist, is reported as having 103-fold higher affinity for sigma(1) (IC(50) = 17.4 nM) than sigma(2) (IC(50) = 1,784 nM) sites in guinea pig brain membranes. Modest structural changes appear to have major effects on sigma(1)/sigma(2) selectivity. The fluoroethyl analog, FE-SA4503, is described as having high primary affinity for sigma(2) sites (IC(50) = 2.11 nM) and a weaker interaction with sigma(1) sites (IC(50) = 6.48 nM). SA4503 and FE-SA4503 have been radiolabeled for PET studies, and both bind selectively to sigma(1) receptors in animal and human brain in vivo. We prepared SA4503 and FE-SA4503 as reference compounds for radioligand development efforts. In our hands, SA4503 is 14-fold selective for sigma(1) (K(i) = 4.6 nM) over sigma(2) (K(i) = 63.1 nM) sites in guinea pig brain homogenates. Further, FE-SA4503 exhibits the same 14-fold selectivity for sigma(1) (K(i) = 8.0 nM) over sigma(2) (K(i) = 113.2 nM) receptors. The main differences from previously reported values stem from sigma(2) affinity determinations. This protocol, displacement of [(3)H]DTG binding to sigma(2) sites using (+)-pentazocine (200 nM) to mask sigma(1) sites, was validated by the proper rank order of sigma(2) inhibitory potencies shown by a panel of additional ligands: ifenprodil > haloperidol > DTG >> (+)-pentazocine. Robust Pearson correlation (r = 1.0, P = 0.002; slope = 0.97) was observed for our pK(i) values against those from a prior study by others. The findings have bearing on structure-activity relationships for this active series, and on conclusions that might be drawn from experiments relying upon defined sigma(1)/sigma(2) binding selectivity.

Effective tumor cell death by sigma-2 receptor ligand siramesine involves lysosomal leakage and oxidative stress

Acquired resistance to classic caspase-mediated apoptosis is a common problem for the treatment of human cancer. Here, we show that siramesine, a novel sigma-2 receptor ligand, effectively induces caspase-independent programmed cell death in immortalized and transformed cells of various origins. Siramesine-treated tumor cells displayed increased levels of reactive oxygen species, lysosomal membrane permeabilization, chromatin condensation, and shrinkage and detachment of cells. Lipid antioxidants (alpha-tocopherol and gamma-tocopherol), but not other tested antioxidants (butylated hydroxyanisol or N-acetyl cysteine), effectively inhibited siramesine-induced morphologic changes and cell death. Cathepsin B inhibitors (CA-074-Me and R-2525) conferred similar, but less pronounced protection, whereas ectopic expression of antiapoptotic protein Bcl-2, lack of wild-type p53 as well as pharmacologic inhibitors of caspases (zVAD-fmk, DEVD-CHO, and LEHD-CHO), calpains (PD150606), and serine proteases (N-tosyl-L-phenylalanine chloromethyl ketone and pefabloc) failed to protect cells against siramesine-induced death. Importantly, transformation of murine embryonic fibroblasts with activated c-src or v-Ha-ras oncogenes greatly sensitized them to siramesine-induced cytotoxicity. Furthermore, p.o. administration of well-tolerated doses of siramesine had a significant antitumorigenic effect in orthotopic breast cancer and s.c. fibrosarcoma models in mice. These results present siramesine as a promising new drug for the treatment of tumors resistant to traditional therapies.

Involvement of sigma (sigma) receptors in the acute actions of methamphetamine: receptor binding and behavioral studies

Methamphetamine interacts with sigma (sigma) receptors, suggesting that the drug produces some of its physiological and behavioral effects through these sites. Therefore, in the present report, receptor binding and pharmacological studies were performed to characterize the interaction between methamphetamine and sigma receptors. Of the two major sigma receptor subtypes, sigma1 and sigma2, competition binding studies showed that methamphetamine has a 22-fold preferential affinity for the sigma1 subtype. Saturation binding studies using the sigma1 selective radioligand [3H]+-pentazocine showed that in the presence of methamphetamine, there was a significant change in Kd, but not Bmax, suggesting competitive interactions. In behavioral studies, pretreatment of Swiss Webster mice with the sigma1 receptor antagonists, BD1063 or BD1047, significantly attenuated the locomotor stimulatory effects of methamphetamine. Mice that were administered an antisense oligodeoxynucleotide to down-regulate brain sigma1 receptors also exhibited a reduced locomotor stimulatory response to methamphetamine, as compared to control mice receiving mismatch oligonucleotides. Together, the data suggest that sigma1 receptors are involved in the acute actions of methamphetamine and that antagonism of this subtype is sufficient to prevent the locomotor stimulatory effects of methamphetamine.

Developmental partitioning of myelin basic protein into membrane microdomains

Specific membrane microdomains (including lipid rafts) exist in myelin but have not been fully characterized. Myelin basic protein (MBP) maintains the compactness of the myelin sheath and is highly posttranslationally modified. Thus, it has been suggested that MBP might also have other functions, e.g., in signal transduction. Here, the distribution of MBP and its modified forms was studied, spatially and temporally, by detailed characterization of membrane microdomains from developing and mature bovine myelin. Myelin membranes were extracted with three different detergents (Brij 96V, CHAPS, or Triton X-100) at 4 degrees C. The detergent-resistant membranes (DRMs), representing coalesced lipid rafts, were isolated as low-buoyant-density fractions on a sucrose density gradient. These myelin rafts were disrupted when cholesterol was depleted with methyl-beta-cyclodextrin. The use of CHAPS detergent led to enrichment of several myelin proteins, including phospho-Thr97-MBP, in the DRMs from mature myelin. Citrullinated and methylated MBP remained in "nonraft" microdomains. In contrast, the DRMs from early myelin were enriched in Golli-MBP, Fyn, Lyn, and CNP. The localization of various proteins in DRMs was further supported by the colocalization of these lipid raft components in cultured mouse oligodendrocytes. Thus, there is a developmental regulation of posttranslationally modified forms of MBP into specific membrane microdomains.