Inositol 1,4,5-trisphosphate and ryanodine receptors mobilize calcium from a common functional pool in human U373 MG cells

The σ1 receptor agonist (+)-pentazocine increases store-operated Ca2+ entry in MCF7σ1 and SK-N-SH cell lines

BACKGROUND

The intracellular [Ca²⁺] is modulated by σ receptors. An important component of the cellular machinery governing the intracellular [Ca²⁺] is Store-Operated Calcium Entry (SOCE). Here we want to investigate whether ligands of σ receptors affect SOCE.

METHODS

The intracellular [Ca²⁺] was monitored, with the fluorescent Ca²⁺-sensitive probe Fura-2, in four cell lines with a different expression of σ receptors, namely MCF7 (expressing σ₁ receptors with a low density and overexpressing σ₂ receptors), MCF7σ₁ (overexpressing σ₁ receptors), SK-N-SH, and HT-29.

RESULTS

When thapsigargin was used to deplete intracellular Ca²⁺ stores, in a Ca²⁺-free incubation medium, the Ca²⁺ influx (following Ca²⁺ re-addition) was significantly increased by 1μM (+)-pentazocine (σ₁ receptor agonist) in MCF7σ₁ (by 22.5%) and SK-N-SH (by 45.6%), but not in HT-29 and MCF7 cells. We have used, as a second approach, the "Mn²⁺ quenching" protocol. In MCF7σ₁ cells, after thapsigargin treatment, the fluorescence quenching induced by Mn²⁺ influx (evidence of Ca²⁺ influx) was significantly increased (by 25.8%) by 1μM (+)-pentazocine, significantly decreased (by 18.0%) by BD1063 (σ₁ receptor antagonist), and not affected by the presence of both ligands. These effects were not observed in MCF7 cells. Finally, in MCF7 cells, 1μM PB28 (σ₂ receptor agonist), did not affect both the Ca²⁺ response after Ca²⁺ re-addition and the fluorescence quenching induced by Mn²⁺ influx.

CONCLUSIONS

We propose that the σ₁ receptor agonist (+)-pentazocine increases SOCE in MCF7σ₁ and SK-N-SH cell lines. The σ₂ receptor agonist PB28 does not affect SOCE in MCF7 cells.

Calcium-release channels in paramecium. Genomic expansion, differential positioning and partial transcriptional elimination

The release of Ca²⁺ from internal stores is a major source of signal Ca²⁺ in almost all cell types. The internal Ca²⁺ pools are activated via two main families of intracellular Ca²⁺-release channels, the ryanodine and the inositol 1,4,5-trisphosphate (InsP₃) receptors. Among multicellular organisms these channel types are ubiquitous, whereas in most unicellular eukaryotes the identification of orthologs is impaired probably due to evolutionary sequence divergence. However, the ciliated protozoan Paramecium allowed us to prognosticate six groups, with a total of 34 genes, encoding proteins with characteristics typical of InsP₃ and ryanodine receptors by BLAST search of the Paramecium database. We here report that these Ca²⁺-release channels may display all or only some of the characteristics of canonical InsP₃ and ryanodine receptors. In all cases, prediction methods indicate the presence of six trans-membrane regions in the C-terminal domains, thus corresponding to canonical InsP₃ receptors, while a sequence homologous to the InsP₃-binding domain is present only in some types. Only two types have been analyzed in detail previously. We now show, by using antibodies and eventually by green fluorescent protein labeling, that the members of all six groups localize to distinct organelles known to participate in vesicle trafficking and, thus, may provide Ca²⁺ for local membrane-membrane interactions. Whole genome duplication can explain radiation within the six groups. Comparative and evolutionary evaluation suggests derivation from a common ancestor of canonical InsP₃ and ryanodine receptors. With one group we could ascertain, to our knowledge for the first time, aberrant splicing in one thoroughly analyzed Paramecium gene. This yields truncated forms and, thus, may indicate a way to pseudogene formation. No comparable analysis is available for any other, free-living or parasitic/pathogenic protozoan.

Dual inhibition of sodium-mediated proton and calcium efflux triggers non-apoptotic cell death in malignant gliomas

Malignant glioma cells maintain an elevated intracellular pH (pH(i)) within hypoxic-ischemic tumor microenvironments through persistent activation of sodium-proton transport (McLean et al., 2000). Amiloride has been reported to selectively kill human malignant glioma cell lines but not primary astrocytes (Hegde et al., 2004). While amiloride reduces pH(i) of malignant gliomas by inhibiting isoform 1 of sodium-proton exchange (NHE1), direct acidification was shown to be cytostatic rather than cytotoxic. At cytotoxic concentrations, amiloride has multiple drug targets including inhibition of NHE1 and sodium-calcium exchange. Amiloride's glioma cytotoxicity can be explained, at least in part, by dual inhibition of NHE1 and of Na(+)-dependent calcium efflux by isoform 1.1 of the sodium-calcium exchanger (NCX1.1), which increases [Ca(2+)](i) and initiates glioma cell demise. As a result of persistent NHE1 activity, cytosolic free levels of sodium ([Na(+)](i)) in U87 and C6 glioma cells are elevated 3-fold, as compared with normal astrocytes. Basal cytosolic free calcium levels ([Ca(2+)](i)) also are increased 5-fold. 2', 4'-dichlorobenzamil (DCB) inhibits the sodium-dependent calcium transporter (NCX1.1) much more potently than NHE1. DCB was employed in a concentration-dependent fashion in glioma cells to selectively inhibit the forward mode of NCX1.1 at ≤1μM, while dually inhibiting both NHE1 and NCX1.1 at ≥20μM. DCB (1μM) was not cytotoxic to glioma cells, while DCB (20μM) further increased basal elevated levels of [Ca(2+)](i) in glioma cells that was followed by cell demise. Cariporide and SEA0400 are more selective inhibitors of NHE1 and NCX1.1 than amiloride or DCB, respectively. Individually, Cariporide and SEA0400 are not cytotoxic, but in combination induced glioma cell death. Like amiloride, the combination of Cariporide and SEA0400 produced glioma cell death in the absence of demonstrable caspase activation.

The sigma-2 receptor agonist PB28 inhibits calcium release from the endoplasmic reticulum of SK-N-SH neuroblastoma cells

In this paper we demonstrate that PB28 abolishes the Ca2+ release through the inositol 1,4,5-trisphosphate (InsP3) receptors and ryanodine receptors in SK-N-SH cells. Sigma receptors are divided into the subtypes sigma-1 and sigma-2, which are expressed in tumor cell lines and characterized by distinct pharmacological profiles. The sigma-1 receptor has been recently cloned, whereas the sigma-2 receptor is less well characterized. The endogenous ligand(s) of both subtypes remain unclear. In isolated guinea pig ileum, PB28 inhibits the contraction induced by carbachol dose dependently and in a non-competitive manner. In SK-N-SH cells PB28 challenge does not affect the intracellular Ca2+ concentration but incubation with PB28 for 45 min abolishes the cytosolic Ca2+ increases evoked by carbachol or histamine. This effect, not sensitive to cycloheximide, is caused by direct inhibition of the InsP3 receptors, since PB28 abolishes the response elicited by InsP3 administration in permeabilized SK-N-SH cells. Finally, incubation for 45 min with PB28 also abolishes the cytosolic Ca2+ increase evoked by caffeine.