Expression of glutamate receptor subunits in human cancers.

Emerging evidence suggests a role for glutamate and its receptors in the biology of cancer. This study was designed to systematically analyze the expression of ionotropic and metabotropic glutamate receptor subunits in various human cancer cell lines, compare expression levels to those in human brain tissue and, using electrophysiological techniques, explore whether cancer cells respond to glutamate receptor agonists and antagonists. Expression analysis of glutamate receptor subunits NR1-NR3B, GluR1-GluR7, KA1, KA2 and mGluR1-mGluR8 was performed by means of RT-PCR in human rhabdomyosarcoma/medulloblastoma (TE671), neuroblastoma (SK-NA-S), thyroid carcinoma (FTC 238), lung carcinoma (SK-LU-1), astrocytoma (MOGGCCM), multiple myeloma (RPMI 8226), glioma (U87-MG and U343), lung carcinoma (A549), colon adenocarcinoma (HT 29), T cell leukemia cells (Jurkat E6.1), breast carcinoma (T47D) and colon adenocarcinoma (LS180). Analysis revealed that all glutamate receptor subunits were differentially expressed in the tumor cell lines. For the majority of tumors, expression levels of NR2B, GluR4, GluR6 and KA2 were lower compared to human brain tissue. Confocal imaging revealed that selected glutamate receptor subunit proteins were expressed in tumor cells. By means of patch-clamp analysis, it was shown that A549 and TE671 cells depolarized in response to application of glutamate agonists and that this effect was reversed by glutamate receptor antagonists. This study reveals that glutamate receptor subunits are differentially expressed in human tumor cell lines at the mRNA and the protein level, and that their expression is associated with the formation of functional channels. The potential role of glutamate receptor antagonists in cancer therapy is a feasible goal to be explored in clinical trials.

Glutamate receptors in pediatric tumors of the central nervous system.

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system (CNS). Experimental evidence indicates that glutamate receptor antagonists may limit tumor growth. This study explores expression of glutamate receptor subunits in pediatric CNS tumors. Samples from eight ependymomas, four glioblastomas, six medulloblastomas and eight low grade astrocytomas were analysed. RNA was used for semiquantitative and quantitative RT-PCR. We examined expression of NMDA receptor subunits NR1-NR3B, AMPA receptor subunits GluR1-GluR4, kainate receptor subunits GluR5-GluR7, KA1, KA2 and metabotropic receptor subunits mGluR1-8. Paraffin embedded samples were immunohistochemically stained for selected subunits. All glutamate receptor subunits were differentially expressed in the tumors examined. Expression of NR2D, NR3A, KA1, GluR4, mGluR1, mGluR4, mGluR5 and mGluR6 was higher in the high grade tumors compared to human brain (HB). In low grade astrocytomas expression of glutamate receptor subunits was comparable or lower than in HB. Immunohistochemistry revealed expression of several glutamate receptor subunit proteins in tumor specimen. This study demonstrates expression of glutamate receptor subunits in pediatric CNS tumors. Together with experimental evidence indicating that interference with glutamate signalling may suppress tumor growth, our findings suggest that adjunctive treatment with glutamate receptor modulators may be a feasible therapeutic option for pediatric patients with CNS tumors.

The human intronless melanocortin 4-receptor gene is NMD insensitive.

Nonsense-mediated decay (NMD) is a phylogenetically widely conserved mechanism that contributes to the fidelity of gene expression. NMD inhibits the accumulation of nonsense- or frameshift-mutated mRNA and thus minimizes the synthesis of truncated proteins with potential dominant negative effects. Yeast and higher eukaryotes use somewhat diverse mechanisms to promote NMD and to discriminate between premature and physiological translation termination codons. NMD in yeast involves the binding of specific RNA-binding proteins to cis-acting exonic elements. In contrast, NMD of the intron-containing genes of higher eukaryotes is splicing-dependent. Here, we investigated the NMD sensitivity of nonsense-mutated transcripts of the naturally intronless human melanocortin 4-receptor (MC4-R) gene. Nonsense-mutated variants of MC4-R transcripts are stable and express truncated proteins that are detectable in the lysates of transfected cells. Thus, the naturally intronless MC4-R gene and probably many other intronless genes fail to be monitored by the NMD pathway.