Abstract 4999: Glutamate receptors in cancer: Stable expression of GluR6 induces growth arrest and senescence in hepatocellular carcinoma HCC 7703 cells

Applying a functional strategy, we identified a gene, SEN6A, located at 6q21, which restores senescence in breast tumor cells. Human genome database search identified SEN6A as GluR6, a kainate type - ionotropic glutamate receptor gene. Kainate receptors constitute a family of ionotropic glutamate receptors that plays a role in the rapid synaptic transmission in the central nervous system. Glutamate receptors have been implicated in several pathophysiological conditions including Epilepsy, Schizophrenia and Parkinson's disease. We cloned the GluR6 gene by RT-PCR and investigated the structure, splicing pattern and its role in the cell senescence and cancer. We show that GluR6A, a splice variant is specifically expressed in brain, and four other variants; namely, GluR6B, GluR6C, GluR6D and GluR6E are expressed in other tissues and fibroblast cell lines. Genomic analysis and cloning of the sequence preceding the transcribed region led to the identification of two tissue specific promoters designated as neuronal promoter Pn and non-neuronal promoter Pnn. GluR6 is expressed in most tissues and its expression is elevated in late passage cells, relative to young human cells. The gene is either turned off or expressed at a reduced level in a large number of tumor cell lines. The chromosomal location of GluR6 at 6q21, a LOH hot spot in multiple cancers, points to its potential role in the etiology of cancer. The analysis for mutations in 22 cancer cell lines revealed a point mutation in exon 15 and deletion of exon 6 in two hepatoma cell lines. Open reading frame of GluR6 was cloned in a retroviral expression vector, in frame with EGFP tag, where EGFP is added after the signal peptide. In this vector GluR6-EGFP gene is expressed as a single transcript, from a CMV promoter, along with puromycin gene for selection. Ecotopic expression of GluR6 (variants A-E) induces growth arrest and senescence in hepatoma HCC7703 cells, and induce premature senescence in normal fibroblasts. Clones of GluR6 transfer cells display doubling time of 72-96 hours that gradually increases to 3-4 days ending in complete growth arrest and senescence. In contrast the transfer of empty vector or a construct carrying truncated GluR6 gene did not affect the growth of HCC7703 or normal cells. These result show that GluR6 is involved in the cascade of signaling circuits that govern cell growth and differentiation. Future studies are designed to investigate signaling pathways responsive to the expr