Induction of Bcl-2 by functional regulation of G-protein coupled receptors protects from oxidative glutamate toxicity by increasing glutathione.

Glutamate treatment depletes hippocampal HT22 cells of glutathione, which renders the cells incapable to reduce reactive oxygen species and ultimately cumulates in cell death by oxidative stress. HT22 cells resistant to glutamate displayed increased phosphorylation of cAMP-response-element binding (CREB) and decreased ERK1/2 suggestive of differences in signal transmission. We investigated the amount of candidate G-protein-coupled receptors involved in this resistance and found an increase in mRNA for receptors activated by the vasoactive intestinal peptide VIP (VPAC2, 12.6-fold) and glutamate like the metabotropic glutamate receptor mGlu1 (5.3-fold). Treating cells with VIP and glutamate led to the same changes in protein phosphorylation observed in resistant cells and induced the proto-oncogene Bcl-2. Bcl-2 overexpression protected by increasing the amount of intracellular glutathione and Bcl-2 knockdown by small interfering RNAs (siRNA) increased glutamate susceptibility of resistant cells. Other receptors upregulated in this paradigm might represent useful targets in the treatment of neurological diseases associated with oxidative stress.

The constitutively active orphan G-protein-coupled receptor GPR39 protects from cell death by increasing secretion of pigment epithelium-derived growth factor.

GPR39 is a constitutively active orphan G-protein-coupled receptor capable of increasing serum response element-mediated transcription. We found GPR39 to be up-regulated in a hippocampal cell line resistant against diverse stimulators of cell death and show that its overexpression protects against oxidative and endoplasmic reticulum stress, as well as against direct activation of the caspase cascade by Bax overexpression. In contrast, silencing GPR39 rendered cells more susceptible to cell death. An array analysis of transcripts induced by GPR39 revealed up-regulation of RGS16 (inhibitor of G-protein signaling 16), which suggested coupling to Galpha(13) and induction of serum response element-mediated transcription by the small GTPase RhoA. In line with this, co-expression of GPR39 with RGS16, dominant-negative RhoA, or serum response factor abolished cell protection, whereas overexpression of the serum response factor protected from cell death. Further downstream the signaling cascade, GPR39 overexpression leads to increased secretion of the cytoprotective pigment epithelium-derived growth factor (PEDF). Medium conditioned by cells overexpressing GPR39 contained 4-fold more PEDF, and when stripped off it lost most but not all of its protective properties. We conclude that GPR39 is a novel inhibitor of cell death, which might represent a therapeutic target with implications for processes involving apoptosis and endoplasmic reticulum stress like cancer, ischemia/reperfusion injury, and neurodegenerative disease.