N. Natsvlishvili., L. Shanshiashvili, D.Mikeladze
1 Institute of Chemical Biology, Ilia State University, Tbilisi, Georgia
2 Ivane Beritashvili Center of Experimental Biomedicine, Tbilisi, Georgia
During inflammatory states, activated innate cells, as well as tumor cells, release amino acid glutamate that induces innate cell migration by activating class I/5 metabotropic glutamate receptors (mGluR1/5). Mutation of mGluR1/5 detected in several tumors triggers a downstream signaling cascade leading to activation of Rho family GTPase, Rac1, and Rac2. Rac overexpression in tumors may play a role in cancer progression. Targeting the Rac pathway is an approach to cancer treatment. The sigma1R (Sig1R) is an endoplasmic reticulum (ER)-resident molecular chaperone expressed at the mitochondria-associated ER membranes (MAM). Cell treatment with sigma antagonists causes rounding, detachment and growth inhibition of several cancer cells, however, whether Sig1R protects cancer cells from death remains to be elucidated. To investigate the interplay between SigR1, Rac, and mGluR5, immunoprecipitation experiments were performed. Sig1R interacted with the Rac1 and Rac2. This interaction was increased in the presence of sigma-agonists, decreased by sigma-antagonists. The sig1-Rac interaction was enhanced by GppNHp, suggesting that active Rac interacted with Sig1R. Specific association/dissociation of Sig1R with an inositol-3-phosphate receptor (IP3R) and BiP was also found. p21-activated kinases (PAK) downstream effectors of Rac. PAK can phosphorylate various regulatory proteins, including pro-apoptotic Bad. Sigma-agonists increase PAK-dependent phosphorylation, causing dissociation of Bad from mitochondria, thus sigma-ligands could operate through SigR1/Rac complex by changing the PAK activity. Rac1 is be involved in the assembly and activation of ROS-generating enzyme – NADPH oxidase (NOX). ROS are increased in the presence of sigma-agonist (pentazocine), and sigma-antagonist (haloperidol) eliminated this effect, suggesting that that sigma-ligands could operate through SigR1/Rac complex by changing the NOX activity. Besides, we have found that SigR1 was immunoprecipitated with mGluR5. We next transfected mGluR5 cDNAs into macrophages. Immunostimulants, like LPS, induced the association of SigR1 with Rac only in non-transformed cells., whereas sigma-agonists increased the binding of SigR1 to Rac in mGluR5-overexpressing macrophages that had a decreased sensitivity to the immunostimulants. Our data suggest that Sig1R dissociates from IP3R and then associates with Rac1 and Rac2, and through this association/dissociation, sigma ligands can regulate Rac activity. The intracellular glutamate receptor may participate in the induction of SigR1-Rac association. This association could lead to activation of downstream effectors (Pak and NOX) of Rac, to initiation of actin rearrangement and production of ROS. This may induce the M1→M2 transition and immunosuppression contributing to the progression of tumorigenesis.