Endoplasmic Reticulum Stress Induced by Toxic Elements-a Review of Recent Developments

Endoplasmic reticulum of all eukaryotic cells is a membrane-bound organelle. Under electron microscope it appears as parallel arrays of "rough membranes" and a maze of "smooth vesicles" respectively. It performs various functions in cell, i.e., synthesis of proteins to degradation of xenobiotics. Bioaccumulation of drugs/chemicals/xenobiotics in the cytosol can trigger ER stress. It is recognized by the accumulation of unfolded or misfolded proteins in the lumen of ER. Present review summarizes the present status of knowledge on ER stress caused by toxic elements, viz arsenic, cadmium, lead, mercury, copper, chromium, and nickel. While inorganic arsenic may induce various glucose-related proteins, i.e., GRP78, GRP94 and CHOP, XBP1, and calpains, cadmium upregulates GRP78. Antioxidants like ascorbic acid, NAC, and Se inhibit the expression of UPR. Exposure to lead also changes ER stress related genes, i.e., GRP 78, GRP 94, ATF4, and ATF6. Mercury too upregulates these genes. Nickel, a carcinogenic element upregulates the expression of Bak, cytochrome C, caspase-3, caspase-9, caspase-12, and GADD 153. Much is not known on ER stress caused by nanoparticles. The review describes inter-organelle association between mitochondria and ER. It also discusses the interdependence between oxidative stress and ER stress. A cross talk amongst different cellular components appears essential to disturb pathways leading to cell death. However, these molecular switches within the signaling network used by toxic elements need to be identified. Nevertheless, ER stress especially caused by toxic elements still remains to be an engaging issue.