Heavy-Metals-Mediated Phospholipids Scrambling by Human Phospholipid Scramblase 3: A Probable Role in Mitochondrial Apoptosis

Abstract

Human phospholipid scramblases are a family of four homologous transmembrane proteins (hPLSCR1–4) mediating phospholipids (PLs) translocation in plasma membrane upon Ca2+ activation. hPLSCR3, the only homologue localized to mitochondria, plays a vital role in mitochondrial structure, function, maintenance, and apoptosis. Upon Ca2+ activation, hPLSCR3 mediates PL translocation at the mitochondrial membrane enhancing t-bid-induced cytochrome c release and apoptosis. Mitochondria are important target organelles for heavy-metals-induced apoptotic signaling cascade and are the central executioner of apoptosis to trigger. Pb2+ and Hg2+ toxicity mediates apoptosis by increased reactive oxygen species (ROS) and cytochrome c release from mitochondria. To discover the role of hPLSCR3 in heavy metal toxicity, hPLSCR3 was overexpressed as a recombinant protein in Escherichia coli Rosetta (DE3) and purified by affinity chromatography. The biochemical assay using synthetic proteoliposomes demonstrated that hPLSCR3 translocated aminophospholipids in the presence of micromolar concentrations of Pb2+ and Hg2+. A point mutation in the Ca2+-binding motif (F258V) led to a ∼60% loss in the functional activity and decreased binding affinities for Pb2+ and Hg2+ implying that the divalent heavy metal ions bind to the Ca2+-binding motif. This was further affirmed by the characteristic spectra observed with stains-all dye. The conformational changes upon heavy metal binding were monitored by circular dichroism, intrinsic tryptophan fluorescence, and light-scattering studies. Our results revealed that Pb2+ and Hg2+ bind to hPLSCR3 with higher affinity than Ca2+ thus mediating scramblase activity. To summarize, this is the first biochemical evidence for heavy metals binding to the mitochondrial membrane protein leading to bidirectional translocation of PLs specifically toward phosphatidylethanolamine.

Mitochondrial Oxidative Stress Is the General Reason for Apoptosis Induced by Different-Valence Heavy Metals in Cells and Mitochondria

“…Cell research. MeHg similarly to Hg 2+ induces mitochondrial apoptosis and oxidative stress, mediating an increase in ROS production, a decrease in cell antioxidant defense, and cytochrome c release from mitochondria [151,152]. Hg 2+ induced apoptosis with caspase-3 activation, a cell viability decrease, mitochondrial integrity decline, and intracellular ATP depletion.…”

Di-(2-Ethylhexyl) Phthalate and Microplastics Induced Neuronal Apoptosis through the PI3K/AKT Pathway and Mitochondrial Dysfunction

“…18 Previous studies have shown that changes in mitochondrial integrity affect changes in the ATP content and ATP enzyme activity. 19 Hg 2+ and Pb 2+ cause apoptosis by enabling the mitochondria to produce large amounts of ROS and by releasing cytochrome-C. 20 plays a vital role in glycogen metabolism and cell proliferation and regulation. 21 GSK-3β regulates apoptosis in SH-SY5Y cells.…”