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Theranostics 2020, Vol. 10, Issue 20 protein-containingmediumarecoveredwithalayer of mixed protein called protein corona [235]. The electrostatic interactions between proteins and nanoparticles contribute to the formation of protein corona [236]. Some proteins may undergo conformational changes during the formation of protein corona [235]. Protein corona has an important effect on the absorption, accumulation and subsequent behaviors of nanoparticles in cells [237]. It is proved that AgNPs with protein coronas enter cells via receptor-mediated endocytosis and subsequently induce mitochondrial dysfunction and cell apoptosis [238]. By comparing nanoparticles without protein coronas, it is realized that the formation of protein coronas around AgNPs can be a prerequisite for their cytotoxicity . Anticancer Mechanisms AgNPs have broad-spectrum anticancer activity via multiple mechanisms [21, 239, 240]. Numerous experiments in vitro and in vivo have proved that AgNPs can decrease the proliferation and viability of cancer cells. AgNPs can cause apoptosis and necrosis by destroying the ultrastructure of cancer cells, inducing the production of ROS and DNA damage [21, 241]. AgNPs can promote apoptosis by up- or down-regulating expression of key genes, such as p53 [242], and regulating essential signaling pathways, such as hypoxia-inducible factor (HIF) pathway [243]. Cancer cells treated with AgNPs may also show cell cycle arrest [160, 244]. Several cancer cells exposed to AgNPs undergo sub-G1 arrest and apoptosis. Besides, AgNPs can also reduce distant metastasis by inhibiting tumor cell migration and angiogenesis [28, 245]. Multiple anticancer mechanisms of AgNPs are described in Figure 4. In order to develop safe and effective anticancer agent, more mechanisms for anti- cancer effects of AgNPs remain to be explored. Here, we summarize the possible anticancer mechanisms of AgNPs both in vitro and in vivo. Ultrastructural Destructions of Cancer Cells Destruction of ultrastructures such as cell membranes and intracellular organelles leads to cell apoptosis and necrosis [21]. Tumor cells exhibit intact cell structure under light microscope, such as round nuclei, intact nuclear membrane, homogeneous chromatin, normal mitochondria and rough endoplasmic reticulum [40]. The ultrastructural changes of AgNPs-exposed tumor cells are in a dose- and time-dependent manner [246]. Generally, the higher the concentration of AgNPs and the longer the exposure time, the more serious the damage of cell ultrastructure. TEM observation showed that AgNPs- exposed cells are suffering morphological change or 9011 cytoplasmic organelle damage, and undergoing different death patterns: apoptosis, necrosis and autophagy [40]. Autophagosomes associated with apoptosis and necrosis are formed in the cytoplasm of AgNPs-treated tumor cells [247]. AgNPs promote autophagosome formation through the PtdIns3K pathway, and induce autophagy in tumor cells without inhibiting lysosomal function [22]. Structural and functional disruption of the actin cytoskeleton may be the cause of morphological deterioration of tumor cells exposed to AgNPs, and may be involved in inhibiting migration and invasion of tumor cells [248]. Free Ag+ released from AgNPs is involved in the destruction of cellular membranes. Ag+ released by AgNPs induces oxidation of glutathione, and increases lipid peroxidation in cellular membranes, resulting in cytoplasmic constituents leaking from damaged cells [249]. Our team found time-dependent morphological changes in cancer cells treated with F-AgÅPs, such as organelle compaction, nuclear fragmentation and cell blebbing [21]. Tumor cells exposed to AgNPs lose their typical shape due to pseudopod contraction, decreased cell adhesion and reduced cell density. Scanning electron microscopy analysis of AgNPs-treated tumor cells reveal spherical appearance, foamed membrane and shorten filopodia [248]. Tumor cells exposed to AgNPs show apoptotic cell characteristics such as loss of intact membrane, decreased contact with adjacent cells, condensed and detached from the culture plate [250]. ROS Production ROS are by-products of biological aerobic metabolism, including oxygen ions, peroxides and oxygenated free radicals [251]. ROS are highly active due to the presence of unpaired free electrons. ROS are controlled at a low level by normal cellular antioxidant defense mechanisms and do not affect the normal physiological activities of the cells. However, excessive ROS can produce oxidative stress that reduces the activity of biological macromolecules and damages subcellular organelles and DNA structures [252, 253]. Oxidative stress trigger lipid peroxidation, impaired mitochondrial function, amino acid oxidation in proteins, enzyme inactivation and DNA/RNA damage [233], which may lead to autophagy, apoptosis and necrosis of cancer cells. AgNPs distributed in tumor cells via endocytosis can result in autophagy and apoptosis through a variety of ROS-mediated stress responses. In addition, AgNPs-induced formation of ROS may affect cellular signal transduction pathways, which may participate in the activation of apoptosis [254]. For example, the mitochondrial function can be inhibited by AgNPs via disrupting mitochondrial respiratory chain, http://www.thno.orgPDF Image | Silver nanoparticles Synthesis medical applications safety
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