Mechanism of the induction of endoplasmic reticulum stress by the anti-cancer agent, di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone (Dp44mT): activation of PERK/eIF2α, IRE1α, ATF6 and calmodulin kinase

The endoplasmic reticulum (ER) plays a major role in the synthesis, maturation and folding of proteins and is a critical calcium (Ca2+) reservoir. Cellular stresses lead to an overwhelming accumulation of misfolded proteins in the ER, leading to ER stress and the activation of the unfolded protein response (UPR). In the stressful tumor microenvironment, the UPR maintains ER homeostasis and enables tumor survival. Thus, a novel strategy for cancer therapeutics is to overcome chronically activated ER stress by triggering pro-apoptotic pathways of the UPR. Considering this, the mechanisms by which the novel anti-cancer agent, Dp44mT, can target the ER stress response pathways were investigated in multiple cell-types. Our results demonstrate that the cytotoxic chelator, Dp44mT, which forms redox-active metal complexes, significantly: (1) increased ER stress-associated pro-apoptotic signaling molecules (i.e., p-eIF2α, ATF4, CHOP); (2) increased IRE1α phosphorylation (p-IRE1α) and XBP1 mRNA splicing; (3) reduced expression of ER stress-associated cell survival signaling molecules (e.g., XBP1s and p58IPK); (4) increased cleavage of the transcription factor, ATF6, which enhances expression of its downstream targets (i.e., CHOP and BiP); and (5) increased phosphorylation of CaMKII that induces apoptosis. In contrast to Dp44mT, the iron chelator, DFO, which forms redox-inactive iron complexes, did not affect BiP, p-IRE1α, XBP1 or p58IPK levels. This study highlights the ability of a novel cancer therapeutic (i.e., Dp44mT) to target the pro-apoptotic functions of the UPR via cellular metal sequestration and redox stress. Assessment of ER stress-mediated apoptosis is fundamental to the understanding of the pharmacology of chelation for cancer treatment

Redox cycling metals: Pedaling their roles in metabolism and their use in the development of novel therapeutics

Essential metals, such as iron and copper, play a critical role in a plethora of cellular processes including cell growth and proliferation. However, concomitantly, excess of these metal ions in the body can have deleterious effects due to their ability to generate cytotoxic reactive oxygen species (ROS). Thus, the human body has evolved a very well-orchestrated metabolic system that keeps tight control on the levels of these metal ions. Considering their very high proliferation rate, cancer cells require a high abundance of these metals compared to their normal counterparts. Interestingly, new anti-cancer agents that take advantage of the sensitivity of cancer cells to metal sequestration and their susceptibility to ROS have been developed. These ligands can avidly bind metal ions to form redox active metal complexes, which lead to generation of cytotoxic ROS. Furthermore, these agents also act as potent metastasis suppressors due to their ability to up-regulate the metastasis suppressor gene, N-myc downstream regulated gene 1. This review discusses the importance of iron and copper in the metabolism and progression of cancer, how they can be exploited to target tumors and the clinical translation of novel anti-cancer chemotherapeutics

Unraveling the mysteries of serum albumin—more than just a serum protein

Serum albumin is a multi-functional protein that is able to bind and transport numerous endogenous and exogenous compounds. The development of albumin drug carriers is gaining increasing importance in the targeted delivery of cancer therapy, particularly as a result of the market approval of the paclitaxel-loaded albumin nanoparticle, Abraxane®. Considering this, there is renewed interest in isolating and characterizing albumin-binding proteins or receptors on the plasma membrane that are responsible for albumin uptake. Initially, the cellular uptake and intracellular localization of albumin was unknown due to the large confinement of the protein within the vascular and interstitial compartment of the body. Studies have since assessed the intracellular localization of albumin in order to understand the mechanisms and pathways responsible for its uptake, distribution and catabolism in multiple tissues, and this is reviewed herein

Membrane transport and intracellular sequestration of novel thiosemicarbazone chelators for the treatment of cancer

Iron is a critical nutrient for DNA synthesis and cellular proliferation. Targeting iron in cancer cells using specific chelators is a potential new strategy for the development of novel anticancer agents. One such chelator, 2-benzoylpyridine 4-ethyl-3-thiosemicarbazone (Bp4eT), possesses potent and selective anticancer activity (J Med Chem 50:3716-3729, 2007). To elucidate the mechanisms of its potent antitumor activity, Bp4eT was labeled with ¹⁴C. Its efficacy was then compared with the 14C-labeled iron chelator pyridoxal isonicotinoyl hydrazone (PIH), which exhibits low anticancer activity. The ability of these ligands to permeate the cell membrane and their cellular retention was examined under various conditions using SK-N-MC neuroepithelioma cells. The rate of [ ¹⁴C]PIH uptake into cells was significantly (p < 0.001) lower than that of [ ¹⁴C]Bp4eT at 37°C, indicating that the increased hydrophilicity of [ 14C]PIH reduced membrane permeability. In contrast, the efflux of [ ¹⁴C]PIH was significantly (p < 0.05) higher than that of [ ¹⁴C]Bp4eT, leading to increased cellular retention of [ ¹⁴C]Bp4eT. In addition, the uptake and release of the 14C-labeled chelators was not reduced by metabolic inhibitors, indicating that these processes were energy-independent. No significant differences were evident in the uptake of [ ¹⁴C]Bp4eT at 37 or 4°C, demonstrating a temperature-independent mechanism. Furthermore, adjusting the pH of the culture medium to model the tumor microenvironment did not affect [ ¹⁴C]Bp4eT membrane transport. It can be concluded that [ ¹⁴C]Bp4eT more effectively permeated the cell membrane and evaded rapid efflux in contrast to [ ¹⁴C]PIH. This property, in part, accounts for the more potent anticancer activity of Bp4eT relative to PIH.10 page(s

Duodenal Cytochrome b (DCYTB) in Iron Metabolism: An Update on Function and Regulation

Iron and ascorbate are vital cellular constituents in mammalian systems. The bulk-requirement for iron is during erythropoiesis leading to the generation of hemoglobin-containing erythrocytes. Additionally; both iron and ascorbate are required as co-factors in numerous metabolic reactions. Iron homeostasis is controlled at the level of uptake; rather than excretion. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance non-heme iron absorption in the gut; ascorbate regulates iron homeostasis. The involvement of ascorbate in dietary iron absorption extends beyond the direct chemical reduction of non-heme iron by dietary ascorbate. Among other activities; intra-enterocyte ascorbate appears to be involved in the provision of electrons to a family of trans-membrane redox enzymes; namely those of the cytochrome b561 class. These hemoproteins oxidize a pool of ascorbate on one side of the membrane in order to reduce an electron acceptor (e.g., non-heme iron) on the opposite side of the membrane. One member of this family; duodenal cytochrome b (DCYTB); may play an important role in ascorbate-dependent reduction of non-heme iron in the gut prior to uptake by ferrous-iron transporters. This review discusses the emerging relationship between cellular iron homeostasis; the emergent “IRP1-HIF2α axis”; DCYTB and ascorbate in relation to iron metabolism

Letter to the Editor: “Analysis of the Interaction of Dp44mT with Human Serum Albumin and Calf Thymus DNA Using Molecular Docking and Spectroscopic Techniques”

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