Nuclear localization signals (NLS) are generally short peptides that act as a signal fragment that mediates the transport of proteins from the cytoplasm into the nucleus. This NLS-dependent protein recognition, a process necessary for cargo proteins to pass the nuclear envelope through the nuclear pore complex, is facilitated by members of the importin superfamily. Here, we summarized the types of NLS, focused on the recently reported related proteins containing nuclear localization signals, and briefly summarized some mechanisms that do not depend on nuclear localization signals into the nucleus.

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Citrus polymethoxyflavone tangeretin (5,6,7,8,4'-pentamethoxyflavone, TAN) displays multiple biological activities, but previous reports showed that TAN failed to induce MCF-7 human breast cancer cells apoptosis. Herein, we prepared 5-acetyl-6,7,8,4'-tetramethylnortangeretin (5-ATAN), and evaluated its cytotoxicity on MCF-7 cells. 5-ATAN revealed stronger cytotoxicity than that of parent TAN in the growth inhibition of MCF-7 cells. 5-ATAN induced apoptosis via both caspase-independent and -dependent pathways, in which 5-ATAN induced the translocation of apoptosis inducing factor and phosphorylation of H2AX as well as poly (ADP-ribose) polymerase cleavage, caspase-3 activation. However, 5-ATAN did not affect extrinsic markers caspase-8, BID, and FADD. Further, 5-ATAN induced the loss of mitochondrial membrane potential (Δψm) by regulating the Bax/Bcl-2 ratio. Loss of Δψm led to the mitochondrial release of cytochrome c which triggered activation of caspase-9. In conclusion, these data indicate that 5-ATAN plays pro-apoptotic cytotoxic roles in MCF-7 cells through both caspase-dependent intrinsic apoptosis and caspase-independent apoptosis pathways.

Glutathione (GSH), γ-Glu-Cys-Gly, is one of the most abundant small non-protein thiol molecules in mammalian tissues, particularly in the liver. Although glutathione is present in thiol-reduced (GSH) and disulfide oxidized (GSSG) forms, the predominant form is GSH and its content can exceed 10 mmol/L in liver cells. As an important intracellular reductant, GSH has many biological functions in cells. Its major function is as an anti-oxidant as it can protect proteins from oxidation by reversible posttranslational modification (glutathionylation) and decrease reactive oxygen species-mediated damage. However, it does have numerous other functions, including to chelate metal irons; enhance the absorption of iron, selenium and calcium; participate in lipid and insulin metabolism; regulate cellular events such as gene expression, DNA and protein synthesis, cell proliferation and apoptosis, redox-dependent signal transduction pathways, cytokine production and the immune response; and control protein glutathionylation. Therefore, GSH plays important roles in cell survival and health, and an imbalance in the GSH level can lead to many diseases. In this review, we provide an overview of the function of GSH in mammalian cells and discuss future research of GSH. Glutathione (GSH) is the most abundant intracellular nonprotein thiol, and was first identified by Hopkins [1] and Kendall [2]. About 85%-90% GSH is freely distributed in the cytosol, but it is also present in organelles including the mitochondria, peroxisomes, nuclear matrix and endoplasmic reticulum (ER). It is mainly produced in cells to help protect against oxidative stress. It is well known that peroxides are highly unstable and readily form very damaging free radicals upon decomposition, and these radical groups are the main source of oxidative stress. As an important reductant, GSH can scavenge these free radicals using the thiol bridge (i.e., via the sulfhydryl [-SH] groups), which generates water and yields the oxidized form of glutathione (GSSG). Via the reducing power of its free sulfhydryl (-SH), GSH plays a key role in many cellular processes. For example, Pan and Berk's research group [3] has shown that glutathionylation regulates TNF-α-induced capase-3 cleavage and apoptosis, and that capase-3 glutathionylation attenuates caspase-3 cleavage and inhibits endothelial cell death. This research group also showed that protein glutathionylation can regulate the cell death pathway [3]. Because GSH can also detoxify metal irons and ROS, it can participate in protein and DNA synthesis, and affect cell proliferation. In the following review, we will describe the characteristics, functions and future prospects for GSH.

Salmonella enterica serovar Typhimurium can inject effector proteins into host cells via type III secretion systems (T3SSs). These effector proteins modulate a variety of host transcriptional responses to facilitate bacterial growth and survival. Here we show that infection of host cells with S. Typhimurium specifically induces the ubiquitination of tumor necrosis factor receptor-associated factor 6 (TRAF6). This TRAF6 ubiquitination is triggered by the Salmonella pathogenicity island 1 (SPI-1) T3SS effectors SopB and SopE2. We also demonstrate that TRAF6 is involved in the SopB/ SopE2-induced phosphorylation of signal transducer and activator of transcription 3 (STAT3), a signaling event conducive to the intracellular growth of S. Typhimurium. Specifically, TRAF6 mediates lysine-63 ubiquitination within the Src homology 2 (SH2) domain of STAT3, which is an essential step for STAT3 membrane recruitment and subsequent phosphorylation in response to S. Typhimurium infection. TRAF6 ubiquitination participates in STAT3 phosphorylation rather than serving as only a hallmark of E3 ubiquitin ligase activation. Our results reveal a novel strategy in which S. Typhimurium T3SS effectors broaden their functions through the activation of host proteins in a ubiquitination-dependent manner to manipulate host cells into becoming a Salmonella-friendly zone.KEYWORDS Salmonella enterica serovar Typhimurium, TRAF6, ubiquitination, STAT3, Salmonella effectors I nfection with bacterial pathogens often induces host inflammatory responses (1), which are initiated by the recognition of microbial products, collectively known as pathogen-associated molecular patterns (PAMPs). PAMPs are recognized by Toll-like receptors (TLRs) or cytosolic NOD-like receptors (NLRs) (1, 2), which activate nuclear factor B (NF-B) and mitogen-activated protein kinases (MAPKs) and induce the production of proinflammatory cytokines critical for host defenses (3, 4). To overcome host defenses, some pathogens have developed strategies to dampen the host innate immune response by inactivating MAPKs or NF-B signaling. For instance, the Shigella flexneri phosphothreonine lyase OspF, which is injected into host cells by a type III secretion system (T3SS), inactivates the innate immune response by dephosphorylating MAPKs (5). The Yersinia protein YopJ/YopP, a T3SS effector protein containing both