信号转导与转录活化蛋白STAT3在肿瘤发生中的作用

STAT3是信号转导与转录活化蛋白（STAT）家族的一个重要成员,也是一个可以被多种调控细胞增殖、分化、发育、存活和炎症的细胞因子、生长因子及肿瘤蛋白所激活的关键信号转导蛋白。在多种肿瘤细胞、细胞性状转化过程和肿瘤形成过程中可以检测到组成性激活的STAT3。我们从STAT3在肿瘤细胞中组成性激活的机制、STAT3作为转录因子对肿瘤细胞调控的机制等方面对STAT3在肿瘤发生中的作用进行简要综述。     

Sustained Submicromolar H2O2 Levels Induce Hepcidin via Signal Transducer and Activator of Transcription 3 (STAT3)

The peptide hormone hepcidin regulates mammalian iron homeostasis by blocking ferroportin-mediated iron export from macrophages and the duodenum. During inflammation, hepcidin is strongly induced by interleukin 6, eventually leading to the anemia of chronic disease. Here we show that hepatoma cells and primary hepatocytes strongly up-regulate hepcidin when exposed to low concentrations of H₂O₂ (0.3–6 μm), concentrations that are comparable with levels of H₂O₂ released by inflammatory cells. In contrast, bolus treatment of H₂O₂ has no effect at low concentrations and even suppresses hepcidin at concentrations of >50 μm. H₂O₂ treatment synergistically stimulates hepcidin promoter activity in combination with recombinant interleukin-6 or bone morphogenetic protein-6 and in a manner that requires a functional STAT3-responsive element. The H₂O₂-mediated hepcidin induction requires STAT3 phosphorylation and is effectively blocked by siRNA-mediated STAT3 silencing, overexpression of SOCS3 (suppressor of cytokine signaling 3), and antioxidants such as N-acetylcysteine. Glycoprotein 130 (gp130) is required for H₂O₂ responsiveness, and Janus kinase 1 (JAK1) is required for adequate basal signaling, whereas Janus kinase 2 (JAK2) is dispensable upstream of STAT3. Importantly, hepcidin levels are also increased by intracellular H₂O₂ released from the respiratory chain in the presence of rotenone or antimycin A. Our results suggest a novel mechanism of hepcidin regulation by nanomolar levels of sustained H₂O₂. Thus, similar to cytokines, H₂O₂ provides an important regulatory link between inflammation and iron metabolism.     

Signal Transducer and Activator of Transcription 3 (STAT3) Mediates Amino Acid Inhibition of Insulin Signaling through Serine 727 Phosphorylation

Nutrient overload is associated with the development of obesity, insulin resistance, and type II diabetes. High plasma concentrations of amino acids have been found to correlate with insulin resistance. At the cellular level, excess amino acids impair insulin signaling, the mechanisms of which are not fully understood. Here, we report that STAT3 plays a key role in amino acid dampening of insulin signaling in hepatic cells. Excess amino acids inhibited insulin-stimulated Akt phosphorylation and glycogen synthesis in mouse primary hepatocytes as well as in human hepatocarcinoma HepG2 cells. STAT3 knockdown protected insulin sensitivity from inhibition by amino acids. Amino acids stimulated the phosphorylation of STAT3 at Ser⁷²⁷, but not Tyr⁷⁰⁵. Replacement of the endogenous STAT3 with wild-type, but not S727A, recombinant STAT3 restored the ability of amino acids to inhibit insulin signaling, suggesting that Ser⁷²⁷ phosphorylation was critical for STAT3-mediated amino acid effect. Furthermore, overexpression of STAT3-S727D was sufficient to inhibit insulin signaling in the absence of excess amino acids. Our results also indicated that mammalian target of rapamycin was likely responsible for the phosphorylation of STAT3 at Ser⁷²⁷ in response to excess amino acids. Finally, we found that STAT3 activity and the expression of its target gene socs3, known to be involved in insulin resistance, were both stimulated by excess amino acids and inhibited by rapamycin. In conclusion, our study reveals STAT3 as a novel mediator of nutrient signals and identifies a Ser⁷²⁷ phosphorylation-dependent and Tyr⁷⁰⁵ phosphorylation-independent STAT3 activation mechanism in the modulation of insulin signaling.     

Infection of Human B Lymphocytes with Lymphocryptoviruses Related to Epstein-Barr Virus

Lymphocryptoviruses (LCVs) naturally infecting Old World nonhuman primates are closely related to the human LCV, Epstein-Barr virus (EBV), and share similar genome organization and sequences, biologic properties, epidemiology, and pathogenesis. LCVs can efficiently immortalize B lymphocytes from the autologous species, but the ability of a given LCV to immortalize B cells from other Old World primate species is variable. We found that LCV from rhesus monkeys did not immortalize human B cells, and EBV did not immortalize rhesus monkey B cells. In this study, baboon LCV could not immortalize human peripheral blood B cells but could readily immortalize rhesus monkey B cells. Thus, efficient LCV-induced B-cell immortalization across distant Old World primate species appears to be restricted by a species-specific block. To further characterize this species restriction, we first cloned the rhesus monkey LCV major membrane glycoprotein and discovered that the binding epitope for the EBV receptor, CD21, was highly conserved. Stable infections of human B cells with recombinant amplicons packaged in rhesus monkey or baboon LCV envelopes were also consistent with a species-restricted block occurring after virus binding and penetration. Transient infections of human B cells with simian LCV resulted in latent LCV EBNA-2 gene expression and activation of cell CD23 gene expression. EBV-immortalized human B cells could be coinfected with baboon LCV, and the simian virus persisted and replicated in human B cells. Thus, several lines of evidence indicate that the species restriction for efficient LCV-induced B-cell immortalization occurs beyond virus binding and penetration. This has important implications for the study of LCV infection in Old World primate models and for human xenotransplantation where simian LCVs may be inadvertently introduced into humans.     

STAT3与在肿瘤耐药中的研究进展

STAT3是信号转导与转录活化蛋白(STATs)家族的重要一员,是一种存在于胞浆并在激活后能够转入核内与DNA结合的蛋白家族,具有信号转导和转录调控双重功能。STAT3在多种肿瘤组织与细胞系中异常表达,并与肿瘤的增殖分化、细胞凋亡密切相关。肿瘤耐药是其治疗失败的重要原因,STAT3能够通过多种途径介导肿瘤耐药。因而,STAT3在近年的抗肿瘤研究中备受关注,成为肿瘤治疗的良好靶点,由传统药物与STAT3抑制剂组成的新型治疗方案使得肿瘤患者大大受益。然而,STAT3介导肿瘤耐药的机制还不是很明确,需要进一步研究。本文就近年来一些化疗药物和靶向药物耐药的发生,对STAT3介导耐药的作用进行综述。     

Cleavage and Secretion of Epstein-Barr Virus Glycoprotein 42 Promote Membrane Fusion with B Lymphocytes

Epstein-Barr virus (EBV) membrane glycoprotein 42 (gp42) is required for viral entry into B lymphocytes through binding to human leukocyte antigen (HLA) class II on the B-cell surface. EBV gp42 plays multiple roles during infection, including acting as a coreceptor for viral entry into B cells, binding to EBV glycoprotein H (gH) and gL during the process of membrane fusion, and blocking T-cell recognition of HLA class II-peptide complexes through steric hindrance. EBV gp42 occurs in two forms in infected cells, a full-length membrane-bound form and a soluble form generated by proteolytic cleavage that is secreted from infected cells due to loss of the N-terminal transmembrane domain. Both the full-length and the secreted gp42 forms bind to gH/gL and HLA class II, and the functional significance of gp42 cleavage is currently unclear. We found that in a virus-free cell-cell fusion assay, enhanced secretion of gp42 promoted fusion with B lymphocytes, and mutation of the site of gp42 cleavage inhibited membrane fusion activity. The site of gp42 cleavage was found to be physically distinct from the residues of gp42 necessary for binding to gH/gL. These results suggest that cleavage and secretion of gp42 are necessary for the process of membrane fusion with B lymphocytes, providing the first indicated functional difference between full-length and cleaved, secreted gp42.Epstein-Barr virus (EBV) is a large DNA virus belonging to the human gammaherpesvirus subfamily. EBV is orally transmitted through saliva and persists for the lifetime of its human host, establishing a latency reservoir in B lymphocytes with intermittent viral reactivation (1, 27). More than 90% of the world''s adult population is infected with EBV, although in healthy individuals, viral reactivation from latency is quickly controlled by the immune system. During primary infection and viral reactivation from latency, EBV infects epithelial cells as well as B lymphocytes (27). Primary infection with EBV can lead to development of infectious mononucleosis, and EBV has also been strongly associated with a number of human malignancies of epithelial and B-cell origin, including Burkitt''s lymphoma and nasopharyngeal carcinoma (4, 9, 10, 33, 36).EBV encodes a number of membrane glycoproteins important in a variety of viral processes, including entry of the virus into target host cells and virus-induced cell-cell fusion. The membrane glycoproteins necessary for fusion with both epithelial and B cells are glycoprotein B (gB), gH, and gL, and together, they form the core virus fusion machinery (7, 20, 24, 29). In addition to these glycoproteins, glycoprotein 42 (gp42) has been shown to play an essential role in membrane fusion with B cells (7, 18, 20). Attachment of EBV virions to B cells occurs through binding of the main envelope protein gp350/220 to CD21 (also known as complement receptor type 2) (5, 23, 34). This interaction enhances the efficiency of EBV infection of B cells but is not required for viral entry (12, 30). Antibodies to gp350/220 inhibit EBV infection of B cells but enhance infection of epithelial cells, possibly by facilitating the access of other viral glycoproteins to the epithelial cell membrane (35). Virus-cell membrane fusion is subsequently triggered by binding of gp42 to human leukocyte antigen (HLA) class II on the B-cell surface (6, 8, 11, 17, 31). Interestingly, gp42 appears to function as a switch of cellular tropism between epithelial and B cells. The presence of gp42 in the viral envelope is necessary for infection of B lymphocytes, and virions that are low in gp42 are better able to infect HLA class II-negative epithelial cells (3). Aside from its role in membrane fusion, gp42 plays a significant role in evasion of the host immune system. Gp42 binds to HLA class II-peptide complexes in infected cells, sterically hindering T-cell recognition of the complex by the T-cell receptor (25). This inhibition may allow EBV to delay detection by the host immune system.Two different mature forms of gp42 are produced by EBV-positive B lymphocytes in the lytic cycle (26). The first form is a full-length type II membrane protein, and the second is a truncated soluble form (s-gp42) (26). s-gp42 is generated by posttranslational cleavage (most likely mediated by a cellular protease resident in the endoplasmic reticulum) and is secreted (26). Both forms of gp42 associate with HLA class II intracellularly, and both inhibit HLA class II-restricted antigen presentation to T cells (26). Both forms of gp42 produced by EBV-positive B cells in the lytic cycle were found to be present in gH-gL-gp42 complexes, indicating that s-gp42 retains the ability to bind gH/gL (26). The physiological significance of s-gp42 is currently unclear, but this form has been suggested to function in infection and immune evasion, blocking EBV entry receptors on lytically infected B cells to prevent reinfection and neutralizing gp42-specific antibodies following its secretion from infected cells (26).Both forms of gp42 have been examined for their functions in mediating evasion from T-cell immunity through binding to HLA class II complexes (26), but the functions of the two forms of the protein in membrane fusion are unknown. To examine how each form of gp42 functions during membrane fusion, we have assayed the effect of gp42 cleavage site mutation on this process. Also, to distinguish residues important for gp42 cleavage from those necessary for association with gH/gL, we have constructed a number of fully secreted gp42 truncation mutants and examined their interaction with gH/gL and their ability to mediate fusion. Mutation or deletion of the gp42 cleavage site inhibited or eliminated cleavage of the protein, which had a direct effect on gp42 function in membrane fusion. An assay of N-terminal truncations of gp42 indicated that the region of gp42 necessary for cleavage was physically distinct from the region of gp42 necessary for association with gH/gL. We show that membrane association of gp42 has an inhibitory effect on gp42 function in membrane fusion and that increased secretion of gp42 stimulates membrane fusion in vitro. Cleavage of gp42 may be necessary for EBV gp42 to assume a functional position, interaction, or conformation for participation in membrane fusion.