[show abstract][hide abstract] ABSTRACT: Many proteins are known to undergo small ubiquitin-related modifier (SUMO) modification by an E1-, E2-, and E3-dependent ligation process. Recognition that protein inhibitor of activated signal transducers and activators of transcription (STATs) (PIAS) proteins are SUMO E3 ligases raised the possibility that STATs may also be regulated by SUMO modification. Consistent with this possibility, a SUMO-ylation consensus site (PsiKxE; Psi indicates hydrophobic residue, and x indicates any residue) was identified in Stat1 (ie, (702)IKTE(705)), but not in other STATs. Biochemical analysis confirmed that Stat1 K(703) could be SUMO modified in vitro. Mutation of this critical lysine (ie, Stat1(K703R)) yielded a protein that, when expressed in Stat1(-/-) mouse embryonic fibroblasts (MEFs), exhibited enhanced DNA binding and nuclear retention. This was associated with modest changes in transcriptional and antiviral activity. However, mutation of the second critical residue in the SUMO consensus site, E(705) (ie, Stat1(E705A)), yielded a protein with wild-type DNA binding, nuclear retention, and transcriptional and antiviral activity. Similar observations were made when these mutants were expressed in primary Stat1(-/-) macrophages. These observations suggest that although Stat1 can uniquely be SUMO-ylated in vitro, this modification is unlikely to play an important role in regulating Stat1 activity in vivo.
[show abstract][hide abstract] ABSTRACT: The transcription factor NF-kappaB is an important regulator of homeostatic growth and inflammation. Although gene-targeting studies have revealed important roles for NF-kappaB, they have been complicated by component redundancy and lethal phenotypes. To examine the role of NF-kappaB in endothelial tissues, Tie2 promoter/enhancer-IkappaBalpha(S32A/S36A) transgenic mice were generated. These mice grew normally but exhibited enhanced sensitivity to LPS-induced toxemia, notable for an increase in vascular permeability and apoptosis. Moreover, B16-BL6 tumors grew significantly more aggressively in transgenic mice, underscoring a new role for NF-kappaB in the homeostatic response to cancer. Tumor vasculature in transgenic mice was extensive and disorganized. This correlated with a marked loss in tight junction formation and suggests that NF-kappaB plays an important role in the maintenance of vascular integrity and response to stress.
Journal of Clinical Investigation 12/2006; 116(11):2955-63. · 12.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Alpha/beta interferon (IFN-alpha/beta) triggers antiviral and antiproliferative responses in target cells through modulation of gene expression. The JAK-STAT pathway is the major mediator of these biological effects through the activation of the transcription factors STAT1 and STAT2, and gene ablation studies have demonstrated that both STAT1 and STAT2 are required for most antiviral responses induced by IFN-alpha/beta. However, additional signaling pathways are also activated by IFN. Here, we show that these additional pathways provoke a proliferative response in activated T lymphocytes. While activation of IFN-stimulated gene factor 3 produces a dominant inhibitory signal capable of overriding the mitogenic response, absence of either STAT1 or STAT2 leads to a proliferative response to IFN. Growth stimulation by IFN-alpha/beta is independent of other STAT proteins, particularly of STAT3, since T lymphocytes from STAT1-STAT3 double-knockout mice are growth stimulated by IFN-alpha/beta treatment. IFN-alpha/beta can cooperate with numerous T-cell mitogens, including interleukin-2 (IL-2), IL-4, IL-7, and IL-12, and can contribute to the rapid restoration of the thymus following glucocorticoid-mediated ablation. These results underscore the complexity of the cellular response to IFN and suggest that the ultimate outcome of IFN action results from a balance between growth-inhibitory and -stimulatory effects.
Molecular and Cellular Biology 08/2005; 25(13):5456-65. · 5.37 Impact Factor
[show abstract][hide abstract] ABSTRACT: Numerous studies have implicated C-reactive protein (CRP) and the acute phase response (APR) in the development of atherosclerotic heart disease. Interleukin (IL)-6, which regulates both CRP expression and the APR, has also been identified as a risk factor for heart disease. To more directly evaluate the role of IL-6 in the development of atherosclerosis, IL-6 knockout mice were crossed with atherosclerosis prone LDL receptor (LDL-R) knockout mice. Lesion development was evaluated on Chow, Western type and Paigen diets. As anticipated, the Paigen diet stimulated the expression of APR genes in LDL-R[-/-] mice, but not IL-6[-/-]/LDL-R[-/-] mice. Despite this difference in acute phase response, only modest and statistically not significant differences were noted in the development of atherosclerotic lesions in LDL-R[-/-] and IL-6[-/-]/LDL-R[-/-] mice. These observations suggest that IL-6 and the acute phase response may not play as significant role in atherogenesis as other studies have indicated.
[show abstract][hide abstract] ABSTRACT: Stat3 is the most pleiotropic member of the signal transducer and activator of transcription (STAT) family of transcription factors and mediates pivotal responses for the cytokine family. In resting cells, STATs, including Stat3, reside largely in the cytoplasm. Upon cytokine stimulation, they rapidly translocate to the nucleus, where they promote the expression of target genes. During the subsequent period of signal decay they are re-exported back to the cytoplasm in preparation for the next round of signaling. This process of nuclear export can be blocked by the fungal toxin leptomycin B (LMB). In contrast to what appears to be the case for Stat1, LMB treatment not only blocks the poststimulation export of Stat3 from the nucleus back to the cytoplasm, but also promotes the nuclear accumulation of Stat3 in resting cells. Remarkably, the LMB-dependent nuclear accumulation of Stat3 in resting cells is independent of tyrosine phosphorylation, highlighting the existence of a "basal" signaling pathway. Subsequent studies identified three nuclear export signal (NES) elements. Two of these elements, Stat3(306-318) and Stat3(404-414), corresponded to those recently identified in Stat1, and a third, Stat3(524-535), is novel. Stat3(306-318) appears to be important in the rapid nuclear export seen after stimulation (poststimulation export), whereas the Stat3(404-414) and Stat3(524-535) play a more important role in regulating basal nuclear export. In summary, these studies indicate that the process of Stat3 nuclear export is dependent on multiple NES elements.
Journal of Clinical Investigation 03/2003; 111(4):553-9. · 12.81 Impact Factor