Dual regulation of Stat1 and Stat3 by the tumor suppressor protein PML contributes to interferon α-mediated inhibition of angiogenesis.

IFNs are effective in inhibiting angiogenesis in preclinical models and in treating several angioproliferative disorders. However, the detailed mechanisms of IFNα-mediated anti-angiogenesis are not completely understood. Stat1/2/3 and PML are IFNα downstream effectors and are pivotal regulators of angiogenesis. Here, we investigated PML's role in the regulation of Stat1/2/3 activity. In Pml knock-out (KO) mice, ablation of Pml largely reduces IFNα angiostatic ability in Matrigel plug assays. This suggested an essential role for PML in IFNα's anti-angiogenic function. We also demonstrated that PML shared a large cohort of regulatory genes with Stat1 and Stat3, indicating an important role of PML in regulating Stat1 and Stat3 activity. Using molecular tools and primary endothelial cells, we demonstrated that PML positively regulates Stat1 and Stat2 isgylation, a ubiquitination-like protein modification. Accordingly, manipulation of the isgylation system by knocking down USP18 altered IFNα-PML axis-mediated inhibition of endothelial cell migration and network formation. Furthermore, PML promotes turnover of nuclear Stat3, and knockdown of PML mitigates the effect of LLL12, a selective Stat3 inhibitor, on IFNα-mediated anti-angiogenic activity. Taken together, we elucidated an unappreciated mechanism in which PML, an IFNα-inducible effector, possess potent angiostatic activity, doing so in part by forming a positive feedforward loop with Stat1/2 and a negative feedback loop with Stat3. The interplay between PML, Stat1/Stat2, and Stat3 contributes to IFNα-mediated inhibition of angiogenesis, and disruption of this network results in aberrant IFNα signaling and altered angiostatic activity.

Identification and characterization of the interferon-beta-mediated p53 signal pathway in human peripheral blood mononuclear cells.

The relationship between the p53 signal pathway and the response of human peripheral blood mononuclear cells (PBMC) to interferon (IFN)-beta has hitherto not been examined. Using an oligonucleotide microarray, we found differential expression of at least 70 genes involved in the p53 signal pathway, including p53, which regulate cell proliferation and cell death following stimulation with IFN-beta. We verified our observations on a limited set of p53-regulated genes at the transcriptional and translational levels. We also examined the consequences of the activation of the p53 signal pathway by IFN-beta in PBMC. When cultured in the presence of T-cell mitogens, IFN-beta restricted the entry of lymphocytes from the G0/G1 phase to the S phase and reduced the number of cells in the G2 phase. The addition of IFN-beta alone did not increase apoptosis. However, in the presence of actinomycin D, a DNA-damaging agent, addition of IFN-beta enhanced the susceptibility of PBMC to apoptosis. These observations suggest that, in spite of the activation of a number of mutually overlapping pathways mediating cell death, cell cycle arrest was the most evident consequence of IFN-beta signalling in PBMC.

Interferons and viruses: signaling for supremacy.

Interferon (IFN)-alpha and IFN-beta are critical mediators of host defense against microbial challenges, directly interfering with viral infection and influencing both the innate and adaptive immune responses. IFNs exert their effects in target cells through the activation of a cell-surface receptor, leading to a cascade of signaling events that determine transcriptional and translation regulation. Understanding the circuitry associated with IFN-mediated signal transduction that leads to a specific biological outcome has been a major focus of our laboratory. Through the efforts of graduate students, postdoctoral fellows, a skilled research technologist, and important collaborations with investigators elsewhere, we have provided some insights into the complexity of the IFN system-and the elegance and simplicity of how protein-protein interactions define biological function.