A Kaposi's sarcoma-associated herpesvirus protein that forms inhibitory complexes with type I interferon receptor subunits, Jak and STAT proteins, and blocks interferon-mediated signal transduction.

Type I interferons (IFNs) are important mediators of innate antiviral defense and function by activating a signaling pathway through their cognate type I receptor (IFNAR). Here we report that lytic replication of Kaposi's sarcoma-associated herpesvirus (KSHV) efficiently blocks type I IFN signaling and that an important effector of this blockade is the viral protein RIF, the product of open reading frame 10. RIF blocks IFN signaling by formation of inhibitory complexes that contain IFNAR subunits, the Janus kinases Jak1 and Tyk2, and the STAT2 transcription factor. Activation of both Tyk2 and Jak1 is inhibited, and abnormal recruitment of STAT2 to IFNAR1 occurs despite the decrement in Tyk2 activity. As a result of these actions, phosphorylation of both STAT2 and STAT1 is impaired, with subsequent failure of ISGF3 accumulation in the nucleus. The presence in the viral genome of potent inhibitors of type I IFN signaling, along with several viral genes that block IFN induction, highlights the importance of the IFN pathway in the control of this human tumor virus infection.

The phytochemical piceatannol induces the loss of CBL and CBL-associated proteins.

Piceatannol is a naturally occurring bioactive stilbene with documented antileukemic properties. It has been extensively used as a Syk-selective protein tyrosine kinase inhibitor for the study of various signaling pathways. Herein, we show that the hydroxystilbene, piceatannol, and related catechol ring-containing compounds are able to induce the loss of the Cbl family of proteins. Normal cellular Cbl-regulatory mechanisms were not involved in this process. Screening of a small library of piceatannol-like compounds indicated that aromaticity and a catechol ring were required for the induction of Cbl loss. Further examination of these two chemical properties showed that the oxidative conversion of the catechol ring of piceatannol into a highly reactive O-benzoquinone was the cause of piceatannol-induced Cbl loss. Characterization of the Cbl selectivity of piceatannol-induced protein loss revealed that this compound was also able to induce the functional loss of specific Cbl-associated proteins involved in signaling pathways commonly associated with cancer. This work uncovers a new, piceatannol-dependent effect and shows a novel way in which this phenomenon can be exploited to inhibit disease-associated signaling pathways.

Isolation and characterization of a novel, transforming allele of the c-Cbl proto-oncogene from a murine macrophage cell line.

The c-Cbl proto-oncogene acts as an E3 ubiquitin ligase via its RING finger domain to negatively regulate activated cellular signal transduction pathways. We have identified an aberrant Cbl-protein of approximately 95 kDa, which we have called p95Cbl, from the murine reticulum sarcoma cell-line, J-774. Cloning of the p95Cbl cDNA revealed that it contains a deletion resulting in the loss of 111 amino acids, eliminating two critical tyrosine residues in the linker region as well as the entire RING finger domain. p95Cbl displays a propensity for its interaction with the Src-family kinase Hck over cellular Cbl expressed in the same cells. Like its wildtype counterpart, p95Cbl is inducibly tyrosine phosphorylated in response to Fcgamma receptor engagement on hematopoietic cells, however this phosphorylation is sustained beyond that of cellular Cbl. NIH3T3 fibroblasts stably expressing p95Cbl acquire the typical refractile morphology associated with cellular transformation and form colonies in a focus-formation assay. The exogenously expressed mutant protein is constitutively phosphorylated in fibroblasts and partitions into the particulate fraction of cells, while cellular Cbl is exclusively cytoplasmic. p95Cbl is a novel, oncogenic mutant of the c-Cbl proto-oncogene, which might act in a dominant negative fashion to prolong normal cellular signaling responses by interfering with the down-regulation of activated signaling complexes through c-Cbl.