Interactions between Epstein-Barr virus and the cell cycle control machinery

Epstein-Barr virus (EBV) persists in the majority of the world's human population. In the majority of cases the infection is asymptomatic, but EBV is associated with a number of human diseases, such as infectious mononucleosis, Burkitt's lymphoma, nasopharyngeal carcinoma, Hodgkin's disease, gastric carcinomas and other lymphomas and lymphoproliferative diseases. In this review the evidence linking EBV with these diseases is reviewed together with recent advances in understanding the interactions between EBV and the cell cycle control machinery.

Kinase suppressor of Ras is ceramide-activated protein kinase

A proline-directed serine/threonine ceramide-activated protein (CAP) kinase mediates transmembrane signaling through the sphingomyelin pathway. CAP kinase reportedly initiates proinflammatory TNF alpha action by phosphorylating and activating Raf-1. The present studies delineate kinase suppressor of Ras (KSR), identified genetically in Caenorhabditis elegans and Drosophila, as CAP kinase. Mouse KSR, like CAP kinase, renatures and autophosphorylates as a 100-kDa membrane-bound polypeptide. KSR overexpression constitutively activates Raf-1. TNF alpha or ceramide analogs markedly enhance KSR autophosphorylation and its ability to complex with, phosphorylate, and activate Raf-1. In vitro, low nanomolar concentrations of natural ceramide stimulate KSR to autophosphorylate, and transactivate Raf-1. Other lipid second messengers were ineffective. Moreover, Thr269 the Raf-1 site phosphorylated by CAP kinase, is also recognized by KSR. Thus, by previously established criteria, KSR appears to be CAP kinase.

Phosphorylation of Raf by ceramide-activated protein kinase

The sphingomyelin pathway, initiated by hydrolysis of sphingomyelin to ceramide and stimulation of a Ser/Thr ceramide-activated protein (CAP) kinase, mediates tumour necrosis factor-alpha (TNF-alpha) and interleukin-1 beta action. CAP kinase is membrane-bound and proline-directed, recognizing the minimal substrate motif Thr-Leu-Pro. TNF may use the sphingomyelin pathway to signal Raf1 to activate the MAP kinase cascade. Evidence shows that cytoplasmic Raf1 binds to GTP-ras upon cellular stimulation, is recruited to the plasma membrane, and activated. How membrane-bound Raf1 is activated is uncertain, but regulation of its kinase activity may involve its phosphorylation. Specific Raf kinases, however, have not hitherto been identified. Here we report that CAP kinase phosphorylates Raf1 on Thr 269, increasing its activity towards MEK (MAP kinase or ERK kinase). Moreover, in intact HL-60 cells, CAP kinase complexes with Raf1 and, in response to TNF and ceramide analogues, phosphorylates and activates Raf1, implicating CAP kinase as a link between the TNF receptor and Raf1.

IL-1 beta inhibits adipocyte formation in human long-term bone marrow culture

Addition of recombinant interleukin-1 beta (IL-1 beta) to human long-term bone marrow culture (LTBMC) was found to have a profound effect on adipocyte formation. Control cultures developed 133 +/- 58 adipocytes/flask compared with 3 +/- 2 adipocytes/flask in IL-1 beta-treated flasks (mean +/- SEM, n = 6 for both groups). In terms of cell number, morphology and colony forming cell, IL-1 beta did not have significant effects on LTBMC, although IL-1 beta slightly enhanced colony-stimulating activity of LTBMC-conditioned medium. To further analyze the effect of IL-1 beta on adipocytes, stromal cells were passaged to reduce hemopoietic cell content, and marrow fibroblasts were converted to adipocytes following addition of hydrocortisone. In both types of assay, IL-1 beta again reduced adipocyte formation: control 1095 +/- 76 adipocytes/flask, IL-1 beta-treated 7 +/- 2 adipocytes/flask (n = 3 for both groups). When preformed adipocytes were treated with IL-1 beta for 1 week, the number of lipid-laden cells decreased to 41 +/- 13% of its value before treatment compared to 87 +/- 9% for untreated controls. The addition of neutralizing concentrations of anti-TNF did not inhibit these effects of IL-1 beta. These results suggest that 1) adipocyte formation is not essential for healthy LTBMC growth, or at least IL-1 beta is an effective substitute for adipocytes; 2) IL-1 beta by itself does not significantly enhance hemopoiesis in LTBMC, and this has implications for clinical trials with this cytokine; 3) IL-1 beta produces major changes in stromal cell development, and 4) overgrowth of fat in marrow may reflect defective cytokine regulation in vivo.