The immunosuppressant rapamycin has been shown previously to inhibit the G 1 /S transition in several cell types by prolonging the G 1 phase of the cell cycle. This process appears to be controlled, in part, by the rapamycin-sensitive FK506-binding protein-rapamycin-associated protein-p70 S6 kinase (p70 S6k ) pathway and the cyclin-dependent kinases (Cdk). We now show that in serum-stimulated NIH 3T3 cells, rapamycin treatment delays the accumulation of cyclin D1 mRNA during progression through G 1 . Rapamycin also appears to affect stability of the transcript. The combined transcriptional and post-transcriptional effects of the drug ultimately result in decreased levels of cyclin D1 protein. Moreover, degradation of newly synthesized cyclin D1 protein is accelerated by rapamycin, a process prevented by inclusion of the proteasome inhibitor, N-acetyl-Leu-Leunorleucinal. The overall effect of rapamycin on cyclin D1 leads, in turn, to impaired formation of active complexes with Cdk4, a process which triggers retargeting of the p27 Kip1 inhibitor to cyclin E/Cdk2. In view of this novel experimental evidence, we discuss a possible mechanism for the rapamycin-induced cell cycle arrest at the G 1 /S transition. Rapamycin, initially characterized as an inhibitor of G 1 cell cycle progression, has been utilized to unravel a growth factorstimulated signaling pathway leading to the preferential translation of a specific subset of mRNAs. The drug forms a stable complex with the immunophilin FK506-binding protein, which binds to a family of kinases, FK506-binding protein rapamycinassociated protein (FRAP) 1 in human cells, the target of rapamycin in yeast (reviewed in Refs. 1 and 2). FRAP is a member of phosphatidylinositol kinase-related kinases, which impinge upon cellular events as diverse as cell cycle regulation in response to stress and DNA recombination (reviewed in Refs. 2 and 3). Although poorly understood, FRAP has intrinsic kinase activity and regulates the activation of the S6 ribosomal protein kinase p70S6k in vivo; this requires both the FRAP kinase domain and the N-terminal domains of FRAP (4). As a result of this interaction, rapamycin causes rapid inactivation of p70 S6k

Endometrial carcinomas (EnCa) predominantly represent a steroid hormone-driven tumor initiated from prestages. The human endogenous retrovirus HERV-W envelope gene Syncytin-1 was significantly increased at the mRNA and protein levels in EnCa and prestages compared to controls. Steroid hormone treatment of primary EnCa cells and cell lines induced Syncytin-1 due to a new HERV-W estrogen response element and resulted in increased proliferation. Activation of the cAMP-pathway also resulted in Syncytin-1 upregulation, but in contrast to proliferation, classic cell-cell fusions similar to placental syncytiotrophoblasts occurred. Cell-cell fusions were also histologically identified in endometrioid EnCa tumors in vivo. Clonogenic soft agar experiments showed that Syncytin-1 is also involved in anchorage-independent colony growth as well as in colony fusions depending on steroid hormones or cAMP-activation. The posttranscriptional silencing of Syncytin-1 gene expression and a concomitant functional block of induced cell proliferation and cell-cell fusion with siRNAs proved the essential role of Syncytin-1 in these cellular processes. TGF-beta1 and TGF-beta3 were identified as main regulative factors, due to the finding that steroid hormone inducible TGF-beta1 and TGF-beta3 inhibited cell-cell fusion, whereas antibody-mediated TGF-beta neutralization induced cell-cell fusions. These results showed that induced TGF-beta could override Syncytin-1-mediated cell-cell fusions. Interactions between Syncytin-1 and TGF-beta may contribute to the etiology of EnCa progression and also help to clarify the regulation of cell-cell fusions occurring in development and in other syncytial cell tumors.

Supplemental material is available at http://www.genesdev.org.

Glial cells missing a (GCMa) belongs to a new transcription factor family. Syncytin was shown to be a target gene of GCMa. Here, we demonstrate that the protein kinase A (PKA) pathway acts upstream of GCMa. After transient transfection of BeWo cells with PKA, GCMa transcriptional activity and both GCMa and syncytin transcripts were upregulated. This increase was accompanied by further cellular differentiation. Using normoxic or hypoxic conditions to mimic pathophysiological settings known to diminish trophoblast differentiation, we found that gene repressive effects of oxygen deficiency were compensated by the induction of the PKA pathway. We propose that GCMa-driven syncytin expression is the key mechanism for syncytiotrophoblast formation.

The anti-inflammatory and antiapoptotic heme degrading enzyme heme oxygenase-1 (HO-1) has been shown recently to interfere with replication of hepatitis C virus (HCV). We investigated the effect of HO-1 products carbon monoxide (CO), iron and biliverdin on HCV replication using the replicon cell lines Huh-5-15 and LucUbiNeo-ET, stably expressing HCV proteins NS3 through NS5B. Incubation of these cell lines in the presence of the CO donor methylene chloride transiently reduced HCV replication, whereas an increase of iron in cell culture by administration of The heme-degrading enzyme heme oxygenase-1 (HO-1) exerts anti-inflammatory and antiapoptotic effects in vitro and in vivo. Induction or overexpression of HO-1 protects kidneys from acute ischemic failure 6 or ischemia-reperfusion injury, 7 cardiac xenografts from rejection, 8 and livers from ischemia-reperfusion injury caused by either transplantation 9 or hemorrhage/resuscitation, 10 as well as from apoptotic damage. 11 Degradation of heme by heme oxygenases results in the production of carbon monoxide (CO), free iron, and biliverdin. HO-1, in contrast to the isoforms HO-2 and HO-3, is inducible by various stimuli, 12,13 such as cobalt-protoporphyrin-IX (CoPP), 14,15 but also by hypoxia, which can be induced by, for example, high amounts of CO. 16 Of the HO-1 products, CO and biliverdin seem to be the major mediators of protective HO-1 effects within the liver. 17-19 CO application in vitro or in vivo can be achieved by special