Actin is abundant in the nucleus and has been implicated in transcription; however, the nature of this involvement has not been established. Here we demonstrate that beta-actin is critically involved in transcription because antibodies directed against beta-actin, but not muscle actin, inhibited transcription in vivo and in vitro. Chromatin immunoprecipitation assays demonstrated the recruitment of actin to the promoter region of the interferon-gamma-inducible MHC2TA gene as well as the interferon-alpha-inducible G1P3 gene. Further investigation revealed that actin and RNA polymerase II co-localize in vivo and also co-purify. We employed an in vitro system with purified nuclear components to demonstrate that antibodies to beta-actin block the initiation of transcription. This assay also demonstrates that beta-actin stimulates transcription by RNA polymerase II. Finally, DNA-binding experiments established the presence of beta-actin in pre-initiation complexes and also showed that the depletion of actin prevented the formation of pre-initiation complexes. Together, these data suggest a fundamental role for actin in the initiation of transcription by RNA polymerase II.

The Schlafen (SLFN) family of proteins includes several mouse and human members. There is emerging evidence that members of this family of proteins are involved in important functions, such as the control of cell proliferation, induction of immune responses, and the regulation of viral replication. These proteins span across all species with great diversity, with 10 murine and 5 human isoforms. Recent work has established that mouse and human SLFN proteins are regulated by interferons (IFNs). Several Slfn genes were shown to be induced as classical interferon-stimulated genes, and emerging evidence suggests that these proteins play important roles in the growth inhibitory and antineoplastic effects of IFNs. In the current review, the known properties of mouse and human SLFNs are reviewed, and the implications of their emerging functions are discussed.

IFN␣ exerts potent inhibitory activities against malignant melanoma cells in vitro and in vivo, but the mechanisms by which it generates its antitumor effects remain unknown. We examined the effects of interferon ␣ (IFN␣) on the expression of human members of the Schlafen (SLFN) family of genes, a group of cell cycle regulators that mediate growth-inhibitory responses. Using quantitative RT-real time PCR, we found detectable basal expression of all the different human SLFN genes examined (SLFN5, SLFN11, SLFN12, SLFN13, and SLFN14), in malignant melanoma cells and primary normal human melanocytes, but SLFN5 basal expression was suppressed in all analyzed melanoma cell lines. Treatment of melanoma cells with IFN␣ resulted in induction of expression of SLFN5 in malignant cells, suggesting a potential involvement of this gene in the antitumor effects of IFN␣. Importantly, stable knockdown of SLFN5 in malignant melanoma cells resulted in increased anchorage-independent growth, as evidenced by enhanced colony formation in soft agar assays. Moreover, SLFN5 knockdown also resulted in increased invasion in three-dimensional collagen, suggesting a dual role for SLFN5 in the regulation of invasion and anchorage-independent growth of melanoma cells. Altogether, our findings suggest an important role for the SLFN family of proteins in the generation of the anti-melanoma effects of IFN␣ and for the first time directly implicate a member of the human SLFN family in the regulation of cell invasion.The interferons (IFNs) are cytokines with important pleiotropic biological effects, including generation of antitumor responses and antiviral activities (1-3). The ability of IFNs to induce antitumor responses in selective systems is highly relevant and, over the years, has had a major impact in the management of certain leukemias and solid tumors in humans. Malignant melanoma is one of the most IFN-sensitive solid tumors. There has been extensive clinical evidence on the ability of IFN␣ to generate antitumor effects in vitro in subset groups of patients with advanced metastatic malignant melanoma (4 -7), and IFN␣ is now a Food and Drug Administration-approved agent for the treatment of this malignancy. An important outstanding issue in the IFN research field has been the identification of specific mechanisms that account for differential sensitivity to the effects of IFNs. Despite the advances in the IFN-signaling field over the last 2 decades, the precise mechanisms and specific signals that account for the unique IFN sensitivity that some tumors exhibit remain largely unknown.It is now well established that IFNs regulate transcription of target genes with important functional relevance via engagement of the JAK-STAT pathway (8 -11). In recent years, additional levels of cellular regulation of IFN-inducible genes and their products have been identified, such as involvement of members of the PKC family (12-16), the MAPK cascades (17-22), translational regulation via mammalian target of rapamycin and 4EBP1 (23-28), modulation of histo...