Growth-associated gene expression is not constant in cells traversing G-1 after exiting mitosis

Autocrine antiapoptotic stimulation of cultured adult T-cell leukemia cells by overexpression of the chemokine I-309

Adult T-cell leukemia (ATL) is an aggressive malignancy of CD4(+) T cells caused by the human T-cell leukemia virus type 1 (HTLV-1). The viral leukemogenesis is critically dependent on its oncoprotein Tax because the protein as well as the virus can immortalize primary human lymphocytes to permanent growth. As a transcriptional transactivator, Tax can stimulate the expression of distinct cellular genes. Alterations in the expression levels of unknown growth-relevant genes may contribute to the changed growth properties of Tax-immortalized and leukemic cells. To identify genes that are linked to Tax transformation and ATL leukemogenesis, this study systematically compared the gene expression of cultured cells from patients with acute ATL with that of stimulated peripheral blood T lymphocytes. Several overexpressed RNAs that encode signal transduction functions were identified. These include a dual-specific protein phosphatase (PAC1), an interferon-inducible factor (ISG15), a basic helix-loop-helix transcription factor (DEC-1), and the secreted antiapoptotic chemokine I-309. The ATL cell culture supernatants contained an antiapoptotic activity that could be specifically inhibited by antibodies directed against I-309. Inhibition of I-309 receptor (CCR8) signaling by pertussis toxin increased the apoptosis rate of ATL cell cultures in the presence and absence of external apoptotic stimuli. Both the I-309--specific antiapoptotic activity and the proapoptotic effect of inhibitors of I-309 signaling suggest the existence of an antiapoptotic autocrine loop in ATL cells. Thus, the overexpression of this chemokine may inhibit apoptosis in ATL cells and could substantially contribute to their growth. (Blood. 2001;98:1150-1159)

Mitotic transcription repression in vivo in the absence of nucleosomal chromatin condensation

All nuclear RNA synthesis is repressed during the mitotic phase of the cell cycle. In addition, RNA polymerase II (RNAP II), nascent RNA and many transcription factors disengage from DNA during mitosis. It has been proposed that mitotic transcription repression and disengagement of factors are due to either mitotic chromatin condensation or biochemical modifications to the transcription machinery. In this study, we investigate the requirement for chromatin condensation in establishing mitotic transcription repression and factor loss, by analyzing transcription and RNAP II localization in mitotic cells infected with herpes simplex virus type 1. We find that virus-infected cells enter mitosis and that mitotic viral DNA is maintained in a nucleosome-free and noncondensed state. Our data show that RNAP II transcription is repressed on cellular genes that are condensed into mitotic chromosomes and on viral genes that remain nucleosome free and noncondensed. Although RNAP II may interact indirectly with viral DNA during mitosis, it remains transcriptionally unengaged. This study demonstrates that mitotic repression of transcription and loss of transcription factors from mitotic DNA can occur independently of nucleosomal chromatin condensation.

Regulation of the resident chromosomal copy of c-myc by c-Myb is involved in myeloid leukemogenesis

c-myb is a frequent target of retroviral insertional mutagenesis in murine leukemia virus-induced myeloid leukemia. Induction of the leukemogenic phenotype is generally associated with inappropriate expression of this transcriptional regulator. Despite intensive investigations, the target genes of c-myb that are specifically involved in development of these myeloid lineage neoplasms are still unknown. In vitro assays have indicated that c-myc may be a target gene of c-Myb; however, regulation of the resident chromosomal gene has not yet been demonstrated. To address this question further, we analyzed the expression of c-myc in a myeloblastic cell line, M1, expressing a conditionally active c-Myb-estrogen receptor fusion protein (MybER). Activation of MybER both prevented the growth arrest induced by interleukin-6 (IL-6) and rapidly restored c-myc expression in nearly terminal differentiated cells that had been exposed to IL-6 for 3 days. Restoration occurred in the presence of a protein synthesis inhibitor but not after a transcriptional block, indicating that c-myc is a direct, transcriptionally regulated target of c-Myb. c-myc is a major target that transduces Myb's proliferative signal, as shown by the ability of a c-Myc-estrogen receptor fusion protein alone to also reverse growth arrest in this system. To investigate the possibility that this regulatory connection contributes to Myb's oncogenicity, we expressed a dominant negative Myb in the myeloid leukemic cell line RI-4-11. In this cell line, c-myb is activated by insertional mutagenesis and cannot be effectively down regulated by cytokine. Myb's ability to regulate c-myc's expression was also demonstrated in these cells, showing a mechanism through which the proto-oncogene c-myb can exert its oncogenic potential in myeloid lineage hematopoietic cells.

Regulation of cellular genes in a chromosomal context by the retinoblastoma tumor suppressor protein

The retinoblastoma tumor suppressor gene product (pRb) is involved in controlling cell cycle progression from G1 into S. pRb functions, in part, by regulating the activities of several transcription factors, making pRb involved in the transcriptional control of cellular genes. Transient-transfection assays have implicated pRb in the transcription of several genes, including c-fos, the interleukin-6 gene, c-myc, cdc-2, c-neu, and the transforming growth factor beta2 gene. However, these assays place the promoter in an artificial context and exclude the effects of far 5' upstream regions and chromosomal architecture on gene transcription. In these experiments, we have studied the role of pRb in the control of cell cycle-related genes within a chromosomal context and within the context of the G1 phase of the cell cycle. We have used adenovirus vectors to overexpress pRb in human osteosarcoma cells and breast cells synchronized in early G1. By RNase protection assays, we have assayed the effects of this virus-produced pRb on gene expression in these cells. These results indicate that pRb is involved in the transcriptional downregulation of the E2F-1, E2F-2, dihydrofolate reductase, thymidine kinase, c-myc, proliferating-cell nuclear antigen, p107, and p21/Cip1 genes. However, it has no effect on the transcription of the E2F-3, E2F-4, E2F-5, DP-1, DP-2, or p16/Ink4 genes. The results are consistent with the notion that pRb controls the transcription of genes involved in S-phase promotion. They also suggest that pRb negatively regulates the transcription of two of the transcription factors whose activity it also represses, E2F-1 and E2F-2, and that it plays a role in downregulating the immediate-early gene response to serum stimulation.

Mitotic repression of RNA polymerase II transcription is accompanied by release of transcription elongation complexes

Nuclear RNA synthesis is repressed during the mitotic phase of each cell cycle. Although total RNA synthesis remains low throughout mitosis, the degree of RNA polymerase II transcription repression on specific genes has not been examined. In addition, it is not known whether mitotic repression of RNA polymerase II transcription is due to polymerase pausing or ejection of transcription elongation complexes from mitotic chromosomes. In this study, we show that RNA polymerase II transcription is repressed in mammalian cells on a number of specific gene regions during mitosis. We also show that the majority of RNA polymerase II transcription elongation complexes are physically excluded from mitotic chromosomes between late prophase and late telophase. Despite generalized transcription repression and stripping of RNA polymerase II complexes from DNA, arrested RNA polymerase II ternary complexes appear to remain on some gene regions during mitosis. The cyclic repression of transcription and ejection of RNA polymerase II transcription elongation complexes may help regulate the transcriptional events that control cell cycle progression and differentiation.

Dynamics of cell cycle regulators: artifact-free analysis by recultivation of cells synchronized by centrifugal elutriation

Studies on the molecular properties of cell cycle regulators in animal cells require cell preparations highly enriched in particular cell cycle phases. Centrifugal elutriation is frequently used to synchronize cells because this technique was thought to cause only minimal distortions in protein expression or metabolic functions. However, in primary chicken erythroblasts, we consistently observed artefacts in mitotic cyclin mRNA expression and p70 S6 kinase activity, which were clearly caused by the elutriation procedure. Therefore, we modified the standard protocol by reseeding various elutriated fractions into preconditioned medium, a process termed recultivation, and harvesting after an appropriate amount of time. This avoided the pleiotropic effects caused by stress and lack of growth factor supply during elutriation. Using this recultivation procedure, highly synchronous progression starting from any given cell cycle phase could be achieved for a variety of cell types, including primary, factor-dependent cells of hematopoietic origin. Mitotic cyclin expression and S6 kinase activity was found to be normal again in recultivated cultures, as opposed to elutriated ones. Finally, monitoring of mitosis-specific cyclin A degradation in recultivated G2 phase cells showed that recultivation provided an excellent tool to follow cells through M phase into G1 without the requirement for a chemical cell cycle block.

Induction of c-fos and c-jun mRNA at the M/G1 border is required for cell cycle progression

The proto-oncogenes c-fos and c-jun have been shown in numerous model systems to be induced within minutes of growth factor stimulation, during the G0/G1 transition. In this report we use the mitotic shake-off procedure to generate a population of highly synchronized Swiss 3T3 cells. We show that both of these immediate-early, competence genes are also induced during the M/G1 transition, immediately after completion of mitosis. While c-fos mRNA levels drop to undetectable levels within 2 hr after division, c-jun mRNA levels are maintained at a basal level which is approximately 30% maximum throughout the remainder of G1. In order to access the functional significance of these patterns of c-fos and c-jun expression, antisense oligodeoxynucleotides specific to c-fos or c-jun were added to either actively growing Swiss 3T3 cells or mitotically synchronized cells, and their ability to inhibit DNA synthesis and cell division determined. Our results show that treatment of Swiss 3T3 cells with either c-fos or c-jun antisense oligodeoxynucleotides, while actively growing, during mitosis, or in early G1, results in a reduction in ability to enter S and subsequently divide. This was also true if Swiss 3T3 cells were treated during mid-G1 with c-jun antisense oligodeoxynucleotides. These results demonstrate that the regulation of G1 progression following mitosis is dependent upon the expression and function of the immediate-early, competence proto-oncogenes c-fos and c-jun.

Use of antisense oligomers to study the role of c-jun in G1 progression

In actively proliferating Swiss 3T3 fibroblasts, expression of the protooncogene c-jun is maximally induced early in G1, immediately after completion of mitosis. Within 2 hours, c-jun mRNA levels drop to a basal amount that is approximately 30% of the maximum. This is maintained throughout the remainder of G1. To access the functional implications of this pattern of c-jun expression, antisense oligomers specific to c-jun were added to either actively proliferating or synchronized Swiss 3T3 cells, and their ability to inhibit DNA synthesis and division was determined. Our results show that if Swiss 3T3 cells are treated with anti-c-jun while actively growing or at any time during G1 after completion of mitosis, they exhibit a reduced ability to enter S-phase and subsequently divide. These results demonstrate that the regulation of G1 progression following mitosis depends on the expression and function of the protooncogene c-jun.