2-(1-Hydroxethyl)-4,8-dihydrobenzo[1,2-b:5,4-b']dithiophene-4,8-dione (BTP-11) is a potent enhancer for all-trans retinoic acid (ATRA)-induced differentiation in HL-60 cells. Combination of BTP-11 and ATRA cut down the concentration of ATRA significantly, and that BTP-11 promoted the progression of ATRA-induced into the terminal granulocytic differentiation. Further, Western blot analysis revealed that combination of BTP-11 and ATRA decreased cyclin D/CDK4 and increased C/EBPvarepsilon protein expression to arrest the cells into G0/G1 phase leading to granulocytic maturation. These results confirmed that BTP-11 is a potent enhancer for ATRA-induced differentiation of HL-60 cells, and the great developmental potential of BTP-11 will be expected.

Mutations in the adenomatous polyposis coli (APC) tumour-suppressor gene occur in most human colon cancers. Loss of functional APC protein results in the accumulation of beta-catenin. Mutant forms of beta-catenin have been discovered in colon cancers that retain wild-type APC genes, and also in melanomas, medulloblastomas, prostate cancer and gastric and hepatocellular carcinomas. The accumulation of beta-catenin activates genes that are responsive to transcription factors of the TCF/LEF family, with which beta-catenin interacts. Here we show that beta-catenin activates transcription from the cyclin D1 promoter, and that sequences within the promoter that are related to consensus TCF/LEF-binding sites are necessary for activation. The oncoprotein p21ras further activates transcription of the cyclin D1 gene, through sites within the promoter that bind the transcriptional regulators Ets or CREB. Cells expressing mutant beta-catenin produce high levels of cyclin D1 messenger RNA and protein constitutively. Furthermore, expression of a dominant-negative form of TCF in colon-cancer cells strongly inhibits expression of cyclin D1 without affecting expression of cyclin D2, cyclin E, or cyclin-dependent kinases 2, 4 or 6. This dominant-negative TCF causes cells to arrest in the G1 phase of the cell cycle; this phenotype can be rescued by expression of cyclin D1 under the cytomegalovirus promoter. Abnormal levels of beta-catenin may therefore contribute to neoplastic transformation by causing accumulation of cyclin D1.

Three mouse cyclin-like (CYL) genes were isolated, two of which are regulated by colony-stimulating factor 1 (CSF-1) during the G1 phase of the macrophage cell cycle. CSF-1 deprivation during G1 leads to rapid degradation of CYL proteins (p36CYL) and correlates with failure to initiate DNA synthesis. However, after entering S phase, macrophages no longer require CSF-1 and can complete cell division without expressing CYL genes. During G1, p36CYL is phosphorylated and associates with a polypeptide antigenically related to p34cdc2. The timing of p36CYL expression, its rapid turnover in the absence of CSF-1, and its phosphorylation and transient binding to a cdc2-related polypeptide suggest that CYL genes may function during S phase commitment.

Oscillations in the activity of cyclin-dependent kinases (CDKs) promote progression through the eukaryotic cell cycle. This review examines how proteolysis regulates CDK activity-by degrading CDK activators or inhibitors-and also how proteolysis may directly trigger the transition from metaphase to anaphase. Proteolysis during the cell cycle is mediated by two distinct ubiquitin-conjugation pathways. One pathway, requiring CDC34, initiates DNA replication by degrading a CDK inhibitor. The second pathway, involving a large protein complex called the anaphase-promoting complex or cyclosome, initiates chromosome segregation and exit from mitosis by degrading anaphase inhibitors and mitotic cyclins. Proteolysis therefore drives cell cycle progression not only by regulating CDK activity, but by directly influencing chromosome and spindle dynamics.

The product (pRb) of the retinoblastoma gene (RB-1) prevents S-phase entry during the cell cycle, and inactivation of this growth-suppressive function is presumed to result from pRb hyperphosphorylation during late G1 phase. Complexes of the cyclin-dependent kinase, cdk4, and each of three different D-type cyclins, assembled in insect Sf9 cells, phosphorylated a pRb fusion protein in vitro at sites identical to those phosphorylated in human T cells. Only D-type cyclins activated cdk4 enzyme activity, whereas cyclins A, B1, and E did not. When Sf9 cells were coinfected with baculovirus vectors encoding human pRb and murine D-type cyclins, cyclins D2 and D3, but not D1, bound pRb with high stoichiometry in intact cells. Introduction of a vector encoding cdk4, together with those expressing pRb and D-type cyclins, induced pRb hyperphosphorylation and dissociation of cyclins D2 and D3, whereas expression of a kinase-defective cdk4 mutant in lieu of the wild-type catalytic subunit yielded ternary complexes. The transcription factor E2F-1 also bound to pRb in insect cells, and coexpression of cyclin D-cdk4 complexes, but neither subunit alone, triggered pRb phosphorylation and prevented its interaction with E2F-1. The D-type cyclins may play dual roles as cdk4 regulatory subunits and as adaptor proteins that physically target active enzyme complexes to particular substrates.

Liver regeneration stimulated by a loss of liver mass leads to hepatocyte and nonparenchymal cell proliferation and rapid restoration of liver parenchyma. Mice with targeted disruption of the interleukin-6 (IL-6) gene had impaired liver regeneration characterized by liver necrosis and failure. There was a blunted DNA synthetic response in hepatocytes of these mice but not in nonparenchymal liver cells. Furthermore, there were discrete G1 phase (prereplicative stage in the cell cycle) abnormalities including absence of STAT3 (signal transducer and activator of transcription protein 3) activation and depressed AP-1, Myc, and cyclin D1 expression. Treatment of IL-6-deficient mice with a single preoperative dose of IL-6 returned STAT3 binding, gene expression, and hepatocyte proliferation to near normal and prevented liver damage, establishing that IL-6 is a critical component of the regenerative response.

Cell cycle phase transitions in eukaryotic cells are driven by regulation of the activity of protein kinases known as cyclin-dependent kinases (Cdks). A broad spectrum of Cdk-inhibitory activity associated with a 28-kilodalton protein (p28lck1) was induced in cells treated with the drug lovastatin or upon density-mediated growth arrest and was periodic in the cell cycle, with peak activity in G1. The p28lck1 protein was shown to be identical to p27Kip1, and the periodic or induced inhibitory activity resulted from a periodic accumulation of the protein. Variations in the amount of p27 protein occurred, whereas the abundance of the p27 messenger RNA remained unchanged. In every instance investigated, the posttranscriptional alteration of p27 protein levels was achieved in part by a mechanism of translational control, although in density-arrested fibroblasts and thymidine-arrested HeLa cells the half-life of the protein was also changed.

In the murine interleukin 3 (IL-3)-dependent myeloid cell line 32D, down-regulation of c-myc and ornithine decarboxylase (ODC) expression is an immediate response to IL-3 deprivation. This is followed by an accumulation of cells in the G1 phase of the cell cycle, and eventual cell death. However, clones of 32D cells harboring an expression vector which constitutively expresses murine c-myc did not down-regulate ODC transcripts in response to IL-3 withdrawal, and they failed to G1 arrest. Moreover, in contrast to control cultures in which the majority of death occurred following G1 arrest, c-myc clones rapidly initiated a program of cell death characteristic of apoptosis following IL-3 deprivation, and their subsequent loss of viability occurred with accelerated kinetics. The premature induction of apoptosis in cells harboring a deregulated c-myc gene suggests that apoptosis may be an important mechanism in the elimination of hematopoietic cells harboring mutations, such as constitutive c-myc expression, which imbalance normal cell cycle regulatory controls.

Chk2 is a protein kinase that is activated in response to DNA damage and may regulate cell cycle arrest. We generated Chk2-deficient mouse cells by gene targeting. Chk2-/- embryonic stem cells failed to maintain gamma-irradiation-induced arrest in the G2 phase of the cell cycle. Chk2-/- thymocytes were resistant to DNA damage-induced apoptosis. Chk2-/- cells were defective for p53 stabilization and for induction of p53-dependent transcripts such as p21 in response to gamma irradiation. Reintroduction of the Chk2 gene restored p53-dependent transcription in response to gamma irradiation. Chk2 directly phosphorylated p53 on serine 20, which is known to interfere with Mdm2 binding. This provides a mechanism for increased stability of p53 by prevention of ubiquitination in response to DNA damage.

The immunosuppressant rapamycin interferes with G1-phase progression in lymphoid and other cell types by inhibiting the function of the mammalian target of rapamycin (mTOR). mTOR was determined to be a terminal kinase in a signaling pathway that couples mitogenic stimulation to the phosphorylation of the eukaryotic initiation factor (eIF)-4E-binding protein, PHAS-I. The rapamycin-sensitive protein kinase activity of mTOR was required for phosphorylation of PHAS-I in insulin-stimulated human embryonic kidney cells. mTOR phosphorylated PHAS-I on serine and threonine residues in vitro, and these modifications inhibited the binding of PHAS-I to eIF-4E. These studies define a role for mTOR in translational control and offer further insights into the mechanism whereby rapamycin inhibits G1-phase progression in mammalian cells.