EBNA-2 and EBNA-LP cooperate to cause G0 to G1 transition during immortalization of resting human B lymphocytes by Epstein-Barr virus

Epstein-Barr virus (EBV) is unusual among DNA tumour viruses in that the virus particle is able to infect and immortalize resting cells with very high efficiency. Mutation of the viral genome has indicated that at least six viral genes (LMP-1 and EBNAs 1, 2, 3A, 3C and LP) are essential for immortalization. We demonstrate that the activation of a G1 cyclin, cyclin D2, is an early event following infection with EBV and that cyclin D2 activation is dependent on the expression of viral genes. The different levels of cyclin D2 transcripts in Burkitt's lymphoma cell lines expressing different subsets of EBV immortalizing genes suggest an involvement of EBNA-2 or EBNA-LP in cyclin D2 regulation. By exposing resting primary B cells to a purified preparation of the EBV surface glycoprotein gp340, we have been able to achieve efficient expression of plasmid DNAs introduced by electroporation. Vectors encoding two viral genes, EBNA-2 and EBNA-LP, are sufficient to activate the expression of cyclin D2 in this system. Thus, the progression of resting B lymphocytes into the G1 phase of the cell cycle can be reconstituted in the absence of virus by the cooperation of two of the six viral genes required for immortalization.

Identification of G1 kinase activity for cdk6, a novel cyclin D partner

A family of vertebrate cdc2-related kinases has been identified, and these kinases are candidates for roles in cell cycle regulation. Here, we show that the human PLSTIRE gene product is a novel cyclin-dependent kinase, cdk6. The cdk6 kinase is associated with cyclins D1, D2, and D3 in lysates of human cells and is activated by coexpression with D-type cyclins in Sf9 insect cells. Furthermore, we demonstrate that endogenous cdk6 from human cell extracts is an active kinase which can phosphorylate pRB, the product of the retinoblastoma tumor suppressor gene. The activation of cdk6 kinase occurs during mid-G1 in phytohemagglutinin-stimulated T cells, well prior to the activation of cdk2 kinase. This timing suggests that cdk6, and by analogy its homolog cdk4, links growth factor stimulation with the onset of cell cycle progression.

Differential expression of D type cyclins during neuronal maturation

The multiple cyclins play essential roles in eucaryotic cell cycle. We previously reported that the expression of a D type G1 cyclin (p36cyclinD1) in neurons was increased at the onset of brain maturation. During neuronal differentiation of PC12h cells induced by nerve growth factor (NGF), expression of p36cyclinD1 and p46cyclinX was enhanced concomitant with neuronal maturation. In contrast, p34cyclinD2, which was highly expressed in undifferentiated cells, decreased as the cells matured. In situ hybridization and Western blot analyses demonstrated that embryonic rat brain strongly expressed cyclin D2 gene, but its expression was dramatically repressed in matured brain. These data suggest differential roles of the D type cyclins in the process of neuronal differentiation and/or function.

Monoclonal antibodies to mammalian D-type G1 cyclins

D-type cyclins are necessary and rate-limiting for G1 progression during the mammalian cell cycle. Cyclins D1, D2, and D3 are encoded by distinct genes and are expressed in proliferating cells in a lineage-specific manner. Monoclonal antibodies (mAbs) generated to bacterially produced recombinant D-type cyclins were able to react with the native proteins expressed in mammalian cells. One mouse and three rat mAbs immunoprecipitated cyclin D1 from mouse macrophages. Only rat mAbs reacted with human cyclin D1 and cross-reacted with cyclin D2 expressed in proliferating T lymphocytes and human tumor cell lines. A single rat mAb to cyclin D2 exhibited a pattern of reactivity reciprocal to that of rat mAbs to D1. Three rat mAbs reacted specifically with mouse or human cyclin D3, but did not cross-react with cyclins D1 or D2 from either species. Representative mAbs were useful for immunoblotting and detected D-type cyclins coprecipitating in complexes recovered with antiserum to cyclin-dependent kinase-4 (CDK4). Because these mAbs detect D-type cyclins in the nuclei of fixed permeabilized cells, they should prove useful in documenting cyclin overexpression in those human tumors in which the genes are amplified or are targets of specific chromosomal rearrangements.

Inhibition of granulocyte differentiation by G1 cyclins D2 and D3 but not D1

Growth factor-induced signals govern the expression of three D-type cyclins, which, in turn, function as regulatory subunits of cyclin-dependent kinases (cdks) to control cell cycle transitions during the late G1 interval. 32D myeloid cells, which self-renew as uncommitted precursors in interleukin 3 (IL-3), express cyclins D2 and D3 (but not D1) in complexes with cdk4 and cdk2. When transferred to granulocyte colony-stimulating factor (G-CSF), 32D cells stop dividing and terminally differentiate to mature neutrophils. Cyclin D and cdk4 expression ceased as cells underwent growth arrest in G-CSF, but cdk2 levels were sustained. 32D cells engineered to ectopically express D-type cyclins exhibited contracted G1 intervals with a compensatory lengthening of S phase but remained IL-3 dependent for cell growth; those overexpressing cyclins D2 and D3 (but not D1) were unable to differentiate and died in G-CSF. Cyclin D2 mutants, which cannot efficiently bind to, or functionally interact with, the retinoblastoma protein (pRb) or its relatives (p107) did not block differentiation. Conversely, the introduction of a catalytically inactive cdk4 mutant into cells overexpressing cyclin D2 restored their G-CSF response. The persistence of cdk2 and its predilection to functionally interact with cyclins D2 and D3 rather than D1 might explain the specificity of the differentiation blockade.

Regulation of G1/S transition by cyclins D2 and D3 in hematopoietic cells

Identification of the genes that control passage through the G1 phase of the cell cycle in mammalian cells is of particular interest because virtually all external events that regulate proliferation act primarily or exclusively during G1. Cyclins are likely to play a key role in controlling cell cycle progression, although their role during G1 in higher eukaryotic cells is unclear. In the hematopoietic cell line 32Dcl3, both cyclins D2 and D3 were expressed in proliferating cells, while cyclin D1 was undetectable. Expression of D2, and to a lesser extent D3, was interleukin 3 (IL-3) dependent and declined rapidly in the absence of this growth factor. To investigate the potential role of D cyclins in regulating cell growth, cell lines overexpressing either D2 or D3 were generated by transfection. Constitutive overexpression of either D2 or D3 did not affect cell viability, rate of cell proliferation, or dependence on IL-3 for growth. However, the distribution of cells through the cell cycle was dramatically altered, with both cyclins causing an increase in the fraction of cells in S phase, apparently related to a shortening of G1. Also, when deprived of IL-3, D3-overexpressing cells failed to arrest in G1, and apoptotic cell death in the absence of IL-3 was delayed. These results suggest a role for cyclins D2 and D3 in controlling passage of hematopoietic cells through G1 in the presence of growth factors and in effecting G1 arrest in the absence of growth factors.

Overexpression of mouse D-type cyclins accelerates G1 phase in rodent fibroblasts

Mammalian D-type cyclins are growth factor-regulated, delayed early response genes that are presumed to control progression through the G1 phase of the cell cycle by governing the activity of cyclin-dependent kinases (cdks). Overexpression of mouse cyclin D1 in serum-stimulated mouse NIH-3T3 and rat-2 fibroblasts increased their rates of G0 to S- and G1- to S-phase transit by several hours, leading to an equivalent contraction of their mean cell generation times. Although such cells remained contact inhibited and anchorage dependent, they manifested a reduced serum requirement for growth and were smaller in size than their normal counterparts. Ectopic expression of cyclin D2 in rodent fibroblasts, either alone or together with exogenous cdk4, shortened their G0- to S-phase interval and reduced their serum dependency, but cyclin D2 alone did not alter cell size significantly. When cells were microinjected during the G1 interval with a monoclonal antibody specifically reactive to cyclin D1, parental rodent fibroblasts and derivatives overexpressing this cyclin were inhibited from entering S phase, but cells injected near the G1/S phase transition were refractory to antibody-induced growth suppression. Thus, cyclin D1, and most likely D2, are rate limiting for G1 progression.

A critical role for the cytoplasmic domain of the granulocyte-macrophage colony-stimulating factor alpha receptor in mediating cell growth

The human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor is composed of an alpha subunit which binds GM-CSF and a beta subunit which allows for high affinity binding. To investigate the role of the short cytoplasmic tail (54 amino acids) of the alpha receptor in mediating signal transduction and in controlling cell growth, we placed a stop codon after the alpha receptor transmembrane domain and expressed this receptor in murine Ba/F3 cells. Unlike the complete alpha subunit, this shortened receptor was unable to stimulate protein phosphorylation or mediate entry into the cell cycle. By comparing Ba/F3 cells expressing the alpha and beta receptors with those expressing the alpha or the terminated alpha receptor, we have been able to correlate specific GM-CSF-induced events with cell cycle commitment. We find that cell growth is correlated with prolonged increases in the cell levels of c-myc, pim-1, and cyclin D2 mRNAs, but not with changes in either immediate early genes or mitogen-activated protein kinase phosphorylation. This suggests that additional signal transduction pathways not mediated by known phosphoproteins are activated by GM-CSF. Since the beta receptor is shared by human interleukins 3 and 5, our data suggest that the specificity of response to GM-CSF is mediated in part by the short cytoplasmic tail of the alpha receptor subunit.

The Vin-1 gene, identified by provirus insertional mutagenesis, is the cyclin D2

The Vin-1 gene was initially identified as a gene whose expression is altered by the integration of proviruses in the Vin-1 common site of integration in retrovirus-induced rodent T-cell leukemias. We have now isolated the Vin-1 cDNA. Sequencing of the Vin-1 cDNA and Vin-1 exons revealed that the proviruses are integrated at the 5' end of the Vin-1 gene in an inverse transcriptional orientation. The sequence of the Vin-1 gene is identical to that of the recently identified G1-phase cyclin D2 gene. The human homolog of the Vin-1/cyclin D2 gene (CCND2) was mapped to chromosome 12, band p13.3, by in situ hybridization, confirming previous mapping data. Our results strongly support a role of the cyclin D2 gene in oncogenesis and thereby implicate altered cell cycle regulation in transformation.

Physical interaction of the retinoblastoma protein with human D cyclins

The retinoblastoma protein (pRb) functions as a regulator of cell proliferation and in turn is regulated by cyclin-dependent kinases. Cyclins D1 and D3 can form complexes with pRb that resemble those formed by several viral oncoproteins and are disrupted by the adenovirus E1A oncoprotein and derived peptides. These cyclins contain a sequence motif similar to the pRb-binding conserved region II motif of the viral oncoproteins. Alteration of this motif in cyclin D1 prevents formation of cyclin D1-pRb complexes while enhancing the biological activity of cyclin D1 assayed in vivo. We conclude that cyclins D1 and D3 interact with pRb in a fashion distinct from cyclins A and E, which can induce pRb hyperphosphorylation, and that cyclin D1 activity may be regulated by its association with pRb.

Functional interactions of the retinoblastoma protein with mammalian D-type cyclins

The retinoblastoma gene product (Rb) can interact efficiently with two of three D-type G1 cyclins (D2 and D3) in vitro. Binding depended upon the minimal regions of Rb necessary for its growth-suppressive activity, as well as upon the D-type cyclin sequence motif shared with Rb-binding DNA tumor virus oncoproteins. Coexpression of the three D-type cyclins with the cyclin-dependent kinase (cdk4) in insect cells generated Rb kinase activity. By contrast, cyclins D2 and D3, but not D1, activated another such kinase, cdk2. Introduction of cyclin D2 and Rb into the Rb-deficient cell line SAOS-2 led to overt Rb hyperphosphorylation, whereas Rb, expressed alone or together with cyclin D1, remained unphosphorylated. Cyclin D2-dependent phosphorylation inhibited its binding to the transcription factor E2F and reversed the Rb G1 exit block in the cell cycle. Thus, all D-type cyclins do not function equivalently, and one of them plays a major role in reversing the cycle-blocking function of a known tumor suppressor.

Cyclins D1 and D2 are differentially expressed in human B-lymphoid cell lines

The cyclin D1 gene can be transcriptionally activated in lymphoid tumours as a result of chromosomal rearrangements but is normally silent in B and T lymphocytes. By isolating cyclin D1-related cDNAs from a B-lymphoid cell line, we identified clones that contain the coding sequences of human cyclin D2. The predicted 289 amino acid protein shares 63% identity with human cyclin D1. Although cyclin D2 transcripts were detected in many lymphoid cell lines, it was not ubiquitously expressed and there was no apparent correlation with cyclin D1 levels. For example, two B-cell leukaemia lines, JVM-2 and Karpas 620, both of which have 11q13 translocations and express cyclin D1, contained markedly different amounts of cyclin D2. An obvious distinction between these cells is that the JVM-2 line, which expresses high levels of cyclin D2, was immortalized by Epstein-Barr virus (EBV). We subsequently found that cyclin D2 is consistently expressed in group III Burkitt's lymphoma (BL) and in lymphoblastoid cell lines immortalized by EBV, but not in group I BLs, in which expression of the EBV genome is more restricted. The data imply that the D-type cyclins may have non-overlapping functions at specific stages of lymphocyte differentiation and that the expression of cyclin D2 may be influenced, directly or indirectly, by EBV.