Expression of cyclins and cdks throughout murine carcinogenesis.

The overexpression and/or amplification of cell cycle regulating genes is an important factor in the progression of cancer. Recent attention has been focused on several cyclin and cdks genes whose expression were increased in many types of tumor. In this study, we investigated the expression kinetics of cyclins A, B, D1, E and cdks 1, 2, 4, 6 by RT-PCR coupled with densitometry and correlated to the growth fraction (percentage of S cells). This analysis was performed using an experimental murine leukemic model, generated by in vivo administration of murine clonogenic cells Wehi-3b injected into balb-c mice. Differential expression of cyclins and cdks was observed between normal and tumoral cells with different patterns of expression between G1 and G2M cyclins-cdks. G1 cyclins cdks expression was significantly increased in tumor cells when compared to normal cells. In the same manner, G2M cyclins cdks expression was only observed in tumor cells at a lower level than for G1 cyclins cdks, but not detected in normal cells. These differences correlated with the growth fraction for both the G1 cyclins cdks (r = 0.91, 0.94, 0.85, 0.90 and 0.96 for cyclin D1, cyclin E, cdk2, cdk4 and cdk6, respectively) and the G2M cyclins cdks (r = 0.96, 0.97 and 0.93 for cyclins A, B and cdkl respectively). Analysis of cyclins cdks expression kinetics during tumoral progression shows that cyclins A, B and cdkl were expressed from the 12th day on of disease, increased until the death of the animals and correlated with the growth fraction (r = 0.94, 0.95 and 0.97 for cyclins A, B and cdk1 respectively) (n = 20). Overexpression of other cyclins cdks were observed, from the 6th day on for cyclin D1, the 12th day for cdk2 and cdk4, the 15th day for cdk6 and the 20th day for cyclin E. These increases persisted during tumoral progression and correlated with the growth fraction (r = 0.85, 0.94, 0.93, 0.96, and 0.98 for cyclin D1, cyclin E, cdk2, cdk4 and cdk6, respectively) (n = 20). Our results demonstrated that G1 and G2-M cyclins cdks mRNA levels were increased at approximately the same time of maximal tumor growth. Only cyclin D1 overexpression occured at the initiation of tumoral development, and could therefore be considered as an early marker of cell proliferation.     

Transforming activity of Fbxo7 is mediated specifically through regulation of cyclin D/cdk6

D cyclins (D1, D2 and D3) and their catalytic subunits (cyclin-dependent kinases cdk4 and cdk6) have a facilitating, but nonessential, role in cell cycle entry. Tissue-specific functions for D-type cyclins and cdks have been reported; however, the biochemical properties of these kinases are indistinguishable. We report that an F box protein, Fbxo7, interacted with cellular and viral D cyclins and distinguished among the cdks that bind D-type cyclins, specifically binding cdk6, in vitro and in vivo. Fbxo7 specifically regulated D cyclin/cdk6 complexes: Fbxo7 knockdown decreased cdk6 association with cyclin and its overexpression increased D cyclin/cdk6 activity and E2F activity. Fbxo7 interacted with p27, but its enhancement of cyclin D/cdk6 activity was p21/p27 independent. Fbxo7 overexpression transformed murine fibroblasts, rendering them tumorigenic in athymic nude mice. Transformed phenotypes were dependent on cdk6, as knockdown of cdk6 reversed them. Fbxo7 was highly expressed in epithelial tumors, but not in normal tissues, suggesting that it may have a proto-oncogenic role in human cancers.     

Explant-induced reactivation of herpes simplex virus occurs in neurons expressing nuclear cdk2 and cdk4

Herpes simplex virus (HSV) establishes productive (lytic) infections in nonneuronal cells and nonproductive (latent) infections in neurons. It has been proposed that HSV establishes latency because quiescent neurons lack cellular factors required for productive infection. It has been further proposed that these putative factors are induced following neuronal stress, as a requirement for HSV reactivation. To date, the identity of these putative cellular factors remains unknown. We have demonstrated that cyclin-dependent kinase (cdk) 1, 2, or 7 is required for HSV replication in nonneuronal cells. Interestingly, cdks 1 and 2 are not expressed in quiescent neurons but can be induced in stressed neurons. Thus, cdks may be among the cellular proteins required for HSV reactivation whose neuronal expression is differentially regulated during stress. Herein, we determined that neuronal expression of nuclear cdk2, cdk4, and cyclins E and D2 (which activate cdks 2 and 4, respectively) was induced following explant cultivation, a stressful stimulus that induces HSV reactivation. In contrast, neuronal expression of cdk7 and cytoplasmic cdk4 decreased during explant cultivation, whereas cdk3 was detected in the same small percentage of neurons before and after explant cultivation and cdks 1, 5, and 6 were not detected in neuronal cell bodies. HSV-1 reactivated specifically in neurons expressing nuclear cdk2 and cdk4, and an inhibitor specific for cdk2 inhibited HSV-1 reactivation. We conclude that neuronal levels of cdk2 are among the factors that determine the outcome of HSV infections of neurons.     

Effects of Cyclic AMP on Components of the Cell Cycle Machinery Regulating DNA Synthesis in Cultured Astrocytes

Cyclic AMP is a second messenger for various hormones that inhibits cell multiplication and DNA synthesis in cultured astrocytes. We examined the effects of increasing intracellular cyclic AMP on the catalytic (cdks) and regulatory (cyclins and ckis) components of cyclin-dependent protein kinases, which regulate progression of the cell cycle before completion of DNA synthesis, in primary cultured astrocytes and in an astrocytic cell line C.LT.T.1.1. The amount of cdk4 changed little during the cell cycle and was not affected by cyclic AMP. There was little cdk1 and cdk2 in quiescent cells, and their expression increased during the G1-S phases. Cyclic AMP strongly inhibited cdk1 and cdk2 expression. Transforming growth factor beta also inhibited cdk1 expression in primary astrocytes. Cyclic AMP did not affect the two ckis p27KIP1 and p21CIP1. There was little cyclin D1 in quiescent cells, but it increased during the G1 phase and was reduced by cyclic AMP. Cyclin E increased during the G1-S phases and was not affected by cyclic AMP in primary astrocytes. The amount of cyclin A was low in quiescent cells and increased during the G1-S phases. Expression of its mRNA and protein was inhibited by cyclic AMP. The protein kinase activities associated with complexes of cyclins and cdks were increased by growth factors and prevented by cyclic AMP. We conclude that cyclic AMP inhibits progression of the cell cycle in astrocytes at least by preventing the expression of the regulatory subunits, cyclins D1 and A, and catalytic subunits, cdk1 and cdk2, of cyclin-regulated protein kinases. Key Words: Cyclin-dependent protein kinases-Glial cells-Cyclic AMP.     

Induction of cyclins E and A in response to mitogen removal: a basic alteration associated with the arrest of differentiation of C2 myoblasts transformed by simian virus 40 large T antigen.

We previously showed that C2 myoblasts transformed by simian virus 40 large T antigen (SVLT) stop the myogenic process after the induction of myogenin and of high Rb levels; the induced Rb, however, becomes notably phosphorylated. We have analyzed the protein levels and activities of cyclin-dependent kinases (cdks) in untransformed C2 cells and in transformants of either SVLT or the cytoplasmic mutant NKT1 (which permits differentiation) upon a shift from growth medium (GM) to mitogen-poor differentiation medium (DM). After the shift, cdk4 levels remained constant and cdk6 levels decreased in all cell types; cdk2 minimally increased only in SVLT cells. Cyclin D1 was downregulated in DM in all cell types, and cyclin D3 was upregulated (albeit less strongly in SVLT cells than in the others). In contrast, a dramatic difference between SVLT cells and the other cells was observed for cyclins E and A, which essentially disappeared (as protein and RNA) in normal C2 and NKT1 cells upon the shift from GM to DM, whereas they increased in SVLT cells. Concurrently, cdk2 activity ceased in C2 and NKT1 cells in DM, whereas it persisted at 20% of the GM level in SVLT cells. cdk4 activity was detectable in all cells only in GM. Cyclin E and A induction thus appeared to sustain enough Rb phosphorylation to interfere with tissue-specific expression, with cdk activity not high enough to activate cyclin self-regulation. In DM, cdk2 complexed to D3 was underphosphorylated in all cells, and SVLT allowed strong inductions of p21 and p27 without affecting their complexes with cdks.     

Evidence for coordinated interaction of cyclin D3 with p21 and cdk6 in directing the development of uterine stromal cell decidualization and polyploidy during implantation.

Uterine decidualization, characterized by stromal cell proliferation, and differentiation into specialized type of cells (decidual cells) with polyploidy, during implantation is critical to the pregnancy establishment in mice. The mechanisms by which the cell cycle events govern these processes are poorly understood. The cell cycle is tightly regulated at two particular checkpoints, G1-S and G2-M phases. Normal operation of these phases involves a complex interplay of cyclins, cyclin-dependent kinases (cdks) and cdk inhibitors (CKIs). We previously observed that upregulation of uterine cyclin D3 at the implantation site is tightly associated with decidualization in mice. To better understand the role of cyclin D3 in this process, we examined cell-specific expression and associated interactions of several cell cycle regulators (cyclins, cdks and CKIs) specific to different phases of the cell cycle during decidualization in mice. Among the various cell cycle molecules examined, coordinate expression and functional association of cyclin D3 with cdk4 suggest a role for proliferation and, that of cyclin D3 with p21 and cdk6 is consistent with the development of polyploidy during stromal cell decidualization.     

Identification and properties of an atypical catalytic subunit (p34PSK-J3/cdk4) for mammalian D type G1 cyclins.

Murine D type cyclins associate with a catalytic subunit (p34PSK-J3) with properties distinct from known cyclin-dependent kinases (cdks). Mouse p34PSK-J3 shows less than 50% amino acid identity to p34cdc2, p33cdk2, and p36cdk3, lacks a PSTAIRE motif, and does not bind to p13suc1. Cyclin D1-p34PSK-J3 complexes accumulate in macrophages during G1 and decline in S phase, whereas complexes involving cyclins D2 and D3 form in proliferating T cells. Although histone H1 kinase activity is not detected in cyclin D or PSK-J3 immunoprecipitates, cyclin D-p34PSK-J3 complexes assembled in vitro stably bind and phosphorylate the retinoblastoma gene product (pRb) and an Rb-like protein (p107) but do not interact with pRb mutants that are functionally inactive. Thus, p34PSK-J3 is a cyclin D-regulated catalytic subunit that acts as an Rb (but not H1) kinase.     

Cell cycle regulation in higher plants

The isolation of plant genes homologous to cdk and cyclin components from yeast and animals proves the existence of a basic cell cycle machinery in all eukaryotes. cdk and cyclin expression has been shown to be involved in the spatial and temporal control of cell division in a variety of developmental processes. In plants, cell division and development are closely interlinked processes that are regulated by phytohormones. cdks and cyclins were found to be under control of phytohormones underscoring their integral role in mediating different developmental pathways. Furthermore, studies on cdk and cyclin expression not only correlate with actual cell cycle activity but also with cell division competence providing a working model to understand regeneration capacity at the molecular level.     

Activation of cyclin-dependent kinases by Myc mediates induction of cyclin A, but not apoptosis.

The activation of conditional alleles of Myc induces both cell proliferation and apoptosis in serum-deprived RAT1 fibroblasts. Entry into S phase and apoptosis are both preceded by increased levels of cyclin E- and cyclin D1-dependent kinase activities. To assess which, if any, cellular responses to Myc depend on active cyclin-dependent kinases (cdks), we have microinjected expression plasmids encoding the cdk inhibitors p16, p21 or p27, and have used a specific inhibitor of cdk2, roscovitine. Expression of cyclin A, which starts late in G1 phase, served as a marker for cell cycle progression. Our data show that active G1 cyclin/cdk complexes are both necessary and sufficient for induction of cyclin A by Myc. In contrast, neither microinjection of cdk inhibitors nor chemical inhibition of cdk2 affected the ability of Myc to induce apoptosis in serum-starved cells. Further, in isoleucine-deprived cells, Myc induces apoptosis without altering cdk activity. We conclude that Myc acts upstream of cdks in stimulating cell proliferation and also that activation of cdks and induction of apoptosis are largely independent events that occur in response to induction of Myc.     

Overexpression of cdk4/cyclin D1 induces apoptosis in PC12 cells in the presence of trophic support.

The induction of apoptosis by cell cycle regulator molecules under conditions optimal for exponential growth was examined in rat pheochromocytoma PC12 cells by overexpression of cyclins and cyclin-dependent kinases (cdks). By flow cytometry and by immunofluorescence, only cells overexpressing cdk4 or cyclin D1 underwent apoptosis, which was not associated with G1-arrest. Cdk4 kinase activity was significantly higher in cdk4-, or cyclin D1-expressing cells. Furthermore, induction of apoptosis by cdk4 was abrogated by co-transfection of p16(INK4), or dominant negative cdk4. These results suggest that upregulation of cdk4 kinase activity is a primary and critical mediator of apoptosis in PC12 cells under physiological conditions.     

Regulation of MyoD function in the dividing myoblast

Proliferating myoblasts express MyoD, yet no phenotypic markers are activated as long as mitogen levels are sufficient to keep the cells dividing. Depending upon mitogen levels, a decision is made in G1 that commits the myoblast to either continue to divide or to exit from the cell cycle and activate terminal differentiation. Ectopic expression of MyoD under the control of the RSV or CMV promoters causes 10T1/2 cells to rapidly exit the cell cycle and differentiate as single myocytes, even in growth medium, whereas expression of MyoD under the weaker SV40 promoter is compatible with proliferation. Co-expression of MyoD and cyclin D1, but not cyclins A, B, E or D3, blocks transactivation of a MyoD responsive reporter. Similarly, transfection of myoblasts with the cyclin-dependent kinase (cdk) inhibitors p16 and p21 supports some muscle-specific gene expression even in growth medium. Taken altogether, these results suggest cell cycle progression negatively regulates myocyte differentiation, possibly through a mechanism involving the D1 responsive cdks. We review evidence coupling growth status, the cell cycle and myogenesis. We describe a novel mitogen-sensitive mechanism that involves the cyclin D1-dependent direct interaction between the G1 cdks and MyoD in the dividing myoblast, which regulates MyoD function in a mitogen-sensitive manner.     

Fbxo7 Gets Proactive with Cyclin D/cdk6

Although all three D cyclins bind and activate cdks 2, 4 and 6, Fbxo7 has been characterised as a selective enhancer of cdk6 activity. It increases activation by directly facilitating cdk6 interaction with viral and cellular D cyclins. Fbxo7 over-expression has transforming activity in murine fibroblasts and is also highly expressed in human cancer, suggesting it is a potential oncogene. Fbxo7 has the ability to activate cell cycle regulators, and is part of an E3 ubiquitin ligase. We postulate Fbxo7 coordinates the ubiquitination of other substrates with cell cycle entry. It may therefore represent a means to integrate cell signals and control disparate biological processes during the early part of the cell cycle.     

Dual phosphorylation of the T-loop in cdk7: its role in controlling cyclin H binding and CAK activity.

A cyclin-dependent kinase (cdk)-activating kinase (CAK) has been shown previously to catalyze T-loop phosphorylation of cdks in most eukaryotic cells. This enzyme exists in either of two forms: the major one contains cdk7, cyclin H and an assembly factor called MAT-1, whilst the minor one lacks MAT-1. Cdk7 is unusual among cdks because it contains not one but two residues (S170 and T176 in Xenopus cdk7) in its T-loop that are phosphorylated in vivo. We have investigated the role of S170 and T176 phosphorylation in the assembly and activity of cyclin H-cdk7 dimers. In the absence of MAT-1, phosphorylation of the T-loop appears to be required for cdk7 to bind cyclin H. Phosphorylation of both residues does not require cyclin H binding in vitro. Phosphorylation of S170 is sufficient for cdk7 to bind cyclin H with low affinity, but high affinity binding requires T176 phosphorylation. By mutational analysis, we demonstrate that in addition to its role in promotion of cyclin H binding, S170 phosphorylation plays a direct role in the control of CAK activity. Finally, we show that dual phosphorylation of S170 and T176, or substitution of both phosphorylatable residues by aspartic residues, is sufficient to generate CAK activity to one-third of its maximal value in vitro, even in the absence of cyclin H and MAT-1, and may thus provide further clues as to how cyclins activate cdk subunits.     

Cyclin-dependent kinases phosphorylate the adenovirus E1A protein, enhancing its ability to bind pRb and disrupt pRb-E2F complexes.

The adenovirus E1A protein of 243 amino acids has been shown to affect a variety of cellular functions, most notably the immortalization of primary cells and the promotion of quiescent cells into S phase. The activity of E1A is derived, in part, from its association with various cellular proteins, many of which play important roles in regulating cell cycle progression. E1A is known to have multiple sites of phosphorylation. It has been suggested that cell cycle-dependent phosphorylation may also control some of E1A's functions. We find now that immune complexes of cyclin-dependent kinases such as cdk4, cdk2, and cdc2 are all capable of phosphorylating E1A in vitro. Additionally, the sites on E1A phosphorylated by these kinases in vitro are similar to the E1A sites phosphorylated in vivo. We have also found that a phosphorylated E1A is far more efficient than an unphosphorylated E1A in associating with pRB and in disrupting E2F/DP-pRB complexes as well. On the basis of our findings and the differences in timing and expression levels of the various cyclins regulating cdks, we suggest that E1A functions at different control points in the cell cycle and that phosphorylation controls, to some extent, its biological functions.     

Crystal structure of a gamma-herpesvirus cyclin-cdk complex

Several gamma-herpesviruses encode proteins related to the mammalian cyclins, regulatory subunits of cyclin-dependent kinases (cdks) essential for cell cycle progression. We report a 2.5 A crystal structure of a full-length oncogenic viral cyclin from gamma-herpesvirus 68 complexed with cdk2. The viral cyclin binds cdk2 with an orientation different from cyclin A and makes several novel interactions at the interface, yet it activates cdk2 by triggering conformational changes similar to cyclin A. Sequences within the viral cyclin N-terminus lock part of the cdk2 T-loop within the core of the complex. These sequences and others are conserved amongst the viral and cellular D-type cyclins, suggesting that this structure has wider implications for other cyclin-cdk complexes. The observed resistance of this viral cyclin-cdk complex to inhibition by the p27(KIP:) cdk inhibitor is explained by sequence and conformational variation in the cyclin rendering the p27(KIP:)-binding site on the cyclin subunit non-functional.     

Cyclin D3 is dispensable for human diffuse large B-cell lymphoma survival and growth: evidence for redundancy with cyclin E

Genomic changes disrupting the expression of cyclin D3 are associated with aberrant growth of several human B-lymphoid malignancies. We demonstrate that the human diffuse large B-cell lymphoma (DLBCL), OCI-LY18 (LY18) expresses cyclin D3 but not cyclins D1 and D2. RNA interference was used to deplete cyclin D3 from LY18 cells. Surprisingly, knockdown of cyclin D3 did not inhibit pRb phosphorylation on cdk4/6- and cdk2-specific residues or measurably affect viability and proliferation. These results suggest that cyclin D3 is dispensable in LY18 cell proliferation and survival. Similar results were obtained following depletion of cyclin E. By contrast, combined knockdown of cyclins D3 and E had substantial consequences leading to G1-phase arrest and inhibition of proliferation. Whereas cell cycle distribution was not affected following individual depletion of cdk4, cdk6, or cdk2, the combined knockdown of cdk4 and cdk6 led to accumulation of LY18 cells in G1-phase of the cell cycle and inhibition of proliferation. Likewise treatment of LY18 cells with 2-Bromo-12,13-dihydro-5H-indolo[2,3-a]pyrrolo[3,4-c]carbazole-5,7(6H)-dione, a selective inhibitor of cdk4/6, led to inhibition of proliferation. Taken together, these results uncover a built-in redundancy with cyclins D3 and E for G1-S progression. Moreover these findings highlight the rationale for simultaneous disruption of cdk4/6 as a potential therapeutic cancer strategy.     

A- and B-type cyclins differentially modulate substrate specificity of cyclin-cdk complexes.

Both cyclins A and B associate with and thereby activate cyclin-dependent protein kinases (cdks). We have investigated which component in the cyclin-cdk complex determines its substrate specificity. The A- and B-type cyclin-cdk complexes phosphorylated histone H1 and their cyclin subunits in an indistinguishable manner, irrespective of the catalytic subunit, p33cdk2 or p34cdc2. In contrast, only the cyclin A-cdk complexes phosphorylated the Rb-related p107 protein in vitro. Likewise, binding studies revealed that cyclin A-cdk complexes bound stably to p107 in vitro, whereas cyclin B-cdk complexes did not detectably associate with p107, under identical assay conditions. Binding to p107 required both cyclin A and a cdk as neither subunit alone bound to p107. These results demonstrate that although the kinase subunit provides a necessary component for binding, it is the cyclin subunit that plays the critical role in targeting the complex to p107. Finally, we show that the cyclin A-p33cdk2 complex phosphorylated p107 in vitro at most of its sites that are also phosphorylated in human cells, suggesting that the cyclin A-p33cdk2 complex is a major kinase for p107 in vivo.