Alternative splicing of human cyclin E

Aberrant Cyclin D1 splicing in cancer: from molecular mechanism to therapeutic modulation

Cyclin D1 (CCND1), a crucial mediator of cell cycle progression, possesses many mutation types with different mutation frequencies in human cancers. The G870A mutation is the most common mutation in CCND1, which produces two isoforms: full-length CCND1a and divergent C-terminal CCND1b. The dysregulation of the CCND1 isoforms is associated with multiple human cancers. Exploring the molecular mechanism of CCND1 isoforms has offer new insight for cancer treatment. On this basis, the alterations of CCND1 gene are described, including amplification, overexpression, and mutation, especially the G870A mutation. Subsequently, we review the characteristics of CCND1 isoforms caused by G870A mutation. Additionally, we summarize cis-regulatory elements, trans-acting factors, and the splice mutation involved in splicing regulation of CCND1. Furthermore, we highlight the function of CCND1 isoforms in cell cycle, invasion, and metastasis in cancers. Importantly, the clinical role of CCND1 isoforms is also discussed, particularly concerning prognosis, chemotherapy, and radiotherapy. Last, emphasis is given to the corrective strategies that modulate the cancerous CCND1 isoforms. Thus, it is highlighting significance of aberrant isoforms of CCND1 as targets for cancer therapy.

An event of alternative splicing affects the expression of two BnCYCD3-1-like genes in Brassica napus

Two full-length cDNAs of the cyclin-D3-1-like gene, named as BnCYCD3-1-like-1 and BnCYCD3-1-like-2 respectively were obtained from Brassica napus, both of which encoded a cell cycle protein CYCD3. Alternative splicing (AS) events of the two genes' transcripts were identified, assigned as BnCYCD3-1-like-1-1, BnCYCD3-1-like-1-2, BnCYCD3-1-like-2-1 and BnCYCD3-1-like-2-2 respectively. BnCYCD3-1-like-1-1 and BnCYCD3-1-like-2-1 were both fully-spliced transcripts which encoded a complete protein containing a LXCXE motif, two cyclin boxes and a PEST domain, while other two alternative splicing transcripts both resulted in the early termination of the protein translation. BnCYCD3-1-like-2-2 retained the third intron, lacking a PEST domain, while BnCYCD3-1-like-1-2 retained all the introns, lacking the C-terminal cyclin domain and a PEST domain. The expression pattern for tissue and development specification of the AS transcripts were investigated. The results showed that the standard splicing transcripts (BnCYCD3-1-like-1-1 and BnCYCD3-1-like-2-1) with complete structural domains were found with the most abundant expression in seeds, followed by leaves, and the least expression in stems. Both of BnCYCD3-1-like-2-1 and BnCYCD3-1-like-2-2 had the highest abundance in leaves, followed by roots. In addition, by applying various biotic and abiotic stresses on Brassica napus, the variations in the expression of each transcript under stress treatment were studied. Also, it was found that AS of the cyclin-D3-1-like gene may play an important role in helping Brassica napus respond to environmental stresses by coordinating the levels of transcripts of standard splicing and alternative splicing.

The 19q12 bladder cancer GWAS signal: association with cyclin E function and aggressive disease

A genome-wide association study (GWAS) of bladder cancer identified a genetic marker rs8102137 within the 19q12 region as a novel susceptibility variant. This marker is located upstream of the CCNE1 gene, which encodes cyclin E, a cell-cycle protein. We performed genetic fine-mapping analysis of the CCNE1 region using data from two bladder cancer GWAS (5,942 cases and 10,857 controls). We found that the original GWAS marker rs8102137 represents a group of 47 linked SNPs (with r(2) ≥ 0.7) associated with increased bladder cancer risk. From this group, we selected a functional promoter variant rs7257330, which showed strong allele-specific binding of nuclear proteins in several cell lines. In both GWASs, rs7257330 was associated only with aggressive bladder cancer, with a combined per-allele OR = 1.18 [95% confidence interval (CI), 1.09-1.27, P = 4.67 × 10(-5)] versus OR = 1.01 (95% CI, 0.93-1.10, P = 0.79) for nonaggressive disease, with P = 0.0015 for case-only analysis. Cyclin E protein expression analyzed in 265 bladder tumors was increased in aggressive tumors (P = 0.013) and, independently, with each rs7257330-A risk allele (P(trend) = 0.024). Overexpression of recombinant cyclin E in cell lines caused significant acceleration of cell cycle. In conclusion, we defined the 19q12 signal as the first GWAS signal specific for aggressive bladder cancer. Molecular mechanisms of this genetic association may be related to cyclin E overexpression and alteration of cell cycle in carriers of CCNE1 risk variants. In combination with established bladder cancer risk factors and other somatic and germline genetic markers, the CCNE1 variants could be useful for inclusion into bladder cancer risk prediction models.

Decreased proliferation kinetics of mouse myoblasts overexpressing FRG1

Although recent publications have linked the molecular events driving facioscapulohumeral muscular dystrophy (FSHD) to expression of the double homeobox transcription factor DUX4, overexpression of FRG1 has been proposed as one alternative causal agent as mice overexpressing FRG1 present with muscular dystrophy. Here, we characterize proliferative defects in two independent myoblast lines overexpressing FRG1. Myoblasts isolated from thigh muscle of FRG1 transgenic mice, an affected dystrophic muscle, exhibit delayed proliferation as measured by decreased clone size, whereas myoblasts isolated from the unaffected diaphragm muscle proliferated normally. To confirm the observation that overexpression of FRG1 could impair myoblast proliferation, we examined C2C12 myoblasts with inducible overexpression of FRG1, finding increased doubling time and G1-phase cells in mass culture after induction of FRG1 and decreased levels of pRb phosphorylation. We propose that depressed myoblast proliferation may contribute to the pathology of mice overexpressing FRG1 and may play a part in FSHD.

Which cyclin E prevails as prognostic marker for breast cancer? Results from a retrospective study involving 635 lymph node-negative breast cancer patients

PURPOSE

To evaluate the prognostic value of cyclin E with a quantitative method for lymph node-negative primary breast cancer patients.

PATIENTS AND METHODS

mRNA transcripts of full-length and splice variants of cyclin E1 (CCNE1) and cyclin E2 (CCNE2) were measured by real-time PCR in frozen tumor samples from 635 lymph node-negative breast cancer patients who had not received neoadjuvant or adjuvant systemic therapy.

RESULTS

None of the PCR assays designed for the specific splice variants of the cyclins gave additional prognosis-related information compared with the common assays able to detect all variants. In Cox multivariate analysis, corrected for the traditional prognostic factors, high levels of cyclin E were independently associated with a short distant metastasis-free survival [hazard ratio (HR), 3.40; P < 0.001 for CCNE1 and HR, 1.76; P < 0.001 for CCNE2, respectively]. After dichotomizing the tumors at the median level of 70% tumor cells, the multivariate analysis showed particularly strong results for CCNE1 in the group of 433 patients with stroma-enriched primary tumors (HR, 5.12; P < 0.001). In these tumors, the worst prognosis was found for patients with estrogen receptor-negative tumors expressing high CCNE1 (HR, 9.89; P < 0.001) and for patients with small (T1) tumors expressing high CCNE1 (HR, 8.47; P < 0.001).

CONCLUSION

Our study shows that both CCNE1 and CCNE2 qualify as independent prognostic markers for lymph node-negative breast cancer patients, and that CCNE1 may provide additional information for specific subgroups of patients.

Expression of a cyclin E1 isoform in mice is correlated with the quiescent cell cycle status of hepatocytes in vivo

Cyclin E1 controls G1/S phase transition of the eukaryotic cell cycle. We report the impact of alternative spliced cyclin E1 isoforms on cell cycle regulation in hepatocytes. We show that expression of new cyclin E1 mRNA variants IN3, Delta4, and Delta5 is associated with retarded proliferation in murine hepatocellular carcinoma. Additionally, we demonstrate that a new cyclin E1 isoform Delta3/8 lacking the central part of wild-type mRNA is expressed predominantly in nonproliferating murine hepatocytes. Following partial hepatectomy, Delta3/8 is downregulated when hepatocytes enter the cell cycle from quiescence. The Delta3/8 protein does not exhibit any cyclin box motif but binds cyclin-dependent kinase 2 without stimulating kinase activity. We demonstrate that Delta3/8 lacks any nuclear localization signal and is exclusively located in the cytoplasm. Overexpression of Delta3/8 in cultured cells leads to a delayed G0-G1 transition, indicating that this splice variant helps to maintain a quiescent state of hepatocytes. In conclusion, we identified an isoform of cyclin E1 involved in G0 maintenance and suggest an additional mechanism for cell cycle control.

Cyclin E in normal and neoplastic cell cycles

Cyclin E-Cdk2 has long been considered an essential and master regulator of progression through G1 phase of the cell cycle. Although recent mouse models have prompted a rethinking of cyclin E function in mammals, it remains clear that cyclin E impacts upon many processes central to cell division. Normal cells maintain strict control of cyclin E activity, and this is commonly disrupted in cancer cells. Moreover, cyclin E deregulation is thought to play a fundamental role in tumorigenesis. In this review, we discuss the regulation and functions of cyclin E in normal and neoplastic mammalian cells.

Novel role for cyclin-dependent kinase 2 in neuregulin-induced acetylcholine receptor epsilon subunit expression in differentiated myotubes

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

Cyclin E

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

DIP-chip: rapid and accurate determination of DNA-binding specificity

We have developed a new method for determining the DNA-binding specificity of proteins. In DIP-chip (DNA immunoprecipitation with microarray detection), protein.DNA complexes are isolated from an in vitro mixture of purified protein and naked genomic DNA. Whole-genome DNA microarrays are used to identify the protein-bound DNA fragments, and the sequence of the identified fragments is used to derive binding-site descriptions. Using objective criteria for assessing the accuracy of DNA-binding motifs, and using yeast Leu3p as a model, we demonstrate that motifs determined by DIP-chip are as effective at predicting the location of bound proteins in vivo as are motifs determined by conventional low-throughput in vitro methods.

Loss of cyclin E requirement in cell growth of an oral squamous cell carcinoma cell line implies deregulation of its downstream pathway

Cyclin E and Cdk2 have been shown to play an important role in G1/S transition of the cell cycle. Two E-type cyclins (E1 and E2) have been identified to date and share functionally similarities. Upregulation of these cyclins has been observed frequently in human cancers. We examined the expression profile of cyclin E1 and E2 in cell lines derived from human oral squamous cell carcinoma (SCC), and found that the expression of cyclin E1 protein was hardly detected in HSC-2 cells. Although cyclin E2 was abundantly expressed, histone H1 kinase activities of both E-type cyclins were virtually undetectable in this cell line. Inhibition of cyclin E1, but not that of E2, by using vectors expressing antisense-oriented their cDNAs induced drastic growth suppression on HOC313 cells that express both E-type cyclins. Inhibition of neither cyclin E1 nor E2 suppressed the growth of HSC-2 cells, and compensatory elevation of cyclin E1 was not evident in cyclin E2-inhibited HSC-2 cells. In contrast, HSC-2 cells expressed cyclin D1 and hyperphosphorylated forms of Rb family proteins, and were arrested in G1 by overexpression of p16(INK4), a specific inhibitor against D-type cyclin activity. These results indicate that HSC-2 cells lost proper growth control specifically mediated by cyclin E and suggest that deregulation of its downstream pathway may contribute to tumorigenesis of oral SCC.

E- and A-type cyclins as markers for cancer diagnosis and prognosis

Cyclin-dependent kinase (CDK)2 interacting cyclins perform essential functions for DNA replication and cellular proliferation. The human genome encodes two E-type cyclins (E and E2) and two A-type cyclins (A1 and A2). Dysregulation of the CDK2-bound cyclins plays an important role in the pathogenesis of cancer. Cyclin A2 is associated with cellular proliferation and can be used for molecular diagnostics as a proliferation marker. In addition, cyclin A2 expression is associated with a poor prognosis in several types of cancer. Cyclin A1 is a tissue-specific cyclin that is highly expressed in acute myeloid leukemia and in testicular cancer. High levels of cyclin E expression are found in many types of cancer. Overexpression of cyclin E at the mRNA level can be based on gene amplification and transcriptional mechanisms. In addition, proteolytically cleaved forms of cyclin E that show oncogenic functions have been described. Cyclin E plays a critical role for G1/S transition. Its overexpression is not only associated with proliferation but rather indicates a more malignant phenotype which is likely to be linked to the induction of chromosomal instability. These biological functions of cyclin E relate to a poor prognosis when high cyclin E levels are found. The link between cyclin E and poor prognosis is well established in breast and lung cancer but is likely to be observed in other cancers as well. The second E-type cyclin, cyclin E2, has been shown to be overexpressed in breast cancers although the potential role as a diagnostic or prognostic marker is unknown. This review provides an overview of the potential of cyclins E and A as markers for diagnosis and prognosis in human cancer.

Overexpression of p21, cyclin E and decreased expression of p27 in DMBA (7, 12-dimethylbenzanthracene)-induced rat ovarian carcinogenesis

Ovarian cancer is a common gynecological malignancy and a leading cause of death in women. Inactivation of the tumor suppressor gene and deregulation of cyclin E are frequent in human ovarian cancer. The objective of this study was to investigate the expressions and roles of cyclin E, p21 and p27 in 7, 12-dimethylbenzanthracene (DMBA)-induced ovarian tumors in rats. The expressions of cyclin E, p21 and p27 were evaluated by immunohistochemistry and western blot analysis. The expressions of cyclin E and p21 in ovarian tumors was higher than that in normal ovarian surface epithelium. In contrast, the expression of p27 in ovarian tumors was lower than that in normal ovarian surface epithelium. But there were no differences among the cancer types. Positive correlation was present between cyclin E and p21. p27 was negatively correlated with cyclin E and p21. These results suggest that the increased expression of cyclin E and p21, and the decreased expression of p27, occur in DMBA-induced rat ovarian carcinogenesis and result in tumor progression.

Cyclin E in breast tumors is cleaved into its low molecular weight forms by calpain

Abundant levels of the hyperactive low molecular weight (LMW) forms of cyclin E contribute to deregulation of Cdk2 in breast tumors, but the mechanism through which they arise is not fully understood. Here, we explored the hypothesis that post-translational processing by a protease generates the LMW forms of cyclin E in breast tumors. In ZR75 tumor cell lysates, calcium-induced cyclin E truncation into peptides corresponding in size with LMW forms of cyclin E in tumor tissues. Calpeptin inhibited calcium-stimulated cyclin E truncation, indicating that cleavage resulted from activity of the calcium-dependent protease, calpain. Consistently, calcium+calpain caused truncation of cyclin E immunoprecipitated from tumor cells and tissues. Calcium also caused truncation of the calpain regulatory subunit in tumor cell lysates, indicating that elevated calpain activity accompanies cyclin E truncation. Increased levels of the calpain small subunit were also observed in breast tumors, and significant amounts of its proteolyzed forms indicated increased calpain activity. While elastase also caused cyclin E truncation, the cleavage pattern was distinct from that generated by calpain, suggesting discrete mechanisms in regulating the formation of LMW cyclin E in breast tumors. Treatment of ZR75 cultures with calcium+A23187 recapitulated the formation of the calcium/calpain-induced LMW forms of cyclin E. Altered calcium homeostasis and/or inability of the endogenous calpain inhibitor to control the activity of high levels of the calpain small subunit may contribute to increased calpain activity in breast tumors, causing abundant levels of LMW cyclin E.

The oncogenic activity of cyclin E is not confined to Cdk2 activation alone but relies on several other, distinct functions of the protein

We have previously shown that cyclin E can malignantly transform primary rat embryo fibroblasts in cooperation with constitutively active Ha-Ras. In addition, we demonstrated that high level cyclin E expression potentiates the development of methyl-nitroso-urea-induced T-cell lymphomas in mice. To further investigate the mechanism underlying cyclin E-mediated malignant transformation, we have performed a mutational analysis of cyclin E function. Here we show that cyclin E mutants defective to form an active kinase complex with Cdk2 are unable to drive cells from G(1) into S phase but can still malignantly transform rat embryo fibroblasts in cooperation with Ha-Ras. In addition, Cdk2 activation is not a prerequisite for the ability of cyclin E to rescue yeast triple cln mutations. We also find that the oncogenic properties of cyclin E did not entirely correspond with its ability to interact with the negative cell cycle regulator p27(Kip1) or the pocket protein p130. These findings suggest that the oncogenic activity of cyclin E does not exclusively rely on its ability as a positive regulator of G(1) progression. Rather, we propose that cyclin E harbors other functions, independent of Cdk2 activation and p27(Kip1) binding, that contribute significantly to its oncogenic activity.

Variability of placental expression of cyclin E low molecular weight variants

Cyclin E, a G(1) cyclin serving to activate cyclin-dependent kinase 2, is the only cyclin gene for which alternative splicing leading to structurally different proteins has been described. Different cyclin E proteins are present in tumor tissues but absent from normal (steady) tissues. Cyclin E contributes to the regulation of cell proliferation and ongoing differentiation and aging. Because trophoblast has invasive properties and differentiates into syncytium and placental aging may develop at term, we examined cyclin E protein variants in human placenta. Placental samples were collected from 27 deliveries between 33 and 41 wk and were compared with ovarian cancer (positive control). Both placental and tumor tissues showed seven cyclin E low molecular weight (LMW) bands migrating between 50 and 36 kDa. Placental expression of cyclin E showed certain variability among cases. Lowest cyclin E expression was detected in normal placentas (strong expression of Thy-1 differentiation protein in villous core and low dilatation of villous blood sinusoids). Abnormal placentas (significant depletion of Thy-1 and more or less pronounced dilatation of sinusoids) showed significant increase either of all (early stages of placental aging) or only certain cyclin E proteins (advanced aging). Our studies indicate that a similar spectrum of cyclin E protein variants is expressed in the placental and tumor tissues. Low cyclin E expression in normal placentas suggests a steady state. Overexpression of all cyclin E proteins may indicate an activation of cellular proliferation and differentiation to compensate for developing placental insufficiency. However, an enhanced expression of some cyclin E LMW proteins only might reflect an association of cyclin E isoforms with placental aging or an inefficient placental adaptation.

Analysis of Drosophila cyclin EI and II function during development: identification of an inhibitory zone within the morphogenetic furrow of the eye imaginal disc that blocks the function of cyclin EI but not cyclin EII

The Drosophila cyclin E (DmcycE) gene gives rise to two transcripts encoding proteins that differ at their N termini, DmcycEII and DmcycEI. This study presents the first in vivo dissection of Cyclin E function. Ectopic expression studies using N- and C-terminal deletions of DmcycEI revealed that a region of 322 residues surrounding the cyclin box is sufficient to induce entry of G1-arrested larval eye imaginal disc cells into S phase. Ectopic expression of DmcycEI in the eye disc has been previously shown to drive anterior, but not posterior, G1-phase cells within the morphogenetic furrow (MF) into S phase. Significantly, ectopic expression of DmcycEII and N-terminal deletions of DmcycEI were able to drive all G1 cells within the morphogenetic furrow into S phase, while a C-terminal deletion of DmcycEI could not. The p21 homolog Dacapo was shown by yeast two-hybrid, coimmunolocalization, and in vivo functional studies not to be the mediator of the DmcycEI inhibition in posterior part of the MF. Taken together, these results reveal a novel zone within the posterior region of the MF where DmcycEI but not DmcycEII function is inhibited, and suggest that DmcycEII is a more potent inducer of S phase.

Chemically induced rat mammary tumor treated with tamoxifen showed decreased expression of cyclin D1, cyclin E, and p21(Cip1)

We studied the effects of tamoxifen (TAM) on the growth of 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumor and the expression of cyclin D1, cyclin E, p21(Cip1), and estrogen receptors (ER) by performing immunohistochemistry and Western blot analysis. When tumor size reached between 10 and 15mm in the largest dimension, the rats were divided into a DMBA-control group and a DMBA-TAM group. The administration of TAM markedly decreased the tumor development and showed decreased expression of bromodeoxyuridine, cyclin D1, cyclin E, and p21(Cip1) when compared with those of the DMBA-control group; however, a few tumors showed progressive growth in spite of TAM treatment. These tumors had decreased expression of ER. This study suggests that TAM suppresses tumor development through the down-expression of cyclin D1 and cyclin E.

The potential of iron chelators of the pyridoxal isonicotinoyl hydrazone class as effective antiproliferative agents, IV: The mechanisms involved in inhibiting cell-cycle progression

Some chelators of the pyridoxal isonicotinoyl hydrazone class have antiproliferative activity that is far greater than desferrioxamine (DFO). In this study, DFO was compared with one of the most active chelators (311) on the expression of molecules that play key roles in cell-cycle control. This was vital for understanding the role of iron (Fe) in cell-cycle progression and for designing chelators to treat cancer. Incubating cells with DFO, and especially 311, resulted in a decrease in the hyperphosphorylated form of the retinoblastoma susceptibility gene product (pRb). Chelators also decreased cyclins D1, D2, and D3, which bind with cyclin-dependent kinase 4 (cdk4) to phosphorylate pRb. The levels of cdk2 also decreased after incubation with DFO, and especially 311, which may be important for explaining the decrease in hyperphosphorylated pRb. Cyclins A and B1 were also decreased after incubation with 311 and, to a lesser extent, DFO. In contrast, cyclin E levels increased. These effects were prevented by presaturating the chelators with Fe. In contrast to DFO and 311, the ribonucleotide reductase inhibitor hydroxyurea increased the expression of all cyclins. Hence, the effect of chelators on cyclin expression was not due to their ability to inhibit ribonucleotide reductase. Although chelators induced a marked increase in WAF1 and GADD45 mRNA transcripts, there was no appreciable increase in their protein levels. Failure to translate these cell-cycle inhibitors may contribute to dysregulation of the cell cycle after exposure to chelators. (Blood. 2001;98:842-850)

Reciprocal expressions of cyclin E and cyclin D1 in hepatocellular carcinoma

Deregulation of the cell cycle by overexpression of G1 cyclins, cyclin E and cyclin D1 genes, has been demonstrated to be a prerequisite for the development of human cancer. Recently, cyclin E is proposed to be sufficient for the progression of the G1 cell cycle without cyclin D1. Here we show that the proposed model system was specifically present in human hepatocellular carcinoma (HCC) unlike other human cancers. Of 31 HCC tissues analyzed, 21 (67.7%) exhibited an overexpression of cyclin E protein. In contrast to cyclin E gene expression, cyclin D1 expression was strongly downregulated in 19 (61.2%) HCCs. Interestingly, 65% of HCC tissues with overexpression of the cyclin E gene exhibited downregulation of cyclin D1, suggesting reciprocal deregulation of these cyclins in the G1 progression of the cell cycle. Southern blot analysis proved the amplification of cyclin E gene in HCC with a high level of overexpression. The present findings suggest that the reciprocal deregulation of cyclin E lacking cyclin D1 expression might play a role in G1 progression and the development of HCC.

Increased expression of cyclin D1, cyclin E and p21(Cip1) associated with decreased expression of p27(Kip1) in chemically induced rat mammary carcinogenesis

We induced rat mammary tumors in 7-week-old female Sprague-Dawley rats by intragastric administration of 7,12-dimethylbenz(a)anthracene (DMBA), and analyzed by immunohistochemistry the expression of cyclin D1, cyclin E, p21(Cip1), and p27(Kip1) in carcinomas, atypical tumors, and benign tumors as well as normal mammary glands from the control group. Proliferation status was assessed by immunohistochemistry using bromodeoxyuridine (BrdU). A sequential increase in cyclin D1-, cyclin E-, and p21(Cip1)-positive epithelial cells was observed from normal mammary glands, to atypical tumors, to carcinomas. In contrast, carcinomas showed a significantly lower number of epithelial cells immunoreactive to p27(Kip1) when compared with atypical tumors, benign tumors and normal mammary glands. The immunoreactivities of BrdU, cyclin D1, cyclin E, and p21(Cip1) were positively correlated, whereas that of p27(Kip1) appeared inversely correlated to those of the others. Reverse transcriptase-polymerase chain reaction (RT-PCR) and western blot analysis were also performed to determine the mRNA and protein levels of cyclins and cyclin-dependent kinase inhibitors in tumors and normal mammary glands. The protein levels for cyclin D1, cyclin E and p21(Cip1) in carcinomas and atypical tumors were significantly higher than those in benign tumors, while normal mammary glands showed negligible expression. On RT-PCR, tumors showed higher mRNA levels of cyclin D1 and cyclin E than those of normal mammary glands. Our results suggest that rat mammary carcinogenesis involves increased expression of cyclin D1, cyclin E, and p21(Cip1), associated with decreased expression of p27(Kip1).

Expression of cyclin E in endomitotic silk-gland cells from mulberry silkworm

The silk glands of mulberry silkworm Bombyx mori are endoreplicating tissues in which the genomic DNA undergoes multiple rounds of replication without mitosis and nuclear division. In the absence of normal mitotic division, the cell cycle essentially alternates between the G1 and S phases. Cyclin E is crucial for the G1/S transition in both mitotic and endoreplicating cycles. We have cloned and characterized cyclin E (cyclin box) from B. mori, which is nearly identical to the Drosophila cyclin E box except for an insertion of 21 amino acids. Two distinct cyclin E transcripts (1.7 and 2.1 kb) were detected in the silk-gland cells of B. mori and in the B. mori-derived embryonic cell line, BmN. Using anti Cyclin E antibodies two protein bands of 52 and 44kDa were detected in silk glands and BmN cells at comparable levels. Both BmN- and the silk-gland cells showed the presence of the interacting kinase Cdk2. Transcripts of the mitotic cyclin, cyclin B, were barely detectable in the endoreplicating silk-gland cells and amounted to only 4-7% of that seen in the mitotically dividing BmN cells. The near absence of cyclin B transcripts and the abundant expression of cyclin E in the silk glands correlate well with the alternation of only G1 and S phases without the intervening mitosis in these cells.

Human CDK10 gene isoforms

The CDK10/PISSLRE gene has been shown to encode two different CDK-like putative kinases. The function(s) of the gene products are unknown, although a role at the G2/M transition has been suggested. We characterised two novel cDNAs. CDK10 mRNA quantity was not found to be correlated with cell proliferation status in HeLa or WI38 cell cultures or in human tissues. Relative levels of the four CDK10 isoforms were studied by RT-PCR, of which three were principally expressed. The two initially cloned isoforms predominated in human tissues, except in brain and muscle. Relative isoform levels did not vary during the cell cycle in culture, except when cells entered into the cell cycle. Finally, the predominant isoforms were shown to have different translation initiation sites and to have different subcellular distribution, due to an alternatively spliced nuclear localisation signal.

Involvement of cyclins in cell proliferation and their clinical implications in soft tissue smooth muscle tumors

Expression of cyclins A and E and cyclin-dependent kinase 2 (cdk2) was examined immunohistochemically in 55 cases of soft tissue smooth muscle tumors, including vascular leiomyoma, and compared to expression of Ki-67 and proliferating cell nuclear antigen. Cyclin A was expressed in 70% of the leiomyoma cases, but with much lower labeling indexes than in leiomyosarcoma. Cyclin E was expressed exclusively in leiomyosarcoma. Although the differences of cyclin A- and cyclin E-labeling indexes between leiomyoma and leiomyosarcoma were statistically significant, no significant differences were found in the other markers. Furthermore, cyclin A- and/or E-positivity predicted a poor prognosis in recurrence- or metastasis-free survivals and overall survival. Immunoblotting revealed that cyclins A and E were expressed, in complex with cdk2, exclusively in tumors. In addition, not only leiomyosarcoma, but also leiomyoma specimens that exhibited negligible levels of complex expression, manifested detectable cdk2 activity. These results suggest 1) up-regulation of active cyclin A/cdk2 expression and associated kinase activity is critical for unrestrained cell proliferation; 2) cyclin E/cdk2 complexes may play a crucial role in leiomyosarcoma; 3) immunohistochemical detection of cyclins can be a more reliable tool for differential diagnosis between leiomyoma versus leiomyosarcoma than that of Ki-67 or proliferating cell nuclear antigen, and be a possible prognostic indicator.

Regulation of G(1) cyclin-dependent kinases in the liver: role of nuclear localization and p27 sequestration

Recent studies suggest that cyclin D1 mediates progression of hepatocytes through G(1) phase of the cell cycle. The present study further examines the regulation of cyclin D1-dependent kinase activity and the interplay between cyclin D1 and other G(1) phase regulatory proteins during liver regeneration. After 70% partial hepatectomy in rats, there was upregulation of kinase activity associated with cyclins (A, D1, D3, and E), cyclin-dependent kinases (Cdk2 and Cdk4), and Cdk-inhibitory proteins (p27, p107, and p130). Although cyclin D1/Cdk4 complexes were more abundant in the cytoplasmic fraction after partial hepatectomy, kinase activity was detected primarily in the nuclear fraction. Cytoplasmic cyclin D1/Cdk4 complexes were activated by recombinant cyclin H/Cdk7. Because endogenous Cdk7 activity was found in the nucleus, this suggests that activation of cyclin D1/Cdk4 requires nuclear importation and subsequent phosphorylation by cyclin H/Cdk7. Recombinant cyclin E/Cdk2 was inhibited by extracts from quiescent liver, and cyclin D1 could titrate out this inhibitory activity. Induction of cyclin D1 was accompanied by increased abundance of cyclin D1/p27 complexes, and most p27 was sequestered by cyclin D1 after partial hepatectomy. Thus cyclin D1 appears to play two roles during G(1) phase progression in the regenerating liver: it forms a nuclear kinase complex, and it promotes activation of Cdk2 by sequestering inhibitory proteins such as p27. These experiments underscore the complexity of cyclin/Cdk regulatory networks in the regenerating liver.

Cyclin E in human cancers

Regulators of the cell cycle such as cyclin E play an important part in neoplasia. The cyclin E protein forms a partnership with a specific protein kinase. This complex phosphorylates key substrates to initiate DNA synthesis. Cyclin-dependent kinase inhibitors (CKIs) are able to suppress the activity of cyclin E. Various substances (including proteins produced by oncogenic viruses) affect cyclin E directly or indirectly through an interaction with CKIs. These interactions are important in elucidating the mechanisms of neoplasia. They may also provide prognostic information in a wide range of common cancers. Cyclin E may even be a target for treatment of cancers in the future.

Cyclins and breast cancer

Cyclins are regulatory subunits for cyclin dependent kinases in the coordination of the cell cycle. Cyclins can also serve non-cell cycle functions, such as the transactivation of estrogen receptor by cyclin D. Evidence for the participation of the G1 cyclins D and E in breast cancer is summarized, including transgenic and knockout mice, transfections, and expression patterns in cohort studies. Overexpression of cyclin D has been reported in ductal carcinoma in situ (DCIS), and similar overexpression of cyclin E is suggested. Strategies to reduce cyclin expression are discussed as potential prevention efforts.

The plant cell cycle in context

Biological scientists are eagerly confronting the challenge of understanding the regulatory mechanisms that control the cell division cycle in eukaryotes. New information will have major implications for the treatment of growth-related diseases and cancer in animals. In plants, cell division has a key role in root and shoot growth as well as in the development of vegetative storage organs and reproductive tissues such as flowers and seeds. Many of the strategies for crop improvement, especially those aimed at increasing yield, involve the manipulation of cell division. This review describes, in some detail, the current status of our understanding of the regulation of cell division in eukaryotes and especially in plants. It also features an outline of some preliminary attempts to exploit transgenesis for manipulation of plant cell division.

Active cyclin A-CDK2 complex, a possible critical factor for cell proliferation in human primary lung carcinomas

Expression of cyclins A and E and cyclin-dependent kinase 2 (CDK2) was examined immunohistochemically in 190 cases of human lung carcinoma. Cyclin A and CDK2 were expressed in the majority of squamous cell carcinomas, small cell carcinomas, and large cell carcinomas, but in significantly fewer cases of adenocarcinomas. Cyclin E was expressed in a minority of all subtypes. In particular, well differentiated cells in squamous cell carcinoma stained positively for cyclin E; in contrast, cyclin A was expressed in the nonkeratinized proliferating areas of the tumor nests. Immunoblotting revealed that all these proteins were expressed at higher levels in tumor tissues than in adjacent normal tissues. Immunoprecipitation also revealed higher levels of cyclin A and cyclin E associated with CDK2 in tumor tissues. Furthermore, tumor tissues which exhibited higher cyclin A and CDK2 expression also had higher CDK2 kinase activity. However, cyclin E-associated kinase activity was barely detectable even in tumor samples exhibiting higher cyclin E expression. Consistent with these data, elevated expression of cyclin A correlated to shorter survival periods in contrast to expression of cyclin E, which correlated to longer survival periods. These results suggest that in human lung carcinomas, elevated expression of active cyclin A-CDK2 complexes with associated higher CDK2 kinase activity is critical for promoting cell cycle progression and unrestrained proliferation of tumor cells and can be a predictive marker for patients' prognosis. On the other hand, immunohistochemical detection of cyclin E-CDK2 reflects accumulation of inactive forms of protein complexes, implying differentiation or senescence of the tumor and the better prognosis.

The role of cyclin E in cell proliferation, development and cancer

Normal cell proliferation is under strict regulation governed by checkpoints located at distinct points in the cell cycle. The deregulation of these checkpoint events and the molecules associated with them may transform a normal cell into a cancer cell. One of these checkpoints whose deregulation results in transformation occurs at the Restriction point, near the G1/S boundary. The periodic appearance of one of the recently identified regulatory cyclins, cyclin E, coincides precisely with the timing of the Restriction point. The deregulation in the expression and activity of cyclin E has been associated with a number of cancers and is thought to be involved in the process of oncogenesis. In this chapter, we summarise the current knowledge on the regulation and apparent function of cyclin E in normal proliferating cells and in developing tissue and alterations of these processes in cancer.

The role of cyclin E in the regulation of entry into S phase

Cyclin E is a crucial regulator of entry into S phase in higher eukaryotes and acts in association with the protein kinase cdk2. Cyclin E expression is transcriptionally controlled in mammalian cells resulting in a maximum just before entry into S phase. Premature expression of cyclin E advances entry into S phase, while lack of cyclin E prevents entry into S phase. Cyclin E/cdk2 activity is regulated at multiple levels (by transcription, phosphorylation and inhibitor proteins) and appears to be involved in triggering initiation of DNA replication and in regulating genes important for proliferation and progression through S phase.

A presumptive developmental role for a sea urchin cyclin B splice variant

We show that a splice variant-derived cyclin B is produced in sea urchin oocytes and embryos. This splice variant protein lacks highly conserved sequences in the COOH terminus of the protein. It is found strikingly abundant in growing oocytes and cells committed to differentiation during embryogenesis. Cyclin B splice variant (CBsv) protein associates weakly in the cell with Xenopus cdc2 and with budding yeast CDC28p. In contrast to classical cyclin B, CBsv very poorly complements a triple CLN deletion in budding yeast, and its microinjection prevents an initial step in MPF activation, leading to an important delay in oocyte meiosis reinitiation. CBsv microinjection in fertilized eggs induces cell cycle delay and abnormal development. We assume that CBsv is produced in growing oocytes to keep them in prophase, and during embryogenesis to slow down cell cycle in cells that will be committed to differentiation.

Cell cycle-mediated regulation of hepatic regeneration

BACKGROUND

Hepatic regeneration after partial hepatectomy (PH) is characterized by a synchronous induction of normally quiescent hepatocytes to reenter the cell cycle, leading to a complete restoration of hepatic mass. Cell cycle progression requires activation of cyclin-dependent kinases (Cdks) that are regulated by cyclins and Cdk inhibitors.

METHODS

Protein expression of the cyclins (D-type and E), Cdks (Cdk2 and 4), and Cdk inhibitors (p21 and p27) was measured by Western blot after SHAM operation or PH in F344 rats. In addition, Cdk2-associated kinase activity was measured.

RESULTS

Rapid induction of D-type and E cyclins, as well as their catalytic partners, Cdk2 and Cdk4, occurred after PH in rats. Complexes containing cyclin E and Cdk2 assembled in the regenerating liver, leading to increased Cdk2-associated kinase activity. The regenerating liver returned to preresection weight by day 7, at which time the Cdk2 activity also returned to SHAM levels. Biphasic induction of the Cdk inhibitor p21 was observed; the first peak occurred as early as 6 hours after PH, with a subsequent peak in expression occurring at 24 to 72 hours after PH.

CONCLUSIONS

Taken together, these data support the concept that cyclins, Cdks, and Cdk inhibitors regulate cell cycle progression in the regenerating liver. In addition, the induction of p21 at two time points suggests that this protein may regulate both early proliferation and subsequent inhibition of hepatocyte regeneration.

Overexpression of Cyclin E in Acute Myelogenous Leukemia

Cyclin E is one of the G1 cyclins that play an important role in cell proliferation. Overexpression of cyclin E protein has been reported in several solid tumors, but little is known about the involvement of cyclin E in leukemia. In this study, we analyzed the expression of cyclin E gene product in 85 patients with acute myelogenous leukemia (AML) by Western blot analysis. In 23 of 85 AML samples (27%), cyclin E expression was enhanced in blasts. Among the French-American-British classification of AML, the ratio of the samples with enhanced cyclin E expression was high in M5 and low in M2 and M3. No rearrangements were observed by Southern blot analysis in these AML blasts with enhanced cyclin E expression. Flow cytometric analysis showed no correlation between overexpression of cyclin E and cell cycle distribution. Immunoblot analysis of cyclin D1 showed no correlation between overexpression of cyclin E and that of cyclin D1. Interestingly, p27 expression detected by Western blotting was apparently enhanced in 18 of 23 AML cells with enhanced cyclin E expression but none of 14 AML cells without enhanced cyclin E exhibited enhanced p27 expression. The rates of complete remission and of disease-free survival of the patients with M4 or M5 leukemia blasts with overexpressed cyclin E seemed to be low. Therefore, we suggest the necessity of a larger-scale study to elucidate the contribution of cyclin E overexpression to the phenotype and the prognosis of certain AML.

Alternative splicing of cyclin transcripts in maize endosperm

Cyclins are the regulatory subunits of cyclin-dependent protein kinases. In investigations of the expression of a cyclin gene during maize endosperm development, we detected a cyclin transcript with a 63-bp deletion in the region encoding the conserved 'cyclin box' where cyclin interacts with p34cdc2, the catalytic domain of the cyclin-dependent protein kinase. Analysis of cDNA and genomic sequences, and other observations, indicated that the deletion was caused by alternative splicing of a retained intron in the normally spliced transcript. Whereas the normally spliced cyclin RNA was mitotically functional, as indicated by its ability to promote maturation of Xenopus oocytes, the alternatively spliced transcript was unable to promote maturation. In addition to maize endosperm, the alternatively spliced cyclin was detected in apical meristem, mature leaf, root tip and mature root.

Differential expression of G1 cyclins during human placentogenesis

Cyclins are proteins that support the progression of cell-cycle stages in proliferating cells. The purpose of this study was to determine which of the cyclin genes is involved in the regulation of normal human trophoblast proliferation. The presence and cellular localization of four G1 cyclins D1, D2, D3 and E, were determined by immunohistochemistry. This analysis indicated that cyclins E and D3 are the predominant cyclins in villous trophoblast. D2 was present only within the villous core, in fetal macrophages. Positive immunoreactivity for cyclin D1 was strongest in second and third trimester placentae, in the cells lining the intravillous vessels with additional reactivity in extravillous cytotrophoblasts. Because cyclin E protein was present in a greater percentage of cells than those that are dividing, Western blot analysis was performed to validate the fidelity of the immunohistochemistry data. The results of the Western analysis revealed that two forms of cyclin E protein of the appropriate size were present. Data collected from this study suggest that within the trophoblast lineage, cyclins D3 and E are important cell cycle regulatory proteins, and further, that cyclin E may function in trophoblast terminal differentiation as well.

Xenopus cyclin E, a nuclear phosphoprotein, accumulates when oocytes gain the ability to initiate DNA replication

The capacity to initiate DNA replication appears during oocyte maturation in Xenopus. Initiation of S phase is driven by several components which include active cyclin/cdk complexes. We have identified three Xenopus cyclin E clones showing 59% amino acid identity with human cyclin E. The recruitment of cyclin E mRNA, like cdk2 mRNA, into the polysomal fraction during oocyte maturation, results in the accumulation of the corresponding proteins in unfertilized eggs. Cyclin E mRNA remains polyadenylated during cleavage and anti-cyclin E antibodies detect Xlcyclin E in embryonic nuclei at this time. Cdk2 protein is necessary for the phosphorylation of radiolabelled cyclin E added to egg extracts. Radiolabelled Xlcyclin E enters interphase nuclei and, though stable through interphase and mitosis, is not associated with condensed mitotic chromatin. In egg extracts, endogenous Xlcyclin E rapidly associates with nuclei before S phase and remains nuclear throughout interphase, becoming nucleoplasmic in G2/prophase. Under conditions where initiation of replication is limiting in extracts, Xlcyclin E associates only with those nuclei that undergo S phase. These features are entirely consistent with the view that Xlcyclin E is required for initiation of S phase.

Constitutive overexpression of CDK2 inhibits neuronal differentiation of rat pheochromocytoma PC12 cells

Changes in the levels of cyclins A, D, and E, p21, and cyclin-dependent kinase 2 (CDK2) were examined in rat pheochromocytoma PC12 cells during neuronal differentiation induced by nerve growth factor (NGF). Expression of cyclin A decreased to an undetectable level after 5 days of exposure to NGF, while expression of CDK2 decreased gradually after day 3. In contrast, the levels of cyclins D1 and E increased gradually through day 10, yet the amount of cyclin E associated with CDK2 decreased concomitant with a decrease in the CDK2 protein level. p21 expression increased gradually after day 7, while the level of CDK2-associated p21 remained unchanged. When human cDNAs encoding cyclins and CDK2 were introduced into PC12 cells, only CDK2 overexpression inhibited NGF-induced differentiation. The cell lines overexpressing CDK2 showed stable and high levels of CDK2 kinase activity during differentiation, whereas parental and vector-transfected cell lines displayed a marked decline in CDK2 kinase activity 1 day after NGF treatment. In cell lines overexpressing cyclins A, D, and E, this reduction of the kinase activity was not apparent until day 3. These results suggest that down-regulation of CDK2 activity is a crucial event for the neuronal differentiation of PC12 cells.

Frequent amplification of the cyclin E gene in human gastric carcinomas

We searched for genetic alterations of the cyclin D1 and cyclin E genes in 45 human gastric carcinoma tissues. Expression of cyclin E mRNA and protein was also analyzed in eight of them by Northern and Western blots and immunohistochemical staining. The cyclin E gene was amplified 3-10 fold in seven gastric cancer tissues (15.6%), of which six were advanced gastric cancers. All of the cases with the cyclin E gene amplification displayed lymph node metastasis. Moreover, the case with the gene amplification overexpressed the cyclin E mRNA and protein. One of eight gastric cancer cell lines, MKN-7, shared the cyclin E gene amplification, and all of the gastric cancer cell lines expressed high levels of the cyclin E mRNA and protein even without gene amplification. Amplification of the cyclin D1 gene was not observed in any of the gastric carcinoma tissues or gastric carcinoma cell lines. These results suggest that the gene amplification and overexpression of cyclin E play an important role in the abnormal growth and progression of gastric carcinoma.

Yeast vectors for the controlled expression of heterologous proteins in different genetic backgrounds

An expression system for Saccharomyces cerevisiae (Sc) has been developed which, depending on the chosen vector, allows the constitutive expression of proteins at different levels over a range of three orders of magnitude and in different genetic backgrounds. The expression system is comprised of cassettes composed of a weak CYC1 promoter, the ADH promoter or the stronger TEF and GPD promoters, flanked by a cloning array and the CYC1 terminator. The multiple cloning array based on pBIISK (Stratagene) provides six to nine unique restriction sites, which facilitates the cloning of genes and allows for the directed cloning of cDNAs by the widely used ZAP system (Stratagene). Expression cassettes were placed into both the centromeric and 2 mu plasmids of the pRS series [Sikorski and Hieter, Genetics 122 (1989) 19-27; Christianson et al., Gene 110 (1992) 119-122] containing HIS3, TRP1, LEU2 or URA3 markers. The 32 expression vectors created by this strategy provide a powerful tool for the convenient cloning and the controlled expression of genes or cDNAs in nearly every genetic background of the currently used Sc strains.

Genomic organization of the mouse cyclin D1 gene (Cyl-1)

To determine the genomic organization of the mouse cyclin D1 locus (Cyl-1), a series of cosmids and cDNAs were recovered by hybridization with a genomic probe representing the 5' end of the homologous human gene, CCND1. Primer extension indicated that transcripts originate from one of three adjacent nucleotides at a single start site. Two overlapping cDNA clones that essentially accounted for the complete sequence of the larger 4.0-kb Cyl-1 transcript were characterized. A combination of RNase protection and sequencing across intron-exon boundaries established that the gene is organized into five coding exons with a long 3' untranslated region. Repeated attempts to isolate clones corresponding to the minor 3.5-kb RNA were compromised by the presence of an internal poly(A) domain. However, hybridization with specific probes revealed that the minor transcript lacks approximately 800 nucleotides from the 3' end of the major transcript and may be generated by a novel mechanism or by RNA processing.

Lineage-specific regulation of cell cycle gene expression in differentiating myeloid cells

We have analysed the expression of 7 cyclin and cyclin-associated kinase (cdk) genes in the human myeloid cell line HL-60 at different stages of the cell cycle in non-synchronised cells and during terminal differentiation. A clear cell cycle-dependent expression was found with cyclins A (S+G2), B (G2) and E (late G1 and S), while other cell cycle genes showed only very weak (cdk2) or no periodic expression (cyclin D1, cyclin D2 and cdk4). Induction of macrophage-like differentiation by TPA or granulocytic differentiation by retinoic acid or DMSO was accompanied by a block in G1 and resulted in distinct patterns of gene expression that were lineage- and inducer-specific. These included: (i) a dramatic decrease in the expression of cyclin A, cyclin B and cdk2, and surprisingly an up-regulation of cyclin D1 in TPA-induced macrophage-like cells; (ii) a down-regulation of cyclin E in retinoic acid-induced granulocytic cells; and (iii) a decreased abundance of cyclin D1 and D2, but high levels of cyclin A, B and E RNA in DMSO-induced granulocytic cells. These observations suggest that the mechanisms leading to a differentiation-associated cell cycle arrest are lineage-specific, and that the sustained expression of cyclin and cdk genes does not interfere with the induction of terminal differentiation.