Physical interaction of the retinoblastoma protein with human D cyclins

Functional domains in cyclin D1: pRb-kinase activity is not essential for transformation

Although cyclin D1 plays a major role during cell cycle progression and is involved in human tumourigenesis, its domain structure is still poorly understood. In the present study, we have generated a series of cyclin D1 N- and C-terminal deletion constructs. These mutants were used to define the domains required for transformation of rat embryonal fibroblasts (REF) in cooperation with activated Ha-ras and and to establish correlations with defined biochemical properties of cyclin D1. Protein binding and REF assays showed that the region of the cyclin box required for the interaction with CDK4 as well as C-terminal sequences determining protein stability were crucial for transformation. Surprisingly, however, the N-terminal deletion of 20 amino acids which impaired pRb kinase activity did not affect the transforming ability of cyclin D1. Likewise, no effect on transformation was observed with mutants defective in p21CIP interaction. These observations argue against a crucial role of pRb inactivation or p21CIP squelching in cyclin D1-mediated transformation.

Distinct cyclin D genes show mitotic accumulation or constant levels of transcripts in tobacco bright yellow-2 cells

The commitment of eukaryotic cells to division normally occurs during the G1 phase of the cell cycle. In mammals D-type cyclins regulate the progression of cells through G1 and therefore are important for both proliferative and developmental controls. Plant CycDs (D-type cyclin homologs) have been identified, but their precise function during the plant cell cycle is unknown. We have isolated three tobacco (Nicotiana tabacum) CycD cyclin cDNAs: two belong to the CycD3 class (Nicta;CycD3;1 and Nicta;CycD3;2) and the third to the CycD2 class (Nicta;CycD2;1). To uncouple their cell-cycle regulation from developmental control, we have used the highly synchronizable tobacco cultivar Bright Yellow-2 in a cell-suspension culture to characterize changes in CycD transcript levels during the cell cycle. In cells re-entering the cell cycle from stationary phase, CycD3;2 was induced in G1 but subsequently remained at a constant level in synchronous cells. This expression pattern is consistent with a role for CycD3;2, similar to mammalian D-type cyclins. In contrast, CycD2;1 and CycD3;1 transcripts accumulated during mitosis in synchronous cells, a pattern of expression not normally associated with D-type cyclins. This could suggest a novel role for plant D-type cyclins during mitosis.

The cytomorphological spectrum of mantle cell lymphoma is reflected by distinct biological features

Mantle cell (centrocytic) non-Hodgkin's lymphoma (MCL) is a malignant tumour with unique biological features. The pathogenesis of MCL seems to be strongly associated with aberrant function of the cell cycle. 110 cases of MCL have been analysed for their cytomorphological features, mitotic and proliferation indices, bcl-1 rearrangements, p53 expression patterns and DNA content by both interphase cytogenetic as well as DNA flow cytometric analyses. According to cytomorphology, three subtypes were recognized: a common, a lymphoblastoid and a pleomorphic variant of MCL. Blastic MCL subtypes were characterized by distinctly elevated mitotic and proliferation indices, frequent bcl-1 rearrangements at the MTC locus, and overexpression of p53. The most interesting finding, however, was a striking tendency of blastoid MCL subtypes to harbour chromosome numbers in the tetraploid range, a feature clearly separating these neoplasms from other types of B-cell NHL and possibly being related to its unphysiological expression of cyclin D1. Although characterised by a uniform immunophenotype and common biological background, MCL shows a broad spectrum of morphological features ranging from small cell to blastic types, and this spectrum is mirrored by distinct biological features.

An artificial cell-cycle inhibitor isolated from a combinatorial library

Understanding the genetic networks that operate inside cells will require the dissection of interactions among network members. Here we describe a peptide aptamer isolated from a combinatorial library that distinguishes among such interactions. This aptamer binds to cyclin-dependent kinase 2 (Cdk2) and inhibits its kinase activity. In contrast to naturally occurring inhibitors, such as p21(Cip1), which inhibit the activity of Cdk2 on all its substrates, inhibition by pep8 has distinct substrate specificity. We show that the aptamer binds to Cdk2 at or near its active site and that its mode of inhibition is competitive. Expression of pep8 in human cells retards their progression through the G1 phase of the cell cycle. Our results suggest that the aptamer inhibits cell-cycle progression by blocking the activity of Cdk2 on substrates needed for the G1-to-S transition. This work demonstrates the feasibility of selection of artificial proteins to perform functions not developed during evolution. The ability to select proteins that block interactions between a gene product and some partners but not others should make sophisticated genetic manipulations possible in human cells and other currently intractable systems.

Walleye retroviruses associated with skin tumors and hyperplasias encode cyclin D homologs

Walleye dermal sarcoma (WDS) and walleye epidermal hyperplasia (WEH) are skin diseases of walleye fish that appear and regress on a seasonal basis. We report here that the complex retroviruses etiologically associated with WDS (WDS virus [WDSV]) and WEH (WEH viruses 1 and 2 [WEHV1 and WEHV2, respectively]) encode D-type cyclin homologs. The retroviral cyclins (rv-cyclins) are distantly related to one another and to known cyclins and are not closely related to any walleye cellular gene based on low-stringency Southern blotting. Since aberrant expression of D-type cyclins occurs in many human tumors, we suggest that expression of the rv-cyclins may contribute to the development of WDS or WEH. In support of this hypothesis, we show that rv-cyclin transcripts are made in developing WDS and WEH and that the rv-cyclin of WDSV induces cell cycle progression in yeast (Saccharomyces cerevisiae). WEHV1, WEHV2, and WDSV are the first examples of retroviruses that encode cyclin homologs. WEH and WDS and their associated retroviruses represent a novel paradigm of retroviral tumor induction and, importantly, tumor regression.

Phosphorylation of retinoblastoma protein assayed in individual HL-60 cells during their proliferation and differentiation

Expression of pRb and its state of phosphorylation were immunocytochemically assayed in individual HL-60 cells during their proliferation and after induction of differentiation, using mAb which detects hypophosphorylated pRb (pRbP-) combined with mAb which reacts with pRb regardless of its phosphorylation (total pRb; pRbT). Correlated measurements of pRbP-, pRbT, a ratio of pRbP-/pRbT, and cellular DNA content by flow cytometry revealed expression of total pRb and its phosphorylation state vis-à-vis the cell cycle position. Following mitosis (during the exponential phase of cell growth) a mixture of hypo- and hyperphosphorylated pRb was present within the cell for less than 2 h, i.e., early in G1; no hypophosphorylated pRb was detected throughout remainder of the cycle. Cellular pRb content was increasing primarily during G1 and the cell entrance to S was correlated with attainment of a distinct threshold level of pRb. No correlation was seen between the content of pRb per cell and its state of phosphorylation during G1. Cell differentiation whether induced by 1,25-dihydroxyvitamin D3, retinoic acid, or phorbol myristate acetate led to cell arrest primarily in G0/1. The G0/1 cells in these cultures, compared to G1 cells from the untreated cultures, had increased level of both pRbT and pRbP-. However, because the relative increase of pRbP- was disproportionally greater than of pRbT, the pRbP-/pRbT ratio of the differentiating cells was markedly elevated. The cells that still were in S and G2/M in the differentiating cultures also showed the presence of hypophosphorylated pRb. Our data suggest that the mechanism of irreversible cell cycle arrest during terminal differentiation involves both the increase in content of pRb and dephosphorylation of pRb already present within the cell. This provides a large pool of hypophosphorylated pRb that can effectively remove all free E2F, thereby precluding activation of the genes whose transcription is needed to pass the G1 restriction point. In contrast to terminal differentiation the transient quiescence (G0 state) manifests only by dephosphorylation of pRb, without a change in its cellular level.

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.

The structural protein ODV-EC27 of Autographa californica nucleopolyhedrovirus is a multifunctional viral cyclin

Two major characteristics of baculovirus infection are arrest of the host cell at G2/M phase of the cell cycle with continuing viral DNA replication. We show that Autographa californica nucleopolyhedrovirus (AcMNPV) encodes for a multifunctional cyclin that may partially explain the molecular basis of these important characteristics of AcMNPV (baculovirus) infection. Amino acids 80-110 of the viral structural protein ODV-EC27 (-EC27) demonstrate 25-30% similarity with cellular cyclins within the cyclin box. Immunoprecipitation results using antibodies to -EC27 show that -EC27 can associate with either cdc2 or cdk6 resulting in active kinase complexes that can phosphorylate histone H1 and retinoblastoma protein in vitro. The cdk6-EC27 complex also associates with proliferating cell nuclear antigen (PCNA) and we demonstrate that PCNA is a structural protein of both the budded virus and the occlusion-derived virus. These results suggest that -EC27 can function as a multifunctional cyclin: when associated with cdc2, it exhibits cyclin B-like activity; when associated with cdk6, the complex possesses cyclin D-like activity and binds PCNA. The possible roles of such a multifunctional cyclin during the life cycle of baculovirus are discussed, along with potential implications relative to the expression of functionally authentic recombinant proteins by using baculovirus-infected cells.

Substrate recruitment to cyclin-dependent kinase 2 by a multipurpose docking site on cyclin A

An important question in the cell cycle field is how cyclin-dependent kinases (cdks) target their substrates. We have studied the role of a conserved hydrophobic patch on the surface of cyclin A in substrate recognition by cyclin A-cdk2. This hydrophobic patch is approximately 35A away from the active site of cdk2 and contains the MRAIL sequence conserved among a number of mammalian cyclins. In the x-ray structure of cyclin A-cdk2-p27, this hydrophobic patch contacts the RNLFG sequence in p27 that is common to a number of substrates and inhibitors of mammalian cdks. We find that mutation of this hydrophobic patch on cyclin A eliminates binding to proteins containing RXL motifs without affecting binding to cdk2. This docking site is critical for cyclin A-cdk2 phosphorylation of substrates containing RXL motifs, but not for phosphorylation of histone H1. Impaired substrate binding by the cyclin is the cause of the defect in RXL substrate phosphorylation, because phosphorylation can be rescued by restoring a cyclin A-substrate interaction in a heterologous manner. In addition, the conserved hydrophobic patch is important for cyclin A function in cells, contributing to cyclin A's ability to drive cells out of the G1 phase of the cell cycle. Thus, we define a mechanism by which cyclins can recruit substrates to cdks, and our results support the notion that a high local concentration of substrate provided by a protein-protein interaction distant from the active site is critical for phosphorylation by cdks.

Dual cyclin-binding domains are required for p107 to function as a kinase inhibitor

The retinoblastoma (pRB) family of proteins includes three proteins known to suppress growth of mammalian cells. Previously we had found that growth suppression by two of these proteins, p107 and p130, could result from the inhibition of associated cyclin-dependent kinases (cdks). One important unresolved issue, however, is the mechanism through which inhibition occurs. Here we present in vivo and in vitro evidence to suggest that p107 is a bona fide inhibitor of both cyclin A-cdk2 and cyclin E-cdk2 that exhibits an inhibitory constant (Ki) comparable to that of the cdk inhibitor p21/WAF1. In contrast, pRB is unable to inhibit cdks. Further reminiscent of p21, a second cyclin-binding site was mapped to the amino-terminal portions of p107 and p130. This amino-terminal domain is capable of inhibiting cyclin-cdk2 complexes, although it is not a potent substrate for these kinases. In contrast, a carboxy-terminal fragment of p107 that contains the previously identified cyclin-binding domain serves as an excellent kinase substrate although it is unable to inhibit either kinase. Clustered point mutations suggest that the amino-terminal domain is functionally important for cyclin binding and growth suppression. Moreover, peptides spanning the cyclin-binding region are capable of interfering with p107 binding to cyclin-cdk2 complexes and kinase inhibition. Our ability to distinguish between p107 and p130 as inhibitors rather than simple substrates suggests that these proteins may represent true inhibitors of cdks.

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

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

Expression patterns of cyclin D1 and related proteins regulating G1-S phase transition in uveal melanoma and retinoblastoma

BACKGROUND/AIMS

A checkpoint mechanism in late G1, whose regulation via loss of retinoblastoma protein (pRB) or p16, or overexpression of cyclin D1 or cyclin dependent kinase 4 (CDK4), has been proposed to constitute a common pathway to malignancy. The aims of this study were (a) to compare markers of cell cycle G1-S phase transition in an intraocular tumour with known pRB deficiency (retinoblastoma) and compare it with one with an apparently functional pRB (uveal melanoma); (b) to determine if one of these markers may have a role in the pathogenesis of uveal melanoma; and (c) to determine if there is a difference in cell cycle marker expression following treatment of uveal melanoma and retinoblastoma.

METHODS

90 eyes were enucleated from 89 patients for retinoblastoma (n = 24) or for choroidal or ciliary body melanoma (n = 66). Conventional paraffin sections were assessed for cell type and degree of differentiation. Additional slides were investigated applying standard immunohistochemical methods with antibodies specific for cyclin D1 protein, pRB, p53, p21, p16, BCL-2, and MIB-1.

RESULTS

Cyclin D1 protein and pRB were negative in retinoblastoma using the applied antibodies. In contrast, cyclin D1 protein expression was observed in 65% of uveal melanomas; a positive correlation between cyclin D1 cell positivity and tumour cell type, location, growth fraction, as well as with pRB positivity was observed. p53, p21, and p16 could be demonstrated in both tumours. An inverse relation between p53 and p21 expression was demonstrated in most choroidal melanomas and in some retinoblastomas. Apart from a decrease in the growth fractions of the tumours as determined by MIB-1, a significant difference in the expression of G1-S phase transition markers in vital areas of uveal melanoma and retinoblastoma following treatment with radiotherapy and/or chemotherapy was not observed.

CONCLUSION

Retinoblastomas and uveal melanomas, two tumours of differing pRB status, differ also in their immunohistochemical pattern for markers of the G1-S phase transition of the cell cycle. The results of the present study support the concept of (a) an autoregulatory loop between pRB and cyclin D1 in tumours with a functional pRB and the disruption of this loop in the presence of pRB mutation, as well as (b) a checkpoint mechanism in late G1, whose regulation via loss of p16 or pRB, or overexpression of cyclin D1 constitutes a common pathway to malignancy. Further, the results raise the possibility of cyclin D1 overexpression having a role in the pathogenesis of uveal melanoma.

Cloning and characterization of a GC-box binding protein, G10BP-1, responsible for repression of the rat fibronectin gene

Fibronectin (FN) is an extracellular matrix protein that connects the extracellular matrix to intracellular cortical actin filaments through binding to its cell surface receptor, alpha5beta1, a member of the integrin superfamily. The expression level of FN is reduced in most tumor cells, facilitating their anchorage-independent growth by still unclarified mechanisms. The cDNA clone encoding G-rich sequence binding protein G10BP-1, which is responsible for repression of the rat FN gene, was isolated by using a yeast one-hybrid screen with the G10 stretch inserted upstream of the HIS3 and lacZ gene minimal promoters. G10BP-1 comprises 385 amino acids and contains two basic regions and a putative zipper structure. It has the same specificity of binding to three G-rich sequences in the FN promoter and the same size as the G10BP previously identified in adenovirus E1A- and E1B-transformed rat cells. Expression of G10BP-1 is cell cycle regulated; the level was almost undetectable in quiescent rat 3Y1 cells but increased steeply after growth stimulation by serum, reaching a maximum in late G1. Expression of FN mRNA is inversely correlated with G10BP-1 expression, and the level decreased steeply during G1-to-S progression. This down regulation was strictly dependent on the downstream GC box (GCd), and base substitutions within GCd abolished the sensitivity of the promoter to G10BP-1. In contrast, the level of Sp1, which competes with G10BP for binding to the G-rich sequences, was constant throughout the cell cycle, suggesting that the concentration of G10BP-1 relative to that of Sp1 determines the expression level of the FN gene. Preparation of glutathione S-transferase pulldowns of native proteins from the cell extracts containing exogenously or endogenously expressed G10BP-1, followed by Western blot analysis, showed that G10BP-1 forms homodimers through its basic-zipper structure.

Retinoblastoma, a tumor suppressor, is a coactivator for the androgen receptor in human prostate cancer DU145 cells

The retinoblastoma protein may function as a tumor suppressor by controlling the progression of the normal cell cycle. Inactivation of Rb has been regarded as an important event in prostate carcinogenesis. However, the detailed mechanism of how Rb is linked to androgen-androgen receptor (A-AR), the major factor in promotion of prostate tumor growth, remains unclear. Using GST-Rb pull down assay and mammalian two-hybrid system, we report here that Rb can bind specifically to AR in an androgen-independent manner. Transient transfection assay demonstrates that cotransfection of AR and Rb can further induce AR transcriptional activity 4-fold in the presence of 1 nM dihydrotestosterone in DU145 cells. Interestingly, cotransfection of Rb and ARA70, the first identified AR coactivator, with AR can additively induce AR transcriptional activity 13-fold (from 5-fold to 64-fold). In conclusion, our discovery that Rb can function as a coactivator to induce AR transcriptional activity in prostate cells may represent the first data to link a negative growth regulatory protein function in a positive manner, by inducing the transcriptional activity of AR.

Functional consequences of cyclin D1 overexpression in human mammary luminal epithelial cells

The proliferation of eukaryotic cells is primarily regulated by a decision made during the G1 phase of the cell cycle as to remain in the cycle and divide, or to withdraw from the cycle and adopt a different cell fate. During this time, environmental signals, which regulate the synthesis of the G1 cyclins, are coupled to cell division. In this context, mammalian D-type cyclins have been shown to control progression through the G1 phase of the mammalian cell cycle. Specifically, cyclin D1 has been reported frequently to be amplified, over-transcribed and overexpressed in human breast carcinomas. Although the effects of cyclin D1 overexpression have been examined in human breast carcinoma cell lines, the biological consequences of cyclin D1 expression in normal human mammary epithelial cells remain to be elucidated. In this study we have stably over expressed cyclin D1 in human mammary luminal epithelial cells in order to more directly address the role of cyclin D1 in cell cycle control and tumorigenesis of the human breast. Here, we demonstrate that the effect of cyclin D1 overexpression in these cells is to reduce their growth factor dependency, as well as shorten the duration of G1 and correspondingly reduce the mean generation time. Collectively, our data indicate that deregulation of cyclin D1 expression in human mammary epithelial cells can provide a growth advantage and hence contribute to the oncogenic potential of these cells.

Clinical implications of cyclins, cyclin-dependent kinases, RB and E2F1 in squamous-cell lung carcinoma

In the search for new risk factors at the molecular and cellular levels, clinical data [lymph-node involvement(LN)and stage] were used and 104 squamous-cell lung carcinomas were analyzed by immuno-histochemistry for expression of cyclin D1, cyclin A, cdk2, cdk4, RB, and E2F1. The results of the univariate analysis of all 8 factors showed that cyclin A and cdk2 gave the best prognostic information, while no prognostic value could be found associated with cyclin D1, cdk4, RB and E2F1. The subsequent multivariate analysis of all possible combinations of the important factors showed that the pairs LN/cyclin A, LN/cdk2 and cyclin A/cdk2, and the triplet LN/cyclin A/cdk2 yielded the best prognostic information. It was essentially better than the information given by a single factor.

Cisplatin-induced apoptosis in rat dorsal root ganglion neurons is associated with attempted entry into the cell cycle

Platinum compounds induce apoptosis in malignant cells and are used extensively in the treatment of cancer. Total dose is limited by development of a sensory neuropathy. We now demonstrate that when rats are administered cisplatin (2 mg/kg i.p. for 5 d), primary sensory neurons in the dorsal root ganglion die by apoptosis. This was reproduced by exposure of dorsal root ganglion neurons and PC12 cells to cisplatin (3 microg/ml) in vitro. Apoptosis was confirmed by electron microscopy, DNA laddering, and inhibition by the caspase inhibitor z-VAD.fmk (100 microM). Cell death in vitro was preceded by upregulation of cyclin D1, cdk4, and increased phosphorylation of retinoblastoma protein; all are indicators of cell cycle advancement. The level of p16(INK4a), an endogenous inhibitor of the cyclin D1/cdk4 complex decreased. Exposure of PC12 cells and dorsal root ganglion neurons to increased levels of nerve growth factor (100 ng/ ml) prevented both apoptosis and upregulation of the cell cycle markers. Cancer cells without nerve growth factor receptors (gp140TrkA) were not protected by the neurotrophin. This indicated that cisplatin may kill cancer cells and neurons by a similar mechanism. In postmitotic neurons, this involves an attempt to re-enter the cell cycle resulting in apoptosis which is specifically prevented by nerve growth factor.

Hypophosphorylation of the RB protein in S and G2 as well as G1 during growth arrest

The RB tumor suppressor protein is a cell cycle regulator, where hypophosphorylated RB is associated with G1/0 arrest and its cyclin-dependent phosphorylation in G1 allows progression from G1 to S. The present report shows that in human leukemia cells induced to undergo growth arrest with sodium butyrate or DMSO, hypophosphorylation of the RB protein is not G1 restricted and also occurs in S and G2/M cells as well as in G1 cells when growth is inhibited. While all of the RB protein in G1/0 cells is hypophosphorylated, residual cells in S and G2 have significant detectable amounts of hypophosphorylated RB as well as still hyperphosphorylated RB protein. Thus RB hypophosphorylation can be induced in S and G2 as well as the G1 phase. The results show that growth retardation in other than the G1 phase is associated with occurrence of hypophosphorylated RB. RB may thus have a broader capability to inhibit proliferation than just in G1.

A critical role for cyclin C in promotion of the hematopoietic cell cycle by cooperation with c-Myc

Cyclin C, a putative G1 cyclin, was originally isolated through its ability to complement a Saccharomyces cerevisiae strain lacking the G1 cyclin gene CLN1-3. Unlike cyclins D1 and E, the other two G1 cyclins obtained by the same approach and subsequently shown to play important roles during the G1/S transition, there is thus far no evidence to support the hypothesis that cyclin C is indeed critical for the promotion of cell cycle progression. In BAF-B03 cells, an interleukin 3 (IL-3)-dependent murine pro-B-cell line, cyclin C gene mRNA was induced at the G1/S phase upon IL-3 stimulation and reached a maximal level in the S phase. Enforced expression of exogenous cyclin C in this cell line failed to alter its growth properties. In the present study, we examined whether cyclin C is capable of cooperating with the cytokine-responsive immediate-early gene products c-Myc and c-Fos in the promotion of cell proliferation. We found that cyclin C is able to cooperate functionally with c-Myc, but not c-Fos, to induce both BAF-B03 cell proliferation in a cytokine-independent fashion and the formation of cell clusters. Furthermore, cyclin C was primarily responsible for the induction of cdc2 gene expression. Our data define a novel role for cyclin C in the regulation of both the G1/S and G2/M phases of the cell cycle, and this effect appears to be independent of the activity of CDK8 in the control of transcription.

Human T-cell leukemia virus type 1 Tax and cell cycle progression: role of cyclin D-cdk and p110Rb

Human T-cell leukemia virus type 1 is etiologically linked to the development of adult T-cell leukemia and various human neuropathies. The Tax protein of human T-cell leukemia virus type I has been implicated in cellular transformation. Like other oncoproteins, such as Myc, Jun, and Fos, Tax is a transcriptional activator. How it mechanistically dysregulates the cell cycle is unclear. Previously, it was suggested that Tax affects cell-phase transition by forming a direct protein-protein complex with p16(INK4a), thereby inactivating an inhibitor of G1-to-S-phase progression. Here we show that, in T cells deleted for p16(INK4a), Tax can compel an egress of cells from G0/G1 into S despite the absence of serum. We also show that in undifferentiated myocytes, expression of Tax represses cellular differentiation. In both settings, Tax expression was found to increase cyclin D-cdk activity and to enhance pRb phosphorylation. In T cells, a Tax-associated increase in steady-state E2F2 protein was also documented. In searching for a molecular explanation for these observations, we found that Tax forms a protein-protein complex with cyclin D3, whereas a point-mutated and transcriptionally inert Tax mutant failed to form such a complex. Interestingly, expression of wild-type Tax protein in cells was also correlated with the induction of a novel hyperphosphorylated cyclin D3 protein. Taken together, these findings suggest that Tax might directly influence cyclin D-cdk activity and function, perhaps by a route independent of cdk inhibitors such as p16(INK4a).

Increased expression of cyclin D2 during multiple states of growth arrest in primary and established cells

Cyclin D2 is a member of the family of D-type cyclins that is implicated in cell cycle regulation, differentiation, and oncogenic transformation. To better understand the role of this cyclin in the control of cell proliferation, cyclin D2 expression was monitored under various growth conditions in primary human and established murine fibroblasts. In different states of cellular growth arrest initiated by contact inhibition, serum starvation, or cellular senescence, marked increases (5- to 20-fold) were seen in the expression levels of cyclin D2 mRNA and protein. Indirect immunofluorescence studies showed that cyclin D2 protein localized to the nucleus in G0, suggesting a nuclear function for cyclin D2 in quiescent cells. Cyclin D2 was also found to be associated with the cyclin-dependent kinases CDK2 and CDK4 but not CDK6 during growth arrest. Cyclin D2-CDK2 complexes increased in amounts but were inactive as histone H1 kinases in quiescent cells. Transient transfection and needle microinjection of cyclin D2 expression constructs demonstrated that overexpression of cyclin D2 protein efficiently inhibited cell cycle progression and DNA synthesis. These data suggest that in addition to a role in promoting cell cycle progression through phosphorylation of retinoblastoma family proteins in some cell systems, cyclin D2 may contribute to the induction and/or maintenance of a nonproliferative state, possibly through sequestration of the CDK2 catalytic subunit.

Structural principles in cell-cycle control: beyond the CDKs

Retinoblastoma protein (Rb) interacts with cyclin-dependent kinases and regulates the transcription of genes necessary for progression through the S phase of the cell cycle. Clues to the atomic mechanisms involved are offered by the structure of the two pocket regions of Rb in complex with a short peptide from a viral oncoprotein. Structures of cyclins, Rb and TFIIB reveal that a common motif occurs in proteins regulating three consecutive events of cell-cycle control.

Negative regulation of DNA replication by the retinoblastoma protein is mediated by its association with MCM7

A yeast two-hybrid screen was employed to identify human proteins that specifically bind the amino-terminal 400 amino acids of the retinoblastoma (Rb) protein. Two independent cDNAs resulting from this screen were found to encode the carboxy-terminal 137 amino acids of MCM7, a member of a family of proteins that comprise replication licensing factor. Full-length Rb and MCM7 form protein complexes in vitro, and the amino termini of two Rb-related proteins, p107 and p130, also bind MCM7. Protein complexes between Rb and MCM7 were also detected in anti-Rb immunoprecipitates prepared from human cells. The amino-termini of Rb and p130 strongly inhibited DNA replication in an MCM7-dependent fashion in a Xenopus in vitro DNA replication assay system. These data provide the first evidence that Rb and Rb-related proteins can directly regulate DNA replication and that components of licensing factor are targets of the products of tumor suppressor genes.

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.

The role of RB in cell cycle control

The retinoblastoma protein is an inhibitor of cell cycle progression from the G1 to the S phase of the cell cycle. It acts through its ability to interact with cellular target molecules such as E2F transcription factors. The function of pRB is negatively regulated by a cell-cycle dependent phosphorylation catalyzed by cyclin-dependent kinases in the late G1 cell cycle phase. Recent evidence indicates that this pRB inactivation is a key molecular event leading to the S-phase commitment at the G1 restriction point in the cell cycle. Deregulated inactivation of pRB in G1 phase may be a universal mechanism underlying cellular transformation.

Intestinal cell cycle regulation

The intestinal epithelium is maintained by a balance between proliferation, differentiation and death that occurs as cells migrate up the crypt-villus axis. Cell cycle regulators such as cyclins, cyclin-dependent kinases (Cdks) and Cdk inhibitory proteins are expressed in a distinct pattern along the crypt-villus structure, suggesting their role in controlling intestinal cells. This is supported by observations that these cell cycle proteins are regulated by growth factors, nutrients and cell-cell contact in cultured intestinal epithelial cells. One of the key regulators of intestinal cell proliferation and differentiation is transforming growth factor-beta, which is expressed in the gut epithelium.

Identification of a substrate-targeting domain in cyclin E necessary for phosphorylation of the retinoblastoma protein

Considerable advances have been made in characterizing the cyclins and cyclin-dependent kinases (CDKs) that are necessary for progression through the cell cycle, but there has been relatively lesser success in identifying the specific biochemical pathways and cell cycle events that are directly under CDK control. To identify physiologically significant CDK substrates we generated mutations in cyclin E that altered the ability of the cyclin to direct the cyclin-CDK holoenzyme to specific in vivo substrates. We show that one of these mutations defines a domain in cyclin E necessary for phosphorylation of the retinoblastoma protein (Rb). These observations confirm the idea that cyclins contribute to substrate recognition by cyclin-CDK complexes, demonstrate the utility of targeting mutants in the identification of essential cyclin-CDK substrates, and put cyclin E squarely into the family of proteins designed to regulate Rb.

Novel mechanisms of E2F induction by BK virus large-T antigen: requirement of both the pRb-binding and the J domains

E2F activity is regulated in part by the retinoblastoma family of tumor suppressor proteins. Viral oncoproteins, such as simian virus 40 (SV40) large-T antigen (TAg), adenovirus E1A, and human papillomavirus E7, can disrupt the regulation of cellular proliferation by binding to pRb family members and dissociating E2F-pRb family protein complexes. BK virus (BKV), which infects a large percentage of the human population and has been associated with a variety of human tumors, encodes a TAg homologous to SV40 TAg. It has been shown that BKV TAg, when expressed at low levels, does not detectably bind to pRb family members, yet it induces a serum-independent phenotype and causes a decrease in the overall levels of pRb family proteins. The experiments presented in this report show that, despite the lack of TAg-pRb interactions, BKV TAg can induce transcriptionally active E2F and that this induction does in fact require an intact pRb-binding domain as well as an intact J domain. In addition, E2F-pRb family member complexes can be detected in both BKV and SV40 TAg-expressing cells. These results suggest the presence of alternate cellular mechanisms for the release of E2F in addition to the well-established model for TAg-pRb interactions. These results also emphasize a role for BKV TAg in the deregulation of cellular proliferation, which may ultimately contribute to neoplasia.

Phosphorylation of retinoblastoma susceptibility gene protein assayed in individual lymphocytes during their mitogenic stimulation

Phosphorylation of the protein encoded by retinoblastoma susceptibility gene (pRb) is the key event of the cell cycle committing the cell to enter S phase and also required for progression through S and G2. We describe a new methodology to monitor pRb phosphorylation in individual cells and correlate it with the cell cycle position. Specifically, pRb phosphorylation in human lymphocytes was assayed immunocytochemically using mAb which recognizes underphosphorylated pRb (pRbP-) conjugated with a fluorochrome of one color combined with mAb which reacts with pRb regardless of its phosphorylation (total pRb; pRbT) tagged with another color fluorochrome. DNA was stained with still another color fluorochrome and cell fluorescence was measured by multiparameter flow cytometry. Specificity of anti-pRbP- mAb was confirmed by preincubation of the permeabilized cells with phosphatase. Analysis of pRbP- or a ratio of pRbP-/pRbT revealed that pRb was underphosphorylated in over 98% of the nonstimulated lymphocytes. The proportion of cells with underphosphorylated pRb dropped to 20% between 3 and 8 h after addition of the mitogen phytohemagglutinin (PHA). Phosphorylation of pRb within a cell was rapid and complete since reactivity of individual lymphocytes with anti-pRbP-mAb was lost abruptly rather than step-wise during stimulation. Phosphorylation of pRb coincided with the appearance of cyclin D3, which was induced 3 h and peaked 12 h after addition of PHA. The nonspecific protein kinase inhibitor staurosporine at a concentration known to arrest lymphocytes in G1 but not to interfere with the induction of cyclin D3 (20 nM) prevented pRb phosphorylation. The present assay can be applied for screening antitumor drugs targeting CDKs and be useful for monitoring pRb phosphorylation in human tumors, the feature of a possible prognostic value in oncology.

Cyclin D1 and human neoplasia

Neoplasia is characterised by abnormal regulation of the cell cycle. Cyclin D1 is a protein derived from the PRAD1, CCND1 or bcl-1 gene on chromosome 11q13, which is involved in both normal regulation of the cell cycle and neoplasia. In the G1 (resting) phase of the cell cycle, cyclin D1 together with its cyclin dependent kinase (cdk) partner, is responsible for transition to the S (DNA synthesis) phase by phosphorylating the product of the retinoblastoma gene (pRB), which then releases transcription factors important in the initiation of DNA replication. Amplification of the CCND1 gene or overexpression of the cyclin D1 protein releases a cell from its normal controls and causes transformation to a malignant phenotype. Analysis of these changes provides important diagnostic information in mantle cell (and related) lymphomas, and is of prognostic value in many cancers. Knowledge of cyclin D1's role in malignancy at the various sites, provides a basis on which future treatment directed against this molecule can proceed.

Mantle cell lymphoma: presenting features, response to therapy, and prognostic factors

BACKGROUND

The goal of this study was to analyze the presenting features, natural history, and prognostic factors in 59 patients with well characterized mantle cell lymphoma (MCL).

METHODS

Cases were classified as nodular or diffuse and as typical or blastic variants. Age, performance status (PS), histologic variants, mitotic index (MI), hematologic parameters, tumor extension data, and International Prognostic Index (IPI) were recorded and evaluated for prognosis.

RESULTS

The median age of the patients was 63 years (range, 39-83 years), and the male to female ratio was 3:1. Fifty-three patients had typical histology (3 nodular and 50 diffuse), and 6 had the blastic variant. Approximately 95% of patients presented with advanced stage disease (Ann Arbor Stage III-IV). Leukemic expression was observed in 58%. Complete and partial response rates were 19% and 46%, respectively. Parameters associated with lower response rate were Stage IV, high/intermediate or high risk IPI, and increased lactate dehydrogenase (LDH) level. In the logistic regression analysis, high LDH level and Stage IV disease were associated independently with lower response rate. Median survival was 49 months. Parameters associated with a short survival were: poor PS, splenomegaly, B-symptoms, MI > 2.5, leukocyte count > 10 x 10(9)/L, high LDH level, blastic variant, and high/intermediate or high risk IPI. In the Cox proportional hazards regression model, only poor PS (relative risk [RR] = 3.3; P = 0.002), splenomegaly (RR = 2.8; P = 0.007), and MI > 2.5 (RR = 2.4; P = 0.012) were associated with short survival.

CONCLUSIONS

In this series, patients with MCL presented with advanced stage and extranodal involvement. Only a minority of patients achieved a complete response. The median survival was 4 years, with PS, splenomegaly, and MI being the most important factors predicting survival. These results show clearly that more effective therapies for MCL are needed.

Functional inactivation of the retinoblastoma protein requires sequential modification by at least two distinct cyclin-cdk complexes

The retinoblastoma protein (pRb) acts to constrain the G1-S transition in mammalian cells. Phosphorylation of pRb in G1 inactivates its growth-inhibitory function, allowing for cell cycle progression. Although several cyclins and associated cyclin-dependent kinases (cdks) have been implicated in pRb phosphorylation, the precise mechanism by which pRb is phosphorylated in vivo remains unclear. By inhibiting selectively either cdk4/6 or cdk2, we show that endogenous D-type cyclins, acting with cdk4/6, are able to phosphorylate pRb only partially, a process that is likely to be completed by cyclin E-cdk2 complexes. Furthermore, cyclin E-cdk2 is unable to phosphorylate pRb in the absence of prior phosphorylation by cyclin D-cdk4/6 complexes. Complete phosphorylation of pRb, inactivation of E2F binding, and activation of E2F transcription occur only after sequential action of at least two distinct G1 cyclin kinase complexes.

Uncoupling of the order of the S and M phases: effects of staurosporine on human cell cycle kinases

The protein kinase inhibitor staurosporine (SSP) was employed to study the involvement of kinases in human cell cycle progression. Thirty to 100 ng/ml SSP blocks entry into S phase and M phase. Lack of entry into S phase is due to impaired activity of the retinoblastoma protein kinase. The requirement for any of the SSP-sensitive kinases for cell cycle progression can be abrogated in tumour cells. Therefore, these kinases act in a checkpoint network negatively controlling the initiation of S phase, M phase and cytokinesis, rather than being inherent parts of a substrate-product chain required for the initiation of the cell cycle phases. As a consequence of the lack of certain checkpoint effectors, tumour cells may endoreduplicate or binucleate in the presence of SSP. The latter processes, as well as meiosis, are naturally occurring in specialized cell types, leading to the idea that this checkpoint network controls the order of the cell cycle phases in normal cells. A model is presented where the cell cycle is envisioned as two independently running cycles, the S and the M cycle, which are controlled by intra and intercycledependent checkpoints in human somatic cells. The model accounts for the dependency of S and M phase initiation on the successful completion of the previous M and S phase, respectively, as well as entry into a resting state.

Inhibition of basal and mitogen-stimulated pancreatic cancer cell growth by cyclin D1 antisense is associated with loss of tumorigenicity and potentiation of cytotoxicity to cisplatinum

Cyclin D1 belongs to a family of protein kinases that have been implicated in cell cycle regulation. Recent studies have demonstrated that elevated cyclin D1 levels correlate with decreased survival in human pancreatic cancer. In this study we expressed in a stable manner a cyclin D1 antisense cDNA construct in PANC-1 human pancreatic cancer cells. Expression of the antisense construct caused a decrease in cyclin D1 mRNA and protein levels and in cyclin D1-associated kinase activity. Antisense expressing clones displayed significantly increased doubling times, decreased anchorage-dependent and -independent basal growth, and complete loss of tumorigenicity in nude mice. EGF, FGF-2, and IGF-I enhanced mitogen-activated protein kinase activity in antisense expressing clones, but failed to stimulate their proliferation. In contrast, all three growth factors were mitogenic in parental cells. Furthermore, the inhibitory effect of cisplatinum on cell proliferation was enhanced markedly in the antisense expressing clones. These findings indicate that cyclin D1 overexpression contributes to abnormal growth and tumorigenicity in human pancreatic cancer and to the resistance of pancreatic cancer to chemotherapeutic agents.

A retinoblastoma susceptibility gene product, RB, targeting protease is regulated through the cell cycle

Degradation of cyclin B and cyclin-dependent kinase inhibitor, p27, at a specific time has been shown to play a critical role in regulating the cell cycle. SPase, a nuclear and cytosol protease with cathepsin B- and L-like proteolytic activity, has been identified in several cell lines. This proteolytic enzyme selectively degraded nuclear proteins such as retinoblastoma susceptibility gene product, RB, and transcription factor, SP-1. High levels of SPase activity were detected at the G1/S, moderate levels at the G1 and S phases, and undetectable activity at the M phase of synchronized CV-1 cells, suggesting that SPase activity is regulated through the cell cycle. Degradation of RB correlated with SPase activity throughout the cell cycle, suggesting that SPase regulates RB, which has a functional role in regulating cell cycle. These results demonstrated that SPase plays an integral role in regulating the nuclear regulator, RB, in controlling cell cycle progression.

Database of mutations within the adenovirus 5 E1A oncogene

The Ad5 E1A database is a listing of mutations affecting the early region 1A (E1A) proteins of human adenovirus type 5. The database contains the name of the mutation, the nucleic acid sequence changes, the resulting alterations in amino acid sequence and reference. Additional notes and references are provided on the effect of each mutation on E1A function. The database is contained within the Adenovirus 5 E1A page on the World Wide Web at: http://www.geocities.com/CapeCanaveral/Hangar /2541/

V3 loop of human immunodeficiency virus type 1 reduces cyclin E expression and induces G1 arrest in interleukin 2-dependent T cells

We previously described that V3 loop derived from the HTLV-III BH10 clone V3-BH10 markedly suppressed IL-2-driven T cell proliferation and produced G1 arrest of the cells. Here, we tested the effect of V3-BH10 on the molecules that are involved in transition from the G1 to S phase of the cell cycle. The effect of V3-BH10 on the IL-2-induced expression of G1 cyclins, Cdk inhibitors, and phosphorylation of retinoblastoma protein (pRb) was tested by immunoblotting, using the IL-2-dependent CD4-positive cell line Kit 225. Furthermore, IL-2-dependent kinase activity of the cyclin E-Cdk2 complex was investigated with histone H1 as a substrate. V3-BH10 reduced the IL-2-dependent expression of cyclin E, but not that of cyclin D and Cdk inhibitors such as p21 and p27. As the result of reduction of cyclin E, histone H1 kinase activity of the cyclin E-Cdk2 complex was markedly reduced even in the presence of rIL-2, followed by incomplete phosphorylation of pRb. The reduction in hyperphosphorylation of pRb by V3-BH10 led to G1 arrest of the cell cycle. Thus, V3-BH10 induced G1 arrest in IL-2-dependent cell cycle progression by reducing cyclin E expression, which may be one of the mechanisms underlying the dysfunction of T cells in HIV-1-infected people.

TCR antigen-induced cell death occurs from a late G1 phase cell cycle check point

Deletion of antigen-activated T cells after an immune response and during peripheral negative selection after strong T cell receptor (TCR) engagement of cycling T cells occurs by an apoptotic process termed TCR antigen-induced cell death (AID). By analyzing the timing of death, cell cycle markers, BrdU-labeled S phase cells, and phase-specific centrifugally elutriated cultures from stimulated Jurkat T cells and peripheral blood lymphocytes, we found that AID occurs from a late G1 check point prior to activation of cyclin E:Cdk2 complexes. T cells stimulated to undergo AID can be rescued by effecting an early G1 block by direct transduction of p16INK4a tumor suppressor protein or by inactivation of the retinoblastoma tumor suppressor protein (pRb) by transduced HPV E7 protein. These results suggest that AID occurs from a late G1 death check point in a pRb-dependent fashion.

Ras-stimulated extracellular signal-related kinase 1 and RhoA activities coordinate platelet-derived growth factor-induced G1 progression through the independent regulation of cyclin D1 and p27

Platelet-derived growth factor (PDGF)-induced Ras activation is required for G1 progression in Chinese hamster embryo fibroblasts (IIC9 cells). Ras stimulates both extracellular signal-related kinase (ERK) activation and RhoA activation in response to PDGF stimulation. Inhibition of either of these Ras-stimulated pathways results in growth arrest. We have shown previously that Ras-stimulated ERK activation is essential for the induction and continued G1 expression of cyclin D1. In this study we examine the role of Ras-induced RhoA activity in G1 progression. Unstimulated IIC9 cells expressed high levels of the G1 cyclin-dependent kinase inhibitor p27(KIP1). Stimulation with PDGF resulted in a dramatic decrease in p27(KIP1) protein expression. This decrease was attributed to increased p27(KIP1) protein degradation. Overexpression of dominant-negative forms of Ras or RhoA completely blocked PDGF-induced p27(KIP1) degradation, but only dominant-negative Ras inhibited cyclin D1 protein expression. C3 transferase also inhibited PDGF-induced p27(KIP1) degradation, thus further implicating RhoA in p27(KIP1) regulation. Overexpression of dominant-negative ERK resulted in inhibition of PDGF-induced cyclin D1 expression but had no effect on PDGF-induced p27(KIP1) degradation. These data suggest that Ras coordinates the independent regulation of cyclin D1 and p27(KIP1) expression by the respective activation of ERK and RhoA and that these pathways converge to determine the activation state of complexes of cyclin D1 and cyclin-dependent kinase in response to mitogen.

Transforming growth factor-alpha enhances cyclin D1 transcription through the binding of early growth response protein to a cis-regulatory element in the cyclin D1 promoter

Cyclin D1 is a critical oncogene involved in the regulation of progression through the G1 phase of the cell cycle, thereby contributing to cell proliferation. This is mediated through interaction of cyclin D1 with its catalytic partners, the cyclin-dependent kinases, and the subsequent phosphorylation of the retinoblastoma protein. Cyclin D1, in turn, is regulated by mitogenic stimuli. We demonstrate that transforming growth factor-alpha (TGFalpha) induces cyclin D1 mRNA in esophageal squamous epithelial cells, and this appears to correlate with increased cyclin D1 protein expression and cyclin-dependent kinase 6 activity. The induction of cyclin D1 transcription by TGFalpha is mediated in part through the induction of the early growth response protein (Egr-1) and its subsequent binding of Egr-1 to a cis-regulatory region spanning nucleotides -144 to -104 of the cyclin D1 promoter. The Egr-1 binding activity to the cyclin D1 promoter appears to require de novo protein synthesis and is not influenced by Sp1 binding to overlapping Sp1 motifs. Taken together, these data provide evidence that TGFalpha enhances cyclin D1 transcription through the induction of Egr-1 binding to a cis-regulatory region in the cyclin D1 promoter. This has important mechanistic implications into the transcriptional regulation of cyclin D1 by an essential proproliferative growth factor and cell cycle progression.

The differential binding of E2F and CDF repressor complexes contributes to the timing of cell cycle-regulated transcription

B- myb and cdc25C exemplify different groups of genes whose transcription is consecutively up-regulated during the cell cycle. Both promoters are controlled by transcriptional repression via modules consisting of an E2F binding site (E2FBS) or the related CDE plus a contiguous CHR co-repressor element. We now show that the B- myb repressor module, which is derepressed early (mid G1), is preferentially recognized by E2F-DP complexes and that a mutation selectively abolishing E2F binding impairs regulation. In contrast, the cdc25C repressor module, which is derepressed late (S/G2), interacts selectively with CDE-CHR binding factor-1 (CDF-1). E2F binding, but not CDF-1 binding, requires specific nucleotides flanking the E2FBS/CDE core, while CDF-1 binding, but not E2F binding, depends on specific nucleotides in the CHR. Swapping these nucleotides between the two promoters profoundly changes protein binding patterns and alters expression kinetics. Thus predominant CDF-1 binding leads to derepression in late S, predominant E2F binding results in up-regulation in late G1, while promoters binding both E2F and CDF-1 with high efficiency show intermediate kinetics. Our results support a model where the differential binding of E2F and CDF-1 repressor complexes contributes to the timing of promoter activity during the cell cycle.

Cyclin D1 antisense RNA destabilizes pRb and retards lung cancer cell growth

To investigate the role of cyclin D1 in the regulation of lung cancer cell growth, we created five stably transfected cell lines carrying a cyclin D1 antisense construct. The transfected cells exhibited a marked decrease in the rate of cell growth, in contrast to the original lines (A549 and NCI-H441). The expression of several cell cycle-regulating proteins, including cyclin A, the cyclin-dependent kinases (cdk) 2 and cdk4, in addition to cyclin D1 itself, was markedly decreased. The expression of one cdk inhibitor, p21WAF1/CIP1, increased in the A549-derived cell lines. A specific target of cyclin D1 activity, the growth-suppressing product of the retinoblastoma gene, pRb, exhibited decreased expression and a decreased level of phosphorylation in the transfected cells. Decreased expression of pRb due to a significant increase in its turnover rate suggested that the stability of the protein may depend on phosphorylation by cyclin D1-dependent cdk activity. In addition to the impact on pRb stability, decreased expression of cyclin D1 induced susceptibility to cell death after withdrawal of exogenous growth factors in the antisense transfected cell lines, a response that was not observed in the original cancer cell lines. We conclude that abrogation of cyclin D1 overexpression in lung cancer cells disrupts several key pathways that are required for uncontrolled cell growth and induces those that lead to cell death after growth factor deprivation. Therefore, we speculate that use of antisense cyclin D1 expression in appropriate gene vectors could be a useful method for retarding lung cancer cell growth in accessible tumors such as those of the lung epithelium.