The E2F transcription factor is a cellular target for the RB protein

Mutation of E2f-1 suppresses apoptosis and inappropriate S phase entry and extends survival of Rb-deficient mouse embryos

Mice mutant for the Rb tumor suppressor gene die in mid-gestation with defects in erythropoiesis, cell cycle control, and apoptosis. We show here that embryos mutant for both Rb and its downstream target E2f-1 demonstrate significant suppression of apoptosis and S phase entry in certain tissues compared to Rb mutants, implicating E2f-1 as a critical mediator of these effects. Up-regulation of the p53 pathway, required for cell death in these cells in Rb mutants, is also suppressed in the Rb/E2f-1 double mutants. However, double mutants have defects in cell cycle regulation and apoptosis in some tissues and die at approximately E17.0 with anemia and defective skeletal muscle and lung development, demonstrating that E2F-1 regulation is not the sole function of pRB in development.

Extracellular matrix-associated transforming growth factor-beta: role in cancer cell growth and invasion

Growth factors of the transforming growth factor-beta (TGF-beta) family inhibit the proliferation of epithelial, endothelial, and hematopoietic cells, and stimulate the synthesis of extracellular matrix components. TGF-beta s are secreted from cells in high-molecular-mass protein complexes that are composed of three proteins, the mature TGF-beta-dimer, the TGF-beta propeptide dimer, or latency-associated protein (LAP), and the latent TGF-beta binding protein (LTBP). Mature TGF-beta is cleaved from its propeptide during secretion, but the proteins remain associated by noncovalent interactions. LTBP is required for efficient secretion and processing of latent TGF-beta and it binds to LAP via disulfide bond(s). LTBP is a component of extracellular matrix microfibrils, and it targets the latent TGF-beta complex to the extracellular matrix. TGF-beta signaling is initiated by proteolytic cleavage of LTBP that results in the release of the latent TGF-beta complex from the extracellular matrix. TGF-beta is activated by dissociation of LAP from the mature TGF-beta. Subsequent signaling involves binding of active TGF-beta to its type II cell surface receptors, which phosphorylate and activate type I TGF-beta receptors. Type I receptors, in turn, phosphorylate cytoplasmic transcriptional activator proteins Smad2 and Smad3, inducing their translocation to the nucleus. Recent evidence suggests that acquisition of resistance to TGF-beta growth inhibition plays a major role in the progression of epithelial and hematopoietic cell malignancies. The role of secretion of TGF-beta in tumorigenesis is more complex. The secretion of TGF-beta s by tumor cells may contribute to autocrine growth inhibition, but on the other hand, it may also promote invasion, metastasis, angiogenesis, and even immunosuppression. Tumor cells may also fail to deposit LTBP:TGF-beta complexes to the extracellular matrix. The elucidation of the mechanisms of the release of TGF-beta from the matrix and its subsequent activation aids the understanding of the pathophysiologic roles of TGF-beta in malignant growth, and allows the development of therapeutic agents that regulate the activity of TGF-beta.

Cellular resistance to adriamycin conferred by enhanced Rb expression is associated with increased MDR1 expression

In order to investigate if the enhanced expression of Rb confers increased cellular resistance to adriamycin, we made Rb stable transfectants from colon carcinoma cells, SW620. Rb stable transfectants exhibited 5- to 10-fold more resistance to adriamycin than the control cells. To study the correlation between enhanced Rb expression and MDR1 expression, products of the Rb gene and the MDR1 gene in Rb stable transfectants were measured by Western blot analysis. These Rb transfectants showed increased MDR1 expression. Transient transfection of the MDR1 promoter-CAT reporter gene and the Rb gene demonstrated that Rb up-regulated MDR1 promoter activity in SW620 cells. Rb may, at least partly, contribute to a role in protecting cells from carcinogen exposure by up-regulating the MDR1 gene.

Unusual proliferation arrest and transcriptional control properties of a newly discovered E2F family member, E2F-6

E2F transcription factors play an important role in the regulation of cell cycle progression. We report here the cloning and characterization of an additional member of this family, E2F-6. E2F-6 lacks pocket protein binding and transactivation domains, and it is a potent transcriptional repressor that contains a modular repression domain at its carboxyl terminus. Overproduction of E2F-6 had no specific effect on cell cycle progression in asynchronously growing Saos2 and NIH 3T3 cells, but it inhibited entry into S phase of NIH 3T3 cells stimulated to exit G0. Taken together, these data suggest that E2F-6 can regulate a subset of E2F-dependent genes whose products are required for entry into the cell cycle but not for normal cell cycle progression.

An oligonucleotide decoy for transcription factor E2F inhibits mesangial cell proliferation in vitro

The transcription factor E2F controls expression of several genes involved in cell proliferation including c-myc, c-myb, proliferating cell nuclear antigen (PCNA), and cdk2 kinase. Having established that both PCNA and cdk2 kinase are induced in rat mesangial cells (MC) by serum stimulation, we attempted to inhibit MC proliferation in vitro by transfecting these cells with cationic liposomes containing a synthetic double-stranded oligodeoxynucleotide (ODN) with high affinity for E2F. Using a gel mobility shift assay, we detected increased specific binding of E2F in MC following serum stimulation. This binding was completely inhibited by preincubation of MC nuclear extracts with the double-stranded ODN with high affinity for E2F but not by preincubation with a missense ODN containing two point mutations. MC were also transfected with a luciferase reporter gene construct containing three E2F binding sites. Luciferase activity was enhanced by serum stimulation of MC, and this effect was specifically abolished by cotransfection of MC with E2F decoy ODN. Furthermore, RT-PCR analysis revealed that serum-induced upregulation of PCNA and cdk2 kinase gene expression was inhibited by E2F decoy ODN transfection but not by transfection of missense ODN. These changes in gene expression were paralleled by a reduction in PCNA and cdk2 kinase protein expression in E2F decoy ODN transfected cells. MC number increased following serum stimulation. This effect was blunted by transfection with E2F decoy ODN but not by transfection of missense ODN. These data suggest that the transcription factor E2F plays a crucial role in the regulation of MC proliferation and that this factor can be successfully targeted to inhibit MC cell cycle progression.

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.

Molecular cloning and characterization of a novel retinoblastoma-binding protein

We describe the isolation and characterization of a novel cDNA encoding a polypeptide that interacts in a yeast two-hybrid system as well as in mammalian cells with the retinoblastoma (RB) protein. This new protein, which we call Rim, consists of 897 amino acids, has two leucine zipper motifs, and has a LECEE sequence previously identified as an RB-binding domain. Rim also has an E1A/CtBP-binding motif and four putative nuclear localization signals. Rim mRNA is expressed ubiquitously at low levels in all human adult tissues tested and at much higher levels in several tumor cell lines. The Rim gene (HGMW-approved symbol RBBP8) is localized on human chromosome 18q11.2.

Molecular mechanisms of impaired stimulation of DNA synthesis in cultured hepatocytes of aged rats

We examined epidermal growth factor (EGF)- and epinephrine-stimulated mitogen-activated protein kinase kinase (MEK) 1 and MEK2 activities, DNA polymerase alpha activity, and EGF-stimulated E2F DNA binding activity in primary cultured hepatocytes from 6- and 24-mo-old rats. MEK stimulation by either EGF or epinephrine was not altered with aging. However, stimulation of DNA polymerase alpha activity by these agents was 70% and 50% lower, respectively, in cells of aged compared with cells of young rats, consistent with a lesser increase in [3H]thymidine incorporation. EGF-stimulated E2F (a transcription factor that regulates expression of the DNA polymerase alpha gene) binding to DNA was reduced with age. PD-098059, a specific inhibitor of MEK, inhibited EGF-stimulated MEK1 and MEK2 activities in hepatocytes from 6- and 24-mo-old rats. Although PD-098059 inhibited EGF-stimulated DNA synthesis in hepatocytes from 6-mo-old rats, it had no effect in 24-mo-old rats. Thus the age-related impairment appears to occur before E2F activation, and signal transduction sequences other than the mitogen-activated protein kinase pathway may be involved in stimulated DNA synthesis in hepatocytes from old rats.

Functional and physical interactions between the estrogen receptor Sp1 and nuclear aryl hydrocarbon receptor complexes

17beta-Estradiol (E2) induces cathepsin D gene expression in MCF-7 human breast cancer cells and previous analyses of the proximal promoter region of this gene identified two functional enhancer sequences; namely an Sp1(N)23estrogen-responsive element (ERE) half-site (-199 to -165) and an imperfect palindromic ERE (-119 to -107). A third region of the cathepsin D gene promoter (CD/L, -145 to -119) was also E2 responsive in transient transfection assays. A GC-rich sequence which contains two overlapping Sp1 binding sites (-145 to -135) was responsible for ER-mediated transactivation and required formation of an ER/Sp1 complex in which only the Sp1 protein bound DNA. E2 responsiveness of the CD/L sequence was also dependent on an adjacent overlapping GCGTG motif corresponding to the dioxin-responsive element (DRE) core binding sequence, which is the cognate response element for the heterodimeric aryl hydrocarbon receptor (AhR)/AhR nuclear translocator (ARNT) transcription factor complex. The results show that ER-mediated transactivation of CD/L was associated with the Sp1(N)2-4DRE (core) motif and involved formation of a multiprotein ER/Sp1-AhR/ARNT complex. These results illustrate a unique example of an endogenous role for AhR/ARNT in the absence of added AhR agonist and indicate that the cathepsin D gene proximal promoter region contains at least three different functional motifs associated with ER-mediated transactivation.

The PITSLRE protein kinase family

A family of p34Cdc2 related protein kinases, the PITSLRE kinases, is generated by alternative splicing and promoter utilization from three duplicated and tandemly linked genes on human chromosome 1p36.3, which is frequently deleted during the late stages of tumorigenesis. PITSLRE mRNA, protein, and enzyme activity are induced during Fas receptor- and glucocorticoid-mediated apoptosis of human T cells. Several PITSLRE isoforms are specific targets of proteolysis during apoptosis, generating an enzymatically active 50 kDa isoform. Inhibition of this protease activity blocks PITSLRE processing and enzyme activation, as well as apoptosis. Thus, PITSLRE kinases may be integral downstream components of apoptotic signal transduction pathway(s). Furthermore, PITSLRE genes, and their products, are physically altered in human neuroblastoma tumors, suggesting that they may be tumor suppressors.

Inhibition of the 3-hydroxy-3-methylglutaryl-coenzyme A reductase pathway induces p53-independent transcriptional regulation of p21(WAF1/CIP1) in human prostate carcinoma cells

Progression through the cell cycle is controlled by the induction of cyclins and the activation of cognate cyclin-dependent kinases. The 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor lovastatin induces growth arrest and cell death in certain cancer cell types. We have pursued the mechanism of growth arrest in PC-3-M cells, a p53-null human prostate carcinoma cell line. Lovastatin treatment increased protein and mRNA levels of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1), increased binding of p21 with Cdk2, markedly inhibited cyclin E- and Cdk2-associated phosphorylation of histone H1 or GST-retinoblastoma protein, enhanced binding of the retinoblastoma protein to the transcription factor E2F-1 in vivo, and induced the activation of a p21 promoter reporter construct. By using p21 promoter deletion constructs, the lovastatin-responsive element was mapped to a region between -93 and -64 relative to the transcription start site. Promoter mutation analysis indicated that the lovastatin-responsive site coincided with the previously identified transforming growth factor-beta-responsive element. These data indicate that in human prostate carcinoma cells an inhibitor of the HMG-CoA reductase pathway can circumvent the loss of wild-type p53 function and induce critical downstream regulatory events leading to transcriptional activation of p21.

p21 is a critical CDK2 regulator essential for proliferation control in Rb-deficient cells

Proliferation in mammalian cells is controlled primarily in the G1-phase of the cell cycle through the action of the G1 cyclin-dependent kinases, CDK4 and CDK2. To explore the mechanism of cellular response to extrinsic factors, specific loss of function mutations were generated in two negative regulators of G1 progression, p21 and pRB. Individually, these mutations were shown to have significant effects in G1 regulation, and when combined, Rb and p21 mutations caused more profound defects in G1. Moreover, cells deficient for pRB and p21 were uniquely capable of anchorage-independent growth. In contrast, combined absence of pRB and p21 function was not sufficient to overcome contact inhibition of growth nor for tumor formation in nude mice. Finally, animals with the genotype Rb+/-;p21(-/-) succumbed to tumors more rapidly than Rb+/- mice, suggesting that in certain contexts mutations in these two cell cycle regulators can cooperate in tumor development.

Cooperative interactions between RB and p53 regulate cell proliferation, cell senescence, and apoptosis in human vascular smooth muscle cells from atherosclerotic plaques

Compared with vascular smooth muscle cells (VSMCs) from normal vessels, VSMCs from human atherosclerotic plaques proliferate more slowly, undergo earlier senescence, and demonstrate higher levels of apoptosis in culture. The tumor suppressor genes p105RB (retinoblastoma, acting through the E2F transcription factor family) and p53 regulate cell proliferation, cell senescence, and apoptosis in many cell types. We have therefore determined whether these stable growth properties of plaque VSMCs reflect altered activity of RB and/or p53. VSMCs were derived from coronary atherectomies or from normal coronary arteries from transplant recipients. Compared with normal VSMCs, plaque VSMCs showed a higher ratio of the active (hypophosphorylated) to the inactive (phosphorylated) form of RB and a lower level of E2F transcriptional activity. Cells were stably transfected with retrovirus constructs that inhibited RB or p53 alone or in combination. Suppression of RB alone increased rates of cell proliferation and apoptosis and inhibited cell senescence in normal VSMCs. Suppression of p53 and RB together had similar effects but, additionally, resulted in immortalization of normal VSMC cultures. In contrast, inhibition of RB binding to E2F or ectopic expression of E2F-1 in plaque VSMCs induced massive apoptosis, which required suppression of p53 to rescue cells. Suppression of RB and p53 together increased cell proliferation and delayed senescence but failed to immortalize plaque VSMCs. Inhibition of p53 alone had minimal effects on plaque VSMCs but increased the lifespan of normal VSMCs. We conclude that human plaque VSMCs have slower rates of cell proliferation and earlier senescence than do cells from normal vessels because of a defect in phosphorylation of RB. Furthermore, both disruption of RB/E2F and inhibition of p53 are required for plaque VSMCs to proliferate without apoptosis. This observation may explain the relatively low level of cell proliferation and high level of apoptosis seen in VSMCs in human atherosclerotic plaques.

Induction of retinoblastoma gene expression during terminal growth arrest of a conditionally immortalized fetal rat lung epithelial cell line and during fetal lung maturation

The process by which fetal lung epithelial cells differentiate into type 1 and type 2 cell is largely unknown. In order to study lung epithelial cell proliferation and differentiation we have infected 20-day fetal lung epithelial cells with a retrovirus carrying a temperature-sensitive SV40 T antigen (T Ag) and isolated several immortalized fetal epithelial cell lines. Cell line 20-3 has characteristics of lung epithelial cells including the presence of distinct lamellar bodies, tight junctions, keratin 8 and 18 mRNA, HFH8, and T1 alpha mRNA and low levels of surfactant protein A mRNA. At 33 degrees C 20-3 grows with a doubling time of 21 h. At 40 degrees C the majority of cells cease to proliferate. Growth arrest is accompanied by significant morphological changes including an increase in cell size, transition to a squamous phenotype that resembles type 1 cells, and an increase in the number of multinucleated cells within the population. Greater than 95% of the cells incorporate [3H]thymidine into DNA at 33 degrees C whereas at 40 degrees C label incorporation drops to less than 20%. When shifted down to 33 degrees C 40% of the cells remain terminally growth arrested. In addition, cells plated at 40 degrees C have a reduced ability to form colonies when replated at 33 degrees C. Treatment with TGF-beta increases the percentage of cells that terminally growth arrest to greater than 80%. Growth arrest is accompanied by an increase in the levels of c-jun, jun D, cyclin D1, C/EBP-beta, transglutaminase type II, and retinoblastoma (Rb) mRNA and an induction of p105, the hypophosphorylated, growth regulatory form of Rb. Evaluation of Rb mRNA in fetal lung indicates that it is induced 2.5-fold between 17 and 21 days of gestation. These studies indicate that 20-3 terminally growth arrests in culture at the nonpermissive temperature and that it may be useful in studying changes in gene expression that accompany terminal growth arrest during lung development.

The role of cyclin D3-dependent kinase in the phosphorylation of p130 in mouse BALB/c 3T3 fibroblasts

We have observed that cyclin D3-dependent kinase activity is increased in the late G1 phase in BALB/c 3T3 fibroblasts. The profile of cyclin D3-associated activity closely parallels that of cyclin D1, which is also induced after mitogenic stimulation of quiescent cells. These activities correlate with the appearance of hyperphosphorylated p130, an Rb family member important in regulating E2F-4 and E2F-5 activity in fibroblastic cells. We demonstrated, however, that only the cyclin D3 activity efficiently phosphorylated p130 in an in vitro kinase assay. This apparent specificity was further demonstrated by experiments which demonstrated that cyclin D3 was physically associated with p130 at the times when D3-dependent kinase activity and p130 hyperphosphorylation were observed. Examination of E2F by electrophoretic mobility shift assay revealed that E2F-4 DNA binding activity existed in a p130.E2F complex at times before D3-dependent kinase activity was apparent and in a free E2F-4 complex after D3 activity developed. Thus, our data suggest that cyclin D3 preferentially phosphorylates p130 and is thereby specifically targeted to overcoming growth-suppressive control mediated through p130 pathways.

Cdkn2a, the cyclin-dependent kinase inhibitor encoding p16INK4a and p19ARF, is a candidate for the plasmacytoma susceptibility locus, Pctr1

Plasma cell tumor induction in mice by pristane is under multigenic control. BALB/c mice are susceptible to tumor development; whereas DBA/2 mice are resistant. Restriction fragment length polymorphisms between BALB/c and DBA/2 for Cdkn2a(p16) and Cdkn2b(p15), and between BALB/c and Mus spretus for Cdkn2c(p18(INK4c)) were used to position these loci with respect to the Pctr1 locus. These cyclin-dependent kinase (CDK) inhibitors mapped to a 6 cM interval of chromosome 4 between Ifna and Tal1. C.D2-Chr 4 congenic strains harboring DBA/2 alleles associated with the Pctr1 locus contained DBA/2 "resistant" alleles of the CDK4/CDK6 inhibitors p16 and p15. On sequencing p16 and p18 cDNAs, two different allelic variants within ankyrin repeat regions of p16 were found between BALB/c and DBA/2 mice. By using an assay involving PCR amplification and restriction enzyme digestion, allelic variants were typed among several inbred strains of mice. One of the variants, G232A, was specific to two inbred strains, BALB/cAn and ABP/Le, of mice and occurred in a highly conserved amino acid in both human and rat p16. When tested with wild-type (DBA/2) p16, both A134C and G232A BALB/c-specific variants of p16 were inefficient in their ability to inhibit the activity of cyclin D2/CDK4 in kinase assays with retinoblastoma protein, suggesting this defective, inherited allele plays an important role in the genetic susceptibility of BALB/c mice for plasmacytoma induction and that p16(INK4a) is a strong candidate for the Pctr1 locus.

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.

Gene expression and cell cycle arrest mediated by transcription factor DMP1 is antagonized by D-type cyclins through a cyclin-dependent-kinase-independent mechanism

A novel 761-amino-acid transcription factor, DMP1, contains a central DNA binding domain that includes three imperfect myb repeats flanked by acidic transactivating domains at the amino and carboxyl termini. D-type cyclins associate with a region of the DMP1 DNA binding domain immediately adjacent to the myb repeats to form heteromeric complexes which detectably interact neither with cyclin-dependent kinase 4 (CDK4) nor with DNA. The segment of D-type cyclins required for its interaction with DMP1 falls outside the "cyclin box," which contains the residues predicted to contact CDK4. Hence, D-type cyclin point mutants that do not interact with CDK4 can still bind to DMP1. Enforced coexpression of either of three D-type cyclins (D1, D2, or D3) with DMP1 in mammalian cells canceled its ability to activate gene expression. This property was not shared by cyclins A, B, C, or H; did not depend upon CDK4 or CDK2 coexpression; was not subverted by a mutation in cyclin D1 that prevents its interaction with CDK4; and was unaffected by inhibitors of CDK4 catalytic activity. Introduction of DMP1 into mouse NIH 3T3 fibroblasts inhibited entry into S phase. Cell cycle arrest depended upon the ability of DMP1 to bind to DNA and to transactivate gene expression and was specifically antagonized by coexpression of D-type cyclins, including a D1 point mutant that does not bind to CDK4. Taken together, these findings suggest that DMP1 induces genes that inhibit S phase entry and that D-type cyclins can override DMP1-mediated growth arrest in a CDK-independent manner.

The retinoblastoma protein-associated cell cycle arrest in S-phase under moderate hypoxia is disrupted in cells expressing HPV18 E7 oncoprotein

We have studied the role of the oxygen-dependent pyrimidine metabolism in the regulation of cell cycle progression under moderate hypoxia in human cell lines containing functional (T-47D) or non-functional (NHIK 3025, SAOS-2) retinoblastoma gene product (pRB). Under aerobic conditions, pRB exerts its growth-regulatory effects during early G1 phase of the cell cycle, when all pRB present has been assumed to be in the underphosphorylated form and bound in the nucleus. We demonstrate that pRB is dephosphorylated and re-bound in the nucleus in approximately 90% of T-47D cells located in S and G2 phases under moderately hypoxic conditions. Under these conditions, no T-47D cells entered S-phase, and no progression through S-phase was observed. Progression of cells through G2 and mitosis seems independent of their functional pRB status. The p21WAF1/CIP1 protein level was significantly reduced by moderate hypoxia in p53-deficient T-47D cells, whereas p16(INK4a) was not expressed in these cells, suggesting that the hypoxia-induced cell cycle arrest is independent of these cyclin-dependent kinase inhibitors. The addition of pyrimidine deoxynucleosides did not release T-47D cells, containing mainly underphosphorylated pRB, from the cell cycle arrest induced by moderate hypoxia. However, NHIK 3025 cells, in which pRB is abrogated by expression of the HPV18 E7 oncoprotein, and SAOS-2 cells, which lack pRB expression, continued cell cycle progression under moderate hypoxia provided that excess pyrimidine deoxynucleosides were present. NHIK 3025 cells express high levels of p16INK4a under both aerobic and moderately hypoxic conditions, suggesting that the inhibitory function of p16(INK4a) would not be manifested in such pRB-deficient cells. Thus, pRB, a key member of the cell cycle checkpoint network, seems to play a major role by inducing growth arrest under moderate hypoxia, and it gradually overrides hypoxia-induced suppression of pyrimidine metabolism in the regulation of progression through S-phase under such conditions.

Rb interacts with histone deacetylase to repress transcription

Previously, we found that Rb can actively repress transcription of cell cycle genes by binding and inactivating transcription factors at the promoter. Here, we demonstrate that Rb can also repress transcription of endogenous cell cycle genes containing E2F sites through recruitment of histone deacetylase, which deacetylates histones on the promoter, thereby promoting formation of nucleosomes that inhibit transcription. These two mechanisms of repression by Rb are selective-some promoters and transcription factors are blocked by this recruitment of histone deacetylase, whereas others are resistant to histone deacetylase activity and are repressed directly by inhibition of transcription factors.

Transcriptional activation of the p34cdc2 gene by cdc2 promoter binding factor/nuclear factor-Y in fetal rat ventricular myocytes

To determine how myocardial terminal differentiation is regulated by cell cycle control genes, we studied cdc2 expression in rat cardiac muscle and found that cdc2 mRNA and protein levels were reduced in neonatal compared with fetal ventricles and became undetectable in juvenile and adult ventricles. To further determine whether cdc2 downregulation is attributed to a decrease in transcription, transient expression assay was performed using the progressively truncated 6.2-, 1.8-, 1.1-, 0.7-, and 0.1-kb human cdc2 5' flanking regions. All five fragments activated reporter expression in fetal myocytes and were significantly less active in neonatal myocytes. The 0.1-kb fragment showed 65% of the activity of the 6.2-kb fragment. A protein binding site that contains an inverted CCAAT box was identified within the 0.1-kb fragment by DNase I footprint assay and named the cdc2 promoter binding factor (CPBF) site. Point mutations within the CPBF site that abolish CPBF binding significantly decreased both 0.1- and 6.2-kb promoter activities. Competition and antibody supershift assays suggested that CPBF was identical or related to the transcription factor, nuclear factor Y (NF-Y). The 0.1-kb promoter activity was suppressed by a dominant-negative NF-Y mutant in fetal myocytes. Taken together, our results demonstrate that cardiac cdc2 expression is downregulated after birth and turned off when the juvenile stage is attained. A 0.1-kb promoter fragment of cdc2 contains major information for both cdc2 transcriptional activation and suppression in fetal and neonatal myocytes, respectively. NF-Y or its related factor plays a critical role in activating the 0.1-kb cdc2 promoter.

Human papillomavirus oncoproteins E6 and E7 independently abrogate the mitotic spindle checkpoint

The E6 and E7 genes of the high-risk human papillomavirus (HPV) types encode oncoproteins, and both act by interfering with the activity of cellular tumor suppressor proteins. E7 proteins act by associating with members of the retinoblastoma family, while E6 increases the turnover of p53. p53 has been implicated as a regulator of both the G1/S cell cycle checkpoint and the mitotic spindle checkpoint. When fibroblasts from p53 knockout mice are treated with the spindle inhibitor nocodazole, a rereplication of DNA occurs without transit through mitosis. We investigated whether E6 or E7 could induce a similar loss of mitotic checkpoint activity in human keratinocytes. Recombinant retroviruses expressing high-risk E6 alone, E7 alone, and E6 in combination with E7 were used to infect normal human foreskin keratinocytes (HFKs). Established cell lines were treated with nocodazole, stained with propidium iodide, and analyzed for DNA content by flow cytometry. Cells infected with high-risk E6 were found to continue to replicate DNA and accumulated an octaploid (8N) population. Surprisingly, expression of E7 alone was also able to bypass this checkpoint. Cells expressing E7 alone exhibited increased levels of p53, while those expressing E6 had significantly reduced levels. The p53 present in the E7 cells was active, as increased levels of p21 were observed. This suggested that E7 bypassed the mitotic checkpoint by a p53-independent mechanism. The levels of MDM2, a cellular oncoprotein also implicated in control of the mitotic checkpoint, were significantly elevated in the E7 cells compared to the normal HFKs. In E6-expressing cells, the levels of MDM2 were undetectable. It is possible that abrogation of Rb function by E7 or increased expression of MDM2 contributes to the loss of mitotic spindle checkpoint control in the E7 cells. These findings suggest mechanisms by which both HPV oncoproteins contribute to genomic instability at the mitotic checkpoint.

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.

Mechanism of retinoblastoma gene inactivation in the spectrum of neuroendocrine lung tumors

The retinoblastoma (RB) gene plays a key role in cell cycle control by regulation of G1 growth arrest. This gene is inactivated in some human cancers and in most small-cell lung carcinoma (SCLC) cell lines. The aim of this study was to analyze the mechanisms of RB silencing in freshly excised neuroendocrine (NE) tumors embracing the entire spectrum of NE lung neoplasms (typical and atypical carcinoids, large-cell neuroendocrine carcinomas [LCNECs], and SCLCs). To study the role and mechanism of RB inactivation in tumor differentiation and malignant potential, the status of the Rb protein was analyzed in 37 NE lung tumors, using immunohistochemistry with five Rb antibodies. Loss or altered expression of Rb protein was more frequently observed in high-grade NE lung carcinoma (23 of 28, 82%) than in typical and atypical carcinoids (1 of 9, 11%) (P < 0.001). Of 24 tumors with abnormal Rb staining, Southern blotting showed 1 to have undergone rearrangement, SSCP (single-strand conformation polymorphism) and sequencing showed that 6 (25%) exhibited mutations in exons 13-18 or 20-24 of the RB gene, and RT-PCR (reverse transcriptase-polymerase chain reaction) revealed that 14 (58%) showed a low level of or entirely absent RB mRNA (messenger RNA) expression, whereas hypermethylation of the CpG-rich island at the 5' end of the RB gene was not observed. Abnormal Rb protein expression was always associated with one of these three alternative mechanisms in the SCLCs analyzed, but in only 50% of LCNECs. These results indicate that inactivation of the RB gene is highly frequent in freshly excised high-grade NE lung tumors through distinct mechanisms including point mutations and frequent abnormal mRNA expression. Different modes of RB inactivation seem to be implicated along the spectrum of NE lung carcinomas, depending on differentiation state, phenotype, and malignancy grade.

Increased RB1 abnormalities in human primary prostate cancer following combined androgen blockade

BACKGROUND

The RB1 proliferation control pathway is a critical determinant of cell cycle progression. Abnormalities of RB1 are found in a variety of cancers, and the association with human prostate cancer (CaP) was examined here.

METHODS

RNA expression levels of RB1 in CaPs were examined by RT-PCR. RNA integrity was assessed by evaluating expression of an endogenous gene standard.

RESULTS

Abnormally low RB1 mRNA expression was found in 12/33 (36%) of CaPs from patients who had received combined androgen blockade (CAB) treatment. In contrast, 6/48 (13%) untreated CaPs showed abnormally low expression. This difference was statistically significant (P = 0.015). In the samples from untreated patients, a higher frequency of abnormal RB1 was found in specimens with a higher Gleason grade (P = 0.038). In addition, one untreated stage C, grade 9 specimen was found to express RB1 transcripts lacking exon 22, as determined by sequencing of DNA from the truncated transcripts.

CONCLUSIONS

These findings suggest that abnormalities of RB1 may contribute to hormone-withdrawal-related survival of CaP cells.

Identification of glucocorticoid- and adenovirus E1A-regulated genes in lung epithelial cells by differential display

Adenovirus infection has been implicated in the pathogenesis of lung inflammatory diseases for which glucocorticoids provide effective antiinflammatory treatment. In this study, the differential display assay was used to identify messenger RNAs (mRNAs) differentially expressed in dexamethasone (1 microM for 24 h)-treated A549 lung epithelial cells compared to A549 cells transfected with the adenoviral E1A gene. Thirty-seven complimentary DNAs (cDNAs) (15 glucocorticoid-regulated, 22 adenovirus E1A-regulated) were isolated. DNA sequence analysis showed that 35 of these were unique, 2 were identical with each other, and 3 were common to the glucocorticoid- and E1A-regulated groups. Genes identified included those involved in transcription/translation, cytoskeletal/contractile element genes, metabolic enzyme genes, and genes associated with cell regulation/signal transduction. After further analysis of the isolated clones by Northern blotting, ribonuclease protection, and semiquantitative RT-PCR (reverse transcriptase-polymerase chain reaction), 10 of the 14 glucocorticoid-regulated and one of the three common to both the adenovirus E1A- and glucocorticoid-regulated cDNAs were confirmed for this control of their expression. We conclude that the strategy of identifying cDNAs regulated by both adenovirus E1A and glucocorticoids provides a promising approach for identifying genes that may be important in the pathogenesis of lung inflammation and therefore targets for glucocorticoid treatment.

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.

Viral transactivating proteins

Many viruses utilize the cellular transcription apparatus to express their genomes, and they encode transcriptional regulatory proteins that modulate the process. Here we review the current understanding of three viral regulatory proteins. The adenovirus E1A protein acts within the nucleus to regulate transcription through its ability to bind to other proteins. The herpes simplex type 1 virus VP16 protein acts within the nucleus to control transcription by binding to DNA in conjunction with cellular proteins. The human T-cell leukemia virus Tax protein influences transcription through interactions with cellular proteins in the nucleus as well as the cytoplasm.

DDB, a putative DNA repair protein, can function as a transcriptional partner of E2F1

The transcription factor E2F1 is believed to be involved in the regulated expression of the DNA replication genes. To gain insights into the transcriptional activation function of E2F1, we looked for proteins in HeLa nuclear extracts that bind to the activation domain of E2F1. Here we show that DDB, a putative DNA repair protein, associates with the activation domain of E2F1. DDB was identified as a heterodimeric protein (48 and 127 kDa) that binds to UV-damaged DNA. We show that the UV-damaged-DNA binding activity from HeLa nuclear extracts can associate with the activation domain of E2F1. Moreover, the 48-kDa subunit of DDB, synthesized in vitro, binds to a fusion protein of E2F1 depending on the C-terminal activation domain. The interaction between DDB and E2F1 can also be detected by coimmunoprecipitation experiments. Immunoprecipitation of an epitope-tagged DDB from cell extracts resulted in the coprecipitation of E2F1. In a reciprocal experiment, immunoprecipitates of E2F1 were found to contain DDB. Fractionation of HeLa nuclear extracts also revealed a significant overlap in the elution profiles of E2F1 and DDB. For instance, DDB, which does not bind to the E2F sites, was enriched in the high-salt fractions containing E2F1 during chromatography through an E2F-specific DNA affinity column. We also observed evidence for a functional interaction between DDB and E2F1 in living cells. For instance, expression of DDB specifically stimulated E2F1-activated transcription. In addition, the transcriptional activation function of a heterologous transcription factor containing the activation domain of E2F1 was stimulated by coexpression of DDB. Moreover, DDB expression could overcome the retinoblastoma protein (Rb)-mediated inhibition of E2F1-activated transcription. The results suggest that this damaged-DNA binding protein can function as a transcriptional partner of E2F1. We speculate that the damaged-DNA binding function of DDB, besides repair, might serve as a negative regulator of E2F1-activated transcription, as damaged DNA will sequester DDB and make it unavailable for E2F1. Furthermore, the binding of DDB to damaged DNA might be involved in downregulating the replication genes during growth arrest induced by damaged DNA.

Alterations in G1/S cell-cycle control contributing to carcinogenesis

Mutations involving the retinoblastoma tumor-suppressor gene (RB-1) have been described in a variety of human neoplasms. In addition, many tumors that retain a wild-type RB-1 allele harbor mutations that indirectly impair the function of the RB-1 gene product (pRB). pRB is a nuclear protein that regulates cell-cycle progression and, in at least certain tissues, differentiation. The former has been linked to its ability to form complexes with members of the E2F transcription factor family. E2F DNA-binding sites have been identified in the regulatory regions of a number of genes involved in cell-cycle progression. pRB-E2F complexes actively repress transcription when bound to these sites. All tumor-derived pRB mutants have lost the ability to bind to E2F. Conversely, activation of E2F-responsive genes is sufficient to overcome a pRB-induced cell-cycle block and, in certain cell types, can lead to transformation. Thus, E2F appears to be a physiologically relevant target of pRB action, and deregulation of E2F-responsive genes is a common, and possibly universal, step in human carcinogenesis.

Cyclin dependent kinase inhibitors and dominant negative cyclin dependent kinase 4 and 6 promote survival of NGF-deprived sympathetic neurons

Neuronal apoptosis plays a critical role in both normal development and disease. However, the precise molecular events controlling neuronal apoptosis are not well understood. Previously, we hypothesized that cell cycle regulatory molecules function in controlling the apoptotic pathways of trophic factor-deprived neurons. To test this hypothesis, we used the RNA alphavirus Sindbis to express three known cyclin dependent kinase inhibitors (CKIs), p16(ink4), p21(waf/cip), and p27(kip1), and dominant negative mutant forms of four known G1 cyclin dependent kinases (CDKs), Cdk2, Cdk3, Cdk4, and Cdk6, in primary cultured rat superior cervical ganglion sympathetic neurons. We demonstrate that expression of each of the CKIs protects the postmitotic cultured neurons from apoptotic death evoked by withdrawal of NGF. In addition, we show that expression of dominant negative forms of Cdk4 or Cdk6, but not Cdk2 or Cdk3, protects NGF-deprived sympathetic neurons from death. Such findings suggest the participation of several CDKs and their cognate cyclins in a neuronal apoptotic pathway.

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.

E1A 12S and 13S of the transformation-defective adenovirus type 12 strain CS-1 inactivate proteins of the RB family, permitting transactivation of the E2F-dependent promoter

The transformation-defective Vero cell host range mutant CS-1 of the highly oncogenic adenovirus type 12 (Ad12) (Ad12-CS-1) has a 69-bp deletion in the early region 1A (E1A) gene that removes the carboxy-terminal half of conserved region 2 and the amino-terminal half of the Ad12-specific so-called spacer that seems to play a pivotal role in the oncogenicity of the virus. Despite its deficiency in immortalizing and transforming primary rodent cells, we found that the E1A 13S protein of Ad12-CS-1 retains the ability to bind p105-RB, p107, and p130 in nuclear extract binding assays with glutathione S-transferase-E1A fusion proteins and Western blot analysis. Like wild-type E1A, the mutant protein was able to dissociate E2F from retinoblastoma-related protein-containing complexes, as judged from gel shift experiments with purified 12S and 13S proteins from transfection experiments with an E1A expression vector or from infection with the respective virus. Moreover, in transient expression assays, the 12S and 13S products of wild-type Ad12 and Ad12-CS-1 were shown to transactivate the Ad12 E1A promoter containing E2F-1 and E2F-5-motifs, respectively, in a comparable manner. The same results were obtained from transfection assays with the E2F motif-dependent E2 promoter of adenovirus type 5 or the human dihydrofolate reductase promoter. These data suggest that efficient infection by Ad12 and the correlated virus-induced reprogramming of the infected cells, including the induction of cell cycle-relevant mechanisms (e.g. E2F activation), can be uncoupled from the transformation properties of the virus.

Subunit composition determines E2F DNA-binding site specificity

The product of the retinoblastoma (Rb) susceptibility gene, Rb-1, regulates the activity of a wide variety of transcription factors, such as E2F, in a cell cycle-dependent fashion. E2F is a heterodimeric transcription factor composed of two subunits each encoded by one of two related gene families, denoted E2F and DP. Five E2F genes, E2F-1 through E2F-5, and two DP genes, DP-1 and DP-2, have been isolated from mammals, and heterodimeric complexes of these proteins are expressed in most, if not all, vertebrate cells. It is not yet clear whether E2F/DP complexes regulate overlapping and/or specific cellular genes. Moreover, little is known about whether Rb regulates all or a subset of E2F-dependent genes. Using recombinant E2F, DP, and Rb proteins prepared in baculovirus-infected cells and a repetitive immunoprecipitation-PCR procedure (CASTing), we have identified consensus DNA-binding sites for E2F-1/DP-1, E2F-1/DP-2, E2F-4/DP-1, and E2F-4/DP-2 complexes as well as an Rb/E2F-1/DP-1 trimeric complex. Our data indicate that (i) E2F, DP, and Rb proteins each influence the selection of E2F-binding sites; (ii) E2F sites differ with respect to their intrinsic DNA-bending properties; (iii) E2F/DP complexes induce distinct degrees of DNA bending; and (iv) complex-specific E2F sites selected in vitro function distinctly as regulators of cell cycle-dependent transcription in vivo. These data indicate that the specific sequence of an E2F site may determine its role in transcriptional regulation and suggest that Rb/E2F complexes may regulate subsets of E2F-dependent cellular genes.

E2F activity is regulated by cell cycle-dependent changes in subcellular localization

E2F directs the cell cycle-dependent expression of genes that induce or regulate the cell division process. In mammalian cells, this transcriptional activity arises from the combined properties of multiple E2F-DP heterodimers. In this study, we show that the transcriptional potential of individual E2F species is dependent upon their nuclear localization. This is a constitutive property of E2F-1, -2, and -3, whereas the nuclear localization of E2F-4 is dependent upon its association with other nuclear factors. We previously showed that E2F-4 accounts for the majority of endogenous E2F species. We now show that the subcellular localization of E2F-4 is regulated in a cell cycle-dependent manner that results in the differential compartmentalization of the various E2F complexes. Consequently, in cycling cells, the majority of the p107-E2F, p130-E2F, and free E2F complexes remain in the cytoplasm. In contrast, almost all of the nuclear E2F activity is generated by pRB-E2F. This complex is present at high levels during G1 but disappears once the cells have passed the restriction point. Surprisingly, dissociation of this complex causes little increase in the levels of nuclear free E2F activity. This observation suggests that the repressive properties of the pRB-E2F complex play a critical role in establishing the temporal regulation of E2F-responsive genes. How the differential subcellular localization of pRB, p107, and p130 contributes to their different biological properties is also discussed.

Reduction in fibronectin expression and alteration in cell morphology are coincident in NIH3T3 cells expressing a mutant E2F1 transcription factor

Fibronectin within the extracellular matrix plays a role in cell attachment, spreading, and shape, while it also affects aspects of cell proliferation. Transcription factors such as E2F1 are also known to regulate cell shape and cell proliferation. Yet, to date no linkage has been established between fibronectin expression and E2F1. We show here that cells constitutively expressing a mutant E2F1 protein (E2F1d87) produce reduced amounts of fibronectin mRNA and protein. The altered expression of fibronectin seen in the E2F1d87 expressing cells is due, in part, to a reduction in transcription from the fibronectin promoter. Providing exogenous fibronectin, but not Type I collagen or laminin, as a substrate for cell adhesion is sufficient to revert the altered morphology and reestablish actin-containing microfilaments lost in the mutant cell line. An additional characteristic of the cells expressing the mutant E2F1 is that they demonstrate slow growth and a doubling in S phase duration. While providing exogenous fibronectin as an adhesion substrate did not shorten the S phase duration in the mutant line, it did significantly shorten the S phase duration in the parental NIH3T3 cell line, implicating a role for the extracellular matrix in regulating S phase transit in normal cells.

Incomplete penetrance of familial retinoblastoma linked to germ-line mutations that result in partial loss of RB function

To study the molecular basis for the clinical phenotype of incomplete penetrance of familial retinoblastoma, we have examined the functional properties of three RB mutations identified in the germ line of five different families with low penetrance. RB mutants isolated from common adult cancers and from classic familial retinoblastoma (designated as classic RB mutations) are unstable and generally do not localize to the nucleus, do not undergo cyclin-dependent kinase (cdk)-mediated hyperphosphorylation, show absent protein "pocket" binding activity, and do not suppress colony growth of RB(-) cells. In contrast, two low-penetrant alleles (661W and "deletion of codon 480") retained the ability to localize to the nucleus, showed normal cdk-mediated hyperphosphorylation in vivo, exhibited a binding pattern to simian virus 40 large T antigen using a quantitative yeast two-hybrid assay that was intermediate between classic mutants (null) and wild-type RB, and had absent E2F1 binding in vitro. A third, low-penetrant allele, "deletion of RB exon 4," showed minimal hyperphosphorylation in vivo but demonstrated detectable E2F1 binding in vitro. In addition, each low-penetrant RB mutant retained the ability to suppress colony growth of RB(-) tumor cells. These findings suggest two categories of mutant, low-penetrant RB alleles. Class 1 alleles correspond to promoter mutations, which are believed to result in reduced or deregulated levels of wild-type RB protein, whereas class 2 alleles result in mutant proteins that retain partial activity. Characterization of the different subtypes of class 2 low-penetrant genes may help to define more precisely functional domains within the RB product required for tumor suppression.

Prognostic implications of cyclins (D1, E, A), cyclin-dependent kinases (CDK2, CDK4) and tumor-suppressor genes (pRB, p16INK4A) in childhood acute lymphoblastic leukemia

Immunohistochemistry was used to analyze samples of 40 newly diagnosed childhood acute lymphoblastic leukemias (ALL) for their expression of cyclins (D1, E, A), cyclin-dependent kinases (cdk2, cdk4) and tumor-suppressor genes (pRb, p16INK4A), in order to discover whether or not the expression of these various proteins may be of prognostic relevance for the survival of children with ALL. Patients with ALL who were strongly positive for cyclin D1 had a lower probability of remaining in first continuous remission than ALL patients who were negative or weakly positive for this trait. There was also a significant correlation between expression of cyclin D1 and frequency of recurrence. For cyclin E and cyclin A, in contrast, there was no difference in the duration of relapse-free-intervals or the frequency of recurrence in patients. Children with cdk4-positive ALL had a lower probability of remaining in first continuous remission than children with cdk4-negative ALL. No prognostic relevance was found for cdk2. Patients with ALL who expressed pRb had a higher probability and patients who expressed p16 a lower probability of remaining in first continuous remission, but the results were not statistically significant. This investigation demonstrated that cyclin D1 and cdk4 were the most important prognostic factors for children with ALL, and that the combination of them showed the strongest prognostic relevance.

Identification of two novel human putative serine/threonine kinases, VRK1 and VRK2, with structural similarity to vaccinia virus B1R kinase

A cDNA library enriched for human fetal-specific liver genes was constructed by suppressive subtractive hybridization. EST fls223 generated from this library was found to represent a novel putative serine/threonine (Ser/Thr) kinase. A full-length clone isolated for this gene encodes a protein of 396 amino acids. The amino acid sequence has 40% identity over 305 amino acids with the B1R Ser/Thr protein kinase of vaccinia virus. This gene has therefore been named VRK1 (vaccinia virus B1R kinase related kinase). VRK1 was also found to have sequence identity (62.0% over 481 nucleotides) to a database EST. A full-length clone for this EST was isolated and sequenced. Conceptual translation predicts a protein of 508 amino acids that, like VRK1, has similarity to B1R kinase (38.7% identity over 300 amino acids). This gene has been named VRK2. Comparison of VRK1 with VRK2 indicates that they encode structurally related putative Ser/Thr protein kinases. Northern analysis shows that expression of both genes is widespread and elevated in highly proliferative cells, such as testis, thymus, and fetal liver. B1R kinase is reported to be essential for DNA replication of vaccinia virus. The similarity of VRK1 and VRK2 to B1R indicates that these genes may have similar functions.

Cell-growth regulation of the hamster dihydrofolate reductase gene promoter by transcription factor Sp1

The dihydrofolate reductase (DHFR) gene (dhfr) promoter contains cis-acting elements for the transcription factors Sp1 and E2F. Given the ability of Sp1 to activate the dhfr promoter, we have evaluated the contribution of Sp1 to the cell-growth regulation of the dhfr gene. Using gel-mobility assays performed with DNA probes from the minimal promoter of the hamster dhfr gene and nuclear extracts from cultured hamster cells (CHO K1) we show that the binding of Sp1 to the dhfr promoter is cell-growth-phase regulated. Accordingly, dhfr transcription and mRNA levels in K1 cells increase upon serum stimulation. Cytological detection of Sp1 by immunofluorescence reveals a decrease of this protein in the process leading to the G0 state, and an increase upon serum stimulation of quiescent cells. These results were confirmed by western blot analysis. It is concluded that Sp1 progressively binds to the hamster dhfr promoter after stimulation of cell proliferation, which can account for the transcriptional regulation of the dhfr gene during the cell cycle. The role of Sp1 in the specific control of dhfr during the cell cycle was confirmed in vivo using cell lines derived from dhfr-negative cells transfected with dhfr plasmids carrying either the wild-type or mutated Sp1-binding or E2F-binding sites in the dhfr minimal promoter.

Hypo-phosphorylation of the retinoblastoma protein (pRb) by cyclin D:Cdk4/6 complexes results in active pRb

In cycling cells, the retinoblastoma protein (pRb) is un- and/or hypo-phosphorylated in early G1 and becomes hyper-phosphorylated in late G1. The role of hypo-phosphorylation and identity of the relevant kinase(s) remains unknown. We show here that hypo-phosphorylated pRb associates with E2F in vivo and is therefore active. Increasing the intracellular concentration of the Cdk4/6 specific inhibitor p15(INK4b) by transforming growth factor beta treatment of keratinocytes results in G1 arrest and loss of hypo-phosphorylated pRb with an increase in unphosphorylated pRb. Conversely, p15(INK4b)-independent transforming growth factor beta-mediated G1 arrest of hepatocellular carcinoma cells results in loss of Cdk2 kinase activity with continued Cdk6 kinase activity and pRb remains only hypo-phosphorylated. Introduction of the Cdk4/6 inhibitor p16(INK4a) protein into cells by fusion to a protein transduction domain also prevents pRb hypo-phosphorylation with an increase in unphosphorylated pRb. We conclude that cyclin D:Cdk4/6 complexes hypo-phosphorylate pRb in early G1 allowing continued E2F binding.

Stage-specific expression and phosphorylation of retinoblastoma protein (pRb) in the rat seminiferous epithelium

To assess the potential role of retinoblastoma protein (pRb) in the regulation of cell cycle during spermatogenesis, the expression of retinoblastoma (Rb) mRNA and protein, as well as the phosphorylation states of pRb, in the rat seminiferous epithelial cycle, were studied. Two transcripts, 5.4 kb and 3.4 kb long, were detected in total RNA from the adult rat testis and only the 5.4 kb transcript was detected in poly (A)+-RNA from 8, 14 and 23-day old rat testes by Northern hybridization. Polysome analysis revealed that only a small portion of both Rb transcripts could be efficiently translated. By in situ hybridization, Rb mRNA was localized to germ cells from stage V pachytene spermatocytes to step 13 spermatids along the epithelial cycle. pRb immunoreactivity was detected in Sertoli cells and spermatogonia at all stages, as well as in the elongated steps 14-19 spermatids by immunohistochemistry. The amount of pRb and the phosphorylation status varied in a stage-specific manner in Western blots. These results show that pRb is expressed in the rat seminiferous epithelium in a cyclic fashion and suggest that it is involved in the regulation of proliferation of spermatogonia and maintenance of the differentiation status of Sertoli cells and spermatids.

Arotinoid mofarotene (RO40-8757) up-regulates p21 and p27 during growth inhibition of pancreatic cancer cell lines

Effective chemotherapy for pancreatic cancer is urgently needed. The anti-proliferative activity of a new retinoid, mofarotene (RO40-8757), was compared with that of other retinoids, such as all trans-retinoic acid, 13-cis retinoic acid and 9-cis retinoic acid, on 9 pancreatic cancer cell lines in relation to the effects on various cell cycle-regulating factors. After treatment with each retinoid, anti-proliferative effect was determined by the MTT method and expression of cell cycle-regulating factors, such as cyclins (D1, E and A), cyclin-dependent kinases (2 and 4), cyclin-dependent kinase inhibitors (p21 and p27) and retinoblastoma protein, was analyzed by Western blotting. Mofarotene showed half-maximal inhibition of cell proliferation at concentrations between 0.14 x 10(-6) and 3.8 x 10(-6) mol/l with little cytotoxicity. In contrast, the other retinoids did not inhibit the growth of all cell lines by over 50% compared to controls. A marked increase in the fraction of cells in G1 phase of the cell cycle was observed after mofarotene treatment; this was associated with marked up-regulation of p21/p27 and a shift of retinoblastoma protein into the hypophosphorylated form. In conclusion, mofarotene inhibits the growth of pancreatic cancer cells by inducing G1-phase cell cycle-inhibitory factors (p21, p27 and hypophosphorylated form of Rb protein) and is considered to be a useful agent for pancreatic cancer treatment.

Apoptosis induced in mammalian cells by small peptides that functionally antagonize the Rb-regulated E2F transcription factor

A variety of studies implicate the E2F transcription factor as a critical regulator of the mammalian cell cycle. The E2F pathway is aberrant in most, if not all, human tumor cells; therefore, therapeutic regimes that modulate E2F activity may provide an approach for reinstating growth control in situations where normal physiological control is lost. To elucidate the role of E2F in the cell cycle and assess its value as a therapeutic target, we have introduced peptides that functionally antagonize E2F DNA binding activity into mammalian cells. Introduction of these peptides into mammalian tumor cells caused the rapid onset of apoptosis, an outcome that correlates with the inactivation of physiological E2F.

In human salivary gland cells, overexpression of E2F1 overcomes an interferon-gamma- and tumor necrosis factor-alpha-induced growth arrest but does not result in complete mitosis

Increased levels of cytokines in the salivary glands have been associated with the loss of secretory cells and reduced salivary function. It has been demonstrated that interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) treatment of a human submandibular gland (HSG) cell line causes growth arrest in the G0/G1 phase of the cell cycle, followed by apoptosis. To stimulate DNA synthesis and reverse this growth arrest, we used an adenovirus vector to overexpress the transcription factor E2F1 in HSG cells. Initially, cells were synchronized by a double thymidine block and then infected with recombinant adenovirus (AdE2F1) expressing E2F1. Cells were harvested at intervals and analyzed by flow cytometry. Greater than 50% of synchronized cells infected with AdE2F1 were in S phase by 18 hours postinfection (hpi) compared to 12% of uninfected cells. Similarly, AdE2F1 infection of HSG cells arrested by IFN-gamma and TNF-alpha treatment caused a fivefold increase in S-phase cells by 48 hpi. However, by 72 hpi, AdE2F1-infected cells showed increases in the subdiploid cell population. Forty-one percent of AdE2F1-infected cells labeled positive by TUNEL, compared to fewer than 6% for controls. Additionally, AdE2F1-infected cells (84 hpi) had low forward-angle and high side scatter light characteristics, similar to apoptotic lymphocytes. These results suggest that E2F1 accumulation in growth-arrested salivary gland cells can stimulate DNA synthesis and overcome a G0/G1 block in the cell cycle. However, E2F1 overexpression did not lead to complete mitosis in HSG cells but, rather, diverted cells into an apoptotic pathway.

Ionizing radiation and teniposide increase p21(waf1/cip1) and promote Rb dephosphorylation but fail to suppress E2F activity in MCF-7 breast tumor cells

Ionizing radiation and the topoisomerase II inhibitor, teniposide (VM-26) both increase levels of the cyclin dependent kinase inhibitor, p21(waf1/cip1) and promote dephosphorylation of the retinoblastoma tumor suppressor protein, Rb, in MCF-7 breast tumor cells, perturbations associated with suppression of the activity of the transcription factor, E2F. However, studies using an E2F binding site-luciferase reporter plasmid transfected into MCF-7 cells failed to demonstrate a reduction in E2F activity in response to VM-26 or to ionizing radiation. In contrast, E2F activity (both basal and E1A stimulated) could be suppressed by transfection with a plasmid expressing Rb, indicating that the capacity of E2F to bind to Rb and to be inactivated by Rb is functionally intact in MCF-7 cells. These findings in MCF-7 breast tumor cells suggest that E2F activity may not be directly susceptible to modulation by endogenous p21(waf1/cip1) and Rb.