The retinoblastoma protein copurifies with E2F-I, an E1A-regulated inhibitor of the transcription factor E2F

The multifunctional role of E1A in the transcriptional regulation of CREB/CBP-dependent target genes

Oncoproteins encoded by the early region 1A (E1A) of adenoviruses (Ads) have been shown to be powerful tools to study gene regulatory mechanisms. As E1A proteins lack a sequence-specific DNA-binding activity, they modulate viral and cellular gene expression by interacting directly with a diverse array of cellular factors, among them sequence-specific transcription factors, proteins of the general transcription machinery, co-activators and chromatin-modifying enzymes. By making use of these factors, E1A affects major cellular events such as cell cycle control, differentiation, apoptosis, and oncogenic transformation. In this review we will focus on the interaction of E1A with cellular components involved in the cAMP/PKA signal transduction pathway and we will discuss the consequences of these interactions in respect to the activation of CREB/CBP-dependent target genes.

Structural basis for the recognition of the E2F transactivation domain by the retinoblastoma tumor suppressor

Repression of E2F transcription activity by the retinoblastoma (Rb) tumor suppressor through its interaction with the transactivation domain of the E2F transcription factor is one of the central features of G1/S arrest in the mammalian cell cycle. Deregulation of the Rb-E2F interaction results in hyperproliferation, lack of differentiation, and apoptosis, and can lead to cancer. The 2.2-A crystal structure of the Rb pocket complexed with an 18-residue transactivation-domain peptide of E2F-2 reveals that the boomerang-shaped peptide binds to the highly conserved interface between the A-box and the B-box of the Rb pocket in a bipartite manner. The N-terminal segment of the E2F-2 peptide in an extended beta-strand-like structure interacts with helices from the conserved groove at the A-B interface, whereas the C-terminal segment, which contains one 3(10) helix, binds to a groove mainly formed by A-box helices. The flexibility in the middle of the E2F-2 peptide is essential for the tight association of E2F to the Rb pocket. The binding of Rb to the E2F-2 peptide conceals several conserved residues that are crucial for transcription activation of E2F. We provide the structural basis for the Rb-mediated repression of E2F transcription activity without the requirement of histone-modifying enzymes.

Cancer genetics

Cancer is a genetic disease of somatic cells. Tumor karyotypes are rarely normal, and most show multiple abnormalities of both number and structure. The first direct evidence for this concept of cancer came from studies of tumor-specific translocations in leukemias and lymphomas, revealing the importance of oncogenes and the regulation of gene transcription in cancer. A second major source of information about human cancer genes is hereditary cancer. Genetic predisposition of the autosomal dominant type imposes a high relative risk for one or more kinds of cancer. In the past decade or so, more than 30 mutant genes for such hereditary cancers have been cloned. Penetrance depends upon additional, somatic, mutations. A few of the genes are oncogenes or DNA repair genes, but most are tumor suppressor genes. Some tumor suppressors regulate transcription, while others operate in signal transduction pathways that are involved in regulating processes of cell birth, differentiation, and death. The knowledge gained is stimulating new approaches to the treatment and prevention of cancer.

The retinoblastoma family of proteins directly represses transcription in Saccharomyces cerevisiae

The retinoblastoma family of proteins are key cell cycle regulatory molecules important for the differentiation of various mammalian cell types. The retinoblastoma protein regulates transcription of a variety of genes either by blocking the activation domain of various activators or by active repression via recruitment to appropriate promoters. We show here that the retinoblastoma family of proteins functions as direct transcriptional repressors in a heterologous yeast system when fused to the DNA binding domain of Gal4. Mapping experiments indicate that either the A or the B domain of the pocket region is sufficient for repression in vivo. As is the case in mammalian cells, a phosphorylation site mutant of the retinoblastoma protein is a stronger transcriptional repressor than the wild type protein. We show that transcriptional repression by pRb is dependent on CLN3 in vivo. Furthermore, the yeast histone deacetylase components, RPD3 and SIN3, are required for transcriptional repression.

Forskolin-mediated G1 arrest in acute lymphoblastic leukaemia cells: phosphorylated pRB sequesters E2Fs

Increased intracellular levels of cAMP, induced by forskolin, lead to permanent G1 arrest of Reh cells. As expected, we observed a rapid dephosphorylation of the retinoblastoma protein (pRB) within 2 hours of forskolin treatment concomitant with reduced activity of the pRB-specific kinases. Interestingly, however, the dephosphorylation of pRB, as well as the inhibition of the kinase activities, was only transient, despite the permanent arrest of cells in G1. Importantly, although the pRB-specific kinases were fully active after 48 hours, pRB became only partially rephosphorylated.

The transient dephosphorylation of pRB could be explained by the transient decrease in the activities of the pRB-specific kinases, but to understand why pRB became only partially rephosphorylated, despite fully activated kinases,we postulated that cAMP could activate a pRB-directed phosphatase. It was therefore interesting to find that the phosphatase inhibitor, tautomycin, was able to abolish the forskolin-mediated dephosphorylation of pRB, without increasing the activities of the pRB-specific kinases.

To understand how Reh cells expressing hyperphosphorylated forms of pRB can remain arrested in G1, we used three different methods to test for the ability of pRB to form functional complexes with the family of E2F transcription factors. As expected, we observed an increased complex formation between E2F-1, E2F-4 and pRB after 2 hours when pRB was in its most dephosphorylated state. Suprisingly, however, prolonged treatment with forskolin, which induced partial rephosphorylation of pRB, in fact further increased the complex formation between the E2Fs and pRB, and this also resulted in reduced E2F-promoter activity in vivo. These data imply that in Reh cells, partially phosphorylated forms of pRB retain the ability to inhibit E2F-promoter activity, and thereby prevent cells from entering into S-phase.

Molecular aspects of bladder cancer III. Prognostic markers of bladder cancer

The current pathological and clinical parameters provide important prognostic information, yet still have limited ability to predict the true malignant potential of most bladder tumors. In the last years, investigation of the basic mechanisms involved in carcinogenesis and tumor progression by molecular biology has provided a host of markers which are of potential diagnostic or prognostic value for bladder carcinoma. These markers may serve as tools for early and accurate prediction of tumor recurrence, progression and development of metastases and for prediction of response to therapy. The precise prediction of tumor biological behavior would facilitate treatment selection for patients who may benefit from radical surgical treatment or adjuvant therapy. We provide a current, comprehensive review of the literature on bladder tumor markers with a special emphasis on their prognostic potential. The literature suggests that currently no single marker is able to accurately predict the clinical course of bladder tumors and thus would serve as a reliable prognosticator. A combination of prognostic markers could predict which superficial tumors need an aggressive form of therapy and which invasive tumors require adjuvant therapy. Altogether, the most promising markers are, at this point, Ki-67 and p53 expression as well as matrixmetalloproteinase complex and angiogenesis.

Okadaic acid suppresses TPA-induced differentiation by stimulating G1/S transition in human myeloblastic leukaemia ML-1 cells

The association between the phosphorylation status of the retinoblastoma protein, pRb and changes in cell cycle control caused by either protein kinase C (PKC) or protein kinase A (PKA) stimulation was evaluated in human myeloblastic leukaemia ML-1 cells. TPA-induced PKC activation resulted in dephosphorylation of pRb and subsequently induced ML-1 differentiation based on morphological changes and CD14 expression. In the present study, we showed that inhibition of protein phosphatases (PP-1 and PP-2a) prevented the TPA-induced differentiation in ML-1 cells. Preinhibition of PP-1 and PP-2a activities with 1-100 nM okadaic acid dose-dependently blunted the decrease in the phosphorylation status of pRb obtained with TPA and overrode cell cycle arrest. PKA stimulation with 8-chlorophenylthio-cAMP (100 microM) decreased cell proliferation by 65% and the distribution of cells in the G1 phase significantly increased from 38% to 83% concomitant with a 34% decline in the number of cells present in the S phase. In addition, PKA stimulation significantly decreased the pRb phosphorylation status but did not elicit CD14 expression, indicating that cAMP-induced dephosphorylation of pRb cannot by itself trigger differentiation in ML-1 cells.

High intensity ERK signal mediates hepatocyte growth factor-induced proliferation inhibition of the human hepatocellular carcinoma cell line HepG2

Hepatocyte growth factor (HGF) induces growth stimulation of a variety of cell types, but it also induces growth inhibition of several types of tumor cell lines. The molecular mechanism of the HGF-induced growth inhibition of tumor cells remains obscure. We have investigated the intracellular signaling pathway involved in the antiproliferative effect of HGF on the human hepatocellular carcinoma cell line HepG2. HGF induced strong activation of ERK in HepG2 cells. Although the serum-dependent proliferation of HepG2 cells was inhibited by the MEK inhibitor PD98059 in a dose-dependent manner, 10 microM PD98059 reduced the HGF-induced strong activation of ERK to a weak activation; and as a result, the proliferation inhibited by HGF was completely restored. Above or below this specific concentration, the restoration was incomplete. Expression of constitutively activated Ha-Ras, which induces strong activation of ERK, led to the proliferation inhibition of HepG2 cells, as was observed in HGF-treated HepG2 cells. This inhibition was suppressed by the MEK inhibitor. Furthermore, HGF treatment and expression of constitutively activated Ha-Ras changed the hyperphosphorylated form of the retinoblastoma tumor suppressor gene product pRb to the hypophosphorylated form. This change was inhibited by the same concentration of MEK inhibitor needed to suppress the proliferation inhibition. These results suggest that ERK activity is required for both the stimulation and inhibition of proliferation of HepG2 cells; that the level of ERK activity determines the opposing proliferation responses; and that HGF-induced proliferation inhibition is caused by cell cycle arrest, which results from pRb being maintained in its active hypophosphorylated form via a high-intensity ERK signal in HepG2 cells.

The Rb pathway in neurogenesis

Cell division during embryogenesis plays a crucial role in the formation of the nervous system. During this developmental process, proliferating neural precursor cells commit to a neuronal fate and, as a consequence, undergo terminal mitosis and adopt a neuronal phenotype. A key cell cycle regulator, the tumor suppressor protein, retinoblastoma (Rb), is involved in both terminal mitosis and neuronal differentiation. Neural development is a complex process involving cell proliferation, cell fate determination and differentiation, as well as programmed cell death. In this review, we will examine each of these processes in turn, focussing on the role of the Rb family proteins to examine their many influences on these events.

Sequential two-step cleavage of the retinoblastoma protein by caspase-3/-7 during etoposide-induced apoptosis

During cellular apoptosis, retinoblastoma protein (RB) is subjected to cleavage near the carboxyl terminus by a caspase-3-like protease. In addition, an heretofore unidentified protease cleaves RB internally, generating fragments of 68 and 48 kDa. Internal cleavage abrogates the ability of RB to associate with E2F. To investigate the mechanism of RB internal cleavage, we developed and employed an in vitro cleavage assay. Incubation of in vitro translated (35)S-RB with apoptotic cell extracts led to RB cleavage at the C-terminus, followed by internal cleavage. The caspase peptide inhibitors z-VAD-FMK or z-DEVD-FMK blocked both cleavage events. Rapid C-terminal and internal cleavage were also observed when recombinant caspase-3 was added to (35)S-RB. Moreover, when caspase-3 was added to nonapoptotic cell extract, efficient internal cleavage of cellular RB was observed. Caspase-mediated internal cleavage occurred following RB residue aspartate(349) in the sequence DSID(349). This sequence is consistent with a DXXD recognition motif for caspase-3-like enzymes. Interestingly, we also observed RB internal cleavage in caspase-3-deficient MCF-7 cells, indicating that other caspases are capable of cleaving RB internally. Indeed, caspase-7, a member of the caspase-3 subfamily, was found to cleave (35)S-RB at both the carboxyl terminus, and following aspartate(349). By contrast, caspases that are not members of the caspase-3 subfamily failed to cleave RB. Taken together, our findings demonstrate that during apoptosis, a caspase-3-like protease is responsible for degradation and functional inactivation of RB by cleaving the protein internally following aspartate(349).

Chasing the cancer demon

Boveri's idea that somatic mutations are at the root of cancer found its first specific support with the investigation of leukemia and Burkitt's lymphoma, and the discovery of the mechanism of oncogene activation by balanced translocation. The study of retinoblastoma later led to the cloning of the first antioncogene, or tumor suppressor gene, and to understanding the mechanisms by which the wild-type genes lose activity. Only a small subset of cancer involves simple mechanisms. A category of hereditary disorders called the phakomatoses provide a perspective on the chain of oncogenic events in such cancers because of two-hit precursor lesions that have a low probability of malignant transformation. The common carcinomas are much more complex and are typically genetically unstable, owing either to mutational instability or chromosomal instability.

Interaction of the retinoblastoma protein (pRb) with the catalytic subunit of DNA polymerase delta (p125)

The retinoblastoma gene product (pRb) interacts with many cellular proteins to function in the control of cell division, differentiation, and apoptosis. Several pRb binding proteins complex with pRb through an amino acid sequence called the LXCXE motif. The catalytic subunit of DNA polymerase delta (p125) contains a LXCXE motif. To further study the biochemical function of this polymerase, we sought to determine if p125 interacts with pRb. Experiments using GST-pRb fusion proteins showed that p125 from breast epithelial (MCF10A) cell extracts associates with pRb. In addition, GST-p125 fusion proteins bound pRb from the same cell extracts. The pRb that associated with GST-p125 was largely unphosphorylated. Coimmunoprecipitation experiments using cell cycle synchronized cells revealed that p125 and pRb form a complex predominantly during G1 phase, the phase during which pRb is mostly unphosphorylated. In vitro phosphorylation of GST-pRb by the cyclin dependent kinases reduced the ability of p125 to associate with GST-pRh. Addition of the LXCXE containing protein SV40 large T antigen to GST-pRb blocks the ability of p125 to associate with pRb, suggesting that it may be through a LXCXE sequence by which p125 interacts with pRb. Finally, in vitro polymerase assays demonstrate that GST-pRb fusion protein stimulates DNA polymerase delta activity.

Target gene specificity of E2F and pocket protein family members in living cells

E2F-mediated transcription is thought to involve binding of an E2F-pocket protein complex to promoters in the G(0) phase of the cell cycle and release of the pocket protein in late G(1), followed by release of E2F in S phase. We have tested this model by monitoring protein-DNA interactions in living cells using a formaldehyde cross-linking and immunoprecipitation assay. We find that E2F target genes are bound by distinct E2F-pocket protein complexes which change as cells progress through the cell cycle. We also find that certain E2F target gene promoters are bound by pocket proteins when such promoters are transcriptionally active. Our data indicate that the current model applies only to certain E2F target genes and suggest that Rb family members may regulate transcription in both G(0) and S phases. Finally, we find that a given promoter can be bound by one of several different E2F-pocket protein complexes at a given time in the cell cycle, suggesting that cell cycle-regulated transcription is a stochastic, not a predetermined, process.

E2F family members are differentially regulated by reversible acetylation

The six members of the E2F family of transcription factors play a key role in the control of cell cycle progression by regulating the expression of genes involved in DNA replication and cell proliferation. E2F-1, -2, and -3 belong to a structural and functional subfamily distinct from those of the other E2F family members. Here we report that E2F-1, -2, and -3, but not E2F-4, -5, and -6, associate with and are acetylated by p300 and cAMP-response element-binding protein acetyltransferases. Acetylation occurs at three conserved lysine residues located at the N-terminal boundary of their DNA binding domains. Acetylation of E2F-1 in vitro and in vivo markedly increases its binding affinity for a consensus E2F DNA-binding site, which is paralleled by enhanced transactivation of an E2F-responsive promoter. Acetylation of E2F-1 can be reversed by histone deacetylase-1, indicating that reversible acetylation is a mechanism for regulation also of non-histone proteins.

Regulation of E2F1 activity by acetylation

During the G(1) phase of the cell cycle, an E2F-RB complex represses transcription, via the recruitment of histone deacetylase activity. Phosphorylation of RB at the G(1)/S boundary generates a pool of 'free' E2F, which then stimulates transcription of S-phase genes. Given that E2F1 activity is stimulated by p300/CBP acetylase and repressed by an RB-associated deacetylase, we asked if E2F1 was subject to modification by acetylation. We show that the p300/CBP-associated factor P/CAF, and to a lesser extent p300/CBP itself, can acetylate E2F1 in vitro and that intracellular E2F1 is acetylated. The acetylation sites lie adjacent to the E2F1 DNA-binding domain and involve lysine residues highly conserved in E2F1, 2 and 3. Acetylation by P/CAF has three functional consequences on E2F1 activity: increased DNA-binding ability, activation potential and protein half-life. These results suggest that acetylation stimulates the functions of the non-RB bound 'free' form of E2F1. Consistent with this, we find that the RB-associated histone deacetylase can deacetylate E2F1. These results identify acetylation as a novel regulatory modification that stimulates E2F1's activation functions.

The E2F transcription factors: key regulators of cell proliferation

Ever since its discovery, the RB-1 gene and the corresponding protein, pRB, have been a focal point of cancer research. The isolation of E2F transcription factors provided the key to our current understanding of RB-1 function in the regulation of the cell cycle and in tumor suppression. It is becoming more and more evident that the regulatory circuits governing the cell cycle are very complex and highly interlinked. Certain aspects of RB-1 function, for instance its role in differentiation, cannot be easily explained by the current models of pRB-E2F interaction. One reason is that pRB has targets different from E2F, molecules like MyoD for instance. Another reason may be that we have not completely understood the full complexity of E2F function, itself. In this review, we will try to illuminate the role of E2F in pRB- and p53-mediated tumor suppression pathways with particular emphasis on the aspect of E2F-mediated transcriptional regulation. We conclude that E2F can mediate transcriptional activation as well as transcriptional repression of E2F target genes. The net effect of E2F on the transcriptional activity of a particular gene may be the result of as yet poorly understood protein-protein interactions of E2F with other components of the transcriptional machinery, as well as it may reflect the readout of the different ways of regulating E2F activity, itself. We will discuss the relevance of a thorough understanding of E2F function for cancer therapy.

Transcriptional regulation of the human DNA Methyltransferase (dnmt1) gene

DNA methylation is an important component of the epigenetic control of genome functions. Understanding the regulation of the DNA Methyltransferase (dnmt1) gene expression is critical for comprehending how DNA methylation is coordinated with other critical biological processes. In this paper, we investigate the transcriptional regulatory region of the human dnmt1 gene using a combination of RACE, RNase protection analysis and CAT assays. We identified one major and three minor transcription initiation sites in vivo (P1-P4), which are regulated by independent enhancers and promoter sequences. The minimal promoter elements of P1, P2 and P4 are mapped within 256 bp upstream of their respective transcription initiation sites. P1 is nested within a CG-rich area, similar to other housekeeping genes, whereas P2-P4 are found in CG-poor areas. Three c-Jun-dependent enhancers are located downstream to P1 and upstream to P2-P4, thus providing a molecular explanation for the responsiveness of dnmt1 to oncogenic signals that are mediated by the Ras-c-Jun oncogenic signaling pathway.

Apoptosis induced by the nuclear death domain protein p84N5 is inhibited by association with Rb protein

Rb protein inhibits both cell cycle progression and apoptosis. Interaction of specific cellular proteins, including E2F1, with Rb C-terminal domains mediates cell cycle regulation. In contrast, the nuclear N5 protein associates with an Rb N-terminal domain with unknown function. The N5 protein contains a region of sequence similarity to the death domain of proteins involved in apoptotic signaling. We demonstrate here that forced N5 expression potently induces apoptosis in several tumor cell lines. Mutation of conserved residues within the death domain homology compromise N5-induced apoptosis, suggesting that it is required for normal function. Endogenous N5 protein is specifically altered in apoptotic cells treated with ionizing radiation. Furthermore, dominant interfering death domain mutants compromise cellular responses to ionizing radiation. Finally, physical association with Rb protein inhibits N5-induced apoptosis. We propose that N5 protein plays a role in the regulation of apoptosis and that Rb directly coordinates cell proliferation and apoptosis by binding specific proteins involved in each process through distinct protein binding domains.

Adenovirus-mediated transfer of wild-type p53 gene sensitizes TNF resistant MCF7 derivatives to the cytotoxic effect of this cytokine: relationship with c-myc and Rb

Tumor suppressor p53 is a nuclear transcription factor that blocks cell cycle progression and induces apoptosis. We have previously shown that the MCF7 resistance to the cytotoxic action of TNF correlates with p53 mutations. In the present study, we used a recombinant adenovirus carrying a wild-type p53 gene (Adwtp53) in order to investigate the effect of wt p53 transfer on modulation of cell resistance to the cytotoxic action of TNF. Our data indicate that infection of TNF resistant MCF7 cells (1001 and MCF7/Adr) with Adwtp53 resulted in the restoration of wt p53 expression and function as respectively revealed by the yeast assay and the induction of p53 inducible genes MDM2 and p21. Furthermore, the restoration of p53 function significantly sensitized TNF resistant cells to TNF cytotoxic action. This correlated with a significant down-regulation of c-myc in both TNF-resistant cell lines and a decrease of Retinoblastoma protein (Rb) in 1001 clone. In contrast, the effect of p53 seems to be independent from Bcl-2 and Bax protein level regulation. The present study suggests that the combination of TNF and Adwtp53 may be a potential strategy to sensitize mutant p53 TNF-resistant tumors to the cytotoxic action of this cytokine.

Regulation of E2F: a family of transcription factors involved in proliferation control

Members of the E2F family of transcription factors are key participants in orchestration of the cell cycle, cell growth arrest and apoptosis. Therefore, an understanding of the regulation of E2F activity is essential for an understanding of the control of cellular proliferation. E2F activity is regulated by the retinoblastoma family of tumor suppressors and by multiple other mechanisms. This review will describe our current knowledge of these mechanisms which together constitute a highly complex network by which the cell cycle and cellular proliferation can be controlled.

Functional interaction between human topoisomerase IIalpha and retinoblastoma protein

DNA topoisomerase II-an essential nuclear enzyme in DNA replication and transcription, chromatin segregation, and cell cycle progression-is also a target of clinically useful anticancer drugs. Preliminary observations of a positive correlation between the expression of topoisomerase (topo) IIalpha and the retinoblastoma protein (Rb) in a series of rhabdomyosarcoma cells prompted us to ask whether these two proteins interact in vivo. Using human rhabdomyosarcoma and leukemic cell lines, we found a physical association between topo IIalpha and Rb protein by reciprocal immunoprecipitation and immunoblotting, in which topo IIalpha appeared to interact primarily with the underphosphorylated form of Rb. Experiments with truncated glutathione S-transferase-Rb fusion proteins and nuclear extracts of Rh1 rhabdomyosarcoma cells indicated that topo IIalpha binds avidly to the A/B pocket domain of Rb, which contains the intact spacer amino acid sequence. To determine whether this interaction has functional consequences in vivo, we expressed wild-type and mutant Rb in human cervical carcinoma cells lacking functional Rb. Wild-type, but not mutant, Rb inhibited topo II activity in nuclear extracts of these transfected cells. Moreover, purified wild-type Rb inhibited the activity of purified human topo IIalpha, indicating a direct interaction between these two proteins. We conclude that topo IIalpha associates physically with Rb in interactions that appear to have functional significance.

Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

Cyclin D3 plays a major role in the development of polyploidy in megakaryocytes. The expression of cyclin D3 gene and the level of cyclin D3 protein are increased by the Mpl ligand in the Y10/L8057 megakaryocytic cell line, as indicated by Northern and Western blot analyses, and by nuclear run-on assays and transfection experiments with cyclin D3 promoter constructs. DNase I footprinting of the promoter region showed protected segments, at −75 to −60 bp and at −134 to −92 bp, which display binding sites for the Sp family of transcription factors. Gel mobility shift assay and supershifts with specific antibodies indicate that Sp1 binds to these regions in the cyclin D3 promoter and that Sp1 binding activity is significantly increased by Mpl ligand. Mutation of either Sp1 site both decreases the basal promoter activity and eliminates the induction by Mpl ligand. We find that the nonphosphorylated form of SP1 has greater affinity for the cyclin D3 promoter and that the majority of Sp1 in the cells is nonphosphorylated. Mpl ligand treatment results in increased levels of Sp1 protein, which also appears as nonphosphorylated. Okadaic acid, which inhibits protein phosphatase 1 (PP1) and shifts Sp1 to a phosphorylated form, decreases cyclin D3 gene expression and suppresses Mpl ligand induction. Our data point to the potential of Mpl ligand to activate at once several Sp1-dependent genes during megakaryopoiesis.

Mpl Ligand Enhances the Transcription of the Cyclin D3 Gene: A Potential Role for Sp1 Transcription Factor

Cyclin D3 plays a major role in the development of polyploidy in megakaryocytes. The expression of cyclin D3 gene and the level of cyclin D3 protein are increased by the Mpl ligand in the Y10/L8057 megakaryocytic cell line, as indicated by Northern and Western blot analyses, and by nuclear run-on assays and transfection experiments with cyclin D3 promoter constructs. DNase I footprinting of the promoter region showed protected segments, at −75 to −60 bp and at −134 to −92 bp, which display binding sites for the Sp family of transcription factors. Gel mobility shift assay and supershifts with specific antibodies indicate that Sp1 binds to these regions in the cyclin D3 promoter and that Sp1 binding activity is significantly increased by Mpl ligand. Mutation of either Sp1 site both decreases the basal promoter activity and eliminates the induction by Mpl ligand. We find that the nonphosphorylated form of SP1 has greater affinity for the cyclin D3 promoter and that the majority of Sp1 in the cells is nonphosphorylated. Mpl ligand treatment results in increased levels of Sp1 protein, which also appears as nonphosphorylated. Okadaic acid, which inhibits protein phosphatase 1 (PP1) and shifts Sp1 to a phosphorylated form, decreases cyclin D3 gene expression and suppresses Mpl ligand induction. Our data point to the potential of Mpl ligand to activate at once several Sp1-dependent genes during megakaryopoiesis.

Prohibitin, a potential tumor suppressor, interacts with RB and regulates E2F function

The retinoblastoma tumor suppressor protein and its family members, p107 and p130, are major regulators of the mammalian cell cycle. They exert their growth suppressive effects at least in part by binding the E2F family of transcription factors and inhibiting their transcriptional activity. Agents that disrupt the interaction between Rb family proteins and E2F promote cell proliferation. Here we describe the characterization of a novel interaction between Rb family proteins and a potential tumor suppressor protein, prohibitin. Prohibitin physically interacts with all three Rb family proteins in vitro and in vivo, and was very effective in repressing E2F-mediated transcription. Prohibitin could inhibit the activity of E2Fs 1, 2, 3, 4 and 5, but could not affect the activity of promoters lacking an E2F site. Surprisingly, prohibitin-mediated repression of E2F could not be reversed by adenovirus E1A protein. A prohibitin mutant that could not bind to Rb was impaired in its ability to repress E2F activity and inhibit cell proliferation. We believe that prohibitin is a novel regulator of E2F activity that responds to specific signaling cascades.

Cumulative effect of phosphorylation of pRB on regulation of E2F activity

The product of the retinoblastoma susceptibility gene, pRB, is a nuclear phosphoprotein that controls cell growth by binding to and suppressing the activities of transcription factors such as the E2F family. Transactivation activity is inhibited when E2F is bound to hypophosphorylated pRB and released when pRB is phosphorylated by cyclin-dependent kinases (CDKs). To determine which of 16 potential CDK phosphorylation sites regulated the pRB-E2F interaction, mutant pRB proteins produced by site-directed mutagenesis were tested for the ability to suppress E2F-mediated transcription in a reporter chloramphenicol acetyltransferase assay. Surprisingly, no one CDK site regulated the interaction of pRB with E2F when E2F was bound to DNA. Instead, disruption of transcriptional repression resulted from accumulation of phosphate groups on the RB molecule.

HLA Class I-Mediated Induction of Cell Proliferation Involves Cyclin E-Mediated Inactivation of Rb Function and Induction of E2F Activity

Chronic rejection of transplanted organs is manifested as atherosclerosis of the blood vessels of the allograft. HLA class I Ags have been implicated to play a major role in this process, since signaling via HLA class I molecules can induce the proliferation of aortic endothelial as well as smooth muscle cells. In this study, we show that HLA class I-mediated induction of cell proliferation correlates with inactivation of the Rb protein in the T cell line Jurkat as well as human aortic endothelial cells. HLA class I-mediated inactivation of Rb can be inhibited specifically by neutralizing Abs to basic fibroblast growth factor (bFGF), suggesting a role for FGF receptors in the signaling process. Signaling through HLA class I molecules induced cyclin E-associated kinase activity within 4 h in quiescent endothelial cells, and appeared to mediate the inactivation of Rb. A cdk2 inhibitor, Olomoucine, as well as a dominant-negative cdk2 construct prevented HLA class I-mediated inactivation of Rb; in contrast, dominant-negative cdk4 and cdk6 constructs had no effect. Furthermore, there was no increase in cyclin D-associated kinase activity upon HLA class I ligation, suggesting that cyclin E-dependent kinase activity mediates Rb inactivation, leading to E2F activation and cell proliferation.

Discovery of a regulatory motif that controls the exposure of specific upstream cyclin-dependent kinase sites that determine both conformation and growth suppressing activity of pRb

The conformation and activity of pRb, the product of the retinoblastoma susceptibility gene, is dependent on the phosphorylation status of one or more of its 16 potential cyclin-dependent kinase (cdk) sites. However, it is not clear whether the phosphorylation status of one or more of these sites contributes to the determination of the various conformations and activity of pRb. Moreover, whether and how the conformation of pRb may regulate the phosphorylation of the cdk sites is also unclear. In the process of analyzing the function and regulation of pRb, we uncovered the existence of an unusual structural motif, m89 (amino acids 880-900), the mutation of which confers upon pRb a hypophosphorylated conformation. Mutation of this structural domain activates, rather than inactivates, the growth suppressor function of pRb. In order to understand the effect of the mutation of m89 on the phosphorylation of cdk sites, we identified all the cdk sites (Thr-356, Ser-807/Ser-811, and Thr821) the phosphorylation of which drastically modify the conformation of pRb. Mutation of each of these four sites alone or in combinations results in the different conformations of pRb, the migration pattern of which, on SDS-polyacrylamide gel electrophoresis, resembles various in vivo hypophosphorylated forms. Each of these hypophosphorylated forms of pRb has enhanced growth suppressing activity relative to the wild type. Our data revealed that the m89 structural motif controls the exposure of the cdk sites Ser-807/Ser-811 in vitro and in vivo. Moreover, the m89 mutant has enhanced growth suppressing activity, similar to a mutant with alanine substitutions at Ser-807/Ser-811. Our recent finding, that the m89 region is part of a structural domain, p5, conserved antigenically and functionally between pRb and p53, suggests that the evolutionarily conserved p5 domain may play a role in the coordinated regulation of the activity of these two tumor suppressors, under certain growth conditions.

Phenotypic differentiation without permanent cell-cycle arrest by skeletal myocytes with deregulated E2F-1

Skeletal muscle terminal differentiation includes expression of muscle cell-specific proteins and concomitant cell-cycle arrest. These two processes require functional retinoblastoma protein (RB). E2F-1 is an RB-associated transcriptional factor and an effector of RB in the regulation of G1 to S-phase transition. Here, we show that proper regulation of E2F-1 is crucial for differentiation-coupled cell-cycle arrest by skeletal myocytes. On induction to differentiate, C2 myoblasts constitutively expressing E2F-1 synthesized muscle cell-specific proteins, fused into myotubes, and upregulated the cdk inhibitor p21. However, unlike control cells, differentiated myocytes expressing exogenous E2F-1 incorporated bromodeoxyuridine into nuclei, indicating S-phase entry. This S-phase entry was accompanied by expression of cyclin A. Our results support the view that RB regulates cell-cycle arrest and muscle cell differentiation through separable mechanisms.

Role of p16/MTS1, cyclin D1 and RB in primary oral cancer and oral cancer cell lines

One of the most important components of G1 checkpoint is the retinoblastoma protein (pRB110). The activity of pRB is regulated by its phosphorylation, which is mediated by genes such as cyclin D1 and p16/MTS1. All three genes have been shown to be commonly altered in human malignancies. We have screened a panel of 26 oral squamous cell carcinomas (OSCC), nine premalignant and three normal oral tissue samples as well as eight established OSCC cell lines for mutations in the p16/MTS1 gene. The expression of p16/MTS1, cyclin D1 and pRB110 was also studied in the same panel. We have found p16/MTS1 gene alterations in 5/26 (19%) primary tumours and 6/8 (75%) cell lines. Two primary tumours and five OSCC cell lines had p16/MTS1 point mutations and another three primary and one OSCC cell line contained partial gene deletions. Six of seven p16/MTS1 point mutations resulted in termination codons and the remaining mutation caused a frameshift. Western blot analysis showed absence of p16/MTS1 expression in 18/26 (69%) OSCC, 7/9 (78%) premalignant lesions and 8/8 cell lines. One cell line, H314, contained a frameshift mutation possibly resulting in a truncated p16/MTS1 protein. pRB was detected in 14/25 (56%) of OSCC but only 11/14 (78%) of these contained all or some hypophosphorylated (active) pRB. In premalignant samples, 6/8 (75%) displayed pRB, and all three normal samples and eight cell lines analysed contained RB protein. p16/MTS1 protein was undetectable in 10/11 (91%) OSCCs with positive pRB. Overexpression of cyclin D1 was observed in 9/22 (41%) OSCC, 3/9 (33%) premalignant and 8/8 (100%) of OSCC cell lines. Our data suggest p16/MTS1 mutations and loss of expression to be very common in oral cancer cell lines and less frequent in primary OSCC tumours. A different pattern of p16/MTS1 mutations was observed in OSCC compared to other cancers with all the detected p16/MTS1 mutations resulting in premature termination codons or a frameshift. The RB protein was expressed in about half (44%) of OSCCs and its expression inversely correlated with p16/MTS1 expression. In conclusion, we show that abnormalities of the RB pathway are a common mechanism of oral carcinogenesis.

Structural basis of DNA recognition by the heterodimeric cell cycle transcription factor E2F-DP

The E2F and DP protein families form heterodimeric transcription factors that play a central role in the expression of cell cycle-regulated genes. The crystal structure of an E2F4-DP2-DNA complex shows that the DNA-binding domains of the E2F and DP proteins both have a fold related to the winged-helix DNA-binding motif. Recognition of the central c/gGCGCg/c sequence of the consensus DNA-binding site is symmetric, and amino acids that contact these bases are conserved among all known E2F and DP proteins. The asymmetry in the extended binding site TTTc/gGCGCc/g is associated with an amino-terminal extension of E2F4, in which an arginine binds in the minor groove near the TTT stretch. This arginine is invariant among E2Fs but not present in DPs. E2F4 and DP2 interact through an extensive protein-protein interface, and structural features of this interface suggest it contributes to the preference for heterodimers over homodimers in DNA binding.

Recognition of an epitope of a breast cancer antigen by human antibody

A novel tumor-associated peptide epitope KASIFLK expressed preferentially by breast cancer cells was identified using an IgG antibody from a breast cancer patient. A cDNA library from a MCF-7 breast cancer cell line was screened to isolate three cDNA clones that were immunoreactive with this antibody. KASIFLK was located in clones 27 and 40, both of which were identical to the cDNA and protein sequence of retinoblastoma binding protein 1 (RBP1, 250-256). An affinity-purified IgG antibody against the peptide epitope was completely absorbed by cytoplasmic extracts of MCF-7 cells. Immunohistochemical staining using this antibody revealed the antigen in MCF-7 cells and in 12 of 15 primary breast cancer tissues and 3 of 34 other cancer tissues, but in none of 6 normal breast tissues. Anti-KASIFLK antibody titers were significantly higher in sera of 55 breast cancer patients than in sera from 30 normal healthy donors (P>0.001). These results suggest that KASIFLK or its cross-reactive epitope is a breast cancer antigen and is immunogenic in humans.

Raf-1 physically interacts with Rb and regulates its function: a link between mitogenic signaling and cell cycle regulation

Cells initiate proliferation in response to growth factor stimulation, but the biochemical mechanisms linking signals received at the cell surface receptors to the cell cycle regulatory molecules are not yet clear. In this study, we show that the signaling molecule Raf-1 can physically interact with Rb and p130 proteins in vitro and in vivo and that this interaction can be detected in mammalian cells without overexpressing any component. The binding of Raf-1 to Rb occurs subsequent to mitogen stimulation, and this interaction can be detected only in proliferating cells. Raf-1 can inactivate Rb function and can reverse Rb-mediated repression of E2F1 transcription and cell proliferation efficiently. The region of Raf-1 involved in Rb binding spanned residues 1 to 28 at the N terminus, and functional inactivation of Rb required a direct interaction. Serum stimulation of quiescent human fibroblast HSF8 cells led to a partial translocation of Raf-1 into the nucleus, where it colocalized with Rb. Further, Raf-1 was able to phosphorylate Rb in vitro quite efficiently. We believe that the physical interaction of Raf-1 with Rb is a vital step in the growth factor-mediated induction of cell proliferation and that Raf-1 acts as a direct link between cell surface signaling cascades and the cell cycle machinery.

Prognostic markers in bladder cancer: a contemporary review of the literature

PURPOSE

We provide a contemporary review of bladder tumor markers and summarize their role as prognostic indicators.

MATERIALS AND METHODS

A comprehensive review of the literature on prognostic markers for transitional cell carcinoma of the bladder was performed.

RESULTS

Intense research efforts are being made to identify and characterize better various bladder cancers and their true biological potential. The need to predict which superficial tumors will recur or progress and which invasive tumors will metastasize has led to the identification of a variety of potential prognostic markers. Blood group antigens, tumor associated antigens, proliferating antigens, oncogenes, peptide growth factors and their receptors, cell adhesion molecules, tumor angiogenesis and angiogenesis inhibitors, and cell cycle regulatory proteins have recently been identified. The potential clinical applications of these tumor markers are under active investigation. Recent attention has focused on which tumor markers may predict the responsiveness of a particular bladder cancer to systemic chemotherapy.

CONCLUSIONS

At present conventional histopathological evaluation of bladder cancer (tumor grade and stage) cannot predict accurately the behavior of most bladder tumors. With a better understanding of the cell cycle, and cell to cell and cell to extracellular matrix interactions as well as improved diagnostic techniques (immunohistochemistry), progress is being made to identify and characterize other potential prognostic markers for transitional cell carcinoma of the bladder. The ultimate goal is to develop reliable prognostic markers that will accurately predict not only the course but also the response of a tumor to therapy. This information may then be used to dictate more aggressive treatment for tumors that are likely to progress and less aggressive treatment for those that are unlikely to progress. In the future these biological markers may also be used in gene therapy for the treatment of bladder cancer.

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.

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.

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.

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.

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.

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.

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.

Transforming region of 243R E1A contains two overlapping but distinct transactivation domains

Conserved regions 1 and 2 as well as the amino terminus of E1A are required for the transforming activity of the E1A oncoprotein. We show here that the amino terminus of 243R E1A has transactivation activity when brought to a promoter in yeast. Recruitment to a specific promoter is essential. Mutagenesis studies correlated the transactivation function with the extreme amino terminus and the conserved region 1 of E1A. Cotransfection assays in rodent cells confirmed that two overlapping but distinguishable domains, amino acids 1-65 and 37-80, can transactivate independently when targeted to a promoter. We also observed that when recruited to the proliferating cell nuclear antigen (PCNA) promoter, the amino-terminal region was sufficient to transactivate the PCNA promoter. On the other hand, deletion of the amino terminus of E1A resulted in failure to induce PCNA expression. Fusion of VP16 with the amino-terminal-deleted E1A mutant was able to restore the ability to induce the PCNA promoter. We further show that the amino-terminal region also is required for 243R E1A to repress the transactivation mediated by a universal transactivator DBD.VP16 and DBD.E1A. This repression could be specifically relieved by overexpression of TBP but not TFIIB. In addition, we show that the amino terminus of E1A is involved in in vitro interaction with the TATA binding protein (TBP). Thus the amino-terminal transforming region of E1A may regulate cellular gene expression in species that are distant in evolution via a common mechanism, functionally targeting TBP.

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.