The retinoblastoma protein region required for interaction with the E2F transcription factor includes the T/E1A binding and carboxy-terminal sequences

Inhibition of retinoblastoma mRNA degradation through Poly (A) involved in the neuroprotective effect of berberine against cerebral ischemia

Berberine is one kind of isoquinoline alkaloid with anti-apoptotic effects on the neurons suffering ischemia. To address the explanation for these activities, the berberine-induced cell cycle arrest during neurons suffering ischemia/reperfusion had been studied in the present study. According to the in vitro neurons with oxygen-glucose deprivation and in vivo ICR mice with cerebral ischemia/reperfusion, it was found that berberine could protect the mRNA of retinoblastoma (Rb) from degradation through its function on the poly(A) tail. The prolonged half-life of retinoblastoma 1 (gene of Rb, RB1) mRNA level secures the protein level of retinoblastoma, which facilitates cell cycle arrest of neurons in the process of ischemia/reperfusion and subsequently avoids cells entering in the apoptotic process. The poly(A) tail of RB1 mRNA, as a newly identified target of berberine, could help people focus on the interaction between berberine and mRNA to further understand the biological activities and functions of berberine.

The nuclear protein ALY binds to and modulates the activity of transcription factor E2F2

E2F transcription factors control the expression of genes involved in a variety of essential cellular processes and consequently their activity needs to be tightly regulated. Protein-protein interactions are thought to be key modulators of E2F activity. To gain insight into the mechanisms that regulate the activity of E2F2, we searched for novel proteins that associate with this transcription factor. We show that the nuclear protein ALY (THO complex 4), originally described as a transcriptional co-activator, associates with DNA-bound E2F2 and represses its transcriptional activity. The capacity of ALY to modulate gene expression was analyzed with expression microarrays by characterizing the transcriptome of E2F2 expressing HEK293T cells in which ALY was either overexpressed or silenced. We show that ALY influences the expression of more than 400 genes, including 98 genes bearing consensus E2F motifs. Thus, ALY emerges as a novel E2F2-interacting protein and a relevant modulator of E2F-responsive gene expression.

PU.1 and pRB interact and cooperate to repress GATA-1 and block erythroid differentiation

PU.1 and GATA-1 are two hematopoietic specific transcription factors that play key roles in development of the myeloid and erythroid lineages, respectively. The two proteins bind to one another and inhibit each other's function in transcriptional activation and promotion of their respective differentiation programs. This mutual antagonism may be an important aspect of lineage commitment decisions. PU.1 can also act as an oncoprotein since deregulated expression of PU.1 in erythroid precursors causes erythroleukemias in mice. Studies of cultured mouse erythroleukemia cell lines indicate that one aspect of PU.1 function in erythroleukemogenesis is its ability to block erythroid differentiation by repressing GATA-1 (N. Rekhtman, F. Radparvar, T. Evans, and A. I. Skoultchi, Genes Dev. 13:1398-1411, 1999). We have investigated the mechanism of PU.1-mediated repression of GATA-1. We report here that PU.1 binds to GATA-1 on DNA. We localized the repression activity of PU.1 to a small acidic N-terminal domain that interacts with the C pocket of pRB, a well-known transcriptional corepressor. Repression of GATA-1 by PU.1 requires pRB, and pRB colocalizes with PU.1 and GATA-1 at repressed GATA-1 target genes. PU.1 and pRB also cooperate to block erythroid differentiation. Our results suggest that one of the mechanisms by which PU.1 antagonizes GATA-1 is by binding to it at GATA-1 target genes and tethering to these sites a corepressor that blocks transcriptional activity and thereby erythroid differentiation.

Integration of the pRB and p53 cell cycle control pathways

Two fundamental molecular pathways, the pRB and p53 pathways, regulate cell growth and cell death. The importance of these pathways in cellular growth control is underscored by the observation that members of these pathways are found mutated in all human cancers. These two pathways have typically been studied and described independently. However, as we discuss here, recent data have revealed an intimate molecular and genetic interaction between the p53 and pRB pathways.

Cells degrade a novel inhibitor of differentiation with E1A-like properties upon exiting the cell cycle

Control of proliferation and differentiation by the retinoblastoma tumor suppressor protein (pRB) and related family members depends upon their interactions with key cellular substrates. Efforts to identify such cellular targets led to the isolation of a novel protein, EID-1 (for E1A-like inhibitor of differentiation 1). Here, we show that EID-1 is a potent inhibitor of differentiation and link this activity to its ability to inhibit p300 (and the highly related molecule, CREB-binding protein, or CBP) histone acetylation activity. EID-1 is rapidly degraded by the proteasome as cells exit the cell cycle. Ubiquitination of EID-1 requires an intact C-terminal region that is also necessary for stable binding to p300 and pRB, two proteins that bind to the ubiquitin ligase MDM2. A pRB variant that can bind to EID1, but not MDM2, stabilizes EID-1 in cells. Thus, EID-1 may act at a nodal point that couples cell cycle exit to the transcriptional activation of genes required for differentiation.

Glutamic acid mutagenesis of retinoblastoma protein phosphorylation sites has diverse effects on function

The retinoblastoma tumor suppressor gene (Rb) has many functions within the cell including regulation of transcription, differentiation, apoptosis, and the cell cycle. Regulation of these functions is mediated by phosphorylation at as many as 16 cyclin-dependent kinase (CDK) phosphorylation sites in vivo. The contribution of these sites to the regulation of the various Rb functions is not well understood. To characterize the effect of phosphorylation at these sites, we systematically mutagenized the serines or threonines to glutamic acid. Thirty-five mutants with different combinations of modified phosphorylation sites were assayed for their ability to arrest the cell cycle and for their potential to induce differentiation. Only the most highly substituted mutants failed to arrest cell cycle progression. However, mutants with as few as four modified phosphorylation sites were unable to promote differentiation. Other mutants had increased activity in this assay. We conclude that modification of Rb phosphorylation sites can increase or decrease protein activity, that different Rb functions can be regulated independently by distinct combinations of sites, and that the effects of modification at any one site are context dependent.

Direct interaction of p21 cyclin-dependent kinase inhibitor with the retinoblastoma tumor suppressor protein

The p21CKI forms a physical complex with the retinoblastoma protein (pRb) both in vitro and in vivo. The A/B pocket region of pRb and the N-terminal region of p21 were indispensable for this interaction. Among p21 family members, p57, but not p27, associated with pRb. Overexpression of cyclin D1, Cdk4, and E2F1 in the cells expressing pRb and p21 did not perturb the interaction between p21 and pRb. Coexpression of p21 in cells expressing pRb, cyclin D1, and Cdk4 prevented pRb hyperphosphorylation by cyclin D1/Cdk4. On the other hand, hyperphosphorylation of pRb by an excess amount of cyclin/Cdk disrupted pRb/p21 complex formation in vitro. These findings suggest that pRb may be dynamically regulated by the relative binding and activities of p21 and Cdks.

In vitro analysis of the E1A-homologous sequences of RIZ

The RIZ (G3B/MTB-Zf) gene was first isolated based on its ability to bind to the retinoblastoma protein (Rb). An acidic, approximately 100-amino-acid region around the Rb-binding motif of RIZ has structural and antigenic similarity to the conserved sequences of the E1A viral oncogene. We show here that this region interacts specifically with the E1A-binding domain of Rb. This interaction could be disrupted by E1A or by a peptide of RIZ homologous to the CR2 motif of E1A which is involved in binding to Rb family proteins. Also like E1A, RIZ can form a ternary complex with Rb and E2F1. Despite this similarity to E1A, however, RIZ could not bind to the Rb family proteins p107 and p130 in vitro. The data show that the RIZ CR2 motif can mediate differential binding to Rb family proteins. We also mapped the shared antigenic determinant between RIZ and E1A to a conserved sequence, designated CE1, which is located in the C terminus of E1A. Unlike that of ETA, the CE1 motif of RIZ is located next to the CR2 motif. Despite this proximity, CE1 and CR2 appear to act independently. The data show similarities as well as differences between the homologous sequences of RIZ and E1A and contribute to an understanding of the biochemistry of these proteins.

HBP1: a HMG box transcriptional repressor that is targeted by the retinoblastoma family

A prominent feature of cell differentiation is the initiation and maintenance of an irreversible cell cycle arrest with the complex involvement of the retinoblastoma (RB) family (RB, p130, p107). We have isolated the HBP1 transcriptional repressor as a potential target of the RB family in differentiated cells. By homology, HBP1 is a sequence-specific HMG transcription factor, of which LEF-1 is the best-characterized family member. Several features of HBP1 suggest an intriguing role as a transcriptional and cell cycle regulator in differentiated cells. First, inspection of the HBP1 protein sequence revealed two consensus RB interaction motifs (LXCXE and IXCXE). Second, HBP1 interaction was selective for RB and p130, but not p107. HBP1, RB, and p130 levels are all up-regulated with differentiation; in contrast, p107 levels decline. Third, HBP1 can function as a transcriptional repressor of the promoter for N-MYC, which is a critical cell cycle and developmental gene. Fourth, because the activation of the N-MYC promoter in cycling cells required the E2F transcription factor, we show that E2F-1 and HBP1 represent opposite transcriptional signals that can be integrated within the N-MYC promoter. Fifth, the expression of HBP1 lead to efficient cell cycle arrest. The arrest phenotype was manifested in the presence of optimal proliferation signals, suggesting that HBP1 exerted a dominant regulatory role. Taken together, the results suggest that HBP1 may represent a unique transcriptional repressor with a role in initiation and establishment of cell cycle arrest during differentiation.

The N-terminal region of E2F-1 is required for transcriptional activation of a new class of target promoter

Because of its expression in numerous cells, the herpes simplex virus thymidine kinase promoter (HSV-TK) is one of the best characterized promoters. Using the HSV-TK promoter as a model system, we have defined a new mode of E2F-1 transcriptional activation which utilizes the N-terminal region of E2F-1. We demonstrate that E2F-1 strongly activated HSV-TK, but in the absence of consensus E2F DNA elements. Nonetheless, E2F-1 could bind to GC-rich elements, which were conclusively identified in classic studies of HSV-TK as SP-1 sites. Second, the transcriptional activation of HSV-TK required the entire E2F-1 protein, including the N-terminal 89 amino acids. In contrast, the N-terminal 89 amino acids of E2F-1 were dispensable for transcriptional activation through consensus E2F sites. Third, we demonstrated that S phase entry is not sufficient for activation of HSV-TK by E2F-1, while the activation through consensus E2F sites is strictly linked to the cell cycle. Taken together, the activation of HSV-TK by E2F-1 proceeds by a different mechanism directed in part through the N-terminal region of E2F-1 and may be uncoupled from the known cell cycle regulatory role.

The human papillomavirus type 16 E7 protein complements adenovirus type 5 E1A amino-terminus-dependent transactivation of adenovirus type 5 early genes and increases ATF and Oct-1 DNA binding activity

We have previously shown that conserved region 1 (CR1) of the adenovirus type 5 (Ad5) E1A protein synergizes with CR3 in the transactivation of Ad5 early genes (H.K. Wong and E. B. Ziff, J. Virol. 68:4910-4920, 1994). CR1 lies within the E1A amino terminus and binds host regulatory proteins such as the RB protein, p107, p130, and p300. Since simian virus 40 (SV40) large T antigen and human papillomavirus type 16 (HPV16) E7 protein also bind host regulatory factors, we investigated whether these viral proteins can complement E1A mutants which are defective in early gene activation. We show that the HPV16 E7 protein but not SV40 T antigen can complement mutations in the Ad5 E1A CR1 in the transactivation of viral early promoters. The inability of SV40 T antigen to complement suggests that RB binding on its own is not sufficient for early promoter transactivation by the E1A amino terminus. Nuclear runoff assays show that complementation by HPV16 E7 restores the ability of the E1A mutants to stimulate early gene expression at the level of transcription. Furthermore, nuclear extracts from the E7-transformed cells show increased binding activity of ATF and Oct-1, factors that can recognize the elements of Ad5 early genes, consistent with gene activation by E1A and E7 at the transcriptional level.

An inverted repeat motif stabilizes binding of E2F and enhances transcription of the dihydrofolate reductase gene

An overlapping inverted repeat sequence that binds the eukaryotic transcription factor E2F is 100% conserved near the major transcription start sites in the promoters of three mammalian genes encoding dihydrofolate reductase, and is also found in the promoters of several other important cellular and viral genes. This element, 5'-TTTCGCGCCAAA-3', is comprised of two overlapping, oppositely oriented sites which match the consensus E2F site (5'-TTT(C/G)(C/G)CGC-3'). Recent work has shown that E2F binding activity is composed of at least six related cellular polypeptides which are capable of forming DNA-binding homo- and heterodimers. We have investigated the binding of cellular E2F activity and of homo- and heterodimers of cloned E2F proteins to the inverted repeat E2F element. We have demonstrated that mutations in this element that abolish its inverted repeat nature, while preserving a single consensus E2F site, significantly decrease the binding stability of all of the forms of E2F tested. The rate of association of E2F-1/DP-1 heterodimers with the inverted repeat wild type site was not significantly different from those with the two single site mutated probes. Furthermore, the mutations decrease in vitro transcription and transient reporter gene expression 2-5-fold, an effect equivalent to that of abolishing E2F binding altogether. These data suggest a functional role that may explain the conservation of inverted repeat E2F elements among the DHFR promoters and several other cellular and viral promoters.

Multiple change in E2F function and regulation occur upon muscle differentiation

We have examined regulation of the E2F transcription factor during differentiation of muscle cells. E2F regulates many genes involved in growth control and is also the target of regulation by diverse cellular signals, including the RB family of growth suppressors (e.g., the retinoblastoma protein [RB], p107, and p130). The following aspects of E2F function and regulation during muscle differentiation were investigated: (i) protein-protein interactions, (ii) protein levels, (iii) phosphorylation of the E2F protein, and (iv) transcriptional activity. A distinct E2F complex was present in differentiated cells but not in undifferentiated cells. The p130 protein was a prominent component of the E2F complex associated with differentiation. In contrast, in undifferentiated cells, the p107 protein was the prominent component in one of three E2F complexes. In addition, use of a differentiation-defective muscle line provided genetic and biochemical evidence that quiescence and differentiation are separable events. Exclusive formation of the E2F-p130 complex did not occur in this differentiation-defective line; however, E2F complexes diagnostic of quiescence were readily apparent. Thus, sole formation of the E2F-p130 complex is a necessary event in terminal differentiation. Other changes in E2F function and regulation upon differentiation include decreased phosphorylation and increased repression by E2F. These observations suggest that the regulation of E2F function during terminal differentiation may proceed through differential interaction within the RB family and/or phosphorylation.

Disruption of retinoblastoma protein function by coexpression of its C pocket fragment

The growth suppression function of the retinoblastoma protein (RB) is mediated by its interaction with a variety of cellular proteins. RB contains at least two protein-binding pockets: the large A/B pocket, which interacts with E2F and the D-type cyclins, and the C pocket, which interacts with the nuclear c-Abl tyrosine kinase. The large A/B pocket and the C pocket are shown here to be functionally distinct and can be occupied simultaneously. A complex containing E2F, RB, and c-Abl is detected in vivo and can be assembled in vitro. We propose that the biological activity of RB not only depends on the inhibition of its targets but also on its ability to properly assemble specific protein complexes. Consistent with this hypothesis, a fragment of RB, SE delta, containing only the C pocket is shown to act as a dominant-negative inhibitor of RB function. SE delta does not have growth inhibitory activity of its own. When coexpressed with full-length RB, SE delta does not disrupt the RB-E2F or RB-D2 complexes nor does it affect the expression, phosphorylation, or nuclear tethering of the full-length RB. SE delta does compete with RB for binding to c-Abl and is fully capable of inhibiting the c-Abl tyrosine kinase. Thus, SE delta can inactivate RB while maintaining the inhibition of E2F and c-Abl. These results suggest that the inhibition of RB-binding proteins is not sufficient to suppress cell growth and that the assembly of RB-mediated protein complexes is also important for the promotion of cell-cycle arrest.

Cyclin A/CDK2 binds directly to E2F-1 and inhibits the DNA-binding activity of E2F-1/DP-1 by phosphorylation

E2F-1, a member of the E2F transcription factor family, contributes to the regulation of the G1-to-S phase transition in higher eukaryotic cells. E2F-1 forms a heterodimer with DP-1 and binds to several cell cycle regulatory proteins, including the retinoblastoma family (RB, p107, p130) and cyclin A/CDK2 complexes. We have analyzed E2F-1 phosphorylation and its interaction with cyclin A/CDK2 complexes both in vivo and in vitro. In vitro, E2F-1 formed a stable complex with cyclin A/CDK2 but not with either subunit alone. DP-1 did not interact with cyclin A, CDK2, or the cyclin A/CDK2 complex. While the complex of cyclin A/CDK2 was required for stable complex formation with E2F-1, the kinase-active form of CDK2 was not required. However, E2F-1 was phosphorylated by cyclin A/CDK2 in vitro and was phosphorylated in vivo in HeLa cells. Two-dimensional tryptic phosphopeptide mapping studies demonstrated an overlap in the phosphopeptides derived from E2F-1 labeled in vitro and in vivo, indicating that cyclin A/CDK2 may be responsible for the majority of E2F-1 phosphorylation in vivo. Furthermore, an active DNA-binding complex could be reconstituted from purified E2F-1/DP-1 and cyclin A/CDK2. Binding studies conducted both in vitro and in vivo demonstrated that the cyclin A/CDK2-binding region resided within the N-terminal 124 amino acids of E2F-1. Because the stable association of E2F-1 with cyclin A/CDK2 in vitro and in vivo did not require a DP-1- or RB-binding domain and because the interactions could be reconstituted from purified components in vitro, we conclude that the interactions between cyclin A/CDK2 and E2F-1 are direct. Finally, we report that the DNA-binding activity of the E2F-1/DP-1 complex is inhibited following phosphorylation by cyclin A/CDK2.

Cyclin A/CDK2 binds directly to E2F-1 and inhibits the DNA-binding activity of E2F-1/DP-1 by phosphorylation

E2F-1, a member of the E2F transcription factor family, contributes to the regulation of the G1-to-S phase transition in higher eukaryotic cells. E2F-1 forms a heterodimer with DP-1 and binds to several cell cycle regulatory proteins, including the retinoblastoma family (RB, p107, p130) and cyclin A/CDK2 complexes. We have analyzed E2F-1 phosphorylation and its interaction with cyclin A/CDK2 complexes both in vivo and in vitro. In vitro, E2F-1 formed a stable complex with cyclin A/CDK2 but not with either subunit alone. DP-1 did not interact with cyclin A, CDK2, or the cyclin A/CDK2 complex. While the complex of cyclin A/CDK2 was required for stable complex formation with E2F-1, the kinase-active form of CDK2 was not required. However, E2F-1 was phosphorylated by cyclin A/CDK2 in vitro and was phosphorylated in vivo in HeLa cells. Two-dimensional tryptic phosphopeptide mapping studies demonstrated an overlap in the phosphopeptides derived from E2F-1 labeled in vitro and in vivo, indicating that cyclin A/CDK2 may be responsible for the majority of E2F-1 phosphorylation in vivo. Furthermore, an active DNA-binding complex could be reconstituted from purified E2F-1/DP-1 and cyclin A/CDK2. Binding studies conducted both in vitro and in vivo demonstrated that the cyclin A/CDK2-binding region resided within the N-terminal 124 amino acids of E2F-1. Because the stable association of E2F-1 with cyclin A/CDK2 in vitro and in vivo did not require a DP-1- or RB-binding domain and because the interactions could be reconstituted from purified components in vitro, we conclude that the interactions between cyclin A/CDK2 and E2F-1 are direct. Finally, we report that the DNA-binding activity of the E2F-1/DP-1 complex is inhibited following phosphorylation by cyclin A/CDK2.

Deregulated transcription factor E2F-1 expression leads to S-phase entry and p53-mediated apoptosis

E2F-1 is a transcription factor suspected of activating genes required for S phase and a known target for the action of RB, the retinoblastoma gene product. Its induction in quiescent fibroblasts led to S-phase entry followed by apoptosis. E2F-1-mediated apoptosis was suppressed by coexpression of wild-type RB or a transdominant negative mutant species of p53. In contrast, coexpression of a naturally occurring loss-of-function RB mutant or wild-type p53 did not suppress the induction of apoptosis under these conditions. Thus, deregulated E2F-1 activity gives rise to proliferative and apoptotic signals. p53 appears to participate in the execution of the latter.

Stabilization of the tumor suppressor p53 during cellular transformation by simian virus 40: influence of viral and cellular factors and biological consequences

To understand the process and biological significance of metabolic stabilization of p53 during simian virus 40 (SV40)-induced cellular transformation, we analyzed cellular and viral parameters involved in this process. We demonstrate that neither large T expression as such nor the cellular phenotype (normal versus transformed) markedly influence the stability of p53 complexed to large T in SV40 abortively infected BALB/c mouse fibroblasts. In contrast, metabolic stabilization of p53 is an active cellular event, specifically induced by SV40. The ability of SV40 to induce a cellular response leading to stabilization of p53 complexed to large T is independent from the cellular phenotype and greatly varies between different cells. However, metabolic stability was conferred only to p53 in complex with large T, whereas the free p53 in these cells remained metabolically unstable. Comparative analyses of cellular transformation in various cells differing in stability of p53 complexed to large T upon abortive infection with SV40 revealed a strong correlation between the ability of SV40 to induce metabolic stabilization and its transformation efficiency. Our data suggest that metabolic stabilization and the ensuing enhanced levels of p53 are important for initiation and/or maintenance of SV40 transformation.

Molecular cloning of cellular genes encoding retinoblastoma-associated proteins: identification of a gene with properties of the transcription factor E2F

The retinoblastoma protein interacts with a number of cellular proteins to form complexes which are probably crucial for its normal physiological function. To identify these proteins, we isolated nine distinct clones by direct screening of cDNA expression libraries using purified RB protein as a probe. One of these clones, Ap12, is expressed predominantly at the G1-S boundary and in the S phase of the cell cycle. The nucleotide sequence of Ap12 has features characteristic of transcription factors. The C-terminal region binds to unphosphorylated RB in regions similar to those to which T antigen binds and contains a transactivation domain. A region containing a potential leucine zipper flanked by basic residues is able to bind an E2F recognition sequence specifically. Expression of Ap12 in mammalian cells significantly enhances E2F-dependent transcriptional activity. These results suggest that Ap12 encodes a protein with properties known to be characteristic of transcription factor E2F.

Molecular cloning of cellular genes encoding retinoblastoma-associated proteins: identification of a gene with properties of the transcription factor E2F

The retinoblastoma protein interacts with a number of cellular proteins to form complexes which are probably crucial for its normal physiological function. To identify these proteins, we isolated nine distinct clones by direct screening of cDNA expression libraries using purified RB protein as a probe. One of these clones, Ap12, is expressed predominantly at the G1-S boundary and in the S phase of the cell cycle. The nucleotide sequence of Ap12 has features characteristic of transcription factors. The C-terminal region binds to unphosphorylated RB in regions similar to those to which T antigen binds and contains a transactivation domain. A region containing a potential leucine zipper flanked by basic residues is able to bind an E2F recognition sequence specifically. Expression of Ap12 in mammalian cells significantly enhances E2F-dependent transcriptional activity. These results suggest that Ap12 encodes a protein with properties known to be characteristic of transcription factor E2F.

Retinoblastoma-repression of E2F-dependent transcription depends on the ability of the retinoblastoma protein to interact with E2F and is abrogated by the adenovirus E1A oncoprotein

The product of the retinoblastoma tumor suppressor gene interacts with the transcription factor E2F. Two distinct types of interactions can be detected between the retinoblastoma gene product (Rb) and E2F. The first type involves an Rb-binding protein, RBP60. The Rb/E2F complex formed in the presence of RBP60 is able to bind DNA and migrates with a distinct mobility in gel retardation assays. The second type of Rb/E2F complex is seen in the absence of RBP60. This second type of Rb/E2F complex does not form a band-shift complex in gel retardation assays and its formation results in an apparent inhibition or loss of the DNA binding activity of E2F. Using a series of Rb-mutants we show that these two types of Rb/E2F complexes depend on common domains of the Rb protein. The T/E1A-binding region as well as the carboxyl-terminus of the Rb protein are critical for these two types of Rb/E2F interactions. We also show that the retinoblastoma protein represses the E2F-dependent transcription, and this Rb-repression of the E2F-dependent transcription depends on the ability of Rb to interact with E2F. Moreover, the adenovirus E1A gene product, which binds Rb, counteracts the Rb-repression and restores E2F-dependent transcription.