Non-NMDA and NMDA receptors in the synaptic pathway between area postrema and nucleus tractus solitarius

Area postrema (AP) modulates cardiovascular function through excitatory projections to neurons in nucleus tractus solitarius (NTS), which also process primary sensory (including cardiovascular-related) input via the solitary tract (TS). The neurotransmitter(s) and their receptors in the AP-NTS pathway have not been fully characterized. We used whole cell recordings in voltage- and current-clamp modes in the rat brain stem slice to examine the role of ionotropic glutamatergic receptors and alpha2-adrenergic receptors in the pathway from AP to NTS neurons receiving visceral afferent information via the TS. In neurons voltage clamped at potentials from -100 to +80 mV, AP stimulation (0. 2 Hz) evoked excitatory postsynaptic currents having a fast component blocked by the non-N-methyl-D-aspartate (NMDA) receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxobenzoquinoxaline-7-sulfonamide (NBQX; 3 microM, n = 7) and a slow component blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 50 microM, n = 8). Although NBQX (3 microM, n = 14) abolished AP-evoked action potentials, APV (50 microM, n = 9 or 500 microM, n = 6) or yohimbine, (200 nM, n = 5 or 2 microM, n = 10) did not. Thus, although AP stimulation activates both non-NMDA and NMDA receptors on NTS neurons receiving TS input, only non-NMDA receptors are required for synaptic transmission.

Effects of hyperdynamic fields on input-output relationships and long-term potentiation in the rat hippocampus

The effects of a 2G force environment on synaptic plasticity were examined in the rat hippocampus. Field potentials from neurons in the CA1 pyramidal cell layer were evoked by stimulation of the afferent Schaffer collateral/commissural fibers in an in vitro slice preparation. Input-output (I-O) relationships of the circuit were determined before and after tetanizing stimuli given to induce long term potentiation (LTP), a form of neural plasticity. I-O curves from animals exposed to 2G via centrifugation for either 2 or 14 days were not different from those obtained in control (1G) animals. Similarly, induction of LTP was equivalent in all groups, showing increases in maximum amplitude, slope and midpoint response of the fitted Boltzmann functions compared to un-tetanized controls. Comparison of slices from dorsal and ventral hippocampus showed the location of the slice had no effect of LTP expression. We conclude that, in contrast to other reports of functional changes in the central nervous system under altered force environments, cellular mechanisms of synaptic plasticity, which may underlie learning and memory, are preserved in the hippocampus.

Effects of cocaethylene on dopamine and serotonin synthesis in Long-Evans and Sprague-Dawley brains

We examined the behavioral and neurochemical effects of cocaethylene treatment in Long-Evans (LE) and Sprague-Dawley (SD) rats. Cocaethylene-induced behaviors were significantly less in LE rats. Cocaethylene caused an inhibition of dopamine synthesis in the caudate nucleus and nucleus accumbens that was equivalent in both rat lines. Serotonin synthesis was also suppressed by cocaethylene treatment, however this phenomenon was less pronounced when compared with the effects on dopamine synthesis.

Activating properties of cocaine and cocaethylene in a behavioral preparation of Drosophila melanogaster

The use of Drosophila as a model to study the behavioral consequences of stimulant drugs was analyzed in an active preparation of decapitated Drosophila. Application of cocaine and cocaethylene to discrete nerve cord cells regulating motor programs of behavior produced striking patterns of behavioral activity in a concentration-related manner. In general, intense circling behavior and significant wing buzzing activity were distinguishable behavioral markers in flies treated with mM concentrations of cocaine or cocaethylene. The significant changes in motor behavior induced by stimulant drugs in decapitated flies were not reproduced by the application of apomorphine, a direct dopamine (DA) agonist, or octopamine, a naturally occurring transmitter in arthropods. Because both cocaine and cocaethylene interfere with DA reuptake in mammals, we characterized the role of DA receptors mediating increased stereotypy and motor behavior in flies. Coadministration of SCH-23390, a specific D1 receptor antagonist, significantly attenuated the behavior-activating properties of cocaine and cocaethylene in this active experimental preparation. Therefore, the receptor protein mediating the behavioral responses to stimulant drugs in Drosophila is pharmacologically similar to the mammalian D1 subtype. In rats, cocaine- and cocaethylene-induced behavioral activity is complex, with increasing evidence that the D1 receptor interacts significantly with N-methyl-D-aspartate (NMDA) receptor pathways to produce an altered behavioral phenotype. To further characterize additional receptor subtypes targeted by the actions of cocaine and cocaethylene, we pretreated flies with MK-801 and dextromethorphan. Both of these drugs are potent, selective noncompetitive NMDA receptor antagonists. Interestingly, MK-801 and dextromethorphan profoundly reduced the behavior-activating properties of cocaine and cocaethylene in Drosophila. Therefore, as in rats, the NMDA (and D1) receptor pathways in this arthropod represent obligatory targets for the behavioral effects of stimulant drugs.

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

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

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.

Effects of hypergravic fields on serotonergic neuromodulation in the rat hippocampus

The effects of 7 day exposure to 2G fields on serotonergic modulation at two synapses on a hippocampal pathway were examined by recording dentate gyrus and CA1 pyramidal cell layer electrical activity. Serotonin decreased the amplitude of the population spike (synchronous action potentials in hundreds of neurons) in both the dentate gyrus and CA1 regions of rats exposed to 2G fields for 7 days. The inhibition, averaging 26 +/- 4% (mean +/- SEM) in the dentate gyrus and 80 +/- 5% in the CA1 region, was not significantly different from inhibitory responses observed in 1G controls. The 5-HT1A agonist 8-OH-DPAT mimicked this inhibition in the dentate and CA1 regions of 1G rats. 8-OH-DPAT responses were not affected by exposure to 2G fields. We conclude that the hippocampus contains surplus 5-HT receptors so that decreases in receptor density reported in receptor binding studies do not result in a decrease in modulatory capability. A model to account for the physiological pathway that relates gravitational field strength to 5-HT receptor density without changing the effectiveness of 5-HT neuromodulation is discussed.

Serotonergic modulation of hippocampal pyramidal cells in euthermic, cold-acclimated, and hibernating hamsters

Serotonergic fibers project to the hippocampus, a brain area previously shown to have distinctive changes in electroencephalograph (EEG) activity during entrance into and arousal from hibernation. The EEG activity is generated by pyramidal cells in both hibernating and nonhibernating species. Using the brain slice preparation, we characterized serotonergic responses of these CA1 pyramidal cells in euthermic, cold-acclimated, and hibernating Syrian hamsters. Stimulation of Shaffer-collateral/commissural fibers evoked fast synaptic excitation of CA1 pyramidal cells, a response monitored by recording population spikes (the synchronous generation of action potentials). Neuromodulation by serotonin (5-HT) decreased population spike amplitude by 54% in cold-acclimated animals, 80% in hibernating hamsters, and 63% in euthermic animals. The depression was significantly greater in slices from hibernators than from cold-acclimated animals. In slices from euthermic animals, changes in extracellular K+ concentration between 2.5 and 5.0 mM did not significantly alter serotonergic responses. The 5-HT1A agonist 8-hydroxy-2(di-n-propylamino)tetralin mimicked serotonergic inhibition in euthermic hamsters. Results show that 5-HT is a robust neuromodulator not only in euthermic animals but also in cold-acclimated and hibernating hamsters.

Sp3 encodes multiple proteins that differ in their capacity to stimulate or repress transcription

The product of the retinoblastoma (Rb) susceptibility gene ( RB-1 ) regulates expression of a variety of growth control genes via discrete promoter elements termed retinoblastoma control elements (RCEs). We have previously shown that RCEs are bound and regulated by a common set of ubiquitously expressed nuclear proteins of 115, 95 and 80 kDa, termed retinoblastoma control proteins (RCPs). We have also previously determined that Sp3 and Sp1, two members of the Sp family of transcription factors, encode the 115 and 95 kDa RCPs respectively and that Rb stimulates Sp1/Sp3-mediated transcription in vivo. In this report we have extended these results by determining that the 80 kDa RCP arises from Sp3 mRNA via translational initiation at two internal sites located within the Sp3 trans -activation domain. Internally initiated Sp3 proteins readily bind to Sp1 binding sites in vitro yet have little or no capacity to stimulate transcription of Sp-regulated genes in vivo. Instead, these Sp3-derived proteins function as potent inhibitors of Sp1/Sp3- mediated transcription. Since cell cycle- or signal- induced expression of a variety of genes, including p21 waf1/cip1, p15 INK4B, CYP11A, mdr1 and acetyl-CoA carboxylase, have been mapped to GC-rich promoter elements that bind Sp family members, we speculate that alterations of the protein and/or DNA binding activities of internally initiated Sp3 isoforms may account in part for the regulation of such differentially expressed genes.

Differential behavioral responses to cocaethylene of Long-Evans and Sprague-Dawley rats: role of serotonin

Cocaethylene is a neuroactive metabolite derived from the concurrent consumption of cocaine and ethanol. The effects of cocaethylene on locomotor activity, stereotypy, and rearing in Long-Evans and Sprague-Dawley rats were compared. A single cocaine injection (molar equivalent of 60 mumol/kg cocaethylene, intraperitoneal) elicited a robust series of motor output behaviors, including locomotion, stereotypy, and rearing over a 30-minute testing period in Long-Evans rats. In contrast, cocaethylene administration, under comparable testing conditions, produced no significant changes in locomotor and investigatory behaviors. Because cocaethylene has relatively little impact on serotonin (5-HT) reuptake as opposed to reuptake of dopamine, we pretreated Long-Evans rats with fluoxetine (10 mg/kg; i.p.), a selective 5-HT reuptake inhibitor. Fluoxetine profoundly augmented cocaethylene-stimulated behaviors in this rat phenotype. To examine whether other rat strains exhibit a similar response to cocaethylene, Sprague-Dawley rats were injected (i.p.) with cocaethylene and their behavior patterns monitored over a 30-minute testing period. Cocaethylene produced marked locomotor and exploratory behaviors in this strain, suggesting therefore that Long-Evans and Sprague-Dawley rat differ in their response to cocaethylene. To relate these behavioral differences to possible structural differences in the neuronal density of dopaminergic or serotonergic neurons, Long-Evans and Sprague-Dawley brains were evaluated for tyrosine hydroxylase and 5-HT immunocytochemistry. No gross morphological differences in neuronal architecture or density were found in the ventral tegmental area or dorsal raphe nucleus of the two rat phenotypes. These results indicate that two commonly used rat strains show a differential response to cocaethylene and the neurochemical basis for this behavioral difference may be related to synaptic 5-HT bioavailability.

NMDA receptors contribute to primary visceral afferent transmission in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) is a principal site for coordinating the reflex control of autonomic function. The nucleus receives and organizes primary visceral (sensory) afferent inputs from the great vessels, heart, lung, and gastrointestinal organs. Glutamate, the excitatory neurotransmitter released by the primary afferent fibers, activates non-N-methyl-D-aspartate (non-NMDA) receptors on second-order neurons in the NTS. Still in question is whether NMDA receptors on the second-order neurons are also activated. Accordingly, the purpose of this study was to directly determine whether NMDA receptors contribute to synaptic transmission of primary visceral afferent input to second-order neurons in the NTS. Whole cell patch-clamp recordings were obtained from intermediate and caudal NTS neurons in rat coronal medullary slices. Excitatory postsynaptic currents (EPSCs) were evoked by stimulation of the solitary tract (1-25 V, 0.1 ms, 0.2 or 0.5 Hz) at membrane potentials ranging from -90 to +60 mV. In 28 of 32 neurons in which current-voltage relationships were obtained for solitary-tract-evoked EPSCs, the currents had short onset latencies (3.42 +/- 1.03 ms, mean +/- SD), indicating that they were the result of monosynaptic activation of second-order neurons. Solitary-tract-evoked EPSCs had both a fast and a slow component. The amplitude of the slow component was nonlinearly related to voltage (being revealed only at membrane potentials positive to -45 mV), blocked by the NMDA receptor antagonist DL-2-amino-5-monophosphovaleric acid (APV, 50 microM; n = 12; P = 0.0001), and enhanced in nominally Mg2+-free perfusate at membrane potentials negative to -45 mV (n = 5; P = 0.016), demonstrating that the slow component was mediated by NMDA receptors. The amplitude of the fast component was linearly related to voltage and blocked by the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX, 3 microM; n = 9; P = 0.0014), demonstrating that the fast component was mediated by non-NMDA receptors. The slow component of the EPSCs was not blocked by NBQX (n = 6; P = 0.134), nor was the fast component blocked by APV (n = 12; P = 0.124). These results show that both NMDA and non-NMDA receptors coexist on the same second-order NTS neurons and mediate primary visceral afferent transmission in the NTS. The participation of NMDA receptors suggests that second-order neurons in the NTS may have previously unrecognized integrative capabilities in the reflex control of autonomic function.

Detection and functional characterization of p180, a novel cell cycle regulated yeast transcription factor that binds retinoblastoma control elements

In recent years it has become apparent that the cellular machinery governing cell cycle progression and transcription control are often homologous in yeast and mammalian cells. We and others have previously shown that the SP family of mammalian transcription factors regulates the transcription of a number of genes whose activities are governed by the product of the retinoblastoma (Rb) susceptibility gene, including c-FOS, c-MYC, TGFbeta-1, IGF-II, and c-JUN. To determine whether a similar pathway of transcriptional regulation may function in yeast, we explored the possibility that transcription factors with nucleotide-binding specificities akin to those of the SP family are expressed in Saccharomyces cerevisiae and Schizosaccharomyces pombe. Here we report the detection of novel yeast proteins (S. cerevisiae, p180; S. pombe, p200) that specifically bind Rb-regulated promoter elements in vitro dependent on nucleotides that are also required for binding and trans-activation by SP family members in vivo. Our results indicate that the S. cerevisiae retinoblastoma control element-binding activity 1) requires zinc for association with DNA; 2) does not bind to SCB, MCB, or E2F sites in vitro; 3) is cell cycle-regulated in a SWI6-independent fashion; and 4) maximally stimulates retinoblastoma control element-mediated transcription in early- to mid-S phase. Taken together, these data suggest that p180 may regulate the transcription of a subset of yeast genes whose expression is coincident with the onset and/or progression of DNA replication.

Dopaminergic and glutamatergic mechanisms mediate the induction of FOS-like protein by cocaethylene

Cocaethylene is a psychoactive metabolite formed during the combined consumption of cocaine and ethanol. As this metabolite has many properties in common with cocaine, it is conceivable that cocaethylene administration may induce the activity of nuclear transcription factors that regulate the expression of late-response genes. Therefore, the temporal induction of FOS-like protein in rat brain was examined following IP administration of 60 micromol/kg cocaethylene. Immunoreactivity for the protein was detectable at 1 h in striatal neurons and had virtually disappeared 6 h after drug treatment. Administration of specific dopaminergic (SCH-23390; 0.5 mg/kg) and glutamatergic (MK-801; 1 mg/kg) receptor antagonists prior to cocaethylene indicated a significant role for dopamine (D1) and N-methyl-D-aspartate receptor subtypes in mediating the nuclear induction of the aforementioned transcription factor protein. In contrast, no significant effects on FOS-like protein in discrete neurons of the caudate putamen were found when spiradoline (U-62066), a kappa opioid-receptor agonist, was administered either IP (10 mg/kg) or directly (50 nmol) into the brain parenchyma. In addition, we uncovered a differential sensitivity of Long-Evans rats to the behavioral effects of cocaethylene, with the psychoactive metabolite producing significantly less behavioral activity (e.g., locomotion, rearing, and continuous sniffing) than that produced by cocaine (molar equivalent of 60 micromol/kg cocaethylene). These findings indicate both common and disparate effects of cocaethylene and its parent compound, cocaine, on receptor pathways that regulate target alterations in gene expression and drug-induced motor behavior.

Cocaethylene stimulates the secretion of ACTH and corticosterone and the transcriptional activation of hypothalamic NGFI-B

Cocaethylene is an active cocaine metabolite formed by hepatic carboxylesterases in the presence of alcohol. The effects of cocaethylene on the hypothalamic-pituitary-adrenal (HPA) axis were investigated in vivo using adrenocorticotropic hormone (ACTH) and corticosterone secretion as indices of peripheral stimulation. To ascertain the central effects of cocaethylene on discrete neurons of the paraventricular nucleus (PVN) of the hypothalamus, a specific cRNA probe was used to follow changes in the transcriptional activation of nerve growth factor I-B (NGFI-B), a member of the family of immediate-early genes. Intravenous (i.v.) injection of cocaethylene (16 mumol/kg) to rats produced a marked but transient increase in plasma levels of ACTH and corticosterone within 10 min of drug exposure. Secretion of these hormones was accompanied by elevated levels of NGFI-B mRNA detected 30 min after i.v. or intraperitoneal (i.p., 60 mumol/kg) cocaethylene administration. The transcriptional stimulation of this immediate-early gene within parvocellular secretory neurons was relatively brief in duration, returning to basal levels by 180 min after drug exposure. As expected both routes of cocaethylene administration produced an increase in locomotor activity compared to saline-vehicle rats, with no differences between i.v. or i.p. routes with respect to duration of behavioral activation. Taken together, these findings indicate that cocaethylene has neuroendocrine properties on its own, targeting a critical region of the brain that regulates stressful events in the body. This, combined with other neurochemical properties, points to the possibility of cocaethylene augmenting the effects of a drug-dependent state.

Combined effects of ethanol and cocaine on FOS-like protein and cocaethylene biosynthesis in the rat

To study the simultaneous effects of ethanol and cocaine on striatal FOS-like protein, rats were exposed to an (8.7%) ethanol solution for 15 days followed by single or daily cocaine injections (20 mg/kg; IP). Ethanol consumption reduced the induction of the nuclear protein under both temporal regimens of cocaine administration. In contrast, sucrose pair-fed or ad libitum control groups exhibited a robust induction of FOS-like protein throughout the striatum, particularly in dorsal-central quadrants of the caudate putamen. This pattern of combined drug use produced blood ethanol concentrations in the range of 22-370 mg/dl, corresponding with those associated with mild intoxication in humans. Under both cocaine regimens, the presence of ethanol led to the transesterification of cocaine into the active metabolite, cocaethylene (31-121 ng/ml). Plasma levels of this metabolite did not exceed those of cocaine (17-1024 ng/ml), suggesting that under this drug regimen at least, cocaethylene formation is relatively low and perhaps dependent upon specific levels of ethanol and cocaine in hepatic microsomes. In addition, systemic administration of cocaethylene to rats (60 mumol/kg; molar equivalent of 20 mg/kg cocaine) induced widespread FOS-like protein in the caudate putamen. Induction of the transcription factor protein by cocaethylene was similar in magnitude and anatomic distribution to that of cocaine, suggesting that these two drug congeners share common molecular mechanisms of gene expression.

DP-2, a heterodimeric partner of E2F: identification and characterization of DP-2 proteins expressed in vivo

E2F is a heterodimeric transcription factor that regulates the expression of genes at the G1/S boundary and is composed of two related but distinct families of proteins, E2F and DP. E2F/DP heterodimers form complexes with the retinoblastoma (Rb) protein, the Rb-related proteins p107 and p130, and cyclins/cdks in a cell cycle-dependent fashion in vivo. E2F is encoded by at least five closely related genes, E2F-1 through -5. Here we report studies of DP-2, the second member of the DP family of genes. Our results indicate that (i) DP-2 encodes at least five distinct mRNAs, (ii) a site of alternative splicing occurs within the 5' untranslated region of DP-2 mRNA, (iii) at least three DP-2-related proteins (of 55, 48, and 43 kDa) are expressed in vivo, (iv) each of these proteins is phosphorylated, and (v) one DP-2 protein (43 kDa) carries a truncated amino terminus. Our data also strongly suggest that the 55-kDa DP-2-related protein is a novel DP-2 isoform that results from alternative splicing. Thus, we conclude that DP-2 encodes a set of structurally, and perhaps functionally, distinct proteins in vivo.

Individual and combined effects of ethanol and cocaine on intracellular signals and gene expression

1. Ethanol and cocaine are drugs of abuse that can produce long-lived changes in behavior, including dependence. 2. A common set of neural pathways appears to mediate the addictive actions of ethanol and cocaine. 3. Many prominent aspects of drug dependence may be the result of alterations in intracellular signals as well as specific patterns of gene expression. 4. For instance, changes in G proteins and cAMP, phosphorylation of proteins and induction of c-fos and zif/268 in specific drug-sensitive brain regions may represent adaptive changes in response to a drug-dependent state. 5. The concurrent use of ethanol and cocaine is the most prevalent pattern of drug abuse in humans. However, the number of studies investigating the behavioral and molecular effects of this combination are few. 6. Emerging evidence indicates a possible antagonistic effect of ethanol and cocaine action on transcription factor function. In addition, cocaethylene (a psychoactive metabolite derived from combined ethanol and cocaine exposure) has significant effects on gene expression as well.

The amino terminus of the retinoblastoma (Rb) protein associates with a cyclin-dependent kinase-like kinase via Rb amino acids required for growth suppression

We have shown previously that a novel cell cycle-regulated histone H1 kinase activity, retinoblastoma kinase (RbK), associates with and phosphorylates the amino terminus of the Rb protein in G2-M. We have shown also that the amino terminus of p107, a Rb-related protein, does not associate with a similar kinase in vitro or in vivo. Here, we report that a RbK-like kinase associates with the amino terminus of p130, another Rb-related protein, only marginally. Moreover, the association of RbK with Rb in vitro is shown to require a discrete portion of the Rb amino terminus, amino acids 89-202. This region has been shown previously to be subject to inactivating mutations in retinoblastoma and to be required for Rb-mediated growth suppression in vitro. Taken together, these data indicate that the formation of Rb-RbK complexes may play an important role in Rb-mediated growth suppression. We have mapped two in vitro sites of Rb phosphorylation by RbK to sites that are phosphorylated in vivo and are targets of cyclin-dependent kinase phosphorylation in vitro. As such, at least some sites of RbK phosphorylation overlap with those of other proline-directed serine and threonine kinases. Consistent with this latter observation, we report that the trans-activation domain of c-myc is phosphorylated specifically by RbK in vitro at a site (serine 62) that is phosphorylated in vivo during G2-M, cell-cycle phases in which RbK activity is maximal.

Functional interactions between the retinoblastoma (Rb) protein and Sp-family members: superactivation by Rb requires amino acids necessary for growth suppression

The transient expression of the retinoblastoma protein (Rb) regulates the transcription of a variety of growth-control genes, including c-fos, c-myc, and the gene for transforming growth factor beta 1 via discrete promoter sequences termed retinoblastoma control elements (RCE). Previous analyses have shown that Sp1 is one of three RCE-binding proteins identified in nuclear extracts and that Rb functionally interacts with Sp1 in vivo, resulting in the "superactivation" of Sp1-mediated transcription. By immunochemical and biochemical criteria, we report that an Sp1-related transcription factor, Sp3, is a second RCE-binding protein. Furthermore, in transient cotransfection assays, we report that Rb "superactivates" Sp3-mediated RCE-dependent transcription in vivo and that levels of superactivation are dependent on the trans-activator (Sp1 or Sp3) studied. Using expression vectors carrying mutated Rb cDNAs, we have identified two portions of Rb required for superactivation: (i) a portion of the Rb "pocket" (amino acids 614-839) previously determined to be required for physical interactions between Rb and transcription factors such as E2F-1 and (ii) a novel amino-terminal region (amino acids 140-202). Since both of these regions of Rb are targets of mutation in human tumors, our data suggest that superactivation of Sp1/Sp3 may play a role in Rb-mediated growth suppression and/or the induction of differentiation.

Detection of a novel cell cycle-regulated kinase activity that associates with the amino terminus of the retinoblastoma protein in G2/M phases

Recent genetic and functional evidence suggests that the amino terminus of the retinoblastoma (Rb) protein plays an important role in Rb-mediated growth suppression. To explore the mechanism(s) by which this portion of Rb may regulate cell growth, we have sought to characterize cellular proteins that associate with the Rb amino terminus using an in vitro protein-binding assay. Here we report that at least one such protein is a cell cycle-regulated Rb/histone H1 kinase (RbK) whose enzymatic and/or Rb association activity is most prevalent in G2/M phases of cells. In contrast to previously characterized cyclin-dependent and Rb-associated kinases, such as cdk1 (cdc2) and cdk2, G2/M RbK 1) is not depleted by incubation with p13suc-beads, 2) is not detected with antisera against several Rb-associated cyclins-cdks, and 3) associated with Rb via the Rb amino terminus, a region that is dispensable for interaction with other Rb-associated kinases. RbK is clearly distinct from previously characterized mitotic cdks since cyclin A-cdc2, cyclin A-cdk2, cyclin B-cdc2, and cyclin B-cdk2 did not associate with the Rb amino terminus. Coprecipitation experiments with Rb antisera confirmed the association of Rb with a RbK-like kinase in metaphase-arrested cells in vivo. Interestingly, G2/M RbK did not appreciably associate with an analogous portion of p107, a Rb-related protein. Taken together, these data indicate that the Rb amino terminus specifically associates with a novel cell cycle-regulated kinase in late cell cycle stages.

Retinoblastoma gene product as a downstream target for a ceramide-dependent pathway of growth arrest

Ceramide, a lipid mediator, has been most closely associated with antiproliferative activities. In this study, we examine the mechanism by which ceramide induces growth suppression and the role of the retinoblastoma gene product (Rb) in this process. Withdrawal of serum from the serum-dependent MOLT-4 cells resulted in significant dephosphorylation of Rb, correlating with the induction of G0/G1 cell cycle arrest. Serum withdrawal resulted in marked elevation in the levels of endogenous ceramide (3-fold at 24 h and 8-fold at 96 h) with little changes in the endogenous levels of sphingosine. The addition of exogenous C6-ceramide resulted in a concentration- and time-dependent dephosphorylation of Rb. Exogenous ceramide was active at levels comparable to endogenous levels achieved with serum withdrawal. Peak activity of exogenous ceramide (at 6 h) correlated with the uptake of C6-ceramide by MOLT-4 cells. Next, a number of studies were conducted to determine whether Rb plays a role in ceramide-induced growth suppression. (i) C6-Ceramide was poorly active in growth suppression of retinoblastoma cells that lack Rb. (ii) Mink lung epithelial cells in which Rb had been sequestered by overexpression of large tumor antigen were resistant to the action of ceramide compared to cells transfected with large tumor antigen mutated in the Rb-binding pocket. (iii) Overexpression of the EIA adenoviral protein, which binds and sequesters Rb, resulted in protection from growth suppression and cell cycle arrest induced by ceramide. Thus, these studies demonstrate that Rb is a downstream target for ceramide and may function in a growth suppressor pathway resulting in cell cycle arrest.

Negative regulation of Sp1 trans-activation is correlated with the binding of cellular proteins to the amino terminus of the Sp1 trans-activation domain

Sp1 is a well characterized and ubiquitously expressed transcription factor that regulates the constitutive and induced expression of a variety of mammalian genes. It is unclear whether Sp1 activity is regulated in vivo; the mechanism by which Sp1 interacts with the basal transcription complex has not been firmly established. We report the identification of a ubiquitously expressed and evolutionarily conserved nuclear protein, p74, that specifically binds Sp1 in vivo and in vitro. p74 interacts with several portions of the Sp1 trans-activation domain in vitro, and we correlate the binding of p74 to the amino-terminal serine/threonine-rich subdomain of Sp1 with the inhibition of Sp1-mediated transcription in vivo.

Sp-1 binds promoter elements regulated by the RB protein and Sp-1-mediated transcription is stimulated by RB coexpression

The retinoblastoma (RB) protein is implicated in transcriptional regulation of at least five cellular genes, including c-fos, c-myc, and transforming growth factor beta 1. Cotransfection of RB and truncated promoter constructs has defined a discrete element (retinoblastoma control element; RCE) within the promoters of each of these genes as being necessary for RB-mediated transcription control. Previously, we have shown that RCEs form protein-DNA complexes in vitro with three heretofore unidentified nuclear proteins and mutation of their DNA-binding site within the c-fos RCE results in an abrogation of RCE-dependent transcription in vivo. Here, we demonstrate that one of the nuclear proteins that binds the c-fos, c-myc, and transforming growth factor beta 1 RCEs in vitro is Sp-1 and that Sp-1 stimulates RCE-dependent transcription in vivo. Moreover, we show that Sp-1-mediated transcription is stimulated by the transient coexpression of RB protein. We conclude from these observations that RB may regulate transcription in part by virtue of its ability to functionally interact with Sp-1.

Regulation of transcription by the retinoblastoma protein

The product of the retinoblastoma gene (RB1) is believed to function as a negative regulator of cell growth. Recent experimental results suggest that RB1 may exert its growth-suppressing activity by regulating the transcription of a variety of growth-related genes, including FOS, MYC, and TGFBI. A series of biochemical and molecular analyses suggest that RB1 indirectly affects gene expression via cell-cycle-regulated interactions with transcription factors, such as E2F and SPI. Determination of the mechanisms regulating such protein-protein interactions and the identification of additional targets of RB1 function will provide vital insights into the role of this tumor-suppressor gene in mammalian cell proliferation.

Functional analysis at the Cys706 residue of the retinoblastoma protein

A missense mutation at cysteine 706, resulting in a retinoblastoma (RB) protein defective in phosphorylation and oncoprotein binding, has been isolated from a human tumor cell line. Since this residue is conserved in murine RB and in the related p107 protein, we studied the activity of in vitro mutants flanking this position. These experiments demonstrated that the thiol atom at codon 706 does not possess intrinsic functional activity as small polar or nonpolar residues could substitute at either codons 706 or 707, while bulkier R-group changes in these positions interfered with in vitro oncoprotein binding or in vivo protein phosphorylation. A series of missense mutants in an adjacent leucine repeat domain also demonstrated a loss of oncoprotein binding that was proportional to the magnitude of amino acid substitutions. To determine whether the cysteine 706 --> phenylalanine RB mutant retained any protein binding activity, we examined its ability to precipitate MYC, which was recently identified as a potential RB-associated protein. These experiments demonstrated that the mutant RB product is capable of binding in vitro to c-myc and L-myc proteins with comparable affinity as wild-type RB. These findings raise questions about the functional role of the RB:MYC interactions and emphasize important differences in the binding patterns between MYC and the other RB-associated proteins.