Analysis of Autophagic Vesicles in Mitotic Cells

The detection of autophagic vesicles in interphase cells is well characterized with markers such as LC3, SQSTM1 (also known as p62) and LAMP2, which are commonly used in immunofluorescence and biochemistry assays to evaluate the status of autophagy in adherent cells. During mitosis, cells undergo important morphological changes which alter the position of the central plane, therefore the imaging of dividing cells has to be specifically designed. Here, we describe a method to label and image autophagic vesicles in mitotic cells to systematically analyze their number, morphology and distribution.

Detection of Nuclear Biomarkers for Chromosomal Instability

Chromosomal instability (CIN) is a hallmark of cancer, which is characterized by the gain or loss of chromosomes as well as the rearrangement of the genetic material during cell division. Detection of mitotic errors such as misaligned chromosomes or chromosomal bridges (also known as lagging chromosomes) is challenging as it requires the analysis and manual discrimination of chromosomal aberrations in mitotic cells by molecular techniques. In interphase cells, more frequent in the cell population than mitotic cells, two distinct nuclear phenotypes are associated with CIN: the micronucleus and the toroidal nucleus. Several methods are available for the detection of micronuclei, but none for toroidal nuclei. Here, we provide a method to quantify the presence of both nuclear biomarkers for the evaluation of CIN status in non-mitotic cells particularly suited for genotoxicity screens.

The 40S-LARP1 complex reprograms the cellular translatome upon mTOR inhibition to preserve the protein synthetic capacity

Ribosomes execute the transcriptional program in every cell. Critical to sustain nearly all cellular activities, ribosome biogenesis requires the translation of ~200 factors of which 80 are ribosomal proteins (RPs). As ribosome synthesis depends on RP mRNA translation, a priority within the translatome architecture should exist to ensure the preservation of ribosome biogenesis capacity, particularly under adverse growth conditions. Here, we show that under critical metabolic constraints characterized by mTOR inhibition, LARP1 complexed with the 40S subunit protects from ribophagy the mRNAs regulon for ribosome biogenesis and protein synthesis, acutely preparing the translatome to promptly resume ribosomes production after growth conditions return permissive. Characterizing the LARP1-protected translatome revealed a set of 5′TOP transcript isoforms other than RPs involved in energy production and in mitochondrial function, among other processes, indicating that the mTOR-LARP1-5′TOP axis acts at the translational level as a primary guardian of the cellular anabolic capacity.

Lysosomal degradation ensures accurate chromosomal segregation to prevent chromosomal instability

Lysosomes, as primary degradative organelles, are the endpoint of different converging pathways, including macroautophagy. To date, lysosome degradative function has been mainly studied in interphase cells, while their role during mitosis remains controversial. Mitosis dictates the faithful transmission of genetic material among generations, and perturbations of mitotic division lead to chromosomal instability, a hallmark of cancer. Heretofore, correct mitotic progression relies on the orchestrated degradation of mitotic factors, which was mainly attributed to ubiquitin-triggered proteasome-dependent degradation. Here, we show that mitotic transition also relies on lysosome-dependent degradation, as impairment of lysosomes increases mitotic timing and leads to mitotic errors, thus promoting chromosomal instability. Furthermore, we identified several putative lysosomal targets in mitotic cells. Among them, WAPL, a cohesin regulatory protein, emerged as a novel SQSTM1-interacting protein for targeted lysosomal degradation. Finally, we characterized an atypical nuclear phenotype, the toroidal nucleus, as a novel biomarker for genotoxic screenings. Our results establish lysosome-dependent degradation as an essential event to prevent chromosomal instability.Abbreviations: 3D: three-dimensional; APC/C: anaphase-promoting complex; ARL8B: ADP ribosylation factor like GTPase 8B; ATG: autophagy-related; BORC: BLOC-one-related complex; CDK: cyclin-dependent kinase; CENPE: centromere protein E; CIN: chromosomal instability; ConcA: concanamycin A; CQ: chloroquine; DAPI: 4,6-diamidino-2-penylinole; FTI: farnesyltransferase inhibitors; GFP: green fluorescent protein; H2B: histone 2B; KIF: kinesin family member; LAMP2: lysosomal associated membrane protein 2; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MEF: mouse embryonic fibroblast; MTOR: mechanistic target of rapamycin kinase; PDS5B: PDS5 cohesin associated factor B; SAC: spindle assembly checkpoint; PLEKHM2: pleckstrin homology and RUN domain containing M2; SQSTM1: sequestosome 1; TEM: transmission electron microscopy; ULK1: unc-51 like autophagy activating kinase 1; UPS: ubiquitin-proteasome system; v-ATPase: vacuolar-type H+-translocating ATPase; WAPL: WAPL cohesion release factor.

Nucleotide depletion reveals the impaired ribosome biogenesis checkpoint as a barrier against DNA damage

Many oncogenes enhance nucleotide usage to increase ribosome content, DNA replication, and cell proliferation, but in parallel trigger p53 activation. Both the impaired ribosome biogenesis checkpoint (IRBC) and the DNA damage response (DDR) have been implicated in p53 activation following nucleotide depletion. However, it is difficult to reconcile the two checkpoints operating together, as the IRBC induces p21‐mediated G1 arrest, whereas the DDR requires that cells enter S phase. Gradual inhibition of inosine monophosphate dehydrogenase (IMPDH), an enzyme required for de novo GMP synthesis, reveals a hierarchical organization of these two checkpoints. We find that the IRBC is the primary nucleotide sensor, but increased IMPDH inhibition leads to p21 degradation, compromising IRBC‐mediated G1 arrest and allowing S phase entry and DDR activation. Disruption of the IRBC alone is sufficient to elicit the DDR, which is strongly enhanced by IMPDH inhibition, suggesting that the IRBC acts as a barrier against genomic instability.

Phosphofructokinases Axis Controls Glucose-Dependent mTORC1 Activation Driven by E2F1

Cancer cells rely on mTORC1 activity to coordinate mitogenic signaling with nutrients availability for growth. Based on the metabolic function of E2F1, we hypothesize that glucose catabolism driven by E2F1 could participate on mTORC1 activation. Here, we demonstrate that glucose potentiates E2F1-induced mTORC1 activation by promoting mTORC1 translocation to lysosomes, a process that occurs independently of AMPK activation. We showed that E2F1 regulates glucose metabolism by increasing aerobic glycolysis and identified the PFKFB3 regulatory enzyme as an E2F1-regulated gene important for mTORC1 activation. Furthermore, PFKFB3 and PFK1 were found associated to lysosomes and we demonstrated that modulation of PFKFB3 activity, either by substrate accessibility or expression, regulates the translocation of mTORC1 to lysosomes by direct interaction with Rag B and subsequent mTORC1 activity. Our results support a model whereby a glycolytic metabolon containing phosphofructokinases transiently interacts with the lysosome acting as a sensor platform for glucose catabolism toward mTORC1 activity.

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Glucose potentiates E2F1-induced mTORC1 by promoting its translocation to lysosomes

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PFKFB3 activity is involved in the regulation of mTORC1 by glucose

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The glycolytic enzymes PFKFB3 and PFK1 were found associated to lysosomal surface

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PFKFB3 and PFK1 activities regulate mTORC1 lysosomal translocation

Oncogenic MYC Induces the Impaired Ribosome Biogenesis Checkpoint and Stabilizes p53 Independent of Increased Ribosome Content

The role of MYC in regulating p53 stability as a function of increased ribosome biogenesis is controversial. On the one hand, it was suggested that MYC drives the overexpression of ribosomal proteins (RP)L5 and RPL11, which bind and inhibit HDM2, stabilizing p53. On the other, it has been proposed that increased ribosome biogenesis leads the consumption of RPL5/RPL11 into nascent ribosomes, reducing p53 levels and enhancing tumorigenesis. Here, we show that the components that make up the recently described impaired ribosome biogenesis checkpoint (IRBC) complex, RPL5, RPL11, and 5S rRNA, are reduced following MYC silencing. This leads to a rapid reduction in p53 protein half-life in an HDM2-dependent manner. In contrast, MYC induction leads to increased ribosome biogenesis and p53 protein stabilization. Unexpectedly, there is no change in free RPL5/RPL11 levels, but there is a striking increase in IRBC complex bound to HDM2. Our data support a cell-intrinsic tumor-suppressor response to MYC expression, which is presently being exploited to treat cancer. SIGNIFICANCE: Oncogenic MYC induces the impaired ribosome biogenesis checkpoint, which could be potentially targeted for cancer treatment.

Autogenous Control of 5′TOP mRNA Stability by 40S Ribosomes

Ribosomal protein (RP) expression in higher eukaryotes is regulated translationally through the 5′TOP sequence. This mechanism evolved to more rapidly produce RPs on demand in different tissues. Here we show that 40S ribosomes, in a complex with the mRNA binding protein LARP1, selectively stabilize 5′TOP mRNAs, with disruption of this complex leading to induction of the impaired ribosome biogenesis checkpoint (IRBC) and p53 stabilization. The importance of this mechanism is underscored in 5q− syndrome, a macrocytic anemia caused by a large monoallelic deletion, which we found to also encompass the LARP1 gene. Critically, depletion of LARP1 alone in human adult CD34+ bone marrow precursor cells leads to a reduction in 5′TOP mRNAs and the induction of p53. These studies identify a 40S ribosome function independent of those in translation that, with LARP1, mediates the autogenous control of 5′TOP mRNA stability, whose disruption is implicated in the pathophysiology of 5q− syndrome.

Abstract PR02: A novel 40S complex involved in the regulation of 5' TOP mRNA stability: A role in mTOR hyperactivated tumors?

The ability of ribosomes to translate genetic information into protein is a hallmark of evolution. However, during cancer progression, tumors often hijack and boost ribosome biogenesis in their favor, which they rely on for high rates of proliferation. In eukaryotes four distinct ribosomal RNAs (rRNAs) and approximately 80 different ribosomal proteins (RPs) are assembled into either the small 40S or large 60S subunit, while more than 300 additional factors participate in the complex process of ribosome biogenesis. Moreover, with the rise of multicellular organisms in higher eukaryotes a hierarchical translational control mechanism of cellular RP mRNAs has evolved. This mechanism has been put into place to guarantee that the building blocks of ribosomes are preferentially translated and integrated with those of the transcriptionally regulated rRNAs, in order to endow the cell with adequate protein synthetic capacity in different tissues upon cellular demand. In higher metazoans this translational mechanism is regulated through a cis regulatory element at the transcriptional start site of RP mRNAs, termed a 5'-terminal oligonucleotide pyrimidine (5' TOP) sequence. We previously demonstrated that 5' TOP mRNA expression at the translational levels is controlled by mTORC1 signaling, a central nexus of cellular growth and proliferation that is often exploited by tumor cells. Although it is thought that 5' TOP translation is mediated by mTORC1, new findings have suggested that their stability also may be selectively controlled by the 5' TOP element. Here we show that the 40S ribosomal subunit, in concert with RNA binding protein LARP1, take on this responsibility, through forming a novel complex with 5' TOP mRNAs. Importantly, we show that the repertoire of 5' TOPs mRNAs recognized by this new 40S complex encompasses categories of transcripts not only belonging to RP and translational factor mRNAs, but also to previously unidentified 5' TOPs involved in distinct cellular processes. LARP1 overexpression, like increased ribosome biogenesis is associated with many aggressive cancers. Thus the functional interaction between LARP1, the 40S subunit and 5' TOPs constitutes a cellular reservoir of RP mRNAs and other factors that can be translationally mobilized or suppressed upon cellular growth demands, particularly in tumors where mTOR signaling is hyperactivated.

This abstract is also being presented as Poster A27.

Citation Format: Antonio Gentilella, Francisco D. Morón-Duran, Guilherme Z. Rocha, Ferran Riaño-Canalias, Joffrey Pelletier, Albert Tauler, Sara Kozma, George Thomas. A novel 40S complex involved in the regulation of 5' TOP mRNA stability: A role in mTOR hyperactivated tumors? [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr PR02.

V-ATPase: a master effector of E2F1-mediated lysosomal trafficking, mTORC1 activation and autophagy

In addition to being a master regulator of cell cycle progression, E2F1 regulates other associated biological processes, including growth and malignancy. Here, we uncover a regulatory network linking E2F1 to lysosomal trafficking and mTORC1 signaling that involves v-ATPase regulation. By immunofluorescence and time-lapse microscopy we found that E2F1 induces the movement of lysosomes to the cell periphery, and that this process is essential for E2F1-induced mTORC1 activation and repression of autophagy. Gain- and loss-of-function experiments reveal that E2F1 regulates v-ATPase activity and inhibition of v-ATPase activity repressed E2F1-induced lysosomal trafficking and mTORC1 activation. Immunoprecipitation experiments demonstrate that E2F1 induces the recruitment of v-ATPase to lysosomal RagB GTPase, suggesting that E2F1 regulates v-ATPase activity by enhancing the association of V₀ and V₁ v-ATPase complex. Analysis of v-ATPase subunit expression identified B subunit of V₀ complex, ATP6V0B, as a transcriptional target of E2F1. Importantly, ATP6V0B ectopic-expression increased v-ATPase and mTORC1 activity, consistent with ATP6V0B being responsible for mediating the effects of E2F1 on both responses. Our findings on lysosomal trafficking, mTORC1 activation and autophagy suppression suggest that pharmacological intervention at the level of v-ATPase may be an efficacious avenue for the treatment of metastatic processes in tumors overexpressing E2F1.

Effect of low doses of actinomycin D on neuroblastoma cell lines

BACKGROUND

Neuroblastoma is a malignant embryonal tumor occurring in young children, consisting of undifferentiated neuroectodermal cells derived from the neural crest. Current therapies for high-risk neuroblastoma are insufficient, resulting in high mortality rates and high incidence of relapse. With the intent to find new therapies for neuroblastomas, we investigated the efficacy of low-doses of actinomycin D, which at low concentrations preferentially inhibit RNA polymerase I-dependent rRNA trasncription and therefore, ribosome biogenesis.

METHODS

Neuroblastoma cell lines with different p53 genetic background were employed to determine the response on cell viability and apoptosis of low-dose of actinomycin D. Subcutaneously-implanted SK-N-JD derived neuroblastoma tumors were used to assess the effect of low-doses of actinomycin D on tumor formation.

RESULTS

Low-dose actinomycin D treatment causes a reduction of cell viability in neuroblastoma cell lines and that this effect is stronger in cells that are wild-type for p53. MYCN overexpression contributes to enhance this effect, confirming the importance of this oncogene in ribosome biogenesis. In the wild-type SK-N-JD cell line, apoptosis was the major mechanism responsible for the reduction in viability and we demonstrate that treatment with the MDM2 inhibitor Nutlin-3, had a similar effect to that of actinomycin D. Apoptosis was also detected in p53(-/-)deficient LA1-55n cells treated with actinomycin D, however, only a small recovery of cell viability was found when apoptosis was inhibited by a pan-caspase inhibitor, suggesting that the treatment could activate an apoptosis-independent cell death pathway in these cells. We also determined whether actinomycin D could increase the efficacy of the histone deacetylase inhibitor, SAHA, which is in being used in neuroblastoma clinical trials. We show that actinomycin D synergizes with SAHA in neuroblastoma cell lines. Moreover, on subcutaneously-implanted neuroblastoma tumors derived from SK-N-JD cells, actinomycin D led to tumor regression, an effect enhanced in combination with SAHA.

CONCLUSIONS

The results presented in this work demonstrate that actinomycin D, at low concentrations, inhibits proliferation and induces cell death in vitro, as well as tumor regression in vivo. From this study, we propose that use of ribosome biogenesis inhibitors should be clinically considered as a potential therapy to treat neuroblastomas.

MYCN concurrence with SAHA-induced cell death in human neuroblastoma cells

BACKGROUND

In the past, the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) has been shown to induce apoptosis in several human tumor types, including neuroblastomas. Amplification and over-expression of the MYCN oncogene is a diagnostic hallmark and a poor prognostic indicator in high-risk neuroblastomas. Here, we studied the relationship between MYCN amplification and over-expression and the anti-tumor effect of SAHA to assess whether this drug may serve as a treatment option for high-risk neuroblastomas.

METHODS

Different human neuroblastoma cell lines, over-expressing or not over-expressing MYCN, were used in this study. Targeted knockdown and exogenous over-expression of MYCN were employed to examine correlations between MYCN expression levels and SAHA responses. After various time periods and concentration exposures to the drug, cell viability was measured by MTS assay, and variations in MYCN mRNA and protein levels were assessed by qPCR and Western blotting, respectively.

RESULTS

We found that SAHA decreased cell viability in all cell lines tested through apoptosis induction, and that SAHA had a stronger effect on cell lines carrying an amplified MYCN gene. A decrease in MYCN mRNA and protein levels was observed in the SAHA treated cell lines. Subsequent silencing and exogenous over-expression of MYCN changed the proliferation rate of the cells, but did not have any significant impact on the effect of SAHA on the viability of the cells. We also found that SAHA blocked the expression of MYCN and, by doing so, reduced the effects mediated by this protein.

CONCLUSIONS

Our results suggest that SAHA may be used as a single-drug treatment option for neuroblastomas with an amplified MYCN gene, and as an adjuvant treatment option for all neuroblastomas.

S6K1 controls pancreatic β cell size independently of intrauterine growth restriction

Type 2 diabetes mellitus (T2DM) is a worldwide heath problem that is characterized by insulin resistance and the eventual loss of β cell function. As recent studies have shown that loss of ribosomal protein (RP) S6 kinase 1 (S6K1) increases systemic insulin sensitivity, S6K1 inhibitors are being pursued as potential agents for improving insulin resistance. Here we found that S6K1 deficiency in mice also leads to decreased β cell growth, intrauterine growth restriction (IUGR), and impaired placental development. IUGR is a common complication of human pregnancy that limits the supply of oxygen and nutrients to the developing fetus, leading to diminished embryonic β cell growth and the onset of T2DM later in life. However, restoration of placental development and the rescue of IUGR by tetraploid embryo complementation did not restore β cell size or insulin levels in S6K1-/- embryos, suggesting that loss of S6K1 leads to an intrinsic β cell lesion. Consistent with this hypothesis, reexpression of S6K1 in β cells of S6K1-/- mice restored embryonic β cell size, insulin levels, glucose tolerance, and RPS6 phosphorylation, without rescuing IUGR. Together, these data suggest that a nutrient-mediated reduction in intrinsic β cell S6K1 signaling, rather than IUGR, during fetal development may underlie reduced β cell growth and eventual development of T2DM later in life.

ROS production is essential for the apoptotic function of E2F1 in pheochromocytoma and neuroblastoma cell lines

In this study we demonstrate that accumulation of reactive oxygen species (ROS) is essential for E2F1 mediated apoptosis in ER-E2F1 PC12 pheochromocytoma, and SH-SY5Y and SK-N-JD neuroblastoma stable cell lines. In these cells, the ER-E2F1 fusion protein is expressed in the cytosol; the addition of 4-hydroxytamoxifen (OHT) induces its translocation to the nucleus and activation of E2F1target genes. Previously we demonstrated that, in ER-E2F1 PC12 cells, OHT treatment induced apoptosis through activation of caspase-3. Here we show that caspase-8 activity did not change upon treatment with OHT. Moreover, over-expression of Bcl-xL arrested OHT-induced apoptosis; by contrast, over-expression of c-FLIP, did not have any effect on OHT-induced apoptosis. OHT addition induces BimL expression, its translocation to mitochondria and activation of Bax, which is paralleled by diminished mitochondrial enrichment of Bcl-xL. Treatment with a Bax-inhibitory peptide reduced OHT-induced apoptosis. These results point out the essential role of mitochondria on the apoptotic process driven by E2F1. ROS accumulation followed E2F1 induction and treatment with the antioxidant N-acetylcysteine, inhibited E2F1-induced Bax translocation to mitochondria and subsequent apoptosis. The role of ROS in mediating OHT-induced apoptosis was also studied in two neuroblastoma cell lines, SH-SY5Y and SK-N-JD. In SH-SY5Y cells, activation of E2F1 by the addition of OHT induced ROS production and apoptosis, whereas over-expression of E2F1 in SK-N-JD cells failed to induce either response. Transcriptional profiling revealed that many of the genes responsible for scavenging ROS were down-regulated following E2F1-induction in SH-SY5Y, but not in SK-N-JD cells. Finally, inhibition of GSK3β blocked ROS production, Bax activation and the down regulation of ROS scavenging genes. These findings provide an explanation for the apparent contradictory role of E2F1 as an apoptotic agent versus a cell cycle activator.

Histone deacetylase inhibition induces apoptosis and autophagy in human neuroblastoma cells

Neuroblastoma (NB) is the most common solid extracranial tumor in children. Here we showed that trichostatin A, a histone deacetylase inhibitor (HDACi), decreases cell viability in three NB cell lines of different phenotypes. The treatment leads to G2/M-phase arrest, apoptosis and autophagy. Autophagy induction accompanies apoptosis in the most proliferative, N-Myc overexpressing cells. In contrast, autophagy precedes apoptosis and acts as a protective mechanism in the less proliferative, non-N-Myc overexpressing cells. Therefore, the autophagy induction is a relevant event in the NB response to HDACis, and it should be considered in the design of new treatments for this malignancy.

Competition between the signal sequence and a 3'UTR localisation signal during redirection of beta-globin mRNA to the endoplasmic reticulum: implications for biotechnology

Secretion of an intracellular protein from a cell factory requires as a first step the redirection of the mRNA for synthesis of the protein on the endoplasmic reticulum. The feasibility of retargeting a mRNA coding for an intracellular protein to the endoplasmic reticulum was investigated using Ltk- fibroblasts stably transfected with gene constructs in which rabbit beta-globin coding region and 5'UTR was linked to albumin signal sequence and different 3'untranslated regions. Globin transcripts with the native globin 3'untranslated region or with the 3'untranslated region of c-myc are present in free/cytoskeletal-bound polysomes. The addition of the signal sequence from rat albumin redirects both these globin transcripts to membrane-bound polysomes but the presence of the c-myc 3'UTR reduces the extent of redirection. Globin transcripts with both the signal sequence and 3'untranslated region from the albumin gene are efficiently redirected to membrane-bound polysomes. The results suggest competition between 5' and 3' localising signals. The addition of the signal sequence does not destabilise the mRNA nor affect translational efficiency. It is concluded that it is possible to retarget an mRNA to the endoplasmic reticulum while maintaining stability and translational capacity. This has important implications for the development of vectors to promote secretion of intracellular proteins from cell factories.

E2F1 regulates cellular growth by mTORC1 signaling

During cell proliferation, growth must occur to maintain homeostatic cell size. Here we show that E2F1 is capable of inducing growth by regulating mTORC1 activity. The activation of cell growth and mTORC1 by E2F1 is dependent on both E2F1's ability to bind DNA and to regulate gene transcription, demonstrating that a gene induction expression program is required in this process. Unlike E2F1, E2F3 is unable to activate mTORC1, suggesting that growth activity could be restricted to individual E2F members. The effect of E2F1 on the activation of mTORC1 does not depend on Akt. Furthermore, over-expression of TSC2 does not interfere with the effect of E2F1, indicating that the E2F1-induced signal pathway can compensate for the inhibitory effect of TSC2 on Rheb. Immunolocalization studies demonstrate that E2F1 induces the translocation of mTORC1 to the late endosome vesicles, in a mechanism dependent of leucine. E2F1 and leucine, or insulin, together affect the activation of S6K stronger than alone suggesting that they are complementary in activating the signal pathway. From these studies, E2F1 emerges as a key protein that integrates cell division and growth, both of which are essential for cell proliferation.

Increase in Fru-2,6-P(2) levels results in altered cell division in Schizosaccharomyces pombe

Mitogenic response to growth factors is concomitant with the modulation they exert on the levels of Fructose 2,6-bisphosphate (Fru-2,6-P2), an essential activator of the glycolytic flux. In mammalian cells, decreased Fru-2,6-P2 concentration causes cell cycle delay, whereas high levels of Fru-2,6-P2 sensitize cells to apoptosis. In order to analyze the cell cycle consequences due to changes in Fru-2,6-P2 levels, the bisphosphatase-dead mutant (H258A) of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase enzyme was over-expressed in Schizosaccharomyces pombe cells and the variation in cell phenotype was studied. The results obtained demonstrate that the increase in Fru-2,6-P2 levels results in a defective division of S. pombe, as revealed by an altered multisepted phenotype. The H258A-expressing cells showed impairment of cytokinesis, but normal nuclear division. In order to identify cellular mediators responsible for this effect, we transformed different S. pombe strains and observed that the cytokinetic defect was absent in cells defective for Wee1 kinase function. Therefore, in S. pombe, Wee1 integrates the metabolic signal emerging from changes in Fru-2,6-P2 content, thus coupling metabolism with cell proliferation. As the key regulators of the cell cycle checkpoints are conserved throughout evolution, these results may help to understand the experimental evidences obtained by manipulation of Fru-2,6-P2 levels in mammalian cells.

Apoptotic action of E2F1 requires glycogen synthase kinase 3-β activity in PC12 cells

Both E2F1 and GSK3beta have been described as essential targets in neuronal apoptosis. Previous studies have demonstrated that GSK3beta binds to E2F1 in vivo. We wanted to investigate whether these proteins could share a common apoptotic signal pathway in neuronal cells. With this intention, we developed a PC12 ER-E2F1 stable cell line in which E2F1 activity was dependent on the presence of 4-hydroxitamoxifen. E2F1 activation produced apoptosis in naive and post-mitotic cells; serum and nerve growth factor respectively protected them from E2F1 apoptotic stimuli. The presence of specific GSK3beta inhibitors SB216763 and LiCl completely protected cells from apoptosis induced by E2F1 activation. In addition, knocked down GSK3beta experiments by small interference RNAs have demonstrated that a reduction of GSK3beta protein levels can lower the apoptotic effect of E2F1. Finally, we demonstrated that the apoptotic effect of E2F1 is not due to the regulation of GSK3beta activity, and that the inhibitory effect of GSK3beta inhibitor SB216763 on E2F1 induced apoptosis could be due to an alteration in the E2F1-regulated transcription gene pattern. In summary, we have demonstrated that the apoptotic action of E2F1 requires GSK3beta activity.

Glycogen synthase kinase-3beta binds to E2F1 and regulates its transcriptional activity

GSK3beta and E2F1 play an important role in the control of proliferation and apoptosis. Previous work has demonstrated that GSK3beta indirectly regulates E2F activity through modulation of cyclin D1 levels. In this work we show that GSK3beta phosphorylates human E2F1 in vitro at serine 403 and threonine 433, both residues localized at its transactivation domain. This phosphorylation was not detected in vivo. However, co-immunoprecipitation experiments do reveal in vivo binding of these proteins. Moreover, uninhibitable and catalitycally inactive GSK3beta forms inhibit the transcriptional activity of a fusion protein containing E2F1 transactivation domain. Both forms of GSK3beta inhibit E2F1 with similar efficiency. Interestingly the effect was independent of the mutation of serine 403 and threonine 433 to alanine. This suggests that this transcriptional modulation is independent of GSK3beta kinase activity and phosphorylation state of serine 403 and threonine 433. The re-targeting of these GSK3beta forms to the nucleus results in a higher capacity to regulate E2F1 transcriptional activity. Depletion of the levels of GSK3beta protein using siRNA activates E2F1 transcriptional activity. The data presented in this study offer a new mechanism of regulation of E2F1 by direct binding of GSK3beta to its transactivation domain.

An E2F-binding site mediates the activation of the proliferative isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by phosphatidylinositol 3-kinase

In the present study, we demonstrate that E2F is implicated in the regulation of the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) during cell division. The expression of this enzyme is induced during the G(1)/S transition of the cell cycle. We identified and monitored the E2F-pocket protein complexes that bind to the E2F site of the F-type promoter during cell-cycle entry, and we analysed their contribution to the phosphatidylinositol 3-kinase (PI 3-kinase)-mediated regulation of the promoter. We found that the predominant E2F complex bound to the F-type promoter in unstimulated/quiescent cells contains E2F4, DP1 and p130 proteins. In serum-stimulated (S-phase) cells, the composition of the complex switched to E2F1/4, DP1 and p107, together with cyclin A and cyclin-dependent kinase 2. Treatment with the PI 3-kinase specific inhibitor LY 294002 prevented the formation of the S-phase complex, suggesting that activation of the PI 3-kinase pathway is essential for the formation of this complex. Further supporting this idea, we obtained results showing that treatment of cycling NIH 3T3 cells with either wortmannin or LY 294002 induces the accumulation of the transcriptionally repressive p130-E2F4-DP1 complex. Using the Rat-1 ER-E2F1 cell line where E2F1 activity can be conditionally induced, we demonstrated that E2F activity is involved in the in vivo transcriptional regulation of the F-type 6PF2K/Fru-2, 6-BPase gene. Taken together, our results show that the F-type 6PF2K/Fru-2, 6-BPase is a genuine E2F-regulated gene, and that its regulation by the PI 3-kinase pathway is at least partially mediated through the E2F transcription factor.

An intronic AP-1 sequence mediates the transcriptional activation of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by serum

The expression of F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is rapidly induced by growth factors. We report here that an AP-1 intragenic sequence located at position +612 of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase transcription initiation site is involved in the transcriptional activation of this gene by serum. We have demonstrated in vitro DNA-protein interaction on this AP-1 site of the F-promoter. Indeed, this element was recognized by c-Fos and JunD in vitro, and mutation or deletion of this element reduced the early response to serum stimulation by 60%. We conclude that the serum response of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene requires the co-ordinated function of ets, E2F and AP-1 binding sites.

Regulation of SGLT1 expression in response to Na(+) intake

In the chicken intestine, the reduction in Na(+) intake led to a decrease in the transport of alpha-methyl-D-glucoside in the ileum (reduction of 42%) and in the rectum (51%). These reductions were reversed within 24 h after resalination and were inversely correlated to the changes in aldosterone plasma concentration. The reduction in intestinal hexose transport in the low Na(+)-fed animals was due to a decrease in the number of Na(+)-dependent D-glucose cotransporters (SGLT1) in the rectum (46%) and in the ileum (38%). Northern blot analysis showed that specific SGLT1 mRNA was expressed in the jejunum, ileum, and rectum. The amount of SGLT1 mRNA was the same in all intestinal regions and was not affected by Na(+) intake, supporting the view that the effects of dietary Na(+) on intestinal hexose transport involve posttranscriptional regulation of SGLT1. This study suggests that changes in SGLT1 expression may be involved in the homeostasis of Na(+).

Insulin inhibits glucocorticoid-stimulated L-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression by activation of the c-Jun N-terminal kinase pathway

The hepatic isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PF2K/Fru-2,6-BPase) is transcriptionally stimulated by glucocorticoids, whereas insulin blocks this stimulatory effect. Although this inhibitory effect has been extensively reported, nothing is known about the signalling pathway responsible. We have used well-characterized inhibitors for proteins involved in different signalling cascades to assess the involvement of these pathways on the transcriptional regulation of glucocorticoid-stimulated PF2K/Fru-2,6-BPase by insulin. Our results demonstrate that the phosphoinositide 3-kinase, p70/p85 ribosomal S6 kinase, extracellular signal-regulated protein kinase (ERK)1/2 and p38 mitogen-activated protein (MAP) kinase pathways are not involved in the inhibitory effect of insulin on glucocorticoid-stimulated PF2K/Fru-2,6-BPase. To evaluate the implication of the MAP kinase/ERK kinase (MEK)-4-stress-activated protein kinase-c-Jun-N-terminal protein kinase ('JNK-SAPK') pathway we overexpressed the N-terminal JNK-binding domain of the JNK-interacting protein 1 ('JIP-1'), demonstrating that activation of JNK is necessary for the insulin inhibitory effect. Moreover, overexpression of MEK kinase 1 and JNK-haemagglutinin resulted in the inhibition of the glucocorticoid-stimulated PF2K/Fru-2,6-BPase. These results provide clear and specific evidence for the role of JNK in the insulin inhibition of glucocorticoid-stimulated PF2K/Fru-2,6-BPase gene expression. In addition, we performed experiments with a mutant of the glucocorticoid receptor in which the JNK phosphorylation target Ser-246 had been mutated to Ala. Our results demonstrate that the phosphorylation of the glucocorticoid receptor on Ser-246 is not responsible for the JNK repression of glucocorticoid-stimulated PF2K/Fru-2,6-BPase gene expression.

Activation of phosphatidylinositol 3-kinase is required for transcriptional activity of F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: assessment of the role of protein kinase B and p70 S6 kinase

Previous studies have demonstrated that the F isoform of<hsp sp=0.5>6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase(6PF2K/Fru-2,6-BPase) is transcriptionally regulated by growth factors. The aim of this study was to investigate the importance of the phosphatidylinositol 3-kinase (PI 3-kinase) pathway in the regulation of 6PF2K/Fru-2,6-BPase gene expression. We have completed studies using chemical inhibitors and expression vectors for the proteins involved in this signalling cascade. Treatment of cells with LY 294002, an inhibitor of PI 3-kinase, blocked the epidermal growth factor (EGF)-dependent stimulation of 6PF2K/Fru-2,6-BPase gene transcription. Transient transfection of a constitutively active PI 3-kinase was sufficient to activate transcription from the F-type 6PF2K/Fru-2,6-BPase promoter. In contrast, co-transfection with a dominant-negative form of PI 3-kinase completely abrogated the stimulation by EGF, and down-regulated the basal promoter activity. In an attempt to determine downstream proteins that lie between PI 3-kinase and 6PF2K/Fru-2,6-BPase gene expression, the overexpression of a constitutively active form of protein kinase B (PKB) was sufficient to activate 6PF2K/Fru-2,6-BPase gene expression, even in the presence of either a dominant-negative form of PI 3-kinase or LY 294002. The over-expression of p70/p85 ribosomal S6 kinase or the treatment with its inhibitor rapamycin did not affect 6PF2K/Fru-2,6-BPase transcription. We conclude that PI 3-kinase is necessary for the transcriptional activity of F-type 6PF2K/Fru-2,6-BPase, and that PKB is a downstream effector of PI 3-kinase directly involved in the regulation of 6PF2K/Fru-2,6-BPase gene expression.

Contractile activity modifies Fru-2,6-P(2) metabolism in rabbit fast twitch skeletal muscle

Modification of muscular contractile patterns by denervation and chronic low frequency stimulation induces structural, physiological, and biochemical alterations in fast twitch skeletal muscles. Fructose 2,6-bisphosphate is a potent activator of 6-phosphofructo-1-kinase, a key regulatory enzyme of glycolysis in animal tissues. The concentration of Fru-2,6-P(2) depends on the activity of the bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2), which catalyzes the synthesis and degradation of this metabolite. This enzyme has several isoforms, the relative abundance of which depends on the tissue metabolic properties. Skeletal muscle expresses two of these isoforms; it mainly contains the muscle isozyme (M-type) and a small amount of the liver isozyme (L-type), whose expression is under hormonal control. Moreover, contractile activity regulates expression of muscular proteins related with glucose metabolism. Fast twitch rabbit skeletal muscle denervation or chronic low frequency stimulation can provide information about the regulation of this enzyme. Our results show an increase in Fru-2,6-P(2) concentration after 2 days of denervation or stimulation. In denervated muscle, this increase is mediated by a rise in liver PFK-2/FBPase-2 isozyme, while in stimulated muscle it is mediated by a rise in muscle PFK-2/FBPase-2 isozyme. In conclusion, our results show that contractile activity could alter the expression of PFK-2/FBPase-2.

Effect of starvation on gene expression of regulatory enzymes of glycolysis/gluconeogenesis in genetically obese (fa/fa) Zucker rats

OBJECTIVE

To study the mechanism that controls fructose-2,6-bisphosphate (Fru-2,6-P2) accumulation, as well as the mRNAs levels of the glycolytic/gluconeogenic regulatory enzymes in the livers of fed and starved lean (fa/-) and obese (fa/fa) Zucker rats.

DESIGN

Rats were fed a standard chow or deprived of food for 24 h.

SUBJECTS

Male lean (fa/-) and genetically obese (fa/fa) rats (nine weeks old).

MEASUREMENTS

Fru-2,6-P2 concentration, 6-phosphofructo-2-kinase (PFK-2), glucokinase (GK), pyruvate kinase (PK) activities and the mRNA levels of GK, PFK-2, L-type pyruvate kinase, fructose-1,6-bisphosphatase (FBPase-1) and phosphoenolpyruvate carboxykinase (PEPCK) were analyzed.

RESULTS

PFK-2/FBPase-2 mRNA decreased during starvation in both fa/- and fa/fa animals. Although PFK-2/FBPase-2 mRNA levels were similar in fed lean and obese rats, PFK-2 concentration and activity were higher in fed obese than in fed lean animals, which might explain the high concentration of Fru-2,6-P2 observed in obese animals. During starvation, PFK-2 protein concentration decreased, correlating with the enzymatic activity and Fru-2,6-P2 levels. The activities of GK and L-pyruvate kinase (L-PK) also increased in fed obese (fa/fa) rats compared with fed lean (fa/-) animals, but decreased during starvation. The mRNA levels of glycolytic enzymes in fed obese rats were similar (PFK-2) or higher than (GK, L-PK) in fed lean animals. During starvation, they decreased in lean and obese rats with one important exception, GK mRNA remained high in obese animals. The mRNA of gluconeogenic enzymes remained constant (FBPase-1) or increased (PEPCK) during fasting.

CONCLUSION

The changes observed might be explained by the hyperinsulinaemia observed in the liver of obese rats, which might lead to the stimulation of glycolysis and lipogenesis.

Expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA during liver regeneration

The expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA was studied during liver regeneration by three independent assays: Northern blot analysis, reverse transcription-polymerase chain reaction and ribonuclease protection. We demonstrate the presence of F-type mRNA in foetal and adult rat livers and a transient increase in its levels with a maximum at 12 h after partial liver resection. The time course of F-type mRNA induction differs from that reported for the L-type isoform, suggesting differences in the regulation of the expression of F- and L-type isoforms of the bifunctional enzyme during liver regeneration.

Effect of growth factors on the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Rat-1 fibroblasts

The activation of glycolytic flux is a biochemical characteristic of growing cells. Several reports have demonstrated the role of fructose 2,6-bisphosphate in this process. In this paper we show that the levels of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase (6PF2K/Fru-2,6-P2ase) mRNA are modulated in response to serum and growth factors and this effect is due to regulation of its transcription rate. The modulation of the expression of this enzyme by growth factors differs according their mitogenic effect; both lysophosphatidic acid and epidermal growth factor, when added alone, increased the mRNA levels, but endothelin had no effect. Furthermore, cAMP, which acts as an antimitogenic signal in Rat-1 fibroblasts, produced a decrease in 6PF2K/Fru-2, 6-P2ase mRNA and inhibited the effects of lysophosphatidic acid and epidermal growth factor on 6PF2K/Fru-2,6-P2ase expression. PD 098059, a specific inhibitor of the activation of the mitogen-activated protein kinase, was able to prevent the effect of EGF on 6PF2K/Fru-2, 6-P2ase gene expression. These results imply that activation of mitogen-activated protein kinase is required for the stimulation of the transcription of 6PF2K/Fru-2,6-P2ase by EGF.

Hepatocyte growth factor and transforming growth factor beta regulate 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression in rat hepatocyte primary cultures

Hepatocyte growth factor (HGF) and transforming growth factor beta (TGF-beta) are believed to be of major importance for hepatic regeneration after liver damage. We have studied the effect of these growth factors on fructose 2,6-bisphosphate (Fru-2,6-P2) levels and the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) in rat hepatocyte primary cultures. Our results demonstrate that HGF activates the expression of the 6PF2K/Fru-2,6-BPase gene by increasing the levels of its mRNA. As a consequence of this activation, the amount of 6PF2K/Fru-2,6-BPase protein and 6-phosphofructo-2-kinase activity increased, which was reflected by a rise in Fru-2,6-P2 levels. In contrast, TGF-beta decreased the levels of 6PF2K/Fru-2,6-BPase mRNA, which led to a decrease in the amount of 6PF2K/Fru-2,6-BPase protein and Fru-2,6-P2. The different actions of HGF and TGF-beta on 6PF2K/Fru-2,6-BPase gene expression are concomitant with their effect on cell proliferation. Here we show that, in the absence of hormones, primary cultures of hepatocytes express the F-type isoenzyme. In addition, HGF increases the expression of this isoenzyme, and dexamethasone activates the L-type isoform. HGF and TGF-beta were able to inhibit this activation.

Role of the N-terminal region in covalent modification of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: comparison of phosphorylation and ADP-ribosylation

The effect of cyclic AMP (cAMP)-dependent phosphorylation and ADP-ribosylation on the activities of the rat liver bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2), was investigated in order to determine the role of the N-terminus in covalent modification of the enzyme. The bifunctional enzyme was demonstrated to be a substrate in vitro for arginine-specific ADP-ribosyltransferase: 2 mol of ADP-ribose was incorporated per mol of subunit. The Km values for NAD+ and PFK-2/FBPase-2 were 14 microM and 0.4 microM respectively. A synthetic peptide (Val-Leu-Gln-Arg-Arg-Arg-Gly-Ser-Ser-Ile-Pro-Gln) corresponding to the site phosphorylated by cAMP-dependent protein kinase was ADP-ribosylated on all three arginine residues. Analysis of ADP-ribosylation of analogue peptides containing only two arginine residues, with the third replaced by alanine, revealed that ADP-ribosylation occurred predominantly on the two most C-terminal arginine residues. Sequencing of the ADP-ribosylated native enzyme also demonstrated that the preferred sites were at Arg-29 and Arg-30, which are just N-terminal to Ser-32, whose phosphorylation is catalysed by cAMP-dependent protein kinase (PKA). ADP-ribosylation was independent of the phosphorylation state of the enzyme. Furthermore, ADP-ribosylation of the enzyme decreased its recognition by liver-specific anti-bifunctional-enzyme antibodies directed to its unique N-terminal region. ADP-ribosylation of PFK-2/FBPase-2 blocked its phosphorylation by PKA, and decreased its PFK-2 activity, but did not alter FBPase-2 activity. In contrast, cAMP-dependent phosphorylation inhibited the kinase and activated the bisphosphatase. These results demonstrate that ADP-ribosylation of arginine residues just N-terminal to the site phosphorylated by PKA modulate PFK-2 activity by an electrostatic and/or steric mechanism which does not involved uncoupling of N- and C-terminal interactions as seen with cAMP-dependent phosphorylation.

Gene expression of enzymes regulating ketogenesis and fatty acid metabolism in regenerating rat liver

Levels of mRNA for mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, carnitine palmitoyltransferase I (CPT I) and carnitine palmitoyltransferase II (CPT II), fatty acid synthase (FAS) and actin were analysed during liver regeneration. mRNA levels for mitochondrial HMG-CoA synthase decreased rapidly, reaching a minimum 12 h after partial hepatectomy and returning to normal at 24-36 h. In contrast, CPT I, CPT II and FAS mRNAs increased throughout the period examined. Expression of actin increased significantly during regeneration. Levels of mRNA for mitochondrial HMG-CoA synthase also decreased as a result of surgical stress, although the effect of hepatectomy was much greater. We determined the levels of mitochondrial HMG-CoA synthase using specific antibodies. The amount of protein rapidly decreased, although less markedly than the corresponding mRNA levels. These results show that the decrease described in ketogenesis in partially hepatectomized rats correlated with the decrease in the expression of mitochondrial HMG-CoA synthase, suggesting that this enzyme may also be a control point in ketogenesis in the regenerating liver, as it is in normal and diabetic rats.

Regulation of hepatic 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene expression by glucagon

The control of hepatic 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene expression by glucagon was studied. Intraperitoneal administration of glucagon rapidly decreased the fructose 2,6-bisphosphate content by phosphorylation of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase and diminution of its Vmax. Immunologic studies using a specific liver antibody showed that the amount of enzyme rapidly decreased. Northern blot analysis showed that the isozyme expressed is the adult liver form. The 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase mRNA content decreased, whereas that of phosphoenolpyruvate carboxykinase increased, and that of albumin did not change. Run-on transcription assays with isolated nuclei showed inhibition in the relative transcription rate of the 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene and a stimulation of phosphoenolpyruvate carboxykinase gene. The regulation of mRNA stability of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase by glucagon was also studied. The half-life of mRNA decreased in the presence of glucagon, suggesting that proteins modulated by a glucagon-dependent process are regulating its stability. The time course of mRNA levels correlated with the transcription inhibition of gene and destabilization of mRNA, indicating that glucagon modulates 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene expression at both transcriptional and posttranscriptional levels.

Gene expression of regulatory enzymes of glycolysis/gluconeogenesis in regenerating rat liver

Levels of mRNA for glucokinase, L-pyruvate kinase, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase were analysed during liver regeneration. Levels of mRNA for glycolytic enzymes (glucokinase and L-pyruvate kinase) decreased rapidly after partial hepatectomy. Glucokinase mRNA increased at 16-24 h, returning to normal values after this time. L-pyruvate kinase mRNA recovered control levels at 168 h. In contrast, phosphoenolpyruvate carboxykinase mRNA increased rapidly after liver resection and remained high during the regenerative process. However, the levels of fructose-1,6-bisphosphatase mRNA were not modified significantly. These results correlate with the reported increased rate of gluconeogenesis and changes in enzyme levels after partial hepatectomy. The effect of stress on the mRNA levels was also studied. All enzymes showed variations in their mRNA levels after the surgical stress. In general, the differences were more pronounced in regenerating liver than in sham-operated animals, being practically normalized at 24 h.

Transcriptional and posttranscriptional regulation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase during liver regeneration

The control of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2; EC 2.7.1.105/3.1.3.46) gene expression during liver regeneration was studied. The level of PFK-2/FBPase-2 mRNA decreased to about 5% of the control value 6 hr after partial hepatectomy. Thereafter the mRNA increased to a maximum at 48 hr and returned to normal levels by 96 hr. In sham-operated animals, only a small increase was observed during the first 4 hr. The mRNA was recognized by a 299-base-pair liver-specific cDNA probe but not by a muscle-specific probe. The time course of mRNA modulation was well correlated with PFK-2/FBPase-2 activity and with the amount of bifunctional enzyme protein determined by immunoblotting with an antibody raised against the N-terminal decapeptide of liver PFK-2/FBPase-2. No alteration in the degradation rate of PFK-2/FBPase-2 mRNA was noted after partial hepatectomy. The modulation of PFK-2/FBPase-2 gene expression during liver regeneration involved changes in the transcription rate. The rate decreased by 50% at 6 hr after liver resection. The rate increased thereafter to a maximum at 72 hr and then returned to control values by 96 hr. The transcription rate of albumin did not change, whereas that of phosphoenolpyruvate carboxykinase increased 12-fold at 6 hr. These results show that PFK-2/FBPase-2 gene transcription is specifically regulated and that this regulation is in part responsible for the alterations in hepatic metabolism seen in regenerating liver.

Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase. Use of site-directed mutagenesis to evaluate the roles of His-258 and His-392 in catalysis

The current model for hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase divides the protein into two functional domains: an N-terminal kinase domain and a carboxyl-terminal bisphosphatase domain. Site-directed mutagenesis was used to evaluate the role of two putative bisphosphatase active site histidyl residues in catalysis. His-258 has been implicated as a phosphoacceptor (Pilkis, S. J., Lively, M. O., and El-Maghrabi, M. R. (1987) J. Biol. Chem. 262, 12672-12675), and the importance of this residue was confirmed when it was mutated to alanine and neither bisphosphatase activity nor a phosphoenzyme intermediate could be detected. Mutation of His-392 to alanine produced an enzyme which had five percent of wild-type fructose 2,6-bisphosphatase activity, and the rate of phosphoenzyme formations was decreased from 4800 nmol/min/mg to 2.9 nmol/min/mg. Mutation of His-392 to phenylalanine, lysine, or aspartic acid also produced proteins that did not hydrolyze fructose 2,6-bisphosphate or form a phosphoenzyme intermediate. These results are consistent with an important role for His-392 in the bisphosphatase reaction, probably as a proton donor, and with its designation as an active site residue based on homology modeling (Bazan (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 9642-9646). H258A had the same Vmax for 6-phosphofructo-2-kinase as the wild-type enzyme, and the mutant's kinase was inhibited by cAMP-dependent phosphorylation. In addition, H392F and H392K did not catalyze the kinase reaction, although H392D had normal kinase activity which was also modulated by cAMP-dependent phosphorylation in the same manner as the wild-type enzyme. Thus, an active bisphosphatase domain is not a necessary condition for phosphorylation-induced changes in 6-phosphofructo-2-kinase activity. The results also suggest that structural and/or active site interactions exist between the two domains of the enzyme.

Expression of rat hepatic glucokinase in Escherichia coli

Rat liver glucokinase was expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The expressed protein starts with the predicted initiator methionine residue and ends at the appropriate carboxyl terminal residue. It was partially purified by ammonium sulfate precipitation and gel filtration and had kinetic and physical properties similar to the purified rat liver enzyme. The efficient expression of this low abundance hepatic protein in bacteria provides a system for in vitro analysis of mutations of the enzyme.

Expression of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and its kinase domain in Escherichia coli

The rat liver bifunctional enzyme, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (ATP:D-fructose-6-phosphate 2-phosphotransferase/D-fructose-2,6-bisphosphate 2-phosphohydrolase, EC 2.7.1.105/EC 3.1.3.46) and its separate kinase domain were expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The bifunctional enzyme (470 residues per subunit) was efficiently expressed as a protein that starts with the initiator methionine residue and ends at the carboxyl-terminal tyrosine residue. The expressed protein was purified to homogeneity by anion exchange and Blue Sepharose chromatography and had kinetic and physical properties similar to the purified rat liver enzyme, including its behavior as a dimer during gel filtration, activation of the kinase by phosphate and inhibition by alpha-glycerol phosphate, and mediation of the bisphosphatase reaction by a phosphoenzyme intermediate. The expressed 6-phosphofructo-2-kinase also started with the initiator methionine but ended at residue 257. The partially purified kinase domain was catalytically active, had reduced affinities for ATP and fructose 6-phosphate compared with the kinase of the bifunctional enzyme, and had no fructose-2,6-bisphosphatase activity. The kinase domain also behaved as an oligomeric protein during gel filtration. The expression of an active kinase domain and the previous demonstration of an actively expressed bisphosphatase domain provide strong support for the hypothesis that the hepatic enzyme consists of two independent catalytic domains encoded by a fused gene.

Sequence of the 5'-flanking region of the rat 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase gene: regulation by glucocorticoids

Dexamethasone addition to cultured hepatocytes caused a 90-fold increase in mRNA for 6-phosphofructo 2-kinase/fructose-2,6-bisphosphatase. Glucocorticoid administration in vivo also increased the enzyme's mRNA in skeletal muscle by 3-4-fold. The sequence of the 5'-flanking region of the enzyme's gene revealed at least one consensus glucocorticoid response element. The amino acid sequence derived from a partial cDNA clone for the rat skeletal muscle bifunctional enzyme was identical to that of the liver isozyme except for an undetermined amount of N-terminal sequence. It is concluded that the rat muscle and liver isozymes, which are postulated to be identical except for the N-terminal region, are both regulated by glucocorticoids.

Glucocorticoid regulation of hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression

The effect of adrenalectomy and triamcinolone treatment on mRNA encoding rat hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was studied. Adrenalectomy decreased both the kinase and the bisphosphatase activities of the bifunctional enzyme to about 30% of the values in livers of normal rats. Triamcinolone treatment restored both activities to normal by 24 h. These changes were caused by alterations in the concentration of the enzyme as determined by immunoblotting and by an assay that measures phosphoenzyme formation. Messenger RNA for liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was markedly decreased by adrenalectomy and was increased 15-fold by triamcinolone administration for 8 h. The rate of transcription of the bifunctional enzyme gene, measured in rat liver nuclei, was also decreased in adrenalectomy, and triamcinolone treatment increased this rate 5-fold within 8 h. Similarly, liver nuclear precursors of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA were decreased by adrenalectomy to 25% of the level in nuclei from normal rats. Triamcinolone treatment restored heterogeneous values by 2 h, while treatment for 30 h increased it 12-fold over the adrenalectomized levels. It was concluded that glucocorticoids regulate the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, at least in part, by modulating the transcription rate of the gene.

Induction of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA by refeeding and insulin

The effects of fasting/refeeding and untreated or insulin-treated diabetes on the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and its mRNA in rat liver were determined. Both enzymatic activities fell to 20% of control values with fasting or streptozotocin-induced diabetes and were coordinately restored to normal within 48 h of refeeding or 24 h of insulin administration. These alterations in enzymatic activities were always mirrored by corresponding changes in amount of enzyme as determined by phosphoenzyme formation and immunoblotting. In contrast, mRNA for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase did not decrease during starvation or in diabetes, but there was a 3-6-fold increase upon refeeding a high carbohydrate diet to starved rats or insulin treatment of diabetic rats. The decrease of the enzyme in starved or diabetic rats without associated changes in mRNA levels suggests a decrease in the rate of mRNA translation, an increase in enzyme degradation, or both. The rise in enzyme amount and mRNA for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase with refeeding and insulin treatment suggests an insulin-dependent stimulation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression. Northern blots of RNA from heart, brain, kidney, and skeletal muscle probed with restriction fragments of a full-length cDNA from liver showed that only skeletal muscle contained an RNA species that hybridized to any of the probes. Skeletal muscle mRNA for 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase was 2.0 kilobase pairs but in contrast to the liver message (2.2 kilobase pairs) was not regulated by refeeding.

Expression of the bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Escherichia coli

The fructose-2,6-bisphosphatase domain of rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (EC 2.7.105/EC 3.1.3.46) was expressed in Escherichia coli by using an expression system based on bacteriophage T7 RNA polymerase. The protein was efficiently expressed (i) as a fusion protein that starts at the T7 major capsid protein initiation site in a pET expression vector and (ii) as a protein that starts within the bisphosphatase sequence by translation reinitiation. Both proteins have similar properties. The protein was purified to homogeneity by anion-exchange chromatography and gel filtration. The purified fructose-2,6-bisphosphatase domain was active and no 6-phosphofructo-2-kinase activity was found associated with it. In contrast to the dimeric bifunctional enzyme, the fructose-2,6-bisphosphatase domain behaved as a monomer of 30 kDa. The turnover number and kinetic properties of the separate bisphosphatase domain were similar to those of the bisphosphatase of the bifunctional enzyme, including the ability to form a phosphoenzyme intermediate. These results support the hypothesis that the rat liver enzyme consists of two independent domains and is a member of a class of enzymes formed by gene fusion.

Functional homology of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, phosphoglycerate mutase, and 2,3-bisphosphoglycerate mutase

The bisphosphatase domain of the rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase has been shown to exhibit a structural similarity to yeast phosphoglycerate mutase and human red blood cell 2,3-bisphosphoglycerate mutase including very similar active site sequences with a histidyl residue being involved in phospho group transfer. The liver bifunctional enzyme was found to catalyze the hydrolysis of glycerate 1,3-bisphosphate to glycerate 3-phosphate and inorganic phosphate. The Km for glycerate 1,3-bisphosphate was 320 microM and the Vmax was 11.5 milliunits/mg. Incubation of the rat liver enzyme with [1-32P]glycerate 1,3-bisphosphate resulted in the formation of a phosphoenzyme intermediate, and the labeled amino acid was identified as 3-phosphohistidine. Tryptic and endoproteinase Lys-C peptide maps of the 32P-phosphoenzyme labeled either with [2-32P]fructose 2,6-bisphosphate or [1-32P]glycerate 1,3-bisphosphate revealed that 32P-radioactivity was found in the same peptide, proving that the same histidyl group accepts phosphate from both substrates. Fructose 2,6-bisphosphate inhibited competitively the formation of phosphoenzyme from [1-32P]glycerate 1,3-bisphosphate. Effectors of fructose-2,6-bisphosphatase also inhibited phosphoenzyme formation. Substrates and products of phosphoglycerate mutase and 2,3-bisphosphoglycerate mutase also modulated the activities of the bifunctional enzyme. These results demonstrate that, in addition to a structural homology, the bisphosphatase domain of the bifunctional enzyme has a functional similarity to phosphoglycerate mutase and 2,3-bisphosphoglycerate mutase and support the concept of an evolutionary relationship between the three enzyme activities.

Enzymes with phosphoglycolate phosphatase activity in chicken skeletal muscle and liver

1. Four enzymes with phosphoglycolate phosphatase (EC 3.1.3.18) activity have been detected in extracts of chicken skeletal muscle and liver analyzed by gel-filtration and ion-exchange chromatography. 2. Two enzymes have been found in muscle extracts. One of them acts on glycerate 2,3-P2, in addition to glycolate 2-P. 3. Liver extracts contain two additional enzymes with broad specificity.

Levels of glycerate 2,3-P2, 2,3-bisphosphoglycerate synthase and 2,3-bisphosphoglycerate phosphatase activities in rat tissues. A method to quantify blood contamination of tissue extracts

The levels of glycerate 2,3-P2 and of 2,3-bisphosphoglycerate synthase and 2,3-bisphosphoglycerate phosphatase activities have been determined in isolated rat hepatocytes and adipocytes and in perfused rat tissues to discard blood contamination. The values obtained are much lower than those previously reported, ranging 0.50-40 nmol/g tissue. No relationship appears to exist between glycerate 2,3-P2 concentration and the levels of the enzymatic activities involved in glycerate 2,3-P2 metabolism. Assay of glycerate 2,3-P2 in tissue extracts constitute a very useful way to quantify blood contamination.