Probing Substrate Transport Effects on Enzymatic Hydrogen Catalysis: An Alternative Proton Transfer Pathway in Putatively Sensory [FeFe] Hydrogenase

[FeFe] hydrogenases, metalloenzymes catalyzing proton/dihydrogen interconversion, have attracted intense attention due to their remarkable catalytic properties and (bio-)technological potential for a future hydrogen economy. In order to unravel the factors enabling their efficient catalysis, both their unique organometallic cofactors and protein structural features, i.e., "outer-coordination sphere" effects have been intensively studied. These structurally diverse enzymes are divided into distinct phylogenetic groups, denoted as Group A-D. Prototypical Group A hydrogenases display high turnover rates (104-105 s-1). Conversely, the sole characterized Group D representative, Thermoanaerobacter mathranii HydS (TamHydS), shows relatively low catalytic activity (specific activity 10-1 μmol H2 mg-1 min-1) and has been proposed to serve a H2-sensory function. The various groups of [FeFe] hydrogenase share the same catalytic cofactor, the H-cluster, and the structural factors causing the diverging reactivities of Group A and D remain to be elucidated. In the case of the highly active Group A enzymes, a well-defined proton transfer pathway (PTP) has been identified, which shuttles H+ between the enzyme surface and the active site. In Group D hydrogenases, this conserved pathway is absent. Here, we report on the identification of highly conserved amino acid residues in Group D hydrogenases that constitute a possible alternative PTP. We varied two proposed key amino acid residues of this pathway (E252 and E289, TamHydS numbering) via site-directed mutagenesis and analyzed the resulting variants via biochemical and spectroscopic methods. All variants displayed significantly decreased H2-evolution and -oxidation activities. Additionally, the variants showed two redox states that were not characterized previously. These findings provide initial evidence that these amino acid residues are central to the putative PTP of Group D [FeFe] hydrogenase. Since the identified residues are highly conserved in Group D exclusively, our results support the notion that the PTP is not universal for different phylogenetic groups in [FeFe] hydrogenases.

Light-Driven [FeFe] Hydrogenase Based H2 Production in E. coli: A Model Reaction for Exploring E. coli Based Semiartificial Photosynthetic Systems

Biohybrid technologies like semiartificial photosynthesis are attracting increased attention, as they enable the combination of highly efficient synthetic light-harvesters with the self-healing and outstanding performance of biocatalysis. However, such systems are intrinsically complex, with multiple interacting components. Herein, we explore a whole-cell photocatalytic system for hydrogen (H₂) gas production as a model system for semiartificial photosynthesis. The employed whole-cell photocatalytic system is based on Escherichia coli cells heterologously expressing a highly efficient, but oxygen-sensitive, [FeFe] hydrogenase. The system is driven by the organic photosensitizer eosin Y under broad-spectrum white light illumination. The direct involvement of the [FeFe] hydrogenase in the catalytic reaction is verified spectroscopically. We also observe that E. coli provides protection against O₂ damage, underscoring the suitability of this host organism for oxygen-sensitive enzymes in the development of (photo) catalytic biohybrid systems. Moreover, the study shows how factorial experimental design combined with analysis of variance (ANOVA) can be employed to identify relevant variables, as well as their interconnectivity, on both overall catalytic performance and O₂ tolerance.

Reversible or Irreversible Catalysis of H+/H2 Conversion by FeFe Hydrogenases

Studies of molecular catalysts traditionally aim at understanding how a certain mechanism allows the reaction to be fast. A distinct question, which has only recently received attention in the case of bidirectional molecular catalysts, is how much thermodynamic driving force is required to achieve fast catalysis in either direction of the reaction. "Reversible" catalysts are bidirectional catalysts that work either way in response to even a small departure from equilibrium and thus do not waste input free energy as heat; conversely, "irreversible" catalysts require a large driving force to proceed at an appreciable rate [Fourmond et al. Nat. Rev. Chem. 2021, 5, 348-360]. Numerous mechanistic rationales for these contrasting behaviors have been proposed. To understand the determinants of catalytic (ir)reversibility, we examined the steady-state, direct electron transfer voltammetry of a particular FeFe hydrogenase, from Thermoanaerobacter mathranii, which is very unusual in that it irreversibly catalyzes H₂ oxidation and production: a large overpotential is required for the reaction to proceed in either direction [Land et al. Chem. Sci. 2020, 11, 12789-12801]. In contrast to previous hypotheses, we demonstrate that in this particular enzyme catalytic irreversibility can be explained without invoking slow interfacial electron transfer or variations in the mechanism: the observed kinetics is fully consistent with the same catalytic pathway being used in both directions of the reaction.

Characterization of a putative sensory [FeFe]-hydrogenase provides new insight into the role of the active site architecture

[FeFe]-hydrogenases are known for their high rates of hydrogen turnover, and are intensively studied in the context of biotechnological applications. Evolution has generated a plethora of different subclasses with widely different characteristics. The M2e subclass is phylogenetically distinct from previously characterized members of this enzyme family and its biological role is unknown. It features significant differences in domain- and active site architecture, and is most closely related to the putative sensory [FeFe]-hydrogenases. Here we report the first comprehensive biochemical and spectroscopical characterization of an M2e enzyme, derived from Thermoanaerobacter mathranii. As compared to other [FeFe]-hydrogenases characterized to-date, this enzyme displays an increased H2 affinity, higher activation enthalpies for H+/H2 interconversion, and unusual reactivity towards known hydrogenase inhibitors. These properties are related to differences in active site architecture between the M2e [FeFe]-hydrogenase and "prototypical" [FeFe]-hydrogenases. Thus, this study provides new insight into the role of this subclass in hydrogen metabolism and the influence of the active site pocket on the chemistry of the H-cluster.

The maturase HydF enables [FeFe] hydrogenase assembly via transient, cofactor-dependent interactions

[FeFe] hydrogenases have attracted extensive attention in the field of renewable energy research because of their remarkable efficiency for H₂ gas production. H₂ formation is catalyzed by a biologically unique hexanuclear iron cofactor denoted the H-cluster. The assembly of this cofactor requires a dedicated maturation machinery including HydF, a multidomain [4Fe4S] cluster protein with GTPase activity. HydF is responsible for harboring and delivering a precatalyst to the apo-hydrogenase, but the details of this process are not well understood. Here, we utilize gas-phase electrophoretic macromolecule analysis to show that a HydF dimer forms a transient interaction complex with the hydrogenase and that the formation of this complex depends on the cofactor content on HydF. Moreover, Fourier transform infrared, electron paramagnetic resonance, and UV-visible spectroscopy studies of mutants of HydF show that the isolated iron-sulfur cluster domain retains the capacity for binding the precatalyst in a reversible fashion and is capable of activating apo-hydrogenase in in vitro assays. These results demonstrate the central role of the iron-sulfur cluster domain of HydF in the final stages of H-cluster assembly, i.e. in binding and delivering the precatalyst.

Discovery of novel [FeFe]-hydrogenases for biocatalytic H2-production

A semi-synthetic screening method for mining the biodiversity of [FeFe]-hydrogenases, expanding the toolbox for biocatalytic H₂-gas production.

A new screening method for [FeFe]-hydrogenases is described, circumventing the need for specialized expression conditions as well as protein purification for initial characterization. [FeFe]-hydrogenases catalyze the formation and oxidation of molecular hydrogen at rates exceeding 10³ s^–1, making them highly promising for biotechnological applications. However, the discovery of novel [FeFe]-hydrogenases is slow due to their oxygen sensitivity and dependency on a structurally unique cofactor, complicating protein expression and purification. Consequently, only a very limited number have been characterized, hampering their implementation. With the purpose of increasing the throughput of [FeFe]-hydrogenase discovery, we have developed a screening method that allows for rapid identification of novel [FeFe]-hydrogenases as well as their characterization with regards to activity (activity assays and protein film electrochemistry) and spectroscopic properties (electron paramagnetic resonance and Fourier transform infrared spectroscopy). The method is based on in vivo artificial maturation of [FeFe]-hydrogenases in Escherichia coli and all procedures are performed on either whole cells or non-purified cell lysates, thereby circumventing extensive protein purification. The screening was applied on eight putative [FeFe]-hydrogenases originating from different structural sub-classes and resulted in the discovery of two new active [FeFe]-hydrogenases. The [FeFe]-hydrogenase from Solobacterium moorei shows high H₂-gas production activity, while the enzyme from Thermoanaerobacter mathranii represents a hitherto uncharacterized [FeFe]-hydrogenase sub-class. This latter enzyme is a putative sensory hydrogenase and our in vivo spectroscopy study reveals distinct differences compared to the well established H₂ producing HydA1 hydrogenase from Chlamydomonas reinhardtii.

B-factor Guided Proline Substitutions in Chromobacterium violaceum Amine Transaminase: Evaluation of the Proline Rule as a Method for Enzyme Stabilization

Biocatalysis is attracting interest in the chemical industry as a sustainable alternative in large‐scale chemical transformations. However, low operational stability of naturally evolved enzymes is a challenge and major efforts are required to engineer protein stability, usually by directed evolution. The development of methods for protein stabilization based on rational design is of great interest, as it would minimize the efforts needed to generate stable enzymes. Here we present a rational design strategy based on proline substitutions in flexible areas of the protein identified by analyzing B‐factors. Several proline substitutions in the amine transaminase from Chromobacterium violaceum were shown to have a positive impact on stability with increased half‐life at 60 °C by a factor of 2.7 (variant K69P/D218P/K304P/R432P) as well as increased melting temperature by 8.3 °C (variant K167P). Finally, the presented method utilizing B‐factor analysis in combination with the proline rule was deemed successful at increasing the stability of this enzyme.

One-pot biocatalytic amine transaminase/acyl transferase cascade for aqueous formation of amides from aldehydes or ketones

A novel biocatalytic amine transaminase/acyl transferase cascade for the formation of amides in aqueous solution has been developed.

Bioinformatic analysis of a PLP-dependent enzyme superfamily suitable for biocatalytic applications

In this review we analyse structure/sequence-function relationships for the superfamily of PLP-dependent enzymes with special emphasis on class III transaminases. Amine transaminases are highly important for applications in biocatalysis in the synthesis of chiral amines. In addition, other enzyme activities such as racemases or decarboxylases are also discussed. The substrate scope and the ability to accept chemically different types of substrates are shown to be reflected in conserved patterns of amino acids around the active site. These findings are condensed in a sequence-function matrix, which facilitates annotation and identification of biocatalytically relevant enzymes and protein engineering thereof.

An efficient single-enzymatic cascade for asymmetric synthesis of chiral amines catalyzed by ω-transaminase

An efficient single-enzymatic cascade approach for the asymmetric synthesis of chiral amines has been developed, which applies the amino donor 3-aminocyclohexa-1,5-dienecarboxylic acid spontaneously tautomerizing to reach reaction completion with excellent ee values.

Gene signature critical to cancer phenotype as a paradigm for anticancer drug discovery

Malignant cell transformation commonly results in the deregulation of thousands of cellular genes, an observation that suggests a complex biological process and an inherently challenging scenario for the development of effective cancer interventions. To better define the genes/pathways essential to regulating the malignant phenotype, we recently described a novel strategy based on the cooperative nature of carcinogenesis that focuses on genes synergistically deregulated in response to cooperating oncogenic mutations. These so-called 'cooperation response genes' (CRGs) are highly enriched for genes critical for the cancer phenotype, thereby suggesting their causal role in the malignant state. Here, we show that CRGs have an essential role in drug-mediated anticancer activity and that anticancer agents can be identified through their ability to antagonize the CRG expression profile. These findings provide proof-of-concept for the use of the CRG signature as a novel means of drug discovery with relevance to underlying anticancer drug mechanisms.

Control of division and differentiation in oligodendrocyte-type-2 astrocyte progenitor cells

Oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells give rise to oligodendrocytes and type-2 astrocytes in cultures of rat optic nerve. These progenitors are one of the few cell types in which most aspects of proliferation and differentiation can be manipulated in a defined in vitro environment. When exposed to platelet-derived growth factor (PDGF), O-2A progenitors divide a limited number of times before clonally related cells differentiate into oligodendrocytes with a timing similar to that seen in vivo. In contrast, O-2A progenitors grown in the absence of mitogen do not divide but differentiate prematurely into oligodendrocytes, and progenitors exposed to appropriate inducing factors differentiate into type-2 astrocytes. O-2A progenitors can become immortalized through at least two different mechanisms. First, when O-2A progenitors are exposed to a combination of PDGF and basic fibroblast growth factor (bFGF) these cells undergo continuous self-renewal in the absence of differentiation. In contrast, the application of bFGF alone is associated with premature oligodendrocytic differentiation of dividing O-2A lineage cells. Thus, cooperation between growth factors can modulate O-2A progenitor self-renewal in a defined chemical environment by eliciting a novel programme of division and differentiation which cannot be predicted from the effects of either factor examined in isolation. A further mechanism which allows prolonged self-renewal in the O-2A lineage is the generation of a stem cell. O-2A progenitors isolated from optic nerves of perinatal rats also have the capacity to give rise to a population of cells called O-2Aadult progenitors, which differ from their perinatal counterparts in many characteristics. Most importantly, O-2Aadult progenitors have a slow cell cycle, divide and differentiate asymmetrically and appear to have the capacity for prolonged self-renewal. Thus, immortalization in this lineage can also be achieved by the generation of a cell with stem cell-like characteristics from a rapidly dividing progenitor population.

Choice and manipulation of retroviral vectors

ABSTACT: During the past few years, retroviral vectors have become a very important and widely used means of gene transfer. In the laboratory, their use has expanded the capabilities of investigators to perform important experiments that have solved previously unanswerable biological questions. Retroviral vectors exploit the inherent capacity of retrovkuses to transfer genetic material stably into a cell's genome and subsequently express it in a manner that is generally not detrimental to the host-cell. Initial entry of genetic material into cells via retroviral vectors is so efficient that it permits successful transfer of genes into limiting numbers of cells, such as hematopoietic stem cells in explanted bone marrow, and transfer approaching 100% in tissue-culture experiments. In comparison, chemical and electrical means of gene transfer require large initial numbers of cells and, under optimal circumstances, function at efficiencies several orders of magnitude lower than retroviral vectors, and only in a limited number of specific cell types.

Recruitment of TRRAP required for oncogenic transformation by E1A

TRRAP links Myc with histone acetylases and appears to be an important mediator of its oncogenic function. Here we show that interaction with TRRAP is required for cellular transformation not only by Myc, but also by the adenovirus E1A protein. Substitution of the 262 N-terminal residues of Myc with a small domain of E1A (residues 12-54) restores Myc transforming function. E1A(12-54) contains a TRRAP-interaction domain, that recruits TRRAP to either E1A-Myc chimeras, or the native 12S E1A protein. Overexpression of a competing TRRAP fragment in vivo blocks interaction of cellular TRRAP with either E1A-Myc or E1A, and suppresses cellular transformation by both oncoproteins. Moreover, E1A(Delta26-35) that fails to bind TRRAP but is capable of binding the Retinoblastoma (Rb)-family and p300/CBP proteins is defective in cellular immortalization, transformation and cell cycle deregulation. Thus in addition to disrupting Rb and p300/CBP functions, E1A must recruit TRRAP to transform cells.

p19ARF-independent induction of p53 and cell cycle arrest by Raf in murine keratinocytes

In tumorigenesis of the skin, activated Ras co-operates with mutations that inactivate the tumour suppressor p53, but the molecular basis for this co-operation remains unresolved. Here we show that activation of the Raf/MAP kinase pathway in primary mouse keratinocytes leads to a p53 and p21Cip1-dependent cycle arrest and to terminal differentiation. Raf activation in keratinocytes lacking p53 or p21Cip1 genes leads to expression of differentiation markers, but the cells do not cease to proliferate. Thus, loss of p53 or p21Cip1 function is necessary to disable growth-inhibitory Raf/MAP kinase signalling. Activation of oncogenes, including Ras, has been reported to stabilize and activate p53 via induction of the tumour suppressor p19ARF. However, the response to Raf in p19ARFI-/- keratinocytes was indistinguishable from wild-type controls. Thus, p19ARF is not essential for Raf-induced p53 induction and cell cycle arrest in keratinocytes, indicating that oncogenes engage p53 activity via multiple mechanisms.

Cyclins D1 and D2 mediate myc-induced proliferation via sequestration of p27(Kip1) and p21(Cip1)

Cyclin E-Cdk2 kinase activation is an essential step in Myc-induced proliferation. It is presumed that this requires sequestration of G(1) cell cycle inhibitors p27(Kip1) and p21(Cip1) (Ckis) via a Myc-induced protein. We provide biochemical and genetic evidence to show that this sequestration is mediated via induction of cyclin D1 and/or cyclin D2 protein synthesis rates. Consistent with this conclusion, primary cells from cyclin D1(-/-) and cyclin D2(-/-) mouse embryos, unlike wild-type controls, do not respond to Myc with increased proliferation, although they undergo accelerated cell death in the absence of serum. Myc sensitivity of cyclin D1(-/-) cells can be restored by retroviruses expressing either cyclins D1, D2 or a cyclin D1 mutant forming kinase-defective, Cki-binding cyclin-cdk complexes. The sequestration function of D cyclins thus appears essential for Myc-induced cell cycle progression but dispensable for apoptosis.

Follow-up at a Dutch addiction hospital and effectiveness of therapeutic community treatment

This paper reports on inpatient treatment of addicts. Attention is paid to the Therapeutic Community (TC) model employed with alcoholics. A sample of 881 patients was assessed at intake and was followed up. The results demonstrate that the patients improved on a variety of outcome measures. Some associations were found between patient variables and improvement. Treatment variables predicting a positive outcome were sustained treatment in a TC and attending AA meetings. The relative efficacy of TCs, originally created by drug users, holds for alcoholics as well. It is concluded that an important precondition to a positive treatment outcome is the continuity of the treatment process. Pursuing that continuity seems to be an excellent mediate goal for both addicts and treatment personnel.

High-intensity Raf signal causes cell cycle arrest mediated by p21Cip1

Activated Raf has been linked to such opposing cellular responses as the induction of DNA synthesis and the inhibition of proliferation. However, it remains unclear how such a switch in signal specificity is regulated. We have addressed this question with a regulatable Raf-androgen receptor fusion protein in murine fibroblasts. We show that Raf can cause a G1-specific cell cycle arrest through induction of p21Cip1. This in turn leads to inhibition of cyclin D- and cyclin E-dependent kinases and an accumulation of hypophosphorylated Rb. Importantly, this behavior can be observed only in response to a strong Raf signal. In contrast, moderate Raf activity induces DNA synthesis and is sufficient to induce cyclin D expression. Therefore, Raf signal specificity can be determined by modulation of signal strength presumably through the induction of distinct protein expression patterns. Similar to induction of Raf, a strong induction of activated Ras via a tetracycline-dependent promoter also causes inhibition of proliferation and p21Cip1 induction at high expression levels. Thus, p21Cip1 plays a key role in determining cellular responses to Ras and Raf signalling. As predicted by this finding we show that Ras and loss of p21 cooperate to confer a proliferative advantage to mouse embryo fibroblasts.

Myc activation of cyclin E/Cdk2 kinase involves induction of cyclin E gene transcription and inhibition of p27(Kip1) binding to newly formed complexes

Induction of the Myc-oestrogen receptor fusion protein (MycER) by 4-OH-tamoxifen (OHT) leads to the activation of Cyclin E/Cyclin-dependent kinase 2 (CycE/Cdk2) complexes followed by the induction of DNA synthesis. As CycE/Cdk2 activity is essential for G1/S transition, we have investigated the mechanism by which Myc can activiate CycE/Cdk2. Our results suggest that this activation may involve at least two Myc-dependent steps: the induction of cyclin E gene transcription followed by accumulation of cyclin E mRNA in a protein synthesis-independent manner and the inhibition of p27(Kip1) association with CycE/Cdk2 complexes containing newly synthesised CycE. As a consequence phosphorylation of CycE-bound Cdk2 by cyclin activating kinase (CAK) is accelerated. We propose a model in which the active newly synthesised CycE/Cdk2 complexes trigger a positive feed-back mechanism to activate preexisting complexes through phosphorylation-dependent p27(Kip1) release.

Cooperating oncogenes converge to regulate cyclin/cdk complexes

The cooperation of oncogenes in the transformation of primary rat Schwann cells is a strikingly synergistic process. We have explored the molecular mechanisms involved. Activation of an inducible Raf kinase results in morphologically transformed cells that are arrested in G1 via the induction of p21(CiP1) and subsequent inhibition of cyclin/cdk activity. In contrast, coexpression of SV40 large T (LT) or a dominant-negative mutant of p53 abolishes p21(CiP1) induction and alleviates the growth arrest. Moreover in this scenario, Raf activation results in an increase in the specific activity of cyclin/cdk complexes with Raf and LT cooperating to superinduce cyclin A/cdk2 activity and stimulate proliferation in the absence of mitogens. Thus, signaling by Raf and its cooperating partners converges at the regulation of cyclin/cdk complexes, with the cellular responses to Raf modulated by p53.

Immortalization and transformation are associated with specific alterations in choline metabolism

Analysis of transformed, immortalized, and primary rat Schwann cells by high-resolution proton nuclear magnetic resonance spectroscopy reveals that immortalization of Schwann cells (by SV40 large T antigen) induced a decrease in sn-glycero-3-phosphocholine (GPCho), whereas H-ras alone, which is known to cause growth arrest in these cells, induced a marked increase in GPCho and a decrease in phosphocholine (PCho). An increase of PCho was found only in cells fully transformed by both oncogenes together. Moreover, we examined 11 human tumor cell lines, all of which expressed a PCho:GPCho ratio similar to that of fully transformed rat Schwann cells. Importantly, neither the absolute levels of PCho nor the ratio of PCho:GPCho were correlated with the rate of cell division across a range of normal (primary cultures) and transformed cells. Thus, raised PCho:GPCho ratios may serve as an indicator of multiple oncogenic lesions and malignancy in noninvasive tumor investigations.

Expression of cyclin D1 mRNA is not upregulated by Myc in rat fibroblasts

Conflicting results have been published regarding the regulation of cyclin D1 mRNA in rat fibroblasts expressing a hormone-regulated Myc protein, MycER. We confirm that activation of MycER with oestrogen rapidly induces cyclin D1 mRNA, even in the presence of cycloheximide. However, we show that this is an artefact resulting from an oestrogen-activated transcriptional activation domain in the oestrogen receptor part of the MycER chimaera. First, addition of 4-hydroxy-tamoxifen (4OHT), which does not activate this domain, allows association of MycER with Max and induces cell proliferation in serum-starved Rat-1-MycER cells without affecting cyclin D1 mRNA levels or the activity of D1 promoter-luciferase constructs. Second, Rat-1 cells expressing a mutant MycER with a hormone-binding domain that still binds 4OHT but no longer binds oestrogen, are driven into the cell cycle in response to 4OHT but fail to up-regulate cyclin D1 mRNA. Finally, Rat-1 cells in which wild-type human c-Myc expression can be induced, also progress into the cell cycle without increased D1 mRNA expression.

Integrated control of cell proliferation and cell death by the c-myc oncogene

Regulation of multicellular architecture involves a dynamic equilibrium between cell proliferation, differentiation with consequent growth arrest, and cell death. Apoptosis is one particular form of active cell death that is extremely rapid and characterized by auto-destruction of chromatin, cellular blebbing and condensation, and vesicularization of internal components. The c-myc proto-oncogene encodes an essential component of the cell's proliferative machinery and its deregulated expression is implicated in most neoplasms. Intriguingly, c-myc can also act as a potent inducer of apoptosis. Myc-induced apoptosis occurs only in cells deprived of growth factors or forcibly arrested with cytostatic drugs. Myc-induced apoptosis is dependent upon the level at which it is expressed and deletion mapping shows that regions of c-Myc required for apoptosis overlap with regions necessary for co-transformation, autoregulation, inhibition of differentiation, transcriptional activation and sequence-specific DNA binding. Moreover, induction of apoptosis by c-Myc requires association with c-Myc's heterologous partner, Max. All of this strongly implies that c-Myc drives apoptosis through a transcriptional mechanism: presumably by modulation of target genes. Two simple models can be invoked to explain the induction of apoptosis by c-Myc. One holds that death arises from a conflict in growth signals which is generated by the inappropriate or unscheduled expression of c-Myc under conditions that would normally promote growth arrest. In this 'Conflict' model, induction of apoptosis is not a normal function of c-Myc but a pathological manifestation of its deregulation. It thus has significance only for models of carcinogenic progression in which myc genes are invariably disrupted.(ABSTRACT TRUNCATED AT 250 WORDS)

Myc-Max-Mad: a transcription factor network controlling cell cycle progression, differentiation and death

The Myc oncoprotein dimerizes with its partner, Max, to bind DNA, activate transcription, and promote cell proliferation, as well as programmed cell death. Max also forms homodimers or heterodimers with its alternative partners, Mad and Mxi-1. These complexes behave as antagonists of Myc/Max through competition for common DNA targets, and perhaps permit cellular differentiation.

The c-Myc protein induces cell cycle progression and apoptosis through dimerization with Max

The c-Myc protein (Myc) is involved in cellular transformation and mitogenesis, but is also a potent inducer of programmed cell death, or apoptosis. Whether these apparently opposite functions are mediated through common or distinct molecular mechanisms remains unclear. Myc and its partner protein, Max, dimerize and bind DNA in vitro and in vivo through basic/helix-loop-helix/leucine zipper motifs (bHLH-LZ). By using complementary leucine zipper mutants (termed MycEG and MaxEG), which dimerize efficiently with each other but not with their wild-type partners, we demonstrate that both cell cycle progression and apoptosis in nontransformed rodent fibroblasts are induced by Myc-Max dimers. MycEG or MaxEG alone are inactive, but co-expression restores ability to prevent withdrawal from the cell cycle and to induce cell death upon removal of growth factors. Thus, Myc can control two alternative cell fates through dimerization with a single partner, Max.

Distinct DNA binding preferences for the c-Myc/Max and Max/Max dimers

The transcription factor c-Myc and its dimerisation partner Max are members of the basic/helix-loop-helix/leucine-zipper (bHLH-Z) family and bind to the DNA core sequence CACGTG. Using a site-selection protocol, we determined the complete 12 base pair consensus binding sites of c-Myc/Max (RACCACGTGGTY) and Max/Max (RANCACGTGNTY) dimers. We find that the c-Myc/Max dimer fails to bind the core when it is flanked by a 5'T or a 3'A, while the Max/Max dimer readily binds such sequences. Furthermore we show that inappropriate flanking sequences preclude transactivation by c-Myc in vivo. In conclusion, Max/Max dimers are less discriminatory than c-Myc/Max and may regulate other genes in addition to c-Myc/Max targets.

Oncogenic activity of the c-Myc protein requires dimerization with Max

c-Myc (Myc) and Max proteins dimerize and bind DNA through basic-helix-loop-helix-leucine zipper motifs (b-HLH-LZ). Using a genetic approach, we demonstrate that binding to Max is essential for Myc transforming activity and that Myc homodimers are inactive. Mutants of Myc and Max that bind efficiently to each other but not to their wild-type partners were generated by either exchanging the HLH-LZ domains or reciprocally modifying LZ dimerization specificities. While transformation defective on their own, complementary mutants restore Myc transforming activity when coexpressed in cells. The HLH-LZ exchange mutants also have dominant negative activity on wild-type Myc function. In addition, wild-type max antagonizes myc function in a dose-dependent manner, presumably through competition of Max-Max and Myc-Max dimers for common target DNA sites. Therefore, Max can function as both suppressor and activator of Myc. A general model for the role of Myc and Max in growth control is discussed.

Transcriptional activation by the human c-Myc oncoprotein in yeast requires interaction with Max

The c-myc protein (Myc) contains an amino-terminal transcriptional activation domain and a carboxy-terminal basic helix-loop-helix-leucine zipper (bHLH-Z) domain that directs dimerization of Myc with its partner, the max protein (Max), and promotes DNA binding to sites containing a CACGTG core consensus sequence. Despite these characteristics and the observation that Myc can modulate gene expression, a direct role for Myc or Max as transcription factors has never been demonstrated. Here we use Saccharomyces cerevisiae as an in vivo model system to show that the Myc protein is a sequence-specific transcriptional activator whose DNA binding is strictly dependent on dimerization with Max. Transactivation is mediated by the amino-terminal domain of Myc. We find that Max homodimers bind to the same DNA sequence as Myc+Max but that they fail to transactivate and thus can antagonize Myc+Max function. We also show that the Max HLH-Z domain has a higher affinity for the Myc HLH-Z domain than for itself, and suggest that the heterodimeric Myc+Max activator forms preferentially at equilibrium.

Max and c-Myc/Max DNA-binding activities in cell extracts

We have examined the interactions and DNA-binding activities of the c-Myc oncoprotein and its partner Max. In cell extracts virtually all c-Myc molecules are associated with Max in heterodimeric complexes. Moreover, DNA-binding studies with in vitro-translated protein and cell extracts show that both Max alone and c-Myc/Max bind the same DNA sequence. Conversely, c-Myc is unable to bind this sequence in the absence of Max. These findings suggest that c-Myc may function via obligate complex formation with Max.

Induction of apoptosis in fibroblasts by c-myc protein

Although Rat-1 fibroblasts expressing c-myc constitutively are unable to arrest growth in low serum, their numbers do not increase in culture because of substantial cell death. We show this cell death to be dependent upon expression of c-myc protein and to occur by apoptosis. Regions of the c-myc protein required for induction of apoptosis overlap with regions necessary for cotransformation, autoregulation, and inhibition of differentiation, suggesting that the apoptotic function of c-myc protein is related to its other functions. Moreover, cells with higher levels of c-myc protein are more prone to cell death upon serum deprivation. Finally, we demonstrate that deregulated c-myc expression induces apoptosis in cells growth arrested by a variety of means and at various points in the cell cycle.

Domains of human c-myc protein required for autosuppression and cooperation with ras oncogenes are overlapping

Amino acids 106 to 143 and 354 to 433 of the human c-myc protein (439 amino acids) were shown to be required for the protein to suppress c-myc gene transcription and were found to exactly overlap with those necessary for c-myc to cooperate with ras oncogenes in the transformation of rat embryo fibroblasts. The essential carboxyl-terminal region harbors structural motifs (a basic region, a helix-loop-helix motif, and a "leucine zipper"), which, in other proteins, can mediate dimerization and sequence-specific DNA binding.

Cooperation between two growth factors promotes extended self-renewal and inhibits differentiation of oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells

Bipotential oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, which give rise to oligodendrocytes and type-2 astrocytes in cultures of rat optic nerve, are one of the few cell types in which most aspects of proliferation and differentiation can be manipulated in a defined in vitro environment. Previous studies have shown that O-2A progenitors exposed to platelet-derived growth factor (PDGF) divide as migratory bipolar cells a limited number of times, with a cell cycle time of 18 hr, before clonally related progenitors differentiate into nondividing oligodendrocytes with a timing similar to that seen in vivo. In contrast, O-2A progenitors grown in the absence of mitogen do not divide but instead differentiate prematurely into oligodendrocytes, and progenitors exposed to appropriate inducing factors differentiate into type-2 astrocytes. We now have found that O-2A progenitors can be induced to undergo continuous self-renewal in the absence of oligodendrocytic differentiation by exposure to a combination of PDGF and basic fibroblast growth factor (bFGF). With the exception of the inhibition of differentiation, the O-2A progenitors exposed to PDGF and bFGF behaved similarly to those exposed to PDGF alone. In contrast, progenitors exposed to basic bFGF alone were multipolar, had a cell-cycle length of 45 hr, showed little migratory behavior, underwent premature oligodendrocytic differentiation, and did not cease division upon expression of oligodendrocyte marker antigens. Thus, inhibition of differentiation required the presence of both mitogens. Our results demonstrate that PDGF and bFGF act on O-2A progenitors as both inducers of division and as regulators of differentiation that modulate multiple aspects of O-2A progenitor development and, additionally, reveal a previously unrecognized means of regulating self-renewal processes, wherein cooperation between growth factors promotes continuous division in the absence of differentiation.

Advanced mammalian gene transfer: high titre retroviral vectors with multiple drug selection markers and a complementary helper-free packaging cell line

We report the development of an advanced system for transfer and expression of exogenous genes in mammalian cells based on Moloney murine leukemia virus (Mo MuLV). Extensive deletion/mutagenesis analysis to identify cis-acting signals involved in virus transmission has led to the design of a family of novel, highly efficient retroviral vectors and a partner helper-free packaging cell line. The pBabe retroviral vector constructs transmit inserted genes at high titres and express them from the Mo MuLV Long Terminal Repeat (LTR). Each of these vectors has been constructed with one of four different dominantly acting selectable markers, allowing the growth of infected mammalian cells in the presence of G418, hygromycin B, bleomycin/phleomycin or puromycin, respectively. The high titre ecotropic helper free packaging cell line, omega E, was designed in conjunction with the pBabe vectors to reduce the risk of generation of wild type Mo MuLV via homologous recombination events. The omega E cell line was generated with separate gagpol and ecotropic env expression constructs with minimal sequence overlap and decreased sequence homology achieved by 'codon wobbling'. Homologous env coding sequences were deleted from the pBabe vectors without diminishing recombinant vector titre. Together, the pBabe vectors and omega E cell line should prove useful in experiments where highest frequencies of gene transfer, or concomitant expression of several different genes within a single cell are required with minimal risk of helper virus contamination.

Establishment of rat endometrial cell lines by retroviral mediated transfer of immortalizing and transforming oncogenes

To study hormone responsive genes in differentiated epithelial cells and as a model for endometrial carcinoma, lines were established from primary rat endometrial cells by infection with replication-defective retroviruses carrying oncogenes and the selectable gene neo. The initial step involved immortalization through the large T antigen of SV40 to generate a line we designate RENT4, or with the E1a gene of adenovirus to generate lines referred to as RENE1 and RENE2. Additionally, lines generated by large T antigen of SV40 were superinfected with a replication-defective retrovirus harboring the v-Ha-ras oncogene and selected by the ability to form colonies in soft agar. The latter cell lines appeared fully transformed and were designated RENTR01 and RENTR03. Five established lines were characterized for steroid hormone receptors, alkaline phosphatase activity and their complement of the intermediate filaments vimentin and cytokeratin. With the exception of the RENE1 cells all other lines have normal levels of glucocorticoid receptor, whereas only RENE1, RENE2 and RENT4 were positive for the progesterone receptor. RENTR01, RENTR03 and, to a lesser extent, RENE1 exhibited differential expression of cytokeratins dependent upon whether the cells were grown on a substrata of NIH3T3 cells. When grown on formalin-fixed NIH3T3 cells, RENTR01 and RENTR03 cells appeared to differentiate or rearrange themselves in culture. Individual islands of cells showed a heterogeneous pattern of intermediate filaments with vimentin-positive cells localized to the outer portion of the islands whereas cytokeratin-positive cells are seen on the insides of these structures.

Negative autoregulation of c-myc transcription

The introduction of activated c-myc and v-myc genes into a variety of non-established and established cells results in the suppression of endogenous c-myc expression. As measured in Rat-1 fibroblasts, the suppression occurs at the level of transcriptional initiation. Moreover, the extent of the down-regulation is proportional to the cellular concentration of c-myc protein, and the critical concentration range in which the endogenous c-myc RNA is effectively suppressed corresponds to that found in non-transformed cells. In addition, the autoregulatory mechanism is not only dependent on c-myc protein, but also requires additional trans-acting factors. These results support a role for c-myc in the regulation of cellular gene transcription and suggest that a negative feedback mechanism can act as a homeostatic regulator of c-myc expression in vivo.

C-MYC: evidence for multiple regulatory functions

The nuclear c-myc proto-oncogene promotes cell proliferation and can inhibit terminal differentiation as well as induce immortalisation in its oncogenic form. There is increasing evidence that c-myc exerts these biological activities by modulating transcription and by directly affecting the initiation of DNA replication. The regulation of these disparate activities may involve the carboxyl end of the c-myc protein, which is essential for transformation and autosuppression of c-myc transcription. Conserved motifs in this region of the c-myc protein may mediate complex formation and sequence-specific nucleic acid binding.