Differential control of proto-oncogene c-myc and c-fos expression in lymphocytes and fibroblasts

PARP-1 inhibition prevents CNS migration of dendritic cells during EAE, suppressing the encephalitogenic response and relapse severity

BACKGROUND

Pharmacological inhibitors of poly(ADP-ribose) polymerase-1 (PARP-1) are currently evaluated in clinical trials for various malignancies but, interestingly, also proved of remarkable efficacy in preclinical models of autoimmune disorders including experimental autoimmune encephalomyelitis (EAE).

OBJECTIVES

The objectives of the study were to determine molecular mechanisms underlying suppression of the encephalitogenic response by these drugs; likewise, whether clinically-relevant post-treatment paradigms with PARP-1 inhibitors could prevent EAE relapses.

METHODS

Adopted both in vitro techniques (bone marrow-derived cultured DC) as well as in vivo models of chronic or relapsing-remitting (RR) EAE.

RESULTS

We report that two structurally unrelated PARP-1 inhibitors negatively regulated NFκB activation, as well as maturation, cytokine production and APC function of cultured mouse bone marrow-derived dendritic cells (DCs). PARP-1 inhibitors also reduced the number and APC function of DCs migrating in the draining lymph nodes of ovalbumin-immunized mice. In C57Bl mice with chronic EAE or SJL mice with RR EAE, pharmacological inhibition of PARP-1 reduced CNS DC migration and demyelination as well as neurological impairment to an extent similar to that achieved with the potent immunosuppressant cyclosporine A. Remarkably, PARP-1 inhibitors injected after the first phase of disease reduced relapse incidence and severity, as well as the spinal cord number of autoreactive Th17 cells. Under this clinically-relevant treatment paradigm, PARP inhibitors also suppressed epitope spreading of the encephalitogenic response.

CONCLUSIONS

Overall, data underscore the potential relevance of PARP-1 inhibitors to MS therapy and suppression of autoimmunity.

IGF-I-induced VEGF expression in HUVEC involves phosphorylation and inhibition of poly(ADP-ribose)polymerase

Insulin-like growth factor-I (IGF-I) has been shown to promote angiogenesis by enhancing vascular endothelial growth factor (VEGF) expression. However, how IGF-I-induces VEGF expression is not yet fully understood. With this investigation, we propose a new possible mechanism involving downregulation of poly(ADP-ribosyl)ation (pADPR). We first demonstrated that IGF-I increased VEGF protein expression in endothelial cells. Inhibitors of mitogen activated kinase (PD 98059), phosphatidyl-3-inositol-kinase (LY 294002), and protein kinase C (staurosporine) diminished the IGF-I effect suggesting the involvement of signal transduction. Since there is an established link between pADPR and transcriptional activity, we focused on a possible role of poly(ADP-ribose)polymerase (PARP). The inhibition of PARP by 3-aminobenzamide or nicotinamide enhanced VEGF expression. Additionally, IGF-I markedly decreased PARP activity. Furthermore, the IGF-I-mediated inhibition of PARP could be demonstrated as a result of protein phosphorylation since phosphorylation of PARP decreased its activity in vitro and IGF-I treatment of endothelial cells induced PARP phosphorylation. The IGF-I-mediated phosphorylation and inhibition of PARP represent a novel mechanism of VEGF protein expression.

Inhibitors of poly(ADP-ribose) polymerase-1 suppress transcriptional activation in lymphocytes and ameliorate autoimmune encephalomyelitis in rats

1. In the presence of genotoxic stress poly(ADP-ribose) polymerase-1 (PARP-1) leads to NAD(+) and ATP depletion, participating in the pathogenesis of several disorders including inflammation. Accordingly, chemical inhibitors of PARP-1 are efficacious anti-inflammatories, albeit the underlying molecular mechanisms are still under debate. 2. This study investigated the effect of the PARP-1 inhibitors 6(5H)-phenanthridinone and benzamide as well as that of benzoic acid, an inactive analogue of benzamide, on development of experimental allergic encephalomyelitis (EAE) in rats. Both 6(5H)-phenanthridinone and benzamide attenuated development of EAE, reducing clinical score, neuroimmune infiltration and expression of inflammatory mediators such as inducible nitric oxide synthase, interleukin-1beta and -2, cyclooxygenase-2, tumour necrosis factor-alpha and interferon-gamma in the spinal cord of myelin-immunized rats. Importantly, no evidence of NAD(+) and ATP depletion as well as poly(ADP-ribose) formation was detected in the spinal cord. 3. By contrast, a robust formation of poly(ADP-ribose) occurred in B- and T-cell areas in lymph nodes of myelin-immunized rats and was suppressed by the treatment with 6(5H)-phenanthridinone and benzamide. In cultures of activated rat lymphocytes, 6(5H)-phenanthridinone and benzamide reduced the DNA-binding activity of NF-kappaB and AP-1 and transcription of pro-inflammatory cytokines such as interleukin-2, interferon-gamma and tumour necrosis factor-alpha. 4. Notably, benzoic acid did not reproduce the in vivo and in vitro effects of its parent compound. 5. These findings indicate that PARP-1 promotes transcriptional activation in lymphocytes and inhibitors of its enzymatic activity are useful for the treatment of autoimmune disorders of the central nervous system.

Interaction of the transcription factor YY1 with human poly(ADP-ribosyl) transferase

Poly(ADP-ribosyl) transferase (ADPRT) is a nuclear enzyme that catalyzes the synthesis of ADP-ribose polymers from NAD+ as well as the transfer of these polymers onto acceptor proteins. The function of ADPRT is thought to be related to a number of nuclear processes including DNA repair and transcription. The transcription factor Yin Yang 1 (YY1) is a potent regulator of RNA polymerase II (Pol II)-dependent transcription. In this study Alu-retroposon-associated binding sites for YY1 located in the distal region of the promoter of the human ADPRT gene have been identified suggesting a possible involvement of this protein in the regulation of ADPRT-gene expression. In the presence of the recombinant automodification domain of the ADPRT the formation of specific YY1 complexes, detected in gel-shift experiments, was strongly inhibited, indicating that this domain of the enzyme may interact directly with YY1. In accordance with this result YY1 was specifically precipitated from nuclear extracts by ADPRT immobilized on sepharose. These results suggest a direct ADPRT-YY1 interaction which may be of importance in the regulation of Pol II-dependent transcription. They also indicate that in some human promoters this regulation may be mediated by retroposons of the Alu family.

Regulation of the human poly(ADP-ribosyl) transferase promoter via alternative DNA racket structures

Human nuclear poly(ADP-ribosyl) transferase (ADPRT) protein content in cells suggests that ADPRT expression is stringently controlled. Analysis of the 3 kb promoter sequence, which is required for high level expression, revealed an extraordinary architecture: several Sp1 motifs are located in the vicinity of the first exon but the closest CCAAT/TATA boxes are several hundred basepairs away. Four Alu type repetitive sequences are in the promoter structure. Within these Alu sequences there exist inverted repeat elements, which could form two mutually exclusive types of DNA tertiary structure consisting of quadruplex DNA and loops resembling rackets. Thereby, a CCAAT/TATA element would be moved to spatial vicinity of the Sp1 site activating the promoter. Deletion analysis showed the functional significance of these racket elements. We also obtained evidence for DNA racket structures when we studied mutational mechanisms in a human adenine phosphoribosyltransferase (APRT) deficient patient. One of his alleles harbours a novel complex type of deletion/insertion mutation. Based on several highly informative sequence features in this genomic region a model is proposed for the generation of this unusual type of mutation involving two steps: an initial targeting step and a subsequent complex rearrangement. This process includes the formation of a DNA racket structure, which resembles that of the ADPRT promoter. Thus we conclude that DNA racket structures seem to be of general importance in nature.

Characterization of the superinduction of the c-myc proto-oncogene in fibroblasts by benzamide derivatives

In mouse fibroblasts stimulated from quiescence into proliferation by serum the induction of expression of the c-myc proto-oncogene is strongly stimulated by 3-methoxybenzamide. Similar superinduction effects are seen with related compounds such as 3-aminobenzamide and the acid analogues, 3-anisic acid and 3-aminobenzoic acid. Whereas the benzamide derivatives are inhibitors of poly(ADP-ribose) polymerase the acid analogues are not, suggesting that inhibition of this enzyme is not the basis for superinduction of the c-myc gene. Analysis of the kinetics of induction of c-myc mRNA indicates that the RNA accumulates more rapidly as well as to a higher level in the presence of serum plus 3-methoxybenzamide than with serum alone. However the stimulation is transient in both cases. Addition of actinomycin D at 30 min or 1 h after serum stimulation shows the c-myc mRNA to be stable at these times, in the presence or absence of 3-methoxybenzamide. Thus the effect of the latter on c-myc mRNA accumulation is likely to be exerted at the level of transcription or RNA processing rather than turnover of the mRNA.

Characterization of the glutamine synthetase amplifiable eukaryotic expression system applied to an integral membrane protein--the human thyrotropin receptor

An amplifiable eukaryotic expression system, based upon glutamine synthetase, has been applied to the production of a complex integral membrane glycoprotein, the human receptor for the polypeptide hormone thyrotropin (TSH). Production of recombinant protein was achieved in chinese hamster ovary (CHO) cells at levels at least 10-fold higher than has been achieved in any other system. After amplification of the inserted gene, the gene copy number was found to be increased in most (but not all) subclones in the range of 3- to 50-fold; mRNA levels of the individual cell lines broadly followed their gene copy number. The level of protein production (measured both functionally and structurally, by radioligand binding and cytofluorimetry, respectively) also reflected these increases in DNA and RNA, but appeared to be limited to a maximum value which we conclude is the maximum that the cells can tolerate without impairing their viability. The receptor is efficiently coupled to adenylate cyclase (22-45 pM TSH producing a 50% response), although the coupling mechanism appeared to be saturated at higher receptor numbers. The high level of expression has allowed, for the first time, the detection of recombinant TSH receptor by immunochemical means. This expression system should prove very useful, not only in facilitating characterization of the TSH receptor, but also for the production of many other integral membrane proteins in their native form.

The use of the amplifiable high-expression vector pEE14 to study the interactions of autoantibodies with recombinant human thyrotrophin receptor

In order to produce significant quantities of the human thyrotrophin (TSH) receptor we have investigated the use of two eukaryotic high expression systems. DNA encoding the receptor was obtained by the polymerase chain reaction (PCR) applied to thyroid cDNA. Receptor DNA was inserted into the baculovirus system; despite high mRNA levels there was little or no demonstrable protein production. However, using a novel amplifiable glutamine synthetase system, clones of transfected Chinese hamster ovary (CHO) cells expressed a high affinity TSH receptor (KD 0.225 +/- 0.046 nM, Bmax 20,000-45,000 sites/cell for individual clones). This was coupled to adenylate cyclase as measured by a TSH-stimulatable increase in extracellular cyclic AMP (cAMP), a detectable response being noted at 1 microU/ml TSH with half-maximal at around 25-50 microU/ml. The high expression allowed detection of both TSH binding inhibition and adenylate cyclase stimulation by autoantibodies in unfractionated sera from patients with Graves' disease.

Changes in mRNA levels of poly(ADP-ribose) polymerase during activation of human lymphocytes

The level of mRNA encoding the nuclear enzyme poly(ADP-ribose) polymerase (ADP-ribosyltransferase, EC 2.4.2.30) was found to be very low in quiescent human lymphocytes and to increase at least 10-fold between 1 and 2 dyas after stimulation with the mitogen phytohaemagglutinin, staying high for several days thereafter. This increase was inhibited by 3-methoxybenzamide (a competitive inhibitor of poly(ADP-ribose) polymerase) but was not affected significantly by aphidicolin. Incubation of activated cells with cycloheximide for 2 h increased the expression slightly. These data demonstrate that, during lymphocyte activation, the level of mRNA of the poly(ADP-ribose) polymerase gene correlates with, and hence is presumably responsible for, the increase in poly(ADP-ribose) polymerase protein detectable by enzyme assay or immunochemistry.

Inhibition of poly(ADP-ribosyl)ation does not prevent lymphocyte entry into the cell cycle

The enzyme poly(ADP-ribosyl)transferase (ADPRT) becomes activated soon after a mitogenic stimulus is applied to lymphocyte cultures. It has also been reported that ADPRT inhibitors prevent cell proliferation when added to cultures at the same time as the mitogen. While this has been ascribed to the need to seal physiologically present DNA strand breaks before cells enter S phase, the presence of DNA strand breaks in quiescent human lymphocytes has been recently questioned. We demonstrate here that non-toxic concentrations of ADPRT inhibitors do not affect lymphocyte blastization and proliferation, as measured by thymidine incorporation and cytofluorimetry. We therefore suggest that ADPRT activation is required for late functions which are not needed for cell cycle progression.

Differential role for ADP-ribosylation in gene expression during the activation of T lymphocytes by various stimuli

Simple competitive inhibitors of the nuclear enzyme ADP-ribosyltransferase, such as 3-methoxybenzamide (3MB), are known to block mitogen-induced activation of lymphocytes by inhibiting an early event. We now report that 3MB affects neither the generation of inositol phosphates nor the increase in cytoplasmic calcium in human T lymphocytes stimulated with phytohaemagglutinin (PHA), indicating that it acts on later or parallel events. The proliferative response to the phorbol ester 12-o-tetradecanoylphorbol-13-acetate (TPA) was much less sensitive to 3MB than was the response to PHA. Similarly, the TPA-induced increases in the expression of the c-myc proto-oncogene and of the Tac polypeptide of the interleukin-2 receptor were not affected by 3MB, whereas the same responses to either PHA or the calcium ionophore A23187 were inhibited by 3MB. The data suggest that 3MB affects the calcium-mediated signal for T-lymphocyte activation, acting after the increase in cytoplasmic calcium, and possibly also affects other signal transduction pathways distinct from the hydrolysis of polyphosphoinositides. There are some similarities between the actions of 3MB and cyclosporin.

Relationship of cellular oncogene expression to inhibition of growth and induction of differentiation of Daudi cells by interferons or TPA

Human alpha or beta interferons inhibit the proliferation of Daudi Burkitt lymphoma cells and induce the differentiation of these cells towards a mature plasma cell phenotype. Similar responses are seen when Daudi cells are treated with the phorbol ester, TPA. Both interferons and TPA down-regulate expression of the c-myc oncogene in these cells. Although TPA can mimic the effect of interferon on cell differentiation, it does not induce 2'5' oligoadenylate synthetase or the interferon-sensitive mRNAs, 6-16 or 9-27. Thus chronic stimulation of protein kinase C by TPA cannot mimic all of the effects of interferon treatment on gene expression. Inhibition of ADP-ribosyl transferase activity by 3-methoxybenzamide impairs interferon- or TPA-induced differentiation of Daudi cells. This agent induces a higher level of c-myc mRNA in the cells and stimulates the incorporation of [3H]thymidine into DNA; although these effects are partially counteracted by interferon or TPA treatment, the elevated expression of the c-myc gene may be sufficient to prevent terminal differentiation and allow cell proliferation to continue.