Low molecular weight, non-peptidic agonists of TrkA receptor with NGF-mimetic activity.

Exploitation of the biologic activity of neurotrophins is desirable for medical purposes, but their protein nature intrinsically bears adverse pharmacokinetic properties. Here, we report synthesis and biologic characterization of a novel class of low molecular weight, non-peptidic compounds with NGF (nerve growth factor)-mimetic properties. MT2, a representative compound, bound to Trk (tropomyosin kinase receptor)A chain on NGF-sensitive cells, as well as in cell-free assays, at nanomolar concentrations and induced TrkA autophosphorylation and receptor-mediated internalization. MT2 binding involved at least two amino-acid residues within TrkA molecule. Like NGF, MT2 increased phosphorylation of extracellular signal-regulated kinase 1/2 and Akt proteins and production of MKP-1 phosphatase (dual specificity phosphatase 1), modulated p38 mitogen-activated protein kinase activation,sustained survival of serum-starved PC12 or RDG cells, and promoted their differentiation. However, the intensity of such responses was heterogenous, as the ability of maintaining survival was equally possessed by NGF and MT2, whereas the induction of differentiation was expressed at definitely lower levels by the mimetic. Analysis of TrkA autophosphorylation patterns induced by MT2 revealed a strong tyrosine (Tyr)490 and a limited Tyr785 and Tyr674/675 activation, findings coherent with the observed functional divarication. Consistently, in an NGF-deprived rat hippocampal neuronal model of Alzheimer Disease, MT2 could correct the biochemical abnormalities and sustain cell survival. Thus, NGF mimetics may reveal interesting investigational tools in neurobiology, as well as promising drug candidates.

Low molecular weight, non-peptidic agonists of TrkA receptor with NGF-mimetic activity.

Exploitation of the biologic activity of neurotrophins is desirable for medical purposes, but their protein nature intrinsically bears adverse pharmacokinetic properties. Here, we report synthesis and biologic characterization of a novel class of low molecular weight, non-peptidic compounds with NGF (nerve growth factor)-mimetic properties. MT2, a representative compound, bound to Trk (tropomyosin kinase receptor)A chain on NGF-sensitive cells, as well as in cell-free assays, at nanomolar concentrations and induced TrkA autophosphorylation and receptor-mediated internalization. MT2 binding involved at least two amino-acid residues within TrkA molecule. Like NGF, MT2 increased phosphorylation of extracellular signal-regulated kinase1/2 and Akt proteins and production of MKP-1 phosphatase (dual specificity phosphatase 1), modulated p38 mitogen-activated protein kinase activation, sustained survival of serum-starved PC12 or RDG cells, and promoted their differentiation. However, the intensity of such responses was heterogenous, as the ability of maintaining survival was equally possessed by NGF and MT2, whereas the induction of differentiation was expressed at definitely lower levels by the mimetic. Analysis of TrkA autophosphorylation patterns induced by MT2 revealed a strong tyrosine (Tyr)490 and a limited Tyr785 and Tyr674/675 activation, findings coherent with the observed functional divarication. Consistently, in an NGF-deprived rat hippocampal neuronal model of Alzheimer Disease, MT2 could correct the biochemical abnormalities and sustain cell survival. Thus, NGF mimetics may reveal interesting investigational tools in neurobiology, as well as promising drug candidates.

Nerve growth factor inhibits apoptosis in memory B lymphocytes via inactivation of p38 MAPK, prevention of Bcl-2 phosphorylation, and cytochrome c release.

Survival of memory B lymphocytes is tightly linked to the integrity of the Bcl-2 protein and is regulated by a nerve growth factor (NGF) autocrine circuit. In factor-starved memory B cells, the addition of exogenous NGF promptly induced p38 mitogen-activated protein kinase (MAPK), but not c-Jun N-terminal kinase (JNK), dephosphorylation. Conversely, withdrawal of endogenous NGF was followed by p38 MAPK activation and translocation onto mitochondria, whereby it combined with and phosphorylated Bcl-2, as assessed by co-immunoprecipitation and kinase assays in vivo and in vitro. Mitochondria isolated from human memory B cells, then exposed to recombinant p38 MAPK, released cytochrome c, as did mitochondria from Bcl-2-negative MDCK cells loaded with recombinant Bcl-2. Apoptosis induced by NGF neutralization could be blocked by the specific p38 MAPK inhibitor SB203580 or by Bcl-2 mutations in Ser-87 or Thr-56. These data demonstrate that the molecular mechanisms underlying the survival factor function of NGF critically rely upon the continuous inactivation of p38 MAPK, a Bcl-2-modifying enzyme.

NGF withdrawal induces apoptosis in CESS B cell line through p38 MAPK activation and Bcl-2 phosphorylation.

The sIgG(+) lymphoblastoid B cell line CESS spontaneously produces a high amount of NGF and expresses both high affinity (p140(Trk-A)) and low affinity (p75(NTR)) NGF receptors. Blocking NGF signals with neutralizing antibodies or specific Trk-A inhibitors induces a rapid phosphorylation of antiapoptotic Bcl-2 protein, followed by caspase activation, and apoptotic death of CESS cells. Bcl-2 phosphorylation in several sites within a approximately 60 aa "loop" domain of protein is known to regulate its antiapoptotic function. Accordingly, CESS cells expressing the loop deletional mutant cDNA constructs Bcl-2 Delta40-91 were completely resistant to apoptosis induced by NGF withdrawal, indicating that Bcl-2 phosphorylation is a critical event. NGF withdrawal induces p38 MAPK, but not JNK, activation in CESS cells, and SB203580, a specific inhibitor of p38 MAPK, is able to prevent both Bcl-2 phosphorylation and apoptosis, indicating that p38 MAPK is the enzyme responsible for these events.

A conserved AU-rich element in the 3' untranslated region of bcl-2 mRNA is endowed with a destabilizing function that is involved in bcl-2 down-regulation during apoptosis.

The control of mRNA stability is becoming recognized as a crucial point of gene expression regulation. A common element responsible for mRNA decay modulation is the adenine- and uracil-rich element that is found in the 3' untranslated region of numerous mRNAs subjected to fast expression changes in response to various stimuli. Previously we identified a post-transcriptional regulation level for the antiapoptotic bcl-2 gene, which could be involved in t(14;18) lymphoma-associated bcl-2 overexpression. Here we demonstrate that bcl-2 mRNA is endowed with an adenine- and uracil-rich element (ARE) characterized by high evolutionary conservation not only among all chordates examined, but even between chordates and the nematode Caenorhabditis elegans (ced-9 gene). As for other well-established destabilizing AREs, the insertion of the bcl-2 ARE downstream from stable beta-globin mRNA causes an enhanced decay of the beta-globin transcript, which proves its functional role. This possibility is corroborated by the fact that the pathway leading to the modulating activity of bcl-2 ARE is influenced by PKC, since the addition of DAG and TPA markedly attenuated the bcl-2 ARE destabilizing potential. Conversely, it is noteworthy that when C(2)-ceramide is added to the culture medium as the apoptotic agent, the beta-globin transcript harboring the bcl-2 ARE undergoes a dramatic increase in decay. This observation clearly indicates that the destabilizing function of bcl-2 ARE is enhanced by apoptotic stimuli and suggests that this element could be involved in a post-transcriptional mechanism of bcl-2 down-regulation during apoptosis. The half-life of the mRNA of bcl-2 in Jurkat cells is prolonged by PKC stimulation and shortened by C(2)-ceramide addition, strongly supporting the view that bcl-2 mRNA stability plays a physiological role in modulating bcl-2 expression, particularly in its down-regulation during apoptosis. Thus, this element becomes a new candidate for mediating those bcl-2 gene expression changes-from apoptosis-associated down-regulation to tumor-associated overexpression-observed thus far that profoundly influence single cell fate and tissue homeostasis.