C81-evoked inhibition of the TNFR1-NFκB pathway during inflammatory processes for stabilization of the impaired vascular endothelial barrier for leukocytes

Chronic inflammation-related diseases are characterized by persistent leukocyte infiltration into the underlying tissue. The vascular endothelium plays a major role in this pathophysiological condition. Only few therapeutic strategies focus on the vascular endothelium as a major target for an anti-inflammatory approach. In this study, we present the natural compound-derived carbazole derivative C81 as chemical modulator interfering with leukocyte-endothelial cell interactions. An in vivo assay employing intravital microscopy to monitor leukocyte trafficking after C81 treatment in postcapillary venules of a murine cremaster muscle was performed. Moreover, in vitro assays using HUVECs and monocytes were implemented. The impact of C81 on cell adhesion molecules and the NFκB signaling cascade was analyzed in vitro in endothelial cells. Effects of C81 on protein translation were determined by incorporation of a puromycin analog-based approach and polysome profiling. We found that C81 significantly reduced TNF-activated leukocyte trafficking in postcapillary venules. Similar results were obtained in vitro when C81 reduced leukocyte-endothelial cell interactions by down-regulating cell adhesion molecules. Focusing on the NFκB signaling cascade, we found that C81 reduced the activation on multiple levels of the cascade through promoted IκBα recovery by attenuation of IκBα ubiquitination and through reduced protein levels of TNFR1 caused by protein translation inhibition. We suggest that C81 might represent a promising lead compound for interfering with inflammation-related processes in endothelial cells by down-regulation of IκBα ubiquitination on the one hand and inhibition of translation on the other hand without exerting cytotoxic effects.

Synthesis of desaza analogues of annomontine and canthin-4-one alkaloids

1-Acetylcarbazoles are readily converted to 3-desazacanthin-4-ones upon treatment with Bredereck's reagent, but in contrast to canthin-4-ones, these do not undergo ring transformation reactions with guanidine. Only after N-protection (methyl or 2-(trimethylsilyl)ethoxymethyl group), 2-desaza analogues of the alkaloid annomontine are accessible via the enaminoketones obtained by condensation with Bredereck's reagent. One of the annomontine analogues is an inhibitor of the Plasmodium falciparum CDC-like kinases (CLK) and shows antimalarial activity.

Inhibition of the SR protein-phosphorylating CLK kinases of Plasmodium falciparum impairs blood stage replication and malaria transmission

Cyclin-dependent kinase-like kinases (CLKs) are dual specificity protein kinases that phosphorylate Serine/Arginine-rich (SR) proteins involved in pre-mRNA processing. Four CLKs, termed PfCLK-1-4, can be identified in the human malaria parasite Plasmodium falciparum, which show homology with the yeast SR protein kinase Sky1p. The four PfCLKs are present in the nucleus and cytoplasm of the asexual blood stages and of gametocytes, sexual precursor cells crucial for malaria parasite transmission from humans to mosquitoes. We identified three plasmodial SR proteins, PfSRSF12, PfSFRS4 and PfSF-1, which are predominantly present in the nucleus of blood stage trophozoites, PfSRSF12 and PfSF-1 are further detectable in the nucleus of gametocytes. We found that recombinantly expressed SR proteins comprising the Arginine/Serine (RS)-rich domains were phosphorylated by the four PfCLKs in in vitro kinase assays, while a recombinant PfSF-1 peptide lacking the RS-rich domain was not phosphorylated. Since it was hitherto not possible to knock-out the pfclk genes by conventional gene disruption, we aimed at chemical knock-outs for phenotype analysis. We identified five human CLK inhibitors, belonging to the oxo-β-carbolines and aminopyrimidines, as well as the antiseptic chlorhexidine as PfCLK-targeting compounds. The six inhibitors block P. falciparum blood stage replication in the low micromolar to nanomolar range by preventing the trophozoite-to-schizont transformation. In addition, the inhibitors impair gametocyte maturation and gametogenesis in in vitro assays. The combined data show that the four PfCLKs are involved in phosphorylation of SR proteins with essential functions for the blood and sexual stages of the malaria parasite, thus pointing to the kinases as promising targets for antimalarial and transmission blocking drugs.