Three homologous cationic porphyrins differently affect the 20S proteasome gating mechanism.

Class III β-tubulin plays a prominent role in the development of drug resistance to paclitaxel by allowing the incorporation of the GBP1 GTPase into microtubules. Once in the cytoskeleton, GBP1 binds to prosurvival kinases such as PIM1 and initiates a signaling pathway that induces resistance to paclitaxel. Therefore, the inhibition of the GBP1:PIM1 interaction could potentially revert resistance to paclitaxel. A panel of 44 4-azapodophyllotoxin derivatives was screened in the NCI-60 cell panel. The result is that 31 are active and the comparative analysis demonstrated specific activity in paclitaxel-resistant cells. Using surface plasmon resonance, we were able to prove that NSC756093 is a potent in vitro inhibitor of the GBP1:PIM1 interaction and that this property is maintained in vivo in ovarian cancer cells resistant to paclitaxel. Through bioinformatics, molecular modeling, and mutagenesis studies, we identified the putative NSC756093 binding site at the interface between the helical and the LG domain of GBP1. According to our results by binding to this site, the NSC756093 compound is able to stabilize a conformation of GBP1 not suitable for binding to PIM1.

The present study provides new evidence that cationic porphyrins may be considered as tunable platforms to interfere with the structural “key code” present on the 20S proteasome α-rings and, by consequence, with its catalytic activity. Here, we describe the functional and conformational effects on the 20S proteasome induced by the cooperative binding of the tri-cationic 5-(phenyl)-10,15,20-(tri N-methyl-4-pyridyl) porphyrin (Tris-T4). Our integrated kinetic, NMR, and in silico analysis allowed us to disclose a complex effect on the 20S catalytic activity depending on substrate/porphyrin concentration. The analysis of the kinetic data shows that Tris-T4 shifts the relative populations of the multiple interconverting 20S proteasome conformations leading to an increase in substrate hydrolysis by an allosteric pathway. Based on our Tris-T4/h20S interaction model, Tris-T4 is able to affect gating dynamics and substrate hydrolysis by binding to an array of negatively charged and hydrophobic residues present on the protein surface involved in the 20S molecular activation by the regulatory proteins (RPs). Accordingly, despite the fact that Tris-T4 also binds to the α3ΔN mutant, allosteric modulation is not observed since the molecular mechanism connecting gate dynamics with substrate hydrolysis is impaired. We envisage that the dynamic view of the 20S conformational equilibria, activated through cooperative Tris-T4 binding, may work as a simplified model for a better understanding of the intricate network of 20S conformational/functional states that may be mobilized by exogenous ligands, paving the way for the development of a new generation of proteasome allosteric modulators.

A new series of nineteen 3-methoxy-1,2-dioxanes containing an amino moiety at C4 was designed, synthesized and tested for in vitro antimalarial activity against chloroquine sensitive (CQ-S) D10 and chloroquine resistant (CQ-R) W2 strains of Plasmodium falciparum (Pf). Cytotoxicity against the human endothelial cell line (HMEC-1) was also evaluated. The introduced modifications resulted in a notable improvement of the antimalarial activity. In particular, compound 9a with an amino-imidazole side-chain at C4 displays antimalarial activity in the high nanomolar range against the CQ-R Pf strain (W2 IC 50 ¼ 200 nM), being more active against CQ-R than CQ-S Pf strains and devoid of cytotoxicity against human HMEC-1 cells. On the other hand, some of the hybrids with 4-amino-7-chloroquinoline (9k-p) show an IC 50 comparable to that of chloroquine against the CQ-S Pf strain (9k-p, D10 IC 50 ¼ 50-90 nM) but without losing potency against the CQ-R Pf strain (9k-p, resistance index ¼ 1.2-2.6), with low cytotoxicity against HMEC-1. Structure-activity relationship studies show that the improved antimalarial activity of the new compounds is the result of a combination of cellular pharmacokinetics and pharmacodynamics effects.