NS5A inhibitors impair NS5A-phosphatidylinositol 4-kinase IIIα complex formation and cause a decrease of phosphatidylinositol 4-phosphate and cholesterol levels in hepatitis C virus-associated membranes.

The hepatitis C virus (HCV) nonstructural (NS) protein 5A is a multifunctional protein that plays a central role in viral replication and assembly. Antiviral agents directly targeting NS5A are currently in clinical development. Although the elucidation of the mechanism of action (MOA) of NS5A inhibitors has been the focus of intensive research, a detailed understanding of how these agents exert their antiviral effect is still lacking. In this study, we observed that the downregulation of NS5A hyperphosphorylation is associated with the actions of NS5A inhibitors belonging to different chemotypes. NS5A is known to recruit the lipid kinase phosphatidylinositol 4-kinase IIIα (PI4KIIIα) to the HCV-induced membranous web in order to generate phosphatidylinositol 4-phosphate (PI4P) at the sites of replication. We demonstrate that treatment with NS5A inhibitors leads to an impairment in the NS5A-PI4KIIIα complex formation that is paralleled by a significant reduction in PI4P and cholesterol levels within the endomembrane structures of HCV-replicating cells. A similar decrease in PI4P and cholesterol levels was also obtained upon treatment with a PI4KIIIα-targeting inhibitor. In addition, both the NS5A and PI4KIIIα classes of inhibitors induced similar subcellular relocalization of the NS5A protein, causing the formation of large cytoplasmic NS5A-containing clusters previously reported to be one of the hallmarks of inhibition of the action of PI4KIIIα. Because of the similarities between the effects induced by treatment with PI4KIIIα or NS5A inhibitors and the observation that agents targeting NS5A impair NS5A-PI4KIIIα complex formation, we speculate that NS5A inhibitors act by interfering with the function of the NS5A-PI4KIIIα complex.

Inhibition of constitutive and BCR-induced Syk activation downregulates Mcl-1 and induces apoptosis in chronic lymphocytic leukemia B cells.

The protein kinase Syk is a key mediator of proximal B-cell receptor (BCR) signaling. Following antigen stimulation, Syk is recruited to the BCR and becomes activated by phosphorylation at Y352. Recently, Syk was found to be constitutively phosphorylated in several common B-cell lymphoma subtypes, indicating a role for antigen-independent Syk activation in the pathogenesis of these diseases. We now report that Syk is constitutively phosphorylated on the activating Y352 residue in chronic lymphocytic leukemia (CLL) B cells. To examine the effects of constitutive Syk activity on intracellular signaling and leukemic cell survival, we performed in vitro studies with the Syk inhibitor R406. Treatment with R406 induced leukemic cell apoptosis in the majority of investigated cases and affected the basal activity or expression of several pro-survival molecules regulated by Syk, including the Akt and extracellular signal-regulated (ERK) kinases, and the anti-apoptotic protein Mcl-1. In addition, R406 prevented the increase in leukemic cell viability induced by sustained BCR engagement and inhibited BCR-induced Akt activation and Mcl-1 upregulation. Collectively, these data identify Syk as a potential target for CLL treatment and suggest that inhibition of this kinase could provide a double therapeutic benefit by disrupting both antigen-dependent and antigen-independent signaling pathways that regulate leukemic cell survival.

The 67 kDa laminin receptor increases tumor aggressiveness by remodeling laminin-1.

The association between expression of the 67 kDa laminin receptor (67LR) and tumor aggressiveness has been convincingly demonstrated although the exact function of this molecule in the metastatic process has remained unclear. In this study, we tested whether the laminin-1, upon interaction with 67LR, promotes tumor cell aggressiveness; the investigation was based on: (i) the previous demonstration that soluble 67LR, as well as a 20-amino-acid peptide corresponding to the 67LR laminin binding site, changes the conformation of laminin upon interaction with this adhesion molecule and (ii) the known relevance of microenvironment remodeling by the tumor, leading to structural modification of extracellular matrix components in tumor progression. MDAMB231 breast carcinoma cells plated on peptide G-treated laminin-1 exhibited a polygonal array of actin filament bundles compared with cells seeded on native laminin-1 which presented the actin bundles organized as multiple cables parallel to margins. Furthermore, in cells seeded on peptide G-treated laminin-1, 67LR was distinct from the alpha6 integrin subunit in filopodia protrusions in addition to colocalizing with this integrin in focal adhesion plaques as it occurs when cells are plated on native laminin-1. In addition to differences in tumor cell adhesion and migration found in cells exposed to peptide G-treated vs native laminin-1, breast carcinoma cells seeded on modified laminin-1 showed a 6-fold increase in invasion capability compared with cells seeded on unmodified laminin-1. Alterations in actin organization as well as adhesion, migration and especially invasion observed in MDAMB231 cells in the presence of peptide G-treated laminin-1 were even found in MDAMB231 cells that, after selection for 67LR high expression, were seeded on native laminin-1. As the 67LR shedding is proportional to its expression level, these findings indicate a role for 67LR in changing laminin structure. Expression analysis of 97 genes encoding proteins that mediate cell matrix interactions, revealed significant differences between cells exposed to modified vs unmodified laminin-1 in 19 genes, 17 of which--including those encoding alpha3 integrin, extracellular matrix protein 1, proteolytic enzymes (such as MT1-MMP, stromelysin-3 and cathepsin L) and their inhibitors--were up-modulated in cells treated with modified laminin-1. Zymogram analysis clearly indicated a significant increase in the activity of the gelatinolytic enzyme MMP-2 in the culture supernatant from cells exposed to modified laminin-1, without an increase in mRNA abundance as observed in microarray analysis. Invasiveness of tumor cells conditioned by modified laminin-1, evaluated as the capability to cross Matrigel basement, was significantly more inhibited by MMPinhibitor TIMP-2 than invasiveness induced by native laminin-1. Taken together, our findings indicate that the role of 67LR in tumor aggressiveness rests in its ability to modify laminin-1 thereby activating proteolytic enzymes that promote tumor cell invasion through extracellular matrix degradation.