Combretastatin A-4 analogues with benzoxazolone scaffold: Synthesis, structure and biological activity.

In order to design and synthesize a new class of heterocyclic analogues of natural combretastatin A-4 and its synthetic derivative AVE8062, the benzoxazolone ring was selected as a scaffold for a bioisosteric replacement of the ring B of both molecules. A library of 28 cis- and trans-styrylbenzoxazolones was obtained by a modified Wittig reaction under Boden's conditions. Structures of the newly synthesized compounds bearing the 3,4,5-trimethoxy-, 3,4-dimethoxy-, 3,5-dimethoxy-, and 4-methoxystyryl fragment at position 4, 5, 6 or 7 of benzoxazolone core were determined on the basis of spectral and X ray data. The in vitro cytotoxicity of styrylbenzoxazolones against different cell lines was examined. Stilbene derivative 16Z, (Z)-3-methyl-6-(3,4,5-trimethoxystyryl)-2(3H)-benzoxazolone, showed highest antiproliferative potential of the series, with IC50 of 0.25 µM against combretastatin resistant cell line HT-29, 0.19 µM against HepG2, 0.28 µM against EA.hy926 and 0.73 µM against K562 cells. Furthermore, the results of flow cytometric analysis confirmed that 16Z induced cell cycle arrest in G2/M phase in the cell lines like combretastatin A-4. This arrest is followed by an abnormal exit of cells from mitosis without cytokinesis into a pseudo G1-like multinucleate state leading to late apoptosis and cell death. Accordingly, synthetic analogue 16Z was identified as the most promising potential anticancer agent in present study, and was selected as lead compound for further detailed investigations.

The activating role of phospho-(Tyr)-calmodulin on the epidermal growth factor receptor.

The activity of calmodulin (CaM) is modulated not only by oscillations in the cytosolic concentration of free Ca(2+), but also by its phosphorylation status. In the present study, the role of tyrosine-phosphorylated CaM [P-(Tyr)-CaM] on the regulation of the epidermal growth factor receptor (EGFR) has been examined using in vitro assay systems. We show that phosphorylation of CaM by rat liver solubilized EGFR leads to a dramatic increase in the subsequent phosphorylation of poly-L-(Glu:Tyr) (PGT) by the receptor in the presence of ligand, both in the absence and in the presence of Ca(2+). This occurred in contrast with assays where P-(Tyr)-CaM accumulation was prevented by the presence of Ca(2+), absence of a basic cofactor required for CaM phosphorylation and/or absence of CaM itself. Moreover, an antibody against CaM, which inhibits its phosphorylation, prevented the extra ligand-dependent EGFR activation. Addition of purified P-(Tyr)-CaM, phosphorylated by recombinant c-Src (cellular sarcoma kinase) and free of non-phosphorylated CaM, obtained by affinity-chromatography using an immobilized anti-phospho-(Tyr)-antibody, also increased the ligand-dependent tyrosine kinase activity of the isolated EGFR toward PGT. Also a CaM(Y99D/Y138D) mutant mimicked the effect of P-(Tyr)-CaM on ligand-dependent EGFR activation. Finally, we demonstrate that P-(Tyr)-CaM binds to the same site ((645)R-R-R-H-I-V-R-K-R-T-L-R-R-L-L-Q(660)) as non-phosphorylated CaM, located at the cytosolic juxtamembrane region of the EGFR. These results show that P-(Tyr)-CaM is an activator of the EGFR and suggest that it could contribute to the CaM-mediated ligand-dependent activation of the receptor that we previously reported in living cells.

O-GlcNAcylation of the human epidermal growth factor receptor.

The reversible O-linked attachment of single ß-D-N-acetylglucosamine (GlcNAc) moieties to serine/threonine residues in target proteins is a frequently occurring post-translational modification affecting the functionality of many cellular systems. In this report we present experimental evidence suggesting that the epidermal growth factor receptor (EGFR) is subjected to O-GlcNAcylation in human carcinoma epidermoid A431 cells and human lung carcinoma A549 cells. However, no signal was detected in human cervix adenocarcinoma HeLa cells or in mouse EGFR-T17 fibroblasts ectopically expressing the human EGFR. We detected a positive O-GlcNAcylation signal in the immunoprecipitated EGFR by Western blotting using two distinct specific anti-O-GlcNAc antibodies even after N-deglycosylation of the receptor using peptide-N-glycosidase F (PNGase F). Conversely, the presence of EGFR was detected by Western blotting using an anti-EGFR antibody in the immunocomplex of O-GlcNAcylated proteins immunoprecipitated with an anti-O-GlcNAc antibody. These signals were enhanced when the O-linked ß-N-acetylglucosaminidase (OGA) inhibitor Thiamet G was added to prevent the deglycosylation of the GlcNAc moiety(ies). Moreover, we also detected a positive signal in the immunoprecipitated and N-deglycosylated EGFR using PNGase F, and tunicamycin when the cells were metabolically labeled with azido-GlcNAc (GlcNAz), biotinylated and probed with a streptavidin-labeled peroxidase. Finally, EGFR and O-linked ß-N-acetylglucosamine transferase (OGT) co-immunoprecipitate, and incubation of the immunoprecipitated EGFR with the immunoprecipitated OGT in the presence of uridine 5'-diphospho-N-acetylglucosamine (UDP-GlcNAc) resulted in a significant enhancement of the EGFR O-GlcNAcylation signal as detected by Western blotting using an anti-O-GlcNAc antibody. We conclude that the human EGFR is subjected to O-GlcNAcylation in the A431 and A549 tumor cell lines.

Ca2+/Calmodulin and Apo-Calmodulin Both Bind to and Enhance the Tyrosine Kinase Activity of c-Src.

Src family non-receptor tyrosine kinases play a prominent role in multiple cellular processes, including: cell proliferation, differentiation, cell survival, stress response, and cell adhesion and migration, among others. And when deregulated by mutations, overexpression, and/or the arrival of faulty incoming signals, its hyperactivity contributes to the development of hematological and solid tumors. c-Src is a prototypical member of this family of kinases, which is highly regulated by a set of phosphorylation events. Other factor contributing to the regulation of Src activity appears to be mediated by the Ca2+ signal generated in cells by different effectors, where the Ca2+-receptor protein calmodulin (CaM) plays a key role. In this report we demonstrate that CaM directly interacts with Src in both Ca2+-dependent and Ca2+-independent manners in vitro and in living cells, and that the CaM antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibits the activation of this kinase induced by the upstream activation of the epidermal growth factor receptor (EGFR), in human carcinoma epidermoide A431 cells, and by hydrogen peroxide-induced oxidative stress, in both A431 cells and human breast adenocarcinoma SK-BR-3 cells. Furthermore, we show that the Ca2+/CaM complex strongly activates the auto-phosphorylation and tyrosine kinase activity of c-Src toward exogenous substrates, but most relevantly and for the first time, we demonstrate that Ca2+-free CaM (apo-CaM) exerts a far higher activatory action on Src auto-phosphorylation and kinase activity toward exogenous substrates than the one exerted by the Ca2+/CaM complex. This suggests that a transient increase in the cytosolic concentration of free Ca2+ is not an absolute requirement for CaM-mediated activation of Src in living cells, and that a direct regulation of Src by apo-CaM could be inferred.

Characterization of phospho-(tyrosine)-mimetic calmodulin mutants.

Calmodulin (CaM) phosphorylated at different serine/threonine and tyrosine residues is known to exert differential regulatory effects on a variety of CaM-binding enzymes as compared to non-phosphorylated CaM. In this report we describe the preparation and characterization of a series of phospho-(Y)-mimetic CaM mutants in which either one or the two tyrosine residues present in CaM (Y99 and Y138) were substituted to aspartic acid or glutamic acid. It was expected that the negative charge of the respective carboxyl group of these amino acids mimics the negative charge of phosphate and reproduce the effects that distinct phospho-(Y)-CaM species may have on target proteins. We describe some physicochemical properties of these CaM mutants as compared to wild type CaM, after their expression in Escherichia coli and purification to homogeneity, including: i) changes in their electrophoretic mobility in the absence and presence of Ca2+; ii) ultraviolet (UV) light absorption spectra, far- and near-UV circular dichroism data; iii) thermal stability in the absence and presence of Ca2+; and iv) Tb3+-emitted fluorescence upon tyrosine excitation. We also describe some biochemical properties of these CaM mutants, such as their differential phosphorylation by the tyrosine kinase c-Src, and their action as compared to wild type CaM, on the activity of two CaM-dependent enzymes: cyclic nucleotide phosphodiesterase 1 (PDE1) and endothelial nitric oxide synthase (eNOS) assayed in vitro.

Targeting the calmodulin-regulated ErbB/Grb7 signaling axis in cancer therapy.

Signal transduction pathways essential for the survival and viability of the cell and that frequently present aberrant expression or function in tumors are attractive targets for pharmacological intervention in human cancers. In this short review we will describe the regulation exerted by the calcium-receptor protein calmodulin (CaM) on signaling routes involving the family of ErbB receptors - highlighting the epidermal growth factor receptor (EGFR/ErbB1) and ErbB2 - and the adaptor protein Grb7, a downstream signaling component of these receptors. The signaling mechanism of the ErbB/Grb7 axis and the regulation exerted by CaM on this pathway will be described. We will present a brief overview of the current efforts to inhibit the hyperactivity of ErbB receptors and Grb7 in tumors. The currently available information on targeting the CaM-binding site of these signaling proteins will be analyzed, and the pros and cons of directly targeting CaM versus the CaM-binding domain of the ErbB receptors and Grb7 as potential anti-cancer therapy will be discussed.