Structural Basis for the Failure of the C1 Domain of Ras Guanine Nucleotide Releasing Protein 2 (RasGRP2) to Bind Phorbol Ester with High Affinity.

The C1 domain represents the recognition module for diacylglycerol and phorbol esters in protein kinase C, Ras guanine nucleotide releasing protein (RasGRP), and related proteins. RasGRP2 is exceptional in that its C1 domain has very weak binding affinity (Kd = 2890 ± 240 nm for [(3)H]phorbol 12,13-dibutyrate. We have identified four amino acid residues responsible for this lack of sensitivity. Replacing Asn(7), Ser(8), Ala(19), and Ile(21) with the corresponding residues from RasGRP1/3 (Thr(7), Tyr(8), Gly(19), and Leu(21), respectively) conferred potent binding affinity (Kd = 1.47 ± 0.03 nm) in vitro and membrane translocation in response to phorbol 12-myristate 13-acetate in LNCaP cells. Mutant C1 domains incorporating one to three of the four residues showed intermediate behavior with S8Y making the greatest contribution. Binding activity for diacylglycerol was restored in parallel. The requirement for anionic phospholipid for [(3)H]phorbol 12,13-dibutyrate binding was determined; it decreased in going from the single S8Y mutant to the quadruple mutant. The full-length RasGRP2 protein with the mutated C1 domains also showed strong phorbol ester binding, albeit modestly weaker than that of the C1 domain alone (Kd = 8.2 ± 1.1 nm for the full-length protein containing all four mutations), and displayed translocation in response to phorbol ester. RasGRP2 is a guanyl exchange factor for Rap1. Consistent with the ability of phorbol ester to induce translocation of the full-length RasGRP2 with the mutated C1 domain, phorbol ester enhanced the ability of the mutated RasGRP2 to activate Rap1. Modeling confirmed that the four mutations helped the binding cleft maintain a stable conformation.

Polar 3-alkylidene-5-pivaloyloxymethyl-5'-hydroxymethyl-gamma-lactones as protein kinase C ligands and antitumor agents.

A series of DAG-lactones with polar 3-alkylidene substituents have been investigated as PKC-alpha ligands and antitumor agents. Extensive analysis of structure-activity relationships for the 3-alkylidene chain revealed that polar groups such as ether, hydroxyl, aldehyde, ester, acyloxy, and amido were tolerated with similar binding affinities and reduced lipophilicities compared to the corresponding unsubstituted alkylidene chain. Among the derivatives, compounds 5, 6 and 8 with an ether type of side chain showed high binding affinities in range of K(i)= 3-5 nM and excellent antitumor profiles, particularly against the colo205 colon cancer and the K562 leukemia cell lines.

Stereospecific high-affinity TRPV1 antagonists: chiral N-(2-benzyl-3-pivaloyloxypropyl) 2-[4-(methylsulfonylamino)phenyl]propionamide analogues.

Previously, we reported the thiourea antagonists 2a and 2b as potent and high affinity TRPV1 antagonists. For further optimization of the lead compounds, a series of their amide and alpha-substituted amide surrogates were investigated and novel chiral N-(2-benzyl-3-pivaloyloxypropyl) 2-[4-(methylsulfonylamino)phenyl]propionamide analogues were characterized as potent and stereospecific rTRPV1 antagonists. In particular, compounds 72 and 73 displayed high binding affinities, with K i values of 4.12 and 1.83 nM and potent antagonism with K i values of 0.58 and 5.2 nM, respectively, in rTRPV1/CHO cells. These values are comparable or more potent than those of 5-iodoRTX under the same assay conditions. A distinctive binding model that includes two hydrophobic pockets is proposed for this series of compounds based on docking studies of 57 and 72 with a homology model of the TM3/4 region of TRPV1.

Alpha-substituted N-(4-tert-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues as potent and stereospecific TRPV1 antagonists.

A series of alpha-substituted N-(4-tert-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. alpha-Methyl substituted analogues showed potent and stereospecific antagonism to the action of capsaicin on rat TRPV1 heterologously expressed in Chinese hamster ovary cells. In particular, compounds 14 and 18, which possess the R-configuration, exhibited excellent potencies (respectively, K(i)=41 and 39.2 nM and K(i(ant))=4.5 and 37 nM).

Branched diacylglycerol-lactones as potent protein kinase C ligands and alpha-secretase activators.

Using as our lead structure a potent PKC ligand (1) that we had previously described, we investigated a series of branched DAG-lactones to optimize the scaffold for PKC binding affinity and reduced lipophilicity, and we examined the potential utility of select compounds as alpha-secretase activators. Activation of alpha-secretase upon PKC stimulation by ligands causes increased degradation of the amyloid precursor protein (APP), resulting in enhanced secretion of sAPPalpha and reduced deposition of beta-amyloid peptide (Abeta), which is implicated in the pathogenesis of Alzheimer's disease. We modified in a systematic manner the C5-acyl group, the 3-alkylidene, and the lactone ring in 1 and established structure-activity relationships for this series of potent PKC ligands. Select DAG-lactones with high binding affinities for PKC were evaluated for their abilities to lead to increased sAPPalpha secretion as a result of alpha-secretase activation. The DAG-lactones potently induced alpha-secretase activation, and their potencies correlated with the corresponding PKC binding affinities and lipophilicities. Further investigation indicated that 2 exhibited a modestly higher level of sAPPalpha secretion than did phorbol 12,13-dibutyrate (PDBu).

Delta(9)-Tetrahydrocannabinol-induced desensitization of cannabinoid-mediated inhibition of synaptic transmission between hippocampal neurons in culture.

Prolonged exposure to cannabinoids results in desensitization of cannabinoid receptors. Here, we compared the desensitization produced by the partial agonist, Delta(9)-tetrahydrocannabinol (THC) to that produced by the full agonist Win55,212-2 on cannabinoid-mediated inhibition of glutamatergic synaptic transmission. Synaptic activity between rat hippocampal neurons was determined from network-driven increases in the intracellular Ca(2+) concentration ([Ca(2+)](i) spikes). To assess the effects of prolonged treatment, cultures were incubated with cannabinoids, washed in 0.5% fatty-acid-free bovine serum albumin to ensure the removal of the lipophilic drug and then tested for inhibition of [Ca(2+)](i) spiking by Win55,212-2. In control experiments, 0.1 microM Win55,212-2 inhibited [Ca(2+)](i) spiking by 93 +/- 5%. Win55,212-2 produced significantly less inhibition of [Ca(2+)](i) spiking following 18-24h treatment with 1 microM THC (48 +/- 5%) or treatment with 1 microM Win55,212-2 (29 +/- 6%). Thus, THC produced significantly less functional desensitization than Win55,212-2. The desensitization produced by THC was maximal at 0.3 microM, remained stable between 1 and 7 days of preincubation and shifted the EC(50) of acute inhibition by Win55,212-2 from 27 to 251 nM. Differences in the long-term effects of cannabinoid receptor agonists on synaptic transmission may prove important for evaluating their therapeutic and abuse potential.

Analysis of structure-activity relationships for the 'A-region' of N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues as TRPV1 antagonists.

The structure-activity relationships for the 'A-region' of N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. The 2-halogen analogues showed enhanced antagonism compared to the prototype antagonist.

Analysis of structure-activity relationships for the 'B-region' of N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]-thiourea analogues as TRPV1 antagonists.

The structure-activity relationships for the 'B-region' of N-(4-t-butylbenzyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues have been investigated as TRPV1 receptor antagonists. A docking model of potent antagonist 2 with the sensor region of TRPV1 is proposed.

Analysis of structure-activity relationships with the N-(3-acyloxy-2-benzylpropyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea template for vanilloid receptor 1 antagonism.

In a continuing effort to elucidate the structure-activity relationships of the lead antagonist N-[2-(3,4-dimethylbenzyl)-3-pivaloyloxypropyl]-N'-[4-(methylsulfonylamino)benzyl]thiourea (1), the distances between the proposed four pharmacophores in 1 have been varied by insertion or deletion of one carbon to optimize their fit to the receptor. In addition, the acyloxy group of the C region was replaced with amide and N-hydroxy amide to identify the pharmacophoric importance of the ester group in the C2 region. The results indicated that the pharmacophoric arrangement of 1 was optimal for receptor binding affinity and antagonism, and the ester of the C2 region was significant for receptor binding. Among the derivatives, compound 19 showed distinct behavior with a 2-fold improvement in antagonism but a 13-fold reduction in binding affinity compared to 1. The partial separation of pharmacophoric requirements of these two assays has been noted before and compound 19 is thus selective for the calcium entry-linked receptor population. The conformational analysis of 1 generated three distinct conformers having different types of hydrophobic interactions, which will be utilized for exploring the active conformation of the VR1 ligand.

Characterization of the interaction of ingenol 3-angelate with protein kinase C.

Ingenol 3-angelate (I3A) is one of the active ingredients in Euphorbia peplus, which has been used in traditional medicine. Here, we report the initial characterization of I3A as a protein kinase C (PKC) ligand. I3A bound to PKC-alpha in the presence of phosphatidylserine with high affinity; however, under these assay conditions, little PKC isoform selectivity was observed. PKC isoforms did show different sensitivity and selectivity for down-regulation by I3A and phorbol 12-myristate 13-acetate (PMA) in WEHI-231, HOP-92, and Colo-205 cells. In all of the three cell types, I3A inhibited cell proliferation with somewhat lower potency than did PMA. In intact CHO-K1 cells, I3A was able to translocate different green fluorescent protein-tagged PKC isoforms, visualized by confocal microscopy, with equal or higher potency than PMA. PKC-delta in particular showed a different pattern of translocation in response to I3A and PMA. I3A induced a higher level of secretion of the inflammatory cytokine interleukin 6 compared with PMA in the WEHI-231 cells and displayed a marked biphasic dose-response curve for the induction. I3A was unable to cause the same extent of association of the C1b domain of PKC-delta with lipids, compared with PMA or the physiological regulator diacylglycerol, and was able to partially block the association induced by these agents, measured by surface plasmon resonance. The in vitro kinase activity of PKC-alpha induced by I3A was lower than that induced by PMA. The novel pattern of behavior of I3A makes it of great interest for further evaluation.

Analysis of structure-activity relationships for the 'B-region' of N-(3-acyloxy-2-benzylpropyl)-N(')-[4-(methylsulfonylamino)benzyl]thiourea analogues as vanilloid receptor antagonists: discovery of an N-hydroxythiourea analogue with potent analgesic activity.

The structural modifications on the B-region of the potent and high affinity vanilloid receptor (VR1) lead ligand N-(3-acyloxy-2-benzylpropyl)-N(')-[4-(methylsulfonylamino)benzyl]thiourea were investigated by the replacement of the thiourea with diverse isosteric functional groups. Structure-activity analysis indicated that the A-region in this series was the primary factor in determining the agonistic/antagonistic activities regardless of the B-region. The N(C)-hydroxy thiourea analogues (12, 13) showed excellent analgesic activities in the acetic acid writhing assay compared to the parent thiourea analogues.

N-(3-acyloxy-2-benzylpropyl)-N'-[4-(methylsulfonylamino)benzyl]thiourea analogues: novel potent and high affinity antagonists and partial antagonists of the vanilloid receptor.

Isosteric replacement of the phenolic hydroxyl group in potent vanilloid receptor (VR1) agonists with the alkylsulfonamido group provides a series of compounds which are effective antagonists to the action of the capsaicin on rat VR1 heterologously expressed in Chinese hamster ovary (CHO) cells. In particular, compound 61, N-[2-(3,4-dimethylbenzyl)-3-pivaloyloxypropyl]-N'-[3-fluoro-4-(methylsulfonylamino)benzyl]thiourea was a full antagonist against capsaicin, displayed a K(i) value of 7.8 nM (compared to 520 nM for capsazepine and 4 nM for 5-iodoRTX), and showed excellent analgesic activity in mice. Structure-activity analysis of the influence of modifications in the A- and C-regions of 4-methylsulfonamide ligands on VR1 agonism/antagonism indicated that 3-fluoro substitution in the A-region and a 4-tert-butylbenzyl moiety in the C-region favored antagonism, whereas a 3-methoxy group in the A-region and 3-acyloxy-2-benzylpropyl moieties in the C-region favored agonism.