Conformationally constrained analogues of diacylglycerol (DAG). 16. How much structural complexity is necessary for recognition and high binding affinity to protein kinase C?

Discovery of Potent DAG-Lactone Derivatives as HIV Latency Reversing Agents

Toward human immunodeficiency virus type-1 (HIV-1) cure, cells latently infected with HIV-1 must be eliminated from people living with HIV-1. We previously developed a protein kinase C (PKC) activator, diacylglycerol (DAG)-lactone derivative 3, with high HIV-1 latency-reversing activity, based on YSE028 (2) as a lead compound and found that the activity was correlated with binding affinity for PKC and stability against esterase-mediated hydrolysis. Here, we synthesized new DAG-lactone derivatives not only containing a tertiary ester group or an isoxazole surrogate but also several symmetric alkylidene moieties to improve HIV-1 latency reversing activity. Compound 9a, with a dimethyl group at the α-position of the ester group, exerted twice higher HIV-1 latency reversing activity than compound 3, and compound 26, with the isoxazole moiety, was significantly active. In addition, DAG-lactone derivatives with moderate hydrophobicity and potent biostability showed high biological activity.

Fluorescence resonance energy transfer-based screening for protein kinase C ligands using 6-methoxynaphthalene-labeled 1,2-diacylglycerol-lactones

Protein kinase C (PKC) is associated with a central cellular signal transduction pathway and disorders such as cancer and Alzheimer-type dementia and is therefore a target for the treatment of these diseases. The development of simple methods suitable for high-throughput screening to find potent PKC ligands is desirable. We have developed an assay based on fluorescence-quenching screening with a solvatochromic fluorophore attached to a competitive probe and its alternative method based on Förster/fluorescence resonance energy transfer (FRET) phenomena. Here, an improved FRET-based PKC binding assay using a diacylglycerol (DAG) lactone labeled with a donor fluorescent dye, 6-methoxynaphthalene (6MN), was developed. The 6MN-labeled DAG-lactone has a higher binding affinity for the PKCδ C1b domain and the fluorescent PKCδ C1b domain labeled by fluorescein as an acceptor fluorescent dye (Fl-δC1b) than the diethylaminocoumarin (DEAC)-labeled DAG-lactone. The combination of the 6MN-labeled DAG-lactone and Fl-δC1b showed a change in fluorescence response larger than that of the DEAC-labeled DAG-lactone and Fl-δC1b. The IC₅₀ values of known PKC ligands calculated by the present FRET-based method using 6MN-labeled DAG-lactone agree well with the K_i values obtained by the conventional radioisotope-based assays. Some false positive compounds, identified by the previous solvatochromic fluorophore-based method, were found to be negative by this method. The present FRET-based PKC binding assay is more sensitive and could be more useful.

Design, Synthesis, and Characterization of Novel sn-1 Heterocyclic DAG-Lactones as PKC Activators

DAG-lactones represent useful templates for the design of potent and selective C1 domain ligands for PKC isozymes. The ester moiety at the sn-1 position, a common feature in this template, is relevant for C1 domain interactions, but it represents a labile group susceptible to endogenous esterases. An interesting challenge involves replacing the ester group of these ligands while still maintaining biological activity. Here, we present the synthesis and functional characterization of novel diacylglycerol-lactones containing heterocyclic ring substituents at the sn-1 position. Our results showed that the new compound 10B12, a DAG-lactone with an isoxazole ring, binds PKCα and PKCε with nanomolar affinity. Remarkably, 10B12 displays preferential selectivity for PKCε translocation in cells and induces a PKCε-dependent reorganization of the actin cytoskeleton into peripheral ruffles in lung cancer cells. We conclude that introducing a stable isoxazole ring as an ester surrogate in DAG-lactones emerges as a novel structural approach to achieve PKC isozyme selectivity.

A Therapeutic Strategy to Combat HIV-1 Latently Infected Cells With a Combination of Latency-Reversing Agents Containing DAG-Lactone PKC Activators

Advances in antiviral therapy have dramatically improved the therapeutic effects on HIV type 1 (HIV-1) infection. However, even with potent combined antiretroviral therapy, HIV-1 latently infected cells cannot be fully eradicated. Latency-reversing agents (LRAs) are considered a potential tool for eliminating such cells; however, recent in vitro and in vivo studies have raised serious concerns regarding the efficacy and safety of the "shock and kill" strategy using LRAs. In the present study, we examined the activity and safety of a panel of protein kinase C (PKC) activators with a diacylglycerol (DAG)-lactone structure that mimics DAG, an endogenous ligand for PKC isozymes. YSE028, a DAG-lactone derivative, reversed HIV-1 latency in vitro when tested using HIV-1 latently infected cells (e.g., ACH2 and J-Lat cells) and primary cells from HIV-1-infected individuals. The activity of YSE028 in reversing HIV-1 latency was synergistically enhanced when combined with JQ1, a bromodomain and extra-terminal inhibitor LRA. DAG-lactone PKC activators also induced caspase-mediated apoptosis, specifically in HIV-1 latently infected cells. In addition, these DAG-lactone PKC activators showed minimal toxicity in vitro and in vivo. These data suggest that DAG-lactone PKC activators may serve as potential candidates for combination therapy against HIV-1 latently infected cells, especially when combined with other LRAs with a different mechanism, to minimize side effects and achieve maximum efficacy in various reservoir cells of the whole body.

Scaffold hopping from (5-hydroxymethyl) isophthalates to multisubstituted pyrimidines diminishes binding affinity to the C1 domain of protein kinase C

Protein kinase C (PKC) isoforms play a pivotal role in the regulation of numerous cellular functions, making them extensively studied and highly attractive drug targets. Utilizing the crystal structure of the PKCδ C1B domain, we have developed hydrophobic isophthalic acid derivatives that modify PKC functions by binding to the C1 domain of the enzyme. In the present study, we aimed to improve the drug-like properties of the isophthalic acid derivatives by increasing their solubility and enhancing the binding affinity. Here we describe the design and synthesis of a series of multisubstituted pyrimidines as analogs of C1 domain–targeted isophthalates and characterize their binding affinities to the PKCα isoform. In contrast to our computational predictions, the scaffold hopping from phenyl to pyrimidine core diminished the binding affinity. Although the novel pyrimidines did not establish improved binding affinity for PKCα compared to our previous isophthalic acid derivatives, the present results provide useful structure-activity relationship data for further development of ligands targeted to the C1 domain of PKC.

Characterization of AJH-836, a diacylglycerol-lactone with selectivity for novel PKC isozymes

Diacylglycerol (DAG) is a key lipid second messenger downstream of cellular receptors that binds to the C1 domain in many regulatory proteins. Protein kinase C (PKC) isoforms constitute the most prominent family of signaling proteins with DAG-responsive C1 domains, but six other families of proteins, including the chimaerins, Ras-guanyl nucleotide-releasing proteins (RasGRPs), and Munc13 isoforms, also play important roles. Their significant involvement in cancer, immunology, and neurobiology has driven intense interest in the C1 domain as a therapeutic target. As with other classes of targets, however, a key issue is the establishment of selectivity. Here, using [³H]phorbol 12,13-dibutyrate ([³H]PDBu) competition binding assays, we found that a synthetic DAG-lactone, AJH-836, preferentially binds to the novel PKC isoforms PKCδ and PKCϵ relative to classical PKCα and PKCβII. Assessment of intracellular translocation, a hallmark for PKC activation, revealed that AJH-836 treatment stimulated a striking preferential redistribution of PKCϵ to the plasma membrane relative to PKCα. Moreover, unlike with the prototypical phorbol ester phorbol 12-myristate 13-acetate (PMA), prolonged exposure of cells to AJH-836 selectively down-regulated PKCδ and PKCϵ without affecting PKCα expression levels. Biologically, AJH-836 induced major changes in cytoskeletal reorganization in lung cancer cells, as determined by the formation of membrane ruffles, via activation of novel PKCs. We conclude that AJH-836 represents a C1 domain ligand with PKC-activating properties distinct from those of natural DAGs and phorbol esters. Our study supports the feasibility of generating selective C1 domain ligands that promote novel biological response patterns.

The C1 domain of Vav3, a novel potential therapeutic target

Vav1/2/3 comprise a protein family with guanyl nucleotide exchange activity for Rho and Rac as well as with motifs conferring adapter activity. Biologically, Vav1 plays a critical role in hematologic cell signaling, whereas Vav2/3 have a wider tissue distribution, but all 3 Vav proteins are implicated in cancer development. A structural feature of Vav1/2/3 is the presence of an atypical C1 domain, which possesses close structural homology to the typical C1 domains of protein kinase C but which fails to bind the second messenger diacylglycerol or the potent analogs, the phorbol esters. Previously, we have shown that five residues in the Vav1 C1 domain are responsible for its lack of phorbol ester binding. Here, we show that the lack of phorbol ester binding of Vav3 has a similar basis. We then explore the consequences of phorbol ester binding to a modified Vav3 in which the C1 domain has been altered to allow phorbol ester binding. We find both disruption of the guanyl nucleotide exchange activity of the modified Vav 3 as well as a shift in localization to the membrane upon phorbol ester treatment. This change in localization is associated with altered interactions with other signaling proteins. The studies provide a first step in assessing the potential for the design of custom C1 domain targeted molecules selective for the atypical C1 domains of Vav family proteins.

Synthesis and Evaluation of Dimeric Derivatives of Diacylglycerol-Lactones as Protein Kinase C Ligands

Protein kinase C (PKC) mediates a central cellular signal transduction pathway involved in disorders such as cancer and Alzheimer's disease. PKC is regulated by binding of the second messenger sn-1,2-diacylglycerol (DAG) to its tandem C1 domains, designated C1a and C1b, leading both to PKC activation and to its translocation to the plasma membrane and to internal organelles. Depending on the isoform, there may be differences in the ligand selectivity of the C1a and C1b domains, and there is different spacing between the C1 domains of the conventional and novel PKCs. Bivalent ligands have the potential to exploit these differences between isoforms, yielding isoform selectivity. In the present study, we describe the synthesis of a series of dimeric derivatives of conformationally constrained diacylglycerol (DAG) analogs (DAG-lactones). We characterize the derivatives in vitro for their binding affinities, both to a single C1 domain (the C1b domain of PKCδ) as well as to the conventional PKCα isoform and the novel PKCδ isoform, and we measure their abilities to cause translocation of PKCδ and PKCε in intact cells. The dimeric compound with the 10-carbon linker was modestly more effective for the isolated PKCδ C1b domain than was the monomeric compound. For the intact PKCα and PKCδ, the shortest DAG-lactone dimer had similar affinity to the monomer and affinity decreased progressively up to the 16-carbon linker. The dimeric derivatives did not cause the Golgi accumulation of PKCδ. The present results provide important insights into the development of new chemical tools for biological studies on PKC.

Exploring the influence of indololactone structure on selectivity for binding to the C1 domains of PKCα, PKCε, and RasGRP

C1 domain-containing proteins, such as protein kinase C (PKC), have a central role in cellular signal transduction. Their involvement in many diseases, including cancer, cardiovascular disease, and immunological and neurological disorders has been extensively demonstrated and has prompted a search for small molecules to modulate their activity. By employing a diacylglycerol (DAG)-lactone template, we have been able to develop ultra potent analogs of diacylglycerol with nanomolar binding affinities approaching those of complex natural products such as phorbol esters and bryostatins. One current challenge is the development of selective ligands capable of discriminating between different protein family members. Recently, structure-activity relationship studies have shown that the introduction of an indole ring as a DAG-lactone substituent yielded selective Ras guanine nucleotide-releasing protein (RasGRP1) activators when compared to PKCα and PKCε. In the present work, we examine the effects of ligand selectivity relative to the orientation of the indole ring and the nature of the DAG-lactone template itself. Our results show that the indole ring must be attached to the lactone moiety through the sn-2 position in order to achieve RasGRP1 selectivity.

Synthesis and Biological Evaluation of Several Bryostatin Analogues Bearing a Diacylglycerol Lactone C-Ring

As an initial step in designing a simplified bryostatin hybrid molecule, three bryostatin analogues bearing a diacylglycerol lactone-based C-ring, which possessed the requisite pharmacophores for binding to protein kinase C (PKC) together with a modified bryostatin-like A- and B-ring region, were synthesized and evaluated. Merle 46 and Merle 47 exhibited binding affinity to PKC alpha with Ki values of 7000 ± 990 and 4940 ± 470 nM, respectively. Reinstallation of the trans-olefin and gem-dimethyl group present in bryostatin 1 in Merle 48 resulted in improved binding affinity, 363 ± 42 nM. While Merle 46 and 47 were only marginally active biologically, Merle 48 showed sufficient activity on the U937 cells to confirm that it was PMA-like for growth and attachment, as predicted by the substitution pattern of its A- and B-rings.

C1 domain-targeted isophthalates as protein kinase C modulators: structure-based design, structure–activity relationships and biological activities

Protein kinase C (PKC) is a serine/threonine kinase belonging to the AGC family. PKC isoenzymes are activated by phospholipid-derived second messengers, transmit their signal by phosphorylating specific substrates and play a pivotal role in the regulation of various cell functions, including metabolism, growth, differentiation and apoptosis. Therefore they represent an interesting molecular target for the treatment of several diseases, such as cancer and Alzheimer's disease. Adopting a structure-based approach on the crystal structure of the PKCδ C1B domain, our team has developed isophthalic acid derivatives that are able to modify PKC functions by binding to the C1 domain of the enzyme. Bis[3-(trifluoromethyl)benzyl] 5-(hydroxymethyl)isophthalate (HMI-1a3) and bis(1-ethylpentyl) 5-(hydroxymethyl)isophthalate (HMI-1b11) were selected from a set of compounds for further studies due to their high affinity for the C1 domains of PKCα and PKCδ. HMI-1a3 showed marked antiproliferative activity in HeLa cells whereas HMI-1b11 induced differentiation and supported neurite growth in SH-SY5Y cells. Our aim in the future is to improve the selectivity and potency of isophthalate derivatives, to clarify their mechanism of action in the cellular environment and to assess their efficacy in cell-based and in vivo disease models. HMI-1a3 has already been selected for a further project and redesigned to function as a probe immobilized on an affinity chromatography column. It will be used to identify cellular target proteins from cell lysates, providing new insights into the mechanism of action of HMI-1a3.

Design and synthesis of protein kinase C epsilon selective diacylglycerol lactones (DAG-lactones)

DAG-lactones afford a synthetically accessible, high affinity platform for probing structure activity relationships at the C1 regulatory domain of protein kinase C (PKC). Given the central role of PKC isoforms in cellular signaling, along with their differential biological activities, a critical objective is the design of isoform selective ligands. Here, we report the synthesis of a series of DAG-lactones varying in their side chains, with a particular focus on linoleic acid derivatives. We evaluated their selectivity for PKC epsilon versus PKC alpha both under standard lipid conditions (100% phosphatidylserine, PS) as well as in the presence of a nuclear membrane mimetic lipid mixture (NML). We find that selectivity for PKC epsilon versus PKC alpha tended to be enhanced in the presence of the nuclear membrane mimetic lipid mixture and, for our lead compound, report a selectivity of 32-fold.

Diacylglycerol lactones targeting the structural features that distinguish the atypical C1 domains of protein kinase C ζ and ι from typical C1 domains

To explore the feasibility of developing ligands targeted to the atypical C1 domains of protein kinase C ζ and ι, we have prepared diacylglycerol lactones substituted with hydrophilic groups on their side chains, which potentially could interact with the arginine residues that distinguish the atypical C1 domains of PKCζ and PKCι from typical C1 domains, and we have measured their binding to mutated versions of the C1b domain of PKCδ that incorporate one or more of these arginine residues. The most selective of the diacylglycerol lactones showed only a 10-fold reduction in binding affinity with the triple arginine mutant (N7R/S10R/L20R) compared to the wild-type, whereas phorbol 12,13-dibutyrate showed a 6000-fold loss of affinity. Molecular modeling confirms that these ligands are indeed able to interact with the arginine residues. Our results show that dramatic changes in selectivity can be obtained through appropriate substitution of diacylglycerol lactones.

Bilayer interaction and protein kinase C-C1 domain binding studies of kojic acid esters

Synthesis of kojic acid ester analogues and their lipid bilayer interaction and PKC-C1 domain binding properties have been demonstrated in this present work.

Peptide-based ligand screening and functional analysis of protein kinase C

Protein kinase C (PKC) plays an important role in cell signaling pathways and is implicated in disorders ranging from cancer to Alzheimer's disease. Highly potent PKC ligands as therapeutic drugs have not been developed to date and useful methodologies for controlling PKC activation in defined areas are necessary to analyze precise PKC functions in cells. Studies focused on the development of PKC ligand screening systems and methods for regulation of PKC activation have been performed in our laboratory. In this review, our ligand screening methods involving synthetic peptides and solvatochromic fluorescent dye-labeled small compounds are summarized and the technique of spatio-temporal manipulation of PKC activation by caging strategies is introduced.

Euphohelioscopin A is a PKC activator capable of inducing macrophage differentiation

To identify small molecules that selectively control hematopoietic stem cell differentiation, we performed an unbiased screen using primary human CD34(+) cells. We identified a plant-derived natural product, euphohelioscopin A, capable of selectively differentiating CD34(+) cells down the granulocyte/monocytic lineage. Euphohelioscopin A also inhibits proliferation and induces differentiation of the myeloid leukemia cell lines THP-1 and HL-60. Mechanistic studies revealed that euphohelioscopin A is an activator of protein kinase C (PKC), and that the promonocytic effects of this natural product are mediated by PKC activation. In addition to shedding insights into normal hematopoiesis, this work may ultimately facilitate the application of stem cell therapies to a host of myeloid dysfunctions.

Development of diacyltetrol lipids as activators for the C1 domain of protein kinase C

The protein kinase C (PKC) family of serine/threonine kinases is an attractive drug target for the treatment of cancer and other diseases. Diacylglycerol (DAG), phorbol esters and others act as ligands for the C1 domain of PKC isoforms. Inspection of the crystal structure of the PKCδ C1b subdomain in complex with phorbol-13-O-acetate shows that one carbonyl group and two hydroxyl groups play pivotal roles in recognition of the C1 domain. To understand the importance of two hydroxyl groups of phorbol esters in PKC binding and to develop effective PKC activators, we synthesized DAG like diacyltetrols (DATs) and studied binding affinities with C1b subdomains of PKCδ and PKCθ. DATs, with the stereochemistry of natural DAGs at the sn-2 position, were synthesized from (+)-diethyl L-tartrate in four to seven steps as single isomers. The calculated EC(50) values for the short and long chain DATs varied in the range of 3-6 μM. Furthermore, the fluorescence anisotropy values of the proteins were increased in the presence of DATs in a similar manner to that of DAGs. Molecular docking of DATs (1b-4b) with PKCδ C1b showed that the DATs form hydrogen bonds with the polar residues and backbone of the protein, at the same binding site, as that of DAG and phorbol esters. Our findings reveal that DATs represent an attractive group of C1 domain ligands that can be used as research tools or further structurally modified for potential drug development.

Identification of diacylglycerol acyltransferase inhibitors from Rosa centifolia petals

Diacylglycerol acyltransferase (DGAT) catalyzes the final step of triacylglycerol (TAG) synthesis, and is considered as a potential target to control hypertriglyceridemia or other metabolic disorders. In this study, we found that the extract of rose petals suppressed TAG synthesis in cultured cells, and that the extract showed DGAT inhibitory action in a dose-dependent manner. Fractionation of the rose extract revealed that the DGAT inhibitory substances in the extract were ellagitannins; among them rugosin B, and D, and eusupinin A inhibited DGAT activity by 96, 82, and 84% respectively, at 10 μM. These substances did not inhibit the activities of other hepatic microsomal enzymes, glucose-6-phosphatase and HMG-CoA reductase, or pancreatic lipase, suggesting that ellagitannins inhibit DGAT preferentially. In an oral fat load test using mice, postprandial plasma TAG increase was suppressed by rose extract; TAG levels 2 h after the fat load were significantly lower in mice administered a fat emulsion containing rose extract than in control mice (446.3 ± 33.1 vs 345.3 ± 25.0 mg/dL, control vs rose extract group; P < 0.05). These results suggest that rose ellagitannins or rose extract could be beneficial in controlling lipid metabolism and used to improve metabolic disorders.

Fluorescence-quenching screening of protein kinase C ligands with an environmentally sensitive fluorophore

A novel fluorescence-quenching screening method for protein kinase C (PKC) ligands was developed utilizing solvatochromic fluorophores. Solvatochromic dyes, highly sensitive to the presence or the absence of competitive ligands in their binding to the C1b domain of PKCδ (δC1b), were combined with a known pharmacophoric moiety of 1,2-diacylglycerol (DAG) lactones, PKC ligands. Addition of δC1b to the fluorescent compounds caused a gradual increase in the fluorescent intensity in proportion to the increase of δC1b. As a competitive ligand was added to the complex of δC1b domain and fluorescent compounds, a gradual decrease in the fluorescent intensity was observed. The relative binding affinities of known ligands were successfully determined by this fluorescent method and corresponded well to the K(i) values measured by a radioisotope method. These results indicate that washing, which is a laborious step in binding evaluations, is not required for this environmentally sensitive fluorophore based system. Screening with the system was performed for 2560 preselected library compounds with possible pharmacophores, and some lead compounds were found. This fluorescence-based method could be applied widely to known ligand-receptor combinations.

Synthetic caged DAG-lactones for photochemically controlled activation of protein kinase C

Switching on kinases: Synthetic caged DAG-lactones have been developed and showed decreases of two orders of magnitude, relative to the corresponding parent compounds, in their binding affinities towards PKC. The caged compounds had no effect on the translocation of PKC until after photoactivation. This approach is a potentially powerful tool for probing the PKC signaling cascade.

Protein kinase C regulates mitochondrial targeting of Nur77 and its family member Nor-1 in thymocytes undergoing apoptosis

Nur77 orphan steroid receptor and its family member Nor-1 are required for apoptosis of developing T cells. In thymocytes, signals from the TCR complex induce Nur77 and Nor-1 expression followed by translocation from the nucleus to mitochondria. Nur77 and Nor-1 associate with Bcl-2 in the mitochondria, resulting in a conformation change that exposes the Bcl-2 BH3 domain, a presumed pro-apoptotic molecule of Bcl-2. As Nur77 and Nor-1 are heavily phosphorylated, we examined the requirement of Nur77 and Nor-1 phosphorylation in mitochondria translocation and Bcl-2 BH3 exposure. We found that HK434, a PKC agonist, in combination with calcium ionophore, can induce Nur77 and Nor-1 phosphorylation, translocation, Bcl-2 BH3 exposure and thymocyte apoptosis. Inhibitors of both classical and novel forms of PKC were able to block this process. In contrast, only the general but not classical PKC-specific inhibitors were able to block the same process initiated by PMA, a commonly used PKC agonist. These data demonstrate a differential activation of PKC isoforms by PMA and HK434 in thymocytes, and show the importance of PKC in mitochondria translocation of Nur77/Nor-1 and Bcl-2 conformation change during TCR-induced thymocyte apoptosis.

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.

Design, synthesis, and biological activity of isophthalic acid derivatives targeted to the C1 domain of protein kinase C

Protein kinase C (PKC) is a widely studied molecular target for the treatment of cancer and other diseases. We have approached the issue of modifying PKC function by targeting the C1 domain in the regulatory region of the enzyme. Using the X-ray crystal structure of the PKC delta C1b domain, we have discovered conveniently synthesizable derivatives of dialkyl 5-(hydroxymethyl)isophthalate that can act as potential C1 domain ligands. Structure-activity studies confirmed that the important functional groups predicted by modeling were indispensable for binding to the C1 domain and that the modifications of these groups diminished binding. The most promising compounds were able to displace radiolabeled phorbol ester ([(3)H]PDBu) from PKC alpha and delta at K(i) values in the range of 200-900 nM. Furthermore, the active isophthalate derivatives could modify PKC activation in living cells either by inducing PKC-dependent ERK phosphorylation or by inhibiting phorbol-induced ERK phosphorylation. In conclusion, we report here, for the first time, that derivatives of isophthalic acid represent an attractive novel group of C1 domain ligands that can be used as research tools or further modified for potential drug development.

Ceramides: branched alkyl chains in the sphingolipid siblings of diacylglycerol improve biological potency

The synthesis of a small number of ceramide analogues containing a combination of linear and highly branched alkyl chains on either the d-sphingosine or the N-acyl core of the molecule is reported. Regardless of location, the presence of the branched chain improves potency relative to the positive control, C2 ceramide; however, the most potent compound (4) has the branched side chain as part of the d-sphingosine core. The induction of apoptosis by 4 in terms of Annexin V binding and DiOC(6) labeling was superior to that achieved with C2 ceramide.

Variability in content of the anti-AIDS drug candidate prostratin in Samoan populations of Homalanthus nutans

Homalanthus nutans, used by Samoan healers to treat hepatitis, produces the antiviral compound 12-deoxyphorbol 13-acetate, prostratin (1). Prostratin is being developed as an adjuvant therapy to clear latent viral reservoirs, the major obstacle to eradication of HIV-AIDS within the human body. A validated reversed-phase HPLC method was developed to assay concentrations of 1 in H. nutans. A survey of four distinct populations on two different Samoan islands revealed significant variability in content. The stem tissue (range 0.2-52.6 microg/g 1), used by healers in indigenous therapies,gave a higher median concentration of prostratin (3.5 microg/g) than root or leaf tissues (2.9 and 2.5 microg/g, respectively).The high variability and skewness of these data indicate that cultivar selection for drug production will be important for this species. The reversed-phase HPLC assay will allow plants to be selected for agricultural development and genetic analysis by identifying those individuals above and below a 95% confidence interval for the median concentration.

Conformationally constrained analogues of diacylglycerol. 30. An investigation of diacylglycerol-lactones containing heteroaryl groups reveals compounds with high selectivity for Ras guanyl nucleotide-releasing proteins

Using a diacylglycerol-lactone (DAG-lactone) template previously developed in our laboratory as a scaffold with high binding affinity for C1 domains, we describe herein a series of novel DAG-lactones containing heterocyclic moieties (pyridines, quinolines, and indoles) as alpha-arylidene fragments. Some of the DAG-lactones obtained show selective binding to RasGRP3 as compared to PKCalpha by more than 2 orders of magnitude and possess subnanomolar affinities. Because activated C1 domains bound to their ligands (DAG or DAG-lactones) insert into membranes, the lipid composition of membranes (cellular, nuclear, and those of internal organelles) is an important determinant for specificity. Therefore, reaching a proper hydrophilic/lipophilic balance for these molecules is critical. This was achieved by carefully selecting partnering acyl fragments for the DAG-lactones with the appropriate lipophilicity. The results clearly show that the combination of chemical and physical properties in these molecules needs to be perfectly balanced to achieve the desired specificity.

Conformationally constrained analogues of diacylglycerol. 29. Cells sort diacylglycerol-lactone chemical zip codes to produce diverse and selective biological activities

Diacylglycerol-lactone (DAG-lactone) libraries generated by a solid-phase approach using IRORI technology produced a variety of unique biological activities. Subtle differences in chemical diversity in two areas of the molecule, the combination of which generates what we have termed "chemical zip codes", are able to transform a relatively small chemical space into a larger universe of biological activities, as membrane-containing organelles within the cell appear to be able to decode these "chemical zip codes". It is postulated that after binding to protein kinase C (PKC) isozymes or other nonkinase target proteins that contain diacylglycerol responsive, membrane interacting domains (C1 domains), the resulting complexes are directed to diverse intracellular sites where different sets of substrates are accessed. Multiple cellular bioassays show that DAG-lactones, which bind in vitro to PKCalpha to varying degrees, expand their biological repertoire into a larger domain, eliciting distinct cellular responses.

Wealth of opportunity - the C1 domain as a target for drug development

The diacylglycerol-responsive C1 domains of protein kinase C and of the related classes of signaling proteins represent highly attractive targets for drug development. The signaling functions that are regulated by C1 domains are central to cellular control, thereby impacting many pathological conditions. Our understanding of the diacylglycerol signaling pathways provides great confidence in the utility of intervention in these pathways for treatment of cancer and other conditions. Multiple compounds directed at these signaling proteins, including compounds directed at the C1 domains, are currently in clinical trials, providing strong validation for these targets. Extensive understanding of the structure and function of C1 domains, coupled with detailed insights into the molecular details of ligand - C1 domain interactions, provides a solid basis for rational and semi-rational drug design. Finally, the complexity of the factors contributing to ligand - C1 domain interactions affords abundant opportunities for manipulation of selectivity; indeed, substantially selective compounds have already been identified.

Oxyl radicals, redox-sensitive signalling cascades and antioxidants

Oxidative stress is an increase in the reduction potential or a large decrease in the reducing capacity of the cellular redox couples. A particularly destructive aspect of oxidative stress is the production of reactive oxygen species (ROS), which include free radicals and peroxides. Some of the less reactive of these species can be converted by oxidoreduction reactions with transition metals into more aggressive radical species that can cause extensive cellular damage. In animals, ROS may influence cell proliferation, cell death (either apoptosis or necrosis) and the expression of genes, and may be involved in the activation of several signalling pathways, activating cell signalling cascades, such as those involving mitogen-activated protein kinases. Most of these oxygen-derived species are produced at a low level by normal aerobic metabolism and the damage they cause to cells is constantly repaired. The cellular redox environment is preserved by enzymes and antioxidants that maintain the reduced state through a constant input of metabolic energy. This review summarizes current studies that have been regarding the production of ROS and the general redox-sensitive targets of cell signalling cascades.

Conformationally constrained analogues of diacylglycerol. 26. Exploring the chemical space surrounding the C1 domain of protein kinase C with DAG-lactones containing aryl groups at the sn-1 and sn-2 positions

Diacylglycerol lactones (DAG-lactones) are known to operate as effective agonists of protein kinase C (PKC), surpassing in potency the activity of natural diacylglycerol (DAG). Localization of activated PKC isozymes in the cell is determined in part by the different cellular scaffolds, the lipid composition of the specific membranes, and the targeting information intrinsic to the individual isoforms bound to DAG. This multifaceted control of diversity suggests that, to develop effective DAG-lactones capable of honing in on a specific cellular target, we need to gain a better understanding of the chemical space surrounding its binding site. Seeking to augment the chemical repertoire of DAG-lactone side chains that could steer the translocation of PKC to specific cellular domains, we report herein the effects of incorporating simple or substituted phenyl residues. A combined series of n-alkyl and phenyl substitutions were used to explore the optimal location of the phenyl group on the side chains. The substantial differences in binding affinity between DAG-lactones with identical functionalized phenyl groups at either the sn-1 or sn-2 position are consistent with the proposed binding model in which the DAG-lactone binds to the C1 domain of PKC with the acyl chain oriented toward the interior of the membrane and the alpha-alkylidene or alpha-arylalkylidene chains directed to the surface of the C1 domain adjacent to the lipid interface. We conclude that DAG-lactones containing alpha-phenylalkylidene side chains at the sn-2 position represent excellent scaffolds upon which to explore further chemical diversity.

E230Q mutation of the catalytic subunit of cAMP-dependent protein kinase affects local structure and the binding of peptide inhibitor

The active site of the mammalian cAMP-dependent protein kinase catalytic subunit (C-subunit) has a cluster of nonconserved acidic residues-Glu127, Glu170, Glu203, Glu230, and Asp241-that are crucial for substrate recognition and binding. Studies have shown that the Glu230 to Gln mutant (E230Q) of the enzyme has physical properties similar to the wild-type enzyme and has decreased affinity for a short peptide substrate, Kemptide. However, recent experiments intended to crystallize ternary complex of the E230Q mutant with MgATP and protein kinase inhibitor (PKI) could only obtain crystals of the apo-enzyme of E230Q mutant. To deduce the possible mechanism that prevented ternary complex formation, we used the relaxed-complex method (Lin, J.-H., et al. J Am Chem Soc 2002, 24, 5632-5633) to study PKI binding to the E230Q mutant C-subunit. In the E230Q mutant, we observed local structural changes of the peptide binding site that correlated closely to the reduced PKI affinity. The structural changes occurred in the F-to-G helix loop and appeared to hinder PKI binding. Reduced electrostatic potential repulsion among Asp241 from the helix loop section and the other acidic residues in the peptide binding site appear to be responsible for the structural change.

Stereoselective synthesis and anti-inflammatory activities of 6- and 7-membered dioxacycloalkanes

A class of 5-trifluoroacetylamino-1,3-dioxacycloalkanes, 5-benzoylamino-1,3-dioxacycloalkanes, and 5-amino-1,3-dioxacycloalkane compounds were stereoselectively synthesized as potential anti-inflammatory drug candidates. The anti-inflammatory activities of these compounds were tested using the xylene-induced mouse ear edema model, from which multiple compounds possessing anti-inflammatory properties which surpass aspirin were identified; these compounds were then compared to establish structure-activity relationships.

Branched Diacylglycerol-Lactones as Potent Protein Kinase C Ligands and α-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).

Conformationally constrained analogues of diacylglycerol (DAG). 25. Exploration of the sn-1 and sn-2 carbonyl functionality reveals the essential role of the sn-1 carbonyl at the lipid interface in the binding of DAG-lactones to protein kinase C

Diacylglycerol (DAG) lactones with altered functionality (C=O --> CH(2) or C=O --> C=S) at the sn-1 and sn-2 carbonyl pharmacophores were synthesized and used as probes to dissect the individual role of each carbonyl in the binding to protein kinase C (PKC). The results suggest that the hydrated sn-1 carbonyl is engaged in very strong hydrogen-bonding interactions with the charged lipid headgroups and organized water molecules at the lipid interface. Conversely, the sn-2 carbonyl has a more modest contribution to the binding process as a result of its involvement with the receptor (C1 domain) via conventional hydrogen bonding to the protein. The parent DAG-lactones, E-6 and Z-7, were designed to bind exclusively in the sn-2 binding mode to ensure the correct orientation and disposition of pharmacophores at the binding site.

Novel synthesis and anti-inflammatory activities of 2,5-disubstituted-dioxacycloalkanes

A novel stereospecific synthetic route to obtain a series of 2,5-disubstituted-dioxacycloalkanes is reported. Using an in vivo inhibition assay by monitoring xylene-induced ear edema in mice, the structure-activity relationship of the dioxacycloalkane compounds was studied, and compounds possessing high anti-inflammatory activity were identified.

A Novel Diacylglycerol-lactone Shows Marked Selectivity in Vitro among C1 Domains of Protein Kinase C (PKC) Isoforms α and δ as Well as Selectivity for RasGRP Compared with PKCα

Although multiple natural products are potent ligands for the diacylglycerol binding C1 domain of protein kinase C (PKC), RasGRP, and related targets, the high conservation of C1 domains has impeded the development of selective ligands. We characterized here a diacylglycerol-lactone, 130C037, emerging from a combinatorial chemical synthetic strategy, which showed substantial selectivity. 130C037 gave shallow binding curves for PKC isoforms alpha, beta, gamma, delta, and epsilon, with apparent Ki values ranging from 340 nm for PKCalpha to 29 nm for PKCepsilon. When binding to isolated C1 domains of PKCalpha and -delta, 130C037 showed good affinity (Ki= 1.78 nm) only for deltaC1b, whereas phorbol 12,13-dibutyrate showed affinities within 10-fold for all. In LNCaP cells, 130C037 likewise selectively induced membrane translocation of deltaC1b. 130C037 bound intact RasGRP1 and RasGRP3 with Ki values of 3.5 and 3.8 nm, respectively, reflecting 8- and 90-fold selectivity relative to PKCepsilon and PKCalpha. By Western blot of Chinese hamster ovary cells, 130C037 selectively induced loss from the cytosol of RasGRP3 (ED50 = 286 nm), partial reduction of PKCepsilon (ED50 > 10 microm), and no effect on PKCalpha. As determined by confocal microscopy in LNCaP cells, 130C037 caused rapid translocation of RasGRP3, limited slow translocation of PKCepsilon, and no translocation of PKCalpha. Finally, 130C037 induced Erk phosphorylation in HEK-293 cells ectopically expressing RasGRP3 but not in control cells, whereas phorbol ester induced phosphorylation in both. The properties of 130C037 provide strong proof of principle for the feasibility of developing ligands with selectivity among C1 domain-containing therapeutic targets.

Anti-tumor-promoting activity of simple models of galactoglycerolipids with branched and unsaturated acyl chains

Six new galactoglycerolipid analogs, in which one or two 4-methylpentanoyl or trans-2-butenoyl groups are linked to the 2-O-beta-D-galactosylglycerol skeleton, were tested for their anti-tumor-promoting activity using a short-term in vitro assay for Epstein-Barr virus early antigen (EBV-EA) activation. All these compounds were more active than their linear or saturated reference compounds in inhibiting the EBV activation promoted by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), the diester 1-O-(4-methylpentanoyl)-2-O-[6-O-(4-methylpentanoyl)-beta-D-galactopyranosyl]-sn-glycerol resulting the most active glycoglycerolipid analog till now tested. Four compounds (three butenoates and one 4-methylpentanoate), when tested in the in vivo two-stage carcinogenesis test, exhibited also inhibitory effects on mouse skin tumor promotion.

Molecular Interaction Model for the C1B Domain of Protein Kinase C-γ in the Complex with Its Activator Phorbol-12-myristate-13-acetate in Water Solution and Lipid Bilayer

Detailed molecular models of the free C1B domain of protein kinase C-gamma (PKC-gamma) and the C1B domain with its activator phorbol-12-myristate-13-acetate (PMA) in water solution and in the presence of dipalmitoylphosphatidylcholine (DPPC) bilayer are presented. Molecular dynamics of the free C1B domain reveals hydrogen bonds, which are critical for the forming of the diacylglycerols/phorbol esters binding site, and indicates the important role of Gln27 for the geometry of this site. According to the model, PMA interacts with the C1B domain by hydrophobic interactions with Pro11 and Tyr22 and by three persistent hydrogen bonds between the C3 carbonyl group of PMA and Gly23 and between the C20 hydroxyl group of PMA and the Leu21 and Thr12 residues of the C1B domain. The C9 hydroxyl group of PMA does not interact with the C1B domain, but it is involved in the interaction with the DPPC bilayer. Two preferential orientations of the C1B-PMA complex toward the DPPC bilayer resulted from our molecular modeling study.

Conformationally constrained analogues of diacylglycerol (DAG). 23. Hydrophobic ligand-protein interactions versus ligand-lipid interactions of DAG-lactones with protein kinase C (PK-C)

The constrained glycerol backbone of DAG-lactones, when combined with highly branched alkyl chains, has engendered a series of DAG-lactone ligands capable of binding protein kinase C (PK-C) with affinities that approximate those of phorbol esters. These branched chains not only appear to be involved in making important hydrophobic contacts with the protein (specific interactions) but also provide adequate lipophilicity to facilitate partitioning into the lipid-rich membrane environment (nonspecific interactions). With the idea of minimizing the nonspecific interactions without reducing lipophilicity, the present work explores the strategy of relocating lipophilicity from the side chain to the lactone "core". Such a transfer of lipophilicity, exemplified by compounds 1 and 3, was conceived to allow the new hydrophobic groups on the lactone to engage in specific hydrophobic contacts inside the binding pocket without any expectation of interfering with the hydrogen-bonding network of the DAG-lactone pharmacophore. Surprisingly, both (E)-3 and (Z)-3 showed a significant decrease in binding affinity. From the molecular docking studies performed with the new ligands, we conclude that the binding pocket of the C1 domain of PK-C is sterically restricted and prevents the methyl groups at the C-3 position of the lactone from engaging in productive hydrophobic contacts with the receptor.

Dual role of prostratin in inhibition of infection and reactivation of human immunodeficiency virus from latency in primary blood lymphocytes and lymphoid tissue

To design strategies to purge latent reservoirs of human immunodeficiency virus type 1 (HIV-1), we investigated mechanisms by which a non-tumor-promoting phorbol ester, prostratin, inhibits infection of CD4(+) T lymphocytes and at the same time reactivates virus from latency. CD4(+) T lymphocytes from primary blood mononuclear cells (PBMC) and in blocks of human lymphoid tissue were stimulated with prostratin and infected with HIV-1 to investigate the effects of prostratin on cellular susceptibility to the virus. The capacity of prostratin to reactivate HIV from latency was tested in CD4(+) T cells harboring preintegrated and integrated latent provirus. Prostratin stimulated CD4(+) T cells in an aberrant way. It induced expression of the activation markers CD25 and CD69 but inhibited cell cycling. HIV-1 uptake was reduced in prostratin-stimulated CD4(+) T PBMC and tissues in a manner consistent with a downregulation of CD4 and CXCR4 receptors in these systems. At the postentry level, prostratin inhibited completion of reverse transcription of the viral genome in lymphoid tissue. However, prostratin facilitated integration of the reverse-transcribed HIV-1 genome in nondividing CD4(+) T cells and facilitated expression of already integrated HIV-1, including latent forms. Thus, while stimulation with prostratin restricts susceptibility of primary resting CD4(+) T cells to HIV infection at the virus cell-entry level and at the reverse transcription level, it efficiently reactivates HIV-1 from pre- and postintegration latency in resting CD4(+) T cells.

The nature of the hydrophobic n-alkanol binding site within the C1 domains of protein kinase Calpha

The activator-binding sites within the C1 domains of protein kinase C (PKC) are also able to bind alcohols and anesthetics. In this study, the nature of the interaction of these agents with the hydrophobic region within the C1 domains was investigated and a structure-activity relationship for the alcohol effects was obtained. The effects of a series of n-alkanols on PKCalpha activity, determined using an in vitro assay system that lacked lipids, were found to be a nonlinear function of the chain length. In the absence of phorbol ester or diacylglycerol, 1-octanol potently activated PKCalpha in a concentration-dependent manner, while 1-heptanol was completely without effect, despite differing by one methylene unit. The minimal structural requirement for the activating effect corresponded to R-CH(OH)-(CH(2))(n)-CH(3), where R = H or an alkyl group and n >or= 6. Consistent with this, 2-octanol, for which n = 5, was without effect on the activity, even though this alcohol is only marginally less hydrophobic than 1-octanol, whereas 2-nonanol, for which n = 6, was able to produce activity. Importantly, it was found that PKCalpha was activated to a greater extent by R-2-nonanol than by the S enantiomer. The potentiation of phorbol ester-induced, membrane-associated PKCalpha activity by long-chain n-alkanols reported previously (Slater, S. J., Kelly, M. B., Larkin, J. D., Ho, C, Mazurek, A, Taddeo, F. J., Yeager, M. D., Stubbs, C. D. (1997) J. Biol. Chem. 272, 6167-6173), was also found here for nonmembrane associated PKC, indicating that this effect is an intrinsic property of the enzyme rather than a result of membrane perturbation. Overall, the results suggest that the alcohol-binding sites within the C1 domains of PKCalpha contain spatially distinct hydrophilic and hydrophobic regions that impose a high degree of structural specificity on the interactions of alcohols and other anesthetic compounds, as well as diacylglycerols and phorbol esters.

Macrocyclic diacylglycerol-bis-lactones as conformationally constrained analogues of diacylglycerol-lactones. Interactions with protein kinase C

A series of macrocyclic diacylglycerol (DAG)-bis-lactones were investigated as extreme conformationally constrained analogues of DAG-lactones in order to seek more potent protein kinase C (PKC) ligands with higher binding affinities and less lipophilicity than previous compounds. The additional constraint achieved the desired objective as exemplified by the macrocyclic DAG-bis-lactone 57, which exhibited a 6-fold higher binding affinity for PKCalpha (K(i) = 6.07 nM) than the corresponding nonmacrocyclic 3-alkylidene DAG-lactone 6. A structure-activity relationship (SAR) analysis of the macrocyclic DAG-bis-lactones demonstrated a parabolic relationship between activity and lipophilicity, as well as a predilection for the Z-alkylidene isomers as the preferred ligands. Molecular docking studies revealed that macrocyclic DAG-bis-lactone 57 bound to the C1b domain of PKCalpha exclusively in the sn-1 binding mode in contrast to DAG-lactone 6, which showed both sn-1 and sn-2 binding modes. It is proposed that the high potency displayed by these macrocyclic DAG-bis-lactones results from a set of more favorable hydrogen bonding and hydrophobic interactions with PKCalpha as well as from a reduced entropy penalty due to conformational restriction.

Automated docking of ligands to an artificial active site: augmenting crystallographic analysis with computer modeling

The W191G cavity of cytochrome c peroxidase is useful as a model system for introducing small molecule oxidation in an artificially created cavity. A set of small, cyclic, organic cations was previously shown to bind in the buried, solvent-filled pocket created by the W191G mutation. We docked these ligands and a set of non-binders in the W191G cavity using AutoDock 3.0. For the ligands, we compared docking predictions with experimentally determined binding energies and X-ray crystal structure complexes. For the ligands, predicted binding energies differed from measured values by +/- 0.8 kcal/mol. For most ligands, the docking simulation clearly predicted a single binding mode that matched the crystallographic binding mode within 1.0 A RMSD. For 2 ligands, where the docking procedure yielded an ambiguous result, solutions matching the crystallographic result could be obtained by including an additional crystallographically observed water molecule in the protein model. For the remaining 2 ligands, docking indicated multiple binding modes, consistent with the original electron density, suggesting disordered binding of these ligands. Visual inspection of the atomic affinity grid maps used in docking calculations revealed two patches of high affinity for hydrogen bond donating groups. Multiple solutions are predicted as these two sites compete for polar hydrogens in the ligand during the docking simulation. Ligands could be distinguished, to some extent, from non-binders using a combination of two trends: predicted binding energy and level of clustering. In summary, AutoDock 3.0 appears to be useful in predicting key structural and energetic features of ligand binding in the W191G cavity.

A case study in conformation design: learning by doing

Efforts are described to design simple, fully flexible but conformationally preorganised omega-hydroxy-nonanoic acids that could serve as the conformation controlling unit in analogues of the potent protein-kinase C activator aplysiatoxin. Such analogues are macrodilactones incorporating the designed omega-hydroxy-nonanoic acid and 3,4-dihydroxy-pentanoic acid, which contains the pharmacophoric groups. The design process (replacement of CH(2) groups by an oxygen atom, annelation of a six-membered ring and placement of alkyl substituents) of the omega-hydroxy-nonanoic acids was monitored by force-field calculations. In the end of this process simple analogues of aplysiatoxin are proposed in which the proper disposition of the pharmacophoric groups is secured by a conformationally flexible but preorganised template structure as part of the macrodilactone ring.

Conformationally Constrained Analogues of Diacylglycerol. 24. Asymmetric Synthesis of a Chiral (R)-DAG-Lactone Template as a Versatile Precursor for Highly Functionalized DAG-Lactones

[structure: see text] Commercially available 2-methylenepropane-1,3-diol was converted to chiral epoxide (R)-2 via Sharpless asymmetric epoxidation in >96% ee. Regiospecific epoxide ring opening and reduction of the intermediate alkyne set the stage for a one-pot lactonization to give (R)-6, a convenient precursor for all functionalized chiral DAG-lactones used as potent PK-C ligands. The synthesis of the most potent DAG-lactones known to date, (Z)-10 and (E)-10, served to confirm PK-C's exclusive preference for the (R)-stereochemistry in this class of compounds.

Design, synthesis, and structure–activity relationship of new isobenzofuranone ligands of protein kinase C

Protein kinase C (PKC) is a family of enzymes, which play important roles in intracellular signal transduction. We have designed novel PKC ligands having an isobenzofuranone template, based on the proposed interaction of DAG (1,2-diacyl-sn-glycerol) with the PKCdelta C1B ligand-binding domain. Several isobenzofuranone derivatives were synthesized and their PKCalpha binding activities were evaluated. The pivaloyl derivative 1f was found to be a strong PKCalpha ligand, and the structure-activity relationship is well explained by our proposed binding model.

Conformationally Constrained Analogues of Diacylglycerol. 21. A Solid-Phase Method of Synthesis of Diacylglycerol Lactones as a Prelude to a Combinatorial Approach for the Synthesis of Protein Kinase C Isozyme-Specific Ligands

A solid-phase method for the synthesis of diacylglycerol lactones as protein kinase C ligands was developed, and a small array of nine compounds were selected with the idea of testing this methodology and forecasting the reliability of the biological data as a preamble for the construction of large chemical libraries to be synthesized under the same conditions. The process started with the loading of 5-(hydroxymethyl)-5-[(4-methoxyphenoxy)methyl]-3,4,5-trihydrofuran-2-one (1) to a 3,4-dihydro-2H-pyran resin packed inside IRORI MacroKan reactors. The elements of diversity were introduced at the alpha-alkylidene (R(1)) and acyl (R(2)) positions using a set of three different aldehydes and three different acid chlorides, respectively. An LDA-mediated aldol condensation with R(1)CHO in the presence of ZnCl(2) followed by a DBU-catalyzed elimination of the triflate of the resulting aldol gave the alpha-alkylidene intermediates as mixtures of geometric isomers. Removal of the aryl-protecting group followed by acylation with R(2)COCl introduced the second element of diversity. Acid-assisted cleavage of the compounds from the resin afforded the final targets. The biological results obtained using the crude samples directly obtained from the resin compared well with those from pure materials, as the K(i) values between the two sets varied only by a factor between 1.5 and 3.7.