Conformationally constrained analogues of diacylglycerol (DAG). 15. The indispensable role of the sn-1 and sn-2 carbonyls in the binding of DAG-lactones to protein kinase C (PK-C)

Structural insights into C1-ligand interactions: Filling the gaps by in silico methods

Protein Kinase C isoenzymes (PKCs) are the key mediators of the phosphoinositide signaling pathway, which involves regulated hydrolysis of phosphatidylinositol (4,5)-bisphosphate to diacylglycerol (DAG) and inositol-1,4,5-trisphosphate. Dysregulation of PKCs is implicated in many human diseases making this class of enzymes an important therapeutic target. Specifically, the DAG-sensing cysteine-rich conserved homology-1 (C1) domains of PKCs have emerged as promising targets for pharmaceutical modulation. Despite significant progress, the rational design of the C1 modulators remains challenging due to difficulties associated with structure determination of the C1-ligand complexes. Given the dearth of experimental structural data, computationally derived models have been instrumental in providing atomistic insight into the interactions of the C1 domains with PKC agonists. In this review, we provide an overview of the in silico approaches for seven classes of C1 modulators and outline promising future directions.

Elucidating the interaction of γ-hydroxymethyl-γ-butyrolactone substituents with model membranes and protein kinase C-C1 domains

The protein kinase C (PKC) family of proteins is an attractive drug target. Dysregulation of PKC-dependent signalling pathways is related to several human diseases like cancer, immunological and other diseases. We approached the problem of altering PKC activities by developing C1 domain-based PKC ligands. In this report γ-hydroxymethyl-γ-butyrolactone (HGL) substituents were investigated in an effort to develop small molecule-based PKC regulators with higher specificity for C1 domain than the endogenous diacylglycerols (DAGs). Extensive analysis of membrane-ligands interaction measurements revealed that the membrane-active compounds strongly interact with the lipid bilayers and the hydrophilic parts of compounds localize at the bilayer/water interface. The pharmacophores like hydroxymethyl, carbonyl groups and acyl-chain length of the compounds are crucial for their interaction with the C1 domain proteins. The potent compounds showed more than 17-fold stronger binding affinity for the C1 domains than DAG under similar experimental conditions. Nonradioactive kinase assay confirmed that these potent compounds have similar or better PKC dependent phosphorylation capabilities than DAG under similar experimental conditions. Hence, our findings reveal that these HGL analogues represent an attractive group of structurally simple C1 domain ligands that can be further structurally altered to improve their potencies.

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.

Membrane Interaction and Protein Kinase C-C1 Domain Binding Properties of 4-Hydroxy-3-(hydroxymethyl) Phenyl Ester Analogues

Protein kinase C (PKC)-C1 domain targeted regulator development is considered as a potential therapeutic strategy for the treatment of cancer and immunological and other diseases. Efforts are underway to synthesize small molecules to achieve higher specificity for the C1-domain than the natural activator, diacylglycerols (DAGs). In this regard, we conveniently synthesized 4-hydroxy-3-(hydroxymethyl) phenyl ester analogues and measured in vitro C1-domain binding properties. We also investigated different physicochemical properties of the synthesized molecules, including aggregation behavior in aqueous solution and interaction with lipid bilayers, and others with an aim for better understanding of their C1-domain binding properties. The results showed that the membrane-active compounds aggregate in aqueous solution at a reasonably lower concentration and strongly interact with the lipid bilayer. The hydrophilic part of the compounds localize at the bilayer/water interface and accessible for C1-domain binding. Biophysical studies revealed that the hydroxyl, hydroxymethyl, and carbonyl groups and acyl chain length are important for their interaction with the C1-domain. The potent compound showed more than 10-fold stronger binding affinity for the C1-domains than DAG under similar experimental conditions. Therefore, our findings reveal that these ester analogues represent an attractive group of C1-domain ligands that can be further structurally modified to improve their binding and activity.

Synthesis and protein kinase C (PKC)-C1 domain binding properties of diacyltetrol based anionic lipids

Protein kinase C-C1 domain binding specificity of the anionic hybrid lipids.

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.

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.

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.

Conformationally Constrained Analogues of Diacylglycerol. 20. The Search for an Elusive Binding Site on Protein Kinase C through Relocation of the Carbonyl Pharmacophore Along the sn-1 Side Chain of 1,2-Diacylglycerol Lactones

Previous studies with 1,2-diacylglycerol (DAG) lactones, which behave as high-affinity ligands for protein kinase C (PK-C), have established the importance of maintaining intact the pharmacophore triad of two carbonyl moieties (sn-1 and sn-2) and the primary alcohol. In addition, docking studies of DAG-lactones into an empty C1b receptor of PK-Cdelta (as it appears in complex with phorbol 13-O-acetate) have revealed that in either of the two possible binding alternatives (sn-1 or sn-2) only one carbonyl group of the DAG-lactone is involved in binding. Therefore, the unknown receptor for the orphaned carbonyl appears to lie outside the boundaries of this binary complex, possibly residing at the membrane or near the membrane-protein interface. A strategy to locate the optimal location of the unengaged carbonyl was conceived by utilizing a small group of DAG-lactones (1-4) with a highly branched chain adjacent to the sn-2 carbonyl such that sn-2 binding is favored. With these compounds, various locations of the sn-1 carbonyl along the side chain were tested for their binding affinity for PK-C. The results indicate that the location of the side chain sn-1 carbonyl in a DAG-lactone must have perfect mimicry to the sn-1 carbonyl of the parent DAG for it to display high binding affinity. A proposed model from this work is that the missing pharmacophore in the ternary complex, which includes the membrane, is close to the membrane-protein interface.

Conformationally constrained analogues of diacylglycerol (DAG). Effect on protein kinase C (PK-C) binding by the isosteric replacement of sn-1 and sn-2 esters in DAG-lactones

In order to determine the importance of the two ester pharmacophores in high affinity, conformationally constrained DAG-lactones (Lac-1-5) as PK-C ligands, we have independently replaced the sn-1 and sn-2 carbonyl esters in these compounds by ketone (2, 10, 11), amide (3, 25-28), and hydroxyl (12, 13) isosteres. Although the ketone analogue of the sn-1 ester (2) exhibited comparable activity to the parent Lac-1 when taking into account the difference in lipophilicities, the other isosteres were significantly poorer PK-C alpha ligands compared to the parent DAG-lactones. This study demonstrates that the ester functionality in DAG-lactone plays an important role in the ligand's capacity to form a strong hydrogen bond with Gly253 at the active site. The discrete K(i) analysis from the sn-1 and sn-2 isosteres further confirms that the DAG-lactones bind preferentially to the C1-domain in the sn-2 binding mode, as previously suggested.

Synthetic diacylglycerols (DAG) and DAG-lactones as activators of protein kinase C (PK-C)

The central role of protein kinase C (PK-C) in cellular signal transduction has established it as an important therapeutic target for cancer and other diseases. We have developed a series of 4,4-disubstituted-gamma-butyrolactones, which contain a constrained glycerol backbone (DAG-lactones) and behave as potent and selective activating ligands of PK-C with affinities that approach those of the structurally complex natural product agonists, such as the phorbol esters. This Account traces the design and construction of these molecules. Initially, we examined the consequences of reducing the entropic penalty associated with the transformation of a DAG into a DAG-lactone. Then, using molecular modeling to extend insights arising from the newly solved crystal structure of a C1 domain complexed with phorbol ester, we incorporated amino acid-specific branched hydrophobic chains to provide a new generation of DAG-lactones that have the capacity to bind to PK-C with low nanomolar affinity. Depending on the specific pattern of hydrophobic substitution, some DAG-lactones are able to induce selective translocation of individual PK-C isozymes to different cellular compartments, and since the specific nature of these hydrophobic interactions influences biological outcome, some of these compounds exhibit cell-specific antitumor activity. The ability to direct specific PK-C isozyme translocation with sets of structurally simple, yet highly potent molecules provides a powerful tool for engineering a plethora of molecules with novel biological functions.

Differential binding modes of diacylglycerol (DAG) and DAG lactones to protein kinase C (PK-C)

Diacylglycerol lactones (DAG lactones), analogous to highly potent diacylglycerols (DAGs) were synthesized to demonstrate the ability of PK-C to discriminate between two differential binding modes, sn-1 and sn-2. While both sn-1 and sn-2 binding modes are allowable in terms of hydrogen bonding, it has been found that in general, DAGs prefer to bind sn-1, while the corresponding analogous DAG lactones prefer to bind sn-2. However, this binding orientation can be directly influenced by the disposition and nature of the acyl substituent, particularly if it is highly branched. When the "binding driving force" (i.e., the larger branched acyl chain) is in the sn-2 position, a dramatic increase in binding affinity is observed in the DAG lactone as compared to its open chain DAG counterpart. As these analogous DAGs and DAG lactones have almost identical log P values, this difference in binding affinity is a direct result of the entropic advantage of constraining the glycerol backbone.

Diacylglycerol (DAG)-lactones, a new class of protein kinase C (PKC) agonists, induce apoptosis in LNCaP prostate cancer cells by selective activation of PKCalpha

Phorbol esters, the archetypical (PKC) activators, induce apoptosis in androgen-sensitive LNCaP prostate cancer cells. In this study we evaluate the effect of a novel class of PKC ligands, the diacylglycerol (DAG)-lactones, as inducers of apoptosis in LNCaP cells. These unique ligands were designed using novel pharmacophore- and receptor-guided approaches to achieve highly potent DAG surrogates. Two of these compounds, HK434 and HK654, induced apoptosis in LNCaP cells with much higher potency than oleoyl-acetyl-glycerol or phorbol 12,13-dibutyrate. Moreover, different PKC isozymes were found to mediate the apoptotic effect of phorbol 12-myristate 13-acetate (PMA) and HK654 in LNCaP cells. Using PKC inhibitors and dominant negative PKC isoforms, we found that both PKCalpha and PKCdelta mediated the apoptotic effect of PMA, whereas only PKCalpha was involved in the effect of the DAG-lactone. The PKCalpha selectivity of HK654 in LNCaP cells contrasts with similar potencies in vitro for binding and activation of PKCalpha and PKCdelta. Consistent with the differences in isoform dependence in intact cells, PMA and HK654 show marked differences in their abilities to translocate PKC isozymes. Both PMA and HK654 induce a marked redistribution of PKCalpha to the plasma membrane. On the other hand, unlike PMA, HK654 translocates PKCdelta predominantly to the nuclear membrane. Thus, DAG-lactones have a unique profile of activation of PKC isozymes for inducing apoptosis in LNCaP cells and represent the first example of a selective activator of a classical PKC in cellular models. An attractive hypothesis is that selective activation of PKC isozymes by pharmacological agents in cells can be achieved by differential intracellular targeting of each PKC.

beta2-chimaerin is a novel target for diacylglycerol: binding properties and changes in subcellular localization mediated by ligand binding to its C1 domain

The members of the chimaerin family of Rac-GTPase-activating proteins possess a single C1 domain with high homology to those present in protein kinase C (PKC) isozymes. This domain in PKCs is involved in phorbol ester and diacylglycerol (DAG) binding. We previously have demonstrated that one of the chimaerin isoforms, beta2-chimaerin, binds phorbol esters with high affinity. In this study we analyzed the properties of beta2-chimaerin as a DAG receptor by using a series of conformationally constrained cyclic DAG analogues (DAG lactones) as probes. We identified analogs that bind to beta2-chimaerin with more than 100-fold higher affinity than 1-oleoyl-2-acetylglycerol. The potencies of these analogs approach those of the potent phorbol ester tumor promoters. The different DAG lactones show some selectivity for this novel receptor compared with PKCalpha. Cellular studies revealed that these DAG analogs induce translocation of beta2-chimaerin from cytosolic (soluble) to particulate fractions. Using green fluorescent protein-fusion proteins for beta2-chimaerin we determined that this novel receptor translocates to the perinuclear region after treatment with DAG lactones. Binding and translocation were prevented by mutation of the conserved Cys-246 in the C1 domain. The structural homology between the C1 domain of beta2-chimaerin and the C1b domain of PKCdelta also was confirmed by modeling analysis. Our results demonstrate that beta2-chimaerin is a high affinity receptor for DAG through binding to its C1 domain and supports the emerging concept that multiple pathways transduce signaling through DAG and the phorbol esters.

Protein kinase C ligands based on tetrahydrofuran templates containing a new set of phorbol ester pharmacophores

A series of substituted tetrahydrofurans with an embedded glycerol backbone carrying additional tetrahydrofuranylideneacetate or tetrahydrofuranylacetate motifs were grouped into four distinct templates (I-IV) according to stereochemistry. The compounds were designed to mimic three essential pharmacophores (C(3)-C=O, C(20)-OH and C(13)-C=O) of the phorbol esters according to a new, revised model. The tetrahydrofuran ring was constructed from glycidyl 4-methoxyphenyl ether, and the structures of the isomeric templates were assigned by NMR spectroscopy, including NOE. The binding affinity for protein kinase C (PKC) was assessed in terms of the ability of the ligands to displace bound [(3)H-20]phorbol 12, 13-dibutyrate (PDBU) from a recombinant alpha isozyme of PKC. Geometric Z- and E-isomers (1 and 3, respectively) containing a tetrahydrofuranylideneacetate motif were the most potent ligands with identical K(i) values of 0.35 microM. Molecular modeling studies of the four templates showed that the rms values when fitted to a prototypical phorbol 12,13-diacetate ester correlated inversely with affinities in the following order: I approximately II > III > IV. These compounds represent the first generation of rigid glycerol templates seeking to mimic the binding of the C(13)-C=O of the phorbol esters. The binding affinities of the most potent compounds are in the same range of the diacylglycerols (DAGs) despite the lack of a phorbol ester C(9)-OH pharmacophore surrogate. This finding confirms that mimicking the binding of the C(13)-C=O pharmacophore of phorbol is a useful strategy. However, since the C(9)-OH and C(13)-C=O in the phorbol esters appear to form an intramolecular hydrogen bond that functions as a combined pharmacophore, it is possible the lack of this combined motif in the target templates restricts the compounds from reaching higher binding affinities.

The transition from a pharmacophore-guided approach to a receptor-guided approach in the design of potent protein kinase C ligands

The pharmacophore-guided approach used in the first phase of the design of novel protein kinase C (PKC) ligands was based on the study of the geometry of bioequivalent pharmacophores present in diacylglycerol (DAG) and in the more potent phorbol ester tumor promoters. A number of potent DAG lactones were generated by this approach, in which the glycerol backbone was constrained into various heterocyclic rings to reduce the entropic penalty associated with DAG binding. Based on the information provided by X-ray and NMR structures of the cysteine-rich, C1 phorbol ester/DAG binding domain, the DAG lactones were further modified to optimize their interaction with a group of highly conserved hydrophobic amino acids along the rim of the C1 domain. This receptor-guided approach culminated with the synthesis of a series of "super DAG" molecules that can bind to PKC with low nanomolar affinities. These compounds provide insight into the basis for PKC ligand specificity. Moreover, some of the compounds reviewed herein show potential utility as antitumor agents.