Diacylglycerols with lipophilically equivalent branched acyl chains display high affinity for protein kinase C (PK-C). A direct measure of the effect of constraining the glycerol backbone in DAG lactones

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.

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.

Ligand Structure-Activity Requirements and Phospholipid Dependence for the Binding of Phorbol Esters to Protein Kinase D

Although protein kinase D (PKD), like protein kinase C (PKC), possesses a C1 domain that binds phorbol esters and diacylglycerol, the structural differences from PKC within this and other domains of PKD imply differential regulation by lipids and ligands. We characterized the phorbol ester and phospholipid binding properties of a glutathione S-transferase-tagged full-length PKD and compared them with those of PKC-alpha and -delta. We found that PKD is a high-affinity phorbol ester receptor for a range of structurally and functionally divergent phorbol esters and analogs and showed both similarities and differences in structure-activity relations compared with the PKCs examined. In particular, PKD had lower affinity than PKC for certain diacylglycerol analogs, which might be caused by a lysine residue at the 22 position of the PKD-C1b domain in place of the tryptophan residue at this position conserved in the PKCs. The membrane-targeting domains in PKD are largely different from those in PKC; among these differences, PKD contains a pleckstrin homology (PH) domain that is absent in PKC. However, phosphatidylinositol-4,5-bisphosphate PIP2, a lipid ligand for some PH domains, reconstitutes phorbol 12,13-dibutyrate (PDBu) binding to PKD similarly as it does to PKC-alpha and -delta, implying that the PH domain in PKD may not preferentially interact with PIP2. Overall, the requirement of anionic phospholipids for the reconstitution of [3H]PDBu binding to PKD was intermediate between those of PKC-alpha and -delta. We conclude that PKD is a high-affinity phorbol ester receptor; its lipid requirements for ligand binding are approximately comparable with those of PKC but may be differentially regulated in cells through the binding of diacylglycerol to the C1 domain.

Rational design of drugs that induce human immunodeficiency virus replication

Drugs that induce human immunodeficiency virus type 1 (HIV-1) replication could be used in combination with highly active antiretroviral therapy (HAART) to reduce the size of the latent reservoir that is in part responsible for viral persistence. Protein kinase C (PKC) is a logical target for such drugs because it activates HIV-1 transcription through multiple mechanisms. Here we show that HIV-1 gene expression can be induced by potent synthetic analogues of the lipid second messenger diacylglycerol (DAG) synthesized on a five-member ring platform that reduces the entropy of binding relative to that of the more flexible DAG template. By varying the alkyl side chains of these synthetic DAG lactones, it was possible to maximize their potency and ability to render latently infected T cells sensitive to killing by an anti-HIV-1 immunotoxin while minimizing the side effects of CD4 and CXCR4 downregulation and tumor necrosis factor alpha upregulation. The two lead compounds, LMC03 and LMC07, regulated a series of PKC-sensitive genes involved in T-cell activation and induced viral gene expression in peripheral blood mononuclear cells from HIV-1-infected individuals. These studies demonstrate the potential for the rational design of agents that, in conjunction with HAART and HIV-specific toxins, can be used to decrease or eliminate the pool of latently infected reservoirs by forcing viral expression.

Pharmacology of the receptors for the phorbol ester tumor promoters: multiple receptors with different biochemical properties

The phorbol ester tumor promoters and related analogs are widely used as potent activators of protein kinase C (PKC). The phorbol esters mimic the action of the lipid second messenger diacylglycerol (DAG). The aim of this commentary is to highlight a series of important and controversial concepts in the pharmacology and regulation of phorbol ester receptors. First, phorbol ester analogs have marked differences in their biological properties. This may be related to a differential regulation of PKC isozymes by distinct analogs. Moreover, it seems that marked differences exist in the ligand recognition properties of the C1 domains, the phorbol ester/DAG binding sites in PKC isozymes. Second, an emerging theme that we discuss here is that phorbol esters also target receptors unrelated to PKC isozymes, a concept that has been largely ignored. These novel receptors lacking kinase activity include chimaerins (a family of Rac-GTPase-activating proteins), RasGRP (a Ras exchange factor), and Unc-13/Munc-13 (a family of proteins involved in exocytosis). Unlike the classical and novel PKCs, these "non-kinase" phorbol ester receptors possess a single copy of the C1 domain. Interestingly, each receptor class has unique pharmacological properties and biochemical regulation. Lastly, it is well established that phorbol esters and related analogs can translocate each receptor to different intracellular compartments. The differential pharmacological properties of the phorbol ester receptors can be exploited to generate specific agonists and antagonists that will be helpful tools to dissect their cellular function.