Distinguishing between Closely Related Protein Kinases: A Variation on the Bisubstrate Inhibitor Theme

Adenosine analogue-oligo-arginine conjugates (ARCs) serve as high-affinity inhibitors and fluorescence probes of type I cGMP-dependent protein kinase (PKGIalpha)

INTRODUCTION

Type I cGMP-dependent protein kinase (PKGIalpha) belongs to the family of cyclic nucleotide-dependent protein kinases and is one of the main effectors of cGMP. PKGIalpha is involved in regulation of cardiac contractility, vasorelaxation, and blood pressure; hence, the development of potent modulators of PKGIalpha would lead to advances in the treatment of a variety of cardiovascular diseases.

AIM

Representatives of ARC-type compounds previously characterized as potent inhibitors and high-affinity fluorescent probes of PKA catalytic subunit (PKAc) were tested towards PKGIalpha to determine that ARCs could serve as activity regulators and sensors for the latter protein kinase both in vitro and in complex biological systems.

RESULTS

Structure-activity profiling of ARCs with PKGIalpha in vitro demonstrated both similarities as well as differences to corresponding profiling with PKAc, whereas ARC-903 and ARC-668 revealed low nanomolar displacement constants and inhibition IC(50) values with both cyclic nucleotide-dependent kinases. The ability of ARC-based fluorescent probes to penetrate cell plasma membrane was demonstrated in the smooth muscle tissue of rat cerebellum isolated arteries, and the compound with the highest affinity in vitro (ARC-903) showed also potential for in vivo applications, fully abolishing the PKG1alpha-induced vasodilation.

Biosensors of protein kinase action: from in vitro assays to living cells

Protein kinases, and the signal transduction pathways in which they participate, are now recognized to be medicinally attractive targets of opportunity. Inhibitors of the protein kinase family not only hold great promise as therapeutic agents, but are also of profound utility in the characterization of signaling pathways. The direct visualization of protein kinase activity in living cells provides a genuine assessment of the efficacy and selectivity of these inhibitors in a physiological setting. In addition, the ability to visualize the activity of a protein kinase in real time furnishes a direct measurement of the activation of specific signaling pathways in response to extracellular stimuli. We have developed two series of fluorescent substrates for protein kinase C (PKC) using a strategy that positions the reporter-group directly on the residue undergoing phosphorylation. The first series of PKC substrates is based, in part, on the Ca(+2) indicators developed by Tsien and his collaborators during the 1980s. In this case, phosphorylation of the substrate creates a divalent metal ion binding site. Upon metal ion coordination, a fluorescence change transpires via a mechanism analogous to that described for the Ca(+2) indicators. The second series of PKC sensors was identified via the preparation and subsequent screen of a library of fluorescently-labeled PKC peptide substrates. The lead derivative displays a phosphorylation-induced fluorescence change that allows the visualization of real-time PKC activity in both cell lysates and living cells. Furthermore, immunodepletion experiments demonstrate that the fluorescently-tagged peptide is selectively, if not exclusively, phosphorylated by the conventional PKCs. Both of the protein kinase biosensor strategies take advantage of the ease with which peptides can be modified to create libraries of structurally altered analogs. However, the inherent synthetic mutability of peptides is not just limited to library construction. For example, it may ultimately be possible to simultaneously monitor multiple protein kinases by affixing fluorophores with distinct photophysical properties to appropriately designed active site-directed peptides.

Synthesis of a bisubstrate analogue targeting estrogen sulfotransferase

Sulfotransferases catalyze the transfer of a sulfuryl group from the eukaryotic sulfate donor 3'-phosphoadenosine 5'-phosphosulfate to an acceptor biomolecule. Sulfotransferases have been linked with several disease states, prompting our investigation of specific sulfotransferase inhibitors. Presented herein is the synthesis and evaluation of a bisubstrate analogue designed to inhibit estrogen sulfotransferase. The synthesis utilizes a novel, orthogonally protected 3'-phosphoadenosine 5'-phosphate (PAP) derivative allowing the selective functionalization of the 5'-phosphate with a sulfate acceptor mimic. Kinetic studies revealed significant inhibitory activity and provide guidance for improved inhibitor design.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

cGMP inhibits IP3-induced Ca2+ release in intact rat megakaryocytes via cGMP- and cAMP-dependent protein kinases

1. Inhibition of inositol 1,4,5-trisphosphate (IP3) receptor-mediated Ca2+ release by cGMP was examined in intact rat megakaryocytes, by using a combination of single cell fluorescence microscopy to monitor intracellular free calcium ([Ca2+]i) and flash photolysis of caged second messengers. 2. Sodium nitroprusside (SNP), a nitric oxide (NO) donor, and the hydrolysis-resistant cGMP analogue 8-(4-chlorophenylthio)guanosine 3',5'-cyclic monophosphate (pCPT-cGMP) inhibited Ca2+ release induced by photolysis of caged IP3. Neither of them affected the rate of Ca2+ removal from the cytoplasm following photolysis of caged Ca2+. 3. Photolysis of the caged NO donor 3-morpholinosydnonimine (SIN-1) during agonist-induced [Ca2+]i oscillations inhibited Ca2+ release without affecting the rate of Ca2+ uptake and/or extrusion. 4. We conclude that the inhibition of IP3-induced Ca2+ release is the principal mechanism of NO-cGMP-dependent inhibition of [Ca2+]i mobilization. 5. IPG, a specific peptide inhibitor of cGMP-dependent protein kinase (cGMP-PK), blocked the inhibitory effect of pCPT-cGMP, indicating that the inhibition of IP3-induced Ca2+ release by pCPT-cGMP is mediated by cGMP-PK. However, the simultaneous application of both IPG and IP20, a specific peptide inhibitor of cAMP-dependent protein kinase (cAMP-PK), was required to block the inhibitory effect of SNP. These data strongly suggest that NO-cGMP-dependent inhibition of [Ca2+]i mobilization is mediated via the activation of both cGMP-PK and cAMP-PK.