Protein kinase D regulates RhoA activity via rhotekin phosphorylation.

The members of the protein kinase D (PKD) family of serine/threonine kinases are major targets for tumor-promoting phorbol esters, G protein-coupled receptors, and activated protein kinase C isoforms (PKCs). The expanding list of cellular processes in which PKDs exert their function via phosphorylation of various substrates include proliferation, apoptosis, migration, angiogenesis, and vesicle trafficking. Therefore, identification of novel PKD substrates is necessary to understand the profound role of this kinase family in signal transduction. Here, we show that rhotekin, an effector of RhoA GTPase, is a novel substrate of PKD. We identified Ser-435 in rhotekin as the potential site targeted by PKD in vivo. Expression of a phosphomimetic S435E rhotekin mutant resulted in an increase of endogenous active RhoA GTPase levels. Phosphorylation of rhotekin by PKD2 modulates the anchoring of the RhoA in the plasma membrane. Consequently, the S435E rhotekin mutant displayed enhanced stress fiber formation when expressed in serum-starved fibroblasts. Our data thus identify a novel role of PKD as a regulator of RhoA activity and actin stress fiber formation through phosphorylation of rhotekin.

A placebo-controlled trial of prucalopride for severe chronic constipation.

BACKGROUND: In this 12-week trial, we aimed to determine the efficacy of prucalopride, a selective, high-affinity 5-hydroxytryptamine4 receptor agonist, in patients with severe chronic constipation. METHODS: In our multicenter, randomized, placebo-controlled, parallel-group, phase 3 trial, patients with severe chronic constipation (< or =2 spontaneous, complete bowel movements per week) received placebo or 2 or 4 mg of prucalopride, once daily, for 12 weeks. The primary efficacy end point was the proportion of patients having three or more spontaneous, complete bowel movements per week, averaged over 12 weeks. Secondary efficacy end points were derived from daily diaries and validated questionnaires completed by patients. Adverse events, clinical laboratory values, and cardiovascular effects were monitored. RESULTS: Efficacy was analyzed in 620 patients. The proportion of patients with three or more spontaneous, complete bowel movements per week was 30.9% of those receiving 2 mg of prucalopride and 28.4% of those receiving 4 mg of prucalopride, as compared with 12.0% in the placebo group (P<0.001 for both comparisons). Over 12 weeks, 47.3% of patients receiving 2 mg of prucalopride and 46.6% of those receiving 4 mg of prucalopride had an increase in the number of spontaneous, complete bowel movements of one or more per week, on average, as compared with 25.8% in the placebo group (P<0.001 for both comparisons). All other secondary efficacy end points, including patients' satisfaction with their bowel function and treatment and their perception of the severity of their constipation symptoms, were significantly improved with the use of 2 or 4 mg of prucalopride as compared with placebo, at week 12. The most frequent treatment-related adverse events were headache and abdominal pain. There were no significant cardiovascular effects of treatment. CONCLUSIONS: Over 12 weeks, prucalopride significantly improved bowel function and reduced the severity of symptoms in patients with severe chronic constipation. Larger and longer trials are required to further assess the risks and benefits of the use of prucalopride for chronic constipation. (ClinicalTrials.gov number, NCT00483886 [ClinicalTrials.gov].).

Efficacy and safety of prucalopride in patients with chronic noncancer pain suffering from opioid-induced constipation.

BACKGROUND: Opioid-induced constipation (OIC) has negative effects on quality of life (QOL). Prucalopride is a new, selective 5-HT(4) agonist and enterokinetic with strong clinical data in chronic constipation. This study investigated the efficacy, safety, and tolerability of prucalopride in patients with noncancer pain and OIC. METHODS: A phase II, double-blind, placebo-controlled study of 196 patients randomized to placebo (n = 66), prucalopride 2 mg (n = 66) or 4 mg (n = 64), for 4 weeks, was carried out. The primary endpoint was the proportion of patients with increase from baseline of ≥ 1 spontaneous complete bowel movement (SCBM)/week. Secondary endpoints [proportion of patients with ≥ 3 SCBM/week, weekly frequency of (SC)BM, severity of constipation, and efficacy of treatment], adverse events (AEs), and safety parameters were also monitored. RESULTS: More patients had an increase from baseline of ≥ 1 SCBM per week (weeks 1-4) in the prucalopride groups [35.9% (2 mg) and 40.3% (4 mg)] versus placebo (23.4%), reaching statistical significance in week 1. Over weeks 1-4, more patients in the prucalopride groups achieved an average of ≥ 3 SBM per week versus placebo (60.7% and 69.0% versus 43.3%), reaching significance at week 1. Prucalopride 4 mg significantly improved patient-rated severity of constipation and effectiveness of treatment versus placebo. Patient Assessment of Constipation-Symptom (PAC-SYM) total scores and Patient Assessment of Constipation-Quality of Life (PAC-QOL) total and satisfaction subscale scores were improved. The most common AEs were abdominal pain and nausea. There were no clinically relevant differences between groups in vital signs, laboratory measures or electrocardiogram parameters. CONCLUSION: In this population with OIC, prucalopride improved bowel function and was safe and well tolerated.

Protein kinase D: an intracellular traffic regulator on the move.

Recent research has identified protein kinase D (PKD, also called PKCmu) as a serine/threonine kinase with potentially important roles in growth factor signaling as well as in stress-induced signaling. Moreover, PKD has emerged as an important regulator of plasma membrane enzymes and receptors, in some cases mediating cross-talk between different signaling systems. The recent discovery of two additional kinases belonging to the PKD family and the plethora of proteins that interact with PKD point to a multifaceted regulation and a multifunctional role for these enzymes, with functions in processes as diverse as cell proliferation, apoptosis, immune cell regulation, tumor cell invasion and regulation of Golgi vesicle fission.

An enzyme-linked immunosorbent assay for protein kinase D activity using phosphorylation site-specific antibodies.

The protein kinase D (PKD) family is a novel group of kinases that are involved in the regulation of cell proliferation and apoptosis, and several other physiological processes. Hence, these enzymes are attractive targets for pharmacological intervention, but no specific PKD inhibitors are known. With this in mind, we have developed a high-throughput, non-radioactive enzyme-linked immunosorbent assay (ELISA) method to monitor the PKD activity with myelin basic protein (MBP) as substrate. We determined that MBP is phosphorylated by PKD on Ser-160 and that this phosphorylation can be quantified in ELISAs, by the use of phosphorylation site-specific antibodies. Antibodies were developed that are highly specific for the MBP peptide sequence surrounding the phosphorylated Ser-160. We show that our high-throughput kinase assay is useful not only for determining the cellular PKD activity but also to screen for PKD-inhibitory compounds. Our ELISA has advantages over the current radioisotope kinase assay in terms of simplicity and environmental safety.

Doxorubicin-induced activation of protein kinase D1 through caspase-mediated proteolytic cleavage: identification of two cleavage sites by microsequencing.

Recent studies have demonstrated the importance of protein kinase D (PKD) in cell proliferation and apoptosis. Here, we report that in vitro cleavage of recombinant PKD1 by caspase-3 generates two alternative active PKD fragments. N-terminal sequencing of these fragments revealed two distinct caspase-3 cleavage sites located between the acidic and pleckstrin homology (PH) domains of PKD1. Moreover, we present experimental evidence that PKD1 is an in vitro substrate for both initiator and effector caspases. During doxorubicin-induced apoptosis, a zVAD-sensitive caspase induces cleavage of PKD1 at two sites, generating fragments with the same molecular masses as those determined in vitro. The in vivo caspase-dependent generation of the PKD1 fragments correlates with PKD1 kinase activation. Our results indicate that doxorubicin-mediated apoptosis induces activation of PKD1 through a novel mechanism involving the caspase-mediated proteolysis.

Getting to know protein kinase D.

The protein kinase D (PKD) enzymes represent a new family of second messenger stimulated kinases, with diacylglycerol as a prime, but not the sole, mediator of activation. Their molecular architecture features a catalytic domain, unrelated to that of all PKC family members, and a large inhibitory, regulatory domain, comprised of two Zinc fingers, and a pleckstrin homology domain. These different sub-domains play distinctive roles in the activation, translocation and biological functions of the kinase. The enzymes have been implicated in signalling mechanisms controlling cell proliferation and programmed cell death and in metastasis, immune responses, and Golgi restructuring and function. A variety of proteins specifically interact with the different sub-domains of the enzymes and direct their wide range of cellular functions.

Protein kinase D: a family affair.

The protein kinase D family of enzymes consists of three isoforms: PKD1/PKCmu PKD2 and PKD3/PKCnu. They all share a similar architecture with regulatory sub-domains that play specific roles in the activation, translocation and function of the enzymes. The PKD enzymes have recently been implicated in very diverse cellular functions, including Golgi organization and plasma membrane directed transport, metastasis, immune responses, apoptosis and cell proliferation.

Protein kinase D induces transcription through direct phosphorylation of the cAMP-response element-binding protein.

Protein kinase D (PKD), a family of serine/threonine kinases, can be activated by a multitude of stimuli in a protein kinase C-dependent or -independent manner. PKD is involved in signal transduction pathways controlling cell proliferation, apoptosis, motility, and protein trafficking. Despite its versatile functions, few genuine in vivo substrates for PKD have been identified. In this study we demonstrate that the transcription factor cAMP-response element-binding protein (CREB) is a direct substrate for PKD. PKD1 and CREB interact in cells, and activated PKD1 provokes CREB phosphorylation at Ser-133 both in vitro and in vivo. A constitutive active mutant of PKD1 stimulates GAL4-CREB-mediated transcription in a Ser-133-dependent manner, activates CRE-responsive promoters, and increases the expression of CREB target genes. PKD1 also enhances transcription mediated by two other members of the CREB family, ATF-1 and CREM. Our results describe a novel mechanism for PKD-induced signaling through activation of the transcription factor CREB and suggest that stimulus-induced phosphorylation of CREB, reported to be mediated by protein kinase C, may involve downstream activated PKD.