The mechanism of action of cantharidin in smooth muscle

Cantharidin increases the force of contraction and protein phosphorylation in isolated human atria

Cantharidin, an inhibitor of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A), is known to increase the force of contraction and shorten the time to relaxation in human ventricular preparations. We hypothesized that cantharidin has similar positive inotropic effects in human right atrial appendage (RAA) preparations. RAA were obtained during bypass surgery performed on human patients. These trabeculae were mounted in organ baths and electrically stimulated at 1 Hz. For comparison, we studied isolated electrically stimulated left atrial (LA) preparations and isolated spontaneously beating right atrial (RA) preparations from wild-type mice. Cumulatively applied (starting at 10 to 30 µM), cantharidin exerted a positive concentration-dependent inotropic effect that plateaued at 300 µM in the RAA, LA, and RA preparations. This positive inotropic effect was accompanied by a shortening of the time to relaxation in human atrial preparations (HAPs). Notably, cantharidin did not alter the beating rate in the RA preparations. Furthermore, cantharidin (100 µM) increased the phosphorylation state of phospholamban and the inhibitory subunit of troponin I in RAA preparations, which may account for the faster relaxation observed. The generated data indicate that PP1 and/or PP2A play a functional role in human atrial contractility.

Hepatotoxic mechanism of cantharidin: insights and strategies for therapeutic intervention

Cantharidin (CTD), a natural compound derived from Mylabris, is widely used in traditional Oriental medicine for its potent anticancer properties. However, its clinical application is restricted due to its high toxicity, particularly towards the liver. This review provides a concise understanding of the hepatotoxic mechanisms of CTD and highlights novel therapeutic strategies to mitigate its toxicity while enhancing its anticancer efficacy. We systematically explore the molecular mechanisms underlying CTD-induced hepatotoxicity, focusing on the involvement of apoptotic and autophagic processes in hepatocyte injury. We further discuss the endogenous and exogenous pathways implicated in CTD-induced liver damage and potential therapeutic targets. This review also summarizes the structural modifications of CTD derivatives and their impact on anticancer activity. Additionally, we delve into the advancements in nanoparticle-based drug delivery systems that hold promise in overcoming the limitations of CTD derivatives. By offering valuable insights into the hepatotoxic mechanisms of CTD and outlining potential avenues for future research, this review contributes to the ongoing efforts to develop safer and more effective CTD-based therapies.

Cantharidin toxicosis in 2 alpacas

Two adult alpacas were presented for recumbency and reluctance to rise. Cantharidin toxicosis was suspected based on clinical and ancillary diagnostic findings. The diagnosis was confirmed by gas chromatography-mass spectrometry of gastric contents and urine. Despite medical treatment, neither alpaca survived. Blister beetle toxicosis has not been previously described in camelids. Challenges in treatment of affected ruminants or pseudoruminants are noted.

Matrix stiffness reverses the effect of actomyosin tension on cell proliferation

The stiffness of the extracellular matrix exerts powerful effects on cell proliferation and differentiation, but the mechanisms transducing matrix stiffness into cellular fate decisions remain poorly understood. Two widely reported responses to matrix stiffening are increases in actomyosin contractility and cell proliferation. To delineate their relationship, we modulated cytoskeletal tension in cells grown across a physiological range of matrix stiffnesses. On both synthetic and naturally derived soft matrices, and across a panel of cell types, we observed a striking reversal of the effect of inhibiting actomyosin contractility, switching from the attenuation of proliferation on rigid substrates to the robust promotion of proliferation on soft matrices. Inhibiting contractility on soft matrices decoupled proliferation from cytoskeletal tension and focal adhesion organization, but not from cell spread area. Our results demonstrate that matrix stiffness and actomyosin contractility converge on cell spreading in an unexpected fashion to control a key aspect of cell fate.

AURKB-mediated effects on chromatin regulate binding versus release of XIST RNA to the inactive chromosome

How XIST RNA strictly localizes across the inactive X chromosome is unknown; however, prophase release of human XIST RNA provides a clue. Tests of inhibitors that mimic mitotic chromatin modifications implicated an indirect role of PP1 (protein phosphatase 1), potentially via its interphase repression of Aurora B kinase (AURKB), which phosphorylates H3 and chromosomal proteins at prophase. RNA interference to AURKB causes mitotic retention of XIST RNA, unlike other mitotic or broad kinase inhibitors. Thus, AURKB plays an unexpected role in regulating RNA binding to heterochromatin, independent of mechanics of mitosis. H3 phosphorylation (H3ph) was shown to precede XIST RNA release, whereas results exclude H1ph involvement. Of numerous Xi chromatin (chromosomal protein) hallmarks, ubiquitination closely follows XIST RNA retention or release. Surprisingly, H3S10ph staining (but not H3S28ph) is excluded from Xi and is potentially linked to ubiquitination. Results suggest a model of multiple distinct anchor points for XIST RNA. This study advances understanding of RNA chromosome binding and the roles of AURKB and demonstrates a novel approach to manipulate and study XIST RNA.

Insulin-inhibited and stimulated cultured vascular smooth muscle cell migration are related to divergent effects on protein phosphatase-2A and autonomous calcium/calmodulin-dependent protein kinase II

Insulin, in the permissive presence of nitric oxide (NO), stimulates cGMP production which inhibits autonomous calcium/calmodulin-dependent protein kinase II (CaM kinase II) thereby inhibiting cultured vascular smooth muscle cell (VSMC) migration. In the presence of angiotensin II (Ang II), insulin stimulates NAD(P)H oxidase activity leading to increased VSMC migration. We wished to see whether insulin-stimulated cGMP stimulates protein phosphatase-2A (PP-2A) thereby inhibiting autonomous CaM kinase II and migration, and whether insulin, in the presence of Ang II, inhibits PP-2A and stimulates autonomous CaM kinase II in a NAD(P)H oxidase-dependent manner. One nanomole per litre of insulin in the presence of NO, or 50 micromol/L 8-Br-cGMP stimulated PP-2A activity by 46+/-6 and 247+/-23%, respectively (both P<0.05), and 8-Br-cGMP inhibited autonomous CaM kinase II activity by 67+/-9% (P<0.05) by a 10 nmol/L okadaic acid-sensitive pathway. Insulin plus Ang II inhibited PP-2A activity by 57+/-7% (P<0.05) and stimulated autonomous CaM kinase II activity by 120+/-14% (P<0.05), both by an apocynin-sensitive pathway. 8-Br-cGMP-inhibited VSMC migration was blocked by okadaic acid. It is concluded that insulin in the presence of NO stimulates cGMP which stimulates PP-2A activity causing inhibition of autonomous CaM kinase II activity and thus VSMC migration, and that insulin in the presence of Ang II inhibits PP-2A and stimulates autonomous CaM kinase II activities by a NAD(P)H oxidase-dependent mechanism which are associated with insulin-stimulated NAD(P)H oxidase-dependent migration.

Postjunctional synergism of norepinephrine with ATP and diadenosine tetraphosphate in Guinea pig vas deferens. Role of protein kinase C and Myosin light chain phosphatase

In isolated guinea pig vas deferens, prior addition of norepinephrine (NE) significantly potentiated the contractile responses to adenosine-5'-triphosphate (ATP) and diadenosine tetraphosphate (AP4A) in a dose-dependent manner up to 240% of the control purine dose. The myosin light chain phosphatase (MLCP) inhibitor cantharidin at a dose of 10 micromol/l caused significant enhancement of ATP at concentrations of 1 and 3 mmol/l by 91 and 95% respectively. Similarly, cantharidin enhanced the contraction to AP4A, 30 and 100 micromol/l by 92 and 100% respectively. Inhibition of protein kinase C (PKC) by the use of chelerythrine (10 micromol/l), incubated at the vas deferens for 60 min, inhibited the NE-induced enhancement of purine-induced contraction. Chelerythrine reversed the NE-ATP and NE-AP4A synergism back close to control ATP and AP4A contraction values respectively. It can be concluded that postjunctional synergism becomes evident not only for adenine mononucleotides and NE but also for diadenosine polyphosphates presented here by AP4A in the guinea pig vas deferens. This synergism involves receptor-mediated activation of PKC and possibly PKC-induced inhibition of MLCP.

Role of Rho-kinase in guinea-pig gallbladder smooth muscle contraction

Guinea-pig gallbladder smooth muscle contractions can be elicited pharmacologically by a range of mechanisms. The involvement of Rho-kinase in contractions mediated by receptor-dependent and receptor-independent mechanisms was investigated using the Rho-kinase inhibitor (+)-(R)-trans-4-(1-aminoethyl)-N-(4-pyridyl) cyclohexane carboxamide (Y-27632). In a separate series of experiments, the role of Rho-kinase in the contractile response to Ca2+ entry through store-operated Ca2+ channels and to electrical field stimulation was also examined. Y-27632 (10 microM), which caused a significant decrease (P<0.0005) in basal resting tone, significantly inhibited gallbladder contractions evoked by cumulative additions of the G-protein-coupled agonists, carbachol (1 nM-100 microM; P<0.05) and cholecystokinin (10 nM-1 microM; P<0.005). Y-27632 also inhibited the contractions evoked by a single addition of the sarcoplasmic reticulum ATPase inhibitor, thapsigargin (1 microM; P<0.0005) and cumulative additions of KCl (10-85 mM; P<0.0005). The contractile response to Ca2+ entry through store-operated Ca2+ channels was significantly inhibited by Y-27632 (P<0.05) as were the contractile responses evoked by electrical field stimulation (2-25 Hz; P<0.0005). In contrast, Y-27632 had no significant effect on contractions evoked by phorbol 12,13-dibutyrate (0.1 nM-1 microM; a protein kinase C activator) or by the phosphatase inhibitor, cantharidin (100 microM). In conclusion, Rho-kinase contributes to the contractile response in guinea-pig gallbladder smooth muscle evoked by both G-protein-coupled and non-G-protein-coupled mechanisms in addition to contributing to the maintenance of basal tone. It also contributes to the contractile responses resulting from electrical field stimulation and store-operated Ca2+ channel entry.

Mechanisms underlying postjunctional synergism between responses of the vas deferens to noradrenaline and ATP

Mechanisms of postjunctional synergism between adenosine 5'-triphosphate (ATP) and noradrenaline were studied in isolated guinea pig vas deferens. Whereas prior exposure to ATP had no significant effect on noradrenaline-mediated contractions, noradrenaline concentration-dependently enhanced ATP-induced contractions. Similarly to noradrenaline, histamine, which also acts via phospholipase-coupled receptors, induced contractions of the vas deferens and enhanced subsequent responses to ATP. Although phorbol-12, 13-dibutyrate (PDBu), a stimulant of protein kinase C (PKC), failed to induce contractions, it significantly potentiated ATP-induced contractions. The PKC inhibitor, Calphostin C, prevented this effect and the noradrenaline-mediated enhancement of ATP-induced contractions. The phosphatase inhibitor cantharidin induced a time- and concentration-dependent tonic contraction and markedly increased subsequent contractions to ATP. It is suggested that noradrenaline potentiates the contractile response of the vas deferens to ATP via a PKC-mediated mechanism. This may involve the inhibition of myosin light chain phosphatase (MLCP) and subsequent calcium sensitisation.

Effects of phosphoprotein phosphatase inhibitors (phenylarsine oxide and cantharidin) on Tetrahymena

The effects of phenylarsine oxide (PAO) (phosphotyrosine phosphatase inhibitor) and cantharidin (serine/threonine phosphatase [PP2A] inhibitor) treatments were analysed on the synthesis of phospholipids and glycolipids, and on the cytoskeletal elements (F-actin and tubulin containing structures) of Tetrahymena pyriformis. Both phosphatase inhibitors reduced the amount of incorporated 32P of the whole phospholipid content, but the ratio of phosphatidylserine (PS) and phosphatidylcholine (PC) to the total phospholipid content increased. Both treatments influenced the phosphatidylinositol (PI) system. These inhibitors also influenced the incorporation of palmitic acid into the phospholipids: in general PAO decreased, whereas cantharidin increased the amount of incorporated palmitic acid; 1 microM cantharidin significantly increased the labelling of PE and PA. The incorporation of mannose and glucosamine was influenced differently by PAO and cantharidin treatments: the latter elevated, while PAO decreased the labelling of glycolipids with these sugars. The effects of these treatments were visible also in the case of confocal scanning laser microscopic (CSLM) images: after treatments with both inhibitors, the F-actin containing cortical elements were destroyed, but the tubulin containing ones (longitudinal and transversal microtubules, oral apparatus and deep fibres) did not display significant alterations. The different effects of phosphatase inhibitors were visible also on the scanning electron microscopic (sEM) images: cantharidin treatments (1 microM) decreased the amount of dissolved membrane lipids after chemical dehydration of the cells with 2, 2-dimethoxy propane (DMP), but in the case of treatments with 10 microM, the surface pattern of cells was similar to the controls. On the other hand, after PAO treatments the surface pattern of Tetrahymena showed significant alterations. Both phosphatase inhibitors inhibited the phagocytotic activity of the cells. On the basis of present experiments we suppose that these treatments are able to influence signalling systems (e.g. PI) of Tetrahymena, and also the structure of the cytoskeleton and the functions (e.g. phagocytosis) which are connected with skeletal elements.

Role of protein phosphatases in regulation of cardiac inotropy and relaxation

We studied the effects of the protein phosphatase (PP) inhibitor cantharidin (Cant) on time parameters and force of contraction (FOC) in isometrically contracting electrically driven guinea pig papillary muscles. We correlated the mechanical parameters of contractility with phosphorylation of the inhibitory subunit of troponin (TnI-P) and with the site-specific phosphorylation of phospholamban (PLB) at serine-16 (PLB-Ser-16) and threonine-17 (PLB-Thr-17). Cant (after 30 min) started to increase FOC (112 +/- 4% of control, n = 10) and TnI-P and PLB-Thr-17 (120 +/- 5 and 128 +/- 7% of control) without any alteration of relaxation time. Cant (10 microM) started to increase PLB-Ser-16, but the relaxation was shortened at only 100 microM (from 140 +/- 9 to 116 +/- 12 ms, n = 9). Moreover, 100 microM Cant, 3 min after application, started to increase PLB-Thr-17, TnI-P, and FOC. Cant (100 microM) began to increase PLB-Ser-16 after 20 min. This was accompanied by shortening of relaxation time. Differences in protein kinase activation or different substrate specificities of PP may explain the difference in Cant-induced site-specific phosphorylation of PLB in isometrically contracting papillary muscles. Moreover, PLB-Thr-17 may be important for inotropy, whereas PLB-Ser-16 could be a major determinant of relaxation time.

Regional expression of protein phosphatase type 1 and 2A catalytic subunit isoforms in the human heart

In mammalian species, including man, the duration of myocardial contraction is shorter in atria than ventricles. Total contraction time depends at least in part on phosphorylation and dephosphorylation of cardiac regulatory proteins. Dephosphorylation reactions are mediated by protein phosphatases. In the mammalian heart more than 90% of the protein phosphatase (PP) activity consists of PP1 and PP2A. Therefore, the aim of this study was to investigate which isoforms of PP1 and PP2A are present in human myocardium and whether their expression is regionally different. RT-PCR and Northern blotting revealed that all isoforms of PP1 and PP2A presently known are expressed in the human heart. Expression levels of PP1 alpha, delta, and gamma as well as 2A alpha were higher in right ventricles than in right atria. However, there was no such difference for PP2A beta. At the protein level PP1 alpha was unchanged, whereas PP2A was by 56% higher in right ventricles compared to atria. The phosphorylation state of TnI was lower in right ventricle than in right atrium. Thus, lower protein expression of PP2A in atrium could contribute to the faster relaxation by increasing the phosphorylation state of TnI. We conclude that expression of PP1 and PP2A isoforms is regionally regulated in the human heart.

Effects of serine/threonine protein phosphatases on ion channels in excitable membranes

This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.

Contractility and inhibition of protein phosphatases by cantharidin

1. Cantharidin is a natural defensive toxicant produced by blister beetles. 2. Cantharidin shares structural similarity with highly toxic commercial herbicides (e.g., endothall, endothall anhydride and endothall thioanhydride). 3. Cantharidin inhibits the activity of purified catalytic subunits of serine/threonine protein phosphatases (PP) type 1 and type 2A. 4. Cantharidin increases force of contraction in isolated myocardial and vascular preparations. 5. Cantharidin enhances the phosphorylation state of myocardial and vascular regulatory proteins. 6. Cantharidin is a valuable tool for studying the function of PP in regulatory phosphorylation-dephosphorylation events.

Postnatal changes in contractile time parameters, calcium regulatory proteins, and phosphatases

Compared with isolated electrically driven neonatal ventricular preparations, the total time of contraction, the time to peak tension, and the time of relaxation were decreased to approximately 50% in adult ventricular preparations. The expression of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) was increased to 133% at the protein level and to 154% at the mRNA level in adult vs. neonatal ventricular preparations, whereas phospholamban was unchanged at both the protein and mRNA levels. Moreover, Ca2+ uptake was increased to 180% in adult vs. neonatal ventricular preparations. Phospholamban phosphorylation was enhanced in adult vs. neonatal ventricular preparations. In adult ventricular preparations, phosphatase activity was reduced to 53% of neonatal preparations, the protein levels of the immunologically detectable catalytic subunits of protein phosphatase types 1 and 2A were reduced to 28 and 61% of neonatal preparations, respectively, and the mRNA levels of type 1alpha, 1beta, 1gamma, 2Aalpha, and 2Abeta phosphatase isoforms were decreased to 69, 68, 54, 67, and 63%, respectively. We conclude that in the adult rat heart, the shortened time parameters of contraction can be explained by an elevated expression of SERCA. In addition, an increased phosphorylation state of phospholamban due to reduced phosphatase activity may be involved.