The relaxant effect of okadaic acid on canine basilar artery involves activation of PKCalpha and phosphorylation of the myosin light chain at Thr-9

Accelerating effects of blebbistatin on relaxation process of cell membrane permeabilized trachea and taenia cecum from guinea pig

Blebbistatin, a potent inhibitor of myosin II, is known to suppress smooth muscle contraction without affecting myosin light chain phosphorylation level. In order to clarify the regulatory mechanisms of blebbistatin on phasic and tonic smooth muscles in detail, we examined the effects of blebbistatin on relaxation process by Ca²⁺ removal after Ca²⁺-induced contraction of β-escin skinned (cell membrane permeabilized) trachea and taenia cecum preparations from guinea pigs. Blebbistatin significantly suppressed the force during relaxation both in skinned trachea and taenia cecum. The data fitting analysis of the relaxation processes indicates that blebbistatin accelerates slow (latch-like) bridge dissociation.

Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Force-inhibiting effect of Ser/Thr protein phosphatase 2A inhibitors on bovine ciliary muscle

Ciliary muscle is a smooth muscle characterized by a rapid response to muscarinic receptor stimulation and sustained contraction. Although it is evident that these contractions are Ca(2+)-dependent, detailed molecular mechanisms are still unknown. In order to elucidate the role of Ser/Thr protein phosphatase 2A (PP2A) in ciliary muscle contraction, we examined the effects of okadaic acid and other PP2A inhibitors on contractions induced by carbachol (CCh) and ionomycin in bovine ciliary muscle strips (BCM). Okadaic acid inhibited ionomycin-induced contraction, while it did not cause significant changes in CCh-induced contraction. Fostriecin showed similar inhibitory effects on the contraction of BCM. On the other hand, rubratoxin A inhibited both ionomycin- and CCh-induced contractions. These results indicated that PP2A was involved at least in ionomycin-induced Ca(2+)-dependent contraction, and that BCM had a unique regulatory mechanism in CCh-induced contraction.

Differential effect of adenosine receptors on growth of human colon cancer HCT 116 and HT-29 cell lines

The study aimed to evaluate the impact of adenosine receptors (ARs) on human colon tumor cells (HCT 116, HT-29) growth and sensitivity to 5-Fluorouracil (5-FU) an anticancer chemotherapeutic drug. The exposure of cancer cells to a selective A(3)-AR agonist (IB-MECA) resulted in an increase in HT-29 cells number, whereas the number of HCT 116 cells decreased significantly. In the presence of IB-MECA (1 μM) the percentage of apoptotic HT-29 cells significantly decreased, whereas the number of apoptotic and necrotic HCT 116 cells increased by 3- and 2,5-fold, respectively. The application of a selective A(2A)-AR agonist resulted in the increased survival of HCT 116 cells, but not HT-29 cells. The blockade of A(2A)-AR with ZM 241385 (0.1 μM) significantly increased the cytotoxicity of 5-FU (1 μM) in HCT 116 cells but not in HT-29 cells. The suppression of A(3)-AR with MRS 1523 (1 μM) increased the sensitivity of HT-29 cells to 5-FU while response of HCT 116 cells to 5-FU decreased. The growth promoting effect of IB-MECA in HT-29 cells was associated with the decreased intracellular cAMP level, whereas IB-MECA growth inhibitory effect in HCT 116 cells was abolished by okadaic acid (2 nM) indicating the involvement of protein phosphatase PP2A.

Role of serine-threonine phosphoprotein phosphatases in smooth muscle contractility

The degree of phosphorylation of myosin light chain 20 (MLC20) is a major determinant of force generation in smooth muscle. Myosin phosphatases (MPs) contain protein phosphatase (PP) 1 as catalytic subunits and are the major enzymes that dephosphorylate MLC20. MP regulatory targeting subunit 1 (MYPT1), the main regulatory subunit of MP in all smooth muscles, is a key convergence point of contractile and relaxatory pathways. Combinations of regulatory mechanisms, including isoform splicing, multiple phosphorylation sites, and scaffolding proteins, modulate MYPT1 activity with tissue and agonist specificities to affect contraction and relaxation. Other members of the PP1 family that do not target myosin, as well as PP2A and PP2B, dephosphorylate a range of proteins that affect smooth muscle contraction. This review discusses the role of phosphatases in smooth muscle contractility with a focus on MYPT1 in uterine smooth muscle. Myometrium shares characteristics of vascular and other visceral smooth muscles yet, during healthy pregnancy, undergoes hypertrophy, hyperplasia, quiescence, and labor as physiological processes. Myometrium presents an accessible model for the study of normal and pathological smooth muscle function, and a better understanding of myometrial physiology may allow the development of novel therapeutics for the many disorders of myometrial physiology from preterm labor to dysmenorrhea.

Regulation of GPCR-mediated smooth muscle contraction: implications for asthma and pulmonary hypertension

Contractile G-protein-coupled receptors (GPCRs) have emerged as key regulators of smooth muscle contraction, both under healthy and diseased conditions. This brief review will discuss some key topics and novel insights regarding GPCR-mediated airway and vascular smooth muscle contraction as discussed at the 7th International Young Investigators' Symposium on Smooth Muscle (2011, Winnipeg, Manitoba, Canada) and will in particular focus on processes driving Ca(2+)-mobilization and -sensitization.

New insights into myosin phosphorylation during cyclic nucleotide-mediated smooth muscle relaxation

Nitrovasodilators and agonists, via an increase in intracellular cyclic nucleotide levels, can induce smooth muscle relaxation without a concomitant decrease in phosphorylation of the regulatory light chains (RLC) of myosin. However, since cyclic nucleotide-induced relaxation is associated with a decrease in intracellular [Ca²⁺], and hence, a decreased activity of MLCK, we tested the hypothesis that the site responsible for the elevated RLC phosphorylation is not Ser19. Smooth muscle strips from gastric fundus were isometrically contracted with ET-1 which induced an increase in monophosphorylation from 9 ± 1 % under resting conditions (PSS) to 36 ± 1 % determined with 2D-PAGE. Electric field stimulation induced a rapid, largely NO-mediated relaxation with a half time of 8 s, which was associated with an initial decline in RLC phosphorylation to 18 % within 2 s and a rebound to 34 % after 30 s whereas relaxation was sustained. In contrast, phosphorylation of RLC at Ser19 probed with phosphospecific antibodies declined in parallel with force. LC/MS and western blot analysis with phosphospecific antibodies against monophosphorylated Thr18 indicate that Thr18 is significantly monophosphorylated during sustained relaxation. We therefore suggest that (i) monophosphorylation of Thr18 rather than Ser19 is responsible for the phosphorylation rebound during sustained EFS-induced relaxation of mouse gastric fundus, and (ii) that relaxation can be ascribed to dephosphorylation of Ser19, the site considered to be responsible for regulation of smooth muscle tone.

Benzyl isothiocyanate (BITC) inhibits migration and invasion of human colon cancer HT29 cells by inhibiting matrix metalloproteinase-2/-9 and urokinase plasminogen (uPA) through PKC and MAPK signaling pathway

Benzyl isothiocyanate (BITC), a component of dietary cruciferous vegetables, has antioxidant and anticancer properties. In this study, we show for the first time the antimetastatic effects of BITC in human colon cancer HT29 cells. BITC had an inhibitory effect on cell migration and invasion. Protein levels of matrix metalloproteinase-2 (MMP-2), matrix metalloproteinase-9 (MMP-9), and urokinase-plasminogen activator (u-PA) were reduced by BITC in a concentration-dependent manner. BITC also exerted an inhibitory effect on phosphorylation of c-Jun N-terminal kinase 1 and 2 (JNK1/2), extracellular signal-regulated kinases 1 and 2 (ERK1/2), phosphatidylinositol 3-kinase (PI3K) and protein kinase C (PKC) that are upstream of nuclear factor kappa B (NF-kappaB). BITC inhibited DNA binding activity of NF-kappaB. Moreover, BITC decreased the levels of c-Fos, c-Jun, Ras, FAK, PI3K and GRB2 in HT29 cells. Reductions in the enzyme activity, protein and mRNA (mRNA) levels of MMP-2 were observed in BITC-treated HT29 cells. BITC also inhibited mRNA levels of MMP-2, -7, and -9 in HT29 cells. Results from zymography showed that BITC treatment decreased MMP-2 expression in a concentration-dependent manner. BITC inhibited PKCdelta activity in HT29 cells. Furthermore, inhibitors specific for JNK (SP600125) reduced expression of MMP-2, MMP-9, and u-PA. These results demonstrated that BITC could alter HT29 cell metastasis by reduction of MMP-2, MMP-9, and u-PA expression through the suppression of a PKC, MAPK signaling pathway and inhibition of NF-kappaB levels. These findings suggest that BITC has potential as an antimetastatic agent.

ARF6-regulated shedding of tumor cell-derived plasma membrane microvesicles

BACKGROUND

Increased mitogen-activated protein kinase (MAPK) signaling, small GTPase activation, cytoskeletal rearrangements, and the directed targeting of proteases to sites of extracellular matrix degradation all accompany the process of tumor cell invasion. Several studies have implicated the small GTP-binding protein ARF6 in tumor cell invasion, although the molecular basis by which ARF6 facilitates this process is unclear.

RESULTS

We show that the ARF6 GTP/GDP cycle regulates the release of protease-loaded plasma membrane-derived microvesicles from tumor cells into the surrounding environment. To enable microvesicle shedding, ARF6-GTP-dependent activation of phospholipase D promotes the recruitment of the extracellular signal-regulated kinase (ERK) to the plasma membrane where, in turn, ERK phosphorylates and activates myosin light-chain kinase (MLCK). MLCK-mediated MLC phosphorylation is required for microvesicle release. Inhibition of ARF6 activation is accompanied by PKC-mediated phosphorylation of MLC, which blocks microvesicle shedding. Protein cargo appears to be selectively sorted into microvesicles, and adhesion to the extracellular matrix (ECM) is facilitated by microvesicle-associated integrin receptors.

CONCLUSIONS

Microvesicle shedding in tumor cells occurs via an actomyosin-based membrane abscission mechanism that is regulated by nucleotide cycling on ARF6. Microvesicle shedding appears to release selected cellular components, particularly those involved in cell adhesion and motility, into the surrounding environment. These findings suggest that ARF6 activation and the proteolytic activities of microvesicles, both of which are thought to correlate directly with tumor progression, could potentially serve as biomarkers for disease.