Signalling pathways responsible for the methylisogermabullone-induced contraction of ileal longitudinal muscles

Characteristics of ginsenoside Rd-induced effects on rat intestinal contractility with irritable bowel syndrome

Irritable bowel syndrome (IBS) is a very common refractory disease. Its exact pathophysiological mechanism is still unclear. Despite the availability of plentiful drugs to control IBS, most patients do not respond well. Ginsenoside Rd is one of the major active components of Panax ginseng, which has been verified to produce various pharmacological actions. However, the role of ginsenoside Rd in modulating smooth muscle contractility is still undefined. The aim of this study is to investigate the effects of ginsenoside Rd on intestinal contractility and related mechanisms in IBS.

ERK and p38MAPK pathways regulate myosin light chain phosphatase and contribute to Ca2+ sensitization of intestinal smooth muscle contraction

BACKGROUND

Mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated protein kinase (ERK) and p38MAPK, are known regulators of smooth muscle contractility. The contraction of smooth muscle is mainly regulated by the phosphorylation of regulatory light chains of myosin II (LC20), which is driven by the balance between myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP). We hypothesized that one possible mechanism for MAPK-dependent modulation of intestinal smooth muscle contractility is via the regulation of MLCP activity.

METHODS

Contractile responses to carbachol (CCh) and effects of MAPK inhibitors on CCh-induced contractions were assessed with isolated rat ileal longitudinal smooth muscle strips. Biochemical assessments of MLCP activity and myosin phosphatse targeting subunit (MYPT1) and CPI-17 phosphorylations were completed.

KEY RESULTS

Treatment of ileal smooth muscle with PD98059 (10 μM; MEK inhibitor) or SB203580 (10 μM; p38MAPK inhibitor) significantly inhibited CCh-induced contractile force. Decreased MLCP activity was observed during sustained contractions induced by CCh; the MLCP activity was recovered by treatment with PD98059 and SB203580. However, MYPT1 (Thr697 and Thr855) and CPI-17 (Thr38) phosphorylations were not affected. Application of ML-7 (MLCK inhibitor) during CCh-induced sustained contraction elicited an MLCP-dependent relaxation, the rate of which was accelerated by application of PD98059 and SB203580 with proportional changes in LC20 phosphorylation levels but not MYPT1 phosphorylation (Thr697 or Thr855).

CONCLUSIONS & INFERENCES

ERK and p38MAPK contribute to CCh-induced sustained contraction in a LC20 phosphorylation dependent manner. Moreover, both kinases inhibit MLCP activity possibly by a novel mechanism.

Effects of ginsenosides on rat jejunal contractility

CONTEXT

Ginsenosides are primary active ingredients of ginseng, which are believed to have various health benefits. It is found that the biotransformation of ginsenosides mainly takes place in the gastrointestinal tract and the information about ginsenosides-exerted effects on intestinal contractility is not sufficient.

AIMS

The present study proposed that ginsenosides could exert stimulatory or inhibitory effects on intestinal motility depending on the assay condition-related intestinal contractile states and was to characterize the effects of ginsenosides on intestinal motility.

METHODS

Jejunal contractility determination, Western blotting analysis, and real-time polymerase chain reaction were performed to test the effects of total ginsenosides isolated from Panax ginseng C. A. Mey (Araliaceae) root.

RESULTS

The results showed that ginsenosides at the fixed concentration of 20 mg/L exerted bidirectional regulation (BR) on the contractility of isolated jejunal segment (IJS), depending on the contractile states. The contractility of IJS was increased by ginsenosides in low contractile states, which were correlated to the cholinergic activation, and the contractility of IJS was decreased by ginsenosides in high contractile states, which were correlated to the adrenergic activation and nitric oxide related mechanisms. Ginsenosides-induced BR was abolished in the absence of Ca(2+) or by using tetrodotoxin, implicating the requirement of Ca(2+) and the enteric nervous system. Effects of ginsenosides on myosin light chain phosphorylation and the mRNA expression of myosin light chain kinase were also bidirectional.

DISCUSSION AND CONCLUSION

Results suggest that ginsenosides may have the potential clinical implication for reliving the symptoms of alternative hypo- and hyper-intestinal motility.

MEKK1-MKK4-JNK-AP1 pathway negatively regulates Rgs4 expression in colonic smooth muscle cells

BACKGROUND

Regulator of G-protein Signaling 4 (RGS4) plays an important role in regulating smooth muscle contraction, cardiac development, neural plasticity and psychiatric disorder. However, the underlying regulatory mechanisms remain elusive. Our recent studies have shown that upregulation of Rgs4 by interleukin (IL)-1β is mediated by the activation of NFκB signaling and modulated by extracellular signal-regulated kinases, p38 mitogen-activated protein kinase, and phosphoinositide-3 kinase. Here we investigate the effect of the c-Jun N-terminal kinase (JNK) pathway on Rgs4 expression in rabbit colonic smooth muscle cells.

METHODOLOGY/PRINCIPAL FINDINGS

Cultured cells at first passage were treated with or without IL-1β (10 ng/ml) in the presence or absence of the selective JNK inhibitor (SP600125) or JNK small hairpin RNA (shRNA). The expression levels of Rgs4 mRNA and protein were determined by real-time RT-PCR and Western blot respectively. SP600125 or JNK shRNA increased Rgs4 expression in the absence or presence of IL-1β stimulation. Overexpression of MEKK1, the key upstream kinase of JNK, inhibited Rgs4 expression, which was reversed by co-expression of JNK shRNA or dominant-negative mutants for MKK4 or JNK. Both constitutive and inducible upregulation of Rgs4 expression by SP600125 was significantly inhibited by pretreatment with the transcription inhibitor, actinomycin D. Dual reporter assay showed that pretreatment with SP600125 sensitized the promoter activity of Rgs4 in response to IL-1β. Mutation of the AP1-binding site within Rgs4 promoter increased the promoter activity. Western blot analysis confirmed that IL-1β treatment increased the phosphorylation of JNK, ATF-2 and c-Jun. Gel shift and chromatin immunoprecipitation assays validated that IL-1β increased the in vitro and ex vivo binding activities of AP1 within rabbit Rgs4 promoter.

CONCLUSION/SIGNIFICANCE

Activation of MEKK1-MKK4-JNK-AP1 signal pathway plays a tonic inhibitory role in regulating Rgs4 transcription in rabbit colonic smooth muscle cells. This negative regulation may aid in maintaining the transient level of RGS4 expression.