TNF-alpha upregulates Gialpha and Gqalpha protein expression and function in human airway smooth muscle cells

Inhibition of geranylgeranylation blocks agonist-induced actin reorganization in human airway smooth muscle cells

To determine whether RhoA isoprenylation (geranylgeranylation) is required for agonist-induced actin cytoskeleton reorganization (measured by an increase in the filamentous F- to monomeric G-actin ratio), human airway smooth muscle cells were treated for 72 h with inhibitors of geranylgeranyltransferase I. Geranylgeranyltransferase inhibitor (GGTI)-2147 or -286 pretreatment completely blocked the increase in the F- to G-actin fluorescence ratio when cells were stimulated with lysophosphatidic acid (LPA), endothelin, or carbachol. In contrast, LPA or endothelin induced actin cytoskeletal reorganization in cells treated with farnesyltransferase inhibitor (FTI)-277 to inactivate Ras. Forskolin-induced adenylyl cyclase activity was inhibited by carbachol in GGTI-2147-pretreated cells, demonstrating that the effect of geranylgeranyltransferase I inhibition on stress fiber formation was not due to uncoupling of signaling between the heterotrimeric G(i) protein (the Ggamma subunit is isoprenylated) and distal effectors. These results demonstrate that selective GGTIs can inhibit agonist-induced actin reorganization.

Calcium sensitization produced by G protein activation in airway smooth muscle

We determined whether activation of G proteins can affect the force developed for a given intracellular Ca(2+) concentration ([Ca(2+)]; i.e., the Ca(2+) sensitivity) by mechanisms in addition to changes in regulatory myosin light chain (rMLC) phosphorylation. Responses in alpha-toxin-permeabilized canine tracheal smooth muscle were determined with Ca(2+) alone or in the presence of ACh, endothelin-1 (ET-1), or aluminum fluoride (AlF; acute or 1-h exposure). Acute exposure to each compound increased Ca(2+) sensitivity without changing the response to high [Ca(2+)] (maximal force). However, chronic exposure to AlF, but not to chronic ACh or ET-1, increased maximal force by increasing the force produced for a given rMLC phosphorylation. Studies employing thiophosphorylation of rMLC showed that the increase in force produced by chronic AlF exposure required Ca(2+) during activation to be manifest. Unlike the acute response to receptor agonists, which is mediated solely by increases in rMLC phosphorylation, chronic direct activation of G proteins further increases Ca(2+) sensitivity in airways by additional mechanisms that are independent of rMLC phosphorylation.

Mechanisms of TNF-alpha stimulation of amiloride-sensitive sodium transport across alveolar epithelium

Because tumor necrosis factor (TNF)-alpha can upregulate alveolar fluid clearance (AFC) in pneumonia or septic peritonitis, the mechanisms responsible for the TNF-alpha-mediated increase in epithelial fluid transport were studied. In rats, 5 microg of TNF-alpha in the alveolar instillate increased AFC by 67%. This increase was inhibited by amiloride but not by propranolol. We also tested a triple-mutant TNF-alpha that is deficient in the lectinlike tip portion of the molecule responsible for its membrane conductance effect; the mutant also has decreased binding affinity to both TNF-alpha receptors. The triple-mutant TNF-alpha did not increase AFC. Perfusion of human A549 cells, patched in the whole cell mode, with TNF-alpha (120 ng/ml) resulted in a sustained increase in Na(+) currents from 82 +/- 9 to 549 +/- 146 pA (P < 0.005; n = 6). The TNF-alpha-elicited Na(+) current was inhibited by amiloride, and there was no change when A549 cells were perfused with the triple-mutant TNF-alpha or after preincubation with blocking antibodies to the two TNF-alpha receptors before perfusion with TNF-alpha. In conclusion, although TNF- alpha can initiate acute inflammation and edema formation in the lung, TNF-alpha can also increase AFC by an amiloride-sensitive, cAMP-independent mechanism that enhances the resolution of alveolar edema in pathological conditions by either binding to its receptors or activating Na(+) channels by means of its lectinlike domain.

beta(2)-adrenoceptor agonists inhibit release of eosinophil-activating cytokines from human airway smooth muscle cells

1. Airway smooth muscle (ASM) is a potential source of multiple pro-inflammatory cytokines during airway inflammation. beta-Adrenoceptor agonist hyporesponsiveness is a characteristic feature of asthma, and interleukin (IL)-1 beta and tumour necrosis factor (TNF)-alpha are implicated in its cause. Here, the capacity of beta-adrenoceptor agonists to prevent release of GM-CSF, RANTES, eotaxin and IL-8, elicited by IL-1 beta or TNF alpha, was examined in human ASM cells. 2. Isoprenaline (approximately EC(50) 150 nM), a non-selective beta-adrenoceptor agonist, and salbutamol ( approximately EC(50) 25 nM), a selective beta(2)-adrenoceptor agonist, attenuated release of GM-CSF, RANTES and eotaxin, but not IL-8 (EC(50) >1 microM). The maximum extent of attenuation was RANTES > or = eotaxin > GM-CSF >> IL-8, and was prevented by either propranolol (1 microM), a non-selective beta-adrenoceptor antagonist, or ICI 118511 (IC(50) 15 nM), a selective beta(2)-adrenoceptor antagonist. 3. The cyclic AMP-elevating agents, dibutyryl cyclic AMP ( approximately EC(50) 135 microM), forskolin ( approximately EC(50) 530 nM) and cholera toxin ( approximately EC(50) 575 pg ml(-1)) abolished IL-1 beta-induced release of GM-CSF, RANTES and eotaxin, but not IL-8. 4. IL-1 beta (1 ng ml(-1)) attenuated early increases (up to 1 h) in cyclic AMP formation induced by salbutamol (1 microM), but not by forskolin (10 microM). The cyclo-oxygenase inhibitor, indomethacin (1 microM) prevented later increases (3 - 12 h) in IL-1 beta-stimulated cyclic AMP content, but did not prevent the attenuation by salbutamol of IL-1 beta-induced cytokine release. 5. We conclude in human ASM cells that activation of beta(2)-adrenoceptors and generation of cyclic AMP is negatively-linked to the release, elicited by IL-1 beta or TNF alpha, of eosinophil-activating cytokines such as GM-CSF, RANTES and eotaxin, but not IL-8.

Decreased adenylyl cyclase protein and function in airway smooth muscle by chronic carbachol pretreatment

Cellular levels of cAMP are an important determinant of airway smooth muscle tone. We have previously shown that chronic (18 h) but not acute (30 min or 2 h) pretreatment with the muscarinic receptor agonist carbachol resulted in decreased adenylyl cyclase activity in response to GTP, isoproterenol, or forskolin via a pathway blocked by the protein kinase C inhibitor staurosporine. The present study was designed to determine if carbachol-induced decreases in adenylyl cyclase activity were due to regulatory events at the level of either G(s)alpha or adenylyl cyclase. Detergent-solubilized G(s)alpha from control or carbachol-pretreated bovine airway smooth muscle had similar adenylyl cyclase activity in response to either NaF or guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) when reconstituted into S49 cyc(-) membranes that lack endogenous G(s)alpha (carbachol pretreated: GTPgammaS, 93 +/- 13% of control; NaF/AlCl(3), 99 +/- 8.6% of control; n = 4). Exogenous G(s)alpha solubilized from red blood cells failed to restore normal adenylyl cyclase activity when reconstituted into carbachol-pretreated bovine airway smooth muscle (carbachol pretreated: GTP, 36 +/- 10% of control; NaF/AlCl(3), 54 +/- 11% of control; n = 4). [(3)H]forskolin radioligand saturation binding assays revealed a decreased quantity of total adenylyl cyclase protein after carbachol pretreatment (maximal binding: 152 +/- 40 and 107 +/- 31 fmol/mg protein in control and carbachol-pretreated airway smooth muscle, respectively). These results suggest that chronic activation of muscarinic receptors downregulates the expression of adenylyl cyclase protein in bovine airway smooth muscle.

TNF-alpha increases transcription of Galpha(i-2) in human airway smooth muscle cells

Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine that has an important role in the regulation of airway smooth muscle tone and reactivity. We have shown previously that TNF-alpha upregulates the expression of Galpha(i-2) protein without significantly increasing G(s)alpha protein and enhances adenylyl cyclase inhibition by carbachol in cultured human airway smooth muscle cells (Hotta K, Emala CW, and Hirshman CA. Am J Physiol Lung Cell Mol Physiol 276: L405-L411, 1999). The present study was designed to investigate the molecular mechanisms by which TNF-alpha upregulates Galpha(i-2) protein in these cells. TNF-alpha pretreatment for 48 h increased the expression of Galpha(i-2) protein without significantly altering the Galpha(i-2) protein half-life (41.0 +/- 8.2 h for control and 46.8 +/- 5.2 h for TNF-alpha-treated cells). Inhibition of new protein synthesis by cycloheximide blocked the increase in Galpha(i-2) protein induced by TNF-alpha. Furthermore, TNF-alpha treatment for 12-24 h increased the steady-state level of Galpha(i-2) mRNA without significantly altering Galpha(i-2) mRNA half-life (9.0 +/- 0.75 h for control and 8.9 +/- 1.1 h for TNF-alpha-treated cells). The transcription inhibitor actinomycin D blocked the increase in Galpha(i-2) mRNA induced by TNF-alpha. These observations indicate that the increase in Galpha(i-2) protein induced by TNF-alpha is due to an increased rate of Galpha(i-2) protein synthesis, most likely as a consequence of the transcriptional increase in the steady-state levels of its mRNA.

Activation of tumor necrosis factor receptor 1 in airway smooth muscle: a potential pathway that modulates bronchial hyper-responsiveness in asthma?

The cellular and molecular mechanisms that are involved in airway hyper-responsiveness are unclear. Current studies suggest that tumor necrosis factor (TNF)-alpha, a cytokine that is produced in considerable quantities in asthmatic airways, may potentially be involved in the development of bronchial hyper-responsiveness by directly altering the contractile properties of the airway smooth muscle (ASM). The underlying mechanisms are not known, but growing evidence now suggests that most of the biologic effects of TNF-alpha on ASM are mediated by the p55 receptor or tumor necrosis factor receptor (TNFR)1. In addition, activation of TNFR1 coupled to the tumor necrosis factor receptor-associated factor (TRAF)2-nuclear factor-kappaB (NF-kappaB) pathway alters calcium homeostasis in ASM, which appears to be a new potential mechanism underlying ASM hyper-responsiveness.