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

Abstract

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.

Leflunomide Prevents Alveolar Fluid Clearance Inhibition by Respiratory Syncytial Virus

“…Indeed, even without considering the absence of effect of 6-MP on AFC, there is little evidence that the decline in AFC on Day 2 is a consequence of the immune response to the virus. Although certain proinflammatory cytokines have been shown to modulate Na ϩ transport in vitro or in vivo (37)(38)(39)(40), analysis of BAL fluid from RSV-infected mice reveals no temporal correlation between pulmonary inflammation and inhibition of AFC. For example, whereas IFN-␥ has been shown to inhibit Na ϩ transport in vitro (38), no IFN-␥ is detectable in BAL fluid on Day 2, when AFC is most depressed, and AFC values are normal on Day 6, when BAL IFN-␥ levels are highest.…”

Functional Regulation of the Epithelial Na+ Channel by IκB Kinase-β Occurs via Phosphorylation of the Ubiquitin Ligase Nedd4-2

“…This inhibition may occur via down-regulation of SGK1 in renal collecting duct epithelial cells and is earliest seen at 6 h (18). However, shorter term, acute effects may occur within 30 min and appear to be stimulatory (13)(14)(15), indicating a probable biphasic response of ENaC to this pathway. Our findings in this study show a stimulatory effect, at least under baseline conditions.…”

“…Interaction between this canonical proinflammatory and stress signaling pathway and ENaC function has been suggested by a number of observations and appears to be biphasic. Acute exposure to the cytokine tumor necrosis factor-␣ has been shown to increase amiloride-sensitive Na ϩ transport in alveolar epithelium (13) and distal tubule cells from diabetic rats (14), whereas LPS acutely increases Na ϩ transport in mouse cortical collecting duct cells (15). Amiloride-sensitive alveolar fluid clearance, a function of ENaC activ-ity, increases in a rat model of pneumonia by a tumor necrosis factor-␣-dependent mechanism (16).…”

Non-Cardiogenic Pulmonary Edema

“…It can also have a beneficial effect by increasing alveolar fluid clearance via an amiloride sensitive, cAMP independent mechanism to enhance alveolar fluid clearance. This is accomplished by binding to its receptors or activating Na+ channels in the epithelium (Fukuda et al, 2001). Other networks of cytokines regulate lung inflammation in lung injury and edema.…”