Glucocorticoids and tumor necrosis factor-alpha synergize to induce absorption by the epithelial sodium channel in the colon.
OBJECTIVE
The epithelial sodium channel (ENaC) mediates electrogenic sodium absorption in distal colon. In patients with inflammatory bowel disease (IBD), ENaC induction is impaired, mainly through transcriptional suppression by proinflammatory cytokines such as tumor necrosis factor (TNF)-alpha. Glucocorticoid therapy rapidly increases sodium absorption; we investigated the molecular mechanisms underlying the interaction among TNF-alpha, glucocorticoids, and ENaC induction.


METHODS
ENaC-mediated sodium transport in glucocorticoid receptor (GR)-expressing HT-29/B6 cells and rat distal colon, under the influence of the synthetic glucocorticoid dexamethasone and TNF-alpha, was quantified in Ussing chambers. ENaC messenger RNA (mRNA) levels were monitored by real-time polymerase chain reaction. GR transactivation and expression were investigated by gene reporter, immunoblot, and confocal immunofluorescence microscopy analyses. The GR mRNA half-life was determined. Signaling pathways were characterized using mitogen-activated protein kinase inhibitors.


RESULTS
Dexamethasone not only prevented TNF-alpha-mediated ENaC suppression but caused synergistic induction of ENaC-dependent sodium absorption in HT-29/B6-GR cells and rat distal colon. This synergy resulted from TNF-alpha-mediated increases in GR protein levels because of GR mRNA stabilization and subsequent GR transactivation by dexamethasone. As a consequence, transcription of the ENaC beta- and gamma-subunits was up-regulated, increasing ENaC-dependent sodium absorption. p38 Mitogen-activated protein kinase is required for this synergistic effect: p38 inhibition blocked the increase in GR protein expression and ENaC-dependent sodium absorption.


CONCLUSIONS
TNF-alpha and dexamethasone induce ENaC, explaining the rapid and intense proabsorptive effect of glucocorticoid therapies.
Bergann
Theresa
T
Department of Gastroenterology, Infectious Diseases, and Rheumatology, Charité, Campus Benjamin Franklin, Berlin, Germany.
Zeissig
Sebastian
S
Fromm
Anja
A
Richter
Jan F
JF
Fromm
Michael
M
Schulzke
Joerg-Dieter
JD
D000818:Animals; D003106:Colon / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D003907:Dexamethasone / Q000494:pharmacology*; D004357:Drug Synergism; D053503:Epithelial Sodium Channels / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression / Q000187:drug effects; D053450:Glucocorticoid-Induced TNFR-Related Protein / Q000494:pharmacology*; D019073:HT29 Cells; D006801:Humans; D007408:Intestinal Absorption / Q000187:drug effects / Q000502:physiology; D007413:Intestinal Mucosa / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D011965:Receptors, Glucocorticoid / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology*; D015854:Up-Regulation / Q000187:drug effects; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
D053503:Epithelial Sodium Channels; D053450:Glucocorticoid-Induced TNFR-Related Protein; D012333:RNA, Messenger; D011965:Receptors, Glucocorticoid; D014409:Tumor Necrosis Factor-alpha; C411470:glucocorticoid receptor alpha; D003907:Dexamethasone; D012964:Sodium; D048051:p38 Mitogen-Activated Protein Kinases