Chenodeoxycholic acid requires activation of EGFR, EPAC, and Ca²⁺ to stimulate CFTR-dependent Cl⁻ secretion in human colonic T84 cells

Abstract

Bile acids are known to initiate intricate signaling events in a variety of tissues, primarily in the liver and gastrointestinal tract. Of the known bile acids, only the 7α-dihydroxy species, deoxycholic acid and chenodeoxycholic acid (CDCA), and their conjugates, activate processes that stimulate epithelial Cl secretion. We have previously published that CDCA acts in a rapid manner to stimulate colonic ion secretion via protein kinase A (PKA)-mediated activation of the dominant Cl channel, the cystic fibrosis transmembrane conductance regulator (CFTR) (Ao M, Sarathy J, Domingue J, Alrefai WA, and Rao MC. Am J Physiol Cell Physiol 305: C447-C456, 2013); however, PKA signaling did not account for the entire CDCA response. Here we show that in human colonic T84 cells, CDCA's induction of CFTR activity, measured as changes in short-circuit current (I), is dependent on epidermal growth factor receptor (EGFR) activation and does not involve the bile acid receptors TGR5 or farnesoid X receptor. CDCA activation of Cl secretion does not require Src, mitogen-activated protein kinases, or phosphoinositide 3-kinase downstream of EGFR but does require an increase in cytosolic Ca In addition to PKA signaling, we found that the CDCA response requires the novel involvement of the exchange protein directly activated by cAMP (EPAC). EPAC is a known hub for cAMP and Ca cross talk. Downstream of EPAC, CDCA activates Rap2, and changes in free cytosolic Ca were dependent on both EPAC and EGFR activation. This study establishes the complexity of CDCA signaling in the colonic epithelium and shows the contribution of EGFR, EPAC, and Ca in CDCA-induced activation of CFTR-dependent Cl secretion.

The Yin and Yang of bile acid action on tight junctions in a model colonic epithelium

“…, ; Domingue et al. ) and others had reported that short‐term (<30 min) bile acid treatment had no effect on TER, we first focused on examining the effect of long‐term (≥18 h) CDCA and LCA on TER. Using the maximal effective concentrations used in our earlier studies (Ao et al.…”

“…; Domingue et al. ) and others (Kelly et al. ) have found that studying signaling cascades in T84 cells in the presence of serum, often leads to inconsistencies in data, due to the undefined and variable composition of serum.…”

“…; Domingue et al. ). Of the two major secondary bile acids, DCA has effects similar to CDCA (Binder and Rawlins ; Dharmsathaphorn et al.…”

“…; Domingue et al. ). While acute exposure to LCA (5–500 μ mol/L) does not alter secretion, it attenuates cAMP‐dependent, but not Ca 2+ ‐mediated Cl − secretion even at 50 μ mol/L.…”

“…; Domingue et al. ), acting largely from the basolateral surface of colonic epithelia, both in cell lines and in ex vivo rabbit and human preparations. This raises the yet unanswered physiological question of how bile acids entering the colon in the lumen, cross the epithelial barrier to reach the basolateral side (Ao et al.…”