Stimulation of electrogenic intestinal dipeptide transport by the glucocorticoid dexamethasone

Abstract

According to recent in vitro experiments, the peptide transporter PepT2 is stimulated by the serum- and glucocorticoid-inducible kinase SGK1. The present study explored the contribution of SGK1 to the regulation of electrogenic intestinal peptide transport. Intestinal PepT1 was expressed in Xenopus oocytes, and peptide transport was determined by dual electrode voltage clamping. Peptide transport in intestinal segments was determined utilizing Ussing chamber. Cytosolic pH (pH( i )) was determined by BCECF fluorescence and Na(+)/H(+) exchanger activity was estimated from Na(+)-dependent pH recovery (pH ( i )) following an ammonium pulse. In PepT1-expressing Xenopus oocytes, coexpression of SGK1 enhanced electrogenic peptide transport. Intestinal transport and pH( i ) of untreated mice were similar in SGK1 knockout mice (sgk1 ( -/- )) and their wild-type littermates (sgk1 ( +/+ )). Glucocorticoid treatment (4 days 10 microg/g body weight (bw)/day dexamethasone) increased peptide transport in sgk1 ( +/+ ) but not in sgk1 (-/-) mice. Irrespective of dexamethasone treatment, luminal peptide (5 mM glycyl-glycine) led to a similar early decrease of pH( i ) in sgk1 (-/-) and sgk1 (+/+) mice, but to a more profound and sustained decline of pH( i ) in sgk1 (-/-) than in sgk1 ( +/+ ) mice. In the presence and absence of glycyl-glycine, pH ( i ) was significantly enhanced by dexamethasone treatment in sgk1 ( +/+ ) mice, an effect significantly blunted in sgk1 ( -/- ) mice. During sustained exposure to glycyl-glycine, pH ( i ) was significantly larger in sgk1 (+/+) mice than in sgk1 (-/-) mice, irrespective of dexamethasone treatment. In conclusion, basal intestinal peptide transport does not require stimulation by SGK1. Glucocorticoid treatment stimulates both Na(+)/H(+) exchanger activity and peptide transport, effects partially dependent on SGK1. Moreover, chronic exposure to glycyl-glycine stimulates Na(+)/H(+) exchanger activity, an effect again involving SGK1.

PI3 Kinase and PDK1 in the Regulation of the Electrogenic Intestinal Dipeptide Transport

“…As shown earlier [7], genetic knockout of the serum and glucocorticoid inducible kinase SGK1 did not appreciably compromise the basal activity of the intestinal peptide transporter but blunted its stimulation by glucocorticoids. Thus, a PI3K and PDK1 regulated kinase other than SGK1 presumably accounts for the present observations.…”

“…As shown earlier [7], the acidification of the cell following the removal of Na + and ammonium was the result of NH 3 and not due to the decrease of extracellular Na + concentration. The standard Hepes solution was composed of (in mM) 115 NaCl, 5 KCl, 1 CaCl 2 , 1.2 MgSO 4 , 2 NaH 2 PO 4 10 glucose, 32.2 Hepes; for sodium-free Hepes 132.8 NMDG, 3 KCl, 1 CaCl 2 , 1.2 MgSO 4 , 2 KH 2 PO 4 , 32 Hepes, 10 mannitol, 10 glucose; in sodium-free ammonium chloride 10 NMDG and 10 mannitol were replaced by 20 mM NH 4 Cl; high K to 7.4 or 7.0 with HCl/NaOH, HCl/NMDG and HCl/KOH, respectively, at 37°C.…”

“…The stimulation of the intestinal peptide transporter by PI3K and PDK1 sensitive signaling is at least in part independent from upregulation of PepT1 transcription, as SGK1 stimulated both peptide transporters PepT1 [7] and PepT2 [8] in the Xenopus oocyte expression system, where the carriers are expressed by injection of cRNA and effects of kinases on carrier activity cannot be due to genomic regulation of expression. Instead, the activity of PepT1 and PepT2 may be enhanced by SGK1 and the related kinases through increase of protein trafficking to and/or protein stability within in the cell membrane or by increase of the transporter activity without affecting the carrier protein abundance.…”

“…SGK1 [9] was originally cloned as a glucocorticoidinducible gene [10] and was shown to mediate the stimulating effect of dexamethasone on several transport systems including gastric acid secretion [11], intestinal glucose transport [12], intestinal Na + /H + exchanger activity [12][13][14], and intestinal peptide transport [7]. Notably, lack of SGK1 did not affect the basal peptide transport but abrogated its stimulation by glucocortioids [7].…”

Human Intestinal PEPT1 Transporter Expression and Localization in Preterm and Term Infants

“…Also, dexamethasone was shown to enhance PepT1 activity in Xenopus oocytes and mice; however, to our knowledge human data are, lacking (Rexhepaj et al, 2009). In our cohort, a large proportion of premature neonates have very likely been exposed prenatally to betamethasone as a routine for the prevention of respiratory distress syndrome in imminent premature birth.…”