Inhibition of intestinal dipeptide transport by the neuropeptide VIP is an anti-absorptive effect via the VPAC1 receptor in a human enterocyte-like cell line (Caco-2)

I Optimal dipeptide and peptidomimetic drug transport across the intestinal mucosal surface is dependent upon the co-operative functional activity of the di/tripeptide transporter hPepT1 and the Na+/H+ exchanger NHE3. The ability of the anti-absorptive enteric neuropeptide VIP (vasoactive intestinal peptide) to modulate dipeptide uptake was determined using human intestinal (Caco-2) epithelial cell monolayers. 2 Uptake of glycylsarcosine (Gly-Sar) across the apical membrane of Caco-2 cell monolayers is inhibited by basolateral exposure to either VIP, pituitary adenylate cyclase-activating polypeptide (PACAP), or the VPAC(1) receptor agonist [(11,22,28)Ala]-VIP. Inhibition of Gly-Sar uptake is observed only in the presence of extracellular Na+. Reverse-transcription polymerase chain reaction (RTPCR) demonstrates that VPAC(1) mRNA is expressed in Caco-2 cells whereas VPAC(2) mRNA is not detected. 3 The VIP-induced inhibition of Gly-Sar uptake is abolished in the presence of the protein kinase A (PKA) inhibitor H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinotinesulfonamide.2HCl). 4 Na-22(+) uptake across the apical membrane is inhibited by the selective NHE3 inhibitor S1611. Experiments with BCECF [2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein]-loaded Caco-2 cells demonstrate that VIP reduces the NHE3-dependent recovery of intracellular pH (pH(i)) after dipeptide-induced acidification. Western blot of Caco-2 cell protein demonstrates expression of the NHE regulatory factor NHERF1 (expression of which is thought to be required for PKA-mediated inhibition of NHE3). 5 VIP has no effect on Gly-Sar uptake in the presence of S1611 suggesting that VIP and S1611 both modulate dipeptide uptake via the same mechanism. 6 These observations demonstrate that VIP (and PACAP) modulate activity of the H+/dipeptide transporter hPepT1 in a Na+-dependent manner consistent with the modulation being indirect through inhibition of NHE3.