hPepT1 mediates bacterial tripeptide fMLP uptake in human monocytes

Here, we examined hPepT1 expression in the monocytic cell line, KG-1. Reverse transcription polymerase chain reaction (RT-PCR) analysis revealed that hPepT1 is expressed in KG-1 cells, while cDNA cloning and direct sequencing confirmed the sequence of KG-1 hPepT1 (accession number, AY634368). Immunoblotting of cell lysates from KG-1 cells or macrophages isolated from human peripheral blood revealed a approximately 100 kDa immunoreactive band mainly present in the membrane fraction. Uptake experiments showed that the transport of 20 microM radiolabeled Gly-Sarcosine ([14C]Gly-Sar) in KG-1 cells was Na+, Cl- dependent and disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS)-sensitive. In addition, hPepT1 activity was likely to be coupled to a Na+/H+ exchanger, as evidenced by the fact that [14C]Gly-Sar uptake was not affected by the absence of Na+ when cells were incubated at low pH (5.2). Interestingly, hPepT1-mediated transport was reduced in KG-1 cells incubated at low pH as it was also observed in nonpolarized Caco2-BBE cells. This pattern of pH-dependence is due to a disruption of the driving force of hPepT1-mediated transport events. This was supported by our finding that nonpolarized cells, Caco2-BBE cells and KG-1 cells, have an increased permeability to H+ when compared to polarized Caco2-BBE cells. Finally, we showed that hPepT1 is responsible for transporting fMLP into undifferentiated and differentiated (macrophage-like) KG-1 cells. Together, these results show that hPepT1 is expressed in nonpolarized immune cells, such as macrophages, where the transporter functions best at the physiological pH 7.2. Furthermore, we provide evidence for hPepT1-mediated fMLP transport, which might constitute a novel immune cell activation pathway during intestinal inflammation.