Membrane transport mechanisms of choline in human intestinal epithelial LS180 cells

Abstract

The aim of the present study was to investigate the membrane transport mechanisms of choline using human intestinal epithelial LS180 cells. The mRNA of choline transporter-like proteins (CTLs) was expressed significantly in LS180 cells, and the rank order was CTL1 > CTL4 > CTL3 > CTL2 > CTL5. In contrast, the mRNA expression of other choline transporters, organic cation transporter (OCT) 1, OCT2 and high-affinity choline transporter 1 (CHT1), was considerably lower in LS180 cells. Five mm unlabelled choline, hemicolinium-3 and guanidine, but not tetraethylammonium, inhibited the cellular uptake of 100 µm choline in LS180 cells. The uptake of choline into LS180 cells was virtually Na(+)-independent. The uptake of choline was significantly decreased by acidification of the extracellular pH; however, it was not increased by alkalization of the extracellular pH. In addition, both acidification and alkalization of intracellular pH decreased the uptake of choline, indicating that the choline uptake in LS180 cells is not stimulated by the outward H(+) gradient. On the other hand, the uptake of choline was decreased by membrane depolarization along with increasing extracellular K(+) concentration. In addition, the Na(+)-independent uptake of choline was saturable, and the Km value was estimated to be 108 µm. These findings suggest that the uptake of choline into LS180 cells is membrane potential-dependent, but not outward H(+) gradient-dependent.

Acetylcholine‐related proteins in non‐neoplastic appearing colonic mucosa from patients with colorectal neoplasia

“…Our study supports previous functional studies in characterizing CTL1 as a plasma membrane transporter of choline in normal colonic epithelial crypt cells . CTL1 has previously been reported localized to plasma membrane of human colon carcinoma cells and small cell lung carcinoma .…”

“…Regarding the third choline uptake family of transporters (CTLs), earlier findings suggested CTL1 and CTL2 to mediate choline transport while functional characteristics of CTL3 and CTL5 are unknown . In the present study, CTL1 had by far the highest expression level, which supports earlier findings indicating that CTL1 is mainly, if not solely, responsible for choline uptake in human colonic mucosa …”

“…Functionally, OCTs have been demonstrated to be important for choline uptake in rat colonic mucosa . Expression levels of OCT2 in the present study were considerably higher as compared to transformed cell lines, indicating an actual functional role of OCT2 in native human colonic mucosa . In contrast, OCT1 seems to be of minor importance.…”

“…7 Expression levels of OCT2 in the present study were considerably higher as compared to transformed cell lines, indicating an actual functional role of OCT2 in native human colonic mucosa. 20,37 In contrast, OCT1 seems to be of minor importance. Regarding the third choline uptake family of transporters (CTLs), earlier findings suggested CTL1 and CTL2 to mediate choline transport while functional characteristics of CTL3 and CTL5 are unknown.…”

“…38 In the present study, CTL1 had by far the highest expression level, which supports earlier findings indicating that CTL1 is mainly, if not solely, responsible for choline uptake in human colonic mucosa. 20,37 Several export mechanisms for the release of ACh have been suggested: OCTs and CTL4 have been shown to be involved in the release of ACh from bronchial epithelium and from lung and colonic cancer cells. 6,10 In agreement with these findings, CTL4 and to a lesser extent OCT2 showed substantial expression in colonic mucosa (Fig.…”