Separate basolateral and apical phosphatidylcholine secretion routes in intestinally differentiated tumor cells

Figure 1 Release of phospholipids (PL) into the apical (bp) and basolateral (bl) media in the presence of BSA or taurocholate as solubilizing agents. The figure illustrates no differences in the amount of PC and Sph secreted regardless of whether taurocholate was applied only apically (left), both apically and basolaterally (middle) compared to control (right) (P = ns).

Figure 2 Temperature dependency of phospholipid (PL) secretion. Caco-2 cells were incubated either at 37°C or 15°C and secreted PC and Sph were measured. Both apical (A) and basolateral secretion (B) were drastically reduced at 15°C; apical PC by 68%, basolateral PC by 75%, apical Sph by 53% and basolateral Sph by 73% (^aP < 0.05).

Figure 3 Time-course of phospholipid secretion. An almost linear secretion up to 12 h with some decline in secretion towards 24 h was registered. The average secretion rate of PC was 24.3 pmol/h on the apical side and 62.2 pmol/h on the basolateral side. Sph was secreted apically at 4.4 pmol/h and basolaterally at 7.4 pmol/h. The regression line for each setup was drawn.

Figure 4 Typical distribution pattern of phospholipid species secreted. The PC species composition is similar in the apical and basolateral compartments. The relative amount of Sph 703 secreted is higher in the apical compartment (P < 0.05 when comparing the ratio of Sph 703 to total PC in the apical and basolateral compartments).

Figure 5 Distribution of secreted PL in pellets and supernatants obtained from the apical (A) and basolateral (B) compartments. The majority of the PL is retrieved in the 100 000 g supernatant resembling a rather soluble fraction. In the medium of the basolateral compartment a reduced amount of PC and Sph could be found in the pellets, but more PC was found in the 100 000 g supernatant.

Figure 6 Effect of cyclosporine and hydrocortisone on PL secretion. Cyclosporine and hydrocortisone did not affect the amount or distribution of PL secretion at the apical (A) and basolateral side (B) (P = ns).