Forskolin blocks the apical expression of dipeptidyl peptidase IV in Caco-2 cells and induces its retention in lamp-1-containing vesicles.

In a previous work, we showed that the differentiation-dependent expression of dipeptidyl peptidase IV (DPP IV) in Caco-2 cells, a human colon adenocarcinoma cell line, was controlled at the mRNA level (D. Darmoul et al. J. Biol. Chem., 1992, 267, 4824-4833). Whether post-translational events may contribute to the final control of DPP IV cell surface expression was explored here by studying the potential effect of forskolin (FK), a drug known to permanently stimulates adenylyl cyclase and to strongly perturbs glucose metabolism in fully differentiated Caco-2 cells. FK treatment reduces by about 50% the amount of active DPP IV present at the brush border membrane, whereas the total amount of active DPP IV remains unchanged. Biosynthesis and maturation of DPP IV were measured using [35S]methionine labeling and were shown to be essentially unaffected by FK treatment. Pulse-chase experiments demonstrate that up to 50% of the neosynthesized DPP IV do not appear at the apical membrane after FK treatment. To get further insight into this phenomenon, we have used confocal laser scanning microscopy. We demonstrate that the blockade of DPP IV transport is associated with the accumulation of this protein in intracellular vesicles. Double-staining experiments demonstrate that these vesicles are not labeled with a monoclonal antibody directed against the Golgi apparatus but display a strong staining with a polyclonal antibody raised against lamp-1, a lysosomal membrane protein. Using a newly developed image analysis procedure, we have been able to quantitate the relative distribution of lamp-1 and DPP IV labels in both control and forskolin-treated cells. We show that the overlap of the two labels dramatically increases in FK-treated Caco-2 cells. These results suggest that, beside the transcriptional level, post-translational events may be involved in the final control of the apical expression of a differentiation-dependent hydrolase.