text: Expression of dipeptidyl aminopeptidase IV during enterocytic differentiation of human colon cancer (Caco-2) cells.
  The human colon cancer cell line Caco-2 spontaneously differentiates to an enterocyte-like cell after confluence under standard culture conditions. This is characterized by polarization of the cell monolayer with the appearance of tight junctions, a brush border membrane and expression of brush-border-membrane-associated hydrolases. Studies have shown that differentiated Caco-2 cells express relatively high levels of dipeptidyl aminopeptidase IV (DPP IV) when compared with other enzymes. However, the biochemical mechanisms involved in the expression of DPP IV in differentiated cells are currently unknown. Therefore, the biosynthesis and expression of membrane-associated DPP IV in undifferentiated (0 day confluent) and differentiated (14 day confluent) Caco-2 cells were examined. Though levels of DPP IV activity in differentiated cells was 5- to 6-fold higher than undifferentiated cells, there was only a 1.6-fold difference in the synthetic rate. Post-translational processing of newly synthesized DPP IV occurred at a slower rate in differentiated cells, though there were no major differences in the type or degree of glycosylation. A comparison of the degradation rates revealed that they were similar with a half-life of approximately 8 to 10 days. We conclude that the high levels of DPP IV expressed in differentiated Caco-2 cells is primarily due to an increase in enzyme synthesis. In addition, accumulation of the enzyme is aided by its slow turnover rate.