Alkaline phosphatase and peptidase activities in Caco-2 cells: differential response to triiodothyronine

Caco-2 cell human colon adenocarcinoma cell line was used to study the hormonal regulation of small intestinal epithelial cell differentiation. We had previously shown that insulin-transferrin-selenium and triiodothyronine (5 x 10(-8) M)-supplemented medium can best replace serum after 2 days of culture for both the maintenance and differentiation of Caco-2 cells. The present study demonstrates that precoating petri dishes with complete serum allows the growth and differentiation of Caco-2 cells seeded directly in serum-free medium. On the other hand, precoating with dialyzed serum inhibits alkaline phosphatase and dipeptidyl-dipeptidase IV activities by more than 50%. The results obtained with complete serum-precoated culture plates indicate that there is no synergy between insulin and triiodothyronine because cells maintained in transferrin-selenium and triiodothyronine-supplemented medium, with or without insulin, express comparable enzyme activities. Moreover, large increases in alkaline phosphatase and dipeptidyl-dipeptidase IV activities were observed when triiodothyronine was added to the culture medium by the time confluency was reached. In contrast, gamma-glutamyltransferase was lowered to a greater extent when triiodothyronine was present from the beginning of culture. These findings show that triiodothyronine preferentially stimulates alkaline phosphatase and dipeptidyl-dipeptidase IV activities during the differentiation period whereas it selectively inhibits gamma-glutamyltransferase during the proliferation phase. Triiodothyronine acts in a dose-dependent manner.