Abstract
The metabolism of human low‐density lipoproteins was studied in 2 subpopulations deriving from cells of HT29, a human colon carcinoma cell line. When grown on standard medium (25 mM glucose), about 95% of these cells are undifferentiated (G^+^ cells). From this heterogeneous population, a subpopulation with features of differentiated small‐intestinal cells was selected by glucose deprivation (G^−^ cells). The characteristics of the LDL receptor were first investigated. The results showed that the binding of ^125^I‐LDL to G^+^ and G^−^ cells performed at 4°C was saturable and specific. The K~d~values were not statistically different in the 2 cell subpopulations. The B~max~ of G^+^ cells was 55 ± 6 ng ^125^I‐LDL/mg cell protein and showed no changes whatever the phase of culture. In G^−^ cells, the B~max~ was higher during the exponential phase of culture and decreased in the post‐confluent phase (82 ± 5 versus 15 ± 6.8 ng ^125^I‐LDL/mg cell protein). Cellular degradation of ^125^I‐LDL was effective in both cell subpopulations but time‐course studies showed that, in post‐confluent G^−^ cells, degradation was slowed as compared to G^+^ cells (4 hr vs. 2 hr to reach maximal degradation). The rate of LDL processing at 37°C was enhanced by pre‐incubation with FCS‐supplemented medium, suggesting the existence of a serum component which stimulates the total degradation of ^125^I‐LDL. Concerning regulation of the LDL receptor activity, we demonstrated that pre‐incubation of G^+^ cells with LDL induced 80% down‐regulation of receptor number in both phases of culture. This was also observed in G^−^ cells during the exponential phase while only a 20% decrease of the receptor number was observed in post‐confluent G^−^ cells. The LDL degradation of G^+^ cells resulted in an inhibition of the cholesterogenic activity by 30% and 60% depending on the phase of culture. In G^−^ cells, LDL pre‐incubation inhibited cholesterol synthesis to the same extent (45%) in the exponential phase but did not affect the rate of cholesterol synthesis when cells were confluent. The defective regulatory role of LDL on receptor number and cholesterol synthesis suggests that, in the post‐confluent differentiated cells, cholesterol derived from LDL does not reach the regulatory pool. Taken together, our findings indicate the existence of functional LDL receptors in the HT29 cell line, either in the differentiated or in the undifferentiated form.