Phenolic and tyrosyl ring iodothyronine deiodination by the caco-2 human colon carcinoma cell line

Abstract

Thyroid hormone metabolism was studied in the human Caco-2 colon carcinoma cell line, which at confluence exhibits several functions of differentiated enterocytes. Cells were harvested two to 17 days after reaching confluence. Intact cells and homogenates were tested for deiodination of [ 125I]-labeled substrates. Small amounts of thyroxine (T 4) were converted by homogenates to 3,3′,5′-triiodothyronine (rT 3), 3,3′-diiodothyronine (3,3′-T 2), and I −, with no detectable production of 3,5,3′-triiodothyronine (T 3) by homogenates or cells. rT 3 was converted to 3,3′-T 2 and I − with an apparent Michaelis constant (K m) for rT 3 of 24 nmol/L; 6-n-propyl-2-thiouracil (PTU) had a 50% inhibitory concentration of 30 nmol/L and abolished rT 3 5′-deiodination at 1 mmol/L in the presence of 20 mmol/L dithiothreitol (DTT). T 3 was deiodinated to 3,3′-T 2 and 3′-monoiodothyronine (3′-T 1) with an apparent Michaelis constant (K m) for T 3 of 5.7 nmol/L; this reaction was not inhibited by 1 mmol/L PTU. Phenolic and tyrosyl ring deiodinating activities were maximal four and six days, respectively, after the cells reached confluence. Homogenates of cells grown in standard medium containing fetal calf serum had fivefold higher rT 3 5′-deiodinating activity than cells grown in a serum-free defined culture medium, reflecting a fivefold difference in the apparent V max with no difference in the apparent K m for rT 3. There was no difference in T 3 5-deiodination rates in homogenates of Caco-2 cells grown in the two media until 12 days postconfluence, when cells grown in standard medium had higher activity. These findings indicate that Caco-2 cells have two deiodinating pathways. rT 3 5′-deiodination more closely resembles the low K m process described in rat kidney microsomes than classic type I deiodination in either rat or human tissues. T 3 5-deiodination occurs via the type III pathway, which is regulated by changes in cell growth and differentiation as in several other cell types.