Mammary gland carcinoma-related increase of type I iodothyronine 5 '-deiodinase activity in Sprague-Dawley rats

Type I, iodothyronine 5'-deiodinase (5'-DI) catalyses deiodination of the p rohormone thyroxine (T-4) to the metabolically active 3,5,3'-triiodo-L-thyr onine (T-3). The present study was undertaken to investigate the activity o f 5'-DI in rat mammary gland tumours representing various combinations of h istologically defined papillary, cribriform or comedo patterns of ductal ca rcinomas. Female Sprague-Dawley rats were given two doses 50 mg.kg(-1) 1-me thyl-1-nitrosourea (MNU) in abdominal parts on the 52(nd) day and 113(th) d ay of age. We have found that in comparison with non-lactating mammary glan d, the activity of 5'-DI in all mammary gland tumours studied was significa ntly (P < 0.0001) increased and that the 5'-DI activity, expressed as pmol of I-125(-) released per min and per mg of protein, in malignant mammary gl and tumours was found to be at least two order higher than that of intact m ammary non-lactating gland. From our data, we suggest that thyroid hormone in mammary gland tumours might play a significant role to support high ener getic expenditure of neoplastic tissues.