3,5,3'-Triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) synthesis in rats hosting the R3230AC mammary tumour.

Abstract

Generation of T3 and rT3 from T4 was studied in R3230AC mammary tumours grown in Fischer 344 rats as well as in the liver and kidney of these tumour-bearing hosts. The primary objective of this study was to determine if reversion of T3 to rT3 synthesis occurs in this experimental tumour model and in organs remote from the tumour site. Tumours, hepatic and renal homogenates were analyzed 14-16 days following tumour implantation for 5- and 5'-iodothyronine deiodinase activity using thyroxine as substrate. It was observed that similar to the liver and kidney, the mammary tumour was capable of generating both T3 and rT3 from T4; renal synthesis of T3 was significantly greater than that of rT3 in tumour hosts and controls. In contrast, there was no significant difference between T3 and rT3 synthesis in the tumour itself and the livers of normal and tumour-bearing animals. Hepatic and renal T3 synthesis were greater in the tumour-bearing than in the normal rats; no difference in the hepatic and renal rT3 synthesis was observed between the tumour-bearing and the normal animals. Despite the fact that serum T3 was significantly lower in the tumour-bearing than in the normal rats, no difference in the serum rT3 level was observed between the two groups of animals. Our data demonstrate that in this particular animal model there is no evidence of dedifferentiation of iodothyronine deiodinase activity either within the tumour or in remote tissues.