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Nuclear Thyroid Hormone Receptors, -Glycerophosphate Dehydrogenases, and Malic Enzyme in N-Nitrosomethylurea-induced Rat Mammary Tumors¹

Division of Clinical Oncology, Wisconsin Clinical Cancer Center, University of Wisconsin, Madison, Wisconsin 53792

Specific L-3,3'-5-triiodothyronine (triiodothyronine) binding by isolated nuclei was determined in N-nitrosomethylurea-induced mammary carcinomas and livers from intact and thyroidectomized animals. Tumors contained a single class of high-affinity nuclear triiodothyronine binding sites with characteristics similar to those of hepatic nuclei. Competition experiments showed the relative affinities of thyroid hormone structural analogues for both tumor and liver receptors to be: triiodothyroacetic acid > triiodothyronine > thyroxine > reverse triiodothyronine. Diiodotyrosine did not compete for the binding sites.

Mammary tumors exhibited a wide range of binding sites, but most were in the range of 50 to 150 fmol/µg DNA. The levels were not affected significantly by hypothyroidism. Liver triiodothyronine receptor concentrations were approximately 5-fold those of tumors; they were unaffected by low serum thyroid hormone levels and were similar to those of non-tumor-bearing, euthyroid controls.

Mitochondrial -glycerophosphate dehydrogenase and cytosolic malic enzyme activities were reduced in both tumors and livers of hypothyroid rats; cytosolic -glycerophosphate dehydrogenase levels were unchanged. Hepatic enzyme activities were similar in euthyroid tumor-bearing animals and in euthyroid healthy controls.

¹ Supported by USPHS Grant CA 14520 awarded to the Wisconsin Clinical Cancer Center by the National Cancer Institute and by Grant CA 17579.

² To whom requests for reprints should be addressed.

Received 12/ 9/82. Accepted 3/24/83.