Thyroid and steroid hormone regulation of proto-oncogene expression

Characterization of 3,5,3'-triiodothyronine receptors in primary cultures of hepatocytes and neurons from chick embryo

We have detected the presence of nuclear 3,5,3'-triiodothyronine (T3) receptors in primary cultures of chick embryo hepatocytes and neurons. Hepatocytes were isolated from livers of embryos of 12, 16 and 19 days by treatment with 0.2% collagenase and hyaluronidase. They were plated at a density of 3-4 x 10(5)/35-mm petri dish in Ham's F-10 medium containing fetal calf serum, tryptose phosphate, and antibiotics. Cells were used for the binding assay at Day 3 of culture. Neurons from 8-day-old embryo brains were cultured in a serum-free medium at a density of 1.2 x 10(6) cells/35-mm petri dish and used for the binding assay after 7 days of culture. Biological activity of hepatocytes was determined by measuring insulin binding, inositol phosphate formation, and 5'-monodeiodinase activity. Neurons or glial cells in culture were identified by immunostaining with anti-neurofilaments and anti-glial fibrillary acidic protein antisera. Binding assay was performed with isolated nuclei and 0.4 M NaCl nuclear extracts. With the latter preparation, the Scatchard analysis showed, in both cells, a single, high-affinity, low-capacity T3 receptor. In the hepatocytes of 12-, 16-, and 19-day-old embryos association constants (Ka) were, respectively, 0.93 +/- 0.02, 0.74 +/- 0.03, and 0.56 +/- 0.04 nM-1, whereas the maximal binding capacities (MBC) were 2.26 +/- 0.2, 2.72 +/- 0.33, and 1.83 +/- 0.19 fmol/microgram DNA (mean +/- SE, n = 3). In neurons Ka was 1.25 +/- 0.53 nM-1 and MBC 0.59 +/- 0.14 fmol/microgram DNA (n = 3). The receptor had a sedimentation coefficient of 3.4 S, an estimated Mr of 59 kDa, and the following relative affinity for thyroid hormone analogues: TRIAC greater than L-T3 greater than L-T4. These data indicate that cultured hepatocytes and neurons of chick embryo contained T3 receptors with properties similar to those described in intact tissues from this and other species. Only the MBC of neurons was 50% lower than that observed in whole brain of embryo, but was comparable to values observed in cultured neurons from other species.

Intracellular actions of steroid hormones and their therapeutic value, including the potential of radiohalosteroids against ovarian cancer

The biological activities of steroid hormones are effected via intracellular receptors. The receptors are part of a ligand-activated family of transcription regulator proteins that are critical for steroid-regulated cell differentiation. With recombinant cDNA technology, yeast and cultured animal cells can be made to express mammalian cDNA steroid receptors from cDNA clones that contain deletions and substitutions. Among the leading problems addressed in these models is the characterization of sequences that promote association or interaction with other transcription regulating molecules, including oncogene products. Recently it has been found that heat shock proteins may serve not only to stabilize the receptor proteins but also to precondition the activation imparted by ligand binding. Aberrant receptor proteins can be found in ovarian cancer. Whether aberrant receptor proteins are associated with transformation in general or with a variable clinical response to steroidal or anti-steroidal therapy is not known. Even after chemotherapy, steroid receptors are expressed in the metastases of ovarian cancers seen clinically, and they may have potential uses for localization and treatment of receptor-rich cancers. Radioligand pharmaceuticals appropriate for imaging or for site-directed radiocytotoxicity can be sequestered to the nuclei of receptor-rich cancers. Initial clinical imaging and therapy trials with such pharmaceuticals have been approved and begun. In the use of halogenated estrogen radiopharmaceuticals, liver metabolism and enterohepatic recirculation are important considerations. Ascites prolongs retention of a radiohalogenated estrogen in the abdominal cavity. Distant metastases have been localized with [123I]-estrogen in breast cancer patients in pre-operative procedures. Receptor-mediated cytotoxicity occurs when estrogen receptor radioligand pharmaceuticals that are Auger electron emitters are used in vitro.

The mammary gland

In vivo studies have shown that the growth of the mammary gland is regulated by a complex synergistic interaction of protein, steroid and thyroid hormones, but it has proved difficult to fully reproduce these effects in vitro. It is becoming apparent that the hormones classically recognized as involved in mammary growth (oestrogen, progesterone, prolactin, GH, adrenal corticoids, triiodothyronine) bring about effects on epithelial cell proliferation at least in part through growth factors produced at distant sites (such as the liver) and also locally by mammary tissue, both parenchyma and stroma. Growth factor receptors can be demonstrated in mammary tissue. Receptor occupancy generates intracellular signals which enable cells to progress through the cell cycle, leading in ways still not understood to DNA synthesis and cell division. Within the mammary gland there probably exists a balance of stimulatory factors (such as IGFs and EGF/TGF-alpha) and inhibitory factors (such as TGF-beta). Interactions between epithelial and stromal cells, involving growth factors and the extracellular matrix, bring about pattern formation. Growth factors may also play some part in mammary differentiation and function, although the evidence here is less clear. Growth factors are also implemented in the failure of growth regulation which neoplastic transformation represents. Breast cancer cells can synthesize and secrete a variety of growth factors which may stimulate tumour growth through local autocrine/paracrine mechanisms. The oestrogen dependence of some breast cancers may involve oestrogen regulation of and interaction with growth factors, progression to hormone independence involving loss of this control. It is significant that the proteins which protooncogenes encode include growth factors and growth factor receptors. Much remains to be learnt about the nature and control of growth factors produced by and acting on the mammary gland. In breast cancer, this research offers the possibility of new methods of diagnosis and treatment.