Effects of Anticancer Agent P-bi-TAT on Gene Expression Link the Integrin Thyroid Hormone Receptor to Expression of Stemness and Energy Metabolism Genes in Cancer Cells.

Chemically modified forms of tetraiodothyroacetic acid (tetrac), an L-thyroxine derivative, have been shown to exert their anticancer activity at plasma membrane integrin αvß3 of tumor cells. Via a specific hormone receptor on the integrin, tetrac-based therapeutic agents modulate expression of genes relevant to cancer cell proliferation, survival and energy metabolism. P-bi-TAT, a novel bivalent tetrac-containing synthetic compound has anticancer activity in vitro and in vivo against glioblastoma multiforme (GBM) and other types of human cancers. In the current study, microarray analysis was carried out on a primary culture of human GBM cells exposed to P-bi-TAT (10-6 tetrac equivalent) for 24 h. P-bi-TAT significantly affected expression of a large panel of genes implicated in cancer cell stemness, growth, survival and angiogenesis. Recent interest elsewhere in ATP synthase as a target in GBM cells caused us to focus attention on expression of genes involved in energy metabolism. Significantly downregulated transcripts included multiple energy-metabolism-related genes: electron transport chain genes ATP5A1 (ATP synthase 1), ATP51, ATP5G2, COX6B1 (cytochrome c oxidase subunit 6B1), NDUFA8 (NADH dehydrogenase (ubiquinone) FA8), NDUFV2I and other NDUF genes. The NDUF and ATP genes are also relevant to control of oxidative phosphorylation and transcription. Qualitatively similar actions of P-bi-TAT on expression of subsets of energy-metabolism-linked genes were also detected in established human GBM and pancreatic cancer cell lines. In conclusion, acting at αvß3 integrin, P-bi-TAT caused downregulation in human cancer cells of expression of a large number of genes involved in electron transport and oxidative phosphorylation. These observations suggest that cell surface thyroid hormone receptors on αvß3 regulate expression of genes relevant to tumor cell stemness and energy metabolism.

How thyroid hormone works depends upon cell type, receptor type, and hormone analogue: implications in cancer growth.

The classical molecular mechanism of thyroid hormone involves the intranuclear interaction of 3,5,3'-triiodo-L-thyronine (T3) with thyroid hormone-specific nuclear proteins and consequent specific gene expression. This mechanism prevails in normal cells. What we emphasize here is that how thyroid hormone acts depends upon the types of cell or cell-like structure, e.g., platelet, under consideration, and that cancer cells, dividing endothelial cells, phagocytes, and platelets respond to the liganding of L-thyroxine (T4) by plasma membrane integrin αvß3. In intact tumor cells, T4 at the integrin can modulate the transcription of a substantial number of specific genes relevant to cancer cell proliferation, cell metabolism, and cancer cell anti-apoptosis defense. T4 may also regulate the interactions of the integrin in the endothelial cell plasma membrane with adjacent vascular growth factor receptors, modulating angiogenesis. T4 activates platelets via αvß3 transferred from the megakaryocyte. It is also possible that, in addition to T4, reverse T3 (rT3) may have actions in cancer cells at the thyroid hormone receptor on αvß3.

In vitro effects of tetraiodothyroacetic acid combined with X-irradiation on basal cell carcinoma cells.

We investigated radiosensitization in an untreated basal cell carcinoma (TE.354.T) cell line and post-pretreatment with tetraiodothyroacetic acid (tetrac) X 1 h at 37°C, 0.2 and 2.0 µM tetrac. Radioresistant TE.354.T cells were grown in modified medium containing fibroblast growth factor-2, stem cell factor-1 and a reduced calcium level. We also added reproductively inactivated (30 Gy) "feeder cells" to the medium. The in vitro doubling time was 34.1 h, and the colony forming efficiency was 5.09 percent. These results were therefore suitable for clonogenic radiation survival assessment. The 250 kVp X-ray survival curve of control TE.354.T cells showed linear-quadratic survival parameters of αX-ray = 0.201 Gy-1 and ßX-ray = 0.125 Gy-2. Tetrac concentrations of either 0.2 or 2.0 µM produced αX-ray and ßX-ray parameters of 2.010 and 0.282 Gy-1 and 2.050 and 0.837 Gy-2, respectively. The surviving fraction at 2 Gy (SF2) for control cells was 0.581, while values for 0.2 and 2.0 µM tetrac were 0.281 and 0.024. The SF2 data show that tetrac concentrations of 0.2 and 2.0 µM sensitize otherwise radioresistant TE.354.T cells by factors of 2.1 and 24.0, respectively. Thus, radioresistant basal cell carcinoma cells may be radiosensitized pharmacologically by exposure to tetrac.

Identification and functions of the plasma membrane receptor for thyroid hormone analogues.

Integrin αvß3 is a heterodimeric structural protein of the plasma membrane that bears a cell surface receptor for thyroid hormone. The functions of this receptor are distinct from those of the classical nuclear receptor (TR) for thyroid hormone. The integrin is expressed primarily by cancer cells, dividing endothelial and vascular smooth muscle cells, and osteoclasts. The hormone receptor on αvß3 enables L-thyroxine (T(4)) and 3, 5, 3'-triiodo-L-thyronine (T(3)) to stimulate cancer cell proliferation and angiogenesis and to regulate the activity of certain membrane ion pumps. Bound to the receptor, the hormone ligand also stimulates protein trafficking within the cell. A deaminated derivative of T(4), tetraiodothyroacetic acid (tetrac), blocks binding and actions of T(4) and T(3) at the receptor on αvß3; tetrac also has anti-proliferative actions at the integrin thyroid hormone receptor beyond the effects of antagonizing actions of agonist thyroid hormone analogues at the receptor. The structure-activity relationships of hormone analogues at the receptor have been computer-modeled and indicate that the receptor includes a site that binds T(3) and a site that binds both T(4) and T(3). Mathematical modeling of the kinetics of hormone-binding also suggests the existence of two sites. Cell proliferation is modulated from the T(4)/T(3) site. Tetrac has been re-formulated as a nanoparticle (nanotetrac) that acts exclusively at the αvß3 receptor and does not enter cells. Nanotetrac disrupts expression of genes in multiple cancer cell survival pathways. The tetrac formulations block human cancer cell proliferation in vitro and in tumor xenografts. Nanotetrac and tetrac inhibit the pro-angiogenic actions in vitro of vascular endothelial growth factor, basic fibroblast factor, and other growth factors. Thus, the receptor described on integrin αvß3 for T(4) and T(3), the function of which is materially affected by tetrac and nanotetrac, provides insight into tumor cell biology and vascular biology.

Overlapping nongenomic and genomic actions of thyroid hormone and steroids.

Nuclear receptors for thyroid hormone and steroids are members of a receptor superfamily with similar molecular organization, but discrete transcriptional functions that define genomic actions of these nonpeptide hormones. Nongenomic actions of thyroid hormone and estrogens and androgens are initiated outside the nucleus, at receptors in the plasma membrane or in cytoplasm; these actions are largely regarded to be unique to the respective hormones. However, there is an increasing number of descriptions of overlapping nongenomic and genomic effects of thyroid hormone and estrogens and testosterone. These effects are concentrated in tumor cells, where, for example, estrogens and thyroid hormone have similar mitogen-activate protein kinase (MAPK)-dependent proliferative actions on ERα-positive human breast cancer cells, and where dihydrotestosterone also can stimulate proliferation. Steroids and thyroid hormone have similar anti-apoptotic effects in certain tumors. But thyroid hormone and steroids also have overlapping or interacting nongenomic and genomic actions in heart and brain cells. These various effects of thyroid hormone and estrogens and androgens are reviewed here and their possible clinical consequences are enumerated.

Propylthiouracil-induced chemical hypothyroidism with high-dose tamoxifen prolongs survival in recurrent high grade glioma: a phase I/II study.

BACKGROUND: High-dose tamoxifen has had disappointing results as a palliative therapy in recurrent glioma. Insulin-like growth factor 1 (IGF-1) is a thyroid hormone modulated naturally occurring antagonist of tamoxifen-induced cytotoxicity. Thyroid function was suppressed to reduce IGF-1 levels in glioma patients and high-dose tamoxifen administered. MATERIALS AND METHODS: Propylthiouracil was used to induce chemical hypothyroidism in 22 patients with recurrent glioma. Tamoxifen was started within one month and given in escalating doses from 40 mg twice a day up to 80 mg 3 times a day. No significant toxicity developed. RESULTS: Eleven out of 22 patients became hypothyroid. No patients experienced symptoms of clinical hypothyroidism. Median survival was significantly longer in the hypothyroid group (10.1 months versus 3.1 months); p = 0.03. There was a significant decrease in blood levels of IGF-1 (p = 0.02). in hypothyroid patients. CONCLUSION: Patients treated for recurrent high-grade gliomas with high-dose tamoxifen had significantly longer survival when chemical hypothyroidism was induced with propylthiouracil.