Polyamine synthesis and transport inhibition in a human anaplastic thyroid carcinoma cell line in vitro and as xenograft tumors.

Polyamines are essential cellular components for neoplastic transformation and cell proliferation. Antineoplastic efforts that inhibit polyamine synthesis are insufficient to induce cytotoxicity, due to compensatory induction of polyamine transport. Treatment of an anaplastic human thyroid carcinoma cell line (DRO90-1) with a novel polymeric spermine conjugate (polyspermine; PSpm) caused in vitro cytotoxicity and inhibited the growth of xenograft tumors at low concentrations. Similar in vitro antineoplastic effects were noted with two other human anaplastic thyroid carcinoma cell lines. This coincided with inhibition of polyamine uptake and synthetic enzyme activities, with reduced ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAM-DC) but increased spermidine/spermine N1-acetyltransferase (SSAT) activities, as measured in DRO90-1 cells. In subsequent studies using these cells, PSpm was effective in reducing the intracellular levels of all polyamines in vitro, resulting in cytotoxicity that was not reversed by administration of extracellular polyamines. Low-dose PSpm inhibited tumor growth in vivo, but high doses of PSpm potentiated xenograft tumor growth. PSpm degradation products produced with in vivo treatment may be produced that function as substrates for polyamine biosynthesis. These studies suggest that polyamine metabolism inhibition is a viable target for antineoplastic therapy of anaplastic thyroid carcinoma, although the in vivo response to PSpm suggests that this agent will have limited clinical utility.

Restoration of iodide uptake in dedifferentiated thyroid carcinoma: relationship to human Na+/I-symporter gene methylation status.

Disseminated dedifferentiated thyroid epithelial carcinoma, which cannot sufficiently concentrate therapeutic radioiodide, is a terminal disease without any effective systemic treatment or chemotherapy. This is a likely consequence of loss of human sodium-iodide symporter (hNIS) function. We hypothesized that hNIS transcriptional failure in thyroid carcinoma could be consequent to methylation of DNA in critical regulatory regions and could be reversed with chemical demethylation treatment. Analysis of hNIS messenger ribonucleic acid (mRNA) expression in 23 tumor samples revealed that although loss of this expression corresponded to loss of clinical radioiodide uptake, some thyroid carcinomas with hNIS mRNA expression did not concentrate iodide, suggesting additional posttranscriptional mechanisms for loss of hNIS function. In addition, analysis of DNA methylation in CpG-rich regions of the hNIS promoter extending to the first intron failed to define specific methylation patterns associated with transcriptional failure in human thyroid tumor samples. In seven human thyroid carcinoma cell lines lacking hNIS mRNA, treatment with 5-azacytidine or sodium butyrate was able to restore hNIS mRNA expression in four cell lines and iodide transport in two cell lines. Investigation of methylation patterns in these cell lines revealed that successful restoration of hNIS transcription was associated with demethylation of hNIS DNA in the untranslated region within the first exon. This was also associated with restoration of expression of thyroid transcription factor-1. These results suggest a role for DNA methylation in loss of hNIS expression in thyroid carcinomas as well as a potential application for chemical demethylation therapy in restoring responsiveness to therapeutic radioiodide.

Expression and glucocorticoid regulation of natriuretic peptide clearance receptor (NPR-C) mRNA in rat brain and choroid plexus.

The natriuretic peptide clearance receptor (NPR-C) binds atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide with high affinity. This receptor lacks an intracellular guanylate cyclase domain, and is believed to exert biological actions by sequestration of released natriuretic peptides and/or inhibition of adenylate cyclase. The present report summarizes the first detailed mapping of NPR-C mRNA in rat brain. In situ hybridization analysis revealed high levels of NPR-C mRNA expression in frontal and retrosplenial granular cortices, medial preoptic nucleus, ventral cochlear nucleus and choroid plexus. NPR-C mRNA expression was also observed in deep layers of neocortex and limbic cortex, posterior cortical amygdala, ventral subiculum, amygdalohippocampal area, and dentate gyrus. Positive hybridization signal was observed in both anterior and intermediate lobes of the pituitary gland. Regulatory studies indicated that expression of NPR-C mRNA was increased in the medial preoptic nucleus of adrenalectomized rats, suggesting negative glucocorticoid regulation. No changes in NPR-C mRNA expression were observed in frontal cortex or choroid plexus. These results suggest a role for the NPR-C in modulation of natriuretic peptide availability and/or adenylate cyclase activity in a subset of central natriuretic peptide circuits concerned with cortical, olfactory and neuroendocrine functions. Response of the NPR-C gene to changes in circulating hormones suggests the capacity for glucocorticoid modulation of natriuretic peptide action at the receptor level.