Octylphenol (OP) Alters the Expression of Members of the Amyloid Protein Family in the Hypothalamus of the Snapping Turtle, Chelydra Serpentina Serpentina. (Articles)

Article excerpt

The gonadal estrogen estradiol-17[beta] ([E.sub.2]) is important for developing and regulating hypothalamic function and many aspects of reproduction in vertebrates. Pollutants such as octylphenol (OP) that mimic the actions of estrogens are therefore candidate endocrine-disrupting chemicals. We used a differential display strategy (RNA-arbitrarily primed polymerase chain reaction) to isolate partial cDNA sequences of neurotransmitter, developmental, and disease-related genes that may be regulated by OP or [E.sub.2] in the snapping turtle (Chelydra serpentina serpentina) hypothalamus. Hatchling and year-old male snapping turtles were exposed to a 10 ng/mL nominal concentration of waterborne OP or [E.sub.2] for 17 days. One transcript [421 base pairs (bp)] regulated by OP and [E.sub.2] was 93% identical to human APLP-2. APLP-2 and the amyloid precursor protein (APP) regulate neuronal differentiation and are also implicated in the genesis of Alzheimer disease in humans. Northern blot analysis determined that the turtle hypothalamus contains a single APLP-2 transcript of 3.75 kb in length. Exposure to OP upregulated hypothalamic APLP-2 mRNA levels 2-fold (p < 0.05) in month-old and yearling turtles. [E.sub.2] did not affect APLP-2 mRNA levels in hatchlings but stimulated a 2-fold increase (p < 0.05) in APLP-2 mRNA levels in yearling males. The protein [beta]-amyloid, a selectively processed peptide derived from APP, is also involved in neuronal differentiation, and accumulation of this neurotoxic peptide causes neuronal degeneration in the brains of patients with Alzheimer disease. Therefore, we also sought to determine the effects of estrogens on the expression of [beta]-amyloid. Using homology cloning based on known sequences, we isolated a cDNA fragment (474 bp) from turtle brain with 88% identity to human APP. Northern blot analysis determined that a single 3.5-kb transcript was expressed in the turtle hypothalamus. Waterborne OP also increased the expression of hypothalamic APP after 35 days of exposure. Our results indicate that low levels of OP are bioactive and can alter the expression of APLP-2 and APP. Because members of the APP gene family are involved in neuronal development, we hypothesize that OP exposure may disrupt hypothalamic development in young turtles.

Key words: alkylphenol polyethoxylates, Alzheimer disease, brain, estrogen, turtles.

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It is increasingly apparent that several common pollutants have profound effects on embryonic development, reproduction, and growth of aquatic animals because they mimic or suppress the actions of the sex steroid estradiol ([E.sub.2]). Of particular concern are the alkylphenol-polyethoxylates (APEOs), a large group of nonionic surfactants in commercial production (approximately 250,000 tons produced per year) that enter the aquatic environment mainly from sewage treatment and pulp and paper mill effluents (1-4). Upon discharge, APEOs are rapidly degraded to form relatively stable, hydrophobic metabolites, principally the alkylphenols nonylphenol (NP) and octylphenol (OP). These estrogenic metabolites competitively bind to both trout and mouse estrogen receptors, stimulate vitellogenin production in trout hepatocytes, inhibit testicular growth in trout, stimulate prolactin gene expression in rat pituitary cells, and induce growth of MCF-7 and ZR-75 breast cancer cell lines (1,5-8). Moreover, Blazquez et al. (8) found that 0.2 [micro]g/mL waterborne OP caused 50% mortality among immature male goldfish 20 days after exposure, but immature females were unaffected. Therefore, there is considerable evidence that indicates that OP over a range of concentrations (0.02-2 [micro]g/mL) is a biologically active contaminant. Indeed, there is reason for concern because OP has been found at significant concentrations in fresh water (< 0.2 pg/mL-0.5 ng/mL), sediments (< 0.010-1.8 [micro]g/g dry weight), sewage treatment effluent (0.1-2.5 ng/mL), and sludge (< 0. …