Effects of pre- and postnatal exposure to the UV-filter Octyl Methoxycinnamate (OMC) on the reproductive, auditory and neurological development of rat offspring

Abstract

Octyl Methoxycinnamate (OMC) is a frequently used UV-filter in sunscreens and other cosmetics. The aim of the present study was to address the potential endocrine disrupting properties of OMC, and to investigate how OMC induced changes in thyroid hormone levels would be related to the neurological development of treated offspring.

Groups of 14–18 pregnant Wistar rats were dosed with 0, 500, 750 or 1000 mg OMC/kg bw/day during gestation and lactation. Serum thyroxine (T₄), testosterone, estradiol and progesterone levels were measured in dams and offspring. Anogenital distance, nipple retention, postnatal growth and timing of sexual maturation were assessed. On postnatal day 16, gene expression in prostate and testes, and weight and histopathology of the thyroid gland, liver, adrenals, prostate, testes, epididymis and ovaries were measured. After weaning, offspring were evaluated in a battery of behavioral and neurophysiological tests, including tests of activity, startle response, cognitive and auditory function. In adult animals, reproductive organ weights and semen quality were investigated.

Thyroxine (T₄) levels showed a very marked decrease during the dosing period in all dosed dams, but were less severely affected in the offspring. On postnatal day 16, high dose male offspring showed reduced relative prostate and testis weights, and a dose-dependent decrease in testosterone levels. In OMC exposed female offspring, motor activity levels were decreased, while low and high dose males showed improved spatial learning abilities. The observed behavioral changes were probably not mediated solely by early T₄ deficiencies, as the observed effects differed from those seen in other studies of developmental hypothyroxinemia. At eight months of age, sperm counts were reduced in all three OMC-dosed groups, and prostate weights were reduced in the highest dose group. Taken together, these results indicate that perinatal OMC-exposure can affect both the reproductive and neurological development of rat offspring, which may be a cause of concern, as humans are systematically exposed to the compound through usage of sunscreens and other cosmetics.

Introduction

The UV-filter Octyl Methoxycinnamate (OMC), also known as Ethylhexyl Methoxycinnamate, is a very frequently used chemical in sunscreens and cosmetics worldwide. OMC is absorbed though the skin and is detectable in human blood and urine samples after topical application (Janjua et al., 2004). OMC has also been found in milk samples of women who have used OMC containing products (Schlumpf et al., 2008a), which indicates that humans are systemically exposed to this compound. Findings in several studies indicate that the substance acts as an endocrine disruptor (Schlumpf et al., 2001, Schlumpf et al., 2008b, Schmutzler et al., 2004, Seidlova-Wuttke et al., 2006, Klammer et al., 2007).

Some in vitro studies have shown OMC to act as an estrogenic compound, as it enhanced proliferation of breast cancer cells (Schlumpf et al., 2001) and activated transcription in human cell lines via the estrogen receptor (Schreurs et al., 2002, Gomez et al., 2005). In an ecotoxicology study, Inui et al. (2003) demonstrated that in male medaka, OMC caused increased production of vitellogenin, a classical marker of estrogenic action in fish. In several studies in rats, OMC exposure has lead to increased uterine weight. This has been observed in an uterotropic test with immature female rats after 4 days of dosing with 1035 mg OMC/kg bw/day (Schlumpf et al., 2001), in ovariectomized (OVX) adult rats treated for 5 days with 1000 mg OMC/kg bw/day (Klammer et al., 2005), and in OVX rats fed OMC for 12 weeks, at a dose of approximately 1100 mg/kg bw/day (Seidlova-Wuttke et al., 2006). These results indicate that OMC possesses estrogenic activity, as estradiol causes similar effects on uterine weight. However, OMC also significantly increased serum LH levels (Seidlova-Wuttke et al., 2006) and upregulated expression of the estrogen receptor beta (Klammer et al., 2005) which are effects opposite those observed after estradiol treatment. Thus, OMC seems to exert both estrogenic and estrogen-independent activities in vivo.

In a large two-generation study, pregnant dams were treated with OMC in doses of 150, 450 or 1000 mg/kg bw/day in the feed. Exposure began before mating and continued throughout gestation, lactation, adolescence, mating of the F1 generation and until weaning of the F2 generation. Exposure caused no adverse effects on reproduction and development. The authors concluded that OMC displayed no estrogenic potential in vivo (Schneider et al., 2005), but no behavioral or thyroid endpoints were included in this study.

There is however good reason to assume that OMC does interfere with the hypothalamo-pituitary-thyroid (HPT) axis, and therefore could also affect brain development. Schmutzler et al. (2004) found that treatment of OVX rats for 12 weeks with 1100 mg OMC/kg bw/day, reduced thyroxine (T₄) levels in the blood as well as activity of 5′-deiodinase (the enzyme that converts T₄ to triiodothyronine (T₃)) in the liver. Reduction of deiodinase activity in peripheral tissues is also one of the mechanisms by which the potent anti-thyroid drug Propylthiouracil (PTU) exerts its effects (Cavalieri and Pitt-Rivers, 1981, Leonard and Rosenberg, 1978, Visser et al., 1979). Decreased T₄ levels were also observed in OVX rats treated with 200 and 1100 mg OMC/kg bw/day for 12 weeks, although a statistically significant effect was only observed in the low dose group (Seidlova-Wuttke et al., 2006). In OVX females treated with 10, 33, 100, 333 or 1000 mg OMC/kg/day for 5 days, the two highest doses also reduced T₄ levels in serum and deiodinase activity in the liver (Klammer et al., 2007).

Altogether, many studies indicate that OMC possesses endocrine disrupting properties, and especially the reductions in T₄ levels seem consistent across studies. The aim of the present study was to address the potential endocrine disrupting properties of OMC on the developing reproductive and thyroid hormone systems, and to investigate how OMC induced changes in thyroid hormone levels would be related to the neurological development of the offspring. Altered hormone levels during shorter periods of adulthood may not lead to adverse effects, but during early pre- and postnatal development, such alterations can affect both the physiological and neurological development of the offspring, as developmental hypothyroidism can cause delayed development, reduced growth, hearing loss and persistent neurobehavioral effects, in both experimental animals and humans (Zoeller and Crofton, 2005, Miller et al., 2009, Pop et al., 1999, Haddow et al., 1999). Previously our group has shown that hearing loss, hyperactivity and impaired maze learning in rats could be directly correlated to decreases in early developmental T₄ levels—when these decreases were induced by developmental exposure to the anti-thyroid drug PTU (Axelstad et al., 2008). In order to gain more knowledge on how hypothyroxinemia in the pre- and postnatal period is related to subsequent neurotoxicity in rats, when caused by an environmentally relevant chemical with more possible modes of action, we wanted to examine if correlations between reduced T₄ levels and altered behavior would also exist after developmental OMC exposure.