Synergy under the Sun? Nanoplastics Enhance Estrogenicity of Common UV-Blocker

Short abstract

Human cells and zebrafish coexposed to nanoplastics and the sunscreen ingredient homosalate showed more plastics in tissues, estrogenic activity, and relevant gene expression changes than they showed after either exposure alone.

The ultraviolet (UV)-light blocker homosalate (HMS), a common ingredient in cosmetics and sunscreen,^1,2 can pass into the bloodstream through the skin it is protecting.³ Mounting evidence suggests homosalate alters the normal activity of estrogen and other hormones.^4,5 Exposures to other chemicals with similar actions have been linked to breast cancer⁶ and reduced fertility⁷ and have long raised serious concerns for public health.⁸

Nanoplastic particles (NPs) are ubiquitous in cosmetics⁹ and in environments across the globe¹⁰; they have also been detected in human blood,¹¹ placenta,¹² testes,¹³ vitreous humor,¹⁴ and other tissues. Experimental research published in Environmental Health Perspectives now shows the estrogenic properties of homosalate are enhanced by coexposure to nanoplastic particles.¹⁵ The particles interact with intracellular proteins and “may complicate the biological actions of HMS,” says Zhenlie Huang, a professor at Southern Medical University in Guangzhou, China, and the study’s senior author. “Our research aimed to investigate not only the influence of NPs on HMS-induced estrogenic effects, but also the underlying mechanisms.”

Scientists who coexposed zebrafish and a human breast cancer cell line to the UV-blocker homosalate in combination with PNSs observed a synergistic effect on estrogenic activity, metabolism, and accumulation of the plastics in tissues from the fish. Image: © iStock.com/Andriy Medvediuk.

Huang’s team evaluated the combined effects of nanoplastic particles and homosalate in several ways. First, they used a standard test that measures how various chemicals affect the growth rates of human breast cancer cells in the lab.¹⁶ During this part of the study, the team exposed the cells to homosalate and manufactured polystyrene nanospheres (PNSs) $50\text{\hspace{0.17em}nm}$ in size, both separately and in combination. The team used environmentally relevant exposures of 2.62 and $262\mathrm{\mu }\mathrm{g}/\mathrm{L}$ for homosalate and $1\mathrm{mg}/\mathrm{L}$ for nanoplastic particles, Huang says, reflecting levels detected in seawater (homosalate¹⁷) and human plasma (homosalate¹⁸ and nanoplastic particles¹¹).

Combined exposure to homosalate and PNSs induced higher rates of cell proliferation than exposure to either compound alone. That finding, the authors wrote, suggests a synergy between homosalate and PNSs in terms of estrogenic activity. Further experiments reinforced that idea of synergy: Compared to each compound by itself, homosalate and PNSs were more effective together at activating estrogen-sensitive genes. One gene in particular, serum and glucocorticoid-regulated kinase 1 (SGK1), “was upregulated through a process involving activation of estrogen receptors, leading to increased … cell growth,” says Huang.

During the next phase of the study, the researchers exposed zebrafish to PNSs and homosalate at varied concentrations in aquarium water. After 21 days, they analyzed blood, brain, gonad, and liver tissues from the fish. Again, combined exposures produced the most striking results, including the highest levels of homosalate accumulation in tissues and disrupted hormone production in both males and females. In that same group, ovarian tissue from females showed the highest SGK1 expression levels. Combined polystyrene nanosphere and homosalate exposures also altered expression of genes across the hypothalamus–pituitary–gonad–liver (HPGL) axis, which controls development and reproduction in animals.¹⁹

The offspring of zebrafish in the combined group at the two highest doses experienced varied effects, Huang says. These included increased rates of embryonic mortality and larval malformations, compared with control, homosalate only–, and polystyrene nanosphere only–exposed fish, as well as poor egg hatchability, or the percentage of eggs surviving to the end of incubation. According to Huang, these findings suggest that disruption of the HPGL axis is a mechanism by which nanosphere exposure worsened the estrogenic effects of homosalate.

Daniel Schlenk, an aquatic toxicologist and professor at the University of California, Riverside, who was not involved in the study, says nanoplastic particles likely function as vectors that bind and transport pollutants throughout the body. In addition to their presence in the environment, nanoplastic particles might co-occur with homosalate in sunscreens and other commercial products, such that “humans could undergo exposure to both these agents simultaneously,” he says.

According to Schlenk, the study’s major limitation is that controlled lab conditions do not accurately reflect the real world, where UV light and other stressors can alter nanoplastic particles and their interactions with biomolecules. At the same time, Huang points out, although environmental forces such as UV light can “age” nanoplastics by changing their physicochemical properties, these changes might lead to a larger specific surface area and pore volume. This increase may enhance the capacity of nanoplastics to transport homosalate into target organs, he says.²⁰ Investigating how homosalate interacts with UV-aged nanoplastics is in fact among Huang’s future research plans.

Still, the new paper “shows nicely that nanoplastics can change the pharmacokinetics of an endocrine disruptor, leading to higher accumulation in the body,” says Iain Drummond, a developmental geneticist who works with zebrafish at the Mount Desert Island Biological Laboratory in Bar Harbor, Maine. “We’re just learning about how ubiquitous NPs have become,” says Drummond, who also did not participate in the research. “The findings speak to our need to understand not just nanoplastics [themselves], but also how they affect our ability to handle other environmental pollutants.”

Biography

Charles Schmidt is a freelance science writer in Portland, Maine.