Hepatotoxic perfluorinated chemicals
Perfluorinated compounds are a group of manmade chemicals that were used for decades before their environmental significance was realised. Once they enter the environment they are incredibly stable and, along with several other groups of chemicals, have been labelled “emerging contaminants.” According to the US EPA, they are characterised by “a perceived, potential, or real threat to human health or the environment or by a lack of published health standards.”
One particular PFC is perfluorooctane sulphonate (PFOS) which has many commercial and industrial applications including as a surfactant and a coating. However, it accumulates in the environment and enters the food chain. Humans can also be exposed by inhalation or drinking water. This is a serious cause for concern because of its subsequent association with a number of health problems, including developmental and reproductive disorders, allergies, ADHD and liver damage.
Although some studies have been carried out on fish and rodents, it is still unclear how PFOS affects the human liver. Now, a group of Chinese scientists has used proteomics to see which hepatic proteins are influenced by the presence of PFOS, especially those involved in cell death. Heqing Shen and coresearchers from the Institute of Urban Environment, Xiamen, and Bruker Scientific Technology Co., Ltd., Beijing, gave their results in Journal of Applied Toxicology.
Protein abundances altered
The researchers used cells derived from normal liver tissue which are considered an ideal model for a Chinese non-malignant liver. The cells were exposed to different concentrations of PFOS for 72 hours and the proteins were extracted and treated with the commercial iTRAQ reagents. These label the groups of proteins at the N-terminus and the amino acid side chains with isotopically labelled tags so that they can be identified and quantified by mass spectrometry.
From a total of 1300 proteins that were identified, the abundances of 18 proteins were modified by PFOS, 11 being up-regulated and the remaining seven down-regulated. The majority were associated with protein synthesis and degradation, while other pathways involved were mRNA processing and splicing, transcriptional regulation, signalling and transport.
Flow cytometry experiments were also carried out, confirming that PFOS induced apoptosis in the cells in a dose-dependent manner and the researchers were keen to find out which proteins were involved in this process.
Proteins involved in apoptosis
A network diagram was generated from the results to gain insight into the mechanism of PFOS-induced apoptosis, implicating the tumour protein p53 and the transcription factor c-myc. Gene expression studies confirmed that the genes for both proteins were regulated by PFOS and Western blotting revealed that p53 and c-myc were markedly more abundant after PFOS exposure.
The network also suggested that several proteins that interact with p53 and c-myc may be involved in cell apoptosis. These were heterogeneous nuclear ribonucleoprotein C1/C2, E3 ubiquitin-protein ligase and ubiquilin-1, all of which interact with p53 and were less abundant after treatment of the cells with PFOS. The researchers proposed that their down-regulation activates expression of p53, leading to apoptosis.
In addition, the protein RNA polymerase II-associated factor 1 homolog, which interacts with c-myc, was up-regulated following PFOS treatment, indicating a stress response.
This information provides an initial insight into the mechanisms behind the toxicological effects of PFOS in the human liver. It could eventually lead to the identification of valuable biomarkers which would permit an assessment of the degree of exposure and liver damage derived from PFOS in the environment.
Related: Journal of Applied Toxicology 2014, 34, 1342-1351: “Proteomic analysis of perfluorooctane sulfonate-induced apoptosis in human hepatic cells using the iTRAQ technique”