Papers of the Month
By Tara Ann Cartwright, Samantha Hall, Gabriel Knudsen, Emily Mesev, and Qing Xu
NTP researchers identify mechanism in vinylidene chloride renal tumor formation
A two-year study, by National Toxicology Program researchers, found that male B6C3F1/N mice exposed to vinylidene chloride (VDC) exhibited dose-dependent increases in renal cell hyperplasia, renal cell adenoma, and renal cell carcinoma (RCC). VDC is widely used in production of food packaging plastics, commercial resins, and flame retardant coatings. Occupational exposure to VDC may occur by inhalation or dermal contact. The general population is exposed by contaminated drinking water.
The objective of this NTP study was to understand the mechanisms of carcinogenesis in kidney, or renal, cells, by examining global gene expression profiles and mutations in mouse RCC resulting from chronic VDC exposure.
Researchers found that in RCC of VDC-exposed mice, there was an overrepresentation of genes from pathways associated with chronic xenobiotic and oxidative stress, as well as c-Myc overexpression and dysregulation of TP53 cell cycle checkpoint, and DNA damage repair. The susceptibility of male but not female mice may be related to the activity of CYP2E1, which differs in male and female mice, and its subsequent metabolism.
Consistent with previous observations that VDC acts as a nongenotoxic carcinogen, the authors found VDC-induced RCC may be secondary to renal cytotoxicity, regenerative hyperplasia, and subsequent neoplasia. (GK)
Citation: Hayes SA, Pandiri AR, Ton TT, Hong HL, Clayton NP, Shockley KR, Peddada SD, Gerrish K, Wyde M, Sills RC, Hoenerhoff MJ. 2015. Renal cell carcinomas in vinylidene chloride-exposed male B6C3F1 mice are characterized by oxidative stress and TP53 pathway dysregulation. Toxicol Pathol 44(1):71-87.
Chronic air pollution associated with increased blood pressure
NIEHS researchers and colleagues found that long-term air pollution exposures are associated with higher blood pressure, potentially explaining the increased incidence of cardiovascular disease (CVD) in previous air pollution studies.
The scientists assessed average annual levels of nitrogen dioxide (NO2) and particulate matter less than 2.5 micrometers in diameter (PM2.5) at residential addresses for women from the NIEHS Sister Study, a national cohort study investigating risk factors for breast cancer and other outcomes. Long-term exposure to these two air pollutants has been linked to CVD, though the mechanism behind this relationship is still unknown. Utilizing advanced modeling with fine-scale gradients, the researchers mapped NO2 and PM2.5 levels with greater resolution than previous studies.
This study showed that chronic exposure to NO2 can increase pulse pressure, while chronic exposure to PM2.5 can increase both pulse pressure and systolic blood pressure. The scientists hypothesize that long-term exposure to air pollution increases blood pressure and ultimately leads to CVD. (SH)
Citation: Chan SH, Van Hee VC, Bergen S, Szpiro AA, DeRoo LA, London SJ, Marshall JD, Kaufman JD, Sandler DP. Long-term air pollution exposure and blood pressure in the Sister Study. Environ Health Perspect 123(10):951-958.
Protein unfolding offers new drug target for treating HIV
NIEHS researchers have dissected the molecular pathway involved in a critical step for the maturation of human immunodeficiency virus (HIV) reverse transcriptase (RT), a key enzyme responsible for viral replication. They revealed crucial factors in the unfolding of a protein domain of RT, ribonuclease H (RH), that are important for the formation of a fully functional RT. The study is the first to demonstrate that maturation of HIV-RT could be targeted for new HIV drug development.
Mature HIV-RT is a heterodimer with two subunits, p66 and p51. The catalytic p66 subunit contains a polymerase domain that copies viral RNA and DNA, and a RH domain that cuts out the RNA after transcription. The structural p51 subunit is derived from p66, but its RH domain is removed during RT maturation. The most highly active form of RT is the mature p66-p51 heterodimer.
By characterizing a new structure of the RH domain, researchers identified important protein sites and conformational changes that contributed to the release and separation of RH domain from the p51 subunit. Furthermore, they found that an inhibitor that stabilized the RH domain delayed RH unfolding and maturation of RT heterodimer. The findings suggest the potential use of similar inhibitors to interfere with RT function and HIV infectivity. (QX)
Citation: Zheng X, Pedersen LC, Gabel SA, Mueller GA, DeRose EF, London RE. 2016. Unfolding the HIV-1 reverse transcriptase RNase H domain — how to lose a molecular tug-of-war. Nucleic Acids Res; doi:10.1093/nar/gkv1538 [Online 14 January 2016].
Interactions between mammalian WDR12 and midasin
Researchers from NIEHS have determined the crystal structure of the ubiquitin-like (UBL) domain of Ytml, the Saccharomyces cerevisiae homologue of the mammalian ribosomal protein WD repeat domain 12 (WDR12). As a ribosome assembly factor, Ytm1 associates with other yeast ribosomal proteins Nop7 and Erb1 to form the Nop7 complex. The Nop7 complex is required for maturation of the 25S and 5.8S ribosomal RNAs and formation of the 60S ribosome. Since ribosomes make all of the proteins within living systems, the research has profound implications for understanding protein synthesis.
Structural analysis revealed that the UBL domain of Ytm1 is homologous with the UBL domain of ribosome assembly protein 4 (Rsa4), the yeast homolog of mammalian Notchless (Nle1). Further studies demonstrated that human midasin binds to both WDR12 and Nle1 through their respective UBL and metal ion-dependent adhesion site (MIDAS) domains. The MIDAS domain of midasin was also shown to contain a conserved extension region required for binding WDR12 and Nle1.
Taken together, these studies demonstrated that the interactions between the UBL domains of WDR12 and Nle1 with midasin are evolutionarily conserved in the ribosome assembly pathway of both yeast and mammalian cells. More importantly, WDR12 and Nle1 have been linked to human health implications and are possible novel targets for therapeutic intervention. (TAC)
Citation: Romes EM, Sobhany M, Stanley RE. 2016. The crystal structure of the ubiquitin-like domain of ribosome assembly factor Ytm1 and characterization of its interaction with the AAA-ATPase midasin. J Biol Chem 291(2):882-893.
Study sheds light on lower birth weight associated with PFAS
In a collaborative study, scientists at NIEHS and other institutions have shown that the glomerular filtration rate (GFR) substantially confounds the association between perfluoroalkyl substances (PFAS) and lower birth weight. This discovery suggests that previous epidemiological studies may have overestimated the potential impact of PFAS on birth weight.
PFAS are pervasive, nondegradable, synthetic surfactants used in products such as nonstick coating for cookware. Prior studies have detected PFAS in maternal blood of pregnant women and in breast milk. Many epidemiological studies have noted a correlation between PFAS and lower birth weight, but have neglected to consider the role of GFR, the flow rate of fluid filtered by the kidneys. PFAS are excreted by the kidney in proportion to GFR. During pregnancy, GFR normally increases in proportion to the size of the fetus and newborn.
In this study, scientists performed a pharmacokinetic analysis of PFAS during pregnancy to determine the association of PFAS concentration with a vast array of variables. Using this model, they simulated an epidemiologic study population and analyzed the data as if they came from an epidemiologic study. The results from the analysis of the simulated data were compared to a meta-analysis of epidemiological studies. Their results showed that about one-third of the reported birthweight-PFAS association was expected on the basis on pharmacokinetics. Future epidemiological studies should consider GFR when addressing effects of prenatal PFAS exposure on fetal growth. (EM)
Citation: Verner MA, Loccisano AE, Morken NH, Yoon M, Wu H, McDougall R, Maisonet M, Marcus M, Kishi R, Miyashita C, Chen MH, Hsieh WS, Andersen ME, Clewell HJ, Longnecker MP. 2015. Associations of perfluoroalkyl substances (PFAS) with lower birth weight: an evaluation of potential confounding by glomerular filtration rate using a physiologically based pharmacokinetic model (PBPK). Environ Health Perspect 123(12):1317-1324.