Oral exposure to polystyrene nanoparticles affectsiron absorptionGretchen J. Mahler1, Mandy B. Esch2, Elad Tako3, Teresa L. Southard4, Shivaun D. Archer2,
Raymond P. Glahn3 and Michael L. Shuler2*
The use of engineered nanoparticles in food and pharmaceuticals is expected to increase, but the impact of chronic oralexposure to nanoparticles on human health remains unknown. Here, we show that chronic and acute oral exposure topolystyrene nanoparticles can influence iron uptake and iron transport in an in vitro model of the intestinal epithelium andan in vivo chicken intestinal loop model. Intestinal cells that are exposed to high doses of nanoparticles showed increasediron transport due to nanoparticle disruption of the cell membrane. Chickens acutely exposed to carboxylated particles(50 nm in diameter) had a lower iron absorption than unexposed or chronically exposed birds. Chronic exposure causedremodelling of the intestinal villi, which increased the surface area available for iron absorption. The agreement betweenthe in vitro and in vivo results suggests that our in vitro intestinal epithelium model is potentially useful fortoxicology studies.
Engineered nanoparticles have unique physical and chemicalproperties and are currently used in a variety of applications,including the food14 and pharmaceutical industries5,6. The
increased surface area, unique crystalline structure, small size andenhanced reactivity of some nanomaterials, however, may lead toharmful interactions with cellular material, and no studies haveaddressed the chronic effects of nanoparticle exposure on thenormal function of the intestinal epithelium7.
It is estimated that the average person in a developed countryconsumes between 1012 and 1014 man-made fine (diameter,0.11 mm) to ultrafine (diameter, ,100 nm) particles every day8.These dietary particles are mainly TiO2, silicates and aluminosili-cates derived from food additives such as stabilizers and anticakingagents8. Because most of these micro- and nanoparticles have nega-tively charged surfaces, they can bind to biomolecules in the gutlumen, absorb across the gastrointestinal tract and accumulate atthe base of Peyers patches, where a large concentration of M cellsare found8. M cells transport microorganisms and particles fromthe gut lumen to immune cells across the intestinal epithelium,and are important for defending the body against ingested toxicsubstances and stimulating mucosal immunity9.
The ingestion of dietary particles has been thought to promotethe development of Crohns disease, which is characterized by trans-mural inflammation of the gastrointestinal tract that first appearsover the Peyers patches10. Lomer et al. have shown that patientswith Crohns disease who followed a diet low in TiO2 and alumino-silicate microparticles for four months had a reduction in theCrohns disease activity index10. Patients with Crohns disease arealso prone to iron deficiency, suggesting a possible link betweennanoparticle consumption, the development of Crohns diseaseand iron absorption11.
Oral delivery is the preferred route of pharmaceutical adminis-tration because it is inexpensive, non-invasive and convenient forpatients12. Polymeric nanoparticle carriers are useful for drug deliv-ery because they are more stable than other colloidal carriers in thegastrointestinal tract and, compared to their micrometre-sized
counterparts, the diameter to surface area ratio of nanoparticlesfavours absorption through the intestinal epithelium13. Furthermore,the physical and chemical characteristics, drug release profile and bio-logical behaviour of polymeric nanoparticles can be manipulatedeasily14. Although many potential peptide and protein therapeuticswill be administered at least daily, very little is known about thechronic effects of nanoparticle ingestion.
The goal of this work was to investigate the effects of oralexposure to nanoparticles on the absorption of iron. Using a physio-logically realistic in vitro model of the intestinal epithelium andin vivo chicken intestinal loop model, we showed that acute exposureto 50 nm polystyrene carboxylated nanoparticles can inhibit irontransport. In chronically exposed chickens, the 50 nm carboxylatedparticles caused a remodelling of the intestinal villi to increase thesurface area available for iron absorption. We chose iron absorptionas a subject because iron is an example of an essential nutrient that istransported across the intestinal epithelium by means of complex,highly regulated, protein-assisted vesicular and non-vesicular mech-anisms15. The polystyrene nanoparticles used in this study (particlecharacterization shown in Table 1) were chosen as a model particleto demonstrate that our in vitro and in vivo experimental systemscan be used for evaluating the subtle effects of nanoparticleconsumption.
Nanoparticle dose calculationsIn vitro and in vivo doses of nanoparticles were formulated to mimicpotential human exposure. The total surface area of the humansmall intestine is 2 106 cm2, and the duodenum, which is thefirst section of the small intestine and the site where most ironabsorption occurs, has 900 cm2 of absorbing surface area1618. If1014 particles are ingested, this represents a dose of 107 particlesper cm2 to the small intestine. If 1012 or 1014 particles are ingested,the dose to the duodenum is 109 or 1011 particles per cm2,respectively. If 0.02 mg kg21 of 50 nm polystyrene particles wereadministered to a 70 kg human as a pharmaceutical, the dose tothe small intestine would be 107 particles per cm2 assuming that
1Department of Bioengineering, Binghamton University, Binghamton, New York 13902, USA, 2Department of Biomedical Engineering, Cornell University,Ithaca, New York 14853, USA, 3Plant, Soil and Nutrition Laboratory, Agricultural Research Services, US Department of Agriculture, Tower Road, Ithaca,New York 14853, USA, 4Department of Biomedical Sciences, Cornell University, Ithaca, New York 14853, USA. *e-mail: [email protected]
ARTICLESPUBLISHED ONLINE: 12 FEBRUARY 2012 | DOI: 10.1038/NNANO.2012.3
NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology 1
2012 Macmillan Publishers Limited. All rights reserved.
the particles are monodispersed and have a density of 1.053 g cm23
(ref. 19). Oral administrations of 2 mg kg21 and 200 mg kg21 oralwould translate to doses of 109 and 1011 particles per cm2 to the smallintestine, respectively.
The doses of 50 nm particles applied to the cell cultures used inthis study (assuming that the presence of microvilli increasesthe surface area by a factor of 20; ref. 16), were 107, 109 and 1011
particles per cm2 for the low (2 109 particles per ml), medium(2 1011 particles per ml) and high (2 1013 particles per ml)experimental concentrations used. The doses of 200 nm particles(1.25 108 particles per ml, 1.25 1010 particles per ml and1.25 1012 particles per ml) were determined by calculating thetotal surface area of particles in the 50 nm particle dose and normal-izing the 200 nm particle dose by surface area. For in vivo experi-ments, chickens were given 2 mg kg21 doses of carboxylatednanoparticles. A 2 mg kg21 dose to a 3 kg chicken exposes theduodenum to 109 particles per cm2 of surface area (the chicken duo-denum is the primary site of iron absorption)20. The surface area ofthe chicken duodenum was estimated to be 1,600 cm2 based onpreviously published values2124, and the effective surface area ofthe duodenum was estimated to be 15 times that value, or24,000 cm2, due to the presence of 105 microvilli per villi25.
In vitro model characterizationOur in vitro model of the intestinal epithelium mimics the absorp-tive enterocytes, goblet cells and M cells that populate the humansmall-intestinal lining. Absorptive enterocytes and mucus-produ-cing goblet cells are the two most common cell types in the intestinalepithelium18. Caco-2 cells absorb and transport iron, mimickingabsorptive enterocytes, and HT29-MTX mucus-secreting cellsmimic goblet cells26,27. We have shown that co-cultures of Caco-2and HT29-MTX have a mucus layer that completely covers thecell monolayer28. Culturing Caco-2 monolayers in the proximityof the Raji B lymphocyte cell line induces Caco-2 differentiationinto cells with an M cell-like morphology9. M cells are known toexpress a greater amount of b1-integrin on the apical surface, andsialyl Lewis A antigen has been shown by Giannasca et al. to dis-tinguish human M cells from other intestinal epithelial celltypes29,30. Supplementary Fig. 1 presents a diagram of the cultureconditions for the Caco-2/HT29-MTX (co-cultures of Caco-2 andHT29-MTX) and M cell (tri-cultures of Caco-2, HT29-MTXand Raji B) monolayers. Supplementary Fig. 2 shows the increasedb1-integrin expression on the apical side of M cell monolayers,and sialyl Lewis A antigen expression in M cell monolayers canbe seen in Supplementary Fig. 3. Image analysis of sialyl Lewis Aantigen staining reveals that the tricultures have an average of2.55% (+0.29%) differentiated M cells. This compares well withhuman physiology, as the human small-intestinal epithelial mem-brane has fewer than 10% M cells31. Figure 1a showsM cell mono-layers after exposure to 50 nm particles, and Fig. 1b andSupplementary Fig. 4 show M cell monolayers after exposure to200 nm particles. The 50 nm particles are localized at the cellmembrane, but more vesicles can be seen in monolayers exposedto 200 nm particles.
Caco-2/HT29-MTX and M cell monolayers were exposed to amedium dose of 50 nm or 200 nm particles at 4 8C and 37 8C to
determine if the particles were transported by an energy-dependentprocess. After 45 min, Caco-2/HT29-MTX monolayers transported4.32 108 50 nm partic
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