To the Editors:
We were delighted to read the article in the European Respiratory Journal by Hussain et al. [1] which demonstrated aggravation of pulmonary inflammation and airway hyperreactivity by gold (Au) nanoparticles (NPs) in a mouse model of chemical-induced asthma. Exposure of sensitised (but not unsensitised) mice to Au NPs was shown to result in increased airway resistance and reactivity to methacholine, associated with Au NP uptake by macrophages, increased oedema and epithelial damage. Overall, these data are consistent with the idea of a potentially toxic, exacerbating effect of nebulised Au NPs within the airway, as discussed in the editorial accompanying the article [2].
While Hussain et al. [1] established the pro-inflammatory risk of Au NPs, the underlying mechanisms were not examined. Increased recruitment and activity of airway inflammatory cells, presumably induced by Au NP uptake, and their subsequent action is probably contributory. However, increased airway reactivity represents much more than inflammation [3, 4]. Increased bronchoconstriction results from enhanced intracellular Ca2+ concentration ([Ca2+]i) and force responses to agonist as well as airway remodelling which, in part, involves airway smooth muscle (ASM) proliferation. Accordingly, enhancement of airway reactivity by Au NPs may involve any or all of these mechanisms. We bring your attention to our recently published study [5], wherein we examined the effect of Au NPs on [Ca2+]i and proliferation/apoptosis of human ASM cells and human bronchial epithelial cells (BECs). The aim of this in vitro study was to investigate how Au NPs interact with cells to mediate their effects. We highlighted three findings that are relevant (and supportive) to the study of Hussain et al. [1]: 1) positively charged Au NPs (+Au NPs) are taken up by BECs and ASM cells within minutes, dependent on plasma membrane potential (with +Au NPs themselves inducing membrane depolarisation); 2) following uptake, only +Au NPs produce rapid elevation in [Ca2+]i; and 3) even 30 min of +Au NP exposure results in substantial reduction in cell proliferation and increased apoptosis of airway cells over the next 24–48 h. We interpreted rapid depolarisation and increased [Ca2+]i induced by +Au NPs in ASM as contributing to enhanced airway contractility. With +Au NP being present intracellularly, such effects could be sustained for prolonged time periods and have downstream effects of altering gene and protein regulation important to cell viability (e.g. caspases). Interestingly, Au NPs of other (or nil) charges are minimally taken up and have only small effects on [Ca2+]i or cell proliferation and survival [5]. These results raise several interesting themes relating to the results of Hussain et al. [1].
First, it may not only be the auric component of Au NPs that contributes to pulmonary inflammation and airway hyperreactivity, but Au NP charge may be important. While Hussain et al. [1] did not explicitly report on Au NP charge, it would be interesting to repeat aspects of their study with nebulisation of Au NPs of different surface charges and then determine the extent of airway inflammation or reactivity.
Secondly, the effects of increased reactivity observed in mouse airways 48 h after Au NP nebulisation may have begun much more acutely (as evidenced by our results), and are sustained. Continued intracellular presence of Au NPs within constitutive cells of the airway (BECs and ASM) may contribute to enhanced contractility, while uptake by inflammatory cells may potentiate and perpetuate increased reactivity by modulating inflammation. It is possible that, akin to airway cells, Au NPs also increase [Ca2+]i within macrophages and other inflammatory cells, modulating pro-inflammatory versus anti-inflammatory mediator production. These issues remain to be examined.
Thirdly, the epithelial damage observed by Hussain et al. [1] may very well be the result of Au NP-induced reduction of BEC viability, as observed in our study. Whether Au NPs also alter BEC function remains to be determined. In pilot studies using diaminofluorescein-based imaging in human BECs, we have not found any evidence for Au NP modulation of nitric oxide.
A major contextual difference between our study and that of Hussain et al. [1] (beyond species differences and our study being in vitro only) is that in the latter study, Au NP exposure occurred in an environment of pre-existing inflammation. It would be interesting to determine whether early pro-inflammatory cytokines, such as tumour necrosis factor-α or interleukin-1β, alter the extent of Au NP (especially +Au NP) uptake and its downstream consequences.
Finally, from the perspective of Au NP toxicity, based on our study, we would conclude that “charge matters”. Accordingly, in designing Au NP-containing preparations, we propose that attention be paid to Au NP charge. Here, the lack of toxic effect of non-positive Au NPs suggests the need to explore favourable modification of Au NP charge and the maintenance of such charge within biologically compatible formulations of drugs and other substances for human use.
Footnotes
Statement of Interest
None declared.
- ©ERS 2011