Goldhas been considered to be a precious material for centuries and have intriguedmankind.
There are suggested reports that colloidal gold nanoparticles havebeen used by artists as these particles were able to interact with visiblelight thereby leading to the formation of radiant colors. Gold nanoparticleshave a broad range of applications in divergent fields. Gold nanoparticlesapplications include therapeutic agents, sensory probes, and drug deliveryplatforms among others. Due to its higher surface area to volume ratio,therapeutic biomarkers can be coated on the surface of GNPs (34). Whenexcited by 700-800 nm wavelength of light, GNPs could generate heat that couldpotentially target tumor cells (35). GNPshave also been used to diagnose cancers, heart diseases and infectious agentsundermining the versatility of GNPs (36). Despitetheir biological applications, anti-inflammatory properties of GNPs have beencharacterized fairly recently (37).
In thisstudy we determined whether gold nanoparticles can be used as immunomodulatorsto treat inflammation linked with cancer.Owingto its size, GNPs have the capacity to pass through blood vessel and eventuallyblood-brain barrier (38).However, both in vitro and in vivo data suggest that GNP can beused as diagnostics to be used in parenteral routes (39). Thisstudy used bio-conjugates of GNPs and tested their toxicity in human PBMCs,HUVECs, PMNs and erythrocytes. Such invitro studies coupled with in vivoexperiments did not show any toxic effects and exhibited ROS scavengingproperty when rat neutrophils were stimulated with LTB4 or PMA along with GNPbioconjugate (39).
Weobserved reduction in ROS production when A549 cells were pre-treated with GNPand then stimulated with either LPS or CpG-ODN (Figure 4, S2). However, when westimulated A549 cells with PMA there was increased ROS production, but pre-treatmentwith GNP did not result in significant reduction in ROS production (data notshown) suggesting that GNPs might be inducing immunomodulation by specificallytargeting TLR pathway in lung cancer (A549) cell lines. Furthermore,GNP-bioconjugate could also potentially have superior ROS scavenging traitowing to efficient cellular uptake of such conjugates. In our studies, weobserved that the 5 nm GNP was most efficiently internalized by A549 cells (Figure2). Future studies would involve comparison between the efficacy of GNP andGNP-bioconjugate in ROS scavenging function.
Cytotoxicityof nanoparticles depends on the size of the nanoparticles (40). Whendifferent sized silver nanoparticles were compared for cytotoxicity, it wasobserved that the 20 nm silver nanoparticles were more toxic than the 80 and113 nm particles (41). Thiseffect could again be cell-type specific as this effect was evident in L929 cellsbut not in RAW264.
7 cells (41). When thecytotoxicity of 1-2 nm GNPs was tested using fibroblasts, macrophages,epithelial cells, drastic changes in IC50 values were observed (42). Incontrast, 15 nm GNPs were nontoxic to a 60-fold higher concentration of 1-2 nmGNPs (42). Similarresults were observed when SK-Mel-28 melanoma cells were used (42). However,in our study spherical GNPs of 5, 15 and 30 nm size did not display anysignificant toxicity in A549 cells (Fig 4). This effect could be dependent onthe cell type being used.
Similarly, when GNPs (5-200 µg/mL) were tested forcytotoxicity in human DCs after 48h incubation, necrosis was not induced even ata high concentration of 200 µg/mL (43). Consistentwith our study, bare non-conjugated gold nanoparticles of various sizes such as4, 11, 19, 35 and 45 nm GNPs did not induce cytotoxicity in murine RAW264.7cells indicating that the cytotoxicity of various sized GNPs is also cell linespecific and varies depending on the type of cells used (16). Shape ofnanoparticles also impacts cell viability due to modifications in geometry.Additionally, distinct chemical and physical interactions with cell membranealso impact cell viability. Gold nanospheres (10 nm) and gold nanorods (41 nm)displayed dose and time-dependent toxicity in HEK293T and HeLa cell lines (44). However,nanoprisms (160 nm), gold nanostars (240 nm), and nanoflowers (370 nm) did notdemonstrate appreciable level of toxicity in HeLa cells implying the sensitivityof HEK293T cells (44).
Similarly, needle-shaped, but not the spherical-shapedPEGylated PLGA polymer NPs were found to induce significant cytotoxicity inHepG2 cell lines (45).Apart from TLR4 and TLR9, otherTLRs are expressed by A549 cells including TLR7, which is also expressed in theendosome. The role of TLR7 in lung cancer has not been understood completely (68).
TLR7 activation has a pro-tumorigenicrole has been associated with poor prognosis in non-small cell lung cancer (69). Type IIFN activates the immune system to target the process of metastasis (70). Whether GNPs can target TLR7pathway and modulate downstream molecules including MyD88 andinterferon-inducible genes need to be ascertained.
Thus,modulation of TLR4-mediated inflammatory responses by 5 nm GNPs suggests their potentialuse an adjunct therapy to treat inflammation-linked lung cancer. Conjugation ofstandard anti-cancer drug on the surface of immunomodulatory (5 nm) GNPs(GNP-anticancer drug conjugates) might be a promising strategy to treat lung cancerwith increased efficacy and minimal side-effects. GNPs in such conjugates mightserve as drug delivery carriers as well as might prevent overt inflammation andassociated cancer progression owing to their immunomodulatory potential. Thus, GNP-anticancerdrug conjugates might aid in maintaining balanced immune responses to killcancer cells as well as prevent hyper-inflammation-mediated tumor promotion.