Synthesis of anisotropic gold nanoparticles for optical hyperthermia: comparing gene expression profile induced by nanoprisms and nanorods after laser irradiation

Hyperthermia (HT) is currently used as a non-invasive technique for cancer therapy, whereby biological tissues are exposed to higher than normal temperatures, for selective ablation of tumoral cells. Heating treatments can be applied using external heating sources such as ultrasounds, however, heating only malignant cells is difficult to obtain. Recently, there has been a growing interest in the use of gold nanoparticles (AuNPs) to selectively generate heat in a spatiotemporal fashion, which is known as photothermal therapy (PTT). When a metal particle is exposed to light, the oscillating electromagnetic field of the light induces a collective coherent oscillation of the free electrons, reaching a maximum at a specific frequency called surface plasmon resonance (SPR). AuNPs can be synthetized to absorb incident light coming from a laser outside the body and generate heat only in the tissues where AuNPs are allocated. In fact, they can be finely tuned to absorb light in the near-infrared (NIR) spectral range, known as the "biological window". In this range tissue is maximally transparent, and therefore, the light itself would not damage the tissue. Although PTT has entered clinical trials, the molecular mechanisms underlying the in vivo cellular responses to heat stress remain unclear to date. Here, and in line with European strategies aimed to reduce vertebrate experimentation, we propose an invertebrate animal, the small freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa) to study the response of different AuNPs to PTT. This organism, more complex than cells, exhibit however functional conservation of main physiological pathways, such as the heat response. To this end, gold nanoprisms (NPr) and nanorods (NR) with SPR absorption bands in the NIR range have been synthetized and characterized (Figure 1). Their heating efficiency upon irradiation with a NIR-laser (1064 nm) has been evaluated, demonstrating that both types of nanoparticles are excellent photothermal agents. To be able to use them in vitro or in vivo, their surface has been engineered with different molecules. After stabilizing them in biological media, toxicity has been evaluated in Hydras and the efficiency of internalization assessed. To study the molecular response to heat once internalized by the animals, the gene expression profile of different genes after laser irradiation have been profiled. Although both NPr and NR showed a high heating efficiency, after NIR irradiation of animals, only NPrs were able to induce the overexpression of genes implied in different pathways, i.e. stress and oxidative responses. These observations demonstrate that Hydra can be a good candidate to test the efficiency of different AuNPs as photothermal agents, and will help to select the most adequate AuNP to be tested in further steps, such as mice.