Killing Cancer in the Heat of the Auric Light

Delivery and activation of genes by gold nanorods. Gold nanorods coated with charged lipids efficiently bind to DNA and penetrate cells. The team designed an artificial gene that is turned on by heat generated by the gold nanorods upon exposure to near infrared light illumination. [Kyoto University iCeMS]

Lasers and gold have been an unwitting combination ever since Auric Goldfinger precariously strapped James Bond to a table laden with the precious metal and threatened to slice the British agent in two in the 1964 classic spy film. Now a team of investigators at Kyoto University's Institute for Integrated Cell-Material Sciences (iCeMS) have developed a new method that could become an efficient cancer killing tool and one that might pique the interest of Mr. Goldfinger.

The new method involves coating gold nanorods, which produce heat when exposed to a near-infrared laser, with the lipids oleate and 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). The lipids enhance the nanorods' ability to interact with and penetrate cells. Findings from the new study were published recently in Scientific Reports in an article entitled “Surface Chemistry for Cytosolic Gene Delivery and Photothermal Transgene Expression by Gold Nanorods.” In addition to identifying the appropriate chemistry to coat the gold nanorods for efficient transfection, the research team also developed a plasmid vector that includes a heat shock protein that is activated in response to heat generated by the laser.

For initial proof of concept, the iCeMS team generated a vector containing the enhanced green fluorescent protein (EGFP) gene under the control of the heat-shock promoter (HSP) and then transfecting it into mammalian cells by the lipid-coated gold nanorods. Exposing the cells to a near-infrared laser heated up the gold nanorods, turning on the HSP promoter and subsequently the EGFP gene. The surrounding, nontargeted cells showed little to no EGFP expression.

Gold nanorods carrying the heat-inducible TRAIL gene are transfected into cancer cells. Cancer cells express TRAIL receptors while normal cells do not. Illumination by a near-infrared laser warms gold nanorods and induces TRAIL expression in transfected cells. TRAIL then kills the surrounding cancer cells. [Kyoto University iCeMS]

“In the present study, we prepared a series of surface-modified gold nanorods (AuNRs) with different cationic dispersants and found that AuNRs functionalized with DOTAP showed comparable transfection efficiency to a commonly used transfection reagent, Lipofectamine 2000,” the authors wrote. “Subsequent near-infrared laser (NIR) illumination of the cells transfected by DOTAP-AuNRs for 10 s induced time- and site-specific [HSP] transgene expression without significant phototoxicity, to a degree similar to that of heating the entire culture dish for 30 min.”

Since the AuNRs/EGFP vector worked so well, the researchers decided to incorporate a known target for anticancer drugs, known as tumor necrosis factor (TNF)-related apoptosis-inducing ligand, or TRAIL. This ligand has been shown to induce apoptotic death in a variety of cancer cell lines. Amazingly, after transfection and NIR illumination, the transfected cells carrying the TRAIL gene showed a high rate of cell death in the surrounding cancer cells.

“Our mechanistic studies suggest that efficient transfection and quick photoactivation of the HSP promoter (HSP70b’) are due to the promoted endosomal escape of DOTAP-AuNRs,” the authors surmised. “We propose a novel protocol for NIR-inducible, site-directed gene expression using an unprecedented complex of the three conventional components capable of both transfection and photothermal heating.”

The investigators were excited by their findings and are optimistic that the lipid-coated gold nanorods will potentially help with new molecular cancer therapies. This new methodology “provides a unique opportunity for site-directed, light-inducible transgene expression in mammalian cells by a NIR laser, with minimal phototoxicity,” conclude the researchers.