Transfection of nonmelanocytic cells with tyrosinase gene constructs for survival studies

Photothermal confocal spectromicroscopy of multiple cellular chromophores and fluorophores

Confocal fluorescence microscopy is a powerful biological tool providing high-resolution, three-dimensional (3D) imaging of fluorescent molecules. Many cellular components are weakly fluorescent, however, and thus their imaging requires additional labeling. As an alternative, label-free imaging can be performed by photothermal (PT) microscopy (PTM), based on nonradiative relaxation of absorbed energy into heat. Previously, little progress has been made in PT spectral identification of cellular chromophores at the 3D microscopic scale. Here, we introduce PTM integrating confocal thermal-lens scanning schematic, time-resolved detection, PT spectral identification, and nonlinear nanobubble-induced signal amplification with a tunable pulsed nanosecond laser. The capabilities of this confocal PTM were demonstrated for high-resolution 3D imaging and spectral identification of up to four chromophores and fluorophores in live cells and Caenorhabditis elegans. Examples include cytochrome c, green fluorescent protein, Mito-Tracker Red, Alexa-488, and natural drug-enhanced or genetically engineered melanin as a PT contrast agent. PTM was able to guide spectral burning of strong absorption background, which masked weakly absorbing chromophores (e.g., cytochromes in the melanin background). PTM provided label-free monitoring of stress-related changes to cytochrome c distribution, in C. elegans at the single-cell level. In nonlinear mode ultrasharp PT spectra from cyt c and the lateral resolution of 120 nm during calibration with 10-nm gold film were observed, suggesting a potential of PTM to break through the spectral and diffraction limits, respectively. Confocal PT spectromicroscopy could provide a valuable alternative or supplement to fluorescence microscopy for imaging of nonfluorescent chromophores and certain fluorophores.

Inhibition of melanogenesis as a radiation sensitizer for melanoma therapy

Melanin pigment displays strong photo- and radioprotective properties, suggesting that inhibition melanogenesis could increase sensitivity of melanoma to ionizing radiation. We tested this concept in human melanoma cells cultured in either Ham's F10 medium to maintain amelanotic phenotype or DMEM to induce/stimulate melanin production, respectively; N-phenylthiourea (PTU) and D-penicillamine were used as an inhibitor of melanogenesis. Melanogenic activity was evaluated by visual inspection (color of cell pellets) or by measurement of tyrosinase (dopa oxidase) activity assay. Amelanotic cells or cells with various melanin content were exposed to gamma radiation and tested for viability and colony forming capability. Gamma radiation at doses of 2-15 Gy inhibited cell viability and colony forming efficiency in dose- and time-dependent manner, but pigmented melanoma cells were significantly more resistant to gamma radiation than nonpigmented cells (p < 0.05-0.001). Both PTU and D-penicillamine inhibited strongly tyrosinase activity and melanin production in melanoma cells (p < 0.05-0.001). Furthermore, inhibition of melanogenesis by PTU or D-penicillamine resulted in enhancement of melanoma cells sensitivity to killing by gamma rays. In conclusion, the results of these cell culture experiments give support to a clinical trial of pharmacologically induced decrease in melanin synthesis to enhance the efficacy of radiotherapy in advanced melanomas.