Thermal phase transitions in plasma membranes of cancer cells measured by means of Qdot fluorescence

Abstract

Abstract Laser scattered-light applicators used for laser-induced tumor therapy (LITT) facilitate precise thermal metastases destructions. Major tissue-optical parameters (absorption coefficient μa, scattering coefficient μs, anisotropy factor g) reveal biological tissues to be strongly scattering media exhibiting a so-called “optical window” in NIR. In this spectral range therapeutic laser radiation is scattered and absorbed at deeper tissue levels leading to virtual enlargement of the laser applicator. In this study breast cancer cells MX1 were heat stressed to verify the maximum phase transition in MX1 plasma membranes. For this purpose, a novel method of quantum dots (Qdots) fluorescence dosimetry was developed. Qdot fluorescence was detected with a confocal laser scan microscope. Evaluation of measured laser-induced fluorescences yielded a first approximation of the phase transition in MX1 cells. Monolayered MX1 cells were thermally stressed at 40, 42, 45, 50 or 56 °C (30 min each). Controls were kept at 37 °C. Fluorescent cell labeling was realized via biotinylated concanavalin A (ConA) targeting glycocalix sited carbohydrate residues covering the plasma membranes. ConA then was sandwiched by red-emitting Qdots™ 605 conjugated to biotin-binding streptavidin. After exposure to low heat stress (40 or 42 °C) plasma membranes were not visibly affected. Heat stressings at 45 or 50 °C induced morphological changes of cells and tissue conformations. Also reorganization of plasma membrane structures were induced leading to more regular Qdot accumulations and higher fluorescence densities. This process coincided with shrinkage and rounding of cell shapes as a result of active stress response which was more pronounced in the 45 °C-stress group than in the 50 °C-stress group. Severe stressing at 56 °C inhibited active responses and caused destruction of membrane integrities shown by necrotic cell phenotypes associated with intracellular Qdot accumulation. Through domain formation during plasma membrane melting, the distribution of differential state transitions was relatively wide (50–56 °C). Qdot labeling of heat stressed cancer cells demonstrated alterations of plasma membrane organizations and integrities, respectively. Extents of cell lesions observed by Qdot fluorescence definitely correlated with stress doses applied during heat treatments. The phase transition temperature calculated from Qdot fluorescence images of MX1 cells was approximately 52 °C and was 20% higher than phase transition of synthetic lipid membranes.