Fluorescent redox-dependent labeling of lipid droplets in cultured cells by reduced phenazine methosulfate

TL;DR

This study demonstrates that phenazine methosulfate (PMS) can be used as a fluorescent probe to label and study lipid droplets in cultured cells, revealing redox dynamics and lipid droplet characteristics.

Contribution

The paper introduces a novel application of PMS as a fluorescent marker for lipid droplets, enabling visualization and analysis of redox processes in live cells.

Findings

PMS fluoresces green when oxidized and blue when reduced, allowing visualization of lipid droplets.

Reduced methyl-phenazine accumulates in lipid droplets, enabling their fluorescent labeling.

Fluorescence shifts from blue to green over time, indicating redox changes within lipid droplets.

Abstract

Natural and synthetic phenazines are widely used in biomedical sciences. In dehydrogenase histochemistry, phenazine methosulfate (PMS) is applied as a redox reagent for coupling reduced coenzymes to the reduction of tetrazolium salts into colored formazans. PMS is also currently used for cytotoxicity and viability assays of cell cultures using sulfonated tetrazoliums. Under UV (340 nm) excitation, aqueous solutions of the cationic PMS show green fluorescence ({\lambda}em: 526 nm), whereas the reduced hydrophobic derivative (methyl-phenazine, MPH) shows blue fluorescence ({\lambda}em: 465 nm). Under UV (365 nm) excitation, cultured cells (LM2, IGROV-1, BGC-1, and 3T3-L1 adipocytes) treated with PMS (5 ug/mL, 30 min) showed cytoplasmic granules with bright blue fluorescence, which correspond to lipid droplets labeled by the lipophilic methyl-phenazine. After formaldehyde fixation blue-fluorescing droplets could be stained with oil red O. Interestingly, PMS-treated 3T3-L1 adipocytes observed under UV excitation 24 h after labeling showed large lipid droplets with a weak green emission within a diffuse pale blue-fluorescing cytoplasm, whereas a strong green emission was observed in small lipid droplets. This fluorescence change from blue to green indicates that reoxidation of methyl-phenazine to PMS can occur. Regarding cell uptake and labeling mechanisms, QSAR models predict that the hydrophilic PMS is not significantly membrane-permeant, so most PMS reduction is expected to be extracellular and associated with a plasma membrane NAD(P)H reductase. Once formed, the lipophilic and blue-fluorescing methyl-phenazine enters live cells and mainly accumulates in lipid droplets. Overall, the results reported here indicate that PMS is an excellent fluorescent probe to investigate labeling and redox dynamics of lipid droplets in cultured cells.