Decreased mitochondrial function in quiescent cells isolated from multicellular tumor spheroids

Area of Science:

• Cell Biology
• Cancer Research
• Metabolic Adaptation

Background:

• Cells in multicellular spheroids experience a stressful microenvironment, leading to reduced oxygen consumption.
• Understanding this metabolic adaptation is crucial for cancer research and in vivo tumor oxygenation.

Purpose of the Study:

• To investigate the mechanism behind reduced oxygen consumption in spheroid cells.
• To differentiate between mitochondrial mass and function changes in tumor cells from different spheroid regions.

Main Methods:

• Utilized mitochondrial-specific fluorescent stains: rhodamine 123 (R123) for activity and 10-nonyl-acridine orange (NAO) for mass.
• Employed flow cytometry to analyze mitochondrial mass and function in cells from various spheroid regions.
• Correlated mitochondrial function (R123 staining) with oxygen consumption rates and cell growth fraction.

Main Results:

• Inner spheroid cells showed a significant reduction in R123 fluorescence (mitochondrial activity), but only a modest decrease in NAO fluorescence (mitochondrial mass).
• This indicates a downregulation of mitochondrial function rather than degradation.
• R123 staining correlated with oxygen consumption and growth fraction in spheroid cells, and R123 uptake recovered upon re-plating.

Conclusions:

• Tumor cells adapt to spheroid environments by reducing mitochondrial function, not mass.
• This functional adaptation explains delayed cell cycle re-entry in quiescent spheroid cells.
• Findings have implications for understanding tumor cell oxygenation in vivo and potential therapeutic strategies.