Nitrosation on the outside: Inhibition of monocarboxylate transport by extracellular S-nitrosation affects breast cancer cell bioenergetics and proliferation

Despite the well-known reliance of cancer cells on glycolysis for ATP production, it has been realized that mitochondrial function is essential for cell proliferation by providing metabolic building blocks for protein, DNA and lipid biosynthesis. Here, we show that pyruvate greatly stimulates MCF7 breast cancer cell proliferation and colony formation when compared to glucose alone. In addition, only in the presence of pyruvate do these cells exhibit a mitochondrial reserve capacity as determined by extracellular flux analysis, suggesting a link between proliferation and pyruvate-driven mitochondrial function. L-CysNO, a membrane-transportable S-nitrosothiol, inhibits cell proliferation, glycolysis and mitochondrial function. Surprisingly, the cell-impermeable isomer D-CysNO, also inhibits proliferation and mitochondrial reserve capacity, but not glycolysis. Further analysis revealed that D-CysNO inhibits pyruvate uptake into cells, and the rate of pyruvate uptake correlates with mitochondrial reserve capacity. In this regard, we show that D-CysNO can act in a similar way to other known monocarboxylate transporter (MCT) inhibitors. Using biotin switch methodology, we show that D-CysNO can modify cellular MCT, suggesting that exofacial S-nitrosation of these transporters can inhibit their function. L-CysNO also inhibits pyruvate transport, but its effects are masked and complicated by its many intracellular effects. Taken together, these data indicate that cell-surface exofacial S-nitrosation can affect cellular metabolism and proliferation, and MCTs may represent a therapeutic target for redox-based thiol modifying agents in cancer.