Reversible Nucleolar Stress and Cell Growth Arrest Triggered by Acidic pH.

The tumor microenvironment is often characterized by hypoxia and extracellular acidosis, which modulate various tumor cell phenotypes. Ribosome biogenesis is a highly energy-demanding process that is essential for protein synthesis and cell proliferation and is sensitive to cellular stress, resulting in a nucleolar stress response. However, whether extracellular acidosis impairs ribosome biogenesis and induces nucleolar stress remains unclear. In this study, we demonstrated that an acidic pH downregulates ribosome biogenesis in cancer cells. RNA sequencing revealed the downregulation of genes related to ribosome biogenesis and cell cycle progression under acidic conditions. Consistently, acidic pH reduced the pre-rRNA levels and induced nucleolar stress, as evidenced by NPM1 translocation from the nucleolus to the nucleoplasm, which led to G1 phase arrest and growth inhibition. Importantly, these effects were reversed upon restoration of neutral pH, with recovery of pre-rRNA expression, NPM1 localization, and cell proliferation. Further, an acidic pH shifted the intracellular redox balance toward an oxidized state. Treatment with the reductant dithiothreitol partially reversed NPM1 translocation, suggesting that oxidative stress contributes, at least partially, to the nucleolar stress response. Overall, our findings reveal a previously unrecognized link between extracellular acidosis and impaired ribosome biogenesis, leading to nucleolar stress and reversible growth arrest. This acidosis-driven stress response represents a therapeutic vulnerability in solid tumors, offering a novel strategy to overcome treatment resistance associated with the acidic tumor microenvironment.