Study target genes of STAT3

This study is to identify downstream genes regulated by STAT3 in response cytosolic acidification. Dysregulated intracellular pH is emerging as a hallmark of cancer. In spite of their acidic environment, cancer cells maintain alkaline intracellular pH (≥7.4) that promotes cancer progression by inhibiting apoptosis and increasing glycolysis, cell growth, migration and invasion. Here, we identify signal transducer and activator of transcription 3 (STAT3) as a key player in the maintenance of alkaline cytosolic pH. STAT3 associates with the vacuolar H+-ATPase on lysosomal membranes in a coiled coil domain-dependent manner and increases its activity in living cells and in vitro. Accordingly, STAT3 depletion disrupts intracellular proton equilibrium by decreasing and increasing cytosolic and lysosomal pH, respectively. This dysregulation can be reverted by reconstitution with wild type STAT3 as well as STAT3 mutants unable to activate target genes (Tyr-705-Phe and DNA binding mutant) or to regulate mitochondrial respiration (Ser-727-Ala). Upon cytosolic acidification, phospho-Tyr-705-STAT3 is rapidly dephosphorylated, transcriptionally inactivated and further recruited to lysosomal membranes to reestablish intracellular proton equilibrium and to enhance cell survival. These data reveal STAT3 as a regulator of intracellular pH, and vice versa intracellular pH as a regulator of STAT3 localization and activity.