The non-gastric H +/K +-ATPase ATP12A (ATP1AL1) is expressed in various tissues. We found by RTPCR and/or western blotting, intracellular pH measurements, electron microprobe analysis, cell volume (CV) measurements and flow cytometry that ATP12A is expressed in human myelomonocytic HL60 cells, rat insulinoma Ins-1E cells, human pancreatic islets, the prostate cancer cell lines LNCaP, PC3 and DU-145 as well as in normal and cancerous human prostate tissue. Treatment of HL60 cells with low (1mM) concentrations of butyrate leads to monocyte-directed differentiation whereas higher (5-10mM) concentrations induce apoptosis as assessed by flow cytometric determination of CD86 expression, CV, cell granularity, caspase activity, phosphatidylserine exposure on the outer cell membrane leaflet, cell cycle analysis and cell proliferation. Ins-1E cells undergo apoptosis upon treatment with dexamethasone, the polyphenol resveratrol or by glucose starvation.Transcriptional up-regulation of ATP12A is evident during apoptosis in HL60 and Ins-1E cells and both cell types exhibit apparent apoptotic volume decrease (AVD). The inhibitor of the H +/K +-ATPase SCH28080 leads per se to induction of apoptosis in HL60 cells and Ins-1E cells and accelerates the time course of induced apoptosis. Moreover ATP12A expression is altered in tissue from benign hyperplasia of human prostate and in prostate cancer. In summary it is shown that ATP12A is functionally active, plays a role during apoptosis in HL60 cells and Ins-1E cells and is differently expressed in normal and pathological prostate tissue.