The putative role of the non-gastric H⁺/K⁺-ATPase ATP12A (ATP1AL1) as anti-apoptotic ion transporter: effect of the H⁺/K⁺ ATPase inhibitor SCH28080 on butyrate-stimulated myelomonocytic HL-60 cells

Background/aims: The ATP12A gene codes for a non-gastric H(+)/K(+) ATPase, which is expressed in a wide variety of tissues. The aim of this study was to test for the molecular and functional expression of the non-gastric H(+)/K(+) ATPase ATP12A/ATP1AL1 in unstimulated and butyrate-stimulated (1 and 10 mM) human myelomonocytic HL-60 cells, to unravel its potential role as putative apoptosis-counteracting ion transporter as well as to test for the effect of the H(+)/K(+) ATPase inhibitor SCH28080 in apoptosis.

Methods: Real-time reverse-transcription PCR (qRT-PCR) was used for amplification and cloning of ATP12A transcripts and to assess transcriptional regulation. BCECF microfluorimetry was used to assess changes of intracellular pH (pHi) after acute intracellular acid load (NH4Cl prepulsing). Mean cell volumes (MCV) and MCV-recovery after osmotic cell shrinkage (Regulatory Volume Increase, RVI) were assessed by Coulter counting. Flow-cytometry was used to measure MCV (Coulter principle), to assess apoptosis (phosphatidylserine exposure to the outer leaflet of the cell membrane, caspase activity, 7AAD staining) and differentiation (CD86 expression).

Results: We found by RT-PCR, intracellular pH measurements, MCV measurements and flow cytometry that ATP12A is expressed in human myelomonocytic HL-60 cells. Treatment of HL-60 cells with 1 mM butyrate leads to monocyte-directed differentiation whereas higher concentrations (10 mM) induce apoptosis as assessed by flow-cytometric determination of CD86 expression, caspase activity, phosphatidylserine exposure on the outer leaflet of the cell membrane and MCV measurements. Transcriptional up-regulation of ATP12A and CD86 is evident in 1 mM butyrate-treated HL-60 cells. The H(+)/K(+) ATPase inhibitor SCH28080 (100 µM) diminishes K(+)-dependent pHi recovery after intracellular acid load and blocks RVI after osmotic cell shrinkage. After seeding, HL-60 cells increase their MCV within the first 24 h in culture, and subsequently decrease it over the course of the next 48 h. This effect can be observed in the overall- and non-apoptotic fraction of both untreated and 1 mM butyrate-treated HL-60 cells, but not in 1 mM butyrate-stimulated phosphatidylserine-positive cells. These cells do not shrink from 24 h to 72 h and have finally a higher MCV than untreated cells unless they are exposed to SCH28080. 10 mM butyrate induces apoptosis within 24 h.

Conclusion: In summary we show that in HL-60 cells ATP12A is a functionally active H(+)/K(+) ATPase that may counteract events during early apoptosis like intracellular acidosis, loss of intracellular K(+) ions and apoptotic volume decrease. Its expression and/or susceptibility to the H(+)/K(+) ATPase inhibitor SCH28080 becomes most evident in cells exposing phosphatidylserine on the outer leaflet of the cell membrane and therefore during early apoptosis.