Question: 

Apoptosis of b-cells is involved in the pathogenesis of both type-1 and type-2 DM. Intracellular acidification, disturbed ion homeostasis and cell shrinkage under isotonicity (apoptotic volume decrease, AVD) are early events during apoptosis and precede caspase activation, DNA degradation and loss of cell membrane integrity. In pancreatic b-cells Na⁺/H⁺ exchangers (NHEs), HCO₃^--dependent mechanisms and H⁺ ATPases regulate cytosolic pH. This study was performed to disclose a possible contribution of H⁺/K⁺ ATPases to pH/volume homeostasis in b-cells. H⁺/K⁺ ATPase a-subunits comprise the gastric (HKa1) and the non-gastric (HKa2; encoded by gene Atp12a) isoforms, being involved in gastric acid secretion, H⁺/K⁺ transport in the distal nephron and colon, acidification of prostate fluids and secretion of HCO₃^- rich fluids in the exocrine pancreas.

Methodology: 

Experiments were performed on INS-1E rat insulinoma cells using RT-PCR, qRT-PCR, Western blot and flow cytometry.

Results: 

On the mRNA level we found expression of HKa2 as well as NHE isoforms 1, 2 and 7, but not HKa1. Western blotting revealed the expression of HKa2 on the protein level. The abundance of Atp12a mRNA increased 4-fold by incubating cells under high medium glucose (25 mM) for 48 hours, indicating regulated gene expression. Using combined Coulter-volume measurements and flow cytometry we found that in absence of other pro-apoptotic stimuli the H⁺/K⁺ ATPase blocker SCH-28080 applied for 24 hours caused a significant reduction of the mean cellular volume (MCV), an increase in annexin-V and caspase-positive cells. Pump inhibition exerted an additive effect on the pro-apoptotic action of the short-chain fatty acid butyrate (10 mM). Cell surface staining with an anti-HKa2 antibody gave a positive signal in 8% of cells under control conditions. The percentage significantly increased to 23% upon treatment with butyrate for 24 hours. Cells expressing Atp12a displayed a significantly reduced MCV compared to HKa2-negative cells.

Conclusion: 

We provide unprecedented evidence for the regulated and functional expression of the non-gastric H⁺/K⁺ ATPase Atp12a in b-cells, which might be associated with apoptosis. The pump might counteract intracellular acidification, K⁺ loss and cell shrinkage, and delay the progression of apoptotic cell death.