TMEM175 activity in BK-deficient macrophages maintains lysosomal function and mediates silica-induced inflammatory response in macrophages

Objective: Lysosomal ion channel function in macrophages contributes to the development of silica-induced inflammation. Recent studies have shown that blocking K+ entry into the lysosome via the BK channel reduces silica-induced damage and inflammation in macrophages. This study aims to explore the mechanisms of particle-induced inflammation in BK-/- macrophages. Methods: Bone marrow derived macrophages (BMdM) from C57BL/6 wildtype (WT) and BK-/- mice were exposed in vitro to silica and IL-1β release and cell death assessed. The effect of BK-/- on lysosomal pH, proteolytic activity, and cholesterol accumulation was evaluated. Results: BK-/- BMdM failed to demonstrate a reduction in IL-1β or cell death following silica exposure. BK-/- BMdM had comparable lysosome function to WT suggesting a compensatory mechanism was maintaining lysosome function. BK-/- macrophages demonstrated an upregulation of a second lysosomal potassium channel, TMEM175. Inhibition of TMEM175 activity caused an increase in lysosomal pH and reduced silica-induced cell death and IL-1β release in both BK-/- and WT BMdM. Conclusion: BK-/- BMdM did not exhibit the same phenotype seen with pharmaceutical abrogation of BK channel activity and showed no differences from WT in response to silica exposure. Upregulation of TMEM175 in BK-/- macrophages appears to prevent changes in lysosomal pH and cholesterol accumulation. Inhibiting TMEM175 activity in both BK-/- and WT BMdM resulted in an increase in lysosomal pH and reduced silica-induced inflammation, suggesting that reduced particle-induced cell damage and inflammation is not dependent on the activity of a single lysosomal ion channel but rather on mechanisms that elevate lysosomal pH.