Regulation of the acid-sensing ion channel 1 by protein kinase c and matriptase

Acid-sensing ion channel 1 (ASIC1) is a member of the Epithelial Na + channel/Degenerin (ENaC/Deg) family of ion channels. Like the other members of this family, it is sensitive to the diuretic amiloride. It was previously shown that both normal astrocytes and glioma cells express ASIC1 and some ENaC subunits. However, only high-grade glioma cells exhibit an inward Na+ current that is sensitive to amiloride and to psalmotoxin 1 (PcTX-1), a specific blocker of ASIC1. Amiloride, PcTX-1, and the knockdown of ASIC1 reduce glioma cell migration. The exact composition of the amiloride-sensitive channel and the molecular mechanisms that regulate its constitutive-expression in glioma cells are not known. In patch clamp experiments, inclusion of PKC in the patch pipette inhibited the inward Na+ current of a cultured glioma cell. This suggests that PKC may phosphorylate a component of the glioma channel, like ASIC1. It has been shown that serine proteases can regulate ENaC/Deg subunits. Matriptase, a trypsin-like type II transmembrane serine protease is overexpressed in many cancers. We have found that matriptase expression is higher in glioma cells than in normal astrocytes. Matriptase activates the ENaC channel, and ASIC1 can also be modulated by proteases. Therefore, in two separate studies I tested the following hypotheses about the regulation of ASIC1: (a) ASIC1 is inhibited by PKC, and (b) matriptase can regulate ASIC1 function through proteolytic cleavage. I found that PKC activation or inhibition both inhibit ASIC1 function, and that two consensus PKC phosphorylation sites (S40 and S499) on ASIC1 are critical sites mediating the modulation of ASIC1 by PKC. In addition, matriptase can cleave ASIC1 protein in three sites in the extracellular loop, R145, K185, and K384, inhibiting its function. The effect of matriptase on ASIC1 is specific, because matriptase does not affect the function of or cleave ASIC2. The modulation of ASIC1 by PKC and by matriptase could lead to a deeper understanding of PKC regulation and/or matriptase cleavage of this channel in glioma cells.