Regulation of the epithelial Na+ channel by paraoxonase-2

Abstract

Paraoxonase-2 (PON-2) is a membrane-bound lactonase with unique anti-oxidative and anti-atherosclerotic properties. PON-2 shares key structural elements with MEC-6, an endoplasmic reticulum-resident molecular chaperone in MEC-6 modulates the expression of a mechanotransductive ion channel comprising MEC-4 and MEC-10 in touch-receptor neurons. Because mRNA resides in multiple rat nephron segments, including the aldosterone-sensitive distal nephron where the epithelial Na channel (ENaC) is expressed, we hypothesized that PON-2 would similarly regulate ENaC expression. We observed PON-2 expression in aquaporin 2-positive principal cells of the distal nephron of adult human kidney. PON-2 also co-immunoprecipitated with ENaC when co-expressed in HEK293 cells. When PON-2 was co-expressed with ENaC in oocytes, ENaC activity was reduced, reflecting a reduction in ENaC surface expression. MEC-6 also reduced ENaC activity when co-expressed in oocytes. The PON-2 inhibitory effect was ENaC-specific, as PON-2 had no effect on functional expression of the renal outer medullary potassium channel. PON-2 did not alter the response of ENaC to extracellular Na, mechanical shear stress, or α-chymotrypsin-mediated proteolysis, suggesting that PON-2 did not alter the regulation of ENaC by these factors. Together, our data suggest that PON-2 regulates ENaC activity by modulating its intracellular trafficking and surface expression.

DUOX1‐mediated hydrogen peroxide release regulates sodium transport in H441 bronchiolar epithelial cells

“…This experiment can be interpreted as a modulation of tonical intracellular ROS level that favours our hypothesis that this level is necessary for basal sodium transport. In addition, H 2 O 2 has been shown to inactivate ubiquitination of lung α‐ENaC, thus increasing its surface expression, which is also consistent with the observations of Rochat et al Furthermore, recently Shi et al have reported that membrane expression of ENaC is regulated by paraoxonase‐2 (PON‐2), a membrane‐bound lactonase with unique antioxidative and antiatherosclerotic properties. They have shown that overexpression of PON‐2 reduces channel density at the cell membrane.…”

“…They have shown that overexpression of PON‐2 reduces channel density at the cell membrane. However, this study did not rule out the possibility that PON‐2 might modify membrane lipid composition and/or quantity of reactive oxygen species in the setting of oxidative stress, which, thus, might alter ENaC gating as well …”

Paraoxonase 3 functions as a chaperone to decrease functional expression of the epithelial sodium channel

“…The average amiloride-sensitive Na + current in oocytes co-expressing ENaC and PON3 (-1.4 ± 1.4 μA, n=60) is approximately 40% lower than that in oocytes expressing ENaC alone (-2.4 ± 1.8 μA, n=62, p < 0.01). This is similar to the extent of inhibition of PON2 on ENaC activity (57). The inhibitory effect of PON3 is specific to ENaC, as PON3 did not alter the renal outer medullary K + (ROMK) channel activity (-2.5 ± 1.8 μA in oocytes expressing ROMK vs -2.2 ± 1.9 μA in oocytes coexpressing PON3 and ROMK, Fig.…”

“…Nonetheless, our results using either cultured epithelial cells or Xenopus oocytes support that PON3 decreases ENaC functional expression. Other members of this family, including PON2 and MEC-6, also inhibit ENaC activity in oocytes (57). This negative regulation of channel activity is of great interest to us, as ENaC hyperactivity has physiological consequences (11,13,(98)(99)(100)(101).…”

“…MEC-6 is a chaperone protein that is required for the proper folding, assembly and surface expression of the touch-sensitive MEC-4/MEC-10 channel in the nematode's touch receptor neurons (55,56). We've recently shown that PON2 inhibits ENaC activity by reducing channel surface expression (57), suggesting that the chaperone function is conserved between mammalian PONs and their nematodes orthologs.…”