Arachidonic acid regulates surface expression of epithelial sodium channels

Epithelial Na+ channels (ENaCs) are regulated by the phospholipase A2 (PLA2) product arachidonic acid. Pharmacological inhibition of PLA2 with aristolochic acid induced a significant increase in amiloride-sensitive currents in Xenopus oocytes expressing ENaC. Arachidonic acid or 5,8,11,14-eicosatetraynoic acid (ETYA), a non-metabolized analog of arachidonic acid, induced a time-dependent inhibition of Na+ transport. These effects were also observed by co-expression of a calcium-independent or a calcium-dependent PLA2. Channels with a truncated alpha, beta,or gamma C terminus were not inhibited by arachidonic acid or ETYA. Furthermore, mutation of Tyr618 in the PY motif of the beta subunit abrogated the inhibitory effect of ETYA, suggesting that intact PY motifs participate in arachidonic acid-mediated ENaC inhibition. Analyses of channels expressing a series of beta subunit C-terminal truncations revealed a second region N-terminal to the PY motif (spanning residues betaVal580-betaGly599) that allowed for ETYA-mediated ENaC inhibition. Analyses of both ENaC surface expression and ENaC trafficking with mutants that either gate channels open or closed in response to [(2-(trimethylammonium) ethyl] methanethiosulfonate bromide, or with brefeldin A, suggest that ETYA reduces channel surface expression by inhibiting ENaC exocytosis and increasing ENaC endocytosis.