Phosphoinositide Binding Differentially Regulates NHE1 Na+/H+ Exchanger-dependent Proximal Tubule Cell Survival

Lipotoxic disruption of NHE1 interaction with PI(4,5)P2 expedites proximal tubule apoptosis

“…Neither palmitic nor oleic acid bound to NHE1 (data not shown). Palmitoyl-CoA binding was weakly pH dependent (Figure 3C), a characteristic that was also observed for PI(4,5)P2 (10). PI(4,5)P2 interacts with 2 Lys/Arg-rich, juxtamembrane NHE1 domains (10,45).…”

“…To determine whether LCCoAs bind directly to the cNHE1, and potentially compete with PI(4,5)P2 for binding, surface plasmon resonance (SPR) was used to assess LC-CoA/NHE1 binding characteristics. Palmitoyl-CoA ( Figure 3A) and oleoyl-CoA ( Figure 3B) bound the NHE1 tail with low affinity, at a K d similar to that in the PI(4,5)P2-NHE1 interaction (10). Neither palmitic nor oleic acid bound to NHE1 (data not shown).…”

“…The initial metabolites of NEFAs, LC-CoAs, are structurally similar to phosphoinositides (Figure 2A), which regulate proximal tubule cell survival through interaction with the NHE1 cytosolic tail (cNHE1) (10). To explore whether LC-CoAs mediate lipotoxicity in vivo, NEFA and LC-CoA concentrations were assayed from renal cortices of eNOS -/-db/db mice and nondiabetic, littermate controls.…”

“…Proximal tubule NHE1 activity is cytoprotective (9) and dependent upon a low-affinity electrostatic interaction with plasma membrane inner leaflet PI(4,5)P2 (10,45). To determine whether LCCoAs bind directly to the cNHE1, and potentially compete with PI(4,5)P2 for binding, surface plasmon resonance (SPR) was used to assess LC-CoA/NHE1 binding characteristics.…”

“…Most forms of chronic kidney disease are initiated by glomerular injury, but progression to end-stage renal disease is more tightly coupled to tubular atrophy and interstitial fibrosis, compared with glomerular pathology (2)(3)(4). Proximal tubule epithelial cell apoptosis is a mechanism of tubular atrophy (5-7), and proximal tubule NHE1 defends against apoptosis through Na + /H + exchange as well as by functioning as a scaffold for the membrane phosphoinositide phosphatidylinositol 4,5-bisphos phate [PI (4,5)P2] and associated cell survival signaling cascades (8)(9)(10). Experimental NHE1 inhibition using genetic or chemical inhibitor approaches causes tubular epithelial cell apoptosis (8,11), but an vivo mechanism of NHE1 inactivation in the pathophysiology of chronic kidney disease has not been described.…”