Lumen LPS inhibits HCO₃⁻absorption in the medullary thick ascending limb through TLR4-PI3K-Akt-mTOR-dependent inhibition of basolateral Na⁺/H⁺exchange

Abstract

Sepsis and endotoxemia induce defects in renal tubule function, but the mechanisms are poorly understood. Recently, we demonstrated that lipopolysaccharide (LPS) inhibits HCO3(-) absorption in the medullary thick ascending limb (MTAL) through activation of different Toll-like receptor 4 (TLR4) signaling pathways in the basolateral and apical membranes. Basolateral LPS inhibits HCO3(-) absorption through ERK-dependent inhibition of the apical Na(+)/H(+) exchanger NHE3. Here, we examined the mechanisms of inhibition by lumen LPS. Adding LPS to the lumen decreased HCO3(-) absorption by 29% in rat and mouse MTALs perfused in vitro. Inhibitors of phosphoinositide 3-kinase (PI3K) or its effectors Akt and mammalian target of rapamycin (mTOR) eliminated inhibition of HCO3(-) absorption by lumen LPS but had no effect on inhibition by bath LPS. Exposure to LPS for 15 min induced increases in phosphorylation of Akt and mTOR in microdissected MTALs that were blocked by wortmannin, consistent with activation of Akt and mTOR downstream of PI3K. The effects of lumen LPS to activate Akt and inhibit HCO3(-) absorption were eliminated in MTALs from TLR4(-/-) and MyD88(-/-) mice but preserved in tubules lacking Trif or CD14. Inhibition of HCO3(-) absorption by lumen LPS was eliminated under conditions that inhibit basolateral Na(+)/H(+) exchange and prevent inhibition of HCO3(-) absorption mediated through NHE1. Lumen LPS decreased basolateral Na(+)/H(+) exchange activity through PI3K. We conclude that lumen LPS inhibits HCO3(-) absorption in the MTAL through TLR4/MyD88-dependent activation of a PI3K-Akt-mTOR pathway coupled to inhibition of NHE1. Molecular components of the TLR4-PI3K-mTOR pathway represent potential therapeutic targets for sepsis-induced renal tubule dysfunction.

Suppression of Mitochondrial Biogenesis through Toll-Like Receptor 4–Dependent Mitogen-Activated Protein Kinase Kinase/Extracellular Signal-Regulated Kinase Signaling in Endotoxin-Induced Acute Kidney Injury

“…We observed rapid (1 hour) activation of TPL-2/MEK/ERK signaling in the renal cortex. These findings are similar to those of previous studies demonstrating TLR4-dependent increases in ERK1/2 phosphorylation after LPS exposure in the medullary thick ascending limb of the kidney (Watts et al, 2013). Although TPL-2 is known to be required for LPS-induced ERK1/2 activation in macrophages, this is the first study to demonstrate increased TPL-2 phosphorylation in the renal cortex in response to systemic endotoxin exposure (Dumitru et al, 2000;Banerjee et al, 2006).…”

Renal effects of treatment with a TLR4-inhibitor in conscious septic sheep

“…Taken together, these observations indicate that neutrophil activation and recruitment into the kidney are potential and perhaps crucial mediators of septic AKI, although the underlying mechanism remains unknown. As shown in a series of elegant experiments by Watts and colleagues [47][48][49], TLR4 activation may also affect renal function by impairing tubular transport. In particular, HCO 3 − reabsorption is inhibited by the direct effect of LPS on TLR4, and this may contribute to sepsisinduced acidosis.…”

Acid-Base Homeostasis

“…First, the thick ascending limb (TAL) reabsorbs a significant amount of HCO 3(e.g., Toll-like receptors, dietary salt, aldosterone, angiotensin II, etc. [36][37][38]), but the integrated role of the thick limb acid-base transport in various acid-base disorders has not been well clarified.…”