Division of Nephrology, University of Texas Medical Branch, 301 University Blvd., Galveston, TX 77555-0562, USA.
Am J Physiol Cell Physiol. 2011 Dec;301(6):C1296-306. doi: 10.1152/ajpcell.00237.2011. Epub 2011 Aug 31.
Sepsis is associated with defects in renal tubule function, but the underlying mechanisms are incompletely understood. Recently, we demonstrated that Gram-negative bacterial lipopolysaccharide (LPS) inhibits HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through activation of Toll-like receptor 4 (TLR4). Here, we examined the mechanisms responsible for inhibition of HCO(3)(-) absorption by basolateral LPS. Adding LPS to the bath decreased HCO(3)(-) absorption by 30% in rat and mouse MTALs perfused in vitro. The inhibition of HCO(3)(-) absorption was eliminated by the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)/ERK inhibitors U0126 and PD98059. LPS induced a rapid (<15 min) and sustained (up to 60 min) increase in ERK phosphorylation in microdissected MTALs that was blocked by PD98059. The effects of basolateral LPS to activate ERK and inhibit HCO(3)(-) absorption were eliminated in MTALs from TLR4(-/-) and myeloid differentiation factor 88 (MyD88)(-/-) mice but were preserved in MTALs from TIR (Toll/interleukin-1 receptor) domain-containing adapter-inducing interferon-β (Trif)(-/-) mice. Basolateral LPS decreased apical Na(+)/H(+) exchanger 3 NHE3 activity through a decrease in maximal velocity (V(max)). The inhibition of NHE3 by LPS was eliminated by MEK/ERK inhibitors. LPS inhibited HCO(3)(-) absorption despite the presence of physiological stimuli that activate ERK in the MTAL. We conclude that basolateral LPS inhibits HCO(3)(-) absorption in the MTAL through activation of a TLR4/MyD88/MEK/ERK pathway coupled to inhibition of NHE3. These studies identify NHE3 as a target of TLR4 signaling in the MTAL and show that bacterial molecules can impair the absorptive functions of renal tubules through inhibition of this exchanger. The ERK pathway links TLR4 to downstream modulation of ion transport proteins and represents a potential target for treatment of sepsis-induced renal tubule dysfunction.
脓毒症与肾小管功能缺陷有关，但潜在机制尚不完全清楚。最近，我们证明革兰氏阴性细菌脂多糖（LPS）通过激活 Toll 样受体 4（TLR4）抑制髓质升支粗段（MTAL）中的 HCO3(-)吸收。在这里，我们研究了基底外侧 LPS 抑制 HCO3(-)吸收的机制。在体外灌流的大鼠和小鼠 MTAL 中，将 LPS 添加到浴液中可使 HCO3(-)吸收减少 30%。MAPK/ERK 抑制剂 U0126 和 PD98059 消除了 HCO3(-)吸收的抑制作用。LPS 在微分离的 MTAL 中诱导 ERK 磷酸化的快速（<15 分钟）和持续（长达 60 分钟）增加，该增加被 PD98059 阻断。TLR4(-/-)和髓样分化因子 88（MyD88)(-/-)小鼠 MTAL 中基底外侧 LPS 激活 ERK 和抑制 HCO3(-)吸收的作用被消除，但在 TIR（Toll/interleukin-1 受体）域包含衔接诱导干扰素-β（Trif)(-/-) 小鼠的 MTAL 中被保留。基底外侧 LPS 通过降低最大速度（V(max)）来降低顶端 Na(+)/H(+)交换器 3 NHE3 活性。MEK/ERK 抑制剂消除了 LPS 对 NHE3 的抑制作用。尽管 MTAL 中存在激活 ERK 的生理刺激，但 LPS 仍抑制 HCO3(-)吸收。我们得出结论，基底外侧 LPS 通过激活 TLR4/MyD88/MEK/ERK 途径抑制 NHE3 来抑制 MTAL 中的 HCO3(-)吸收。这些研究确定 NHE3 是 MTAL 中 TLR4 信号的靶标，并表明细菌分子可以通过抑制这种交换器来损害肾小管的吸收功能。ERK 途径将 TLR4 与离子转运蛋白的下游调节联系起来，代表治疗脓毒症引起的肾小管功能障碍的潜在靶点。
