Eicosanoids and tumor necrosis factor-alpha in the kidney

Abstract

The thick ascending limb of Henle's loop (TAL) is capable of metabolizing arachidonic acid (AA) by cytochrome P450 (CYP450) and cyclooxygenase (COX) pathways and has been identified as a nephron segment that contributes to salt-sensitive hypertension. Previous studies demonstrated a prominent role for CYP450-dependent metabolism of AA to products that inhibited ion transport pathways in the TAL. However, COX-2 is constitutively expressed along all segments of the TAL and is increased in response to diverse stimuli. The ability of Tamm-Horsfall glycoprotein, a selective marker of cortical TAL (cTAL) and medullary (mTAL), to bind TNF and localize it to this nephron segment prompted studies to determine the capacity of mTAL cells to produce TNF and determine its effects on mTAL function. The colocalization of calcium-sensing receptor (CaR) and COX-2 in the TAL supports the notion that activation of CaR induces TNF-dependent COX-2 expression and PGE₂ synthesis in mTAL cells. Additional studies showed that TNF produced by mTAL cells inhibits ⁸⁶Rb uptake, an in vitro correlate of natriuresis, in an autocrine- and COX-2-dependent manner. The molecular mechanism for these effects likely includes inhibition of Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2) expression and trafficking.

Figure 1. Major transport pathways in the TAL

Selected transporter molecules in the TAL that contribute to NaCl reabsorption in this segment of the nephron include the apically expressed NKCC2 and ROMK K⁺ channel; the latter is important for K⁺ recycling that helps maintain NKCC2 function. Also note the basolateral localization of the CaR, Na⁺-K⁺-ATPase, and CLC-K chloride channel. The dashed arrow indicates paracellular transport of ions.

Figure 2. Colocalization of Calcium-sensing Receptor and COX-2 in the TAL

Double label immunostaining of TAL segments for COX-2 (blue) and Calcium-sensing Receptor (CaR) (brown); a similar protocol was used in our previous study . CaR is observed on the basolateral membrane, and each cell expressing COX-2 co-expresses CaR; scale bars = 50 μm. Tissue sections were observed and photographed on a Nikon Eclipse 600 microscope with a Nikon DXM1200 digital photographic system (Nikon Corporation, Tokyo, Japan).

Figure 3. Schematic of the CaR/TNF/COX-2 regulatory system in the TAL

The diagram shows that activation of CaR increases TNF production, via activation of the NFAT5 (Ton/EBP) transcription factor, which subsequently increases COX-2-derived PGE₂ synthesis. The regulation of NKCC2 by TNF is postulated to include a mechanism involving COX-2 as well as a COX-independent pathway that may be important under basal conditions.