Urea transport and clinical potential of urearetics

Abstract

Purpose of review: Urea is transported by urea transporter proteins in kidney, erythrocytes, and other tissues. Mice in which different urea transporters have been knocked out have urine-concentrating defects, which has led to the development and testing of urea transporters Slc14A2 (UT-A) and Slc14A1 (UT-B) inhibitors as urearetics. This review summarizes the knowledge gained during the past year on urea transporter regulation and investigations into the clinical potential of urearetics.

Recent findings: UT-A1 undergoes several posttranslational modifications that increase its function by increasing UT-A1 accumulation in the apical plasma membrane. UT-A1 is phosphorylated by protein kinase A, exchange protein activated by cyclic AMP, protein kinase Cα, and AMP-activated protein kinase, all at different serine residues. UT-A1 is also regulated by 14-3-3, which contributes to UT-A1 removal from the membrane. UT-A1 is glycosylated with various glycan moieties in animal models of diabetes mellitus. Transgenic expression of UT-A1 into UT-A1/UT-A3 knockout mice restores urine-concentrating ability. UT-B is present in descending vasa recta and urinary bladder, and is linked to bladder cancer. Inhibitors of UT-A and UT-B have been developed that result in diuresis with fewer abnormalities in serum electrolytes than conventional diuretics.

Summary: Urea transporters play critical roles in the urine-concentrating mechanism. Urea transport inhibitors are a promising new class of diuretic agent.

Figure 1

Diagram of transporters involved in urine concentration. In the center is a diagram of the nephron with cortex (CTX), outer medulla (OM) and inner medulla (IM) marked. Oval enlargements show locations of the sodium potassium 2 chloride cotransporter (NKCC2) and the renal outer medullary potassium channel (ROMK) in the thick ascending limb, aquaporins 2-4 (AQP2-4) in the inner medullary collecting duct (IMCD) and urea transporters (UT-A1 and UT-A3) in the IMCD. Top left enlargement shows IMCD cells with UT-A1 located apically and UT-A3 located basolaterally.

Figure 2

PKA and PKC consensus phosphorylation sites in UT-A1. This diagram presents UT-A1 in its theoretical 12 membrane spanning domain structure. Consensus phosphorylation sites for PKA are designated with open arrows; those for PKC are designated with closed arrows. The proven PKA (S486, S499) and PKC (Ser 494) sites are located in the large intracellular loop and designated by larger arrows.

Figure 3

Proposed model for the negative feedback of vasopressin stimulated UT-A1 activity by 14-3-3ɣ. This schematic shows vasopressin (AVP) binding to the type 2 vasopressin receptor (V2R) and stimulating cAMP production, which stimulates PKA to activate/phosphorylate UT-A1. It also shows the PKA stimulation of 14-3-3ɣ which then binds to UT-A1 promoting ubiquitination (Ub) and subsequent degradation, effectively reversing the original PKA-mediated activation. The data showing these opposite effects of PKA are established, but whether there is a negative feedback loop, or whether only one effect is dominant in vivo, remains to be determined.

Figure 4

Urearetics: comparison of parent structures and maximal IC₅₀’s. Shown are 5 classes of synthetic inhibitors of urea transporters. The IC₅₀’s of the most potent of each of these “urearetics” is also provided.