UT-A urea transporter protein expressed in liver: Upregulation by uremia

In perfused rat liver. there is phloretin-inhibitable urea efflux, but whet her it is mediated by the kidney UT-A urea transporter family is unknown, T o determine whether cultured HepG2 cells transport urea, thiourea influx wa s measured. HepG2 cells had a thiourea influx rate of 1739 +/- 156 nmol/g p rotein per min; influx was inhibited 46% by phloretin and 32% by thionicoti namide, Western analysis of HepG2 cell lysate using an antibody to UT-A1. U T-A2, and UT-A4 revealed two protein bands: 49 and 36 kD, The same bands we re detected in cultured rat hepatocytes, freshly isolated mt hepatocytes, a nd in liver from rat, mouse, and chimpanzee. Both bands were present when a nalyzed by native gel electrophore sis, and deglycosylation of rat liver ly sate had no effect on either band, Differential centrifugation of tat river lysate showed that the 49-kD protein is in the membrane fraction and the 3 6-kD protein is in the cytoplasm, To determine whether the abundance of the se UT-A proteins varies in I vivo, rats were made uremic by 5/6 nephrectomy . The 49-kD protein was significantly increased, 5.5-fold in live rs from u remic mts compared to pair-fed control mts, It is concluded that phloretin- inhibitable urea flux in liver may occur via a 49-kD protein that is specif ically detected by a UT-A antibody. Uremia increases the abundance of this 49-kD UT-A protein in rat liver in vivo.