Endothelin and calciotropic hormones share regulatory pathways in multidrug resistance protein 2-mediated transport

Abstract

The kidney of vertebrates plays a key role in excretion of endogenous waste products and xenobiotics. Active secretion in the proximal nephron is at the basis of this excretion, mediated by carrier proteins including multidrug resistance protein 2 (Mrp2). We previously showed that Mrp2 function is reduced by endothelin-1 (ET-1) through a basolateral B-type receptor, nitric oxide (NO), cGMP and protein kinase C (PKC). This pathway was rapidly activated by several nephrotoxicants and appeared to be calcium dependent. In the present study we studied the effect of the calciotropic hormones, parathyroid hormone (PTH), PTH-related protein (PTHrP) and stanniocalcin (STC), to interfere with ET-regulated Mrp2 transport. Like ET-1, PTH reduces Mrp2-mediated transport for 40% in killifish renal proximal tubules. When given combined an additive effect was seen, which is partially reversed by the PKC inhibitor calphostin C. Recombinant PTHrP shows a comparable inhibitory effect, which is concentration-dependent and additive to the inhibition by ET. STC fully reverses PTHrP inhibited transport as does a guanylyl cyclase inhibitor. Finally, to confirm PTHrP bioactivity in a homologous assay we performed immunolocalization and transport studies in sea bream kidney tubules. Mrp2 immunoreactivity was observed in about 40% of the tubules and is associated with the brush border and apical plasma membrane of cells. Both proximal tubules and distal (collecting) tubules express the antigen. A highly significant 40% inhibition of Mrp2-mediated transport was observed with PTHrP in sea bream tubules. In conclusion, ET-regulated Mrp2 transport is influenced by calciotropic hormones and involves PKC and cGMP signaling. Key words: endothelin, stanniocalcin., parathyroid hormone related protein, parathyroid hormone, multidrug resistance-associated protein 2.