ABNORMAL POLARIZATION OF EGF RECEPTORS AND AUTOCRINE STIMULATION OF CYST EPITHELIAL GROWTH IN HUMAN ADPKD

The underlying mechanism of the hyperproliferative response of human a utosomal dominant polycystic kidney disease (ADPKD) epithelia was stud ied. Epidermal growth factor (EGF) protein is highly expressed in ADPK D cyst epithelia in vivo, and primary cultures are hyperesponsive to m itogenic stimulation by EGF in vitro. Doses of >1 ng/ml EGF were highl y mitogenic to ADPKD epithelia. H-3-labeled thymidine proliferation as says showed that cyst fluids and ADPKD epithelial cell-conditioned med ia also stimulated renal epithelial cell proliferation and contained E GF immunoreactivity (6, 30, and 37 kDa) as detected by Western blots. Radioimmunoassays detected mean levels of 2.87 and 1.4 ng/ml EGF in cy st fluids from early (proliferative) and end-stage ADPKD cysts, respec tively. Scatchard analysis of I-125-labeled EGF binding to apical and basolateral membrane showed high-affinity binding to basolateral membr anes of normal and ADPKD kidneys but additional unique high-affinity r eceptor binding to apical membranes of ADPKD but not normal kidneys. C ross-linking analysis and antiphosphotyrosine Western analysis demonst rated functionally active apical EGF receptors at 150-170 kDa. These r esults suggest mediation of cyst expansion via an autocrine loop invol ving EGF synthesis and processing by cyst epithelial cells, apical sec retion into cyst lumens, and subsequent binding to and phosphorylation of apical membrane EGF receptors. These findings are consistent with a membrane protein polarization defect in ADPKD cyst epithelia.