ROLE OF THE Na,K-ATPase IN POLYCYSTIC KIDNEY DISEASE

Abstract

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic disease, and is characterized by multiple fluid-filled cysts that impair the organ, ultimately leading to renal failure. Formation and enlargement of the cysts require abnormal proliferation and cell death, as well as changes in the transport properties of the renal tubule epithelial cells. Because of its primary role in the vectorial movement of salt and water in the kidney, the Na,K-ATPase has been the focus of investigation to understand the pathophysiology of ADPKD. However, the precise role of the transporter has not been identified. In this dissertation, we describe studies designed to characterize the Na,K-ATPase in ADPKD, and examine the mechanisms underlying its role in the disease. The transport properties of the Na,K-ATPase is regulated by the hormone ouabain, which inhibits its movement of the cations. In addition, ouabain binding to the Na,K-ATPase has been been found to activate a cascade of phosphorylating events, leading to cell growth. Interestingly, we have found a population of the Na,K-ATPase in human cystic epithelial cells to have a higher affinity for ouabain, at concentrations consistent with circulating levels of the hormone. Thus, nanomolar concentrations of ouabain that do not normally affect the activity of the Na,K-ATPase in the kidney partially inhibit the enzyme of cystic tissue and cells. The Na,K-ATPase has been found to interact with polycystin-1, and we have found this to association to increase the sensitivity of the enzyme to ouabain. Our hypothesis is that due to their increased sensitivity to the hormone in ADPKD, the cystic renal epithelium is more susceptible to the effects of endogenous ouabain. We found ouabain to stimulate both proliferation and apoptotic death of the ADPKD cells, causing a disbalance that favors increased cell growth. While the mitogenic effect of ouabain is mediated by activation of the epidermal growth factor receptor (EGFR), Src kinase and extracellular signal-regulated kinase (ERK) pathway, its apoptotic effect was found to be through activation of the intrinsic pathway of apoptotic cell death. We also found ouabain to exacerbate the development and growth of cysts, both in cultured human ADPKD cells and metanephric organ cultures from Pkd1m1Bei mice, a well characterized model of ADPKD. Collectively, these results demonstrate ouabain, acting through the Na,K-ATPase, is a novel agent that can adversely affect the progression of ADPKD.