The regulation of aldosterone secretion is complex. Its biosynthesis is stimulated acutely and chronically primarily by angiotensin II (A-II) and potassium (K+) and acutely by ACTH. Excessive and autonomous production of aldosterone occurs in disorders of the adrenal zona glomerulosa (ZG) producing Primary Aldosteronism (PA). PA is the most common cause of secondary hypertension and is associated with greater risk of cardiovascular complications and death than essential hypertension of similar severity and duration. Most PA is caused by aldosterone-producing adenomas (APA) and idiopathic hyperaldosteronism (IH). The incidence of APA and IH is similar. The most recent breakthrough in understanding the pathophysiology of PA has been the discovery of somatic mutations in APAs, including those in the selectivity filter of the potassium channel KCNJ5, the sodium/potassium ATPase gene (ATP1A1), calcium ATPase, ATP2B3, CACNA1D, CACNA1H and CLCN2. The mechanism by which mutations of the sodium/potassium ATPase increases aldosterone secretion and cell proliferation for adenoma formation has been studied with contradictory results, suggesting either a loss-of-function or a gain-of-function produced by the mutations. ATP1A1 mutations occur in 25% of adenomas from white men. We will address the following hypothesis: ?Somatic mutations of the ATP1A1 gene (?-subunit of the Na/K- ATPase) found in some aldosterone producing adenomas increase aldosterone biosynthesis and cell proliferation through two different mechanisms: creation of functional defects in the pump (loss-of-function), causing unregulated zona glomerulosa cell membrane depolarization, and alteration the signal transduction function of the Na/K-ATPase complex, resulting in Src and MAPK activation (gain-of-function)?.
Our aims address the mechanisms by which these mutations regulate steroidogenesis and cell proliferation. 1: Elucidate the mechanisms of increased aldosterone biosynthesis and adrenal cell proliferation produced by the mutations of the human ATP1A1: L104R, V332G, del100-104, EETA963S and G99R. Do these mutations cause a gain-of-function or a loss-of-function of the ATP1A1 pump? Do different gene mutations cause a pump loss-of-function but a gain-of-function of the signaling pathways also associated with the ATP1A1. 2. Elucidate the effects of ATP1A1 and its mutations on the role of c-Src in aldo synthesis. Study the role of the sodium/calcium exchanger which is associated with the ATP1A1 on the alterations in aldo synthesis. These alternate explanations for the increase in aldo secretion will be tested in cells in which normal ATP1A1 is decreased by the shRNA and in those transduced with the various ATP1A1 mutations. These studies will elucidate mechanisms by which mutations of the ATP1A1 participate in the genesis of the hyperaldosteronism and adenoma formation and may help explain other causes of inappropriate aldo synthesis, thus lead to the development of more specific treatments than the mineralocorticoid receptor antagonists.
Primary aldosteronism is the most common cause of secondary hypertension (5-10%) and is associated with a very high cardiovascular morbidity and mortality. Aldosterone-producing adenomas occur in about half of these patients. We propose to study the mechanism(s) by which mutations the sodium/potassium ATPase found in many of these adenomas contribute to the excessive unregulated production of aldosterone and promote growth of the adenoma cells. Results of these studies should lead to more specific treatments of excessive aldosterone production in other forms of hypertension and cardiovascular disease associated with inappropriate aldosterone synthesis.
