Mesangial AT1 receptors: expression, signaling, and regulation

Mesangial cells are one of the main targets of angiotensin II (AngII) in the renal cortex. AngII receptors on mesangial cells are of high affinity (nanomolar range). They belong to the AT1 subtype as shown by the inhibitory effect of AT1 antagonists on [125I]-Sar1, Ala8 AngII binding and on all of the biologic effects mediated by AngII, such as cytosolic calcium stimulation, inositol phosphate formation, prostaglandin production, and cell contraction. AngII also exerts long-term effects on mesangial cells, including stimulation of cell growth and synthesis of a variety of proteins, essentially the components of the extracellular matrix (collagen, fibronectin) and the type 1 inhibitor of plasminogen activator. These effects are mediated, at least in part, by autocrine products, in particular endothelin, platelet-derived growth factor, and transforming growth factor-beta, whose synthesis is enhanced by AngII. Treatment by an AT1 receptor blocker of mice with experimental nephritis inhibits activation of type I collagen alpha2 chain promoter and prevents the development of glomerulosclerosis. AngII receptors in rat mesangial cells are equally distributed between the AT1A and AT1B isoforms. Treatment of these cells by AngII or losartan, an AT1 receptor blocker, has no effect on AT1A and AT1B receptor mRNA expression, whereas candesartan, another AT1 receptor blocker, increases and dexamethasone decreases this expression.