Vacuolar Hϩ -ATPases (V-ATPases) are a family of electrogenic ATP-driven proton pumps which function in almost every eukaryotic cell. V-ATPases are required to maintain proton gradients between intracellular compartments and for proton secretion from the plasma membrane of certain specialized cells (reviewed in references 32, 42, and 43). In the intracellular compartments, V-ATPases acidify early and late endosomes, lysosomes, and Golgi-derived secretory vesicles, providing the motive force and optimal pH for internalization and dissociation of ligand-receptor complexes, pH-driven secretion, and activation of lysosomal enzymes for protein processing and degradation (43, 48). Plasma membrane VATPases are essential for acid secretion and bicarbonate transport in the proximal tubule and collecting duct of the kidney, pH homeostasis in macrophages and neutrophils, acidification of the extracellular environment by certain tumor cells, and K ϩ secretion in insect midgut cells, (24, 43). V-ATPase-dependent acidification of the extracellular compartment between the osteoclast ruffled membrane and the bone surface is crucial for bone remodeling (23).V-ATPase is a large, multisubunit protein. It is composed of at least 13 subunits with a total molecular mass of about 900 kDa and has two domains, the V 1 domain (catalytic, membrane-associated, 640 kDa) and V o domain (membrane spanning, 260 kDa). The V 1 domain is responsible primarily for ATP hydrolysis. It consists of eight different subunits (A to H) in a stoichiometry of A 3 B 3 CDEFG 2 H 1-2 . V 1 attaches to the proton-translocating V o domain, which is composed of five subunits (a, b, c, cЈ, and cЉ) in a stoichiometry of abcЈcЉc 4 (20, 32, 43). Regulation of V-ATPase function in response to physiological stimuli is thought to be a multilevel process. It includes control of the expression of V-ATPase subunit genes (34, 62), intracellular targeting and translocation from vesicles to the plasma membrane (2, 9, 10), and reversible dissociation of the V o and V 1 domains, entailing inactivation of the pump (30,31,55).Several extrinsic regulatory factors have been reported to control V-ATPase-mediated proton transport. They include bicarbonate concentration and the closely related parameters pH and pCO 2 , mineralocorticoid hormones, endothelin, angiotensin II (reviewed in references 23 and 24), and cytokines, such as interleukin-1 (IL-1) (7) and IL-4 and -13 (59). The underlying intracellular signaling mechanisms remain largely unknown. Involvement of G proteins and protein kinases A and C has been implicated in several studies (23,24). The E subunit of V-ATPase is able to interact directly with the Dbl homology domain of the guanine nucleotide exchange factor

Transplant-related lung fibrosis is characterized by excessive fibro-collagenous deposition. Induction of arginase, an enzyme that metabolizes L-arginine to urea and L-ornithine, is vital for collagen synthesis. Pirfenidone is an investigational anti-fibrotic agent shown to be effective in blocking pulmonary fibrosis. The purpose of this study was to determine if pirfenidone was protective against the development of fibro-collagenous injury in rat lung orthotopic transplants through altering L-arginine-arginase metabolic pathways. Lung transplants were performed using Lewis donors and Sprague-Dawley recipients (allografts) or the same strain (isografts). Recipients were given pirfenidone (0.5% chow) 1-21-day posttransplantation. A significantly increased peak airway pressure (PawP) with excessive collagen deposition was found in untreated lung allografts. Pirfenidone treatment decreased PawP and collagen content in lung allografts. The beneficial effects were associated with downregulation of arginase protein expression and activity. In addition, pirfenidone decreased endogenous transforming growth factor (TGF)-b level in lung allografts, and TGF-b stimulated arginase activity in a dose-dependent manner in both lung tissue and fibroblasts. These results suggest that pirfenidone inhibits local arginase activity possibly through suppression of endogenous TGF-b , hence, limiting the development of fibrosis in lung allografts.

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