MMP9 and Peritoneal Fibrosis: Early studies on immortalized and primary human peritoneal mesothelial cells demonstrated that upon incubation with hyperosmolar solutions (osmolytes: glucose, mannitol, or NaCl) the matrix metalloproteinase 9 (MMP9) was downregulated favoring accumulation of collagen type IV, thereby potentially promoting the development of peritoneal fibrosis [29].