MMP9 and diabetes mellitus: Recently, several studies documented that MMP-9 expression is regulated by extracellular-signal-regulated kinases- (ERK-) dependent pathways [24–27] and in the retina RAF protooncogene serine/threonine-protein kinase- (Raf-) mitogen-activated protein kinase kinase (MEK)-ERK cascade is activated by diabetes [13, 24].