Angiopoietin-like protein 2 is a potent hemangiogenic and lymphangiogenic factor in corneal inflammation

We determined the plausible functional role of angiopoietin-like protein 2 (Angptl2) in inflammatory corneal hemangiogenesis and lymphangiogenesis in vivo.

Corneal hemangiogenesis and lymphangiogenesis were induced by suturing 10-0 nylon 1 mm away from the limbal vessel in Angptl2 knockout and K14-Angptl2 transgenic mice. We analyzed Angptl2 and interleukin 1β (IL-1β) expressions in normal and vascularized corneas by real-time RT-PCR and immunohistochemistry. Corneal hemangiogenic and lymphangiogenic responses, and macrophage infiltration were assessed by immunofluorescent microscopic studies using specific antibodies against CD31, LYVE-1, and F4/80, and compared to their corresponding background. Subconjunctival injection of Angptl2 siRNA to the sutured corneas was also performed.

Angptl2 mRNA expression increased markedly in the neovascularized corneas compared to the normal cornea. Angptl2 protein was expressed strongly in the corneal epithelium and stroma of the vascularized cornea. The regions showing hemangiogenesis and lymphangiogenesis were increased significantly in K14-Angptl2 mice and reduced in Angptl2(-/-) mice compared to their corresponding background strains. In contrast to control mice, the number of F4/80-positive cells, as well as the expressions of F4/80 and IL-1β were found to be higher in K14-Angptl2 mice and lower in Angptl2(-/-) mice. Subconjunctival injection of Angptl2 siRNA significantly inhibited hemangiogenesis and lymphangiogenesis in the sutured corneas.

Our findings demonstrated Angptl2 to be upregulated in corneal inflammation, and highlight that corneal hemangiogenesis and lymphangiogenesis may be driven by Angptk2 overexpression via macrophage infiltration and IL-1β expression. Angptl2 may be a novel therapeutic target for preventing blindness.