Prolactins are natural inhibitors of angiogenesis in the retina.

PURPOSE Disruption of the normally antiangiogenic environment of the retina leads to aberrant angiogenesis, the major cause of vision loss throughout the world. Prolactin (PRL), the hormone originally associated with milk production, can be proteolytically processed to 16K-PRL, a 16 kDa N-terminal PRL fragment with potent antiangiogenic and vasoconstrictive actions. This study was conducted to determine whether 16K-PRL is found naturally in the retina and plays a role in angiogenesis and vasodilation. METHODS Expression of PRL mRNA in rat retina was determined by RT-PCR and in situ hybridization. Western blot was used to examine the expression of PRL and derived fragments in retinal homogenates. The role of PRL and 16K-PRL in the retina was studied by intravitreal injection of either antibodies against PRL or small interfering RNAs (siRNA), to suppress expression of retinal PRL mRNA. RESULTS Rat retina expressed PRL mRNA in the outer nuclear, outer plexiform, inner nuclear, and ganglion cell layers. Both full-length PRL and N-terminal 16K-PRL were detected in retinal homogenates by polyclonal and monoclonal antibodies. The intravitreal injection of antibodies able to neutralize the actions of 16K-PRL increased the number of retinal blood vessels and capillary area by threefold. Furthermore, siRNA-mediated inhibition of PRL mRNA increased retinal neovascularization threefold and resulted in a significant increase in vasodilation. CONCLUSIONS These results demonstrate that PRL is synthesized and cleaved to antiangiogenic 16K-PRL by retinal tissue and that these molecules play a key role in preventing angiogenesis in the healthy retina.